No abstract

No abstract

No abstract

No abstract

Sulfated zirconia has been prepared according to three different procedures, viz., (i) conventional impregnation with sulfuric acid and calcination (3 h at 773 K) of two zirconia's (50 and 217 m2/g), (ii) reaction of zirconium tetrachloride with sulfuric acid giving bulk anhydrous zirconium sulfate, and (iii) deposition−precipitation of highly dispersed zirconia on silica and subsequent reaction with either H2S and O2, or SO2 and O2, or SO3. The latter two procedures lead to essentially water-free catalysts. Thermogravimetry showed that the impregnated and calcined zirconia's loose sulfate above 830 K (50 m2/g) and 910 K (217 m2/g). In a gas flow containing water, the sulfated silica-supported zirconia loses sulfate already at 673 K because of the reaction to more volatile sulfuric acid. The catalysts were employed in the gas-phase trans-alkylation of benzene (1) and diethylbenzene (2) to ethylbenzene (3) at 473 and 673 K and in the solvent-free, liquid-phase hydro-acyloxy-addition of acetic acid to camphene (4) to camphene (5) to isobornyl acetate (6) at 338 K. The water-free catalysts were not active; only after addition of water was catalytic activity exhibited. The catalytic activity of the differently prepared sulfated zirconia's is governed by the equilibrium:  Zr(SO4)2 + 4H2O ⇌ Zr(SO4)2·4H2O + nH2O ⇌ ZrO2 + 2H2SO4.aq. Addition of water vapor to the bulk sulfate at 473 K led to the reaction to the tetrahydrate, which was not active, whereas the highly dispersed silica-supported zirconium sulfate reacted to form sulfuric acid. The supported catalyst rapidly released the water at 473 K, which resulted in a rapid drop in catalytic activity. Transport of water through the porous system dominates the activity of the impregnated zirconia's. Accordingly, the slight activity of the zirconia of 50 m2/g rapidly dropped at 473 K, whereas the zirconia of 217 m2/g displayed a high and stable activity. At 673 K, the transport is much more rapid. The activity of the highly porous zirconia was therefore at 673 K much lower than at 473 K. Whereas the gas-phase reaction is governed by transport of water vapor, the liquid-phase reaction is dominated by transport of the reactants to the active sites. Consequently, the sulfated zirconia of 50 m2/g showed a considerably higher activity than that of 217 m2/g. Also the silica-supported catalyst exhibited a higher activity. The consistent results demonstrate that sulfated zirconia needs water to display activity in the gas-phase and liquid-phase reaction studied.

SummaryTo assess whether Lewis acid sites are present on zirconium sulfate, we prepared water-free bulk zirconium sulfate. Furthermore, a water-free silica-supported zirconium sulfate catalyst was prepared by deposition-precipitation of zirconia on silica and subsequent gas-phase reaction with SO 3 . The activity of these catalysts was compared with that of two conventionally prepared sulfated zirconia catalysts.The different catalysts were extensively characterized. XPS indicated that the conventionally prepared sulfated zirconia catalysts contained sulfuric acid. The activity of the catalysts was determined with the gas-phase trans-alkylation of diethylbenzene with benzene and the solvent-free liquid-phase addition of acetic acid to camphene.Both water-free zirconium sulfate catalysts did not exhibit a significant activity; Lewis acid sites are therefore not active in these sulfated zirconia catalysts. Upon exposure to water vapor the initially water-free catalysts were active. The stability of the conventional sulfated zirconia catalysts appeared to be determined in the gas-phase by the volatilization of sulfuric acid. As a result, a highly porous catalyst was more effective than a catalyst based on zirconia of a relatively low porosity. With liquidphase reactions extraction of sulfuric acid proceeds leading to an acid liquid, which is catalytically active also after separation of the solid catalyst from the reaction mixture.