Elsevier

Journal of Catalysis

Volume 215, Issue 1, 1 April 2003, Pages 57-65
Journal of Catalysis

Structure–reactivity correlations in sulphated-zirconia catalysts for the isomerisation of α-pinene

https://doi.org/10.1016/S0021-9517(02)00150-1Get rights and content

Abstract

A range of mesoporous sulphated zirconias with tuneable structural and catalytic properties have been prepared by direct impregnation. The surface sulphate coverage can be readily varied, achieving a maximum value of ∼0.2 monolayers. High-temperature calcination induces the crystallisation of tetragonal zirconia while suppressing the monoclinic phase and enhances surface acidity. Superacid sites only appear above a critical threshold SO4 coverage of 0.08 mL (corresponding to 0.44 wt% total S). Sulphated zirconias show good activity towards α-pinene isomerisation of under mild conditions. Conversion correlates with the number Brønsted acid sites, while the selectivity towards mono- versus polycyclic products depends on the corresponding acid site strength; superacidity promotes limonene formation over camphene.

Introduction

The syntheses of many fine and speciality chemicals often rely on homogeneous mineral acids, bases, or metal salts, which are frequently used in stoichiometric amounts. Tightening legislation on the treatment and disposal of excessive toxic waste, produced during the separation and neutralisation of products from these reaction media, is driving industry to consider cleaner technologies, including the use of heterogeneous catalysis. Of particular concern are the wide range of liquid-phase industrial reactions which rely on the use of inorganic or mineral acids. While many of these processes are catalytic, some require stoichiometric amounts of acid (e.g., acylation using AlCl3). While zeolites are widely employed as solid acid catalysts in gas-phase chemistry, their application in liquid-phase organic synthesis is limited by their small pore sizes (<8 Å), which make them unsuitable for reactions involving bulky substrates. However, recent developments in materials chemistry have led to the discovery of the M41S family of mesoporous molecular sieves [1], [2] offering pore sizes in the range 20–100 Å and thus new avenues for liquid-phase solid acid catalysis.

In contrast to liquid acids, which possess well-defined acid properties, solid acids may contain a variety of acid sites. Generally they are categorised by their Brønsted and/or Lewis acidity, the strength and number of these sites, and the textural properties of the support (surface area and porosity) [3]. The synthesis of pure Brønsted and pure Lewis acid catalysts has attracted great academic interest, although the latter has proven more difficult because Brønsted acidity often arises from Lewis acid–base complexation [4]. In order to achieve high selectivity towards the desired products in a synthesis all these properties must be considered and if possible controlled. For example, acetal formation and hydrolysis reactions generally require medium-acid-strength sites, while electrophilic additions of alcohols or water to olefins, skeletal rearrangements, and esterification and alkylation reactions require strong acid sites. The importance of the type of acid site has likewise been shown for Friedel Crafts alkylation reactions, where Lewis acid sites are required for toluene alkylation by benzyl chloride, while Brønsted sites are preferred for reactions using benzyl alcohol [5].

There has been much interest in the use of sulphated metal oxides as strong solid acids, in particular sulphated zirconias, which have been suggested to possess superacidic properties [6]. While most research has focused on gas-phase transformations [7], [8] for applications in cracking and isomerisation reactions, their use has been recently extended to liquid phase processes [9], [10]. However, the nature of the active site(s) remains contentious, with both sol–gel and impregnation methods reported as effective preparation routes [11], [12]. Sulphated zirconias' superacid nature has also been questioned; Hino and Arata first reported their low-temperature activity towards n-butane isomerisation [13] and evaluated their acid strength using Hammett indicators [14], the application of which to solids has been latterly criticised. A wide range of alternative techniques including NMR [15], NH3 microcalorimetry [16], [17], and ESR analysis [18] have been employed to address this issue; however, there is still no consensus. The super/strong acid sites, which are the active sites of the catalyst, have also been attributed to either Lewis [19], [20], Brønsted [21], [22], or a combination of both acid sites [23], [24]. Alternative local zirconia geometries have also been postulated [7]. To our knowledge no systematic study of structure–reactivity relationships within these materials has been performed wherein acid strength has been correlated with SO4 content and concomitant selectivity in liquid-phase chemistry.

To address this issue, we report the effect of controlled sulphation of a mesoporous ZrO2 support on the structural, acidic, and catalytic properties of the resulting material. The rearrangement of α-pinene was selected to probe the effect of acid strength on catalytic performance [25]. This is a particularly interesting reaction as the product selectivity is known to vary with catalyst acid strength, with weak acids favouring camphene formation, while stronger acids result in monocyclic products [26], [27], as shown in Scheme 1. The only previous study of this system, by Ponzi et al. [28], [29], demonstrated that sulphated zirconias were active above 100 °C, producing mainly camphene. However, their catalysts were ill-defined and neither textural properties, surface sulphate loading, or acid strength were reported. Here we find that direct wet impregnation permits the preparation of mesoporous sulphated zirconias exhibiting tuneable surface acidity and concomitant selectivity control in the isomerisation of α-pinene under mild conditions.

Section snippets

Experimental

A series of sulphated zirconias were prepared by wet impregnation of 5 g zirconia (99%+, Engelhard) with 50 cm3 of sulphuric acid (0.01–2.5 M). The resulting slurry was filtered and dried at 80 °C overnight before calcination at 550 °C for 3 h. The calcination temperature was selected to maximise the degree of surface ordering while minimising sulphur loss, based on the thermal analysis results presented in this paper. Samples were subsequently stored in air prior to characterisation and

Results and discussion

The catalytic performance of sulphated zirconia hinges upon the successful incorporation of sulphoxy functionalities into the zirconia framework. Fig. 1 shows the total and surface sulphur content as a function of molarity of impregnating solution. It is evident that the total sulphur loading shows a rapid increase for low sulphuric acid concentrations (in line with the theoretical loading), demonstrating efficient S inclusion during the impregnation process. The surface sulphur levels were

Conclusions

Direct wet impregnation routes permit the synthesis of mesoporous sulphated zirconias with tuneable structural and catalytic properties. The molarity of H2SO4 impregnating solution controls the surface sulphate coverage, which attains a maximum of ∼0.2 ML SO4 for acid concentrations >2.5 M. Sulphation induces a concomitant crystallisation of amorphous zirconia to the tetragonal phase, while suppressing the monoclinic phase, and the formation of superacidic sites. The emergence of these

Acknowledgements

Financial support by the UK Engineering and Physical Sciences Research Council under Grant GR/M20877 and by BP-Amoco Chemicals is gratefully acknowledged. MAE thanks the University of Hull for financial assistance.

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