It is possible to say that zeolites are the most widely used catalysts in industry They are crystalline microporous materials which have become extremely successful as catalysts for oil refining, petrochemistry, and organic synthesis in the production of fine and speciality chemicals, particularly when dealing with molecules having kinetic diameters below 10 A The reason for their success in catalysis is related to the following specific features of these materials:1 (1) They have very high surface area and adsorption capacity (2) The adsorption properties of the zeolites can be controlled, and they can be varied from hydrophobic to hydrophilic type materials (3) Active sites, such as acid sites for instance, can be generated in the framework and their strength and concentration can be tailored for a particular application (4) The sizes of their channels and cavities are in the range typical for many molecules of interest (5-12 A), and the strong electric fields2 existing in those micropores together with an electronic confinement of the guest molecules3 are responsible for a preactivation of the reactants (5) Their intricate channel structure allows the zeolites to present different types of shape selectivity, ie, product, reactant, and transition state, which can be used to direct a given catalytic reaction toward the desired product avoiding undesired side reactions (6) All of these properties of zeolites, which are of paramount importance in catalysis and make them attractive choices for the types of processes listed above, are ultimately dependent on the thermal and hydrothermal stability of these materials In the case of zeolites, they can be activated to produce very stable materials not just resistant to heat and steam but also to chemical attacks Avelino Corma Canos was born in Moncofar, Spain, in 1951 He studied chemistry at the Universidad de Valencia (1967−1973) and received his PhD at the Universidad Complutense de Madrid in 1976 He became director of the Instituto de Tecnologia Quimica (UPV-CSIC) at the Universidad Politecnica de Valencia in 1990 His current research field is zeolites as catalysts, covering aspects of synthesis, characterization and reactivity in acid−base and redox catalysis A Corma has written about 250 articles on these subjects in international journals, three books, and a number of reviews and book chapters He is a member of the Editorial Board of Zeolites, Catalysis Review Science and Engineering, Catalysis Letters, Applied Catalysis, Journal of Molecular Catalysis, Research Trends, CaTTech, and Journal of the Chemical Society, Chemical Communications A Corma is coauthor of 20 patents, five of them being for commercial applications He has been awarded with the Dupont Award on new materials (1995), and the Spanish National Award “Leonardo Torres Quevedo” on Technology Research (1996) 2373 Chem Rev 1997, 97, 2373−2419

From Microporous to Mesoporous Molecular-Sieve Materials and Their Use in Catalysis

The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts. © 2001 Elsevier Science B.V. All rights reserved.

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Mechanisms of catalyst deactivation

Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

The hydrothermal synthesis of zeolites: precursors, intermediates and reaction mechanism

ZSM-5 zeolite having a SiO2/Al2O3 molar ratio of 39.4 was treated in a NaOH alkali solution and the changes in structural and acidic properties were investigated. A siliceous species was selectively dissolved from the framework of zeolite, although a lower amount of Al was also eluted. In this procedure, mesopores with a uniform size were formed on the zeolite, while the microporous structure remained. The acidic property was changed very little quantitatively or qualitatively, even though the catalytic activity for cracking of cumene was enhanced by the alkali-treatment. This can be explained by the facts that adsorptive and diffusive properties of cumene through micropores of the ZSM-5 are increased by the creation of mesopores.

Alkali-treatment technique — new method for modification of structural and acid-catalytic properties of ZSM-5 zeolites

Selective catalytic reduction of nitrogen monoxide by methane on zeolite catalysts in an oxygen-rich atmosphere

Steam reforming of methane for the purpose of hydrogen production was performed using nickel catalysts supported on a variety of perovskites, including LaAlO3, LaFeO3, SrTiO3, BaTiO3, La0.4Ba0.6Co0.2Fe0.8O3−δ, to compare the catalytic activity and resistance to coking of these catalysts to those of the conventional Ni/α-Al2O3 catalyst. Ni/LaAlO3 and Ni/SrTiO3 showed high catalytic activities among the Ni/perovskites and longer-term stabilities than the conventional catalyst. Temperature programmed oxidation of carbon deposited on used catalysts revealed that inactive carbon species detected on Ni/α-Al2O3 were not formed in the case of Ni/LaAlO3. The results of temperature programmed reduction confirmed that consumption and recovery of the lattice oxygen in perovskites occurred during the reaction, and that the reducibility of perovskites had a great impact on the steam reforming activity. The lattice oxygen in perovskites is considered to play important roles in promoting the oxidation of CHx fragments adsorbed on metallic nickel.

Eiichi Kikuchi

Papers

Alkali-treatment technique — new method for modification of structural and acid-catalytic properties of ZSM-5 zeolites

Selective catalytic reduction of nitrogen monoxide by methane on zeolite catalysts in an oxygen-rich atmosphere

Catalytic activities and coking resistance of Ni/perovskites in steam reforming of methane

Palladium/ceramic membranes for selective hydrogen permeation and their application to membrane reactor

Reactivity and structural change of coal char during steam gasification