Sulfated zirconia preparation

Figure 17.1 summarizes the effect of the calcination temperature on the catalytic activity for butane isomerization over sulfated zirconias prepared from different zirconia gels dried at the optimum temperatures. The figure indicates that the maximum activities are approximately the same for different catalysts even though... 

The same behaviour has been found with Cu/ZrOa. A highly dispersed Cu phase was obtained at the surface of zirconla by reacting the support with Cu acetylacetonate [19]. This procedure yields an active catalyst. This catalyst was selective for Na formation at low temperature (< 550 K), but produced only NO2 when the temperature becomes higher than 650 K. However, the same type of catalyst prepared from sulfated zirconia did not produce NO2 but selectively reduces NO to N2 whatever the temperature, with a yield of about 40% at 670 K, and a GHSV of 70000 h l, using only 300 ppm of decane. 

Sulfated zirconia sol was prepared by placing 10 g of ethanol and 10 g of zirconium butoxide (80%) into a flask under magnetic stirring at room temperature, followed by adding a mixture of 6.5 g of ethanol, 1.5 g of distilled water, and 5 g of H2SO4 (18 M). 

D. Farcasiu and J. Q. Li, Preparation of sulfated zirconia catalysts with improved control of sulfur content, 111 effect of conditions of catalyst synthesis on physical properties and catalytic activity,... 

H. Althues, and S. Kaskel, Sulfated zirconia nanoparticles synthesized in reverse microemulsions preparation and catalytic properties, Langmuir 18, 7428—7435 (2002). 

C. Morterra, G. Cerrato, andF. Pinna, Preparation and characterization of sulfated zirconia catalysts obtained via various procedures, Appl. Catal. A 176(1), 27 3 (1999). 

The sulfate promoted transition metal oxides focussed considerable attention in recent years due to attractive catalytic properties. Most of the research carried out to date centered on sulfated zirconias,1 5 not surprisingly perhaps, as they exhibit the highest surface acidity (Ho <-16.04) among the members of this family of materials and appear to be able to initiate isomerization reactions in temperatures as low as 298 K. Far less interest attracted sulfated porous titanias, mainly owing to a lower surface acidity,6 although it may be a useful property in many catalytic situations. Thus closer inspection of the preparation procedures for sulfated titanias may be of interest, in particular as the reports on preparation and properties of these materials are scarce and we are not familiar with any work dealing with titania-sulfate aerogels. 

M. Signoretto, F. Pinna, G. Strukul, P. Chies, G. Cerrato, S. Di Ciero, and C. Morterra, Platinum - Promoted and Unpromoted Sulfated Zirconia Catalysts Prepared by a One - Step Aerogel Procedure, J. Catal. 167, 522-532 (1997). 

Both aluminum oxide and zirconium oxide are catalytically interesting materials. Pure zirconium oxide is a weak acid catalyst and to increase its acid strength and thermal stability it is usually modified with anions such as phosphates. In the context of mesoporous zirconia prepared from zirconium sulfate using the S+X I+ synthesis route it was found that by ion exchanging sulfate counter-anions in the product with phosphates, thermally stable microporous zirconium oxo-phosphates could be obtained [30-32]. Thermally stable mesoporous zirconium phosphate, zirconium oxo-phosphate and sulfate were synthesized in a similar way [33, 34], The often-encountered thermal instability of transition metal oxide mesoporous materials was circumvented in these studies by delayed crystallization caused by the presence of phosphate or sulfate anions. 

Sulfated zirconias and sulfated titanias are interesting solids that were first reported to be superacids. The acidity of such samples has been determined by NH3 adsorption calorimetry [47, 53-56]. Depending on the preparation method, the quantity of sulfate ions, calcination temperature and hydroxylation degree, the initial heats of adsorption of NH3 observed can vary from -120 up to 200kJ moT, but most of these materials display heats of adsorption close to those of H-ZSM-5 zeolite. 

Among the superacids listed in Table 17.3, sulfated zirconia (S04/Zr02) has been most frequently investigated, modified, and applied to various reactions. This may be because the sulfated zirconia possesses strong acid sites, is relatively easy to prepare, and was found to be a superacid early in the history of solid superacid development. Sulfated zirconia is now commercially available and used as a catalyst for organic synthesis in industry. 

Several sulfated zirconias were prepared by changing the drying temperature of gel in the range 100-400 °C and the final calcination temperature in the range 475-700 °C. It was found that the drying temperatures exhibiting the highest activity for the butane conversion are not always fixed, for instance 200 °C for one and 300 °C for another. 

New approach to preparation and investigation of active sites in sulfated zirconia catalysts for skeletal isomerization of alkanes... 

We have recently suggested a new approach to the preparation of active sites in sulfated zirconia catalysts [5, 6]. In this case, the catalysts are prepared by deposition of sulfate ions on crystalline zirconium dioxide samples with highly defective structure. According to numerous reports, the monoclinic phase typical for ZrOa is not suitable for this purpose. We have shown that active materials could be obtained by impregnation of zirconia-based oxides with cubic crystalline structure. It should be noted that the cubic structure is not thermodynamically stable for pure zirconia at low temperatures. It can be stabilized by introducing different additives, in particular, alkaline-earth metal cations [7]. Recently, similar results have been obtained for ZrOa stabilized by Y2O3 [8]. 

The textural and catalytic properties of the most active Zr02-Ca0-S04 sample prepared by our technique are similar to those of a traditional sulfated zirconia catalyst... 

To assess whether Lewis acid sites are present on zirconium sulfate, we prepared water-free bulk zirconium sulfete. Furthermore, a water-free silica-supported zirconium sulfrite catalyst was prepared by deposition-precipitation of zirconia on silica and subsequent gas-phase reaction with SO3. The activity of these catalysts was compared with that of two conventionally prepared sul ted zirconia catalysts. 

The presence of catalytically active Lewis acid sites in sulfated zirconia catalysts is much debated [1-5]. The conventional preparation of sulfated zirconia catalysts involves reaction of freshly precipitated zirconium hydroxide with diluted sulfiiric acid or impregnation of zirconium hydroxide with sulfuric acid or ammonium sulfate [6,7]. The final solid acid catalyst results by calcination at a temperature of 723 to 873 K. Provided thermodynamic equilibrium has been reached, all water and free sulfuric acid should have evaporated upon calcination at 673 to 873 K and only chemically bonded sulfete groups remain [8]. Above 890 K, bulk anhydrous Zr(S04)2 decomposes [1]. When uptake of water by the calcined catalyst is prevented or after loading of the catalyst in the reactor physisorbed water is removed by thermal treatment, only Lewis acid sites are present. Since it is difficult either to prevent the uptake of water vapor or to remove adsorbed water completely, it is difficult to attribute the acid activity of sulfeted zirconia catalysts unambiguously to Lewis acid sites. 

The anhydrous bulk zirconium sulfate preparation did not display any activity in the trans-alkylation of benzene (1) and diethylbenzene (2) to ethylbenzene (3). At 473 K the silica-supported, gas-phase sulfated zirconia showed a very small activity, which rapidly dropped to a negligible level (Fig. 2). The conclusion is that Lewis acid sites are not active with sulfated zirconia catalysts. The low activity of the silica-supported catalyst is due to adsorption of some water leading to Bronsted acid sites. Desorption of water at 473 K leads to the decrease in activity with time. Pre-hydration of the supported catalyst brings about a slightly higher activity as apparent from Fig. 2 the activity drops again due to the loss of water. 

The anomalous behavior of the sulfated zirconia catalysts is due to the loss of sulfuric acid at elevated temperatures. The catalyst prepared fi-om calcined zirconia looses its sulfuric acid at 673 K and, consequently, is not active at this temperature. As a result, decreasing the temperature of the catalyst to 473 K does not restore the activity. Also the more highly porous catalyst prepared from zirconium hydroxide releases sulfuric acid, but in narrow pores some sulfuric acid is left. The loss of sulfuric acid at 673 K is obviously irreversible. When the catalyst prepared from zirconium hydroxide is, however, kept at 473 K, the transport of water out of the porous structure is thus low that a stable activity is exhibited. Pre-hydration of the bulk anhydrous zirconium sulfate does not provide an active catalyst. That no catalytic activity is induced in this case is due to the fact that bulk anhydrous zirconium sulfete readily reacts to a stable tetrahydrate, viz., Zr(S04)2.4H20 [1]. As a result the hydrolysis of the sulfate by water vapor is suppressed. 

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