铈基复合金属氧化物强化糠醛影响下乙酸酮基化反应的研究

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

  • 摘要: 酮基化反应可以将生物油中的有机羧酸转化为易增链的酮类,是生物油高值化利用的重要环节之一。然而,生物油中的呋喃类物质对酮基化反应具有不利影响。针对这一问题,本研究以乙酸和糠醛作为模化物,考察了CeO2-MOx(M=Mg、Zr、Mn、Fe)复合金属氧化物催化剂在酸-呋喃混合反应物酮基化反应中的催化活性。在CeO2催化剂上,糠醛的加入使乙酸的转化率从94.68%降至79.33%,主要原因是呋喃类物质与乙酸在活性位上的竞争吸附效应,此外部分乙酸与糠醛还发生了协同转化。在铈基复合金属氧化物催化剂中,CeO2-MnO2对乙酸的转化和羰基化合物的生成均表现出最佳的催化活性,在反应温度为350 ℃,12 h连续反应中乙酸转化率为90.79%,羰基化合物碳收率可达61.95%。其原因是CeO2-MnO2具有最高的比表面积和表面氧空位活性位浓度,能有效削弱糠醛由于竞争吸附对乙酸酮基化反应的不利影响,提高了催化剂抵抗失活的能力,同时较平衡的表面酸碱性位点分布有利于酮基化反应的进行。

     

    Abstract: 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-MOx (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 (Odefect) concentration of 29.25% among the CeO2-MOx 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-MOx 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.

     

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