Selectivity in hydrocarbon catalytic reforming: a surface chemistry perspective

A brief overview of the recent advances in the understanding of the reaction mechanisms of hydrocarbon reforming processes is provided. Emphasis is placed on the knowledge developed by studies using model single-crystal metals and modem surface analytical techniques. An argument is presented for the early definition of reaction selectivities in these processes because of subtle relative changes in dehydrogenation rates, Specifically, the ability of platinum to promote gamma-hydride elimination steps from chemisorbed alkyl groups is proposed to be the reason for its unique behavior in isomerization and cyclization reactions. Nickel, in contrast, facilitates dehydrogenation preferentially at the a-position, and catalyses hydrogenolysis instead. Nevertheless, by far the most favorable reaction of alkyl moieties on transition metals is beta-hydride elimination. It is this step the one that accounts for the fast equilibrium reached between alkanes and alkenes under most reaction conditions. Additional mechanistic complications in hydrocarbon reforming under catalytic conditions a-re introduced by the strongly bonded carbonaceous deposits that cover the surface of the active catalyst. The role that those deposits play in reforming is not yet entirely clear, but it is believed to involve passivation of the surface to facilitate the adsorption of pi-bonded olefins and other weakly adsorbed intermediates, and also the provision of a reservoir for hydrogen, The working reforming catalysts is therefore likely to display bifunctional character, with rapid hydrogenation-dehydrogenation steps taking place on the hydrocarbon-covered surface, and more demanding skeletal rearrangement steps occurring on patches of bare metal. (C) 2002 Elsevier Science B.V. All rights reserved.