Elsevier

Applied Catalysis A: General

Volume 308, 10 July 2006, Pages 216-222
Applied Catalysis A: General

Acylation of veratrole over promoted SZ/MCM-41 catalysts: Influence of metal promotion

https://doi.org/10.1016/j.apcata.2006.04.039Get rights and content

Abstract

Acylation of veratrole with acetic anhydride over sulphated zirconia (SZ) supported on MCM-41 catalysts (SZ/MCM-41) was investigated. In order to study the effect of metal oxides promotion on catalytic activity of the systems, modified SZ/MCM-41 catalysts were synthesized by addition of small amount of Ga and Fe oxides. The catalysts were characterized by N2 adsorption, X-ray diffraction and TPO-MS analyses. Sulphate content was determined by ion exchange chromatography. The effect of the veratrole/acetic anhydride ratio and reaction temperature on the reaction activity were studied.

Introduction

Friedel–Crafts acylation of aromatic compounds is widely used in the synthesis of aromatic ketones, important intermediates for the production of pharmaceuticals, dyes, agrochemicals and fragrances. Acylation of anisole and veratrole (1,2-dimethoxy benzene) leads to the synthesis of p-acetoanisole and acetoveratrole, respectively, precursors of a sun protector (Parsol) and of a component in an insecticide formulation (Verbutin) [1]. Moreover, acetoveratrole is used for the synthesis of papaverine, an opium-alkaloid antispasmodic [2].

Aromatic ketones are conventionally prepared using Lewis (AlCl3, FeCl3, BF3, …) or Brønsted (HF, H2SO4, …) acids, added in more than a stoichiometric amount in order to achieve satisfactory ketone yields. These acids cannot be recovered and reused because of the formation of a stable complex with the ketone, destroyed only with acid water. In view of the increasing interest in clean processes, the use of these compounds is not suitable because of the formation of corrosive acid wastes in large amounts resulting from the destruction of Lewis acids by water and the formation of HCl as by-product [3], [4].

Solid acid catalysts have been found to be a good alternative to liquid acids because they are easily recoverable, reusable and non-toxic. The Rhodia company operates on an industrial scale a fixed bed process for acylation of anisole and veratrole with acetic anhydride using the H-form of zeolites Y and β [5]. The main drawback related to their use is the deactivation of the catalyst due to the adsorption of reaction products and heavy by-products on the surface and inside the pores [4]. Furthermore, zeolites with small pore size have a limited use in reaction involving large molecules.

Sulphated zirconia, SO42−/ZrO2 (SZ), has been shown to be active in a great number of gas and liquid phase reactions (isomerization of light paraffines, alkylation, acylation, esterification, etc.) [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Recently, Signoretto et al. studied the effect of preparative parameters on texture and on acylation activity of sulphated zirconia. In particular, they concluded that there is a close correlation between pore size dimension [15] and catalytic activity of the system. If SZ is synthesized by a conventional method it is very difficult to control its structural properties and large pores are formed with wider pore size distribution. Various ways can be followed to gain a catalysts that has narrow pore size distribution and ordered pore structure. At this purpose some researchers proposed to support sulphated transition metal oxides, such SZ, on mesoporous materials [21]. The use of ordered mesoporous materials like M41s, which have uniform mesopores and very high surface area, as a support for SO42−/ZrO2, should increase the catalytic capabilities of SZ system. MCM-41, with its hexagonal array, is the most widely studied among the members of M41s family because of its stability and ease in preparation. Several papers reported the synthesis of SZ supported on siliceous mesoporous materials, especially on MCM-41 [22], [23], [24], [25], [26], [27]. The MCM-41 system promoted with Al2O3, Ga2O3 and In2O3 [28], [29], [30], [31] was used to catalyse the liquid phase Friedel–Crafts acylation of aromatic compounds, but the SZ/MCM-41 catalysts have never been tested in these reactions. Recently, SZ/MCM-41 catalysts, also promoted with metal oxides (Ga2O3 and Fe2O3), have been synthesized in our laboratory. These systems showed activity in the liquid phase acylation reaction of anisole with benzoic anhydride; the promotion with Ga2O3 and Fe2O3 enhanced both the catalytic activity and the reusability of the catalysts, in particular for the Ga2O3 promoted sample [32].

In this work SZ/MCM-41 promoted with metal oxides (Ga2O3 and Fe2O3) was employed in the catalytic acylation of veratrole with acetic anhydride. Data are present in the literature, concerning the acylation of veratrole with zirconia-supported heteropoly acids [33] and tungsten oxide [34], heteropoly acid supported on hexagonal mesoporous silica [2], cation-exchanged clays [35] and zeolites H-Y and H-β [36], [37], [38]. However, to our knowledge, the acylation of veratrole with acetic anhydride over SZ or SZ/MCM-41 has never been reported before. The present paper is aimed to fill such a lack of information.

Section snippets

MCM-41 synthesis

MCM-41 was synthesized starting from tetraethy orthosilicate (TEOS) as source of silica and an alkaline aqueous solution of the template (cetyltrimethylammonium bromide, CTA-Br). CTA-Br (0.007 mol) was dissolved in 600 ml of deionised water previously basified with NaOH (0.03 mol). The mixture was stirred for about 30 min, in order to dissolve the template. TEOS (0.056 mol) was added to the solution by a peristaltic pump with a flow rate of 1 ml/min. Molar ratio of the reagents is 1 SiO2:0.12

Morphological and structural characterization

Characterization results are reported in Table 1.

The metal promotion showed almost no direct influence on the sulphate retention: only a low increment in the sulphate content was detected for GSZM catalyst.

The MCM-41 sample has a BET surface area larger than 1000 m2/g. Both BET surface area and pore volume of SZM catalysts are smaller than those of MCM-41, and a further decrease is observed for the promoted catalysts. N2 adsorption–desorption isotherms for all samples are reported in Fig. 1:

Conclusions

MCM-41/SZ catalysts were synthesized using a reproducible and easy method.

All samples were found to be active in acylation reaction of veratrole with acetic anhydride, with good conversions and yields. The effects of reaction parameters were studied in order to obtain the optimal conditions (T = 80 °C; molar ratio, veratrole:anhydride = 10:1). The modification of SZ/MCM-41 system with Ga and Fe oxides influenced the catalytic behaviour and their reusability. In fact, the best performance (higher

Acknowledgments

The financial supports from MURST (Project FIRB-2003) and INSTM (Prisma Project) are gratefully acknowledged. We wish to thank Dr. Giuseppina Cerrato for her kindness in doing the FTIR measurements.

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