Review
Ordered mesoporous materials in catalysis

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Abstract

Ordered mesoporous catalysts could open the door for new catalytic processes, based partly on novel principles, owing to their hitherto unprecedented intrinsic features. For the preparation of ordered mesoporous catalysts, many strategies have been described. These strategies and the essential properties of the resulting materials are described in the first part of this review. Catalytic processes over such mesoporous materials, especially such reactions where the specific features of ordered mesoporous catalysts are exploited, are described in the second part.

Introduction

Porous materials have been intensively studied with regard to technical applications as catalysts and catalyst supports. According to the IUPAC definition, porous materials are divided into three classes; microporous (pore size < 2 nm), mesoporous (2–50 nm), and macroporous (>50 nm) materials [1]. In addition, also the term “nanoporous” is increasingly being used. However, it is not clearly defined and loosely refers to pores in the nanometer size range. Many kinds of porous materials such as (pillared) clays, anodic alumina, carbon nanotubes and related porous carbons and so on, have been extensively described in the literature [2]. Among the family of microporous materials, the best known members are zeolites which have a narrow and uniform micropore size distribution due to their crystallographically defined pore system.

In recent years, environmental and economic considerations have raised strong interest to redesign commercially important processes so that the use of harmful substances and the generation of toxic waste could be avoided. In this respect, there is no doubt that heterogeneous catalysis can play a key role in the development of environmentally benign processes in petroleum chemistry and in the production of chemicals, for instance by substitution of liquid acid catalysts by solid materials. Especially zeolites have attracted strong attention as such acids, but also as base and redox catalysts. However, zeolites present severe limitations when large reactant molecules are involved, especially in liquid-phase systems as is frequently the case in the synthesis of fine chemicals, due to the fact that mass transfer limitations are very severe for microporous solids. Attempts to improve the diffusion of reactants to the catalytic sites have so far focused on increasing the zeolite pore sizes [3], on decreasing zeolite crystal size [4], or on providing an additional mesopore system within the microporous crystals [5], [6]. An important line of research has focused on the enlargement of the pore sizes into the mesopore range, allowing larger molecules to enter the pore system, to be processed there and to leave the pore system again.

The first synthesis of an ordered mesoporous material was described in the patent literature in 1969. However, due to a lack of analysis, the remarkable features of this product were not recognized [7], [8]. In 1992, a similar material was obtained by scientist in Mobil Oil Corporation who discovered the remarkable features of this novel type of silica and opened up a whole field of research [9]. MCM-41, which stands for Mobil Composition of Matter No. 41, shows a highly ordered hexagonal array of unidimensional pores with a very narrow pore size distribution [10], [11]. The walls, however, very much resemble amorphous silica. Other related phases such as MCM-48 and MCM-50, which have a cubic and lamellar mesostructure, respectively, were reported in these early publications as well. At approximately the same time, an alternative, but less versatile approach to mesoporous materials was described by Yanagisawa et al. [12]. Kanemite, a layered silicate, serves as a silica source, the pathway leading to the ordered mesoporous material is thought to proceed via surfactant intercalation into the silicate sheets, warping of the sheets and transformation to the hexagonally packed material. Modifying and optimizing the reaction conditions yielded highly ordered mesoporous silicates and aluminosilicates as well [13], [14]. The obtained materials are designated as FSM-n, Folded Sheet mesoporous Materials-n, here n is the number of carbon atoms in the surfactant alkylchain used to synthesize the material. Since these early discoveries a large research effort has been invested in the synthesis and characterization of a variety of different, although related materials.

Many reviews have been published covering various aspects of ordered mesoporous materials, such as their synthesis, surface modification, application as host materials, and in catalysis [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. In this review, we will first describe the general methods for the preparation of ordered mesoporous solids, then the pathways used to introduce catalytic functions, and finally highlight their applications in catalysis, with a focus on such applications which exploit the special features of ordered mesoporous materials. The treatment will cover primarily the more recent literature, because surveys of older publications can be found in previous reviews on the catalytic behavior [15], [28], [29]. It will not be attempted to cover the patent literature, because it is almost impossible to follow this extensive body of work and—more importantly—although many valuable findings are reported in the patents, it is generally not easy to assess the validity of the claims. It should, however, be mentioned, that the patents are covered on a rather broad basis in the early review by Corma [15].

Section snippets

Synthesis of ordered mesoporous materials

The discovery of ordered mesoporous solids of the MCM-41 type and related materials in the early 1990s has been a breakthrough in materials engineering, and since then there has been impressive progress in the development of many new mesoporous solids based on templating mechanisms related to the one used for the original MCM-41 synthesis. Depending on the synthesis conditions, the silica source or the type of surfactant used, many other mesoporous materials can be synthesized following the

Control of local environment and morphology

One of the interesting features of ordered mesoporous solids for catalysis is the multitude of possibilities to modify them. The modifications can be used to adjust surface functionality, to incorporate catalytic functions or to change textural properties. These methods, with a focus on such modifications which are useful for the application of the materials as catalysts, will be discussed in the following.

Recently, the thermal, hydrothermal, and mechanical stabilities of various mesoporous

Preparation of catalysts—strategies and properties

Ordered mesoporous silicas are not often used as catalysts as such. Much more frequently, additional catalytic functions are introduced, by incorporation of active sites in the silica walls or by deposition of active species on the inner surface of the material. The advantage of using ordered mesoporous solids in catalysis are the relatively large pores which facilitate mass transfer and the very high surface area which allows a high concentration of active sites per mass of material. There are

Highly dispersed active sites — as an extension of microporous zeolite catalysts

Possibly the greatest hope in the initial phase of research on ordered mesoporous materials was their expected use in FCC catalysis. In this respect, substantial efforts have focused on the potential activity of Al-MCM-41 in processing bulky hydrocarbon molecules. However, their relatively low hydrothermal stability and the relatively low acid strength limit their suitability in this field. In other reactions that require milder acidity and also involve bulky reactants and products, such as

Conclusion

There are many strategies for the design and the preparation of mesoporous catalysts. The narrow and uniform pore size of mesoporous materials with extremely high surface area holds much promise for the development of novel solid catalysts. Substitution of elements in the framework, impregnation of active components, and immobilization of active species with pre-determined structure can create well-isolated sites with uniform properties. Varying wall components, as well as surface coating or

Acknowledgment

We would like to thank for the generous continuous funding of the DFG and the FCI over the years we have worked on ordered mesoporous materials, in addition to the basic funding provided by the MPI für Kohlenforschung.

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