Zeolites general description

Various ways to modify ZSM-5 catalyst in order to induce para-selectivity have been described. They include an increase in crystal size (15,17,20) and treatment of the zeolite with a variety of modifying agents such as compounds of phosphorus (15,18), magnesium (15), boron (16), silicon (21), antimony (20), and with coke (14,18). Possible explanations of how these modifications may account for the observed selectivity changes have been presented (17) and a mathematical theory has been developed (22). A general description of the effect of diffusion on selectivity in simple parallel reactions has been given by Weisz (23). 

In order to design a zeoHte membrane-based process a good model description of the multicomponent mass transport properties is required. Moreover, this will reduce the amount of practical work required in the development of zeolite membranes and MRs. Concerning intracrystaUine mass transport, a decent continuum approach is available within a Maxwell-Stefan framework for mass transport [98-100]. The well-defined geometry of zeoHtes, however, gives rise to microscopic effects, like specific adsorption sites and nonisotropic diffusion, which become manifested at the macroscale. It remains challenging to incorporate these microscopic effects into a generalized model and to obtain an accurate multicomponent prediction of a real membrane. 

The separation of a reactant system (solute) from its environment with the consequent concept of solvent or surrounding medium effect on the electronic properties of a given subsystem of interest as general as the quantum separability theorem can be. With its intrinsic limitations, the approach applies to the description of specific reacting subsystems in their particular active sites as they can be found in condensed phase and in media including the rather specific environments provided by enzymes, catalytic antibodies, zeolites, clusters or the less structured ones found in non-aqueous and mixed solvents [1,3,6,8,11,12,14-30],... 

Virial Isotherm Equation. No isotherm equation based on idealized physical models provides a generally valid description of experimental isotherms in gas-zeolite systems (19). Instead (6, 20, 21, 22) one may make the assumption that in any gas-sorbent mixture the sorbed fluid exerts a surface pressure (adsorption thermodynamics), a mean hydrostatic stress intensity, Ps (volume filling of micropores), or that there is an osmotic pressure, w (solution thermodynamics) and that these pressures are related to the appropriate concentrations, C, by equations of polynomial (virial) form, illustrated by Equation 3 for osmotic pressure ... 

Precipitation is a method often used for producing both support precursors and catalyst precursors (including precursory forms of zeolites) and occurs when two or more solutions are mixed in a suitable way. In addition to providing general details of the method (e.g., concentration, temperature, pH, etc.) it is necessary to indicate specifically the order and rate of addition of one solution into the other, a description of the mixing procedure and the details of the ageing procedure, if... 

Numerous reviews dealing with the adsorption capacities and acid-base proper-hes of zeolites have been published [4, 8, 10-14, 17] so we will not give a detailed description of these systems. Only some case studies will be given in order to assess the possibilihes of thermal techniques for characterizing such materials. In general the total number of acid sites is greater in zeolites than in amorphous silica-aluminas for a similar Si Al ratio. 

Because charge defects will polarize other ions in the lattice, ionic polarizability must be incorporated into the potential model. The shell modeP provides a simple description of such effects and has proven to be effective in simulating the dielectric and lattice dynamical properties of ceramic oxides. It should be stressed, as argued previously, that employing such a potential model does not necessarily mean that the electron distribution corresponds to a fully ionic system, and that the general validity of the model is assessed primarily by its ability to reproduce observed crystal properties. In practice, it is found that potential models based on formal charges work well even for some scmi-covalent compounds such as silicates and zeolites. 

M. Dubinin (Academy of Sciences of the USSR, Moscow, USSR) The survey paper considers the fundamental problems of adsorption of vapors on zeolites within the framework of a general theory of molecular adsorption on nonporous and porous adsorbents. Among other things, it quotes examples of description of adsorption equilibria on zeolites over temperature ranges not exceeding 50° for initial and intermediate regions of isotherms by an exponential series with virial coefficients taking into account both adsorbent—adsorbate and adsorbate—adsorbate interactions. It is assumed that these equations can be used for practical calculations of adsorption equilibria. 

Naturally, for adsorption on cations in zeolite voids, which we have called electrostatic adsorption, the concept of the invariance of characteristic curves is a rather crude approximation. This is directly indicated by characteristic curves constructed from experimental adsorption isotherms for a sufficiently wide temperature range. However, for the purposes of practical description of adsorption equilibria, such an approximate assumption in a controlled temperature range is reasonable. A general evaluation will be made below. 

Structural studies have been relatively few in number these are also typically performed at low temperature when sorbates are reasonably well localized. For examples, aromatic hydrocarbons in zeolites X, L, Y and good agreement with docking simulations is found. At higher temperatures the effective description of disordered sorbate distributions so as to reproduce the measure difiraction data remains, in general, a challenge [75]. 

Our refined models allow a good description of the concentration dependence of rate and e.e. with different Pt-catalysts and additives in various solvents. This is well in line with results reported for various modifiers [6a, 6c, 6d] and supports like Pt/Al203 [4], Pt/Si02 [6b], Pt-zeolite [6e] or Pt-colloids [1]. With etpy as substrate, a qualitatively similar behavior is observed in all cases. Therefore, the concept of reversible adsorption of the modifier on the catalyst seems to be generally applicable. 

Descriptions of the syntheses of zeolites and zeotypes are very widely disseminated in the journal and patent literature. Searches carried out on, for example, zeolite A or ZSM-5 would reveal hundreds of examples. In general, it is often instructive to look both at some of the original synthesis procedures (often as patent examples) and also at some of the most recently published methods. For all materials, the extent of information increases with the length of time since first disclosure. This means that the amount of data on (for example) recently-discovered zeotype phases may be quite limited. A very useful source for experimental methods and reliable synthetic procedures for zeolites and the more common zeotypes can be found in the handbook issued by the Synthesis Commission of the International Zeolite Association (IZA) [51]. 

Due to the large size of the zeolite crystals the rigorous quantum chemical methods cannot be used for the description of the entire zeolite crystal. Therefore, simplified models are used for the zeolite description. Model is defined as a set of simplifying approximations adopted for the description of a specific system. In general, model definition includes (i) specification of the set of atomic nuclei representing the system, (ii) set of constraints applied (e. g., boundary conditions, constraints used in geometry optimization, etc.), (iii) number of electrons explicitly treated, and (iv) interaction potentials between particles in the system. It is sometimes advantageous to limit the model definition to items (i) and (ii) and refer to items (iii) and (iv) as method . The notation model/method defined by items (i)-(ii)/(iii)-(iv), respectively, is adopted here. First we describe quantum chemical methods applicable on zeolites followed by the discussion of various models used for the zeolite representation. 

The description of the system in terms of this set of variables implies that we have to regard the solid phase as a solid solution of the sorbate in the zeolite, the concentration of the solid solution being given by Wg and. The mass of the solid sorbent is an essential variable, whereas the area of the phase boundary between solid and gas is not important. The situation is thus different from the case of adsorption on the surface of a solid, where we can define a surface concentration as the amount of matter from the gas per unit area of phase boundary, which is in excess of the concentration in the gas. In the case of sorption by zeolites, the sorbed amount is generally found to be proportional to the mass of the solid sorbent, independent of crystallite size or extent of phase boundary sorption in the bulk volume is generally large compared with adsorption on the crystal faces, and therefore the latter will be neglected in the present considerations. 

The thus established, four-divided nomenclature of deep-sea sediments with threshold limits of 10 %, 25 % and 50 % easily permits a quite detailed categorization of the sediment which is adaptable to generally rare, but locally frequent occurrences of components, e g. zeolite, eventually important for a more complete description. 

When investigating single-file diffusion in zeolites, one must face not only the general experimental problems of diffusion studies with zeolites, but also additional aggravations. These are due to possible transitions between the regimes of single-file and normal diffusion and the influence of the real structure on these transitions. There is also substantial need for a satisfactory and handy theoretical description of the expected processes. Sects. 2 and 3 of this chapter are devoted to this problem, while Sect. 4 provides a survey of the available experimental data in this rather new held of research. 

Diffusion in micropore is assumed to be driven by the chemical potential gradient of the adsorbed species, instead of the concentration gradient. This is not a general rule, but it has been shown in many systems (Ruthven, 1984) that the chemical potential gradient is the proper description for the driving force of diffusion in zeolite, especially zeolites A, X, Y. Diffusion in other zeolites, and molecular sieve particles, there are still some discrepancies in the description of the diffusion. Solid structure and properties of the diffusing molecule may all contribute to these discrepancies. 

This description might well be generalized to remove the word organic, since more recent work has revealed a wealth of inorganic hosts, such as zeolites and polyoxometallates," or mixed metal-organic coordination compounds, such as metal-organic frameworks (MOFs)... 

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