Influence of chlorine on the catalytic properties of supported rhodium, iridium and platinum in ring opening of naphthenes

Pt, Ir and Rh were deposited on SiO2 or Al2O3 using chlorinated precursors and various amounts of HCl in the impregnation medium. The Bronsted and Lewis acidities increased with the chlorine content of the alumina supported catalysts. The silica-supported catalysts only presented Lewis acid sites. The catalysts were evaluated in methylcyclopentane (MCP) and methylcyclohexane (MCH) ring-opening (RO) under pressure (2.85 and 3.95 MPa, respectively), from 200 to 425 degrees C. For MCP conversion, the acidity of the alumina support had no sensitive effect on the activity and selectivity to RO products, and few effects on the distribution of RO products. No isomerization or hydrocracking products were observed, confirming that these reactions occurred mainly on the metal function, which was not modified by the presence of chlorine. The nature of the support, SiO2 or Al2O3, had a strong effect on both the activity (1.9 against 0.5 mol h(-1) g(-1) (metal) for Ir/Al2O3 and Ir/SiO2, respectively at 225 degrees C) and selectivity to RO products (99.6% against 97.5% for Ir/Al2O3 and Ir/SiO2, respectively, at 80% of MCP conversion) for Ir catalysts only. Interestingly, the Rh/SiO2 exhibited a high selectivity for converting MCP to RO products, similar to Ir/Al2O3, i.e. 99.6% at 80% of conversion. Depending on the metal and the supports, three types of behavior were observed for MCH ring-opening: (i) a direct ring-opening on the metal function whatever the support for Ir, (ii) a first step of isomerization, and then a need of a sufficiently acidic support, for Pt and (iii) an intermediate behavior for Rh, which was able to either directly convert MCH in absence of acidic support or favor a bifunctional mechanism on chlorinated alumina. (C) 2013 Elsevier B.V. All rights reserved.