Ring opening of methylcyclopentane on alumina-supported Rh catalysts of different metal loadings

Hydrogenolytic ring opening of methylcyclopentane (MCP) was investigated on Rh/Al2O3 catalysts, prepared by the incipient wetness method. The catalyst s, with different metal loadings (11.3%, 3%, and 10%) and altered reduction temperatures (573 K, LTR; 973 K, HTR), were further characterized by tempe rature-programmed reduction (TPR). With high metal loading (10Rh) we observ ed a second TPR peak, which appeared after the impregnated as well as the s ubsequently oxidized form was reduced. This second peak was absent with 0.3 and 3% Rh/Al2O3. Strong dependence could be seen in the distribution of ri ng-opening products as a function of temperature and hydrogen pressure with 10Rh and 3RhHTR. We attribute these variations to changing selectivities t oward fragmentation. This selectivity varies with the reaction temperature, hydrogen pressure, and catalyst preparation. Another behavior pattern appe ared with 0.3Rh and 3RhLTR, exhibiting no selectivity variation in the ring -opening product distribution. This was caused by the random fragment produ ction from the ring-opening surface species. We compared the ring-opening d istribution with the fragmentation pattern and found correlation in the cas e of the first type of catalysts and suggest a common active site on Rh/alu mina for the two main reactions, single and multiple C-C bond rupture, resp ectively. Due to the parallel variation in catalytic and TPR behavior of Rh /Al2O3, we attribute the variations in reaction mechanisms to changes in me tal loadings and pretreatments, leading to different particle morphologies. By changing the metal loading and altering the reduction temperature, we s uggest two forms of rhodium based on (i) temperature-programmed reduction s tudy and (ii) the behavior of different samples in MCP reactions. (C) 2000 Academic Press.