Elsevier

Applied Catalysis A: General

Volume 299, 17 January 2006, Pages 137-144
Applied Catalysis A: General

Active and recyclable sulphated zirconia catalysts for the acylation of aromatic compounds

https://doi.org/10.1016/j.apcata.2005.10.019Get rights and content

Abstract

Mesoporous sulphated zirconia catalysts have been prepared by precipitation of Zr(OH)4 at constant basic pH and tested for the acylation of anisole with benzoic anhydride. All samples have been characterized by N2 physisorption, ion chromatography and thermal analysis coupled with MS analysis of evolved gases (EGA). The samples possessed high specific surface areas and mean pore size in the mesopore range. TG/DSC/EGA analysis allowed us to observe all phenomena related to the calcination process and to better understand the catalyst behavior. Observed phenomena are physisorbed water elimination, dehydroxylation, sulphating agent decomposition, phase transitions and sulphate species removal. Catalytic tests were carried out at different temperatures (30–50 °C) after a thermal treatment. The effects of the precipitation pH, calcination temperature and activation temperature on the catalytic performances were studied. Reuse of the catalyst was also appraised.

Introduction

Liquid-phase reactions, such as Friedel–Crafts acylation [1], are important unit processes for the preparation of many industrially valuable chemicals. Traditionally, these reactions have been conducted using stoichiometric amount of liquid Brønsted acids (such as H2SO4) or Lewis acids (such as AlCl3 or BF3). Nowadays, the restrictions imposed by the waste minimization laws and economic considerations drive to the development of a new catalytic technology. Modern processes are, in fact, based on solid acids. In particular, sulphated zirconia (SZ) has been shown to be active for a number of reactions [2], [3] including isomerization [4], [5], [6], [7], cracking [8], alkylation [9], [10], [11], [12] and also Friedel–Crafts acylation [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. This work deals with the preparation of sulphated zirconia catalysts which are active and selective towards the acylation of aromatic compounds. Thermal techniques coupled with the analysis of the evolved gas (EGA) helped us to understand the phenomena which occurred during the calcination process and to better explain the catalytic behavior of our samples.

Section snippets

Catalysts synthesis

Catalysts were prepared via a conventional precipitation method [24], [25], [26]. ZrOCl2·8H2O (99%, FLUKA) was dissolved in distilled water and added with a peristaltic pump under vigorous stirring to an ammonia (30%, AnalaR) solution. During the entire course of the precipitation, the pH value was kept constant at 8.0 ± 0.1 or 10.0 ± 0.1 by the continuous addition of a 5N ammonia solution. In both cases, after the complete addition of the salt solution, the hydroxide suspension was aged for 20 h at

Surface features

Physisorption measurements results are summarized in Table 1. All isotherms relative to calcined samples belong to type IV IUPAC classification, with hysteresis loop of type H2–H3. As it is well known, the specific surface area decreased as the calcination temperature increased and the mean pore size followed the opposite trend. This is because of the crystallite growth and pore wall thickening, as reported by other authors [30], [31]. It is instead interesting to notice that the total pore

Conclusions

High conversion and p-selectivity in the acylation of anisole with benzoic anhydride were achieved in the presence of SZ catalyst under mild conditions.

The catalytic activity is influenced by the calcination and the activation temperatures, the first affected the sulphate content and the surface features, whereas the latter influenced the extent of hydration that favours the anhydride hydrolysis. The precipitation pH had only a minor influence. Thermal analyses coupled with EGA allowed the

Acknowledgement

The authors would like to acknowledge Dr. G. Cerrato (University of Turin, Italy) for the DRIFT measurements.

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