Efficacy of mechanochemically prepared ceria–zirconia catalysts in ketonisation of acetic acid

Abstract

This work presents a comprehensive study on the catalytic and kinetic aspects of the ketonisation of acetic acid, a model volatile fatty acid, using Ce_1−xZr_xO₂ as catalysts. Volatile fatty acids are promising biomass derived feedstock for production of drop-in sustainable aviation fuels through a series of cascade reactions, with ketonisation as the first step followed by aldol condensation and subsequent hydrogenation. A series of Ce_1−xZr_xO₂ catalysts for ketonisation were prepared using a mechanochemical technique of ball milling, and their performance was evaluated for varying Ce/Zr mole ratios. Among the catalysts tested, Ce_0.75Zr_0.25O₂ exhibited the highest conversion and selectivity towards the desired product, acetone. The catalyst characterisation showed the formation of nano-aggregates with an average particle size of 340.8 nm and a specific surface area of 66.2 m² g⁻¹. The kinetics of the reaction indicated a second-order dependence on acetic acid, while the products (acetone, water, and CO₂) exhibited negative orders, suggesting competitive adsorption on the active sites of the catalyst. The activation energy for the reaction was determined to be 103.4 kJ mol⁻¹ suggesting the surface reaction as the rate controlling step. These findings provide valuable insights into the catalytic behaviour and kinetics of the ketonisation reaction.

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1. Introduction

The globally increasing demand for energy and dwindling crude oil reserves combined with the climate emergency have sparked rapid development of low carbon technologies to secure clean energy from renewable sources. Biomass has become a key focus as renewable feedstock for producing hydrogen, liquid transportation fuels, and platform chemicals.^1,2 Annually, the increase in greenhouse gas emissions continues to rise, causing difficulties in climate mitigation. A joint effort from all sectors is imperative to prevent dangerous climate crisis, and in this regard, the aviation industry holds a major contribution. The global consumption of aviation fuels, currently at 360 billion litres per year, is expected to double by 2050, posing to be a major challenge to the ambitious 2050 net zero target due to increasing greenhouse gas emissions. With the aviation industry committing to reduce its carbon footprint by 50% before 2050, increasing the production of sustainable aviation fuel (SAF) has taken centre stage. However, current SAF production is at less than 1% of global jet fuel demand. Hence, new chemical technologies using waste and renewable feedstocks are urgently needed to increase SAF production and meet the 2050 net zero target.^1,2

Volatile fatty acids (VFAs), which are carboxylic acids ranging from C₂ to C₈, can be generated from wet waste sources such as food waste, animal manure, and wastewater sludge, making them potential feedstock for SAF.^3,4 Typically, anaerobic digestion of wet waste by microbial consortia produces CO₂ and CH₄, but this process can be modified to produce VFAs instead by arresting methanogenesis.^5–7 Biomass conversion pathways for aviation fuel, collectively referred to as “sustainable”, require quantification of carbon intensity to ensure significant reductions compared to fossil fuel usage and to evaluate the benefits of each pathway comprehensively.⁷

In a recent life cycle analysis (LCA) of the investigated pathway, which involves VFA upgrading to SAF through sequential ketonisation and hydro-treating, researchers found that the process exhibits a carbon emission intensity below zero (−55 g CO₂ eq. MJ⁻¹). This is primarily due to the avoidance of methane emissions that would otherwise occur if wet waste was landfilled.⁸ Similar LCA studies on the upgrading of VFAs to diesel blendstocks and fatty acids to waxes and surfactants, have also demonstrated substantial reductions in carbon intensity when waste acids are valorised for fuel and chemical production.^9–11 Therefore, the upgrading of VFAs derived from wet waste for use as drop in blendstocks in SAF presents a promising pathway for decarbonisation.

The ketonisation reaction has gained recent attention due to its capability to upgrade bio-oil and its aqueous phase.^12–21 Ketonisation proves to be an effective technique for the upgrading of organic acids by converting them into ketones while simultaneously removing oxygen in the form of water and CO₂. This process results in an increase in the heating values of the bio-oil and a reduction in acidity. The produced ketones are valuable as they can serve as building blocks for further reactions such as C–C coupling and hydro-deoxygenation, enabling the production of hydrocarbons suitable for transportation fuels. By utilizing the ketonisation process, organic acids can be efficiently transformed into ketones, providing a promising pathway for the upgrading of bio-oil and the generation of high-quality hydrocarbon products for sustainable energy applications.^8,21–24 The reaction scheme for ketonisation of acetic acid is shown in Fig. 1.

Fig. 1 Reaction scheme for ketonisation of acetic acid.

Building on our ongoing research in environmental catalysis and low carbon clean energy technologies,^25–34 we have investigated the ketonisation of acetic acid using mechanochemically synthesised ceria–zirconia (Ce_1−xZr_xO₂) mixed metal oxide catalysts as a model reaction. This transformation is highly relevant for upgrading VFAs into sustainable aviation fuels through a sequential process involving ketonisation, aldol condensation, and subsequent hydrogenation to produce C₈–C₁₄ hydrocarbons. Mechanochemical synthesis offers distinct advantages over conventional hydrothermal methods for preparing ceria–zirconia mixed metal oxide catalysts. It reduces the number of synthesis steps and eliminates the need for solvents, making the process more sustainable and economically attractive, particularly for scale-up.^35,36 The solvent-free nature also aligns with green chemistry principles by minimising waste. Furthermore, the intense mechanical grinding promotes defect formation and reduces particle size, leading to higher surface area and enhanced lattice oxygen vacancies, both of which are critical for achieving high activity and selectivity in ketonisation reactions. Initially, efficacy of various compositions of Ce_1−xZr_xO₂ catalysts is explored for establishing the optimal catalyst for ketonisation. Different catalyst compositions are prepared using the ball milling technique, followed by experimental investigations. The subsequent phase focuses on studying the kinetics of the reaction using the selected catalyst to determine the kinetic parameters. Both differential and integral reactor studies are conducted to gather the necessary kinetic data for fitting into a kinetic model. The kinetic modelling is performed using the Langmuir–Hinshelwood–Hougen–Watson (LHHW) mechanism, and a non-linear regression analysis is applied to determine the parameters based on the obtained experimental data. Through this approach, valuable insights into the catalytic ketonisation of acetic acid and the associated kinetic behaviour are obtained.

2. Experimental section

2.1 Materials

Acetic acid (>99%), acetone (99%), cerium oxide (CeO₂), and zirconium oxide (ZrO₂) were purchased from Molychem Chemicals, Mumbai, India. CO₂ and deionised water were also co-fed into the reactor. Nitrogen was used as a sweep gas. All other reagents were purchased from commercial sources and used as received, unless otherwise stated.

2.2 Experimental setup

The experimental setup comprises a phase change reactor designed to convert acetic acid from a liquid phase to a gas phase, facilitated by co-feeding of sweep gas nitrogen. Subsequently, the gaseous mixture is directed towards the catalyst bed within the main reactor for the conversion of acetic acid. After the conversion process, the gases undergo condensation in a condenser unit, where samples are extracted for analysis. The scheme for the experimental setup is shown in Fig. 2. The experimental setup used in this study involved the fixed-bed reactor configuration. Inside the main reactor, the catalyst is placed between quartz wool plugs, as shown in Fig. 3.

Fig. 2 Scheme for the experimental setup.

Fig. 3 Catalyst arrangement inside the reactor.

The flow rate of acetic acid was regulated using a peristaltic pump, with the system initially set at a stable flow rate for 60 minutes before taking subsequent samples. Furthermore, the gases CO₂ and N₂ were introduced into the system by co-feeding them through the gas cylinders via rotameters. The reactor temperature was controlled using a PID temperature controller. To collect the samples for analysis, the feed exiting the reactor was condensed using cold water from a condenser.

2.3 Analysis of products

Analysis of acetic acid and acetone was carried out using a ThermoFisher UltiMate™ 3000 HPLC system equipped with a refractive index detector (ERC RefractoMax 520). The column selected for this analysis is the Hypersil GOLD™ C18 Selectivity HPLC Column (250 × 4.6 mm maximum pressure 400 bar and maximum temperature 60 °C) packed with spherical, fully porous, and ultrapure silica. The mobile phase consists of 0.005 M H₂SO₄, and a flow rate of 0.06 ml min⁻¹ with a column temperature of 30 °C is maintained. The mobile phase and temperature were selected based on previous work by Kumar et al. for the analysis of acetic acid and acetone.³⁷ The calibration plots of acetic acid and acetone are shown in Fig. S1 of the ESI.†

The acetic acid conversion and selectivity towards acetone are estimated based on the concentrations obtained by HPLC. Equations are formulated by combining material balance and chemical reaction engineering principles, considering the reaction stoichiometry. Here, it is assumed that the products are formed in stoichiometric amount and the concentration of water and CO₂ in the system are calculated by material balance.

C_In = inlet concentration of acetic acid (g mL⁻¹)

C_Out = outlet concentration of acetic acid (g mL⁻¹)

2.4 Catalyst preparation

The catalysts were prepared using commercially purchased cerium oxide (CeO₂) and zirconium oxide (ZrO₂) powders. A ball-milling method suggested by Puértolas et al. was used to prepare catalysts with varying Ce/Zr mole ratios at 150 rpm for 60 minutes.³⁸ The different catalysts prepared are listed below in Table 1.

Table 1 List of prepared catalysts

Ce/Zr (mole ratio)	Formula	Code
0 : 100	ZrO2	100Zr
10 : 90	Ce0.1Zr0.9O2	10CeZr
25 : 75	Ce0.25Zr0.75O2	25CeZr
50 : 50	Ce0.5Zr0.5O2	50CeZr
75 : 25	Ce0.75Zr0.25O2	75CeZr
90 : 10	Ce0.9Zr0.1O2	90CeZr
100 : 0	CeO2	100Ce

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3. Results and discussion

3.1 Effect of catalyst composition

This study evaluated the prepared catalysts with varying compositions for their performance in a fixed-bed reactor under similar reaction conditions. The reaction was carried out at a temperature of 350 °C with a catalyst loading of 2 g and a flow rate of 0.1 ml min⁻¹. Before initiating the reaction, the reactor was brought to a steady-state in terms of temperature and flow rate, which required around one hour. In this section, a solution of acetic acid with a weight-to-weight ratio of 50% was employed to investigate the effects of the aqueous stream on both the catalyst's stability and its subsequent yield. The results of this study are shown below in Fig. 4.

Fig. 4 Effects of catalyst composition. Reaction conditions: catalyst loading 2 g, reaction temperature 350 °C, and feed flow rate 0.1 ml min⁻¹.

The results show the conversion and selectivity of acetic acid towards acetone for each of the catalysts tested. The conversion of the acetic acid varies in the order 75CeZr > 100Ce > 25CeZr > 100Zr > 50CeZr > 90CeZr > 10CeZr while the selectivity to acetone varies in the order 75CeZr > 100Ce > 50CeZr > 10CeZr > 90CeZr > 25CeZr > 100Zr.

We believe it can be attributed to the combination of the structural characteristics of the Ce_1−xZr_xO₂ mixed oxide catalysts, which includes a fine balance between the surface acid–base sites, available surface area and dispersion of ZrO₂ nanoclusters on the surface of CeO₂. The results obtained from the study indicate that the 75CeZr catalyst exhibits the highest conversion and selectivity towards the target product, thus making it the optimal catalyst for the target reaction. Thus, for studying kinetics, all further experiments were performed using 75CeZr as the catalyst.

3.2 Kinetic study

3.2.1 Differential reactor. The aim of this study was to determine the reaction order with respect to acetic acid and with respect to the products. It should be noted that at lower conversion, acetone, water, and CO₂ were the only products detected. Thus, we can assume that the converted acetic acid exhibited ≈100% selectivity towards the aforementioned products at lower conversion. Hence, the total conversion of acetic acid was maintained below 20% to ensure complete selectivity and prevent any unwanted side reactions. The ESI† shows that the obtained kinetic data were free from mass transfer limitations.

The rate of the reaction was calculated using eqn (3):

where,

r = rate of the reaction (mol min⁻¹ g_cat⁻¹)

X = conversion

F = inlet molar flow rate (mol min⁻¹)

W = catalyst weight (g)

To ascertain the reaction order with respect to acetic acid, the partial pressure of acetic acid was varied by modifying the inlet flow of the carrier gas (nitrogen). The total pressure was kept at a constant 760 mmHg with the total temperature at 300 °C. The acetic acid flow rate was kept at a constant value.

The impact of varying acetic acid partial pressures on the rate of the reaction is presented below in Fig. 5. As the conversion was low, it was assumed that the partial pressure of the products was insignificant compared to the partial pressure of acetic acid. Therefore, it is evident from Fig. 5 that the reaction order with respect to acetic acid is 1.675, which is consistent with previous findings reported in the literature.^20,39–41

Fig. 5 Rate vs. partial pressure of acetic acid.

To further investigate the impact of product concentration on the reaction rate, varying quantities of acetone, water, and CO₂ were injected into the reactor, and the corresponding rates were measured. In the case of acetone and water, the feed proportions were adjusted to create different quantities of the mixture, which were then introduced into the reactor. To vary the CO₂ partial pressure, a calibrated cylinder of CO₂ was connected to the setup and CO₂ was fed into the reactor along with acetic acid.

The results indicate that all the products exerted an inhibitory effect on the reaction rate, with acetone demonstrating the most pronounced inhibition. On the other hand, the effect of water and CO₂ was significantly lower than that of acetone. The reaction orders with respect to acetone, water, and CO₂ were determined to be −0.418, −0.132, and −0.202, respectively. The findings are illustrated below in Fig. 6.

Fig. 6 Rates vs. partial pressures of acetone, water and carbon dioxide.

3.2.2 Integral reactor. In addition to conducting differential reactor studies, an integral reactor study was also conducted to explore the reaction, following the methodology outlined by Pham et al.^39,40 and Gaertner et al.²⁰ The integral reactor study was carried out at four different temperatures: 300 °C, 325 °C, 350 °C and 375 °C. To analyse the variation in partial pressure with changing residence time W/F (h), the residence time of the feed acetic acid was varied by adjusting the volumetric flow rate and calculating the corresponding residence time. The partial pressure was measured based on the conversion of acetic acid to acetone. The residence time has been determined by dividing the catalyst loading in grams (g) by the inlet mass flow rate of acetic acid in grams per hour (g h⁻¹). To verify if the calculated partial pressures were accurate, a material balance was performed based on the conversion of acetic acid.

The results at four different reaction temperatures are shown in Fig. 7. As expected, it is evident that as the residence time increases, the conversion of acetic acid also increases, leading to a decrease in the partial pressure of acetic acid and an increase in the partial pressure of acetone. The dotted lines represent the general trend of the graph.

Fig. 7 Variation in partial pressures of acetone and acetic acid (mmHg) as a function of residence time (W/F) (h) on the 75CeZr catalyst (a: 300 °C, b: 325 °C, c: 350 °C, and d: 375 °C).

3.3 Development of the kinetic model

From the differential reactor data, it can be inferred that the reaction follows a second-order kinetics with respect to acetic acid. Furthermore, the presence of reaction products hinders the reaction, indicating that there is a competition for the active sites on the catalyst. To account for these observations, a set of elementary reactions can be proposed and integrated into the LHHW kinetic model to effectively describe the reaction kinetics. This model incorporates a series of elementary reactions that describe the reaction kinetics, accounting for the competitive adsorption of reactant and product molecules on the catalyst surface. The proposed model is expected to provide valuable insights into the underlying reaction mechanisms and facilitate the optimisation of reaction conditions to improve reaction performance. The series of elementary reactions can be shown below based on the reaction behaviour observed in the differential reactor study and literature.^19,39,40Here, it should be noted that K_acid, K_Ketone, K_CO2, and K_H2O are the adsorption constants while the rate-determining step is eqn (8) where k is the kinetic factor. From the equations mentioned above, eqn (4)–(7) highlight the competitive adsorption of the reactants and products on the surface active sites of the catalysts as the same can be observed in the differential reactor expressions. Eqn (8) depicts the surface reaction considered to be the rate-determining step.

It is important to note that in the LHHW kinetic model for the reaction, the adsorption constants K_acid, K_Ketone, K_CO2, and K_H2O have been incorporated. The kinetic factor k for step 5 has been identified as the rate-determining step. Based on this rate-determining step, the overall rate of the reaction can be expressed using the appropriate mathematical expression.

Rate = k(Θ_CH3COOH)² (9)

In this context, the parameter Θ_CH3COOH represents the fraction of active sites on the catalyst surface that are covered by acetic acid molecules. This value can be calculated using standard equations based on the adsorption equilibrium of the molecules.

Θ_CH3COOH = K_CH3COOHP_CH3COOHΘ_V (10)

Θ_CH3COCH3 = K_CH3COCH3P_CH3COCH3Θ_V (11)

Θ_CO2 = K_CO2P_CO2Θ_V (12)

Θ_H2O = K_H2OP_H2OΘ_V (13)

Θ_CH3COOH + Θ_CH3COCH3 + Θ_CO2 + Θ_H2O + Θ_V = 1 (14)

Here, Θ_V represents the fraction of vacant sites, while Θ_CH3COOH, Θ_CH3COCH3, Θ_CO2, and Θ_H2O denote the fractional surface coverages of the respective species. Substituting these values of Θ_CH3COOH, Θ_CH3COCH3, Θ_CO2, and Θ_H2O into the above expression and factoring out Θ_V as a common term, the rate expression can be derived based on the conventional LHHW mechanism as follows:

3.4 LHHW model fitting

To validate the developed kinetic model, a multi-variable non-linear regression model was developed in MATLAB®. The model was made based on the experimental data that was fed into the software. The input data utilised for the multi-variable non-linear regression model included both differential and integral reactor data obtained at 300 °C, as well as integral reactor data obtained at 325 °C, 350 °C, and 375 °C, respectively. Partial pressures (P_{Acetic Acid}, P_Acetone, P_Water and P_CO2) along with the rate of reaction (r_A) were fed into the software. For this purpose, an lscurve fit (a solver for nonlinear least-squares problems) was utilised to estimate the unknown constants with the constraints of the given upper and lower bounds. The temperature dependence of the adsorption constants was modelled by using an Arrhenius type expression eqn (16) as shown below:Fig. 8 demonstrates that the calculated reaction rates by the model fit well with the actual measured reaction rates. Table 2 presents the results of the fitted data for the adsorption constants, including K_CH3COOH, K_CH3COCH3, K_CO2, and K_H2O, and the rate constant k with their tolerance ranges.

Fig. 8 Parity plots at (a) 300 °C, (b) 325 °C, (c) 350 °C and (d) 375 °C.

Table 2 Values of the rate and adsorption constants

Rate constant (k)	KCH3COOH	KCH3COCH3	KH2O	KCO2	Temp.
mol gcat^−1 h^−1	mmHg^−1				K
0.0636	0.0211 ± 0.079	0.0557 ± 0.02	0.0012 ± 0.01	0.0042 ± 0.001	573
0.1498	0.0193 ± 0.011	0.0342 ± 0.003	0.0062 ± 0.001	0.0042 ± 0.001	598
0.3312	0.0114 ± 0.0009	0.0211 ± 0.081	0.0050 ± 0.001	0.0040 ± 0.002	623
0.8168	0.0178 ± 0.013	0.0132 ± 0.0076	0.0037 ± 0.001	0.0038 ± 0.003	648

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Based on the rate constants obtained from nonlinear regression, we plotted ln k (mol g_cat⁻¹ s⁻¹) versus 1/T (K), as shown in Fig. 9, in accordance with the Arrhenius equation. The apparent activation energy was estimated to be 103.4 kJ mol⁻¹, with an average frequency factor of 4.54 × 10⁴. The value falls within the range reported in previous studies, such as Gaertner et al.,²⁰ who reported an activation energy of 57.7 kJ mol⁻¹ for hexanoic acid ketonisation, and Pham et al.,⁴⁰ who predicted activation energies ranging from 156.4 to 220 kJ mol⁻¹ for the ketonisation of acetic, propanoic and butyric acids. It is important to note that the range of activation energies observed signifies the variability in catalyst properties and their effectiveness in promoting the ketonisation reaction, as well as the variation in the reaction pathways. Also, the higher activation energy values indicate that the reaction is controlled by surface kinetics.

Fig. 9 ln k vs. 1/T.

3.5 Catalyst characterization

The catalyst's true and bulk densities were estimated using Archimedes' principle. The true density, representing the density of individual catalyst particles, was found to be 6500 kg m⁻³, while the bulk density, which accounts for interparticle voids in the packed bed, was measured at 5314 kg m⁻³. Brunauer–Emmett–Teller (BET) analysis using N₂ physisorption, Scanning Electron Microscopy (SEM) and particle size analysis were performed to estimate the necessary properties of the catalyst. The catalyst showed a BET surface area of 66 m² g⁻¹, with an average pore size of 19 nm. Dynamic Light Scattering (DLS) analysis indicated an average particle size of 340.8 nm. The elemental composition of all catalysts prepared was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis using a Perkin-Elmer 4300. Additional catalyst characterisation studies of 75CeZr are provided in the ESI.†

The scanning electron microscope (SEM) images provide valuable insights into the particle morphology of the fresh and spent 75CeZr catalysts (Fig. 10). The images show no noticeable changes, indicating excellent morphological stability. This observation is further supported by the catalyst's stable performance under reaction conditions, as evidenced by the time-on-stream data in Fig. 11.

Fig. 10 SEM images of the (a) fresh catalyst and (b) spent catalyst.

Fig. 11 Time on stream study. Reaction conditions: catalyst loading 2 g, reaction temperature 350 °C, and feed flow rate 0.1 ml min⁻¹.

4. Conclusions and future scope

A comprehensive study of the ketonisation reaction was carried out using Ce_1−xZr_xO₂ mixed metal oxide catalysts prepared via a mechanochemical ball milling approach. Among the catalysts tested, 75CeZr demonstrated the highest conversion and selectivity toward the desired product, acetone. The order of catalyst performance in terms of conversion was: 75CeZr > 100Ce > 25CeZr > 100Zr > 50CeZr > 90CeZr > 10CeZr, while selectivity followed: 75CeZr > 100Ce > 50CeZr > 10CeZr > 90CeZr > 25CeZr > 100Zr. Kinetic analysis indicated that the reaction follows a second-order rate law, with product inhibition observed due to competitive adsorption on active catalyst sites. The apparent activation energy of 103.4 kJ mol⁻¹ further confirms the catalytic efficiency of the mechanochemically prepared materials.

Future studies will focus on evaluating catalyst performance with volatile fatty acids (VFAs) of higher chain lengths and assessing catalyst stability under conditions that closely mimic the complex composition of real VFA feedstocks such as hydrothermal liquefaction or anaerobic digestate aqueous phases. These studies will provide deeper insight into catalyst durability and activity in realistic applications.

Overall, the mechanochemically developed catalysts and the kinetic insights gained underscore the promising potential of this ketonisation process for upgrading waste streams from hydrothermal liquefaction.

Data availability

Data are available within the article and its ESI.†

Author contributions

K. P., G. D. and D. W.: methodology, investigation, and writing – original draft preparation. V. C. and S. K. M.: catalyst characterisation, analysis, and draft preparation. A. B., A. P., and N. A.: methodology and draft revision. H. M.: conceptualisation, visualisation, validation, writing – review & editing and resources.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the funding and support provided by the Leverhulme Trust research grant RPG-2020-301, as well as the UK Catalysis Hub via our membership of the UK Catalysis Hub Consortium funded by EPSRC grant: EP/R026645/1.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4re00181h

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