FTIR and Reaction Studies of Styrene and Toluene over Silica–Zirconia-Supported Heteropoly Acid Catalysts

The interactions of styrene and toluene with a sol–gel-prepared SiO2–ZrO2 mixed oxide and with two supported heteropolytungstic acid samples have been studied by FTIR. The results of the IR study of styrene adsorption and toluene/styrene coadsorption indicate that the oligomerization of styrene, which proceeds through a carbenium intermediate, takes place over the support and the supported catalysts. The alkylation of toluene by styrene, which also proceeds via the formation of a carbenium ion from styrene, is only detected for the supported heteropoly acid catalysts. These spectroscopic observations, which are in good agreement with the product distribution obtained for reaction experiments conducted in the liquid phase, suggest that the reaction of the adsorbed, activated styrene with toluene or with a second molecule of styrene depends on the relative coverage of the two competing reagents. The incorporation of the HPA to the silica–zirconia support increased the density and strength of the acid sites favoring the retention of a higher fraction of toluene and consequently the production of higher yields of alkylated products. Differences between the two supported HPA catalysts could be ascribed to the nature of the supported heteropoly acid species and the modification of the acid characteristics due to the interaction with the silica–zirconia support.