Unraveling the steric effects of confined water within zeolite and their consequences on hydrogen isotope exchange

Confined water clusters within the microporous framework of zeolite positively or negatively influence the catalytic reaction rates and selectivities, because of their steric effects to the reactive intermediates and transition states differently. In this study, a series of supported Pt-zeolites (e.g. 1Pt/USY, 1Pt/MOR, 1Pt/Beta, 1Pt/ZSM-5 and 1Pt/ZSM-35) with the analogous Pt cluster size and the concentration of Brønsted acid sites (BAS) and the different pore structures were employed to investigate the catalytic roles of confined water clusters during H 2 -HDO isotope exchange. The comparable reactive environments upon these bifunctional Pt catalysts enable them to share the identical reaction pathway during hydrogen isotope exchange. Specifically, water clusters are protonated on BAS to deliver deuteron and the subsequent hydrogen isotope exchange occurs at the interface of metal-water clusters by a synchronously bidirectional transfer, leading to the similar activation energy of 30.5 ± 4.0 kJ mol −1 . Nevertheless, the pore architectures of as-used zeolites affect the structure of confined water clusters in the form of one-dimensional oligomeric chains or three-dimensional bulk-like networks within the microporous frameworks, which constrains the hydrogen intrapore diffusion in the sub-nanometer scale and the subsequent reaction kinetic events to the different extents. As a result, the measured reaction rates upon these Pt-zeolite catalysts follow this order: 1Pt/USY>1Pt/MOR>1Pt/Beta>1Pt/ZSM-5>1Pt/ZSM-35. Collectively, this work shows that how the confined water within microporous environment affects the catalytic process, which also offers the mechanistic understanding of reaction chemistry in the restricted space.