Zeolite catalysis structure-reactivity

The major effect of new advanced techniques on catalyst structure is found in zeolite catalysis. NMR techniques, especially MASNMR, have helped to explain aluminum distribution in zeolites and to increase our understanding of critical parameters in zeolite synthesis and crystallization. MASNMR, combined with TEM, STEM, XPS, and diagnostic catalytic reaction probes, has advanced our knowledge of the critical relationship between the structure and reactivity patterns of zeolites in the chemical fuels industry. Throughout the symposium upon which this book is based, many correlations were evident between theoretical quantum mechanical calculations and the structures elucidated by these techniques. 

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. 

We use the activation of linear alkanes and their conversion to isomers and cracked products as the main motive of our discussion. This class of reactions catalyzed by acidic zeolites is an ideal choice to illustrate the state of the art of theoretical molecular heterogeneous catalysis, because the reaction mechanisms, zeolite micropore structure, and structure of the catalytically reactive sites are rather well understood. 

Venuto, P.B., 1997, Structure-reactivity-selectivity relationships in reaction of organics over zeolite catalysts, in Progress in Zeolite and Microporous Materials, eds H. Chon, S.-K. Ihm and Y.S. Uh, Vol. 105 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 811-852. 

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

L. Kubelkova, J. Novakova, and P. Jiru, in Studies in Surface Science and Catalysis Structure and Reactivity of Modified Zeolites, P. A. Jacobs, N.I. Jaeger, P. Jiru, V.B. Kazansky, and G. Schulz-Ekloff (Eds), Elsevier Science Publishers, Amsterdam, Vol. 18, p. 217 (1984). 

Large zeolite crystals with dimensions of tens and hundreds of micrometers have proven to be irreplaceable as model materials for reactivity and diffusion studies in the field of zeolite science and heterogeneous catalysis [1-3], These large crystallites often possesses complex structures consisting of several intergrown subunits and since the pore orientations of the different elements are not always aligned, this phenomenon can have a considerable effect on the accessibility of the pores in different crystallite regions [4]. 

The characterization OF catalyst structures has undergone revolutionary developments in recent years. Powerful novel techniques and instrumentation are now used to analyze catalyst structure before, during, and after use. Many of these advances are responsible for placing the field of catalysis on an improved scientific basis. These developments have resulted in a better understanding of catalytic phenomena, and therefore improvements in commercial catalysts and the discovery of new systems. The application of advanced electronics and computer analysis has optimized many of these analytical tools. These developments are especially evident in spectroscopy, zeolite structure elucidation, and microscopy several other techniques have also been developed. Thus, the difficult goal of unraveling the relationships between the structure and reactivity of catalytic materials is finally within reach. 

A., Navarro, M.T., Renz, M., and Valencia, S. (2009) Reactivity in the confined spaces of zeolites the interplay between spectroscopy and theory to develop structure-activity relationships for catalysis. Phys. Chem. Chem. Phys., 11, 2876-2884. 

Bhan, A. and Iglesia, E.A. (2008) link between reactivity and local structure in acid catalysis on zeolites. Acc. Chem. Res., 41 (4), 559-567. 

Predictability of activity, selectivity, and stability based on known structures of catalysts can be considered the main aim of the theoretical approaches applied to catalysis. Here, for a particular class of heterogeneous catalysts, namely, acidic zeolites, we present the theoretical approaches that are available to accomplish this goal, which lead to a better understanding of molecular motion within the zeolitic micropores and the reactivity of zeolitic protons. It is not our aim to introduce the methods as such, since introductory treatments on those can be found elsewhere. Rather we focus on their application and use to solve questions on mechanisms and reactivity in zeolites. The discussion is focused on an understanding of the kinetics of zeolite-catalyzed reactions. 

Packet, D. Schoonheydt, R.A. in Structure and Reactivity of Modified Zeolites Jacobs, P.A. Jaeger, N. Jiru, P. Kazanskii, V.B. Schulz-Ekloff, G., Eds. Studies in Surface Science and Catalysis no. 18 Elsevier Amsterdam, 1984 p 41-8. 

Hydroxylation of phenol by hydrogen peroxide over solid acids exhibits an autocatalysis that has never been described in earlier works. The induction period is dependent on the acidity and is reduced by initial addition of dihydroxybenzenes or other electron-transfer agents. A new mechanism, initiated by the slow formation of dihydroxybenzenes in the induction period, should be considered. Comparison of various catalysts shows that the reaction is also dependent on the structure of the solid. Zeolites with too small a porosity are not active, according to a large space demand of the reaction. Catalysis by titanium silicalites does not show such behaviour the reactivity is low but regular. Thus, our results show that valuable comparison between catalysts cannot be deduced from tests performed by stopping the reaction at a determined time, but that kinetic studies are essential. 

Boccuti M, Rao K, Zecchina A, Leofanti G, Petrini G (1989) In Morterra C, Zecchina A, Costa G (eds) Structure and reactivity of surfaces, Proc European Conf, Trieste, Italy, Sept 13-16,1988, Elsevier, Amsterdam, 1989 p 1 Stud Surf Sci Catal 48 1 Zecchina A, Spoto G, Bordiga S, Ferrero A, Petrini G, Leofanti G, Podovan M (1991) In Jacobs PA, Jaeger NI, Kubelkovd L, WichterJova B (eds) Zeolite chemistry and catalysis, Proc Int Symp, Prague, Czechoslovakia, Sept 8-13,1991, Elsevier, Amsterdam, 1991, p 251 Stud Surf Sci Catal 69 251... 

Sauer, J. (1994) Structure and reactivity of zeolite catalysts Atomistic modeling usin ab initio techniques, in J. Weitkamp et al (eds.), Zeolithes and Related Micro-porous materials State of the Art, Studies in Surface Science and Catalysis, vol. 84, Elsevier, Amsterdam, pp. 2039-2057. 

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