The chemistry of selective ring-opening catalysts

Bitumen-derived crude and heavy oils require severe processing and produce middle distillate product with poor ignition quality. This becomes a concern to refiners as tighter specifications on transportation fuel are promulgated. One process to address this issue is selective ring opening of cycloparaffins to reduce the number of ring structures, while retaining the carbon number of a product molecule. The process involves bifunctional catalysts, both metal and acid sites, working in high-pressure, high-temperature reactor systems in the presence of hydrogen. The acidic sites catalyze dehydrogenation, cracking, isomerization and dealkylation, while the metal sites promote hydrogenation, hydrogenolysis and isomerization. Various compounds containing single, double and multiple rings have been used to model the ring-opening reactions and a number of mechanisms have been proposed. The five-membered ring readily undergoes ring-opening reaction on either acid or metal catalysts with the selectivity and activity dependent on the nature of the supported metal catalyst. The ring opening of six-membered ring compounds is secondary, requiring an acidic function to isomerize a six-membered ring cycloparaffin to a five-membered ring. A balanced metallic-acidic function catalyst is necessary to achieve optimal performance. A system dominated by acid function results in excess cracking, while a catalytic system with high concentration of metals leads to mainly hydrogenation. Commercial hydrocracking catalysts using transition metal sulfides on acidic supports usually require severe operating conditions due to their low activities of the metal sulfide compared to metal sites, leading to extensive cracking of cycloparaffin side chains. Noble metals supported on acidic oxides are the most active catalysts for selective ring opening, but these catalysts are very sensitive to poisoning by sulfur compounds in petroleum feedstocks. An understanding of the chemistry of selective ring-opening catalysts, combined with theoretical studies of structure-activity relationships and high throughput experimentation methods, provides opportunities in searching for new generations of selective ring-opening catalysts with high-performance and sulfur resistance. Crown Copyright (c) 2005 Published by Elsevier B.V. All rights reserved.