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The conversion of methylcyclopentane (MCP) has been studied overzeolite Y supported rhodium catalysts with a variety of protonconcentrations. Ring opening (RO) of MCP to hexane isomers iscatalyzed by metal sites, but ring enlargement (RE) of MCP tocyclohexane and benzene requires metal sites and acid protons.The activity for RE of Rh/HY is 2.5 times higher than that ofphysical mixtures of neutralized Rh/NaY(neutr)and HY with the samenumber of metal and acid sites at 200°C. This illustrates theimportance of proximity between metal and acid sites andsuggests that rhodium–proton adducts,[RhnHx]x+,behave ascollapsed bifunctional sites.Initial overall activity at 200°Cdecreases with increasing proton concentration in the orderRh/SiO2> Rh/NaY(neutr)> Rh/NaY > Rh/HY.In contrast to RE, theintrinsic activity of rhodium–proton adducts for RO is muchlower than that of proton-free Rh clusters. This difference isnot caused by preferential site blocking with coke precursors, asfollows from temperature-programmed oxidation of the catalystafter MCP reaction. Likewise, cyclopentane H/D exchange, areaction that specifically probes for uncovered metal sites,shows that the free metal surface in Rh/HY decreases only by25% upon exposure to MCP. Since the activity differences betweenRh/HY and Rh/NaY cannot be attributed to different metalcoverages with carbonaceous deposits, it follows that thedecrease in RO activity with increasing proton concentration ofthe zeolite results from coverage of metal sites with carbeniumions, which is a direct consequence of metal–proton adduct formation. |
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