Sulfated zirconia aerogels

The calcination temperature plays an important role in the stabilization of a particular chromium oxidation state. This state seems to be responsible of the improvement of the catalytic performances of sulfated zirconia aerogels doped with chromium (Figure 6.5) [45]. 

Figure 6.5. influence of chromium oxidation state on the catalytic activity of sulfated zirconia aerogels doped with chromium A. AZ873/Cr(III)-Al B. AZ873/Cr(V)-Al [45],... 

Recently, sulfated zirconia aerogels were also doped with cerium. In order to optimize the catalytic performances of these materials, prepared by the sol-gel method, Mejri et al. [48] studied the impact of the solvent evacuation mode on their properties. The xerogel solids (XZC) obtained by ordinary gel drying and calcined at different temperatures exhibit a very low surface areas. On the contrary, the aerogels (AZC) obtained by solvent evacuation under supercritical conditions show a more developed surface areas (Table 6.2). 

Signoretto M, Oliva L, Pinna F, Strukul G (2001) Synthesis of sulfated-zirconia aerogel Effect of the chemical modification of precursor on catalyst porosity. J Non-Cryst Solids 290 145-152... 

Bedilo A F, Klabunde K J (1998) Synthesis of catalytically active sulfated zirconia aerogels. J catal 176 448 58... 

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). 

The effect of the Zr/Si molar ratio is studied both for aerogels and xerogels, and a comparison allows to determine the effect of solvent evacuation procedure on the textural properties of silica supported sulfated zirconia samples. 

Also in this case, the sulfated zirconia-silica xerogels exhibit a decrease of the BET surface area compared to the original support with molar ratio Zr/Si=0, which agrees with the results obtained for the aerogels and with literature data [7,10,15]. 

Fig. 5 shows the diffraction patterns corresponding to aerogels of silica-supported sulfated zirconia with Zr/Si molar ratios of 0.05 and 0.1 and calcined at different temperatures. 

Comparative study of the sulfur loss in the xerogel and aerogel sulfated zirconia calcined at different temperatures effect on n-hexane isomerization... 

Table 2 shows the XPS measurements of the relative amount of sulfur present at the surface of sulfated zirconia heated at different temperatures. These data show that there is an impoverishment in sulfur both for the xerogel and the aerogel when the calcination temperature increases. However, for a given temperature of calcination, the amount of sulfiir detected always appears higher for the aerogel than for the xerogel, thus supporting the h) othesis that the sulfur loss is easier and more important for the latter, or in other words, that... 

Platinum-Promoted and Unpromoted Sulfated Zirconia Catalysts Prepared by a One-Step Aerogel Procedure 1. Physico-Chemical and Morphological Charaderization. Journal of Catalysis, Vol.165, No.2, 0anuary 1997), p>p. 172-183, ISSN 0021-9517... 

Table 6.2. Textural properties of aerogel (AZC)- and xerogel (XZC)-sulfated zirconia doped...

Table 6.3. Activity and selectivity of aerogel- and xerogeZ-sulfated zirconia doped with cerium in n-hexane isomerization reaction [48]...

Zirconia aerogels are known to possess several properties needed for applications as a catalyst or catalyst support. The most reported catalytic applications of zirconia aerogels are related to the modification of their surfaces by metals or ions, in particular sulfate, phosphate, and tungstate anions. 

Mortem C, Cerrato G, DiCiero S, Signoretto M, Pinna F, Strukul G (1997) Platinum-piomoted and unpromoted sulfated zirconia catalysts prepared by a one-step aerogel procedure 1. Physico-chemical and morphological characterization. J Catal 165 172 3... 

Mejri 1, Younes M K, Ghorbel A, Eloy P, Gaigneaux E M (2006) Comparative study of the sulfur loss in the xerogel and aerogel sulfated zirconia calcined at different temperatures effect on n-hexane ismnerizatimi. Stud Surf Sci Catal 162 953-960... 

Attenuation length A (optical) The distance traveled across in an aerogel, where the probability that a photon has not been absorbed has dropped by 1/e (where e = 2.71828) AZC Aerogel sulfated zirconia doped with cerium AZS Aerogel sulfated zirconia... 

AZSCr Aerogel sulfated zirconia doped with chromium... 

The presence of sulfate ions markedly affects the nanopore structure of titania-sulfate aerogels. In Ti02-S042 materials, unlike in zirconia-sulfate aerogels, the larger sulfate load stimulates formation of a more consolidated structure. The XRD analysis shows that even a crystalline phase (anatase) may be present in fresh, dry aerogels, which, perhaps, is the first observation of this phase in sol-gel titania obtained from the low temperature drying process. 

The ability to introduce several components into solution during the sol-gel step makes this approach especially attractive for the preparation of multicomponent oxides and bimetallic catalysts. Of course, the solution chemistry becomes more complex with additional components. But many of the concepts that we have discussed, such as matching relative precursor reactivity and changing the microstructure of the gel network, remain valid in principle. Furthermore, a promoter or an active species can be introduced the same way. There have been recent reports on the onc-step preparation of zirconia-sulfate aerogels [45] and Li/MgO catalysts [46], These samples arc active in the isomerization of n-butane and the oxidative coupling of methane, respectively. 

For the xerogels (spectra b, c), the main Si-O-Si band moves toward lower wavenumbers as the zirconia content of sulfated mbted oxide increases. These spectral changes are consistent with a weakening of the silica network by zirconium atoms [5,12]. This shift to lower frequencies is sometimes attributed to the formation of Zr-O-Si bonds [5]. For the aerogels (spectra d, e), the Si-O-Si band shifts to lower frequencies when a small amount... 

Mesoporous aerogels based rai sidfated zirconia doped with chromium have been synthesized by sol-gel method and dried in supercritical conditions of the solvent by Ghorbel et al. [45]. The characterization results revealed that Zr02 tetragonal phase is stabilized by both sulfate groups and metal at high temperature. The physisorption of N2 showed that the sol-gel procedure coupled to the drying under supercritical conditions of the... 

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