Zeolite catalysis structural models

It seems that the zeolites have been well screened in a qualitative sense, for their catalytic properties. This paper is concerned with the quantitative aspects of catalytic reaction rates in zeolites. The question whether the model of coupled surface adsorption and reaction is still meaningful in the case of zeolite catalysis was already raised by Weisz and Frilette (4) when they wrote In conventional surface catalysis the termination of a three-dimensional solid structure is considered to be the locus of activity. For these zeolites the concept of surface loses its conventional meaning.. . It is the purpose of the present article to examine critically some possibile models representing equilibrium and rate phenomena in gas-zeolite systems, in order to obtain an understanding of the kinetics of chemical reactions in zeolites. Sorption equilibria, on the one hand, and rates of sorption/desorption, exchange, and catalytic reaction on the other hand are closely related and therefore have to be represented in terms of the same model. 

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. 

The state-of-the-art methodologies in computational zeolite catalysis are based on DFT calculations with periodic boundary conditions. These allow theoretical analysis of structure and chemical properties of zeolites with moderate-sized unit cells using a real crystal structure as a model (Fig. 2C). Such periodic DFT calculations of zeolites are mostly limited to the GGA density functionals. 

Models of regular structures, such as zeolites, have been extensively considered in the catalysis literature. Recently, Garces [124] has developed a simple model where the complex pore structure is represented by a single void with a shell formed by n-connected sites forming a net. This model was found to work well for zeolites. Since polymer gels consist of networks of polymers, other approaches, discussed later, have been developed to consider the nature of the structure of the gel. 

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]. 

Tlhe importance of zeolites in research on heterogeneous catalysis is A based mainly on the fact that the structure of the active surface is a defined part of the crystal structure and does not represent a more or less severe lattice defect as most catalyst surfaces do. The crystal structure, and therefore the structure of the zeolite surface, can be determined by x-ray diffraction. Knowledge of this structure allows the construction of simple models of the distribution of electric fields in the holes of the zeolite by which wide ranges of experimental results can be explained, as is shown by the pioneering work of Barrer 1-5) and Kiselev 6-9) on calculation of the heats of adsorption of various substances. 

Zeolite matrices have appeared recently to be of fascinating interest both for industrial and universitary purposes. For the industry many applications, particularly for acidic zeolites, have been found saving billions of dollars each year. For university or basic research the well defined structure of the zeolite materials constitutes a relatively simple model. The main properties of the zeolites in catalysis arise from (70)... 

Quantitative structure-activity or structure-performance (property) relationships (QSAR and QSPR respectively) are of increasing interest in a wide diversity of technology areas. With some notable exceptions, most QSA(P)R studies associated with heterogeneous catalysis are restricted to zeolite-based catalysts. The reasoning is simple -the well defined 3-D structures, of molecular-sieve zeolites, are a reasonable representation of the catalyst "surface", hence bulk characterisation provides information on the catalyst. It is for a similar reason that the majority of modelling studies involves zeolites and zeotypes. 

As platinum is especially important in catalysis, strong experimental efforts have been made to characterize small Pt particles entrapped in zeolite cages, using the whole arsenal of spectroscopic, structural and chemical methods (e.g. Refs. 200, 241, 242 and references therein). However, the interpretation of experimental data often remained ambiguous because the measured values commonly reflect more than one effect as a rule, when the electronic state of an encapsulated metal species changes due to interactions with the zeolite host, so do its size and shape. Model calculations enable to separate these effects. 

Large coffin-shaped zeolite H-ZSM-5 crystals have been chosen as suitable model systems to study the structural motifs and related intergrowth structure, which are also present in smaller powdered zeolite materials used in industrial catalysis. The advantage of large model crystals, which can be prepared with adjustable Si/Al ratio and size, is that the catalytic properties of each growth unit of a zeolite can be studied separately by means of optical microscopy methods, such as UV-vis microspectroscopy and confocal fluorescence microscopy [114, 122]. As both microspectroscopy methods obey the optical Abbe diffraction limit, features can be studied in the size of approximately X/2. The resolution d of an optical microscope is given by... 

The goal of this chapter is to gather the methods most commonly used for the preparation of gold catalysts supported on common oxide powders. For the sake of brevity, deposition on more specific supports, such as structured mesoporous supports, zeolites, and carbon is barely reported in this chapter, and we refer to Chapter 4 of the book on gold catalysis [7]. The methods described are mainly those performed in the liquid phase. The preparations of model catalysts, that is, on films or single crystals and those of bimetallics are excluded. 

The development of new or improved processes in catalysis and adsorption were in many cases induced by the development of new catalytic materials and adsorbents. In this context, the synthesis of new aluminosilicates is a continuing challenge in zeolite science. The present review, discussing the synthesis principles of selected more recent zeoUtes, has shown that there is still much room for innovation in this field. It can be expected that by the use of new classes of templates (one recent example is that of the metallocenes) new structures wiU be synthesized in the future. Moreover, with the availability of more and more sophisticated tools for modelling zeolite and template structures and their interactions, it will probably be possible to tailor templates for a given (or a theoretical) zeolite structure. Finally, beside the exploration of new templates and new reaction compositions, the influence of the synthesis conditions on the products should not be overlooked, e.g. changing the reaction parameters from subcriti-cal to supercritical conditions could well have an influence on the materials which are formed. 

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