Location of the Active Sites for Ethylcyclohexane Hydroisomerization by Ring Contraction and Expansion in the EUO Zeolitic Framework,ACS Catalysis

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Location of the Active Sites for Ethylcyclohexane Hydroisomerization by Ring Contraction and Expansion in the EUO Zeolitic Framework
ACS Catalysis ( IF 11.3 ) Pub Date : 2019-01-14 00:00:00 , DOI: 10.1021/acscatal.8b04462
Ester Gutierrez-Acebo ₁ , Jérôme Rey ₁ , Christophe Bouchy ₁ , Yves Schuurman ₂ , Céline Chizallet ₁

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Identifying the location of the active sites in a zeolite is a current challenge, impeding the design of optimal catalysts. In this work, we identify the location of the most active sites of 1-ethylcyclohexene isomerization in the EUO framework (10 MR channels, 12 MR side pockets) thanks to DFT calculations corroborated by experiments. Skeletal isomerization of cycloalkenes is a crucial industrial reaction for the bifunctional isomerization of ethylbenzene. Ethylcyclohexene is protonated by framework protons into cyclic carbenium ions, which undergo ring contraction–expansion reactions through protonated cyclopropane (PCP) like transition states. Ab initio calculations clearly show that the acid sites located at the intersection between the channel and the pocket stabilize much less the cyclic carbenium ions involved in the reaction than 12 MR pockets and 10 MR channel sites due to stronger dispersion stabilizing interactions. This computational finding is fully confirmed experimentally by the comparison of the catalytic performances of the H-EU-1 and H-ZSM-50 zeolites in ethylcyclohexane hydroisomerization. Both zeolites possess the EUO structure but with different location of the acid sites. The ratio in turnover frequencies is quantitatively rendered by the DFT-calculated free energy profiles. Diffusion measurements reveal similar ethylcyclohexane diffusion times for the two zeolites, supporting that the difference in activity is primarily driven by the location of the active sites.

中文翻译：

在EUO沸石骨架中通过环的收缩和膨胀进行乙基环己烷加氢异构化的活性位点的位置

鉴定沸石中活性位点的位置是当前的挑战，这阻碍了最佳催化剂的设计。在这项工作中，由于实验证实了DFT计算，我们确定了EUO框架中1-乙基环己烯异构化最活跃的位置（10个MR通道，12个MR侧袋）。环烯烃的骨架异构化是乙苯双功能异构化的关键工业反应。乙基环己烯被骨架质子质子化为环状碳鎓离子，通过过渡态质子化的环丙烷（PCP）经历环的伸缩反应。从头算就清楚地表明，由于较强的分散稳定作用，与12个MR腔和10个MR通道位点相比，位于通道和腔室之间的交点处的酸位点稳定的反应中涉及的环状碳鎓离子的稳定性差得多。通过比较H-EU-1和H-ZSM-50沸石在乙基环己烷加氢异构化中的催化性能，可以从实验上完全证实这一计算发现。两种沸石都具有EUO结构，但是酸位点的位置不同。通过DFT计算的自由能概图定量表示周转频率的比率。扩散测量显示两种沸石的乙基环己烷扩散时间相似，这表明活性差异主要由活性位点的位置驱动。

更新日期：2019-01-14

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