Gas and liquid phase reactions on MCM-41/SZ catalysts

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Abstract

A series of sulphated zirconia (SZ) catalysts supported on MCM-41 were prepared by the liquid-crystal template method (LCT), using cetyltrimethylammonium bromide (CTABr) as template. The effect of metal oxides promotion (Al2O3, Fe2O3 and Ga2O3) on structural and textural property was studied. These materials were characterized by ion chromatography (for the determination of sulphates content), nitrogen-adsorption measurements, X-ray diffraction, FT-IR spectroscopy, TG–DSC analysis and TPR–MS measurements. The catalytic performance of the catalysts in both gas and liquid phase reactions was investigated. A correlation between morphological and chemical physical features of the systems and their catalytic activity is here proposed.

Introduction

There is a strong interest in replacing dangerous liquid acids, such as HF and H2SO4, (which are still used in the industrial acidic catalysis) by more environmentally friendly solid acids [1]. Sulphated zirconia (SZ) has been shown to be active in a number of reactions like, for instance, isomerization [2], cracking [3], alkylation [4], acylation [5]. Unfortunately, there is a limit for a commercial application of SZ mainly due to its short life, but it has been shown that the catalytic behaviour of SZ can be greatly improved by promotion with small amounts of transition metals. In recent years, SZ systems promoted with Al, Ga and Fe have been reported in the literature [6], [7], [8], [9], [10], [11]. The use of sulphated zirconia in acylation of aromatic compounds have been investigated by several authors [5], [12], [13], [14], [15]. Recently, Signoretto et al. have 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 [16] and on catalytic activity of the system. If SZ is synthesized by a conventional method it is very difficult to control their textural properties and large pores are formed with wider pore size distribution. Various ways can be followed to gain a catalyst that has narrow pore-size distribution and ordered pore structure. At this purpose and some researchers proposed to support sulphated transition metal oxides, such as SZ, on meso-porous materials [17].

A major breakthrough in the synthesis of well-defined meso-porous materials came about in 1992 with the development of supramolecular templating, which involved the use of molecular aggregates instead of individual molecules as the framework-directing agents. Supramolecular templating was first reported by Mobil researchers, who used surfactants to guide the formation of meso-structures from solubilized silicate precursors [17]. For example, hexagonally packed rod-shaped micellar templates were used to derive a meso-structured silicate–surfactant composite. Upon removal of surfactants by heat treatment, silicates with hexagonally packed cylindrical pores (designated MCM-41) were obtained with ultrahigh surface areas of 1000 m2/g, which were of interest for a number of applications: adsorption–desorption processes, ion exchange, catalysis [18], [19].

Recently SZ catalysts supported on MCM-41 [20] have been prepared successfully by several groups. Sun et al. [21], for example, have used the dispersion method to prepare ZrOCl2·8H2O supported in the meso-pores of MCM-41. After hydrolysis and calcination, as well as sulphation, the precursor was converted into MCM-41 supported SZ.

Chen et al. [22], instead, have synthetized SZ on MCM-41 systems by using an one-step incipient wetness impregnation method with zirconium sulphate as precursor.

Moreover, Wang et al. [23], [24] reported that SZ/MCM-41 systems, and SZ supported samples promoted with small amounts of Ga2O3 and Al2O3, show an improved catalytic activity in the n-butane and n-pentane isomerization reaction. Choudhary et al. [25] have studied catalysts of Ga2O3 and In2O3 supported on MCM-41, as well as on other inert micro-porous and meso-porous supports, concluding that these catalysts are highly active and reusable for the acylation of aromatic compounds. Moreover, the same authors [26] report that Ga- and In-modified Hβ zeolite catalysts are very active in benzylation and benzoylation of aromatic compounds. They concluded that the some red-ox properties of metal oxides, such as Ga2O3 or In2O3, are expected to play a significant role in activating both reactants in the benzylation or benzoylation process.

The purpose of this work is to prepare a solid acid catalyst characterized by high surface area and ordered pore structure active in gas and liquid phase reaction.

In order to obtain the aim we have supported SZ on MCM-41. To improve the catalytic activity we have promoted this system with Fe, Al, Ga.

The catalytic behaviour of the systems of interest has been investigated towards two different reactions: (i) the gas-phase isomerization of n-butane and (ii) the liquid-phase acylation of anisole with benzoic anhydride.

Section snippets

Synthesis of MCM-41

The MCM-41 meso-porous support was synthesized as follows: a given amount of cetyltrimethylammonium bromide (CTABr, Aldrich) was added to a NaOH aqueous solution. The mixture was stirred slowly at room temperature for 40 min. The resulting solution was combined with tetraethylorthosilicate (TEOS, Aldrich), as source of silica and the reaction mixture was stirred at room temperature for 3 h. The molar composition of the resultant gel is: 1: SiO2, 0.12: CTABr, 0.53: NaOH, 590: H2O. The gel was

Chemical and physical features

Some characterisation results are reported in Table 1.

The MCM-41 sample has a BET surface area larger than 1000 m2/g. Both BET surface area and pore volume of SZM catalyst 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. 1a: they are similar in shape and all belong to the type IV curves (BDDT classification [27]), as expected for meso-porous systems.

The corresponding pore-size

Conclusions

All samples show a narrow meso-pore size distribution and high surface area; the regular arrangement of meso-pores is preserved in all supported catalysts. The addition of metal oxides (Ga, Fe or Al) stabilizes the ZrO2 tetragonal phase and this is particularly evident for Ga- and Fe-promoted samples.

The addition of small amounts of Ga and Fe greatly improves the catalytic activity in both n-butane isomerization reaction and liquid-phase acylation reaction, whereas for the Al-promoted catalyst

Acknowledgements

This research was partly financed with funds from INSTM (Project PRISMA 2002) and from the Italian Ministry MIUR (Project FIRB 2001 RBAU01X7PT). The authors want to thank Prof. Giuseppe Cruciani (University of Ferrara) for supplying XRD data.

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