Supported Pt-Rh bimetallic catalysts as efficient systems for methylcyclohexane ring opening

International audienceA series of supported Pt-Rh bimetallic systems was prepared either (i) by the refilling method (a surface redox reaction), or (ii) by the classical coimpregnation or (ii) by mechanical mixture of supported monometallic Pt and Rh catalysts and acidic support (chlorinated alumina). Two oxide supports were used for these preparations, i.e. alumina and silica. All these systems, largely characterized in a previous work dedicated to methylcyclopentane (MCP) hydrogenolysis, were studied for the methylcyclohexane (MCH) ring opening (RO) performed under high pressure (39.5 bar). During MCH transformation, synergetic performances were observed with some supported Pt-Rh bimetallic catalysts, since better performances in terms of activity and RO selectivity were obtained compared to those of monometallic Pt and Rh systems. The metallic particle size acts as a determining parameter modulating the catalytic properties, since the best RO performances were obtained on bimetallic catalysts presenting the largest particle sizes, for which a Pt surface enrichment and the presence of a Pt-Rh alloy were previously detected. On these bimetallic catalysts, the ring opening occurs mainly according to a bifunctional mechanism, the chlorinated alumina support bringing the required acidic function for the first isomerization step of C6 ring to alkylcyclopentanes further opened on the metallic function. The acidic function and the metal sites should not be necessarily in close vicinity since the mechanical mixture of non-acidic Pt-Rh/SiO2 catalyst with chlorinated alumina leads also to high RO selectivities. Finally, as for MCP ring opening, the refilling method allows synthesizing supported bimetallic Pt-Rh catalysts particularly efficient in terms of activity and RO selectivity. (C) 2011 Elsevier B.V. All rights reserved