Catalysis zeolite functionality

As can be seen, the catalytic process over a zeolite-supported cation, or an oxide-supported cation, can be considered as a supported homogeneous catalysis, as far as adsorbed reactants and products behave like reactive ligands. The model developed for lean DcNO. catalysts over supported cations (function 3), as well as this supported homogeneous catalysis approach, is also suitable for stoichiometric mixture (TWC) comprising CO and H2 as reductants over supported transition metal cations [20-22],... 

In this communication a study of the catalytic behavior of the immobilized Rhizomucor miehei lipase in the transesterification reaction to biodiesel production has been reported. The main drawbacks associated to the current biodiesel production by basic homogeneous catalysis could be overcome by using immobilized lipases. Immobilization by adsorption and entrapment have been used as methods to prepare the heterogeneous biocatalyst. Zeolites and related materials have been used as inorganic lipase supports. To promote the enzyme adsorption, the surface of the supports have been functionalized by synthesis procedures or by post-treatments. While, the enzyme entrapping procedure has been carried out by sol-gel method in order to obtain the biocatalyst protected by a mesoporous matrix and to reduce its leaching after several catalytic uses. 

Although beyond the scope of this book, a vast amount of work has been directed to supporting homogeneous catalysts on solid supports including silica, alumina and zeolites, and functionalized dendrimers and polymers [19]. These give rise to so-called solid-liquid biphasic catalysis and in cases where the substrate and product are both liquids or gases then co-solvents are not always required. In many ways solvent-free synthesis represents the ideal method but currently solvent-free methods can only be applied to a limited number of reactions [20],... 

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

Hafner, J., Benco, L., and Bucko, T. (2006) Add-based catalysis in zeolites investigated by density-functional methods. Top. Catal, 37 (1), 41-54. 

Boronat, M., Virrruela, M., and Corma, A. (2004) Reaction intermediates in acid catalysis by zeolites prediction of the relative tendency to form alkoxides or carbocations as a function of hydrocarbon nature and active site structure. 

One of the most studied aspects of catalysis science is the relationship between structure and function. Some general themes are weU estabUshed by now, but specific connections between catalyst characteristics and performance attributes remain elusive in most cases. The crystalline geometry of zeolites makes them relatively more amenable to study by a variety of powerful modern characterization tools, but there remain many key unanswered questions in the catalytic application... 

This is the first book to offer a practical overview of zeolites and their commercial applications. Each chapter is written by a globally recognized and acclaimed leader in the field. The book is organized into three parts. The first part discusses the history and chemistry of zeolites, the second part focuses on separation processes and the third part explores zeolites in the field of catalysis. AH three parts are tied together by their focus on the unique properties of zeolites that allow them to function in different capabilities as an adsorbent, a membrane and a catalyst. Each of the chapters also discusses the impact of zeolites within the industry. 

Reglospeclflc functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [1-5]. Catalysis using zeolites is the most promising way to prepare sterlcally small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Sterlc restrictions by zeolites Increase the formation of preferred products and prevent the formation of undesirable products [6]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL In the alkylation of biphenyl. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

The three themes of the symposium selective hydrogenation, selective oxidation and acid-base catalysis were introduced by four plenary lectures and two invited communications. A panel concerned with the future of zeolites and other shape-selective materials for fine chemical synthesis was conducted by specialists in the field D. Barthomeuf (University of Paris 6), E. Derouane (University of Namur), L. Forni (University of Milan), M. Gubelmann (Rhone-Poulenc, St Fons), W. Hoelderich (BASF, Ludwigshafen) and G. Perot (University of Poitiers). An exhibition of equipment was held during the symposium on October 3 and 4. Over 20 firms exhibited equipment, chemicals and catalysts which were of interest to researchers involved with the synthesis of functional compounds by heterogeneous catalysis. 

Endowing these polymolecular entities with recognition units and reactive functional groups may lead to systems performing molecular recognition or supramolecular catalysis on external or internal surfaces of organic (molecular layers, membranes, vesicles, polymers, etc.) [7.1-7.13, A.41] or inorganic (zeolites, clays, sol-gel preparations, etc.) [7.14-7.20] materials. 

The catalysis by protonated zeolites, used in industrial cracking, isomerization and alkylation of hydrocarbons, involves proton transfer and formation of carbenium or car-bonium ions as reactive intermediates488,489. To understand the function of the zeolite, the reactions between CD4 and acidic hydrogens of OH groups of two zeolite samples have been studied recently490. 

Figure 3.6 Yields, X (%) of phenyl acetate, PA ( ), o-hydroxyacetophenone, o-HAP (o) and p-hydroxyacetophenone, p-HAP ( ) as a function of the conversion of phenol, Xp (%), in an equimolar mixture with acetic acid over HMFI at 553 K. Reprinted from Journal of Molecular Catalysis, Vol. 93, Neves et al., Acylation of phenol with acetic acid over a HZSM-5 zeolite, reaction scheme, pp. 169-179, Copyright (1994), with permission from Elsevier...

Of particular interest for shape-selective catalysis is the modification of the zeolite by means of cation exchange as well as the modification of the inner and/or outer crystallite structure by a treatment with chemically reacting agents which leads to a deposition of additional functional groups or compounds. This can be done either in the gas phase (i.c. chemical vapor deposition (CVD)) or in the liquid phase. 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

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