Hydroisomerization of benzene-containing paraffinic feedstocks over Pt/WO3-ZrO2 catalysts

This report studies the feasibility of carrying out the elimination of benzene contained in paraffinic feedstocks (3-15%) with economy of equipment and sparing of pretreatment steps by hydrogenating benzene to cyclohexane and isomerizing partly the latter to methylcyclopentane in an isomerization reactor loaded with a Pt/WO3-ZrO2 catalyst. The results indicate that the temperature of calcination of the catalysts affects differently the acid and metal functions. The hydrogenating capacity of Pt mildly decreases at higher temperatures because of an increasing interaction with the support, while the isomerization capacity is enhanced because of the creation of strong acid sites. The optimum catalyst is the one calcined at 800 degrees C because of the formation of strong acid sites at this temperature. With respect to the reaction temperature, there exists a narrow range in which both the hydrogenation of benzene and the isomerization of n-paraffins are thermodynamically feasible with nonnegligible yield. At 200 degrees C, the conversion of benzene is greatly favored but the activity of the acid function for the acid-catalyzed reactions, i.e., the ring contraction of cyclohexane and the isomerization of n-hexane, is too small. At 300 degrees C, the acid activity is high but the conversion of benzene is low, even at high pressure, due to thermodynamic reasons. 250 degrees C seems to be the best temperature for performing both reactions simultaneously. The inhibition of the hydrogenolytic activity due to the interaction of Pt with the WO3-ZrO2 support suppresses the ring-opening activity of supported Pt; therefore, benzene transforms only into cyclohexane and methylcyclopentane over Pt/WO3-ZrO2. Addition of Pt/Al2O3 to form a composite catalyst enhances the metal activity and ring-opening products appear. However, this is not considered convenient in this case, because methylcyclopentane has a conveniently high octane number and most ring-opening products have similar or lower values. The presence of benzene partly inhibits the conversion of n-hexane because of the adsorption over the strong acid sites of WO3-ZrO2. The selectivity is also modified because of the suppression of most of the cracking activity.