Enhanced furfural-affecting ketonization of acetic acid by cerium composite metal oxides

Ketonization reaction can transform the organic carboxylic acids in bio-oil into ketones, which are important platform chemicals because they can easily undergo chain growth through Aldol reaction to produce jet fuel precursors; and this is an important pathway for the high-value utilization of bio-oil. However, the presence of furanic compounds in bio-oil has negative effects on the ketonization reaction. To solve this problem, CeO₂-MO_x (M=Mg, Zr, Mn, Fe) composite metal oxide catalysts were prepared, and their catalytic activities in the ketonization of acid-furan mixture were investigated in this study. Over the unmodified CeO₂ catalyst, the addition of furfural in the feedstock led to the decrease of acetic acid conversion from 94.68% to 79.33%, and the acetone selectivity also reduced from 88.33% to 79.87%. This was mainly attributed to the competitive adsorption of furfural and acetic acid on the active sites. Additionally, some acetic acid and furfural molecules also underwent joint transformation to form complicated condensed products, some of which retained the carbonyl group. When CeO₂ was modified by the doping of second metals (M=Mg, Zr, Mn, Fe) through the coprecipitation method, it was found that all second metals were successfully introduced into the lattice of CeO₂, forming CeO₂-based solid solutions. As a result, the oxygen vacancies on the catalyst surface increased, and CeO₂-MnO₂ had the highest surface defect oxygen (O_defect) concentration of 29.25% among the CeO₂-MO_x catalysts. Meanwhile, N₂ physisorption and H₂-TPR characterizations showed that CeO₂-MnO₂ also had the largest specific surface area and good reducibility. Furthermore, according to the results of NH₃-TPD and CO₂-TPD characterizations, the amounts of acid and base sites on the surface of CeO₂- MnO₂ were close, resulting in an acid-base ratio of 1.17. The catalytic activities of CeO₂-MO_x and CeO₂ catalysts in ketonization were tested using the mixture of acetic acid and furfural as the feedstock. CeO₂-MnO₂ exhibited the highest catalytic activities in the conversion of acetic acid, the selectivity of acetone and the formation of compounds with carbonyl group: during the operation at 350 ℃ with the lasting time of 12 h, the conversion of acetic acid was 90.79%, the selectivity of acetone was 81.95% and the carbon yield of compounds with carbonyl group reached 61.95%. Two reasons could explain the superior performance of CeO₂-MnO₂ in the ketonization of acetic acid-furfural mixture. First, the high specific surface area and surface oxygen vacancy concentration of CeO₂-MnO₂ facilitated the adsorption of acetic acid on the active sites, which could effectively overcome the problem of restricted acetic acid conversion caused by the competitive adsorption of acetic acid with furfural. Second, CeO₂-MnO₂ had a more balanced distribution of surface acid-base sites, which could benefit the ketonization reaction because the cooperation of acid (the oxygen vacancies acting as Lewis acid sites) and base sites were more efficient in catalyzing ketonization than individual acid sites. This study could offer reference values for the selection and design of catalysts applied in the ketonization of bio-oil acid-rich fractions such as aqueous fraction and vacuum distilled fraction, and the refined fraction could be further converted into hydrocarbons fuels like jet fuel through the processes of carbon chain growth and hydrodeoxygenation.