Selectivity in mechanistic studies of alkanes on “classical” and “inorganic cluster-derived” platinum catalysts

Abstract

Skeletal isomerization of 2-methylpentane (2MP) and hydrogenolysis of methylcyclopentane (MCP) have been studied over a series of Pt/Al₂O₃ catalysts, mono- or bi-metallic “cluster-derived” catalysts and well-characterized single-crystal faces of platinum. ¹³C labelling allowed an estimation of the relative contributions of cyclic or bond-shift mechanisms in isomerization. The bond-shift mechanisms predominate on large crystallites and on single-crystal faces of platinum. The highly dispersed catalysts can be easily simulated by “inorganic cluster-derived catalysts”. “Chini's cluster-derived catalysts” showed differences in selectivity due to the number of active sites. “Parshall and Wilkinson cluster-derived catalysts” have the same catalytic behaviour as industrial bimetallic Pt–Sn catalysts. New platinum catalysts derived from Chatt clusters led to a very selective demethylation and to a total disappearance of the cyclic mechanisms in spite of the very high dispersion of these catalysts (d_s≈ 20 Å). Geometric and electronic factors are taken into account to explain the differences in the selectivities obtained on “classical” and “inorganic cluster-derived” catalysts.