A Kinetic Study of Propylene Metathesis over WO3-SiO2 Gel

Recent workers have reported anomalous results for the metathesis of propylene over WO3-SiO2 gel in a flow system. One anomaly, confirmed in the present work, is an increase in catalytic activity with an increase in flow rate. The reaction was studied at a propylene partial pressure of 0.37-1 atm between 435 and 493 degrees C. Two reaction mechanisms have recently been proposed; one includes a bimolecular surface step and the other is a carbene chain mechanism. The rate law for the carbene mechanism has been developed. Attempts were made to fit the catalytic activities to the rate laws for these two mechanisms. An attempt was also made to determine whether or not the activities meet the site density criterion. This criterion states for a postulated slow step that the calculated density of active sites on the surface, determined by analyzing kinetic data using transition state theory, must be a physically possible site density. Our results are consistent with the bimolecular, but not with the carbene, rate law. However, our data are not consistent with the site density criterion for the bimolecular rate law. The theoretical methods used enable us to detect anomalies in our kinetic data, in this case the flow rate anomaly. Applying the same tests to systems reported in the literature indicates that inconsistency with a rate law or a physically possible site density is not found when there is no flow rate anomaly