Selective Ring Opening of Naphthenic Molecules

Abstract

Ring opening as practiced in hydrocracking of naphthenic molecules results from multiple cleavages of both endo- and exocyclic C–C bonds. Selective ring opening requires that only one endocyclic C–C bond per naphthene ring be severed, preserving thereby reactant molecular weight. The products of selective ring opening are alkanes and alkylnaphthenes. Over hydrocracking catalysts the yield of alkanes with the same number of carbon atoms as the reactant naphthenes is unacceptably low as a result of extensive dealkylation of alkylnaphthenes and secondary cracking of alkanes. Alkylcyclopentanes, in contrast, selectively ring open by hydrogenolysis over a number of noble metal catalysts. Under similar reaction conditions the selective ring-opening rates of alkylcyclohexanes are, however, one to two orders slower than those of alkylcyclopentanes. Addition of a ring-contraction acidity function converts alkylcyclohexanes into more easily ring-opened alkylcyclopentanes, greatly facilitating selective ring opening. Selective ring-opening rates and selectivities are optimum when ring isomerization occurs by a nonbranching ring contraction. Branching ring contraction, creating increased numbers of ring substituents, is detrimental to both ring-opening rates and selectivities. When an effective acidity function is coupled with a high-activity hydrogenolysis metal, such as iridium, the resulting bifunctional catalyst system greatly outperforms conventional hydrocracking catalysts for the selective conversion of naphthenes to alkanes.