Reaction pathways in n-pentane conversion catalyzed by tungstated zirconia:effects of platinum in the catalyst and hydrogen in the feed

摘要

The catalytic isomerization of n-pentane catalyzed by tungsted zirconia (WZ)was investigated to elucidate the effects of H_2 in the feed and platinum in the catalyst.In the reaction catalyzed by WZ with or without platinum,when no H_2 was present,a complex reaction network was observed,associated with organic deposits on the catalyst,yielding mainly cracked products.The alkane is inferred to be activated in a redox step forming W~(5+)on the catalyst and unsaturated intermediates that react to form polyalkenylic surface species,which were detected by in situ UV-visible spectroscopy.Promotion of WZ with platinum dramatically improved its catalytic activity and the isomerization selectivity inn-pentane conversion.The improvement was ony marginal in the absence of H_2,but it became substantial in the presence of H_2,with the conversion increasing from3 to 55% for the platinum-promoted catalyst,which underwent only little deactivation.The selectivity for isopentane was about 95% at 523 K.Teh results indicate that the complex reaction network operative with the WZ catalyst is supressedon the platinum-containing catalysts in the presence of H_2.A fast and selective monomolecular isomerization reaction takes over in this case.The adsorbed unsaturated C_5 intermediats are rapidly desorbed via hydrogenation on the reduced tungstate surface.This rapid desorption minimizes the formation of higher-molecular-weight organic species such as polyakenyls that are necessary for the complex reaction path observed with the unpromoted catalyst.The observed side products are interpreted not as cracking products accompanying the acid-catalyzed isomerization reaction but instead as hydrogenolysis products formed directly on the platinum particles.