(544o) Understanding Catalytic Bifunctionality of Cu/ZSM5 and Cu/Y Zeolites for Biomass Conversions
The biomass-derived 2,3-butanediol (2,3-BDO) can be obtained in a large quantity economically. This compound can be further utilized to produce valuable molecular precursors, such as 1,3-butadiene and butenes for polymerization, and other important chemicals. Kinetic measurements have shown that the Cu-loaded H-ZSM5 (or Cu/ZSM5) is more selective for butene production, while the Cu-loaded H-Y zeolite (Cu/Y) is more selective for methyl ethyl ketone (MEK) formation. In order to understand the catalytic roles played by the loaded copper ions and their interactions with existing Brønsted acid sites within the zeolite framework, 2-butanol is used as a probe molecule to test catalytic functionalities of these sites by examining respective dehydration and dehydrogenation reaction pathways. Our strategy is to generate reaction potential energy landscape using density functional theory calculations to describe 2-butanol dehydration (forming butene) and dehydrogenation (forming MEK) at the Brønsted acid site and the copper site in ZSM5 and Y cluster models, respectively. The modeling results showed that, individually, the copper site promotes the dehydrogenation, while proton in the Brønsted acid site promotes the dehydration reaction to form butene. In models combining these two sites, DFT calculations indicate that proton in the Brønsted acid site tends to migrate onto Cu in a reverse hydrogen spillover process, which facilitates alcohol dehydration. We attribute the different catalytic behaviors exhibited by Cu/H-ZSM5 and Cu/H-Y to the structures of these zeolitic frameworks. In ZSM5, the interactions among Cu, proton, and Al-O site enhance dehydration. In Y zeolite, because of its larger cavity, Cu may aggregate to form larger clusters, and hinder the alcohol access to the Brønsted acid site, hence, the dehydration reaction. We anticipate that the same principle can be applied to explain 2,3-BDO conversions in these catalysts as well.