Renewable aromatic production from waste: exploring pathways, source materials, and catalysts

Abstract

The growing demand for aromatic compounds, essential in industries like pharmaceuticals, agrochemicals, and materials manufacturing, coupled with rising environmental concerns, has driven significant research toward renewable production pathways. Traditionally, aromatic synthesis relies on fossil fuels, contributing to resource depletion, climate change, and ecological damage. This review explores the emerging field of producing aromatic compounds from waste materials, such as biomass, agricultural residues, plastic waste, and industrial by-products, offering a sustainable alternative to conventional methods. The major key processes for converting waste into valuable aromatics have been covered in this review, including pyrolysis, lignin depolymerization, cellulose depolymerization, hemicellulose to furfural, CO₂ capture and chemocatalytic techniques. A central focus is placed on the role of catalysts, discussing the latest advancements in catalyst design, selectivity, stability, and recyclability to improve conversion efficiency. This review gives insights into current developments, highlighting the potential of waste-to-aromatic processes to address environmental and economic challenges. It identifies research gaps and future directions to further advance the field. By promoting waste valorization, resource efficiency, and sustainability, this paper contributes to the growing efforts toward a circular economy.

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Green foundation

1. The manuscript explores green chemistry advancements for sustainable aromatic production from waste feedstocks, focusing on catalytic processes like pyrolysis, lignin depolymerization, and CO₂ capture. It discusses advances in catalyst design—improving selectivity, stability, and recyclability.

2. This study addresses global challenges by transforming waste into valuable aromatics via renewable pathways, aligning with sustainability and circular economy principles. Advances in catalysis enhance efficiency, promoting greener practices and reducing environmental footprints.

3. The future of sustainable aromatic production involves waste-derived feedstocks and green chemistry. This review provides insights into renewable feedstocks, catalytic advancements, and material selection, fostering collaboration for scalable, sustainable processes.

1. Introduction

The growing global demand for aromatic compounds as the platform chemicals for the manufacture of various day-to-day products, coupled with increasing environmental concerns and the need to switch to sustainable practices, has propelled research efforts toward exploring renewable pathways for their production.^1–3 Aromatic compounds are essential to several industries, such as agrochemicals,⁴ materials manufacture,⁵ pharmaceuticals,⁶etc. However, most of the feedstocks used in traditional aromatic chemical synthesis processes come from fossil fuels, posing severe threats to the economy; this reliance on limited resources causes environmental deterioration and heightens worries about climate change.⁷ Traditional methods for synthesizing aromatic compounds, which mostly rely on feedstocks sourced from fossil fuels, significantly impact air pollution, greenhouse gas emissions, and ecological disturbance.⁸ It is becoming increasingly necessary to switch to sustainable methods, considering these serious environmental issues. Thus, the search for sustainable methods for producing aromatic compounds has become a significant area of interest for research and innovation.⁹ We may simultaneously promote economic resilience and resource efficiency by reducing reliance on fossil fuels and utilizing renewable resources, such as waste materials, to minimize the adverse environmental effects of traditional synthesis approaches.¹⁰

Using waste materials as renewable sources for aromatic compound production has received much attention in recent years as a solution to these difficulties.⁹ First and foremost, it tackles the crucial problem of waste management by recycling items that would otherwise be thrown away or dumped in landfills, which lessens the load on waste management infrastructures and mitigates environmental degradation.¹¹ Second, recycling garbage into beneficial aromatic compounds minimizes resource depletion and closes the material usage loop.¹² This is a sustainable approach to resource conservation. Furthermore, waste as a feedstock is renewable, ensuring long-term availability and providing a dependable and robust supply chain for the manufacture of aromatic compounds. Furthermore, the circular economy paradigm—which separates economic expansion from environmental deterioration and maximizes resource efficiency—benefits from converting waste into high-value products.¹³ Overall, using waste as a renewable resource for aromatic compound production holds immense potential to simultaneously address environmental, economic, and societal challenges holistically and sustainably. Thus, with an emphasis on investigating routes, resources, and catalysts for the sustainable synthesis of aromatic compounds from waste, this review paper offers a thorough summary of the most recent developments.¹⁴

This review begins by examining the industrial process for converting various forms of waste, such as biomass, agricultural residues, plastic waste, and industrial by-products, into valuable aromatic compounds. Furthermore, the review delves into different processes, like pyrolysis, lignin depolymerization, cellulose depolymerization, and chemocatalytic approach. It also delves into the intricate role of catalysts in facilitating efficient conversion processes. It discusses the design principles, catalytic mechanisms, and recent developments in catalyst technologies tailored for renewable aromatic production. Emphasis is placed on catalyst selectivity, stability, and recyclability, which are crucial for achieving high yields and minimizing environmental impact.

Although reviews have been done, a detailed understanding of the material for each type of biomass source needs to be compiled. Our review provides a comprehensive analysis of renewable aromatic compound production using waste-derived feedstocks, addressing the pressing need for sustainable alternatives to traditional fossil fuel-based processes. To compile current information, identify significant obstacles, and suggest future research paths in renewable aromatic production from waste carbon, this review paper critically analyses the literature that has already been published the research findings. It synthesizes insights from environmental science, engineering, and chemistry to give scholars, decision-makers, and industry stakeholders working toward a sustainable and circular economy a comprehensive resource.

2. Scope of the review

In the past decades, reviews have been done on various topics like pyrolysis of cellulose for biofuel production,¹⁵ furfural production from hemicellulose,¹⁶ aromatics from lignin,¹⁷ aromatics from the utilization of CO₂.¹⁸ This review explores a range of waste materials, including biomass, agricultural residues, plastic waste, and industrial by-products, focusing on their potential as renewable sources for aromatic production. It emphasizes the production of aromatics through all possible routes, such as pyrolysis, lignin, cellulose, and hemicellulose depolymerization, CO₂ capture, and chemocatalytic methods. A central aspect of the review is the role of catalysts in enhancing the efficiency of these conversion processes. We also explain the types of renewable aromatics produced and the specific catalysts best suited for each source. This combined approach not only connects these processes but also provides a clearer and more complete understanding of renewable aromatics production and catalyst development. It highlights advances in catalyst design, focusing on selectivity, stability, and recyclability—key factors in achieving high yields while reducing environmental impact. By integrating insights from chemistry, engineering, and environmental science, the paper aims to present a holistic view of current developments in this field. This review synthesizes the latest research, identifies challenges, and suggests future directions for the sustainable synthesis of aromatic compounds from waste. It serves as a valuable resource for researchers, industry professionals, and policymakers interested in promoting resource efficiency and advancing a circular economy. A detailed review outline is shown in Fig. 1.

Fig. 1 Scope of the review.

3. Industrial process for aromatic production

Industrial production techniques are constantly changing to suit the growing demand for these compounds in an effective and sustainable manner. Traditionally, catalytic reforming and steam cracking are used to create aromatic chemicals from crude oil.¹⁹ However, there has been a change in the direction of creating alternate pathways for the synthesis of aromatics due to growing concerns about sustainability and its impact on the environment.²⁰ The base materials used in the chemical industry to produce aromatic compounds are fossil raw materials such as crude oil, coal, and natural gas.²¹ The worldwide demand for carbon-containing feedstocks in the chemical industry is approximately 245 Mt oil equivalent, with crude oil being the primary source, followed by natural gas, coal, and renewable raw materials.²² The origin of fossil raw materials, including crude oil, coal, and natural gas, is rooted in photosynthesis, leading to the formation of organic materials.²³ The production of aromatic compounds has a long history and has evolved significantly over time.²⁴ Distillation of wood was one of the important methods for producing aliphatic and aromatic compounds.²⁵ The petrochemical sector has grown to be a significant force in the manufacturing of aromatic compounds over time. Nowadays, fossil fuels—most notably crude oil and natural gas—are used to manufacture aromatic chemicals like benzene, toluene, and xylene. These fossil fuels are refined using a variety of techniques, including steam cracking and catalytic reforming, to produce a large number of aromatic chemicals which are widely utilized in the manufacturing of synthetic textiles, plastics, medicines, and numerous other industrial goods.²⁶ The synthesis of aromatic compounds has been greatly influenced by the dominance of fossil fuel-based technologies despite the existence of traditional processes utilizing biomass sources. This shift towards fossil fuel feedstocks began during World War I when the production of aromatic compounds from coal through destructive hydrogenation was pioneered in Germany.²⁷ During World War II, the demand for toluene for the manufacturing of trinitrotoluene led to the petroleum industry entering the field of aromatics production on a large scale.²⁸ The use of fossil fuels for the production of aromatic compounds has greatly expanded since then.

The dominance of fossil fuel-based production methods has been further solidified by advancements in refining processes. For example, catalytic reforming and steam cracking of crude oil & natural gas have enabled the large-scale production of benzene, toluene, and xylene.²⁴ Additionally, fossil fuel feedstocks are readily available and have high levels of purity, which contributes to the overall efficiency and quality of aromatic compound production. The industry has used several conventional techniques to produce aromatic chemicals, such as catalytic reforming & steam cracking of natural gas and crude oil.²⁹ By using catalysts and high temperatures and pressures, these techniques transform hydrocarbon feedstocks into aromatic chemicals.³⁰ Comparably, another popular technique is steam cracking,²⁹ which involves heating hydrocarbon fractions to high temperatures in order to break them down into smaller molecules, including aromatic compounds. Because of their effectiveness and affordability,³¹ these techniques have been effectively applied to the large-scale synthesis of aromatic molecules. For many years, the petrochemical, pharmaceutical, and agrochemical sectors have relied on conventional methods to deliver a consistent supply of aromatic compounds.²³

Large-scale production occurs for the most often utilized aromatic chemicals, toluene and xylene. Reformer gasoline, pyrolysis gasoline, & coke oven benzole are the main suppliers of Benzene, Toluene, and Xylene (BTX) aromatics.³² These mixtures’ compositions differ based on the initial feedstock and processing parameters. Several techniques, including liquid–liquid extraction, extractive distillation, and azeotropic distillation, are used to recover aromatics from these mixtures.³³ By utilizing their solubility in particular solvents and their respective melting and boiling temperatures, these procedures seek to separate the different BTX aromatics. Reduction of mixed aromatics to their component parts, like benzene, toluene, and xylenes, is accomplished via crystallization and distillation techniques.³⁴ Dealkylation, isomerization, and disproportionation are among the specific processes that are used in addition to direct synthesis from aromatic mixtures.³⁵ Overall, the detailed production of aromatics from BTX involves a combination of extraction, distillation, and reaction processes to isolate and purify BTX mixture from coal and petroleum feedstocks.²³

Thermal and catalytic procedures are the primary means of manufacturing aromatics.³⁴ In thermal procedures, the hydrocarbons undergo treatment at temperatures ranging from 400 to 2000 °C, which causes the carbon skeleton to reorganize.³⁶ Naturally, the hydrocarbon feedstock's structure has a significant impact on how much the rearranged products aromatize.³⁶ However, when cracking severity is plotted against the aromaticity of the generated liquids, a qualitative relationship may be inferred (Fig. 2) as a function of reaction temperature (T) and residence time (τ) of the hydrocarbon mixture in the thermal reaction.²³ Visbreaking is the mildest thermal cracking method, with comparatively little aromatization.³⁷ Thermal cracking, thermal reforming (formerly used to make gasoline), low-temperature carbonization, and coal gasification all have an intermediary role in aromatization.³⁸ Processes that cause more severe cracking, like steam cracking, crude oil cracking, and high-temperature carbonization of coal, cause an increase in aromatization.³⁴ It is important to distinguish between hydrogenation processes and reforming and catalytic cracking processes among the catalytic processes. Increasing the hydrogen content, especially of the distillates, is the goal of hydrogenation techniques, whether they are used on coal or petroleum residues.^34,39

Fig. 2 Formation of aromatics during the thermal transformation of hydrocarbons.

As a result, the aromaticity of the product's distillable portion falls. On the other hand, because isomerization reactions take place under the reaction circumstances & cyclic aliphatic hydrocarbons are transformed into aromatics, there is an increase in aromatization during the catalytic cracking and reforming processes. Compared to solely thermal techniques for generating aromatics, it is less convenient to infer a reliance on product aromaticity on reaction parameters in catalytic processes. The primary processing data for the thermal and catalytic methods used to convert hydrocarbons to aromatics are replicated in Fig. 3.

Fig. 3 Major processes review and condition for aromatics productions.

Moving the aromatic compound production process away from fossil fuels is still a major concern for the industry as it looks to the future. A more sustainable and eco-friendly future for the aromatic compounds industry will be made possible by sustained research and innovation, leading to the development of scalable and financially feasible technologies utilizing renewable feedstocks. A multifaceted strategy is necessary for the transition to sustainable aromatic compound production. Creating catalytic techniques that transform renewable feedstocks into aromatic chemicals is one element. This is consistent with the research conducted by Ratnasari et al.,⁴⁰ who emphasized the possibility of catalytically producing aromatic chemicals from lignocellulosic biomass. Utilizing both thermal and catalytic processes, biomass can be effectively transformed into valuable aromatic compounds, presenting a viable and sustainable substitute for conventional technologies that rely on fossil fuels. An essential component of the sustainable synthesis of aromatic chemicals is catalytic reactions. Jia et al. research highlights the possibility of zeolite catalysts in the production of aromatic chemicals from renewable feedstocks.⁴¹ Zeolites are a viable option for the effective synthesis of aromatic compounds from bio-based sources due to their excellent selectivity and activity.

4. Need for the transition from fossil-based to waste sources for the aromatics production

Fossil fuel-derived aromatics rely on expensive and finite crude oil resources, whereas waste-derived aromatics often utilize low-cost or negative-value feedstocks (e.g., agricultural residues or industrial by-products).⁴² Waste-derived aromatics make use of feedstocks such as municipal solid waste, food industry byproducts, and agricultural residues, which are inexpensive or even negative-cost resources because of the costs involved in disposing of garbage. In contrast, fossil fuel-derived aromatics rely on crude oil, a finite and increasingly expensive resource subject to market volatility.⁴³ The earth's surface is expected to warm by at least 3 °C by the end of this century as long as people continue to rely on fossil fuels for energy.⁴⁴ Presently, the vast majority of aromatic compounds are derived from fossil sources. According to recent data, approximately 97% of aromatics are produced as by-products from crude oil refining, with the remaining 3% sourced from coal-based coke oven oil. This heavy reliance on fossil feedstocks underscores the need for alternative, more sustainable production methods.⁴⁵ Although production methods based on fossil fuels, like catalytic reforming, are extremely efficient, developments in technology sourced from waste are closing the efficiency gap. For example, the production of heterogeneous catalysts, such as functionalized carbons and zeolites, has increased the yield and selectivity of bio-aromatics.⁴⁶ When compared to fossil fuel-based methods, waste-derived aromatic synthesis dramatically lowers greenhouse gas emissions, water use, and energy consumption, according to life-cycle assessments (LCA).⁴⁷ According to recent research, mixing bio-oil with conventional fossil fuels can cut carbon emissions by as much as 30%.⁴⁸ Additionally, waste valorization reduces the environmental burden associated with trash disposal by turning waste into valuable compounds, which is consistent with the principles of the circular economy. Fig. 4 shows fossil-based and bio-based aromatics production paths, which are represented by the dark grey and green arrows, respectively. Aromatics are produced using diverse technologies from fossil and biomass sources. Lignocellulosic biomass is referred to as LCB.

Fig. 4 Aromatics production pathways from fossil-based and bio-based sources.⁴⁹ Copyright 2022, Elsevier.

A comprehensive life cycle assessment revealed striking differences in emissions profiles. Crude oil-based naphtha catalytic reforming (NACR) routes emit 43.4–43.9 t CO₂ equivalent per tonne (eq per t) aromatics.⁴⁵ In contrast, biomass-based routes incorporating carbon capture and storage (CCS) technology can achieve negative emissions, ranging from −6.1 to −1.1 t CO₂ eq. per t aromatics.⁴⁵ The gasification-methanol-aromatics (GMA) pathway with CCS demonstrates the lowest emissions at −14.6 t CO₂ eq. per t aromatics.⁴⁵ These figures highlight the potential for waste-derived aromatics to not only reduce emissions but also potentially act as carbon sinks when combined with appropriate technologies. While environmental benefits are clear, the economic viability of waste-derived aromatics remains a challenge. Current estimates place the production costs of bio-based aromatics between 1480 to 4121 $ per t.⁴⁵ This wide range reflects the variability in feedstocks, technologies, and scale of production. In comparison, fossil-based aromatics typically have lower and more stable production costs due to established infrastructure and economies of scale. The maturity of production technologies is a critical factor in the transition to waste-derived aromatics. Currently, most biomass-based aromatics production techniques remain at laboratory or demonstration stages. Catalytic pyrolysis has progressed to the pilot plant stage, indicating a higher level of technological readiness.⁴⁵ In contrast, fossil-based production methods benefit from decades of optimization and scale-up, operating at full commercial scale. The transition from fossil-based to waste-derived aromatic production offers significant environmental benefits, particularly in terms of GHG emissions reduction. However, challenges remain in terms of production costs, technological readiness, and ensuring consistent product quality.⁵⁰ As technologies mature and supportive policies are implemented, the economic viability of waste-derived aromatics is likely to improve.⁵¹ Future research should focus on scaling up promising technologies, optimizing production processes, and conducting more comprehensive comparisons of product quality and performance across various applications.^51,52

5. Renewable routes

The development of environmentally friendly and sustainable processes to transform waste biomass into commodity chemicals, biofuels, and novel bio-based materials like bioplastics is one of the primary challenges.³⁹ The increasing use of petroleum-based materials and goods has led to an increase in the rate of depletion of fossil fuels and the environmental problems they cause, such as inappropriate waste disposal and climate change. Despite these problems, human lifestyle changes are driving demand for petroleum-based products and fuels. The manufacturing of substitutes based on sustainable biomass has drawn more attention than ever before in order to meet this rising demand.⁵³

Using lignocellulosic biomass is one strategy that shows promise because it can be hydrodeoxygenated and liquefied to produce aromatic chemicals. Utilizing feedstocks sourced from biomass offers the possibility of lowering dependency on fossil fuels and promoting a more environmentally responsible and sustainable approach to producing aromatic compounds.⁵⁴ Another potential pathway towards sustainable manufacturing of aromatic compounds is the utilization of bio-based feedstocks, like vegetable oils and sugars.⁵⁵ These feedstocks can be processed enzymatically and by fermentation to yield important aromatic chemicals.³⁹ In addition to these pathways utilizing waste materials as feedstocks, renewable aromatics production offers an environmentally friendly alternative to the traditional petrochemical industry. This strategy not only lessens reliance on fossil fuels but also contributes to the solution of the expanding waste management problem. Waste materials such as biomass, agricultural wastes, and food waste can be converted into valuable aromatic compounds by the application of novel technologies and methods. The manufacture of plastics, paints, and scents are just a few of the businesses that can employ these renewable aromatics. Moreover, by establishing a closed-loop system, the generation of renewable aromatics from trash can support the circular economy. The waste materials are gathered, processed, and transformed into renewable aromatic compounds in this system, which are subsequently utilized to create new products.⁵⁶Fig. 5 shows the overall publication on aromatic production from waste over the time generated by lens.org. This shows the growing demand for adopting sustainability and using renewable feedstocks for aromatic production. Various renewable routes have been adopted for the production of industrially valued aromatic compounds. The detailed renewable routes are explained in the subsequent sections.

Fig. 5 Publications over time (Jan 2004–Dec 2024) in the field of aromatics production from waste by using the keywords aromatics, biomass, waste, sustainability, and renewable routes generated through lens.org.

5.1 Pyrolysis

Pyrolysis is a potential method that recovers valuable energy and products while also addressing the pollution issue. It is an endothermic reaction that involves heating feedstock to temperatures typically between 400 °C and 650 °C in the absence of oxygen to obtain bio-oil, aromatics, and solid char.^{9,14,15,40,41,57} The various factors affect the product yield, like particle size of feedstock, heating rate, temperature, etc. Mainly, two types of pyrolysis are performed. Slow pyrolysis is done at a low heating rate, whereas fast pyrolysis is done at a high heating rate. Fast pyrolysis is best to produce aromatics as it is the most economically feasible and easy to handle.⁵⁸ Detailed studies on pyrolysis by using different feedstocks are explored in a further section. Pyrolysis of biomass, plastics waste, and both co-pyrolysis of biomass and plastic wastes are being explored. The type of plastic waste and the respective catalyst are studied in detail, giving more perspective about the method and reaction conditions to be operated along with the selections of the appropriate feedstocks for the pyrolysis processes.

5.1.1 Pyrolysis of biomass. The world has never needed more ecologically friendly and renewable energy sources as it struggles to address the twin issues of depleting fossil fuel supplies and growing environmental concerns. For the purpose of producing sustainable energy, biomass—which includes a broad variety of organic resources like forestry waste, energy crops, and agricultural residues—represents a huge and mostly unexplored resource. Pyrolysis is a well-known technology for converting biomass into useful energy products. Pyrolysis is a type of oxygen-free thermal breakdown.¹⁵ Various products, such as biogas, bio-liquid, and bio-solid fuels, can be produced through pyrolysis and used in place of fossil fuels in transportation and power generation.⁵⁹Fig. 6 shows the overall publication on aromatic production by pyrolysis of biomass over the time generated by lens.org.

Fig. 6 Publications over time (Jan 2004–Dec 2024) in the field of aromatics production by pyrolysis of biomass using the keywords aromatics, biomass, pyrolysis, catalyst, and thermochemical generated through lens.org.

Pyrolysis is a well-known technology for converting biomass into useful energy products. Pyrolysis is a type of oxygen-free thermal breakdown.⁴⁰ According to Uddin et al., biomass pyrolysis is defined as the thermal breakdown of solid fuel that results in the breakage of carbon–carbon bonds and the creation of carbon–oxygen bonds.⁶⁰ Although greater temperatures can potentially be used, the procedure usually takes place in the 400–550 °C range.⁶⁰ A flexible and appealing way to transform biomass into energy products that are easily utilized for heat, power, and chemical production processes is through pyrolysis.⁶¹ Choosing the right feedstock is a crucial step in the pyrolysis of biomass.⁶⁰ Pyrolysis methods can make use of a variety of lignocellulosic biomass feedstocks, including energy crops, forestry waste, and agricultural wastes.⁶² According to Zaman et al. the feedstock is normally crushed, dried, and then put into the reactor to be pyrolyzed using a heated inert gas.⁶³ Optimizing heat transmission to the feedstock is a major difficulty in biomass pyrolysis since it can have a big impact on the final products’ production and quality.⁶³ Solutions to the technical problems with employing pyrolysis-derived biofuels as transportation fuels are now being investigated. Catalysts like zeolites, metal oxides, or bifunctional catalysts are introduced directly into the pyrolysis reactor.⁶⁴ They assist in deoxygenating the pyrolysis vapors as they are formed, resulting in bio-oils with a lower oxygen content and higher stability.⁶⁵ Catalytic pyrolysis increases the yield of aromatics, olefins, and other valuable hydrocarbons over unwanted oxygenates using ZSM-5 zeolite in direct catalytic pyrolysis promotes the formation of aromatics like benzene, toluene, and xylene (BTX), which are important precursors in the petrochemical industry.⁶⁶ Most researchers have used HZSM-5 for the pyrolysis of various biomass. Usually, at temperatures more than 500 °C, a mixture of BTX has been formed. Y. Li et al. used Fe-modified hierarchical ZSM-5 zeolite to produce mono aromatics and olefins.⁶⁷ They used poplar sawdust as biomass. Introducing Fe to the hierarchical samples increased the acid value and the number of strong acid sites. Fe/Hie-ZSM-5 increased the selectivity of mono-aromatics and olefins, while 4 wt% Fe loading was regarded as a superior choice for catalytic applications.⁶⁷ Lu et al. also used iron-doped Ca/Silica oxide for the pyrolysis of wheat straw. They found that Fe–Ca/SiO₂ enhances the oxygen transfer, as shown in Fig. 7.⁶⁸

Fig. 7 Pyrolysis pathway of wheat straw using Fe–Ca/SiO₂ catalyst. Reproduced with permission.⁶⁸ Copyright 2020, Elsevier.

Vichaphund et al. did interesting research by using Jatropha residues from local oil plants as biomass feedstock.⁶⁹ The increase in catalyst content had excellent efficiency for hydrocarbon production, notably aromatics, and encouraged the reduction of oxygenated and nitrogen-containing compounds.⁶⁹ Yan et al. obtained gasoline range hydrocarbons from wood chips of oak trees.⁷⁰ The creation of aromatic hydrocarbons is favoured by increases in reaction temperature and pressure, whereas the selectivity of aromatic hydrocarbons is decreased by increases in GHSV (gas hourly space velocity). The range of the aromatic hydrocarbon selectivity was 29 to 45%, depending on the conditions of the reaction.⁷⁰ However, in order to support potential future applications, the yield of particular bio-aromatics needs to be enhanced, making up for the high cost of separation in the process. It is crucial to remember that adding different metals to the zeolite support, such as Pd, Ga, Fe, Ni, or Co, might cause hydrodeoxygenation activity and moderate-severe Brønsted acidity.⁷¹ Additionally, it might have regulated acid–base characteristics, which would have a big impact on how bio-aromatics distribute their products. Ultimately, model compounds are often selected to unveil the method of bio-aromatics synthesis due to the complicated composition of biomass. Table 1 summarizes the different types of biomass pyrolysis and product distribution.

Table 1 Summary of different types of biomass pyrolysis and product distribution

Source	Catalyst	Temperature/°C	Heating rate/°C min^−1	Product	Ref.
Wheat straw	Fe–Ca/SiO2	750	10	Light aromatics	68
Pinewood	HZSM-5/Al2O3	500	—	BTX	72
Cedarwood	Ni–Mo2N/HZSM-5	700	10	BTX	73
Poplar sawdust	Fe/Hie-ZSM-5	550	20	Monoaromatics and olefins	67
Cereal	HZSM-5	600	—	BTX	74
Oak	Pd-promoted Fe/HZSM-5	311	—	Gasoline	70
Pistacia khinjuk seed	BP3189	450	300	Bio oil	75
Jatropha residues	Mo/HZSM-5	500	5	BTX	69

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Gueudré et al. research group did an interesting study on dual-pathway mechanism of co-processing of fossil hydrocarbons and biomass-derived oxygenated compounds in catalytic cracking.⁷⁶ Fossil hydrocarbon route occurs mainly in USY zeolite micropores and it involves the acid cracking of the hydrocarbons to form olefins, dienes and polyaromatics. But this route leads to the formation of graphite like coke deposits that block access to the Brønsted acid sites. Whereas biomass oxygenated compounds route occurs primarily in the outer volume of zeolite micropores due to the size constraints of biomass molecules. It involves cracking of lignin fragments and other oxygenates on Lewis acid sites of extra-framework aluminium and produces lighter hydrocarbons and substituted phenolic molecules through hydrogen transfer from alkanes, phenolic compounds undergo thermal repolymerization and condensation to form polyaromatic hydrocarbons as “bio” coke precursors. The synergy between these routes is explained by hydrogen transfer from alkanes produced in the fossil route to oxygenated compounds in the biomass route. This leads to formation of both lighter hydrocarbons and phenolic molecules observed in the liquid products.⁷⁷Fig. 8 outlines the mechanistic scheme of these two routes.

Fig. 8 Mechanistic scheme highlighting two primary co-processing routes: (i) model nC7 acid cracking within micropores producing liquid hydrocarbons and graphitic fossil coke, and (ii) conversion of lignin fragments in mesopores into hydrocarbons, residual oxygenates (phenolics), and amorphous bio coke. Reproduced with permission.⁷⁶ Copyright 2015, Elsevier.

To understand much deeper about the role of catalyst in pyrolysis process and its interaction with the biomass components Wang et al. proposed the mechanism as shown in Fig. 9.⁷⁸ In cellulose conversion, pyrolysis produces levoglucosan as the predominant product. The conversion of levoglucosan to smaller furanic compounds occurs by the dehydration, dearbonylation and decarboxylation by the acid sites of HZSM-5. Further production of aromatics and olefins occurs by the acid catalyzed reactions inside the zeolites pores by decarbonylation and oligomerization. Whereas in hemicellulose, pyrolysis produces double-hydrated xylose as the major product and other low molecular weight compounds which diffuse into zeolites pores without the further reactions. In case of lignin conversion pyrolysis generates the monomeric phenolic compounds. The limited cracking of phenols produces the aromatics and predominant acid dehydration of phenols leads to the coke formation.

Fig. 9 Catalytic pyrolysis mechanism of HZSM-5 with different biomass components. Reproduced with permission.⁷⁸ Copyright 2014, Royal Society of Chemistry.

The role of HZSM-5 overall on the lignocellulosic biomass on catalytic pyrolysis is simplifies in the Fig. 10.

Fig. 10 HZSM-5 role on the catalytic pyrolysis of lignocellulosic biomass to obtain the aromatics.

Although zeolites act as the best catalyst for the biomass pyrolysis, it deactivates quickly due to the coke accumulation during the pyrolysis process.⁷⁹ By doing the partial combustion coke content is controlled.⁷⁹ Also, catalyst is regenerated by oxidation to remove coke, followed by washing with a liquid (e.g., water, acidic/basic solutions, or organic solvents) to remove minerals and restore activity.⁸⁰ Calcination in a muffle furnace at 600 °C for 5 hours can regenerate the catalyst.⁸¹ Phosphorus modifies mesoporous HZSM-5 showed the improved stability maintaining effectiveness after 50 runs, whereas unmodified HZSM-5 was permanently deactivated after 50 runs.⁸² H-ZSM-5/Al-MCM-41 catalyst mixture retained 94% of the surface area compared to fresh catalyst, indicating effective regeneration.⁸¹ It was also found that catalyst cracking abilities decreased with increased number of reaction cycles.⁸¹ These findings demonstrate that while HZSM-5 catalysts do experience deactivation over multiple cycles, various regeneration methods can effectively restore their activity. Modified versions of HZSM-5, such as phosphorus-modified mesoporous HZSM-5, show improved stability and recyclability compared to unmodified catalysts.

Fig. 11 summarises the various routes available for thermochemical conversion of biomass to value-added products. Fast pyrolysis involves rapidly heating biomass to decompose it into vapors, aerosols, and char. The vapors are condensed to form bio-oil, which typically contains a complex mixture of oxygenated compounds that reduce its stability and calorific value.⁴¹ Catalytic upgrading by this route transforms bio-oil into more valuable and stable products with higher energy density, reduced oxygen content, and improved fuel properties by using catalysts like zeolites for deoxygenation and cracking.⁷³ In direct catalytic pyrolysis, biomass is directly subjected to pyrolysis in the presence of a catalyst during the thermal decomposition stage. The key goals of catalytic pyrolysis are to improve product selectivity (particularly to bio-oil and biofuels), reduce oxygenated compounds, and enhance the overall efficiency of the conversion process.⁸³

Fig. 11 Routes for the thermochemical conversion of biomass to liquid fuels and highly valuable compounds.

Platform molecules are key intermediates derived from biomass, such as sugars (e.g., glucose), furfural, levulinic acid, or glycerol.⁸⁴ Catalytic pyrolysis can be applied to these molecules to convert them into high-value chemicals and fuels. Platform molecules can be upgraded to hydrocarbons, contributing to bio-based fuels, solvents, and specialty chemicals used in industrial applications. This route allows for tailored conversion processes where platform molecules undergo further catalytic refinement, leading to a wide range of valuable products, including fuel additives, polymers, and fine chemicals.⁶³Fig. 12 shows the reaction pathways for biomass pyrolysis.

Fig. 12 Reaction pathways for lignocellulosic biomass pyrolysis. Reproduced with permission.⁸⁵ Copyright 2021, Elsevier.

At temperatures around 300 °C, researchers have reported the formation of Gasoline.⁸⁶ Also, bio-oil formation is reported from the pyrolysis of biomass. Robinson et al., reported about the microwave pyrolysis of biomass.⁸⁷ They mentioned at 130 °C hemicellulose content can be decomposed and at 180 °C cellulose can be decomposed using microwave heating.⁸⁷ A novel biomass thermochemical conversion process called fast pyrolysis primarily produces high-grade liquid products (bio-oil).⁷² Bio-oil has emerged as a significant sustainable alternative to petroleum fuel due to its high energy density and ease of storage and transportation when compared to raw biomass sources.²⁵ Boilers and engines have run on bio-oil as fuel.⁸⁸ It can be utilized as high-grade aviation fuel and transportation fuel with additional hydrorefining treatment.³¹ In addition, bio-oil's chemical makeup is incredibly complicated. Bio-oil has a wide range of oxygenated organic molecules, including high-value compounds that are hard to come by through traditional chemical manufacture. Onay et al. reported a maximum bio-oil yield of 57.6% at a heating rate of 300 °C min⁻¹. They have used commercial catalysts, which decrease the formation of oxygenated species.⁷⁵ Qiu et al. reported the various factors affecting the fast pyrolysis of biomass.⁸³ Biomass pretreatment, the initial stage of pyrolysis, can enhance biomass's physicochemical characteristics and increase bio-oil's stability and yield.⁸⁹ For instance, pyrolysis can be carried out more thoroughly, and biomass particle size can be improved with mild crushing.⁸⁹ Drying can speed up the pyrolysis reaction and improve the stability of the characteristics of the liquid bio-oil products.⁹⁰ Numerous studies demonstrate the importance of pickling, torrefaction, and biological preparation in terms of the distribution and output of bio-oil.⁹¹ Simultaneously, the co-pyrolysis of polyethylene, methane, and ethanol facilitates the production of compounds with elevated values.^92,93 The circumstances of the pyrolysis process significantly impact the distribution and content of bio-oil.⁸³ Circumstances, including raw materials, pretreatment method, reaction conditions, and co-pyrolysis, affect the yield of bio-oil. The type of biomass used directly impacts the composition and quantity of bio-oil produced. Lignin content tends to increase biochar yield, while cellulose and hemicellulose are responsible for higher bio-oil production.⁹⁴ Municipal solid waste, food waste, or industrial by-products offer cost-effective alternatives, although their heterogeneous composition can lead to variability in bio-oil yield and quality.⁹⁵ Reducing particle size via grinding or milling increases surface area and improves heat transfer during pyrolysis, enhancing bio-oil production.⁹⁶ The operating conditions during pyrolysis significantly affect the distribution of products between bio-oil, biochar, and syngas. Higher temperatures (above 500 °C) favor gas production and biochar, while lower temperatures (350–500 °C) are optimal for bio-oil yield, particularly in fast pyrolysis. Very low temperatures (below 300 °C) can cause incomplete thermal degradation of biomass.⁹⁷ Mixing biomass with plastics (like polyethylene or polypropylene) can increase the hydrogen content in bio-oil, improving its stability and reducing oxygenated compounds.⁹⁸ Waste oils (such as used motor oil or cooking oil) can act as hydrogen donors, improving the deoxygenation and overall quality of bio-oil.^99,100 Understanding these factors will help us to understand and modify the reaction accordingly to enhance the bio-yield. Fig. 13 summarizes the various factors that are explained above. Each of these elements plays a critical role in optimizing bio-oil production, composition, and upgrading potential.

Fig. 13 Summary of various factors affecting the fast pyrolysis of biomass.

5.1.2 Pyrolysis of plastic waste. Plastic is one of the greatest innovations of the century, and presently, it is difficult to replace owing to its exceptional qualities, including lightweight and low cost. With the increasing concern over environmental pollution and the pressing need for sustainable solutions, finding an efficient method for treating plastic waste has become essential. Finding a productive way to handle plastic garbage has become crucial due to the growing concern about environmental pollution and the urgent demand for sustainable solutions.¹⁰¹ Aromatic compounds can be produced from plastic waste using pyrolysis, a thermo-chemical method of treating offers a sustainable approach. Pyrolysis, which breaks down the C–C and C–H bonds in plastic waste by heating it to temperatures typically between 800 °C and 1200 °C, turns the trash into smaller intermediate species. Saturated or unsaturated molecules can be produced by additional reactions involving these intermediate species, also referred to as unstable radicals. Diolefins are another product of high-temperature pyrolysis, and they can cyclize with other olefins to produce aromatic compounds. In a nutshell, pyrolysis is a lucrative process that recycles plastic waste and yields aromatic chemicals that are used in a variety of industries.¹⁰² In addition, a recent study by Brown et al. examined the economic feasibility of using pyrolysis as a method to produce aromatic chemicals, providing insight into how cost-effective this strategy may be in comparison to more conventional waste management techniques.¹⁰³ These results highlight the increasing amount of research that supports the use of pyrolysis to produce aromatic chemicals sustainably from garbage, further highlighting the technology's potential to revolutionize how we see and handle plastic waste. Fig. 14 shows the overall publication on aromatic production by pyrolysis of plastic waste over the time generated by lens.org.

Fig. 14 Publications over time (Jan 2004–Dec 2024) in the field of aromatics production by pyrolysis of plastic waste using the keywords aromatics, plastic waste, pyrolysis, catalyst, and thermochemical generated through lens.org.

In a review paper, Abnisa et al. analyzed the chemical properties of different plastic compounds based on parameters like moisture, fixed carbon, and volatile and ash content.¹⁰⁴ They concluded that high ash content decreased the amount of liquid content by increasing the char formation and that high volatile compounds increased the liquid oil production. Pyrolysis mainly depends on the parameters that affect the yield of aromatics, such as temperature and catalyst. In general, to degrade the plastics thermally, a large amount of energy and high temperature is required to avoid char formation.¹⁰⁵ In addition to this, the cracking of macromolecules into small molecules hinders their utilization at the industrial scale.¹⁰⁶ Because of all those stated reasons, it is essential to use a catalyst in the pyrolysis process as the apparent activation energy is lowered along with the frequency factor.¹⁰⁷ When catalysts are used, the pyrolysis mechanisms for polyolefins (PE, PP, and PS) switch from the free radical commenced chain scission to the carbonium ion intermediate chain scission.¹⁰⁸ The ability of catalysts to lower the optimal temperature increases with substituent size (–H for PE, –CH₃ for PP, and –C₆H₅ for PS). The order of this ability to decrease the optimal temperature is PE > PP > PS.¹⁰⁹ The catalyst active sites’ steric obstacles to contacting the polymer chain have been suggested as the substituents. Because of this, the pyrolysis of PET (polyethylene terephthalate) is least affected by the aromatic ring in the structure since the starting process, whether it occurs with or without catalysts, is relatively comparable. It involves a hydride shift from either the acid sites or nearby C_β hydrogen to C=O oxygen.¹⁰⁹ It has been observed that catalytic pyrolysis enhances gaseous yield and diminishes the liquid fraction, producing the distributions of lighter hydrocarbon.¹¹⁰ This loss in liquid yield is counterbalanced by quality improvement and the formation of more industrially valued chemicals like plastic monomers or jet/diesel/gasoline fuel products.¹¹⁰

When pyrolyzing plastic wastes, the composition and yield of the final product are greatly influenced by the type of catalyst used. Catalysts such as acidic catalysts like zeolites, functionalized activated carbon, and basic catalysts such as metal oxides and alkali carbonates play a crucial role in improving the pyrolysis process.

Acid catalysts. These microporous substances have a distinct structure and can function as catalysts during the pyrolysis process. They offer active areas for plastic molecules to break down, producing chemicals and other desired products like liquid fuels.¹⁰⁷ Because of their large surface areas and ability to selectively catalyze particular reactions, zeolites can increase the production of useful chemicals while decreasing undesirable byproducts.¹¹¹ Inayat et al. have successfully pyrolyzed LDPE (low-density polyethylene) plastic into aromatics by two-stage thermos-catalytic pyrolysis.¹¹² They concluded that high acid site density is responsible for aromatics formation in LDPE and low acid site density for light olefins in the pyrolysis of polyethylene. HZSM-5 exhibits those acid density sites that act as a driving force for tuning the shape selectivity of desired products. A graphical illustration by Inayat et al. has been shown in Fig. 15 to show the two-stage thermos-catalytic pyrolysis in LDPE.¹¹² Another interesting work has been laid out by using the waste COVID-19 masks.¹¹³ Polypropylene type of plastic is found in the masks. In their work, they found that compared to the HZSM-5, the selectivity of the HBETA and HY catalysts for BTEX were significantly higher. When compared to HZSM-5, the HBETA and HY zeolite catalysts have more pores, surface area, and acid sites. Since the kinetic diameters of the branched hydrocarbons were less than the pore sizes of HBETA and HY, the thermally derived branched hydrocarbons were able to diffuse inside the pores and change into aromatic hydrocarbons over the acid sites that were primarily inside the pores. Because the HBETA is more acidic than the HY, more aromatics are formed. Porosity, surface area, and acidity, among other variables, worked in concert to increase the number of aromatic compounds that were produced from the mask. Fig. 16 shows the composition (GC/MS area %) of the products that are formed during the pyrolysis of the mask over various zeolite catalyst combinations that are found in the oil. Apart from zeolites, researchers also used functionalized activated carbon. One study by Sun et al. produced aromatics from mixed plastics by using biochar activated by H₃PO₄, and it showed good aromatics yield and selectivity.¹¹⁴ They showed the compositions of aromatics in pyrolysis oils, as shown in Fig. 17. In addition to this, they concluded that by creating Lewis/Brønsted acid sites on the char surface, H₃PO₄ treatment encouraged the conversion of alkenes to aromatics and supported other reactions such as the Diels–Alder reaction, hydrogen transfer reaction, and dehydrogenase process.¹¹⁴

Fig. 15 A graphical illustration of the thermo-catalytic pyrolysis of LDPE in one- (top) and two-stage (bottom) processes. Reproduced with permission.¹¹² Copyright 2022, Elsevier.

Fig. 16 Composition (GC/MS area %) of the products that are formed during the pyrolysis over various zeolite catalyst combinations. Reproduced with permission.¹¹³ Copyright 2021, Elsevier.

Fig. 17 Pyrolysis oil aromatics distributions. Reproduced with permission.¹¹⁴ Copyright 2018, American Chemical Society.

Base catalysts. In plastic pyrolysis, base catalysts such as metal oxides, alkali carbonates, and metal complexes are frequently employed to enhance monomer recovery.¹¹⁵ These catalysts help polymers depolymerize, which produces oils that are low in aromatics and branching isomers and high in 1-olefins.¹¹⁵ Base catalysts can support more sustainable plastic waste management by assisting in the repurposing of recovered monomers for the production of new polymers. Decarbonylation and deoxygenation reactions are promoted, which enhance the quality of bio-oil obtained.¹¹⁶ Also, it promotes cracking initiation by random scission facilitation and carbanion formation by accelerating depolymerization. This leads to the breakdown of larger molecules into value-added chemicals.¹¹⁷ Alkali metal earth oxides like CaO are proven to be effective for the removal of sulfur from the products obtained through pyrolysis. This has a good impact on the environment by improving the quality of end products. Kumagai et al. used CaO as a catalyst for the pyrolysis of PET.¹¹⁸ They found that strong basicity is required for the high selectivity of aromatic compounds as it enhances the decarboxylation and decreases the time required, whereas weakly basic metal oxide dehydroxylation competes against decarboxylation.

In general, these catalysts can be used in plastic waste pyrolysis processes to reduce the activation energy needed for plastic degradation, boost selectivity towards desired products, and enhance the overall efficiency of processing plastic wastes into useful chemicals and fuels. Table 2 summarises the studies on plastic pyrolysis at different temperatures and catalysts.

Table 2 Summary of different types of waste plastic pyrolysis

Type of plastics	Catalyst	Temperature (°C)	Heating rate (°C min^−1)	Duration	Product	Ref.
PET	ZnO	T1 = 450	—	—	88.8% benzene	119
		T2 = 700
PET	CaO	T1 = 450	10	20 min	99.7% BTX	118
		T2 = 800
HDPE	Ni/HZSM-5	400	3	60 min	28.9% aromatic yield	120
LDPE	HZSM-5	500	—	—	32% aromatics (BTEX)	112
PP (from waste COVID-19 mask)	HBeta	500	200	30 min	49.4% BTEX yield	113
PS	Zn–Al2O3	450	25	120 min	90.2 wt% liquid yield	121
Polyethylene (PE)	H3PO4-activated carbon	T1 = 550	20	60 min	41.8% oil yield, 23.8% aromatic composition	122
		T2 = 600
22% PP + 59%PE + 19%PS	H3PO4-activated carbon	500	20	60 min	66% aromatics selectivity, 47.6% aromatics yield	114

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5.1.3 Co-pyrolysis of biomass and plastic waste. The only renewable energy source that can be transformed into many fuels, such as liquid, char, and gas, and which also offers flexibility in production and marketing, is biomass, which is widely acknowledged as a potential energy source.¹²³ Usually, pyrolysis is selected as the suggested method. Fig. 18 shows the overall publication on aromatic production by co-pyrolysis of biomass and plastic waste over the time generated by lens.org.

Fig. 18 Publications over time (Jan 2004–Dec 2024) in the field of aromatics production by co-pyrolysis of biomass and plastic waste using the keywords aromatics, biomass, plastic waste, co-pyrolysis, catalyst, and thermochemical generated through lens.org.

Fast pyrolysis results in low heating value, high viscosity, high acidity, and high reactivity, which is why it can't be used directly or by current engines as a transportation fuel.¹²⁴ Thus, two techniques emerged to upgrade the quality of catalytic deoxygenation: catalytic cracking and hydrodeoxygenation (HDO). However, in catalytic cracking, grade products were obtained with high coke and low carbon yield, resulting in catalyst deactivation in a short time,¹⁰⁴ whereas HDO requires high reaction conditions in temperature and pressure, and using of noble catalysts leads to an increase in the cost of operation.¹²⁵ It is found that the oxygen-enriched intrinsic nature and hydrogen deficit of lignocellulosic biomass are the primary causes of the petrochemicals with the poor carbon yield of aromatics and significant coke formation.¹²⁶ In addition, coke formation and the effectiveness of converting biomass into advanced biofuels are significantly influenced by the hydrogen-to-carbon effective (H/C_eff) ratio.¹²⁷ Therefore, when lignocellulose is converted over zeolite catalysts, the hydrogen deficient (H/C_eff typically less than 0.3) biomass yields low carbon yields of petrochemicals and a substantial production of coke. It makes sense that adding high H/C_eff ratio co-reactants with lignocellulosic biomass to the catalytic pyrolysis could assist enhance the carbon efficiency of aromatics and reduce the generation of coke. Co-feeding lignocellulosic biomass with hydrogen-rich feedstock during catalytic pyrolysis has been shown to alter the oxygen removal reaction mechanism by replacing dehydration with decarbonylation and decarboxylation.¹²⁸ In order to decrease the generation of coke during catalytic co-pyrolysis and increase the carbon efficiency of aromatics, synthetic polymers, such as waste plastics, offer more affordable and plentiful hydrogen sources. Globally, enormous amounts of garbage made of synthetic polymers are produced annually. Presently, the only options for handling polymer waste are incinerators and landfills.¹²⁹ The most advanced tertiary recycling methods, such as catalytic rapid pyrolysis, present a viable substitute for the waste management of polymers. These premises suggest that co-feeding lignocellulosic biomass and polymer waste during catalytic pyrolysis has remarkable environmental and energy-recapture benefits. Also, it reduces the char formation which is helpful for maximizing the selectivity of aromatics. Here, plastic waste enhances deoxygenation, leading to less oxygen content, which is suitable for transportation fuel. During this process, synergistic effects enhance the overall efficiency. Fig. 19 shows the promising routes for the conversion of biomass and polymers to value-added chemicals.

Fig. 19 Promising routes for the conversion of biomass and polymers to value-added chemicals.

In addition to this, co-pyrolysis can produce commodity chemicals like aromatic hydrocarbons and bio-oil. These substances can help produce high-value goods and have a variety of industrial uses. Fig. 20 shows the schematic diagram of co-pyrolysis.

Fig. 20 Schematic diagram of co-pyrolysis.

Mechanism and catalysts. Complex interactions between biomass and polymers during thermal degradation, as well as among the pyrolytic volatiles at the catalytic sites, are a part of the process of catalytic co-pyrolysis. Zheng et al. observed the positive synergy for aromatic production due to the interaction of cellulose-derived oxygenates and LDPE-derived olefins in the presence of HZSM-5.¹³⁰ Also, the addition inhibits the coking, thereby increasing the volatile content.¹³⁰ Wang et al. found the maximum monocyclic aromatic compound when zeolites are impregnated with metal oxides.¹³¹ Sekyere et al. found that the base-acid tandem catalyst enhances the yield of BTX along with light olefins.¹³² It is evident that light olefins (ethylene and propylene) developed from LDPE and may react with furan compounds (furan and furfural, for example) produced from cellulose to form aromatics (BTX, for example) through Diels–Alder reactions followed by dehydration reactions.¹³¹ Furan compounds and light olefins function as diene and dienophile chemicals in the catalytic co-pyrolysis, respectively.¹³³ Also, LDPE-derived hydrocarbons play the role of hydrogen donors for oxygenates derived from cellulose and decrease the formation of coke in zeolite-catalyzed reactions.¹³⁴ A study by Iftikhar et al. and Shafaghat et al. showed that PS, compared to PP, cannot generate enough olefins for the catalytic co-pyrolysis of oxygenates produced from biomass to produce aromatics.^135,136 Diels–Alder condensation, benzene, furan dehydration, and a cascade of alkylation between toluene and the intermediate allene are the usual processes that yield naphthalene. Zhang et al. reviewed the different results obtained from the co-pyrolysis of biomass and plastics and showcased the reaction network demonstrated in Fig. 21.¹³⁴ In order to produce furan compounds during thermal breakdown, cellulose may go through a series of processes, including dehydration, decarbonylation, and decarboxylation for the predominant route in lignocellulosic biomass. Because of this, it has been demonstrated that hemicellulose was probably going to depolymerize into furan compounds, which is similar to what happens when cellulose degrades.¹³⁷

Fig. 21 Reaction pathways proposed for the conversion of biomass and plastics in co-pyrolysis. Reproduced with permissions.¹³⁴ Copyright 2016, Royal Society of Chemistry.

In lignocellulosic biomass, lignin was mainly broken down into phenolic chemicals, in contrast to cellulose and hemicellulose.¹³⁸ Regarding the other pathways of plastic degradation, random scission, and chain-end scission are the two typical mechanisms by which plastic deterioration occurs at high temperatures.¹³⁹ The lengthy carbon chains and free radicals were produced concurrently by the two aforementioned processes. Hydrogen transfer processes could also convert the radical fragments into straight-chain hydrocarbons at the same time.¹⁴⁰ Those hydrogen acts as a strong acceptor, thereby inhibiting the formation of char. The hydrogen-deficient oxygenates from the thermal breakdown of cellulose and hemicellulose could easily polymerize and react with phenolic chemicals to generate coke during the catalytic pyrolysis of lignocellulosic biomass separately;¹⁴¹ Similar to this, the majority of phenolic compounds resulting from the thermal breakdown of lignin are too big to fit through the pores in zeolite. These compounds, being relatively unstable, might readily deposit on the surface of the catalyst and then polymerize or react with oxygenates, which are tiny molecules that create coke. However, instead of using polymerization processes, the resulting furan compounds might combine with olefins generated from plastic through Diels–Alder reactions. This would lessen the amount of coke that forms during the catalytic co-pyrolysis of lignocellulosic biomass with plastics.¹⁴²Table 3 summarises the co-pyrolysis done with different catalysts, giving the overall idea of selecting the catalyst with different classes of plastics.

Table 3 Summary of the co-pyrolysis done with different catalysts

Biomass	Waste	Catalyst	Operating conditions	Optimized conditions	Ref.
Pine sawdust	LDPE	HZSM-5	Fixed bed reactor 450 °C	The addition of LDPE inhibited the coking reaction of biomass effectively, resulting in an increase in the volatile content	130
Bamboo sawdust	LDPE	HZSM-5, CeO2/γ-Al2O3	Pyroprobe pyrolyzer coupled with GC 600 °C	Catalyst/biomass ratio of 4, CeO2/-Al2O3-to-HZSM-5 mass ratio of 1 : 3, 75% Linear LDPE percentage; maximum contents of aromatic HCs were obtained	131
Choerospondias axillaris seeds	LDPE	FeCl3 and MCM-41	600 °C	Co-pyrolysis can promote the Diels–Alder reaction between furans and olefins and increase the yield of monocyclic aromatics	143
Pine	LDPE	CaAl and ZSM-5	Tandem catalyst at 600 °C, with CaAl to Z40 ratio of 1 : 1	Catalytic co-pyrolysis can reduce the energy input and increase desired products. The base–acid tandem catalyst gave the maximum light olefins and BTX yield	132
Sugarcane bagasse	HDPE	Mesoporous FAU	Fixed-bed reactor, 400–700 °C	The catalyst-to-feedstock ratio was 1 : 6, the HDPE-to-SCB ratio was 40 : 60, and the temperature was 500 °C; the maximum bio-oil yield was achieved	144
Pine sawdust	HDPE	MgCl2, HZSM-5	Fixed-bed reactor 400–700 °C	Pyrolysis temperature: 600 °C, biomass to-HDPE ratio: 1 : 2, and feedstock-to-catalyst ratio: 1 : 1; maximum oil-phase product yield of 20.6% and aromatics’ selectivity (area %) of 95.9% was obtained	145
Sugarcane bagasse	PET	HZSM-5/Na2CO3/γ-Al2O3	Tandem reactor coupled with GC 400–800 °C	700 °C, biomass-to-PET ratio of 4, and HZSM-5-to-Na2CO3/-Al2O3 ratio of 5; maximum BTX yield of 18.3% was obtained	146
Sugarcane bagasse	PS	HZSM-5, MgO, CaO	Fixed-bed reactor 500 °C	Mass ratio of 1 : 3 HZSM-5 : MgO; maximum (56.8 wt%) MAHs’ yield and lowest (20.8 wt%) PAHs’ content was observed	135
Empty fruit bunches	PP	Al-MSU-F, Al-SBA-15, and meso-MFI	In situ catalytic pyrolysis at 600 °C	In situ pyrolysis produces more aromatics than ex situ pyrolysis, and the coke yield is lower than the theoretical value	136
Cellulose, lignin, and pine wood	PE, PP, and PS	ZSM-5	Fast pyrolysis with reaction temperature 550 °C	ZSM-5 promotes the synergistic effect. Diels–Alder reaction of cellulose-derived furans with LDPE-derived olefins greatly enhances aromatic production	147
Cellulose	PET, PP, LDPE, HDPE, PS	HZSM-5	Catalytic fast pyrolysis at 650 °C	Olefins produced by PE can react with oxygenates produced by biomass, which is conducive to the transfer of C in biomass to aromatics rather than coke	148
Black-liquor lignin	PE, PP, and PS	LOSA-1, spent FCC, gamma-Al2O3, sand	Reaction temperature 600 °C	PE pyrolysis produces a lot of products with a high H/Ceff ratio, which can offer H to lignin	149
Camellia shell	Take-out solid waste	HZSM-5, CaO, MgO	Pyroprobe pyrolyzer coupled with GC 700 °C	The mixing ratio of biomass and plastic was 3 : 7, and the mixture of HZSM-5 and CaO (mixing ratio of 1 : 1); aliphatic HCs and MAHs were generated, and acids’ formation was inhibited	150
Kitchen waste	Waste tire	ZSM-5	Fast pyrolysis at 600 °C in N2 atmosphere	When kitchen waste and waste tire are 5 : 5, the yield of hydrocarbon products is the highest	151
Grape seeds	Waste tire	CaO	Auger reactor (pilot scale)	Catalyst calcination temperature: 900 °C, 20 wt% waste tires, feedstock: CaO mass ratio of 2 : 1; deoxygenated bio-oil (0.5 wt% of oxygen content) was obtained with a heating value of 41.7 MJ kg^−1	152

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One method that shows promise for producing useful chemicals and biofuels from lignocellulosic biomass is the catalytic co-pyrolysis of biomass with plastics. With the help of a catalyst, biomass, and polymers are thermally degraded simultaneously in this process, increasing liquid production, improving product quality, and fostering synergistic effects between the feedstocks. Several advantages are achieved by switching from biomass pyrolysis to co-pyrolysis, including higher production of aromatics, decreased waste output, and enhanced energy security. Overall, catalytic co-pyrolysis presents a viable and effective method for managing waste, producing renewable energy, and valuing biomass, underscoring its promise as a crucial technology in the shift to a more sustainable and environmentally friendly future. Fig. 22 concludes the overall pyrolysis process of renewable aromatics production from biomass and plastics wastes which requires carbon material, metal oxides and chlorides as the catalyst in the temp. range of 400–800 °C.

Fig. 22 Renewable aromatics production from biomass and waste plastics through pyrolysis route in the presence of catalysts like carbon/metal oxides/chlorides.

5.2 Biomass to aromatics: chemo catalytic approach

As discussed earlier presently, fossil fuels are the basic source of energy for humankind. Its excessive consumption and effect on the environment have raised the issue of finding an alternative. To address this, researchers came up with biomass as an abundant renewable source as a replacement for fossil fuels.¹³⁸ There are three main types of biomass generation.¹⁵³ First generation refers to edible sources like food crops or other edible raw materials; examples include corn, sugarcane, and vegetable oils. The second generation refers to non-edible sources like agricultural residue, wood, straws, etc.¹⁵⁴ Third generation refers to microorganisms like algae, mostly for enzymatic reactions.¹⁵⁵ For biomass conversion into value-added chemicals, the first generation is avoided as it raises concerns about the competition in food production and the potential impact on food prices. Hence, the second generation prefers to be the more valuable and sustainable option as it depends on non-edible sources and waste products. From this generation, one of the most important types of biomasses is lignocellulosic biomass, mainly composed of cellulose (35%–50%), hemicellulose (25%–30%), and lignin (15%–30%).¹⁵⁶Fig. 23 shows the structure and main components with monomeric units of lignocellulose.

Fig. 23 Structure and main components with monomeric units of lignocellulose.

As discussed earlier, the composition of lignocellulose, cellulose, and hemicellulose mainly consists of complex polymers of sugar units that are connected by glycosidic bonds.¹⁵⁷ Along with lignin, researchers are focussing on cellulose and hemicellulose conversion.^16,138 Cellulose is formed by the glucose monomers linked with each other β-1,4-glycoside bonds by rigid packing of cellulose chains.¹⁵⁷ On the other hand, hemicellulose is a highly branched carbohydrate that is composed of C5 and C6 sugars linked by different forms of glycosidic bonds.¹⁵⁸ For the production of aromatics from cellulose, several routes are there, out of which there are mainly two routes that have gained attention recently, one is through the synthesis of 5-hydroxymethylfurfural (HMF), which can further be converted into various compounds that can replace petroleum-based compounds which are used to make valued added chemicals and plastics; second is selective aldol condensation of alkyl methyl ketones to aromatics.¹⁵⁹

5.2.1 Cellulose to HMF. From cellulose, different furan types of compounds are produced from cellulose, and HMF gained more attention as it serves as a platform molecule.¹⁵⁹ The conversion of cellulose to HMF happens in three major steps. First is the formation of monosaccharides by hydrolysis of cellulose, followed by the isomerization of glucopyranose to form fructopyranose and, in the end, dehydration of fructopyranose to form HMF.¹⁶⁰ Several researchers reported that HMF is the key intermediate for the production of renewable aromatics chemicals.¹⁶¹ At present, the production cost of HMF is too high, which is a major reason for limiting its availability for various industrial uses. The present process of producing HMF involves the use of acid catalysts, but there are several drawbacks to using them, as they cause several side reactions along with an increase in the cost of purifications.¹²⁵ A Brønsted acid catalyst can be used to convert fructose directly into HMF, whereas glucose must first be converted into fructose by isomerization using the basic or Lewis acidic sites of the catalyst, and then fructose must be dehydrated into HMF using a Brønsted acid catalyst. This process can take two steps.¹⁶²Fig. 24 shows the overall publication on aromatic production from cellulose via HMF formation over the time generated by lens.org.

Fig. 24 Publications over time (Jan 2004–Dec 2024) in the field of aromatic production from cellulose via HMF formation using the keywords aromatics, cellulose, HMF, and catalyst, generated through lens.org.

Rezayan et al. explain the mechanism, as shown in Fig. 25, as a convenient procedure for converting fructose and glucose into HMF in mono- or biphasic systems by utilizing acid–basic catalysts.¹⁶² Comparing a one-pot synthesis versus multiple steps for HMF synthesis, the latter is more efficient in terms of time and energy conservation. Consequently, the goal of the study is to identify effective catalytic systems that can improve productivity, decrease waste, and raise overall profitability for the direct one-pot synthesis of HMF from cellulose. However, there are a number of difficulties because cellulose is a complex material, and there are several side reactions that can occur throughout the process, like the creation of humins or oligomers, as well as the rehydration of HMF to levulinic acid (LA) and formic acid (FA).¹⁶³ The overall process depends on reaction conditions and the type of catalysts used. Various types of catalysts have been utilized like carbon-based catalysts, zeolites, metal oxides, etc.^163,164 A few challenges have also been observed for achieving maximum yield, like the crystallinity of cellulose, which limits the interaction linking catalyst active sites and cellulose.¹⁶⁵ To avoid this, researchers employed pretreatment processes like ball milling,¹⁶⁶ microwave,¹⁶⁷ mechanocatalysis,¹⁶⁸etc. Table 4 summarises the literature review of HMF production from waste-derived cellulose.

Fig. 25 Mechanism for conversion of cellulose to HMF. Reproduced with permission.¹⁶² Copyright 2023, Wiley.

Table 4 Summary of the literature review of HMF production from waste-derived cellulose

Source	Catalyst	Reaction conditions	Conv./yield (%)	Ref.
Banana plant waste	Al2O3–TiO2–W	175 °C, 180 min, Ar pressure at 30 bar	45 mg mL^−1 HMF	164
Filter paper	[bi-C3SO3HMIM][CH3SO3] (IL-2)/(MnCl2)	120 °C, 1 h	40.2	169
Corn straw biomass	BT300S sulfonated solid acid carbonaceous	200 °C, 1 h	16.2 mg 5-HMF per g biomass	170
Straw	[bi-C3SO3HMIM][CH3SO3] (IL-2)/(MnCl2)	120 °C, 1 h	30.5%	169
Corn stalk	Biochar-Mg–Sn	100 °C, 3 h	63.57%	171
Vegetable waste	SnCl4	140 °C, 20 min	54–73% HMF and glucose	172

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Homogeneous catalysts give good yields, but they have certain drawbacks, such as limiting the reusability of the catalyst and being corrosive in nature.¹⁷³ Thus, heterogeneous catalysts proved to be better than homogeneous catalysts as they are versatile.

Carbon-based catalysts. Carbon-based catalysts are considered to have good chemical and mechanical stability as well as high specific surface area (SSA). Zhang et al. found that carbon without functionalization doesn't shows desirable catalytic activity. The reason he stated is presence of insufficient surface acidity.¹⁷⁴ Ozsel et al. modified carbon catalyst with sulfuric acid and obtained sulfonated solid acid carbonaceous material as their catalyst.¹⁷⁰ They used corn stalk and filter paper as the biomass source for cellulose and found the good yield of HMF. AC–SO₃H treated catalyst shows good yield due to increasing if Lewis acidic sites, increasing of hydrophobic nature and high surface area.¹⁷⁰ In addition to this, addition of transition metals also improves the HMF yield. Transition metals have small ionic radius as compared to alkali metals which gives the suitable contact between cellulose and catalyst acidic active sites. Using bimetallic also improves the reaction. The distribution of metal ions promotes the degradation and further conversion into aromatics. Liu et al. used bimetallic transition metal on biochar and obtained 63.57% HMF yield.¹⁷¹Fig. 26 shows the Scanning electron microscope (SEM) image of the catalyst prepared with bimetallic doped biochar and it shows the different morphologies. Also, carbon supported catalyst provides both catalytic and binding sites. Yang et al. explained how carbon catalyst can act as cellulase-mimetic catalysts. These catalysts decrease the apparent activation energy. Fig. 27 shows the functional groups being act as binding and catalytic sites.¹⁷⁵ Thus, preparing enzyme mimetic catalyst considered to be interesting and holds as a sustainable and green approach.

Fig. 26 Biochar structure of (A) corn stalk, (B) biochar-Mg, (C) biochar-Sn, (D) biochar-Mg–Sn observed on SEM. Reproduced with permission.¹⁷¹ Copyright 2018, Elsevier.

Fig. 27 Functional groups as binding and catalytic sites.¹⁷⁵ Copyright 2021, Frontiers.

Metal oxides. Recently metal oxides gained the attention for the cellulose depolymerization due to their preparation techniques, water resistance, high porosity and surface area and tunable acidity/basicity which enhances the catalytic activity. Flores-Velázquez et al. synthesized Al₂O₃–TiO₂–W catalyst which successfully results in formation of HMF from Banana waste as cellulose source.¹⁶⁴ The catalyst's ability to break down glycosidic bonds indicates that it is effective at hydrolyzing cellulose oligomers into glucose units. They explained the three phases by which cellulose was converted into HMF: (1) short-chained, low-molecular-weight cellulose molecules known as oligomers spread out on the catalyst surface, whereupon hydrolysis in glycosidic bonds produces glucose molecules; (2) the glucose formed isomerizes to fructose through interactions with basic sites on the catalyst surface; and (3) fructose dehydrates through interactions with acid sites in the catalyst to finally form HMF. Fig. 28 explains the interaction between the catalyst surface and cellulose chains.¹⁶⁴ Furthermore, these reactions have not yet made use of several other metal oxides that, because of their acid–base characteristics, showed promise activity. For future research, it could be quite intriguing to investigate these unknown metal oxides.

Fig. 28 Interaction between catalyst surface and cellulose chains. Reproduced with permission.¹⁶⁴ Copyright 2019, Elsevier.

Metal chlorides. By combination of metal chlorides interesting results were obtained. Yu et al. used SnCl₂ to convert HMF from vegetable waste and obtained the good yield.¹⁷² Similarly, Shi et al. used MnCl₂ for conversion to HMF using corn stalk and filter paper as biomass source.¹⁶⁹ The synergetic effects of both Lewis and Brønsted acid sites of metal are responsible for the valorisation of vegetable waste to HMF. Protons liberated from metal catalyst hydrolysis and the resulting production of levulinic acid and formic acid enhanced fructose dehydration and disaccharide hydrolysis. The electronegativity, electrical configuration, and charge density of metal ions controlled the rate-determining intramolecular hydride movement that occurred during fructose dehydration & glucose isomerization, which the Lewis acid sites might catalyse. Lewis acid sites, however, caused less carbon selectivity and unfavourable polymerization, necessitating process improvement.¹⁷²

HMF and LA are the most demanding platform chemicals, having more future scope for various applications in industries like polymers, coatings, paints, lubricants, batteries, biopesticides, printing ink, photography, drug delivery, antifouling compounds, corrosion inhibitors, etc.¹⁷⁶Fig. 29 shows some important valuable chemicals that can be synthesized from HMF and LA.

Fig. 29 Valuable chemicals that can be synthesized from HMF and LA. Reproduced with permission.¹⁷⁶ Copyright 2014, Wiley.

5.2.2 Cellulose and plastic waste to aromatics. Cycloadditions have become a well-known method for coupling chemical intermediates produced from biomass and plastic waste. This is caused by the abundance of readily available biomass-derived addends, especially dienes, and plastic waste dienophile for the reliable pathway towards cyclic products with high atom economy.^177,178 Dehydration or dehydrogenation reactions can produce aromaticity after cyclo-addition of reactants, which can be accelerated simultaneously over a single heterogeneous catalyst.¹⁷⁹Fig. 30 shows the overall publication on aromatic production from cellulose and plastic waste via cycloadditions over the time generated by lens.org.

Fig. 30 Publications over time (Jan 2004–Dec 2024) in the field of aromatic production from cellulose via cycloadditions using the keywords aromatics, cellulose, cycloadditions, and catalyst, generated through lens.org.

Section 5.1.3, which discusses the co-pyrolysis of biomass and plastic waste, mostly follows the metal oxide catalysed cycloadditions. Developments in engineered catalysts are significantly increasing the pool of biomass-derived intermediates available for coupling reactions. This could lead to the synthesis of new aromatics with a variety of chemicals for direct replacement of commodity aromatics.¹⁸⁰ The extensive and diverse range of functional groups found in native biomass may change the mechanical and thermal properties of the resulting polymeric materials, opening up new commercial opportunities that petroleum cannot easily address. Fig. 31 shows the scheme of Biomass and plastic waste conversion to aromatics.

Fig. 31 Biomass and plastic waste conversion to aromatics.

To support continuous flow reactor operations and enable economically viable industrial processes for commodity aromatic synthesis, heterogeneous catalyst system development is of great interest. Stable metal oxides development has gained recent attention as they are available in different structural configurations through heteroatom framework substitutions and can be tuned to develop active sites.^181,182 Several studies have doped the metal to enhance the yield of aromatics. Wherein, for the pyrolysis of both biomass and plastic, metal oxides were found suitable to work at temperatures above 500 °C.

Table 5 shows the various works that have been laid out by using waste to produce aromatics via cycloaddition reactions.

Table 5 Summary of aromatics production via cycloaddition reactions

Dienes	Dienophiles	Catalyst	Reaction conditions	Conv./yield (%)	Ref.
Cellulose	Low-density polyethylene	Fe/HZSM-5	∼600 °C	77.8% BTX	183
Fraxinus mandshurica sawdust	Waste polypropylene plastic	Fe/HZSM-5	600 °C, 20 min	74.8% BTEX	182
Textile waste	Plastic waste	USY/zeolites	650 °C	68% BTX	184
Rubber waste	Polyacrylonitrile	Zn/HZSM-5	500 °C	22.01 wt% BTEX	185

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5.2.3 Cellulose to aromatics via ketones formation. Another alternative method for aromatics production from biomass is selective aldol condensation of alkyl methyl ketones. Generally, the self-aldol condensation is catalyzed by acids and bases for biomass-derived ketones, and it yields water as the sole condensation product.¹⁸⁶ It is one of the sole strategies for deoxygenation without requiring any additional metal catalyst or hydrogen.¹⁸⁷Fig. 32 shows the overall publication on aromatic production from cellulose via Ketones formation over the time generated by lens.org.

Fig. 32 Publications over time (Jan 2004–Dec 2024) in the field of aromatic production from cellulose via ketones formation using the keywords aromatics, cellulose, ketones, and catalyst, generated through lens.org.

For the formation of aromatics, an acid catalyst is needed. For aromatics formation, an acid catalyst is needed for the sequential cyclotrimerization of three alkyl methyl ketones. Tsai et al. suggested that longer ketones like butanone, 2-hexanone, C10-ketones, etc. yield alkyl aromatics which have the capacity to further upgrade to the desired product by transalkylation reactions.¹⁸⁸ Fuhse and Bandermann investigated the conversion of different ketones to aromatics over H-ZSM-5 at 400 °C using high-temperature gas-phase conditions.¹⁸⁹ Accessibility from biomass and appropriate reaction conditions are key factors in producing aromatics from biomass through the self-condensation of alkyl methyl ketones. Sacia et al. obtained jet fuel from biomass derived ketones by using metal oxides. Fig. 33 shows the various methyl ketones production from biomass.¹⁹⁰ Different pathways are summarized.

Fig. 33 Different methyl ketones production from biomass. Reproduced with permission.¹⁹⁰ Copyright 2015, Wiley.

Mostly, ketones serve as a valuable solvent. More than 6 million tons of acetone, the most significant alkyl methyl ketone, are produced yearly as a byproduct of the cumene oxidation process, which is used to create phenol.¹⁹¹ The elevated requisites for phenol have directly expanded the acetone production scale. The second most significant alkyl methyl ketone is 2-butanone, sometimes referred to as methyl ethyl ketone. It is made by dehydrogenating 2-butanol. Reductive condensation of acetone with butyraldehyde and pentanal yields 2-heptanone and 2-octanone, which are industrially manufactured since there is a market for ketones with longer alkyl chains.¹⁹¹ Similar to the majority of the chemical sector, fossil materials are used in the synthesis of ketones. Benzene and propene serve as the starting ingredients for the cumene oxidation process, which produces acetone. To counter this, alternative routes are developed, like exploring biomass for ketone yields, which got the yield of 67.5% 2-butanone by oxidative decarboxylation of LA by using cupric oxides. Gong et al. synthesized large alkyl methyl ketones (2-hexanone, 2-pentanone) by ring-opening hydrogenolysis of derivatives of furan.¹⁹² Researchers also used vegetable oils as the long chain alkyl methyl ketones source for hydrogenated long chain fatty acids by doing alkylation with acetone.^193,194 The upgrading process known as ketonization of carboxylic acids enables the conversion of widely available chemicals from biomass and waste sources into ketones, such as acetic acid into acetone.¹⁹⁵ It is carried out with metal catalysts present at elevated reaction temperatures of 350–500 °C. Based on this, Fufachev et al. employed a tandem method to first ketonize butyric acid on TiO₂ and then aromatization to produce aromatics via successive aldol condensation processes on Ga/ZSM-5.¹⁹⁶ After that, the ketones can be oligomerized to create fuel or utilized as feedstock for bigger molecules. Acetone is a good model chemical since it is the most basic alkyl methyl ketone, despite having less steric hindrance and therefore greater reactivity.

When a nucleophile attacks the carbonyl group of another acetone molecule, the ketone molecules are activated by acids or bases, forming the adduct diacetone alcohol.¹⁹⁷ Diacetone alcohol may frequently not be isolated under reaction circumstances because it quickly dehydrates to the dimer mesityl oxide. The α, β-unsaturated ketone can create two isomers: isomesityl oxide and mesityl oxide. Under normal circumstances, principal product is mesityl oxide. It has the ability to react with another acetone molecule to produce trimeric phorones, which may have many isomers.¹⁹⁸ It is possible for phorones to undergo successive aldol condensations with acetone to further oligomerize into tetra-, penta-, and oligomers. Dehydration of phorones to the desired aromatic trimer mesitylene is favoured when acids are present.¹⁸⁶ On the other hand, the synthesis of the non-aromatic oxygenated cyclotrimer isophorone is promoted by base catalysts like KOH. In the presence of acids, Faba et al. discovered that α-isophorone can dehydrate to mesitylene, but this is not the case for the normally produced β-isophorone.¹⁹⁹Fig. 34 shows the reaction network for the acetone self-condensation.

Fig. 34 Reaction network of acetone self-condensation. Reproduced with permission.¹⁹⁸ Copyright 2013, Elsevier.

By Using acetone condensation in conjunction with liquid mineral acids like HCl and sulfuric acid, considerable yields of mesitylene (10–36%) were obtained over longer reaction periods of 24 hours.²⁰⁰ During the early stages of research, traditional Brønsted acid catalysts were most frequently utilized for mesitylene production because the use of strong acid catalysts greatly increases dehydration and aromatization.¹⁸⁶ However, there are a number of drawbacks to homogeneous acid catalysts, including gypsum production during downstream neutralization, catalyst separation, and corrosiveness.

The optimal studied reaction conditions for acetone condensation are represented by the high temperature (300–500 °C) gas-phase conditions, for which a variety of solid acids have been tested, including zeolites and metal oxides. Among them, Cs-doped TiO₂ at 300 °C had the most mesitylene yield (28%).²⁰¹ Under these circumstances, aromatics selectivity is enhanced; however, rapid coking also promotes catalyst deactivation, which limits the creation of a continuous catalytic process. Salvapati et al. claim that an excess of acid, a high temperature, and high pressure all promote aromatization.¹⁸⁶ In order to investigate the impact of Lewis acidity, Benson et al. reported a 27.8% mesitylene yield for the catalytic deoxygenation of acetone at 500 °C and 71 bar utilizing 100% anatase TiO₂.²⁰² The idea behind the catalyst stacking was that TiO₂ preferentially catalyzes acetone dimerization, while Al-MCM efficiently encourages aromatization.¹⁹⁸ This tactic was adopted to stop polycondensation on acid sites, which would have caused a quick deactivation of the catalyst. It has been discovered that using larger pore catalysts, like mesoporous MCM-41 or zeolite H-Y, can prolong catalyst stability.²⁰³ As an alternative, high aromatics yields, and extended catalytic lives are possible with the addition of hydrogen and metal-doping of catalysts, which is frequently used in dual bed configurations with acid catalysts. In order to avoid catalyst deactivation and evaluate the fundamental processes in acetone dimerization on aluminosilicates, Herrmann and Iglesia employed Pt functions and excess hydrogen.²⁰⁴ Not only do strong acid sites facilitate the selective production of mesitylene, but large catalyst holes also allow for high mass transport. Catalyst stability is an issue because of rapid deactivation, even though mesitylene selectivity increases at reaction temperatures beyond 300 °C. One way is to look more closely at the deactivation and utilize lower reaction temperatures while keeping good acetone selectivity. Another is to employ specific catalysts like tantalum phosphate.

Herrmann and Iglesia included Pt-functions and clarified the acetone dimerization under hydrogen-controlled reaction conditions, which prevented coking, in order to address catalyst deactivation under reaction circumstances and investigate the condensation over zeolite catalysts.²⁰⁴ If not, the Brønsted acidic sites on the zeolite catalysts caused quick deactivation. They discovered that the condensation reaction only happened on Brønsted acidic sites and that Lewis acidic sites had no role in it. The kinetically important step is the establishment of a C–C bond, which happens over an H-bonded acetone with another acetone molecule. A bimolecular transition state is made up of a protonated acetone molecule, and a C-3 alkenol mediates it. The quantity of available protons and the acetone pressure both affect the rate of condensation.²⁰⁵ Their research led to the proposal of a simple step mechanism for the acetone condensation, which explains the process leading to C6 products on zeolitic Brønsted acidic sites. The mechanism omitted the sequential chemical steps required to create mesitylene. It was reported that under reaction conditions, mesitylene rapidly deactivates the active sites on Brønsted acidic sites due to its great binding affinity. Kosslick and colleagues discovered that while higher Brønsted acidity promotes the synthesis of isobutene, lower Brønsted acidity is beneficial for aromatization when using various metal-substituted MCM-materials, such as Fe-MCM-41.²⁰⁶ The type of catalyst and the density of active sites determine how acetone in aldol condensation occurs. On both acid and base catalysts, the basic processes are as follows: (1) reactant adsorption; (2) carbonyl compound enolization; (3) C–C coupling reaction and bond formation; (4) adduct dehydration; and (5) desorption of α,β-unsaturated oxygenates.²⁰⁷

Dellon et al. found that the activated cation dimer condensation product is the main branching point in the catalytic self-condensation mechanism of acetone through their analysis of microkinetic reaction models.²⁰⁸ When β-scission takes place in the process, acylium ions and isobutene are produced, which eventually result in the synthesis of xylene. The physiosorbed dimer intermediates, mesityl and isomesityl oxide, are created when deprotonation is predominant. These intermediates are necessary for the subsequent mesitylene production pathway through trimeric phorones.²⁰⁸ A complicated product mixture is the outcome of the different competing and following processes in the catalytic self-condensation of acetone. Since it enables the synthesis of aromatic compounds that are entirely biobased, the acid-catalysed aromatization of acetone to mesitylene is a very intriguing process. Meanwhile, the large-scale acetone manufacturing that occurs as a by-product of the phenol production process provides a conveniently available and inexpensive acetone stream. Since trans- and dealkylation processes were developed and put into operation commercially, the key intermediate step in replacing the various fossil-based (alkyl) aromatics at the basis of the chemical industry is understanding and optimizing the mesitylene formation from acetone in a continuous catalysed process.

5.2.4 Hemicellulose to aromatics. Hemicellulose, the second most abundant component of lignocellulosic biomass, is a polysaccharide that contains a mixture of C5 and C6 sugars, primarily xylose, arabinose, mannose, and glucose. Due to its abundance and structural complexity, hemicellulose is an attractive feedstock for furfural production—a key platform molecule with broad applications in bio-based chemicals, polymers, and biofuels, as shown in Fig. 35.¹⁶ The depolymerization of hemicellulose leads to the release of pentose sugars, primarily xylose, which can be dehydrated into furfural. This conversion process can be catalyzed by a wide range of catalysts, including homogeneous acids, heterogeneous materials, and bifunctional systems designed for enhanced selectivity and activity. Fig. 36 shows the research articles published over the years obtained through lens.org.

Fig. 35 Furfural as a key platform molecule for conversion into high-value chemicals and fuels. Reproduced with permission.¹⁶ Copyright 2018, Elsevier.

Fig. 36 Research article publication count over the year from Jan 2004 to Dec 2024 with the keywords using hemicellulose, furfural, aromatics, and catalyst from lens.org.

Traditionally, mineral acids such as H₂SO₄ and HCl have been used to catalyze the dehydration of hemicellulose into furfural.^209,210 Sweygers et al. obtained a yield of 45.79% by using HCl as the catalyst.²¹¹ These catalysts have high conversions but poor selectivity and tremendous environmental and operational disadvantages. These acids corrode the equipment, and any process involving such acids requires downstream neutralization and purification, which in turn complicates and increases waste generation. Moreover, it is also impossible to recycle homogeneous catalysts, thereby reducing the process's economic and environmental sustainability.

In contrast, heterogeneous catalysts offer several advantages that address the limitations of mineral acids. Solid acid catalysts, such as zeolites, sulfonated carbon, and metal oxides, are reusable, facilitating easier catalyst recovery and reducing waste generation.^212,213 Their solid nature allows for catalyst separation through simple filtration, eliminating the need for energy-intensive neutralization and purification steps. Additionally, heterogeneous catalysts can be designed with tailored porosity and acidity, allowing for better control over reaction selectivity and minimizing unwanted side reactions such as humin formation. These catalysts also tend to be less corrosive, enabling the use of standard materials in reactor construction and reducing operational costs. Given these benefits, the development and application of heterogeneous catalysts are essential for making biomass conversion processes like furfural production more sustainable, economically viable, and aligned with green chemistry principles. Table 6 shows the various works that have been carried out using waste to produce aromatics from hemicellulose.

Table 6 Summary of aromatics production from hemicellulose

Source	Catalyst	Reaction conditions	Conv./yield (%)	Ref.
Corn stover	AlCl3	140 °C	26 (conversion)	213
Vegetative biomass	Sulfonated graphene oxide	200 °C, 35 min	62	212
Corncob	Biochar	170 °C, 60 min	81.4	214
Corn stover	SC-CaCt-700	200 °C, 100 min	93	215
Carbohydrates	g-CN	200 °C, 100 min	96	216
Corncob	HSO3-ZSM-5	160 °C, 120 min	89.4	217
Eucalyptus sawdust	H-SAPO-34	210 °C, 120 min	99.28	218
Xylose	SBA-15-SO3H	160 °C, 240 min	70	219
Xylose	SO3H/ZrO2–Al2O3/SBA-15	160 °C, 240 min	53	220
Xylose	Cr/Mg hydrotalcite	160 °C, 720 min	50.3	221
Olive stone	γ-Al2O3–CaCl2	150 °C, 50 min	55	222
Xylose	Nb2O5	160 °C, 360 min	60.1	223
Xylose	TPA-TiO2	190 °C, 60 min	76.71	224
Wheat straw	CrPO4	160 °C, 60 min	88	225
Corncob	Cu/SBA-15-SO3H	140 °C, 360 min	62.6	226
Xylose	Al2O3–Ni–Al layered double hydroxide	100 °C	46	141

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Heterogeneous acid catalysts. Zeolites, sulfonated carbon materials, and metal oxides are the most researched heterogeneous catalysts. It is well established that H-ZSM-5 zeolites offer high catalytic activity through strong acidity and shape selectivity, ensuring maximal furfural yield. The pentose sugar conversion process in zeolites gets efficiently carried out without forming any unwanted side products, such as humin. Hoang et al. used sulfonated ZSM-5 and obtained a good yield of 89.4% of furfural. The prepared catalyst has enhanced surface acidity groups, which enhanced the yield.²¹⁷ Li et al. used H-SAPO-34 to convert sawdust to furfural using GVL solvent. They obtained a maximum yield of 99.28%.²¹⁸ The catalyst has both Lewis and Brønsted acid sites. The Lewis acid sites improved the isomerism of xylose to xylulose, while the Brønsted acid sites promoted dehydration of xylulose to furfural.²¹⁸Fig. 37 shows the direct production of furfural from lignocellulosic residues and hemicellulose.

Fig. 37 Direct production of furfural from lignocellulosic residues and hemicellulose.²²⁷ Copyright 2022, Elsevier.

Sulfonated carbon materials derived from biomass or waste resources have emerged as promising alternatives due to their high acidity, tunable porosity, and eco-friendly nature. These –SO₃H type functionalizations make up groups that are analogous to those that give sulfuric acid its catalytic property. Lam et al. tested with various kinds of carbon-based catalysts and found that sulfonated graphene oxide (SGO) shows a good result with 62% furfural yield.²¹² It has been established that SGO is a quick and effective catalyst for increasing furfural production from D-xylose aqueous solution, even at extremely low catalyst loadings of 0.5 wt% relative to D-xylose. Additionally, compared to COOH or OH groups, SO₃H groups—the active acidic sites for dehydrating D-xylose to furfural—have demonstrated better water tolerance and greater temperature stability during the reaction.²¹² Deng group obtained the 81.4% furfural yield from corncob by using a biochar catalyst, which is also prepared from corncob, as shown in Fig. 38.²¹⁴ Li et al. studied the effect of solvent on furfural yield. When raw corn stover was dehydrated in GVL (γ-Valerolactone) at 200 °C, the utilization of sulfonated carbonaceous residue—which is produced by calcining bio-based calcium citrate—produced furfural (93%) in the GVL. The authors noted that a furfural yield of just 51% was achieved under the same reaction conditions when GVL was substituted with water.²¹⁵ Verma group reported the use of graphitic carbon nitride, and they obtained a furfural yield of 96% with water as the solvent at 100 °C for 30 min.²¹⁶

Fig. 38 Xylose conversion into furfural by biochar catalyst.²¹⁴ Copyright 2016, Elsevier.

Further, their attractivity towards potential large-scale industrial operations is supported by catalyst recyclability and low environmental impact. Functionalized mesoporous silica materials, such as SBA-15-SO₃H, are equally effective with a remarkably high surface area and strength acidity in producing furfural.²¹⁹ Shi et al. introduced Al and ZrO₂ on SBA-15-SO₃H and found that Al improves the quantity and strength of acid sites by stabilizing the ZrO₂ tetragonal phase. They obtained the maximum yield of 53% furfural at 160 °C, 240 min.²²⁰

In the case of metal oxides, Lewis and Brønsted acid sites can be occupied by surface unsaturated coordination metal species and terminal hydroxyl groups that donate a proton.^228,229 In addition to this hydrotalcite, metal oxides like alumina (Al₂O₃) and titania (TiO₂) have also been demonstrated to be catalytically active in the formation of furfural.^221,222,224 Effective xylose to furfural dehydration was proposed by Lu et al. using a titania-supported tungstophosphoric acid (TPA-TiO₂) nanocomposite catalyst, which has benefits in acidity flexibility and catalytic stability as shown in Fig. 39.²²⁴ By mixing different metal oxides yield of Yi et al. used an AlCl₃ catalyst for the selective conversion of corn stover stalk by 85.1%.²¹³ These oxides typically support other active species or function in conjunction with another material in bifunctional systems. In addition to this, in most cases, GVL was found to be the appropriate green solvent for the conversion of hemicellulose to aromatics, mainly furfural. Hu group demonstrated high catalytic activity by the hydrotalcite-derived catalyst through calcination for benzylating xylose.²²¹ The Cr-Mg-LDO catalyst material that was synthesized demonstrated a mesoporous structure, possessing both basic and acidic bifunctional surface groups, with an average pore diameter of 13.73628 nm. It also had a specific surface area of 39.2168 m² g⁻¹. It demonstrated good stability and yielded a 50.3% conversion rate when utilized as a catalyst to convert xylose to furfural. Fúnez-Núñez et al. studied the synergistic effect between CaCl₂ and γ-Al₂O₃ for furfural production.²²² In comparison to other salts, CaCl₂ raises the α-xylopyranose/β-xylopyranose ratio, which favors xylose dehydration. γ-Al₂O₃ Lewis's acid sites facilitate the synthesis of furfural. After 50 minutes at 150 °C, the combined action of CaCl₂ and γ-Al₂O₃ results in complete conversion and a 55% furfural yield. CaCl₂ and γ-Al₂O₃ gave liquors made from olive stones an 83% furfural yield, as shown in Fig. 40.²²² De Lima et al. worked on amorphous Nb₂O₅ and obtained a yield of 60.1% furfural.²²³ They found that the distribution of acid site strength and types plays an important role in furfural production. The amount of converted xylose increased when isopropanol was present in the reaction medium.²²³

Fig. 39 TiO₂-supported heteropolyacids catalyst for the efficient conversion of xylose to furfural. Reproduced with permission.²²⁴ Copyright 2022, American Chemical Society.

Fig. 40 Synergistic effect between CaCl₂ and γ-Al₂O₃ for furfural production by dehydration of olive stones. Reproduced with permission.²²² Copyright 2019, Elsevier.

Bifunctional catalysts. Recent trends in catalyst design have targeted bifunctional catalysts that integrate two catalytic functionalities into the molecules to enhance both furfural production and its upgrading into aromatics. The catalysts often combine acidic sites with metallic sites in which both functions favor consecutive reactions, including the dehydration of xylose to furfural, followed by the hydrogenation and decarbonylation of furfural into aromatics like benzene, toluene, and xylene (BTX). Xu et al. used a CrPO₄ catalyst to obtain a yield of 88% furfural from wheat straw.²²⁵ For instance, bifunctional acid sites catalysts like zeolites or sulfonated carbons combined with hydrogenation metals like Ni and Cu have been found to be very efficient for converting hemicellulose into furfural followed by upgrading it further to valuable aromatic products.^230,231 Deng et al. reported the cascade conversion of hemicellulose-derived xylose by the co-existing of acidic –SO₃H sites and Cu sites in a balanced manner, as shown in Fig. 41.²²⁶ The impacts of the Cu/acid ratio and mesopore size on the product distribution demonstrated that the excessive acidic site and large pore size might increase the xylose conversion but lead to a low furfuryl alcohol yield. The most active copper-based catalysts are highly active in the decarbonylation step, selectively removing the formyl group from furfural, giving the intermediate furan compounds ready for cyclization and aromatization. Nickel and cobalt-based catalysts have also been used for hydrogenation and decarbonylation to produce furfural, showing promising outcomes regarding selectivity toward the desired product and stability under reaction conditions.^232,233 Tang group obtained biomass-derived furfural which is further converted to ethyl levulinate over bifunctional Nb/Ni@OMC.¹⁴² Similarly, Kurniawan et al. produced 1,5-pentanediol from biomass-derived furfural over a nickel–cobalt oxide alumina trimetallic catalyst.²³³ The best yields are obtained in the temp. range of 150–210 °C with a maximum duration of 120 min.

Fig. 41 The acidic –SO₃H sites and Cu sites co-existed and maintained a balance for the one-pot conversion of xylose to furfuryl alcohol. Reproduced with permission.²²⁶ Copyright 2020, Elsevier.

Catalytic performance and challenges. Although significant progress has been made in developing catalysts for the conversion of hemicellulose to furfural and aromatics, several challenges remain. Catalyst deactivation due to humin formation, the need for more cost-effective catalyst preparation methods, and the development of scalable processes are areas of ongoing research. Furthermore, achieving high selectivity for aromatic products without excessive side reactions, such as over-hydrogenation or polymerization, requires fine-tuning of catalyst properties and reaction conditions. Future research is expected to focus on improving catalyst stability, optimizing process conditions, and integrating green chemistry principles for sustainable production routes.

In summary, the conversion of hemicellulose to furfural and its upgrading to aromatics using various catalysts represents a promising approach for the valorization of biomass. The development of heterogeneous and bifunctional catalysts, as well as process intensification techniques, has the potential to improve efficiency and sustainability, aligning with the goals of green and sustainable chemistry. However, further research is needed to overcome existing challenges and scale these processes for industrial applications.

5.2.5 Lignin depolymerization. The most promising biomass source for obtaining aromatic compounds is lignin, which is the second most abundant and inexpensive lignocellulosic resource. Rich in aromatic components (i.e., high carbon content), high biodegradability, and thermal stability, lignin pockets the cellulose and hemicellulose in vascular plants and gives cell walls rigidity.²³⁴ Lignin has an amorphous, heavily branching structure. It is a 3-dimensional phenylpropane macromolecule structure.²³⁵ It is created when three monolignol radicals polymerize via various C–C and C–O linkages, including β–β, β-1, 5–5′, β-O-4, α-O-4, and 4-O-5.²³⁶ Rath et al. illustrated the structure of lignin, as shown in Fig. 42.²³⁷

Fig. 42 Molecular structure of lignin. Reproduced with permission.²³⁷ Copyright 2024, Springer Nature.

These groups make lignin a promising candidate for the production of a wide range of compounds, including simple phenols (catechols, eugenol, vanillin, and quinones), hydrocarbons (benzene and homologues of benzene), polymeric macromolecules (carbon fibers and thermosets), nutritional products, cosmetics and pharmaceuticals.²³⁸ Different plant species, extraction techniques, ambient conditions, and even plant genotypes can affect the structure and characteristics of lignin. This variance is primarily associated with the amounts of coumaryl, pineal, and sinapyl alcohol.²³⁹ Paper and Pulp industry generates approx. 50 million tons and from ethanol and textile industries almost 11 million tons lignin from industries by-products.²⁴⁰ A variety of technical lignins are produced as byproducts of the pulp and paper industry, including lignosulfonates, organosolvent lignin, and kraft lignin. Every year, over 55 million tons of kraft lignin are generated worldwide; only 2 percent of this is utilized to make binding agents and dispersants; the remaining 2 percent is burned in the cooking reagent regeneration system.²⁴¹Fig. 43 shows the overall publication on aromatic production from lignin over the time generated by lens.org.

Fig. 43 Publications over time (Jan 2004–Dec 2024) in the field of aromatic production from lignin using the keywords aromatics, lignin, and catalyst, generated through lens.org.

Lignin depolymerization is a multifaceted procedure that converts lignin's inflexible and asymmetrical structure into chemical building blocks that are helpful. There are various techniques for depolymerizing lignin, such as thermal, enzymatic, and chemical methods. Researchers are always looking for novel ways to increase the effectiveness of lignin depolymerization, as each approach has benefits and drawbacks.²⁴² Using catalysts to help break up lignin bonds is one method that shows promise for producing more valuable products like phenolic monomers and aromatic molecules.²⁴³ Enzymatic routes for lignin depolymerization are a subject of interest due to their ability to function in moderate settings and generate minimum waste.²⁴⁴ Comprehending the underlying mechanisms of lignin depolymerization is essential for refining current tactics and creating new ones. Through the clarification of the chemical reactions and elements influencing lignin degradation, researchers can endeavor to devise economically feasible and sustainable procedures for the usage of lignin in diverse industries.²⁴⁵ Significant progress has been made in the field of lignin depolymerization in recent years, opening the door to more effective and sustainable procedures. The creation of heterogeneous catalysts, which have demonstrated exceptional activity in increasing lignin bond breakage, is one noteworthy field of advancement.²⁴⁶ These catalysts have the ability to selectively produce high-value aromatic chemicals while simultaneously improving the efficiency of lignin depolymerization.²⁴⁷ Ether bond breaks are the main application for commonly used acid or base catalysts.²³⁶ There are obstacles to the industrial use of acid or alkali catalytic lignin depolymerization, including corrosion and the difficulty of recovering the catalyst.²⁴⁸ Additionally, using acid or alkali catalysts usually calls for extreme reaction conditions, such as high pressure and temperature, which drives up the cost of depolymerization significantly.²⁴⁹ The energy needed to keep these conditions can be the reason for the cost increase. Metals and metal oxides have the ability to maximize lignin depolymerization yield and selectivity while lowering the energy barrier in the conversion process.²⁵⁰ As catalysts, metals and metal oxides have special chemical characteristics. The addition of specific metal oxides improves the reaction's selectivity toward particular products while simultaneously lowering the activation energy of the process.²⁵¹ Under diverse conditions, metals and metal oxides can selectively break particular chemical bonds in lignin, but they also demonstrate varied processes for depolymerizing lignin. Almost 50% or more of all the chemical bonds in lignin are made up of the β-O-4 bond, which is a major bond in the lignin structural unit. Cleavage of the β-O-4 bond is regarded as a crucial phase in the depolymerization of lignin.²⁵²

Noble metal catalysts. Pt, Ru, Pd, Rh are among few noble metals that are used widely as the catalyst for the conversion of lignin. Out of these researchers mostly use Ru as it is economical compared to others. Kong et al. used 5Ru/30WOx/N–C catalyst and found that the intermediate phenols were consistently transformed into arenes.²⁵³ These findings demonstrated the effectiveness of the 5Ru/30WOx/N–C catalyst for the efficient cleavage of C–O–C bonds in the lignin network structure, followed by the selective cleavage of C_aromatic–OH bonds in phenolic compounds. The hydrogenolysis of C–O bonds and the hydrogenation of aromatic rings in the phenolic compounds hydrodeoxygenation reaction were confirmed to be in competition with one another by the tendency for the concentration of cyclic alcohols to increase slowly. To investigate the C–O bond cleavage process, dimer model compounds (β-O-4) were converted using a 5Ru/30WOx/N–C catalyst under the same conditions. Fig. 44 show the overall mechanism done by them.²⁵³ Shen et al. found that Ru/AC resulting in formation of less coke than other noble metal catalysts for the depolymerization of organsolov lignin, acid hydrolysis lignin, soda lignin.²⁵⁴ Ru/AC, on the other hand, exhibits significant phenol hydrogenation. The explanation is that during the catalytic conversion of diphenyl ether, hydrogenolysis and hydrogenation processes take place simultaneously but at distinct active sites. While hydrogenation of aromatic rings occurs at successive places on the platform, hydrogenolysis of C–O bonds usually occurs at edge and corner sites.²⁵⁵ Vitaly V. Ordomsky et al. modified Ru/C with Br atoms and found that by poisoning the Ru nanoparticles’ platform sites, the addition of Br specifically prevents the hydrogenation of aromatic rings.²⁵⁶ Nonetheless, the C–O bond cleavage has greater intrinsic activity at the edge and corner defect sites, which are still accessible. With this alteration, the C–O bond in diphenyl ether can be broken directly and selectively without hydrogenating any aromatic rings. As a result, there is a 90.3% yield of benzene and phenol (with about 0% selectivity on Ru/C).²⁵⁶ Xu et al. used Pt/C catalyst to depolymerize from switchgrass lignin.²⁵⁷ They combined it with formic acid and found that it boosts the production of higher fractions of low molecular weight products. This combination of catalyst with formic acid inhibits the solid products formation and also the char formation.

Fig. 44 Mechanism of lignin depolymerization into aromatics. Reproduced with permission.²⁵³ Copyright 2023, Elsevier.

Non-noble metal catalysts. Other than noble metals, transition metals can be used as they as abundant and non-noble metals. Non-noble metals are essential for the catalytic activity and selectivity of the lignin depolymerization process. For cleavage of lignin bonds nickel, cobalt, iron act as active sites, the lignin molecules are hydrogenated and cleaved more easily by these metals, producing smaller, more valuable compounds like monomers and bio-oils. Non-noble metals can have synergistic effects that improve catalytic performance overall when paired with noble metals. It has been demonstrated that bimetallic catalysts, which combine noble metals with inexpensive metals, enhance catalytic activity in the hydrogenolysis of lignin. In addition to lowering the number of required noble metals, this synergy increases catalytic activity above that of single metal catalysts. Kong et al. found this in their result. In addition to this, non-noble metals are cost-commercial and abundantly available compared to the noble metals. By using this overall cost can be deduced in the lignin depolymerization process without affecting the catalytic efficiency. Also, by tuning them we can achieve the specific target compounds. Zhang et al. used FeCl₃/NaNO₃/O₂ catalyst and with low temperature they depolymerized kraft lignin to aromatics.²⁵⁸ The Mechanism they found is shown in Fig. 45. They found vanillic acid and vanillin to be the major product which is formed by the cleavage of β-O-4 bond present in lignin and further fragmentation undergoes the transformation to vanillin.²⁵⁸ Song et al. used nickel catalyst and via fragmentation–hydrogenolysis they depolymerized the birch wood lignin.²⁵⁹ They concluded that lignin fragmented into smaller species via alcoholysis and further they converted to monomeric phenols over the catalyst. Ni based catalysts prevents hydrogenation of aromatic hydrocarbons along with breaking C–O bonds. β-O-4 and α-O-4 can be broken by Ni/AC catalyst as they exhibit strong depolymerization activity.²⁶⁰

Fig. 45 Schematic of lignin degradation to aromatics. Reproduced with permission.²⁵⁸ Copyright 2020, Elsevier.

Other catalysts. Apart from these researchers also adopted zeolites as they have well-defined pores which acts as molecular sieves to selectively adsorb and trap unwanted molecules. It exhibits the shape selectivity property. This characteristic helps with lignin depolymerization by promoting the selective breaking of lignin linkages to create desired monomeric products and preventing undesirable reactions like disproportionation. Also, its acidity influences the catalytic and product distribution. In bimetallic catalytic systems, zeolites can function as efficient supports for catalytic metals. Dehydration and deoxidation property of zeolites beneficial in lignin depolymerization operations where the extraction of functional groups containing oxygen is necessary to produce important compounds and biofuels. By giving the catalytic reactions involved in lignin depolymerization a stable and selective environment, the combination of zeolites and metal catalysts might improve catalytic performance. Chaudhary et al. used alkali metal substituted zeolites to depolymerize lignin from coconut coir to aromatics.²⁶¹ Rana et al. used K₂CO₃ as a catalyst. Due to its functions as a base catalyst, its capacity to promote hydrogen transfer processes, its solvent interaction, its ability to change pH, and its prospective application as a catalytic support, K₂CO₃ may play a significant role in the depolymerization of lignin. Through the utilization of K₂CO₃ distinct characteristics, scientists can enhance the efficiency of lignin conversion procedures and augment the production of valuable bio-based products.²⁶²Table 7 summarises the various catalysts used for lignin depolymerization.

Table 7 Summary of the various catalysts for lignin depolymerization

Source	Catalyst	Operating condition	Major product	Aromatics yield	Ref.
Coconut coir	Alkali metal-substituted zeolite catalyst	200 °C, 1 h	—	64%	258
Kraft lignin	K2CO3	300 °C, 40 min	—	48.5 wt%	262
Wheat straw alkali lignin	H2SO4	120 °C, 40 min	Monophenolic compounds	15.77 wt%	263
Switchgrass lignin	Pt/C	350 °C, 1/4/8/20 h	Lower molecular weight compounds	21 wt%	257
Birch sawdust	Ni/C	200 °C, 6 h,0.1 MPa Ar	Aromatic products	>90%	259
Soda lignin	TiN	300/340 °C	Aromatic monomers	19 wt%	264
Lignosulfonate (from pulp industry)	Ni/MgAlO–C	1 MPa of H2, 200 °C, 2 h.	Aromatic monomers	22 wt%	265
Kraft lignin	Biochar (BC) derived from lignin supported Ni–Ce catalysts	6.5 MPa N2, 280 °C, 4 h	Guaiacol and 4-alkyl guaiacols	<60%	266
Kraft lignin	5Ru/30WOx/N–C	310 °C, 5 h	Aromatic hydrocarbon	96.89 wt% bio-oil, 36.85 wt% monomer, 20.85 wt% arenes	253

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Thus, it can be concluded that Lignin molecules can change chemically with the use of catalysts, which offer active sites for bond cleavage, hydrogenation, and deoxygenation. The intricate lignin structure is broken down by these processes into more manageable, smaller molecules that can then be treated further to produce the desired results. Because catalysts reduce the activation energy needed for chemical transformations to occur, they quicken the pace at which lignin depolymerization events occur. As a result, lignin converts into target products more quickly, increasing process efficiency overall. In these reactions, catalysts can affect the selectivity of product production. Researchers can manipulate the types and yields of products produced via catalyst composition, structure, and reaction conditions, thereby improving the process for particular purposes. Reactive intermediates created during lignin depolymerization can be stabilized by catalysts, which helps to promote the synthesis of desired products and inhibit undesirable side reactions. This selective stabilization aids in guiding the reaction pathways in the direction of the intended results. Without catalysts, some lignin depolymerization events, such breaking strong C–C and C–O bonds, can be difficult. The environment and energy channels that catalysts offer help to get past these obstacles and allow lignin to be converted into useful compounds. Synergistic interactions between various metals in bimetallic or multimetallic catalyst systems can improve catalytic performance beyond what can be accomplished by any one metal alone. By enhancing the catalyst's activity, selectivity, and stability, these synergies can result in lignin depolymerization methods that are more effective.

Further lignin can be used as the platform for different industrially valued chemicals due to its abundance in biomass and unique macromolecular structure.²⁶⁷ By depolymerizing lignin, valuable aromatic chemicals like guaiacol, vanillin, and syringaldehyde are produced. These compounds are used in the flavor, fragrance, and pharmaceutical sectors.²⁶⁸ Lignin can also be converted into benzene, a versatile chemical that is employed in the manufacture of several industrial products, by certain catalytic methods. It can be used as a precursor to produce hydrocarbons, which are necessary for the petrochemical sector.²⁶⁹ By modifying and combining lignin and its derivatives with polymers, biocomposites can be produced for use in materials science applications.²⁷⁰ For example, lignin-epoxy composite resins, which are thermosetting materials, can be used to make sports equipment and aircraft parts.²⁷¹ The antioxidant and UV shielding qualities of lignin-polysaccharide composites have been studied, making them promising materials for the development of oxidants and sunscreens.²⁷² The promise of lignin-based materials in environmental remediation applications has been demonstrated by their effective use as adsorbents for ecologically hazardous metals.²⁷³ Researchers are investigating novel methods to transform lignin into important platform chemicals for a variety of commercial applications by utilizing the chemical and structural features of lignin. Rath et al. illustrated the diverse industrial valued chemicals that can be produced from lignin as shown in Fig. 46.²³⁷

Fig. 46 Lignin as a platform for different chemicals.

The prospective uses and environmental advantages of materials and chemicals derived from lignin are set to transform sustainable resource utilization and influence the course of numerous sectors as the research landscape continues to change. In addition to influencing current research, the conclusions and ramifications of seminal articles in this area have spurred the creation of novel approaches, thereby reinforcing lignin's status as a fundamental component of sustainable innovation. Fig. 47 shows the overall biomass valorization into renewable aromatics from various LCB components, cellulose, hemicellulose, and lignin, with the presence of zeolites, metal oxides, and carbon catalysts.

Fig. 47 Renewable aromatics production from lignocellulosic biomass.

5.3 Glycerol to aromatics

There is an increasing demand for creative ways to use waste materials for the synthesis of useful chemicals due to the growing global concern over waste reduction and the detrimental effects of toxic emissions on ecological systems and public health. Glycerol is one such waste product that is produced in enormous amounts as a byproduct of the biodiesel industry.²⁷⁴ The use of a catalyst to transform waste-derived glycerol into aromatics presents a significant opportunity to address resource sustainability and waste management.²⁷⁵ Three hydroxyl groups make glycerol, a polyol molecule that is useful as a feedstock for a variety of chemical reactions.²⁷⁶ It is a desirable option for the manufacturing of sustainable chemicals due to its abundance as a waste product. Because catalysts have the ability to convert glycerol into aromatic compounds with excellent selectivity and efficiency, this process has attracted a lot of interest. Numerous catalysts, each with their own benefits and reaction routes, have been investigated for this purpose, including zeolites, metal oxides, and supported metal catalysts.^275,277,278 The development of efficient and sustainable techniques for the synthesis of aromatics can help to promote a circular economy and lessen the environmental impact of waste disposal by utilizing the potential of glycerol obtained from waste.²⁷⁹Fig. 48 shows the overall publication on aromatic production from glycerol over the time generated by lens.org.

Fig. 48 Publications over time (Jan 2004–Dec 2024) in the field of aromatic production from glycerol using the keywords aromatics, glycerol, and catalyst, generated through lens.org.

The many catalyst types used in the glycerol conversion process function according to different processes and paths of reactions. Comprehending these mechanisms is crucial in order to optimize the conversion process and customize the catalyst in order to attain the intended level of product selectivity. Significant improvements in catalyst stability, longevity, and selectivity have been made as a result of ongoing research in the development of catalysts for the conversion of glycerol. The process of converting glycerol to aromatics is being continuously improved in terms of efficiency and economic feasibility through the development of new catalyst designs and modifications. Fig. 49 shows the glycerol to aromatics conversion over zeolite catalyst.

Fig. 49 Glycerol to aromatics conversion. Reproduced with permissiom.²⁸⁰ Copyright 2023, American Chemical Society.

5.3.1 Zeolites based catalyst. The catalytic pyrolysis technique has garnered significant attention in the conversion of glycerol to bio-aromatics, as it was discovered to be a means of improving the quality of pyrolysis-derived products. Mostly pyrolysis of glycerol was performed in the range of 400–550 °C, with the presence of nitrogen gas flow in flow reactor. Almost all the reactions zeolites-based catalyst are utilized. He et al. used ZSM-5 catalyst with methanol as solvent and found the yield around 34.6% and later modifying the zeolite with metal oxides he found the yield in Benzene, Toluene, Xylene (BTX) almost the same.²⁸¹ Jang et al. used unmodified HZSM-5 and found the yield around 45%. He found that yield can be tuned by adjusting the glycerol content.²⁷⁵ Xiao and Verma doped Platinum and Palladium in HZSM-5 and found the increase in yield of the BTX production. They proposed that conversion of glycerol to hydrocarbons is followed by hydrodeoxygenation and aromatization.²⁸² Pan et al. shows the reaction pathway as shown in Fig. 50.²⁷⁷ Singh et al. obtained the highest selectivity of 64% by doping Ga and Zn in ZSM-5.²⁸⁰ By doping SiO₂ and Zn in ZSM-5 they obtained 57% pXylene selectivity. Increased shape selectivity of ZSM-5's micropores, increased Lewis's acidity, and decreased surface Brønsted acidity all contributed to the SiO₂–Zn/ZSM-5 catalyst's increased p-X selectivity during the glycerol–methanol aromatization process.²⁸⁰ Zeolite-based catalysts are beneficial because of their acidic characteristics, which allow for a wide range of modification in their Lewis and Brønsted acid features. Because of their acidic qualities and small holes that allow for shape selectivity, several series of zeolites are therefore widely employed to convert glycerol to bio-aromatics. Additionally, the distribution of the end products is influenced by the pore structure of the zeolites.

Fig. 50 Reaction pathway for glycerol to aromatics (GTA) over HZSM-5 as catalyst. Reproduced with permission.²⁷⁷ Copyright 2022, Elsevier.

Zeolites have several advantages as catalysts, including a large surface area, tunable acid characteristics, and sometimes simple regeneration. Zeolites exhibit strong catalytic activity in a variety of processes, including the conversion of glycerol into high-value chemicals.²⁸³ Zeolites are a highly effective catalyst for producing bio-aromatics by the selective pyrolysis of glycerol, as per their comprehensive description. The bio-aromatics which were obtained were significantly impacted by the acid strength sites on zeolite. Three-dimensional pores, like HZSM-5, were the most selective catalysts in catalytic pyrolysis of glycerol when compared to one-dimensional pore zeolites, including HZSM-22 and Mordenite.²⁸⁴ Strong Brønsted acid and weak Lewis acid sites provided by HZSM-5 result in a high yield of liquid aromatics containing C9–C12 aromatics.²⁸⁵

Zeolites having a hierarchical pore structure can enhance bulk molecular diffusion, leading to higher BTX yields and longer catalyst life.²⁸⁶ The acidity, porosity, and morphological of HZSM-5 were significantly impacted by soft mesoporogens, which also had an effect on the catalyst's lifespan and product dispersion.²⁸⁷ With the highest BTX yield, the structure of the HZSM-5 catalyst outperformed the other catalysts in terms of performance. Rapid intra-mesopore mass transfer combined with strong micropore shape selectivity relates to effective micropore-mesopore interconnection.²⁸⁸ Lastly, we suggest developing a hierarchical zeolite that has a nanosheet structure to avoid coke formation and taking into consideration developing the method at a pilot scale via stability, recyclability, and regeneration of the catalyst in order to improve the catalyst's performance in the conversion of glycerol to aromatics. Table 8 summarizes the using of zeolites catalyst for aromatics production from glycerol.

Table 8 Summary of aromatics production from crude glycerol

Source	Catalyst	Reaction conditions	Yield (%)	Ref.
Crude glycerol	ZSM-5(23)	520 °C, N2	34.6% BTX	281
Crude glycerol	H-ZSM-5/Al2O3	550 °C, 12 h	32.3 BTX	289
Crude glycerol	H-ZSM-5	440 °C, 3 h	37–45% BTX	275
Crude glycerol	H-ZSM-5/Al2O3	550 °C, 12 h	26.7 BTX	290
Crude glycerol	Pt/H-ZSM-5 and Pd/H-ZSM-5	400 °C, N2 and H2	60 BTX	282
Crude glycerol	ZIF-8 + ZSM-5	400 °C, 11 h	59.5 BTX	277
Crude glycerol	HZSM-5/CNT	440 °C, 3 h	28.6 BTX	291
Crude glycerol	HZSM-5/TPOAC	450 °C, 8 h	38.5 BTX	292
Crude glycerol	NaOH/HZSM-5	400 °C, 3 h	35 BTX	283
Crude glycerol	0.1Ba–1Zn/ZSM-5	420 °C, 3 h	57	278
Crude glycerol	SiO2–Zn/ZSM-5	420 °C, 3 h	57% p-xylene selectivity	280
Crude glycerol	Ga–Zn/ZSM-5	420 °C, 3 h	64% (selectivity)	293

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One promising step in the design of a biorefinery superstructure is the use of atom economy as a performance measure. Although atom economy is a popular strategy in chemistry, especially in biochemical conversion or green chemistry, thermochemical biorefinery designs have not yet made extensive use of it. Mukeru et al. used the concepts of the atom and carbon economies along with the Gibbs energy and economic potential to propose feasible conversion scenarios in the area of glycerol pyrolysis that would not generate waste while still accounting for energy performance.²⁹⁴ A more thorough examination of how the biomass's C, H, and O contents changed during the conversion process may provide information for bettering the design of biorefineries and enhancing the utilization, accessibility, and control of atoms, molecules, and chemicals throughout a process. With the assumption of a proportionate reduction in waste, the yield of a target chemical is a key parameter in process design to assess the efficacy of a reaction, disregarding the molecular efficiency of reactants in the conversion. Naveenkumar et al. used the green chemistry balance equation in their study to study the techno-economic analysis.²⁹⁵Fig. 51 shows the renewable aromatics production mainly BTX from the crude glycerol with the presence of Metal/Zeolites with the temperature range of 400–550 °C under 3–12 h.

Fig. 51 Renewable aromatics production from glycerol.

5.4 CO₂ to aromatics

Growing concern surrounding climate change and the pressing need to cut greenhouse gas emissions have made the process of turning carbon dioxide (CO₂) into useful compounds seem like a potential way forward.²⁹⁶ Yet, the chemical stability of CO₂ presents a substantial obstacle, frequently necessitating severe reaction conditions and large energy inputs that raise the conversion process's financial, operational, and environmental costs.²⁹⁷Fig. 52 shows the overall publication on aromatic production from CO₂ over the time generated by lens.org.

Fig. 52 Publications over time (Jan 2004–Dec 2024) in the field of aromatic production from CO₂ using the keywords aromatics, CO₂, and catalyst, generated through lens.org.

One of the most practical ways to cut CO₂ emissions is to use CO₂ as a carbon source for chemical synthesis. Usually, CO₂ uses the reverse water–gas shift reaction to react with hydrogen from hydrocarbons, releasing CO instead of directly adding carbon to the product creation process, which results in a lower atom economy.²⁹⁸ However, recent studies have shown a great deal of progress in creating efficient catalytic processes that can convert CO₂ into a range of useful compounds and fuels, such as gasoline fractions, dimethyl ether, ethanol, methanol, lower paraffin, and lower olefins.^299,300 Olajire et al. analysis offers a thorough synopsis of the several valorization systems that turn CO₂ into these useful goods.³⁰¹ Both the direct hydrogenation of CO₂ to methanol and the indirect conversion of CO₂ to methanol, which entails the synthesis of gas production by reforming as an intermediate step, have been extensively explored.³⁰² These procedures show how important catalysis is to making CO₂ conversion technology commercially viable. Zangeneh et al. process simulations and economic assessments are crucial for assessing the viability and bounds of economic viability for these technologies, as noted by Weisberg et al.^299,300 This is especially true in light of the price volatility of raw materials like CO₂ and hydrogen.

The importance of using CO₂ as a feedstock for aromatic synthesis has been highlighted by the pressing need to switch from fossil-based feedstocks in favor of more sustainable alternatives. One appealing and sustainable carbon source is CO₂, a common and abundant greenhouse gas. In addition to addressing the urgent problem of CO₂ emissions, its conversion into high-value aromatics offers a sustainable route for the production of necessary industrial chemicals.³⁰³ The petroleum refining sector is the primary source of aromatics, which are the platform chemicals used in the polymer industry. Apart from the petroleum refining sector, aromatics can also be generated by the alkenes’ dehydrogenation and cyclization processes. Chemical engineers argue that the step-by-step aromatics production strategy (I. alkenes formation from petroleum refining or methanol; II. aromatics formation from alkene dehydrogenation and cyclization reactions) is more inconvenient and expensive than the single-pass CO₂ to aromatics process as shown in Fig. 53. Thus, in terms of the reaction mechanism, chemical engineering, environmental evaluation, etc., the creation of the CO₂ to aromatics method represents a thermodynamically feasible, economical, and ecologically acceptable pathway for aromatics synthesis.³⁰⁴

Fig. 53 (A) Aromatics synthesis from crude oil (I), coal, biomass, or natural gas (II), and greenhouse gas CO₂ (III); (B) CTA process realized by a methanol-mediated pathway on the bifunctional catalyst and modified FTS on the multifunctional catalyst.³⁰⁴ Copyright 2021, Wiley.

It can be observed that mostly zeolites act as a good catalyst when combined with various transition metals. Yan et al. explain the role of zeolites as the catalyst for targeted value-added products.³⁰⁵ The type of zeolite selected is the deciding factor for obtaining the type of product. Fig. 54 explains the various zeolites and metals roles in obtaining the product.³⁰⁵ Metal–zeolite catalysts have proven to be highly effective in CO₂ conversion, owing to their distinct characteristics like high surface area, consistent pore size, and tunable acidity. They have also shown encouraging results in terms of activity and selectivity. A key element influencing the production distribution is the zeolites’ framework topology, which regulates which molecules can enter and exit the pores. Zeolites that have large internal pores, like MCM-22 and HBEA, preferentially create C₅₊ hydrocarbons, whereas those with small holes, like SAPO, SSZ-13, and HRUB-13, encourage the creation of C₂–C₄ olefins. The density, strength, and kind of zeolite acid sites—Brønsted or Lewis—as well as the zeolite structure, play a critical role in determining the products’ selectivity. Iron-based catalysts are commonly employed for CO₂ hydrogenation via the FTS pathway, and the addition of alkali metals to it improves the basic properties.³⁰⁶ Other compound like ZnO and ZrO₂ favors the MTH pathways. The creation of CH_XO species (such as CH₃O, CHO, and CH₃OH) is facilitated by the metal–oxide interfaces. These species are then transported to zeolites, where they are transformed into olefins and aromatics.³⁰⁷

Fig. 54 Illustration of metals and zeolites used in CO₂-MTH and CO₂-FTS pathways for hydrogenation of CO₂. Reproduced with permissiom.³⁰⁵ Copyright 2023, Royal Society of Chemistry.

The distribution of the products is greatly impacted by the acid sites, which are important in the hydrogenation of CO₂. The zeolite contains two different kinds of acid sites: Lewis acid sites (electron pair acceptors) and Brønsted acid sites (proton donors). According to reports, zeolites with the right design can provide increased catalytic activity for CO₂ hydrogenation.³⁰⁸ Zeolites include Brønsted acid sites, which are essential for oligomerization, isomerization, cyclization, and alkylation. Through CO₂ hydrogenation, these sites help create n-paraffins, iso-paraffins, olefins, light aromatics, and heavy aromatics.³⁰⁹ At the same time as avoiding coke production and catalyst deactivation, achieving the proper balance of Brønsted acid sites (in terms of strength and concentration) becomes crucial to optimizing intended product output.³¹⁰Fig. 55 shows the influence of Brønsted sites on product selectivity and yield.

Fig. 55 Influence of zeolite acidity on the product selectivity and catalysts.

Dai et al. used a Fe-based catalyst with the addition of potassium.³¹¹ They reported that the Fe–K bimetal is better dispersed when alkaline Al₂O₃ is added as the support. This encourages CO₂ adsorption, which in turn prevents H₂ adsorption and aids in the synthesis of lower olefin intermediates. Low Si/Al ratio HZSM-5 zeolites’ strong acid sites are necessary for the synthesis of aromatic compounds. The highly selective catalytic process that converts CO₂ to aromatics depends on the proper proximity of two active components in the tandem catalyst. The granule-mixing catalyst accelerates the hydrogenation of CO₂ to aromatics while retaining robust acidity and CO₂ adsorption capability. Phosphorus alteration increases the number of medium-strength acid sites and decreases the acid strength of HZSM-5 zeolites, which further encourages the production of aromatics.³¹¹ Wang et al. also used Fe catalyst with Na and obtained good yield of 50% yield.³¹² Na-modified Fe-based catalyst, which is made by pyrolyzing Fe-based metal–organic frameworks (Fe-MOFs), has highly accessible active sites and catalytic interfaces that are finely tuned, which can enhance the generation of alkene intermediates.³¹² The generated alkenes can then diffuse to the acid sites of H-ZSM-5 and undergo dehydrogenation and cyclization reactions, which can turn them into aromatics. The high output of aromatics was ensured by the hollow H-ZSM-5, which had short diffusional channels, the right density, and strong acid sites.³¹² Similarly, Cui et al. used Zn, Na, and Fe-based catalysts on the ZSM-5 catalyst and obtained the highest selectivity of 75.6% as shown in Fig. 56.³¹³

Fig. 56 Selective production of aromatics by Zn alkali Fe-based catalyst on HZSM-5. Reproduced with permission.³¹³ Copyright 2019, American Chemical Society.

Zn and Zr oxides also show the good selectivity of aromatics. Zhou et al. took ZnO–ZrO₂ aerogels integrated with HZSM-5 and obtained the highest selectivity of 76%. They reported that their catalyst provides a high surface area and more oxygen vacancies.³¹⁴ The overall quantity of oxygen vacancies determines how quickly the methanol intermediate forms over ZnO–ZrO₂. ZnO encourages the dissociation of hydrogen, whereas ZrO₂ is primarily in charge of CO₂ adsorption. In addition to converting methanol to aromatics, the incorporation of H-ZSM-5 promotes the conversion of CO₂ over ZnO–ZrO₂ catalysts. Furthermore, the bifunctional catalyst's consistent performance under settings relevant to industry points to bright futures for industrial uses. Fig. 57 shows the possible mechanism as reported by them.³¹⁴

Fig. 57 Possible mechanism of direct hydrogenation of CO₂ into aromatics over bifunctional ae-ZnO–ZrO₂/Z5 catalyst. Reproduced with permission.³¹⁴ Copyright 2020, American Chemical Society.

However, Ni et al. reported that they obtained 58.1% p-xylene over the composite catalyst prepared by them as a result of Si-H-ZSM-5.³¹⁵ ZnAlO_x & H-ZSM-5 have promising roles in future industrial manufacturing of aromatics by CO₂ and H₂. Li et al. obtained 75% selectivity; they created combined ZnZrO_x/ZSM-5 catalysts for CO₂ hydrogenation to aromatics with movable proximity between the two components. By varying the quantity of biomass during the catalyst production process, the intimacy may be tuned using the rice husk-derived SiO₂. It was discovered as shown in Fig. 58, that the aromatics selectivity was much enhanced by intimate optimization.³¹⁶

Fig. 58 Role of SiO₂ platform for aromatics selectivity. Reproduced with permission.³¹⁶ Copyright 2023, Elsevier.

Researchers also tried the use of Chromium on ZSM-5 and obtained a good yield of around 76%. Wang et al. tried by combining it with Zn and obtained the selectivity of 70%.³¹⁷ In order to increase the driving force towards aromatics in the tandem reaction process, the CO₂ reactant can function as a hydrogen acceptor to hasten the dehydrogenation of alkenes and intermediates in the synthesis of aromatics.³¹⁷ The technique is becoming more and more feasible for industrial use thanks to advancements in the study and improvement of catalytic mechanisms, which have enhanced catalyst selectivity, yield, and stability. The use of CO₂ as a feedstock supports international efforts to mitigate climate change by lowering greenhouse gas emissions and decreasing reliance on fossil fuels. Even with these developments, a number of obstacles still exist. Important areas that need greater investigation and creativity are the creation of more affordable and effective catalysts, reaction condition optimization, and process scalability. To improve the process’ overall sustainability, catalytic systems’ interaction with renewable energy sources also has to be improved. By investigating innovative catalytic materials, utilizing cutting-edge computational and experimental approaches, and encouraging interdisciplinary collaborations, future research should concentrate on addressing these issues. The catalytic conversion of CO₂ to aromatics has the potential to become a key component of sustainable chemical production if efforts are made to further push the boundaries of catalyst design and process optimization.³¹⁸

In conclusion, a viable route to a more sustainable and circular economy is provided by the catalytic conversion of CO₂ to aromatics. This method helps the chemical sector grow sustainably while simultaneously protecting the environment by converting a greenhouse gas into useful chemical products. In order to fulfil this field's promise and ensure a sustainable future, more research and innovation are required. Table 9 summarises the production of the aromatic from CO₂ by using various catalysts.

Table 9 Summary of aromatics production from CO₂

Source	Catalyst	Reaction conditions	Conv./yield (%)	Ref.
CO2	Fe–K/α-Al2O3 and P/ZSM-5	400 °C, 4 h, 3 MPa pressure	35.5% selectivity	311
CO2	Na–Fe@C & H-ZSM-5	320 °C, 12 h, 3 MPa pressure	50.2%	312
CO2	ZnFeOx–nNa/H-ZSM-5	320 °C, 3 MPa pressure	75.6% selectivity	313
CO2	ZnO–ZrO2/H-ZSM-5	340 °C, 4 MPa pressure	76% selectivity	314
CO2	Si-H-ZSM-5, ZnAlOx & H-ZSM-5	319.85 °C, 3 MPa	58.1% p-xylene	315
CO2	ZnZrOx/ZSM-5	340 °C, 3 MPa	75% selectivity	316
CO2	ZnO–ZrO2/HZSM-5	420 °C, 5 MPa	20.7%	319
CO2	HO–ZnZrO@C and H-ZSM-5	360 °C, 3 MPa	73% selectivity	320
CO2	Cr2O3/H-ZSM-5	350 °C, 3 MPa	75.9% selectivity	321
CO2	Cr2O3/Zn-ZSM-5@SiO2	350 °C, 3 MPa	70% selectivity	170

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Fig. 59 shows the production of renewable aromatics from the CO₂ capture under the temperature range of 300–400 °C using metal oxides and zeolites as catalysts.

Fig. 59 Renewable aromatics production from the CO₂ capture.

6. Separation of aromatics compounds from complex mixtures

Separating aromatic compounds from complex hydrocarbon mixtures presents significant challenges due to the overlapping physical and chemical properties of the components. Several key issues and approaches in aromatic separation include:

6.1 Extractive distillation

Extractive distillation using selective solvents is a common method for separating aromatics.³²²N-Substituted morpholines with up to 7 carbon atoms in the substituent group have been proposed as effective solvents. This method can work across a wide range of aromatic concentrations in the feed mixture.

6.2 Liquid–liquid extraction

Traditional liquid–liquid extraction often requires water addition to adjust solvent selectivity, which increases energy costs for solvent recovery.³²² Newer solvents like ionic liquids show promise for aromatic extraction without water addition.³²³

6.3 Chromatographic techniques

Gas chromatography, particularly using capillary columns, has been utilized for separating complex aromatic mixtures.³²⁴ For more challenging separations, two-dimensional gas chromatography (GC × GC) coupled with time-of-flight mass spectrometry (ToF-MS) has shown improved resolution of complex polycyclic aromatic hydrocarbon (PAH) mixtures.³²⁵

6.4 Membrane technology

Pervaporation using oriented monolayer membranes shows potential for aromatic–aliphatic separations, which are particularly challenging due to similar boiling points.³²⁶

Bio-oils can be separated into mainly two fractions, as suggested by Onay, as soluble and insoluble compounds using n-pentane.⁷⁵ Adsorption chromatography was used to further separate the n-pentane soluble components into polar, aromatic, and aliphatic fractions. Olefins and paraffins make up the majority of the aliphatic portion. The polar fraction comprises oxygenated molecules, while the aromatic fraction contains low molecular weight aromatics, often benzene or its derivatives. In general, Py-GC/MS was utilized to examine the distribution and composition of volatile products in order to better understand the pyrolysis mechanism and catalytic efficiency. Based on their functional structure, the compounds are divided into five distinct groups: aromatic oxygenates, non-aromatic oxygenates, non-aromatic hydrocarbons, aromatic hydrocarbons, and compounds that contain nitrogen.⁶⁷ Torrefaction pretreatment was performed to separate the solid product, as explained by Liu et al.⁷³ To obtain the purified product, Yan et al. performed the purification process where, in a pilot-scale downdraft gasifier, biosyngas were produced from oak-tree wood chips with a moisture content of 8.3–10.8%.⁷⁰ It was then cleaned to remove impurities. To get rid of fine particles, the syngas generated from biomass was passed through parallel bag filters. Following filtration, tars were eliminated from the biomass-derived syngas by passing it through an activated carbon filter. Prior to the Fischer-Tropsch reactor, a deep purification procedure was created and put in place to get rid of tar, sulfur, oxygen, and ammonia.⁷⁰

Pyrolysis bio-oil is a highly complex mixture containing hundreds of oxygenated compounds including phenolics, aromatics, acids, and sugar derivatives. This chemical complexity makes separation of specific aromatics challenging.³²⁷ The high acidity and instability of bio-oil can complicate separation processes and equipment design.^322,327 In addition to this many separation techniques that work well at lab-scale face difficulties in scaling up to industrial processes.³²⁸ Addressing these challenges will be crucial for developing economically viable processes to obtain pure aromatic compounds from pyrolysis bio-oil. Further research is needed on innovative separation technologies tailored to the unique properties of bio-oil. Apart from these challenges, aromatics and aliphatic often have similar boiling ranges, making distillation difficult.³²⁹ Advanced analytical techniques, and process optimizations need to be achieve efficient for the economical aromatic separations from complex mixtures.

7. Green chemistry metrics in process chemistry

The foundational idea of Green Chemistry is “benign by design”, which refers to creating ecologically friendly products and procedures in accordance with the twelve principles of Green Chemistry established by Anastas and Warner.³³⁰ It consists of three fundamental components. Reducing waste in the first place by using raw materials effectively. Secondly, evading risks to health, safety, and the environment by avoiding hazardous or toxic materials, including solvents, and utilizing renewable biomass by replacing natural gas, coal, or crude oil as non-renewable fossil feedstocks. In the chemical process, renewable carbon is not only the factor need to be considered for designing bio-based products. In addition to this, other factors are needed to assess the greenness of synthesis process to finding the waste and hazardous materials. These include atom economy and environmental factor (E factor) which are accepted widely to measure the greenness of the chemical process. E factor (EF) can be used to quantify the amount of generated waste from the process as shown in Table 12. It is expressed as kg of waste per kg of product.³³¹ For calculating the EF, experimental weight of all the reagents and solvents used are considered along with the chemical yield obtained in the chemical process.³³¹ The higher the E factor, the greater the generation of waste, leading to a negative environmental impact. In general, solvent discharged in the chemical reaction contributes to the major portion of waste because of the toxic, non-renewable nature of solvent. To overcome this, researchers have came across for using the green solvents like polyethylene glycol,³³² supercritical fluids,³³³ ionic liquids.³³⁴ Along with these, biomass derived solvents gained significant attention in the industry for commercialisation. In the section 5.3 BTX is produced from glycerol which obtained from the waste of biorefinery waste thereby reducing the E factor value. Simple E factor (sEF) doesn't consider the solvents for the calculation of waste generated. This is generally used in the primary evaluation of the chemical process.³³⁵ In the case of pyrolysis as mentioned above in section 5.1, the EF can be addressed through pyrolysis, as it converts most of the input waste into usable products.³³⁶ Catalytic fast pyrolysis of lignocellulosic biomass can be energy-intensive but offers rapid conversion.³³⁷ Microwave-assisted pyrolysis has gained attention due to its energy efficiency and rapid processing capability.³³⁸

Atom economy (AE) is another factor which is a theoretical number based on the raw materials stochiometric quantities and with the assumption of 100% yield. This is a rapid evaluation of number of the atoms of the raw material entered into the reactor to the desired product formed.³³⁹ Fadlallah and the team did a thorough study and calculated the E factor, sEF for the vanillin-derived monomers for various precursors which is shown in the Table 10.³³¹ From this analysis one can study about the best route for the sustainable chemical process. In the section 5.1, pyrolysis offers high atom economy for plastic waste treatment. It allows virtually all atoms present in the waste material to be reused to produce useful chemicals in the form of gas, oil, and char.³³⁶ For example, the sulfur-assisted pyrolysis strategy developed for upcycling waste plastics into high-value carbons achieves an atom economy of over 90%.³⁴⁰ The heat generated by combustion of pyrolysis gas can be recovered and used in the pyrolysis process itself, improving overall energy efficiency. However, pyrolysis still requires high temperatures, so careful energy balance calculations are needed to optimize efficiency.

Table 10 Green metrics for the vanillin-derived monomers for various precursors. Reproduced with permission.³³¹ Copyright 2021, Royal Society of Chemistry

Steps	Yield (%)	AE (%)	sEF	EF	Solvent contribution (%)	Ref.
1	76	81	3.48	3.89	8	341
1	84	75	0.64	0.64	90	342
1	97	92	1.41	38	94	343
1	95	93	0.13	13.7	92	344

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Sheldon illustrates the table as shown in Table 11 describing the magnitude of the waste generated in the chemical industry.³⁴⁵

Table 11 Standard values of EF for various industries

Industry	Product volume (tons per annum)	E factor (kg waste per kg product)
Bulk chemicals	10^4–10^6	<1 to 5
Fine chemical industry	10^2–10^4	5 to >50
Pharmaceutical industry	10–10^3	25 to >100

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Apart from EF, there is another factor that is used called “Environmental quotient” (EQ). EQ is calculated by multiplying the E factor by an unfriendliness quotient, Q, that is chosen at random.³⁴⁵ For instance, based on its toxicity and ease of recycling, one could arbitrary assign a Q value of 1 to NaCl and, say, 100–1000 to a heavy metal salt, like chromium. In theory, it is conceivable to perform a “quantitative assessment” of the environmental impact of trash, even though the exact value of Q is controversial and challenging to measure. Q depends on a number of factors, including how simple it is to dispose of or recycle garbage; in general, organic waste is easier to do so than inorganic waste.³⁴⁵

Process mass intensity (PMI) assesses the overall mass of the materials employed (catalysts, solvents, and reactants). A low PMI indicates the resource-efficient processes. It can be done by optimizing the solvent and by using recycled catalyst.³⁴⁶ Reaction mass efficiency is another metrics used which considers the reaction yield, atom economy, and stochiometric factors to evaluate the overall mass efficiency. It is particularly useful for multi-step processes common in converting waste biomass into aromatics.³⁴⁷ Life cycle assessment (LCA) is the most common when it comes to the sustainability. It provides a comprehensive evaluation of environmental impacts across a product's lifecycle, including raw material sourcing, energy use, emissions, and end-of-life disposal. For renewable aromatics, LCA helps compare waste-derived processes with petrochemical routes to identify greener alternatives.³⁴⁸ Eco-scale is the effective tool for benchmarking green processes in renewable aromatic production.³⁴⁷ It assigns a score based on the factors like yield, cost, safety, and environmental impact. Another metrics is the green aspiration level (GAL). It compares a process's actual performance against an ideal green process benchmark. It is particularly relevant for industrial applications where renewable feedstocks are scaled up for aromatic production.³⁴⁶ Following the green chemistry principles of use of renewable feedstocks, carbon efficiency (CE) assess how effectively carbon from renewable sources (e.g., lignocellulosic biomass or glycerol) is incorporated into aromatic products. This metric underscores the importance of maximizing resource utilization while minimizing carbon losses.³⁴⁷ In the section 5.1, a better CE was obtained when olefins are produced by PE and cellulose by using the HZSM-5 catalyst, where C in biomass is transferred to the aromatics rather than the coke.¹⁴⁸ Ga-promoted HZSM-5 zeolites have shown improved efficiency in the aromatization of biomass-derived furans, with increased aromatic yield and reduced coke formation.³⁴⁹Table 12 summarizes all the green metrics that can be used for analysis.^350–358

Table 12 Green metrics formulas and the preferred values in process chemistry

Metric	Formula	Preferred value
Environmental factor		0
Simple E factor		0
Atom economy		100%
Real atom economy		1
Process mass intensity		1
Reaction mass efficiency		100%
Mass intensity		1
Mass productivity		100%
Stoichiometric factor		1
Carbon efficiency		-
Effective mass yield		-
Solvent and catalyst environmental impact parameter		-

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Kreuder and the research group developed principle specific algorithms to calculate the 12 principles of green chemistry.³⁴⁸Table 13 summarizes all these algorithms.

Table 13 Green chemistry principles algorithms for green metrics analysis

Principles	Algorithms
Waste prevention
Atom economy
Less hazardous chemical synthesis
Designing safer chemicals
Safer solvents and auxiliaries
Energy efficiency
Use of renewable feedstock
Reduce derivatives
Catalysis
Design for degradation focuses on environmental hazard criteria
Real time analysis for pollution prevention
Inherently safer chemistry for accident prevention

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In this review paper we report the aromatics production from the waste sources, where the reactions are carried out in a various solvent out of which those carried out with the water as the friendly solvent under mild condition leads to the less energy intensive and waste free techniques that are more appealing from an economic and environmental standpoint than traditional methods. Unfortunately, less data is available for green chemistry metrics study for the renewable aromatics from waste making it difficult to compare between the various reports. It can be concluded that green chemistry principles—such as waste prevention, atom economy, renewable feedstocks, and safer chemical synthesis—lead to reduced hazardous waste, lower emissions, and minimized ecosystem disruption, thereby protecting the environment. Economically, adopting green chemistry approaches results in higher yields, reduced energy consumption, lower waste management costs, and increased competitiveness through innovation and safer products.³⁵⁹

8. Artificial intelligence and machine learning in renewable aromatic production

Artificial intelligence (AI) and machine learning (ML) are rapidly transforming the chemical and energy sectors, where these technologies are being leveraged across the value chain to optimize processes, accelerate research, and drive sustainability.^360,361 AI and ML models, particularly artificial neural networks (ANNs), are increasingly used to predict and optimize the complex kinetics of biomass and plastic pyrolysis. These models can handle nonlinear relationships among variables such as feedstock composition, temperature, and reaction time, leading to more accurate predictions of product yields and process efficiency. For instance, ANN-based models have demonstrated high accuracy of R² ∼ 0.94–0.99 for predicting pyrolysis behaviour of biomass and in predicting activation energies and yields during biomass pyrolysis, outperforming traditional kinetic modelling methods.^362,363 ML algorithms, trained on large datasets from experiments and simulations, accelerate the discovery of novel catalysts for renewable aromatic production. It rapidly identifies promising catalyst compositions and reaction conditions, reducing experimental workload and time-to-discovery.^364,365 In addition, ML models can predict toxicity, environmental impact, and sustainability metrics (e.g., atom economy, E-factor, life cycle assessment), supporting green chemistry initiatives and regulatory compliance.³⁶⁵ However, the effectiveness of AI/ML models depends heavily on the quality and quantity of available data. Incomplete or inconsistent datasets can limit model accuracy and generalizability, especially for novel feedstocks or underexplored reaction pathways.^360,364,365 In conclusion, integrating artificial intelligence and machine learning into renewable aromatic production represents a transformative step toward process optimization, catalyst discovery, and sustainability assessment. While challenges remain in data quality, model interpretability, and experimental integration, ongoing advancements are rapidly expanding the practical applications of AI/ML in this field. Harnessing these technologies will be crucial for realizing the full potential of waste-to-aromatics processes and achieving a circular, sustainable chemical industry.

9. Conclusion and future outlook

Petrochemical chemicals find their way into a wide range of everyday products, including digital devices, tires, plastics, fertilizers, packaging, and medical equipment. It is the primary driver of the world's industrial oil demand, contributing over one-third of the growth in global oil demand by 2030, and is expected to rise by nearly half by the year 2050. Olefins and aromatics also referred to as basic chemicals, are the principal products of the fossil carbon feedstock utilized in industry. These basic compounds are mostly utilized in the synthesis of polymeric hydrocarbons, such as polystyrene and polyolefins. A lesser percentage is transformed into intermediates in the polymer industries with the addition of oxygen, nitrogen, and chlorine to produce engineering polymers like nylon, polyurethane, polycarbonates, etc. It can be concluded that polymers are used in most of the petrochemical industry's products.

As a result of the overuse of fossil fuels, which is causing a sharp rise in carbon emissions, industrial processes are shifting more and more toward using renewable resources as a substitute, including geothermal, wind, hydropower, solar, and biomass. Because biomass can produce a wide range of useful chemicals and fuels, it has received special attention as a renewable carbon source. After natural gas, coal, and crude oil, biomass has emerged as the fourth-most significant energy source for electricity and heating in recent decades. Because biomass is produced through natural photosynthetic processes, which require the consumption of atmospheric CO₂, it has many advantages when used. Thus, through the interaction of photosynthesis and biorefinery processes, biomass conversion not only offers sustainable routes for the synthesis of chemicals and fuels but also plays a role in the ongoing cycle of CO₂ mitigation. As a result, it is anticipated that by 2030, the percentage of biofuels and biopolymers will surpass 10%. In the framework of sustainability, using biomass as a raw source to make significant industrial chemicals can also reinforce the ideas of green chemistry. Lipid, starch, and lignocellulose make up the majority of the biomass.

For the conversion of biomass, a number of chemocatalytic, biological, and thermochemical techniques have been discovered. For example, the non-edible portion of biomass, known as lignocellulosic matter (cellulose, hemicellulose, and lignin), is prioritized for use in the production of new chemicals, while the edible portion, known as starch and lipids, is given less attention to prevent rivalry with food for fuels and chemicals. However, the process of fermenting corn or sugarcane to create bio-ethanol is an established one. On the other hand, a great deal of effort has been put into valorizing plant-based vegetable oils as a raw material for transesterification, which produces biodiesel. However, there aren't many methods for improving inedible vegetable oils. The effective conversion of inedible vegetable oils to biodiesels has been highlighted in some recent work. However, the limited supply of feedstock and high fuel demand make these methods unappealing. For this reason, it is highly appealing to directly convert non-edible vegetable oils into other components like aromatics for the polymer industries. Lately, it has been concluded that direct conversion of inedible oils to aromatics is performed by using heterogeneous catalysts. It has been discovered that a number of metal-based catalysts, including Y zeolite, Zn, Cr, Ga, Co, Mo-ZSM-5, and others, are effective for converting undesired oils into important aromatic compounds for industry. Porosity and active site design of the catalyst are essential for good conversion and excellent aromatic selectivity. Due to unregulated integration between active sites and structurally complex feedstocks, selectivity in guiding a reaction route to preferred aromatics remains a key difficulty in many situations of these oil conversion. Furthermore, lots of byproducts are formed for glycerol conversion. Finding a suitable pathway to increase the selectivity of aromatics will enhance and offer a suitable pathway for biorefinery wastes.

The future of aromatic production from plastic waste and biomass via pyrolysis offers a sustainable solution to reduce waste and produce valuable chemicals. Pyrolysis, a thermal decomposition process, can convert both plastic waste and lignocellulosic biomass (comprising cellulose, hemicellulose, and lignin) into aromatic compounds like BTX. For plastic waste, innovations in catalyst design will improve selectivity toward aromatics, while process intensification methods like microwave-assisted pyrolysis will enhance efficiency. Biomass pyrolysis will focus on maximizing the conversion of lignin (rich in aromatic structures) into phenolic compounds and BTX. Integrating plastic waste and biomass co-pyrolysis can optimize feedstock utilization and reduce environmental impact. Advances in catalytic fast pyrolysis, coupled with recycling technologies, will ensure high yields of aromatics while addressing global waste challenges. The future of aromatic production from CO₂ holds great promise for sustainable chemical manufacturing. Advances in catalytic technologies such as electrocatalysis and heterogeneous catalysis will enable more efficient and selective conversion of CO₂ into valuable aromatics like benzene, toluene, and xylene. Innovations in methanol-to-aromatics processes and renewable energy-powered systems (like photocatalysis) will further enhance the viability of CO₂-based aromatic production.

However, to fully assess the sustainability of these processes, it is crucial to incorporate green chemistry metrics. Atom economy and reaction mass efficiency should be calculated for each conversion process. E-factor and mass intensity metrics will help quantify waste generation and resource utilization. Life cycle assessments should be conducted to evaluate overall environmental impacts, including global warming potential, eutrophication, and ozone depletion. Government policies, carbon credits, assessing the economic viability of scaled-up renewable aromatic production, and life cycle assessments (LCA) will drive commercialization, making these processes both environmentally and economically viable. Ongoing research should focus on optimizing reaction conditions to maximize yield and selectivity while minimizing energy input, improving solvent recycling and utilization of all biomass fractions to minimize waste and integrating renewable energy sources into production processes to reduce overall carbon footprint. Also, the one can focus on the developing novel catalysts with improved selectivity and stability for waste-to-aromatic conversion.

In conclusion, our study uniquely synthesizes the recent advancements in the production of renewable aromatics from waste through processes like pyrolysis and chemocatalytic methods presents a sustainable and viable alternative to fossil-based chemicals. By utilizing plastic waste, biomass, and CO₂, coupled with advancements in catalytic technologies and process intensification, this approach contributes to waste reduction, carbon management, and the development of a circular economy. Renewable aromatic production holds great potential for addressing global environmental challenges while supplying valuable chemicals for industrial use.

Abbreviations

BTX	Benzene, toluene, and xylene
LCB	Lignocellulosic biomass
GHSV	Gas hourly space velocity
PE	Polyethylene
PP	Polypropylene
PS	Polystyrene
PET	Polyethylene terephthalate
LDPE	Low-density polyethylene
GC/MS	Gas chromatography-mass spectrometry
HDO	Hydrodeoxygenation
H/Ceff	Hydrogen-to-carbon effective ratio
HMF	5-Hydroxymethylfurfural
LA	Levulinic acid
FA	Formic acid
SSA	Specific surface area
SEM	Scanning electron microscope
SGO	Sulfonated graphene oxide
GVL	γ-Valerolactone
TPA	Tungstophosphoric acid
CTA	Carbon into aromatics
FTS	Fischer-Tropsch synthesis
MTH	Methanol to hydrocarbons
LCA	Life cycle assessment
EF	Environmental factor
AE	Atom economy
EQ	Environmental quotient
CE	Carbon efficiency
GAL	Green aspiration level
PMI	Process mass intensity
AI	Artificial intelligence
ML	Machine learning
ANN	Artificial neural networks

Author contributions

Ripsa Rani Nayak: data curation, methodology, investigation, software, visualization, conceptualization, writing – original draft. Navneet Kumar Gupta: conceptualization, supervision, validation, resources, project administration, funding acquisition, writing – review and editing.

Data availability

Data sharing not applicable – no new data generated.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

RRN is thankful to the Ministry of Education for the Ph.D. scholarship. NKG expresses gratitude for the startup grant from the Indian Institute of Science (10183) and the financial support provided by the Saroj Poddar Trust through the Saroj Poddar Young Investigator Award.

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