Single file diffusion in mordenite channels: Neopentane conversion and H/D exchange as catalytic probes

When small molecules inside one-dimensional micropores of zeolites are unable to pass along each other, transport is limited by single file diffusion. Product molecules generated inside such micropores are unable to escape to the gas phase if the pores are plugged with physisorbed molecules. As a consequence, the apparent activation energy, E(app), is higher than the true activation energy. This is experimentally confirmed by comparing neopentane reactions (H/D exchange, isomerization and hydrogenolysis) over Pt/H-mordenite and Pt/SiO2 catalysts. At 150 degrees C the Arrhenius line for Pt/H-mordenite shows a break, indicating the switch from the single file diffusion controlled to the chemically controlled regime. The isosteric heat of physisorption of neopentane is -11.5 kcal/mol on Na-mordenite, whereas physisorption on SiO2 is negligible. In the regime of significant physisorption of neopentane, the increase in E(app) caused by single file diffusion overcompensates the decrease in E(app) caused by pre-equilibration between gaseous and physisorbed neopentane.