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 unab le to escape to the gas phase if the pores are plugged with physisorbe d molecules. As a consequence, the apparent activation energy, E(app), is higher than the true activation energy. This is experimentally con firmed 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, indicat ing the switch from the single file diffusion controlled to the chemic ally controlled regime. The isosteric heat of physisorption of neopent ane is -11.5 kcal/mol on Na-mordenite, whereas physisorption on SiO2 i s negligible. In the regime of significant physisorption of neopentane , the increase in E(app) caused by single file diffusion overcompensat es the decrease in E(app) caused by pre-equilibration between gaseous and physisorbed neopentane.