WO3/ZrO2 catalysts by sol–gel processing

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Abstract

Sol–gel preparation of ZrO2–WO3 is reported. The sol–gel technique allows preparation to be simplified and yields a more reliable and reproducible product. Catalysts were characterized by X-ray, BET and were tested in the isomerization of n-butane and 1-butene. Catalysts prove to be active with respect to analogous samples prepared by traditional methods.

Introduction

Metal oxides have many applications in chemical and petrochemical processing both as supports for catalytically active materials and as catalysts themselves. Mixed oxides, with two or more components, are also of significant scientific and commercial interest. Incorporation of a minor oxide component can enhance the thermal stability of a major component, allowing use at higher operating temperatures [1]. They can limit deterioration of textural properties, most notably surface area and pore volume, with thermal treatment [2]. The catalytic activities of mixed oxides have also been reported to be superior to those of individual components [3]. For example, it is known that a high surface acidity often develops when different oxides are combined 4, 5, 6. Such materials, especially silica–alumina compounds, have been employed as industrial solid acid catalysts. The relative amounts of each oxide component and mixing generally affect the textural properties and the catalytic performance of the mixed oxides. Conventional mixed oxide preparation techniques do not usually produce homogeneous, high-surface-area materials. In contrast, sol–gel synthesis of mixed oxide aerogels allows control of nanoscale homogeneity, and stabilizes high surface areas and pore volumes.

In previous works we have synthesized zirconia-sulfate and platinum promoted zirconia sulfate catalysts, using the sol–gel method 7, 8. We applied the sol–gel method to synthesis of aerogel Pt/ZrO2–SO4 catalysts, because this allows to reduce the number of preparation steps providing a more reliable and reproducible synthetic method, while maintaining catalytic activity in the samples. We report the single step preparation of WO3/ZrO2 catalysts by sol–gel technique and their characterization and reactivity.

Section snippets

Experimental

Zirconia tungsten oxide samples were prepared by mixing i-PrOH (60 ml), Zr(OC3H7)4 solution (18.4 ml), W(OC3H7)6 solution (40 ml), HNO3 (68%, 0.6 ml). To this solution, a mixture of i-PrOH (30 ml) and H2O (4.7 ml) was slowly added dropwise with stirring, and the mixture was vigorously stirred for 15 min. Gelation occurred on standing at room temperature overnight. Three solvent extraction procedures were applied to different gel batches. The prescript X is used to indicate the xerogel samples,

Results

A summary of the analytical and morphological properties of the samples is reported in Table 1. It is noteworthy that the surface area of all WO3/ZrO2 samples is larger than that of the AZ sample which contains no tungsten. Samples prepared by aerogel, show higher surface areas than carbogels and xerogels. In the case of the xerogel sample, prepared by conventional drying, various detrimental effects occur, causing differential microscopic and macroscopic shrinkage. Thus, the drying stress

Discussion

In general, the ZWO samples show higher surface area than the ZW and ZWI samples. As in the first case both precursors are metal alkoxides, it is possible that the reactivities of zirconium and tungsten precursors are better matched. Indeed, gel formation from the individual alkoxides, when reacted separately, seem to occur rapidly in both cases. This would allow tungsten to be incorporated homogeneously into the growing zirconia network without altering zirconia normal gelling patterns [17].

Conclusions

Preparation influences structure, texture and catalytic behaviour of catalysts. Supercritical drying yields an oxide network which is resistant to sintering and crystallization upon thermal treatment.

The highest values of selectivity found for aerogel samples for the reaction at 400°C may indicate that these samples have stronger acid sites than the other samples. This sites are those necessary for skeletal isomerization.

The highest values of conversion found for aerogel samples for the

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