Ga2O3-promoted sulfated zirconia systems: Morphological, structural and redox properties
Introduction
Acid catalysis is of fundamental industrial importance. Some metal oxides, when sulfated [1], develop the ability to catalyze at low temperatures reactions characteristic of very strong acid catalysts, although with limited lifetimes. Besides promoting a variety of acid-catalyzed reactions, sulfation has been also observed to affect some physical properties. For instance, surface sulfation of ZrO2 has been reported to lower crystallinity and to increase SSA [2], and the latter effect is certainly important for heterogeneously catalyzed processes in which reactions occur at the solid/fluid interface. In metal oxides that present polymorphism, sulfation has been observed to stabilize one form over the other(s). In the case of zirconium oxide, that can exist in monoclinic, tetragonal, cubic and orthorhombic crystal form depending on preparation method and conditions [3], sulfate is one of a range of dopants that have been observed to favor the tetragonal phase over the more commonly encountered monoclinic one [4]. The acidity, so far reported for several sulfated metal oxides, is thought to be highest in the case of SZ systems. While earlier studies indicated super-acidity as the primary source of SZ catalytic activity [5], [6], there is now growing evidence that no real super-acidity is actually involved and that a stable tetragonal phase is a necessary condition to obtain catalytically active samples [7]. Also the method of SZ preparation has been reported to play a vital role on the structural and reactivity features of the resulting systems [8]. Moreover, it has been shown that, when an adequate promoter is introduced during the preparation step, improved redox properties may arise and influence the catalytic behaviour of SZ systems. Gao et al. [9] reported that the addition of small amounts of Al2O3 to a SZ system leads to catalysts that are both more active than the corresponding unpromoted ones and very stable with reaction time, if the n-butane isomerization reaction is carried out at 250 °C in the presence of hydrogen. According to these authors, the remarkable activity and stability of Al-promoted SZ catalysts are due to a different distribution of acid sites strength and, in particular, to the enhanced population of acid sites with an intermediate strength. Canton et al. [10] reported the influence of different Al2O3 amounts on the ZrO2 structure, and pointed out that the optimum Al2O3 concentration in SZ catalysts lies in the 2.2–3.7 wt.% range. In particular, they showed that the catalytic system containing 2.2 wt.% Al2O3, that is the most active one, has the highest SO4 content and exhibits the largest ratio as well as the highest Lewis/Brønsted acid sites ratio. The cited authors concluded that the strongest catalytic activity could be due to these different factors acting together. Recently, some works reported the good promoting effect of Ga2O3 on the catalytic features of mixed oxides. For example, Poncelet and coworker [11] showed that Ga2O3-promoted SZ system exhibits an improved catalytic performance in the dehydrocyclization of n-hexane [12] or in the isomerization of n-butane, while Cao et al. [13] showed that similar Ga-promoted catalysts are highly active in the hydroisomerization of n-hexane. Also some characterization studies of Ga-containing mixed oxides have appeared in recent years, reporting data concerning the acidity features of Ga-doped mixed oxides [14], [15]. In the present contribution we report a study of the influence of Ga2O3 content (in the 0–15 mol% range) and of the calcination temperature on the structure and activity of SZ catalysts.
Section snippets
Samples preparation
Ga-promoted samples were prepared by a co-precipitation method. ZrOCl2 · 8H2O and the required amount of Ga(NO3)3 · nH2O, roughly corresponding to 1; 3; 5; 9; 15 mol% Ga2O3 (nGZ), were dissolved in distilled water and added dropwise under vigorous stirring to an ammonia solution. As Ga nitrate is very hygroscopic, the salt was kept in a dry glove-box until weighing. During precipitation, the pH value was kept constant at 8 by simultaneous addition of aqueous ammonia. This pH value was chosen because
Chemical and physical features
Table 1 shows some results of the physico-chemical characterization of Ga-promoted SZ and of some selected reference samples.
ICP measurements gave us the real Ga2O3 content of the samples, and this turned out to be in slight excess but not to differ much from the nominal content, in spite of the highly hygroscopic nature of the Ga precursor.
Sulfur loading ranges between 2.1 and 2.6SO4 groups/nm2 for the samples standard calcined at 650 °C, and between 1.3 and 2.1 for the samples calcined at 720
Conclusions
Sulfated zirconia (Z) promoted with 1–5 mol% Ga2O3 exhibits enhanced catalytic activity (with respect to plain Z catalysts), whereas Z systems with higher Ga2O3 loadings are characterized by a declining promotion effect. The improved catalytic behaviour of low-loading Ga2O3–ZrO2/SO4 systems (GZ) is thought to be due to several morphological and chemical factors, among which are important the enhanced redox properties.
The presence of the Ga2O3 promoter modifies also some other physical and/or
Acknowledgement
This research was partly financed with funds from MIUR (Project FIRB 2001, code RBAU01X7PT_001) and with funds from INSTM (Project PRISMA 2003).
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