Insights into the Complexity of Heterogeneous Liquid-Phase Catalysis: Case Study on the Cyclization of Citronellal

详细信息    查看全文

• 作者：Philipp Müller ; Patrick Wolf ; Ive Hermans
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：May 6, 2016
• 年：2016
• 卷：6
• 期：5
• 页码：2760-2769
• 全文大小：523K
• 年卷期：0
• ISSN：2155-5435

文摘

Heterogeneously catalyzed liquid-phase reactions are highly complex chemical systems. As the local molecular composition close to the active site is often unknown, sophisticated spectroscopic tools are needed to gain insights on a molecular level. One solution to these challenges is the use of modulation excitation spectroscopy with attenuated total reflection infrared spectroscopy. We use this highly sensitive and selective technique to study the Lewis acid-catalyzed cyclization of citronellal over mesoporous Sn-SBA-15 and microporous Sn-Beta. We find that the reaction mechanism is generally similar for the two materials. However, the confined space at the active site within the zeolite stabilizes the coordination of citronellal to the Sn^IV site and prevents byproduct formation, as well as the reverse reaction due to size-exclusion of the product isopulegol. The use of the Lewis base acetonitrile as solvent reduces the catalytic performance with Sn-SBA-15 drastically, while Sn-Beta remains highly active. Infrared spectra reveal a simultaneous coordination of citronellal and acetonitrile to the tin site in Sn-Beta, whereas in Sn-SBA-15, the more-accessible Sn^IV site leads to much stronger and hence detrimental competitive adsorption. The results obtained in this study indicate that the substrate–catalyst–solvent combination needs to be optimized in order to maximize the performance of solid–liquid reactions.