Mesoporous sulphated zirconia by liquid-crystal templating method

https://doi.org/10.1016/j.micromeso.2005.11.004Get rights and content

Abstract

A series of sulphated zirconia (SZ) samples were prepared by the liquid-crystal templating (LCT) method, using linear alcohols with different alkyl chain length as co-solvents and neutral ammine as template. The type and duration of aging was also varied in order to study their effects on the catalytic activity and texture of the synthesized catalysts. These materials were characterized by ion chromatography for determination of sulphate content, nitrogen-adsorption measurements, X-ray diffraction, and transmission electron microscopy. The results showed that the sulphated zirconia catalysts were mesoporous and that their surfaces area and pore size distribution varied as a function of the synthesis conditions. In particular, we found a direct correlation between alkyl chain length of co-solvent and the pore size. The catalytic activity was tested in the toluene and anisole acylation and it was found to increase with the pore size.

Introduction

The synthesis of aromatic ketones constitutes a very important route to the preparation of a wide variety of fine chemicals. This is usually accomplished through Friedel–Craft acylation of aromatic substrates like phenol, anisole and many others [1], [2]. Traditional catalysts are inorganic acids such as sulphuric acid, polyphosphoric acid or metal halides like AlCl3, BF3. These are used in at least stoichiometric amounts to achieve satisfactory reaction rates and yields. Work-up procedures result in losses of the catalyst and acidic wastes. There is considerable interest in developing clean technologies, minimizing economical and environmental problems. The replacement of the acids mentioned above by heterogeneous catalysts is a research topic of high interest. Sulphated zirconia (SZ) may catalyse reactions of industrial importance. It has already been applied to the acylation of aromatics with acylation agents of high reactivity like benzoyl chloride and to acylations of aromatic ethers with carboxylic anhydrides [3](a), [3](b), avoiding the formation of the undesirable highly corrosive by-product HCl.

Conventional preparations of sulphated zirconia lead to microporous material suitable for reactions of small molecules in the vapour phase. If SZ from conventional synthesis is calcined at high temperatures, the micropores collapse and larger pores are formed with wider pore-size distributions. To form a catalyst that has shape selectivity for larger molecules, in the same way that zeolites do for smaller molecules, narrow pore-size distributions need to be created.

Since the discovery of MCM-41 materials in the early 1990s by researchers at Mobil, a great deal of work has been devoted to the use of liquid-crystal templating (LCT) method in the formation of mesoporous metal oxides [4]. Many researchers have attempted to extend the LCT technique to other metal oxides such as tungsten, iron, zirconium in order to make a strongly acidic mesoporous material [5], [6], [7], [8].

Tanev and Pinnavaia first used the neutral templating method in the synthesis of hexagonal mesoporous silica [9]. This approach was based on hydrogen bonding and self-assembly between the neutral primary amine micelles and neutral inorganic precursors. Sachtler et al. were applied this method to ZrO2 with success. The material formed by Sachtler and co-workers had a narrow pore-size distribution centred around 3.6 nm and surface area of 95 m2/g [8]. McIntosh and Kydd have reported the neutral templating method to make mesoporous sulphated zirconia. They have studied various synthesis parameters, such as alkyl chain length of the surfactant, and concentration of the surfactant and co-solvent to determine how they affect the pore size of the materials [7].

Several authors have found that when attempting synthesis with surfactant assisted method, structure collapsed upon attempts to remove the surfactant either by solvent extraction or calcination. Rish and Wolf [10] attempting modifications of the synthesis reported by Knowles and Hudson [11] observed that a high surface area mesoporous zirconia could be formed in the absence of surfactant by refluxing a solution of zirconyl chloride and ammonium hydroxide at high pH. Recently Melada et al. [12] reported the effect of pH precipitation and conditions of aging in the surface area and porosity of sulphated zirconia and the effect of these in the isomerisation and acylation reactions. By coupling alcohothermal and supercritical fluid drying techniques, Yong Cao et al. have recently demonstrated that it is possible to prepare thermally stable ultrafine zirconia powder with a high surface area and well developed textural mesoporosity with a narrow pore size distribution [13].

Moreover, the review of the literature shows that still there seems to be a significant lack in knowledge on relations between catalytic and structural properties of sulphated zirconia.

In this work, the neutral templating method was used to make mesoporous sulphated zirconia. Various synthesis parameters, such as alkyl chain length of the co-solvent, temperature and aging times were investigated to determine how they affected on the pore size of the material and activity.

Section snippets

Catalysts synthesis

Mesoporous SZ samples were synthesized by sol–gel techniques, starting from Zr(O–Pr)4 and acetylacetone using water as solvent and a neutral amine (hexadecylamine) as template (C16), following the method of Sachtler and co-workers [8]. The co-solvents we used were linear alcohols (ethanol, 1-butanol, 1-hexanol, 1-octanol, 1-decanol). A synthesis using 0.010 mol of the C16 template in the reaction mixture is described. Hexadecylamine (2.42 g, 0.010 mol) was dissolved in a mixture of 70 ml of

Results and discussion

The textural properties of samples prepared with a same type of surfactant but with a co-solvent with a different chain lengths are given in Table 1. From these data, it emerges that the modification of the chain length had, as a main effect, the modification of the pore size distribution and the increase of the surface area. As the alkyl chain length of the linear alcohol co-solvent increased from 2 to 6 carbon atoms (by two-carbon atoms increment), the surface area of the SZ materials varied,

Conclusions

We found that the porosity of sulphated zirconia can be tailored by means of a relatively simple synthetic route. There is a correlation between alkyl chain length of co-solvent, pore size dimension and activity.

The acylation activity turned out to be appreciably dependent at RT and in on the pore size distribution of the catalyst. The catalyst could be regenerated with a simple treatment. On the whole, SZ proved to be a versatile heterogeneous catalyst in aromatic acylation and a potential

Acknowledgment

This research was financed with funds from INSTM (Project PRISMA 2003).

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