Kinetics of propylene disproportionation over a tungsten oxide on silica catalyst

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Virginia Polytechnic Institute and State University


This investigation consisted of a study of the kinetics of propylene disproportionation over a tungsten oxide on silica catalyst. A catalyst of ten percent WO₃ on silica gel (223 square meters per gram B.E.T. surface area) was used in a microcatalytic reactor. Electron probe scans of the commercially prepared catalyst showed that the standard liquid impregnation technique used in preparation of this catalyst can result in large radial variations in the distribution of the promoter within the pellet. Both flow and pulse reactor techniques were used.

It had been reported that external mass transfer effects could not be eliminated in this system. Here linear velocities in excess of those used previously were investigated, and it has been found that both external and intraparticle mass transfer effects can be eliminated, though exceptionally high linear velocities are required to eliminate the external mass transfer effects.

Initial rate data were obtained for the disproportionation of propylene by this catalyst. Temperatures of 399° to 454° Centigrade and pressures from one to nine atmospheres were used. The experimental data were well correlated by assuming that a Langmuir-Hinshelwood, dual-site surface reaction was the rate controlling step in the reaction mechanism. The mechanism parameters and their temperature dependence were extracted from the experimental data using a linear least squares technique. An apparent activation energy of 24.73 Kcal per mole was found for this catalytic system.

During the initial contacting of freshly activated samples of this catalyst with propylene, significant increases in disproportionation activity were observed for periods up to twenty four hours. The rate of catalyst break-in was found to depend on both the temperature and pressure with an activation energy of 47.17 Kcal/mole and a first order propylene partial pressure dependency.

Data are presented to establish that both a reduction of the catalyst to WO₂.₉ and the strong adsorption of an olefin are responsible for this period of transient activity. Prior reduction to the normally thermodynamically unfavorable oxidation state of WO₂ was found to give a thirty percent increase in the steady-state activity of this catalyst. The strong adsorption of an olefin was found to be at least partly reversible in an inert atmosphere.