Zeolite Catalysts For Cleaner Technologies

Michel Guisnet

University of Poitiers, Poitiers, France


The name of zeolites, which originates from the Greek words zeo (to boil) and lithos (stone), was given some 250 years ago to a family of minerals (hydrated aluminosilicates) that exhibited intumescence when heated in a flame.1 However, the history of zeolites really began 60 years ago with the development of synthesis methods. Commercial applications in three main fields—ion exchange, adsorption, and catalysis—were rapidly developed, the corresponding processes being more environmentally friendly than their predecessors.

Thus, the largest use of zeolites (~70%) is as a water-softener substitute for sodium tripolyphosphate (STPP) in laundry detergents.2 Contrary to STPP, which disrupts the biological equilibrium of rivers and lakes by eutrophication, the NaA zeolite, which is now generally employed, has no negative effect on the environment. Furthermore, the high efficiency of zeolites for the ultimate drying of gases and liquids led to a large variety of applications; separation of isomers (e.g., n-butane/isobutane) through adsorption over zeolites was substituted for the classic methods of distillation and crystallization, which consume energy.

The highest market value for zeolites, however, is in the field of catalysis: zeolites are now involved as basis components of most of the catalysts used in the production of fuels and petrochemicals; moreover they are playing an increasing

Methods and Reagents for Green Chemistry: An Introduction, Edited by Pietro Tundo, Alvise Perosa, and Fulvio Zecchini

Copyright © 2007 John Wiley & Sons, Inc.

role in the synthesis of intermediate and fine chemicals, as well as in pollution abatement.3 This is due to three main factors:

1. The molecular-size pore system of zeolites, in which the catalytic reactions occur: zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages, or channel intersections). The result is that the rate, selectivity, and stability of all the zeolite catalyzed reactions are affected by the shape and size of the nano reactors and of their apertures. This effect has two main origins: (i) spatial constraints on the diffusion of reactant or product molecules or on the formation of intermediates or transition states that limit the formation of undesired products; (ii) confinement within the micropores of reactant molecules, with a positive effect on the rates of the reaction, especially the bimolecular ones, but also of product molecules with sometimes autoinhibition of the desired reaction. These characteristics of zeolite catalyzed reactions are generally grouped under the label shape selectivity, which was proposed 45 years ago by Weisz and Frilette.4 This name, which considers only the shape of the micropores, but not their size and that of their apertures, as well as only the selectivity, although both reaction rate and stability are also significantly affected, could seem too restrictive. However, it has the great advantage of being simple and striking. The concept of shape selectivity, which was at the origin of the design and development of many processes, remains an important source for conceiving new processes and improving existing ones. This is clearly demonstrated by the major breakthrough in the field established by the discovery that the shape selective properties of zeolites were not limited to their inner micropores, but could also originate from the external surface of their crystallites. This discovery that was made possible by the development of methods for the synthesis of very small crystallites (^20-50 mm instead of 0.3-1 mm for conventional zeolites), hence with a large external surface, has already led to commercial applications.

2. The rich variety of active sites that can be present in zeolites: (i) protonic acidic sites, which catalyze acid reactions; (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis); (iii) basic sites; (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosilicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes.

3. The large number of different zeolites that have been synthesized (more than 130), as well as the variety of well-mastered postsynthesis treatments that have been developed for tailoring their composition, porosity and acidity. Other characteristics of zeolites—their easy handling (no corrosivity even for the most acidic ones, etc.) and setup in continuous processes, their high thermal stability, which allows their use under a wide range of operating conditions, as well as their regeneration by coke oxidation, etc., have also played a significant role in the development of zeolite-catalyzed processes.

After a short description of the main features of zeolites, the significant contribution of zeolite catalysts in green chemistry will be shown in examples of commercial or the potential processes of refining, petrochemicals, and fine chemicals involving acid or metal acid bifunctional catalysts.

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Solar Panel Basics

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