Promoting Effect of Al on SO2−4/MxOy (M=Zr, Ti, Fe) Catalysts

The promoting effect of Al on various types of sulfated metal oxide catalysts has been studied. Incorporation of an appropriate amount of Al2O3 into zirconia, titania, and iron oxide before sulfation increases the acidity, activity, and stability of the catalysts for isomerization of n-butane and n-pentane significantly. The steady-state conversion of n-butane at 250°C and n-pentane at 300°C over 3 mol% Al2O3-promoted sulfated zirconia (SZA-3) are 2.2 and 4.5 times higher than those over unpromoted sulfate zirconia (SZ), respectively. In benzoylation of toluene with benzoyl chloride, after running for 10 h at 110°C the yield of methylbenzophenones on SZA-3 and SZ catalysts are 91.2 and 70.9%, respectively. The addition of Al2O3 helps to stabilize the surface sulfate complex on the oxides and increases the number of catalytically effective acid sites on the catalysts. Different from the promoting mechanism of Fe and Mn incorporated sulfated zirconia catalyst, the high catalytic activity and slow deactivation of the Al-promoted catalysts for n-alkane isomerization are caused by an increase in the number of intermediate–strong acid sites with differential heat of adsorption of NH3 between 125 and 140 kJ mol−1, providing long-term activity to the reaction.