Methylcyclopentane conversion catalysis over zeolite-Y encaged rhodium: A test for the metal-proton adduct model

The conversion of methylcyclopentane (MCP) has been studied over zeolite Y supported rhodium catalysts with a variety of proton concentrations. Ring opening (RO) of MCP to hexane isomers is catalyzed by metal sites, but ring enlargement (RE) of MCP to cyclo-hexane and benzene requires metal sites and acid protons. The activity for RE of Rh/HY is 2.5 times higher than that of physical mixtures of neutralized Rh/NaY(neutr) and HY with the same number of metal and acid sites at 200 degrees C. This illustrates the importance of proximity between metal acid acid sites and suggests that rhodium-proton adducts, [RhnHx](x+), behave as collapsed bifunctional sites. Initial overall activity at 200 degrees C decreases with increasing proton concentration in the order Rh/SiO2 > Rh/NaY(neutr) > Rh/NaY > Rh/HY. In contrast to RE, the intrinsic activity of rhodium-proton adducts for RO is much lower than that of proton-free Rh clusters. This difference is not caused by preferential site blocking with coke precursors, as follows from temperature-programmed oxidation of the catalyst after MCP reaction. Likewise, cyclopentane HID exchange, a reaction that specifically probes for uncovered metal sites, shows that the free metal surface in Rh/HY decreases only by 25% upon exposure to MCP. Since the activity differences between Rh/HY and Rh/NaY cannot be attributed to different metal coverages with carbonaceous deposits, it follows that the decrease in RO activity with increasing proton concentration of the zeolite results from coverage of metal sites with carbenium ions, which is a direct consequence of metal-proton adduct formation. (C) 1996 Academic Press, Inc.