Hydroisomerization of Benzene-Containing Paraffinic Feedstocks over Pt/WO_3-ZrO_2 Catalysts

摘要

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/WO_3—ZrO_2 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℃ 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℃, 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℃, the acid activity is high but the conversion of benzene is low, even at high pressure, due to thermodynamic reasons. 250℃ 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 WO_3—ZrO_2 support suppresses the ring-opening activity of supported Pt; therefore ,benzene transforms only into cyclohexane and methylcyclopentane over Pt/WO_3—ZrO_2. Addition of Pt/Al_2O_3 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 WO_3—ZrO_2. The selectivity is also modified because of the suppression of most of the cracking activity.