Ga-promoted sulfated zirconia systems. II. Surface features and catalytic activity

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Abstract

Sulfated zirconia samples having a variable Ga2O3 content (in the 1–15% molar range) were synthesized. The promoting effect of gallium was studied in the catalytic isomerization of n-butane at 523 K, by feeding n-butane and H2 (with a 1:4 ratio). Catalytic activity was found to be greatly dependent on gallium loading. Catalysts containing 3–5 mol% Ga2O3 doubled the activity of sulfated zirconia, whereas for a sample with 15 mol% Ga2O3 the catalytic activity was completely lost. Surface chemistry of these materials was studied by means of FTIR spectroscopy and adsorption microcalorimetry, using selected probe molecules (CO and 2,6-dimethylpyridine). IR spectroscopy showed that gallium-containing sulfated zirconia samples exhibit both Lewis and Brønsted acidity. Lewis acidity is attributed to coordinatively unsaturated Zr4+ ions located in defective surface sites, whereas Brønsted acidity is associated to surface sulfate groups. Samples with a Ga2O3 content between 1 and 9 mol% show a combination of Lewis and Brønsted acidity that quantitatively decreases with increasing gallium oxide content. Samples having a Ga2O3 content equal to 9 mol% or greater show the following specific features: (i) these samples are much more difficult to dehydrate than those with smaller gallium content, and the hydroxy groups interact by hydrogen bonding, (ii) the sulfate groups progressively lose their covalent character, and (iii) both Lewis and Brønsted acidity of the samples decreases drastically. Couples of Lewis and Brønsted acid sites appear to be needed for catalytic activity in n-butane isomerization, and they present an optimum ratio when the catalyst is brought to a medium-high dehydration degree and when its Ga2O3 content is of about 3–5 mol%.

Introduction

Catalytic systems based on sulfated zirconia (SZ) are of high interest in the petrochemical industry, since these catalysts play an important role in the isomerization of light linear alkanes, such as n-butane [1], n-pentane and n-hexane [2]. Catalysts currently employed in the above mentioned reactions consist of Pt supported on chlorinated Al2O3 or similar systems, such as Pt supported on mordenite [3]. However, some of these systems operate at an elevated temperature and require constant addition of alkyl chlorides to recover acid functionalities. Owing to increasingly strict environmental regulations, researchers are paying increasing attention to solid acids in the search for stable and environmentally friendly catalysts. Among solid acids, SZ systems attract considerable interest because they are almost environmentally benign, highly active, and quite selective for the transformation of hydrocarbons. In order to overcome the fast deactivation affecting plain SZ systems, the addition of small amounts of Pt was tested [4], and was found to be of great help in enhancing both activity and stability of SZ. Successively, in order to avoid the use of Pt for economical reasons, promotion with other elements, such as Fe [5], Ni [6] and, more recently, either Al [7], [8] or Ga [2], [8](a), [9] was investigated.

Aims of the present contribution are: (i) to investigate the physico-chemical aspects of the introduction of Ga2O3 as a promoting agent in SZ systems; (ii) to find out the range of Ga2O3 concentration that leads to the most effective promotion role; and (iii) to see whether a correlation can be found between surface acidity features and promoting effects.

Section snippets

Catalyst preparation

Ga-promoted SZ systems, termed SZGn (where n stands for the mol% Ga2O3) were prepared by a co-precipitation method, starting from solutions of ZrOCl2 · 8H2O (Alfa Aesar, 99.9%) and Ga(NO3)3 · xH2O (Alfa Aesar, 99.9%) mixed in the appropriate proportion to obtain nominal n values of 1%, 3%, 5%, 9%, and 15%, after precipitation with an ammonia solution at pH 8 (±0.1), as described in detail elsewhere [9b]. The precipitate was dried at 383 K for 20 h and then sulfated by incipient wetness impregnation

Catalytic activity

The effect of gallium content on the catalytic activity for n-butane isomerization at 523 K was investigated for all samples. The profile of activity vs. time on stream is shown in Fig. 1. After an initial decrease, the activity stabilizes and no further deactivation is observed within 20 h. Steady-state activity was found to be dependent on gallium loading: a promoting effect is clearly evident for Ga2O3 loadings between 1 and 9 mol%. SZGn samples with n = 3 or 5 showed the best conversion output,

Conclusions

Sulfated zirconia samples having a variable Ga2O3 content (from 1 to 15% molar) were synthesized, with the aim of studying the promoting effect of gallium in the isomerization of n-butane. FTIR spectroscopy was used to study surface functionalities (OH and sulfate groups) of the samples; the adsorption of carbon monoxide and 2,6-dimethylpyridine on gallium promoted sulfated zirconia materials showed that these catalysts combined Brønsted with Lewis acidity. Lewis acidity was attributed to

Acknowledgments

This research was partly financed with funds from MIUR (Project FIRB 2001, code RBAU01X7PT_001) and from INSTM Consortium (Project PRISMA 2002). M.R.D. thanks the Spanish Ministry of Education for a Ph.D. fellowship. Thanks are also due to Dr. C.L. Bianchi (University of Milan, Italy) for XPS measurements.

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      These results indicate that the existence of small hafnia content plays a primordial role in increasing the amount and strength of acid sites and this situation is different from the other researchers who report that sulfated zirconia and promoted sulfated zirconia with different transition metal usually contain intermediate acid sites only [23]. Several authors attribute the increase in the amount and strength of Brönsted acid sites to the concentration of sulfate and to stabilization of tetragonal phase [31,38–44]. Morterra et al. indicated that only Lewis acidity is present if the concentration of the SO42− groups corresponds to values lower than two sulfur atoms per nm2.

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    Part I: Ref. [9b].

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