Daria
Lebedeva
a,
Lars William
Schick
a,
Daniel
Cracco
a,
Withsakorn
Sangsuwan
a,
Gonzalo
Castiella-Ona
a,
Dagoberto O.
Silva
c,
Alessandro
Marson
d,
Erik
Svensson Grape
b,
A. Ken
Inge
*b,
Liane M.
Rossi
*c,
Elena
Subbotina
*a,
Alessandro
Manzardo
*d and
Joseph S. M.
Samec
*a
aStockholm University, Department of Organic Chemistry, Svante Arrhenius väg 16C, SE 106 91 Stockholm, Sweden. E-mail: joseph.samec@su.se
bDepartment of Materials and Environmental Chemistry, Stockholm University, Stockholm SE 10691, Sweden
cDepartamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil
dCESQA (Quality and Environmental Research Center), University of Padova, Department of Civil, Environmental and Architectural Engineering, Via Marzolo 9, 35131 Padova, Italy
First published on 22nd May 2024
Maximizing products of high value and minimizing incineration of side-streams is key to realize future biorefineries. In current textile production from forestry, hemicellulose is removed by prehydrolysis before delignification. The resulting prehydrolysis liquor is incinerated in the recovery boiler at low efficiency. This additional burden on the limiting recovery boiler reduces the pulp production. In this study, we demonstrate that prehydrolysis liquor can be upgraded, in 5 steps, to yield aviation fuels. Prehydrolysis liquors were dehydrated to furfural by zeolite catalysis. Furfural was selectively reduced to furfuryl alcohol by Au@NC. Rhenium-catalysed Achmatowicz rearrangement gave a C5 intermediate susceptible to self [2 + 2] cycloaddition to give the C10 oxygenated precursor. By using a combination of Ru/C and zeolites, full hydrodeoxygenation was achieved. The overall transformation from furfural to hydrocarbons resulted in a 48% carbon yield. The resulting hydrocarbons, containing an anticipated strained four-membered ring, are preferred aviation fuel components. This is an important step to show that aviation fuels can be produced sustainably from existing industrial side-streams. A comparative life cycle assessment was applied to evaluate the environmental impact of the proposed valorization approach, demonstrating benefits in the climate change impact category when implementing this technology in a pulp mill compared to the incineration of pre-hydrolysis liquor scenario.
The generation of prehydrolysis liquor, from hemicelluloses, is a crucial wood pre-treatment step in the dissolving grade pulp production of the kraft process. Dissolving grade pulp is used for generation of textile fibers to substitute cotton production, which has lately been questioned for environmental concerns. The prehydrolysis liquor produced in this process, together with lignin, is currently incinerated to generate heat and power, however at low efficiencies. This brings an additional burden on the rate limiting recovery boiler, reducing the overall production capacity of pulp.4 Thus, there is a potential to simultaneously increase the value of prehydrolysis liquor beyond its heating value, and debottleneck pulp production (Fig. 1).
The state-of-the-art synthetic pathways for SAF:s includes pioneering research by Dumesic and co-workers, demonstrating the approach for utilizing sugars, such as glucose, to synthesise linear hydrocarbons within aviation boiling point range.5 Another approach was described by Corma and co-workers, who converted sugar-derived furanics into branched alkanes (Scheme 1, entries 1 and 2).6 Recent advancements have underscored the advantages of cyclic hydrocarbons in fuel applications, owing to their increased energy density and higher heat of combustion. The work by Li's research group demonstrated the synthesis of multicyclic hydrocarbons containing 5-membered rings from cellulose (Scheme 1, entry 3).7 Nevertheless, it's worth mentioning that cyclobutane's relative strain energy of 26.5 kcal mol−1 is notable when compared to cyclopentane (6.2 kcal mol−1) and cyclohexane (0 kcal mol−1).8 Thus, cyclobutane derivatives could be desired candidates for aviation fuel, as they afford a higher heat of combustion. Several research groups proposed synthetic pathways for preparing cyclobutane-based hydrocarbons from sources that are not as available as the sugars, vide supra. These approaches include a [2 + 2] cycloaddition to yield cyclobutane-derivatives from acetone derived isopherone followed by hydrodeoxygenation (HDO), when applicable (Scheme 1, entry 4).9–12 However, no reports disclose high energy 4-membered SAF:s produced from available side-streams of low value.
In this work, we report a novel value chain from industrially relevant prehydrolysis liquor to produce cyclobutane-containing hydrocarbons suitable for aviation fuel via furfural (Scheme 1, entry 5). Our approach involves the multi-step transformation of furfural into C5 precursors 3 and 4, susceptible to [2 + 2] cycloaddition, resulting in the formation of cyclobutane 5, that was successfully hydrotreated to produce high energy C10 SAF:s. To ascertain the sustainability of the novel route, a comparative LCA has been performed showing significant reduction in climate change, compared to conventional aviation fuels.
A consequential LCA approach is adopted as it more effectively captures the environmental implications of decision-making in prehydrolysis liquor management.15
A functional unit of 1 kg of unbleached kraft pulp was chosen to focus on the determining product of the system,16,17 treating all the co-products by substitution. A cradle to gate approach was chosen, thereby excluding the bleaching process, usage phase, and end-of-life of dissolving grade pulp from our analysis. Since these life cycle stages are identical for both scenarios being compared, our chosen approach does not compromise the comparative validity. This assessment is contextualized within Northern Europe, with a subsequent extension to a broader European context examined during the sensitivity analysis. Detailed information on the choices made for scope definition and a comprehensive review of both primary and secondary data sources (Table S8) can be found in ESI.†
The impact assessment results are displayed using 16 midpoint impact categories from the Environmental Footprint (EF) method 3.0.18 In the main text, only five categories are presented and discussed: climate change (GWP-total), land use (SQP), water use (WDP), resource use of fossils (ADP-fossil), and mineral and metal resources (ADP-min&met). The complete set of indicators and results is available in ESI.†
Previously we reported a successful transformation of prehydrolysis liquors to furfural using beta zeolites in a dioxane–water mixture as solvent.19,23 In this work, we explored EtOAc, a more sustainable solvent for the transformation of xylose into furfural.24
Dehydration to furfural 1 was performed through catalysis by beta zeolites at 180 °C in EtOAc giving 74% yield under optimized conditions (ESI, Table S3†), slightly lower than using dioxane (83%). It was found that addition of small amounts of water was important for obtaining high yields. Stopping the reaction after 1 h was also crucial to avoid humin formation.
Hydrogenation catalysed by Ru/C and Pd/C were tested using 2-propanol as the hydrogen donor (Table 1, entries 1 and 2). Reactions catalysed by Ru/C showed higher conversion of furfural and higher selectivity towards furfuryl alcohol, however, the maximum yield observed was only 53% (Table 1, entry 1).27 Among the hydrogen donors tested (2-propanol, EtOH, MeOH, HCOOH), 2-propanol showed the best performance. When molecular hydrogen was used as a hydrogen source, full conversion was achieved with Ru/C, although selectivity dropped significantly (Table 1, entry 3). Thus, seemingly straight forward reduction proved challenging. Selectivity issues are known in most traditional heterogeneous hydrogenation catalysts, e.g., ruthenium, palladium, and nickel, and we decided to explore a less active metal, such as gold. Gold has been considered catalytically inactive towards H2, but it can develop into a very selective catalyst once H2 heterolytic activation occurs across a metal–ligand interface.28,29 Base ligands, such as ammonia and pyridine, were suggested to be used as additives to improve hydrogenation of aldehydes over gold-based catalysts.30 DFT calculations suggested that the base allows the chemical adsorption of H2 and its cleavage via an heterolytic dissociation mechanism, forming NH4+ and H− that will be adsorbed on the active metal site. Finally, H+ (of NH4+) and H− will be transferred to the aldehyde and produce the corresponding alcohol. Later, Rossi and co-workers developed a fully heterogenous gold embedded on N-doped carbon (Au@NC/TiO2) catalyst, which was active and selective for alkynes semihydrogenation.13 It was suggested that the heterolytic cleavage of molecular H2 occur at the basic nitrogen species on the N-doped carbon material in close vicinity of Au nanoparticles. Later, this catalyst was tested for the hydrogenation of various aldehydes to alcohols, showing high selectivity.14 A similar mechanism was also proposed by Nagpure et al.,31 for hydrogenation of cinnamaldehyde, based on the heterolytic dissociation of H2, followed by the transfer of nucleophilic H− ion to electrophilic carbonyl carbon of the aldehyde, whereas the electrophilic H+ ion transfers to nucleophilic carbonyl oxygen, leading to the desired alcohol.
Gratifyingly, the Au@NC/TiO2 catalyst showed very high selectivity in the conversion of furfural to furfuryl alcohol, reaching full conversion and no other side-products (Table 1, entry 4). It is worth to mention that the heterogenous catalyst can be recycled and reused, as demonstrated in the hydrogenation of benzaldehyde.14
Heterogeneous titanium silicalite (TS-1) has been reported in this transformation.40 Attempts to use TS-1 catalyst, as previously reported, were unsuccessful. Only 48% yield could be observed, compared to the reported 94%.40,41 A possible explanation could be the difference in the crystal structure of TS-1 that can significantly influence the activity of the catalyst.42,43 Even though several different procedures were used to prepare TS-1, higher yields were not achieved, showing challenges with this approach.
We were able to successfully transform unsubstituted furfuryl alcohol into compound 3via epoxidation by hydrogen peroxide using MTO catalyst. MTO together with hydrogen peroxide forms active peroxo-metal species able to catalyse double bond epoxidation.44 The reaction gave full conversion of the starting material within 5 h (ESI, Table S4†). After 6 h, selectivity towards the product 3 dropped. If the reaction was left for 2 days, the yield of product 3 decreased to 48% (ESI, Table S4†). This is not surprising as product 3 also contain a double bond that can undergo further reactions.
To confirm product degradation, a stability test was performed. After 3 h using standard reaction conditions, half of the product 3 was degraded, and after 12 h, only 28% of the product remained. Therefore, quenching the reaction after 5 h was essential to prevent side-reactions. Attempts to stop the reaction by simple work-up after 5 h were unsuccessful. Perhaps product degradation in the presence of oxidative agents occurred. We proposed to quench the reaction using molecular sieves once the reaction was completed. The reaction and quenching procedure were optimized, and it was shown that 1 h treatment with molecular sieves after 5 h reaction gave 98% isolated yield of compound 3.
To prevent waste generation at each stage of our transformations, we hypothesized the possibility of recycling the molecular sieves used. When the optimized reaction was completed and quenched, molecular sieves (4 Å) were recovered and dried at 70 °C under reduced pressure for 30 h to release solvent and moisture. Dried molecular sieves were re-used twice, without significant loss in the yield of product 3 (Fig. 2).
In this work, we report, for the first time, the cycloaddition of enone-containing substrate 4 for creating 4-membered adducts (Scheme 2). The resulting adducts are desired precursors for C10 cyclobutane-containing aviation fuels.
The substrate 4 was selected for [2 + 2] cycloaddition due to its more convenient handling and the ease of isolating of the isomeric cycloadducts compared to substrate 3. Compound 4 was synthesised through the methylation of substrate 3 using the procedure described in the Experimental section.
The light absorption spectrum of compound 4 was analysed. In accordance with the anticipated behaviour of the conjugated π-system, the compound showed π–π* absorption band in the range of 300–400 nm, which allows dimerization to occur (ESI, Fig. S1†).
The substrate 4 was irradiated at 365 nm with eight high-pressure mercury lamps (4 W each), resulting in the formation of cycloadducts. The photoreactions were conducted for a total of 48 h, although shorter reaction times can be achieved by employing more powerful lamps. The photoreactions were performed in solution and as neat reactions. The neat reaction gave lower conversion of starting material 4 than the reaction performed in acetonitrile where 79% and 96% conversion, respectively, were observed.
In the concerted light-driven [2 + 2] cycloaddition, the substrate 4 can form two regioisomers: head-to-head (HH) and head-to-tail (HT). Further complexity arises from the possibility of syn- or anti-addition, leading to syn-HH, anti-HH, syn-HT, and anti-HT isomers.46 In this study, we successfully separated and isolated two anti-HH diastereomers (Scheme 3, cycloadducts 1 and 2, both were racemic mixtures) and one anti-HT isomer (Scheme 3, cycloadduct 3). The structures as well as relative stereoisomers of isolated cycloadduct were confirmed by single-crystal X-ray analysis (ESI, Fig. S2–S4†).
![]() | ||
Scheme 3 Isolated cycloadducts. The reaction was performed using the following conditions: In a quartz tube, 6-methoxy-2H-pyran-3(6H)-one (154 mg, 1.2 mmol) was dissolved in MeCN (5 mL, 0.24 M) and irradiated using UV lamps (365 nm) for 48 h. The cycloadducts were isolated by column chromatography (40% EtOAc![]() ![]() |
Before optimizing the HDO conditions for cycloadduct transformation, catalyst screening was performed using substrate 3. In our initial experiments, we employed Pd/C while running the reaction at 220 °C at 20 bar of hydrogen pressure using cyclohexane as carrier liquid. After 16 h treatment, we observed full conversion of the starting material and the formation of hydroxy-substituted tetrahydropyran (THP-OH) as a major product (Fig. 3). This confirms that Pd/C facilitates hydrogenation, but not full deoxygenation.
![]() | ||
Fig. 3 The distribution of products after hydrotreating the compound 3. Reaction conditions: compound 3 (0.88 mmol), cyclohexane (3 mL), 220 °C, 20 bar of hydrogen pressure, 16 h. |
When zeolites were introduced together with Pd/C, a shift in the product selectivity towards hydrocarbon formation was observed (Fig. 3). Zeolite screening revealed that the addition of HZSM-5 zeolites (Si/Al ratio of 23) still yielded THP-OH as a major product, while HY zeolites (Si/Al ratio of 30) shifted selectivity towards THP and C5 hydrocarbons. It is likely that larger channels in HY type of zeolites facilitate better diffusion of the substrates through zeolites, making active sites more accessible. Changing the Si/Al ratio of zeolites from 30 to 80 shifted the selectivity back to THP-OH. This shift is connected to the decreased Brønsted acidity in HY with a Si/Al ratio of 80, compared to a Si/Al of 30.
We then tested other noble metals in tandem with HY zeolites (Si/Al of 30). The performance of Pt/C was similar to Pd/C. The more oxophilic Ru/C demonstrated the capacity to hydrodeoxygenate the THP ring, resulting in selective transformation to hydrocarbons (Fig. 3).
With an optimized catalytic system in hand, we performed HDO on the dimeric cycloadducts. The reaction showed full conversion of the starting material and high selectivity towards fully deoxygenated molecules, resulting in a 51% yield of detected hydrocarbons, which included 48% of C7–C10 compounds. Notably, C9 hydrocarbons were the major product, followed by C8, suggesting that C–C bond cleavage took place in the form of decarbonylation of reaction intermediates (Table 2, entry 1).1 Further improvements were attempted by varying carrier liquid (Table 2, entry 2), pressure (Table 2, entries 3 and 4), and reaction temperature (Table 2, entries 5 and 6).
However, no improvement in yields of hydrocarbons in aviation fuel range (C8–C10) was observed. Increasing the pressure to 80 bar resulted in a shift in selectivity towards C8 and C7 products, possibly due to cracking reactions. A similar trend was observed when the temperature was increased to 245 °C. In reactions conducted at lower pressure (Table 2, entry 4) and temperature (Table 2, entry 6), complete deoxygenation was not achieved.
The mass balance of the complete value chain was assessed for two distinct scenarios (Fig. 4). In one scenario (S1-Burn), prehydrolysis liquor, generated during a wood pretreatment step, is utilized as a source of energy through combustion in an external co-generation plant. In the second scenario (S2-AF), the prehydrolysis liquor is utilized to produce furfural with residual side-streams contributing to energy co-generation. Subsequently, furfural undergoes a multi-step transformation process to yield hydrocarbons suitable for aviation fuel applications (S2-AF). Quantitative values, expressed in kilograms, were calculated relative to the functional unit, defined as the kilogram of unbleached pulp.
Detailed representations of the process model with the system boundaries are available in ESI (Fig. S6–S9†).
To determine the carbon content [kgC kg−1] in cellulose, hemicellulose and lignin and their Low Heating Value [MJ kg−1] to be used in the model secondary data from Ecoinvent 3.9.1 consequential were exploited. From the values retrieved the theoretical energy that can be produced from incineration phases was calculated and the efficiency of the recovery boiler was calculated to be 13% and used in the modelling of the scenarios. From the same source, secondary data for the values of efficiencies to heat and electricity from a co-generation plant were retrieved and are 45% and 15% respectively.
Fig. 5 graphically illustrates the Life Cycle Impact Assessment outcomes for the primary five categories, with comprehensive data detailed in ESI (Tables S10–S13†). S2-AF emerged as the superior performer in the GWP-total (−5.63 × 10−2 kg CO2eq kgUSP−1) and ADP-min&met categories, while S1-Burn was the most performing in SQP, WDP and ADP-fossil categories.
In GWP-total for S2-AF, the principal factor contributing to reduced impacts was the avoidance of fossil CO2 emissions relative to conventional aviation fuel, decreasing emissions by 0.57 kg CO2eq kgUSP−1 (USP stands for unbleached sulfate pulp). The efficacy of S1-Burn in the GWP-total, which utilizes prehydrolysis liquor as an energy source in a cogeneration plant, is highly contingent on the replaced energy mix. It should be noted that prehydrolysis liquors are currently burnt in the recovery boiler. When this scenario was calculated, the environmental sustainability was worse in all impact categories; mainly, because the recovery boiler has a lower efficiency. Thus, to study the effect of valorization, incineration in an external co-generation plant was modelled.
Sensitivity analysis reveals that substituting this with a European energy mix enhances environmental benefits, outstripping S2-AF with a GWP-total of −0.634 kg CO2eq.
Conversely, in the SQP category, S1-Burn outperforms S2-AF, attributable to the increased combustion of hemicellulose for heat production, thereby supplanting biomass-derived heat prevalent in Northern Europe's energy mix. This substitution mitigates wood chip consumption and reduces land use impacts. S2-AF's intensified use of solvents and catalysts leads to greater WDP impacts than S1-Burn.
In terms of ADP-fossil, while biobased aviation fuel production substantially reduces impacts, it does not suffice for an overall benefit in this category due to solvent usage in the value chain upgrade. S1-Burn prevails due to enhanced energy yield from hemicellulose. For ADP-min&met, S2-AF outstrips S1-Burn since metal extraction for hydrogenation catalyst concurrently yield other metals concurrently yield other metals, diminishing the need for their primary production, as per the Ecoinvent marginal background data. A full discussion of the results in the other impact categories is available in ESI.†
Four sensitivity analyses assessed the effects of various modeling and operational parameters on the outcomes. The most significant involved an average European energy mix scenario for the integrated pulp mill, resulting in notable shifts for both S1-Burn and S2-AF, particularly in GWP-total, WDP, and ADP-fossil. The impact on GWP-total is attributed to the higher emission factor of the European marginal energy mix (0.218 kgCO2eq kW−1 h−1) compared to the Northern mix (0.07 kgCO2eq kW−1 h−1), suggesting a net reduction in greenhouse gas emissions when replacing an energy mix dominated by fossil fuels. A separate sensitivity analysis evaluated the impact of a 10% lower yield to aviation fuels in the hydrodeoxygenation (HDO) section on S2-AF. Although the GWP-total and ADP-fossil indicators deteriorated, as indicated in Table S15,† S2-AF still maintains a climate change advantage over S1-Burn, albeit with increased impacts on fossil resource use.
Another analysis for S2-AF assessed the effects of producing aviation fuels in an isolated facility, predicting increased GWP-total and ADP-fossil due to higher energy demands. However, the results showed a decrease, contrary to expectations, due to varying efficiencies between recovery boilers and co-generation plants in hemicellulose combustion.
Lastly, altering the assessment to the ReCiPe 2016 (midpoint), H method in the final sensitivity analysis did not yield significant differences in the relevant impact categories. ReCiPe 2016 (midpoint) is a Life-Cycle Impact Assessment method which name stands for the initials of the institutes that were the main contributors to the design and development of the method: RIVM and Radboud University, CML, and PRé Consultants. This method calculates 18 midpoint indicators which focus on single environmental issues and express their relative severity.
Herein, a full value chain from an industrial side-stream to high energy content aviation fuel is demonstrated. The prehydrolysis liquors were transformed to furfural using zeolites. Furfural was reduced using homogeneous catalysis, selectively yielding furfuryl alcohol by transfer hydrogenation. Furfuryl alcohol was then rearranged to the Achmatowicz product using MTO and hydrogen peroxide. The resulting adduct was dimerized using a photo [2 + 2] cycloaddition. HDO of the cycloadducts gave desired hydrocarbons in aviation fuel range. Catalyst screening revealed that Ru/C and zeolites were required to cleave all C–O bonds, including the challenging tetrahydropyran bonds. LCA showed benefits in climate change when implementing this technology in a pulp mill instead of incineration.
This study makes progress in mild and selective HDO of challenging C–O bonds. We hope that this inspires researchers to further development of value chains from available side streams.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2307840, 2307841 and 2308513. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4gc01257g |
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