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Hydrogenolytic ring opening of methylcyclopentane (MCP) was investigated on Rh/Al 2 O 3 catalysts, prepared by the incipient wetness method. The catalysts, with different metal loadings (0.3%, 3%, and 10%) and altered reduction temperatures (573 K, LTR; 973 K, HTR), were further characterized by temperature-programmed reduction (TPR). With high metal loading (10Rh) we observed a second TPR peak, which appeared after the impregnated as well as the subsequently oxidized form was reduced. This second peak was absent with 0.3 and 3% Rh/Al 2 O 3 . Strong dependence could be seen in the distribution of ring-opening products as a function of temperature and hydrogen pressure with 10Rh and 3RhHTR. We attribute these variations to changing selectivities toward fragmentation. This selectivity varies with the reaction temperature, hydrogen pressure, and catalyst preparation. Another behavior pattern appeared with 0.3Rh and 3RhLTR, exhibiting no selectivity variation in the ring-opening product distribution. This was caused by the random fragment production from the ring-opening surface species. We compared the ring-opening distribution with the fragmentation pattern and found correlation in the case of the first type of catalysts and suggest a common active site on Rh/alumina for the two main reactions, single and multiple C-C bond rupture, respectively. Due to the parallel variation in catalytic and TPR behavior of Rh/Al 2 O 3 , we attribute the variations in reaction mechanisms to changes in metal loadings and pretreatments, leading to different particle morphologies. By changing the metal loading and altering the reduction temperature, we suggest two forms of rhodium based on (i) temperature-programmed reduction study and (ii) the behavior of different samples in MCP reactions. |
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