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

Abstract

The catalytic isomerization of n-pentane catalyzed by tungsted zirconia (WZ) was investigated to elucidate the effects of H2 in the feed and platinum in the catalyst. In the reaction catalyzed by WZ with or without platinum, when no H2 was present, a complex reaction network was observed, associated with organic deposits on the catalyst, giving 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 UVvisible spectroscopy. Promotion of WZ with platinum dramatically improved the catalytic activity and the isomerization selectivity in n-pentane conversion. The improvement was only marginal in the absence of H2, but it became substantial in the presence of H2, with the conversion increasing from 3 to 55% for the platinum-promoted catalyst, which underwent only little deactivation. The selectivity for isopentane was about 95% at 523 K. The results indicate that the complex reaction network operative with the WZ catalyst is suppressed on the platinum-containing catalysts in the presence of H2. A fast and selective monomolecular isomerization reaction takes over in this case. The adsorbed unsaturated C5 intermediates are rapidly desorbed via hydrogenation on the reduced tungstate surface. This rapid desorption minimizes the formation of highermolecular-weight organic species such as polyalkenyls 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.

Bruce Gates: A Career in Catalysis

“…Reaction paths A and B for isomerization on the tungsten oxide surface. Redrawn with permission from ref . Copyright 2003 Elsevier.…”

“…The reaction paths on Pt-promoted WZ were elucidated by observing the n-pentane reaction with or without H 2 in the feed, and by prereduction of the catalyst. 208 The Pt was assumed to act either (1) as a site for dehydrogenation and hydrogenolysis, giving alkenes that isomerize on acid sites with hydrogenolysis products C1 and C4 or C2 and C3; 209 or (2) as a locus for hydrogen spillover that then affects reactions on the acid sites. 210 Four reaction paths, A−D, were hypothesized to explain the reaction chemistry.…”

Biomass Gas Cleanup Using a Therminator

“…The pretreatment in H 2 /CO seems to result in two opposite effects: partial reduction of WC (by H 2 ) and carburization of the surface by CO to yield WC and perhaps W 2 C domains [10][11][12][13][14].…”

“…38%. Presence of Pt and H 2 is known to increase the cracking activity [10]. When CO was co-fed with H 2 , the toluene conversion dropped to 10% but the rate of CH 4 formation was higher, possibly due to the reaction between H 2 and CO.…”

Characterization of Iron Promoter in Tungstated Zirconia Catalysts by Mössbauer Spectroscopy at Very Low Temperatures

“…While tungstated zirconia as solid acid catalyst has been discovered almost two decades ago, it has not been given much attention because of its lower activity and higher temperature of utilization compared to sulfated zirconia. Recently tungstated zirconia catalysts found renewed interest because of their high thermal stability and regenerability. − Furthermore, doping tungstated zirconia with noble metals such as Pt or Pd was shown to promote the activity of the catalysts in the presence of a hydrogen containing feed, rendering them possible candidates for practical alkane isomerization catalysts. These noble metals form at the surface of zirconia large clusters of 100 nm diameter whose role is to provide atomic hydrogen to contribute to the reduction of the dispersed tungsten cations involved in the catalytic conversion of the hydrocarbon. − …”