Key Roles of Lewis Acid–Base Pairs on ZnxZryOz in Direct Ethanol/Acetone to Isobutene Conversion

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• 作者：Junming Sun ; Rebecca A. L. Baylon ; Changjun Liu ; Donghai Mei ; Kevin J. Martin ; Padmesh Venkitasubramanian ; Yong Wang
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：January 20, 2016
• 年：2016
• 卷：138
• 期：2
• 页码：507-517
• 全文大小：675K
• ISSN：1520-5126

文摘

The effects of surface acidity on the cascade ethanol-to-isobutene conversion were studied using Zn_xZr_yO_z catalysts. The ethanol-to-isobutene reaction was found to be limited by the secondary reaction of the key intermediate, acetone, namely the acetone-to-isobutene reaction. Although the catalysts with coexisting Br?nsted acidity could catalyze the rate-limiting acetone-to-isobutene reaction, the presence of Br?nsted acidity is also detrimental. First, secondary isobutene isomerization is favored, producing a mixture of butene isomers. Second, undesired polymerization and coke formation prevail, leading to rapid catalyst deactivation. Most importantly, both steady-state and kinetic reaction studies as well as FTIR analysis of adsorbed acetone-d₆ and D₂O unambiguously showed that a highly active and selective nature of balanced Lewis acid–base pairs was masked by the coexisting Br?nsted acidity in the aldolization and self-deoxygenation of acetone to isobutene. As a result, Zn_xZr_yO_z catalysts with only Lewis acid–base pairs were discovered, on which nearly a theoretical selectivity to isobutene (～88.9%) was successfully achieved, which has never been reported before. Moreover, the absence of Br?nsted acidity in such Zn_xZr_yO_z catalysts also eliminates the side isobutene isomerization and undesired polymerization/coke reactions, resulting in the production of high purity isobutene with significantly improved catalyst stability (<2% activity loss after 200 h time-on-stream). This work not only demonstrates a balanced Lewis acid–base pair for the highly active and selective cascade ethanol-to-isobutene reaction but also sheds light on the rational design of selective and robust acid–base catalyst for C–C coupling via aldolization reaction.