Relationship between the structural properties of supported bimetallic Pt–Rh catalysts and their performances for methylcyclopentane ring opening

Bimetallic Pt–Rh catalysts supported on alumina and silica were synthesized either by the refilling method or by coimpregnation. They were characterized by transmission electron microscopy (TEM), H2 chemisorption, temperature-programmed reduction (TPR), Fourier transform infrared (FTIR) of adsorbed CO and the test reaction of cyclohexane dehydrogenation. Varying the preparation procedure and the nature of the support leads to the formation of bimetallic Pt–Rh particles of different size in the range 1.2–2.7 nm. A Pt surface enrichment and the presence of a Pt–Rh alloy were detected by FTIR of adsorbed CO on the largest particles. The catalytic performances of the Pt–Rh systems were evaluated in methylcyclopentane (MCP) ring opening (RO) under pressure (28.5 bar). All the bimetallic catalysts present an activity much higher than that of the Pt monometallic sample, close to that of the Rh one. At 300 °C, the turnover frequencies are the followings: 115 h−1 for Pt, 4094 h−1 for Rh, between 2264 and 5563 h−1 for the Pt–Rh catalysts whatever the preparation procedure and the nature of the support. On Pt–Rh samples presenting particles with large enough size (>1.7 nm), the RO selectivity at iso-conversion (>50%) is higher than those of monometallic Pt/Al2O3 and Rh/Al2O3 systems, reaching the performances of a monometallic iridium-based sample considered as the most selective catalyst in these conditions. Finally, the refilling method allows obtaining supported Pt–Rh catalysts with the structural properties required to obtain both high activity and RO selectivity during methylcyclopentane hydrogenolysis.