Zeolite structural parameters

Toluene alkylation with isopropyl alcohol was chosen as the test reaction as we can follow in a detail the effect of zeolite structural parameters on the toluene conversion, selectivity to cymenes, selectivity to para-cymene, and isopropyl/n-propyl ratio. It should be stressed that toluene/isopropyl alcohol molar ratio used in the feed was 9.6, which indicates the theoretical toluene conversion around 10.4 %. As you can see from Fig. 2 conversion of toluene over SSZ-33 after 15 min of T-O-S is 21 %, which is almost two times higher than the theoretical toluene conversion for alkylation reaction. The value of toluene conversion over SSZ-33 is influenced by a high rate of toluene disproportionation. About 50 % of toluene converted is transformed into benzene and xylenes. Toluene conversion over zeolites Beta and SSZ-35 is around 12 %, which is due to a much smaller contribution of toluene disproportionation to the overall toluene conversion. A slight increase in toluene conversion over ZSM-5 zeolite is connected with the fact that desorption and transport of products in toluene alkylation with isopropyl alcohol is the rate controlling step of this reaction [9]... 

The elementary building block of the zeolite crystal is a unit cell. The unit cell size (UCS) is the distance between the repeating cells in the zeolite structure. One unit cell in a typical fresh Y-zeolite lathee contains 192 framework atomic positions 55 atoms of aluminum and 1atoms of silicon. This corresponds to a silica (SiOj) to alumina (AI.O,) molal ratio (SAR) of 5. The UCS is an important parameter in characterizing the zeolite structure. 

Table 1 Structural Parameter as Derived from the Mo K-Edge EXAFS for Mo and Co-Mo Sulfide Catalysts Encaged in a NaY Zeolite...

One difficulty with many synthetic preparations of semiconductor NCs that complicates any interpretation of NMR results is the inevitable distribution of sizes (and exact shapes or surface morphologies). Therefore attempts to make semiconductors as a sort of molecular cluster having a well-defined stoichiometry are of interest to learn potentially about size-dependent NMR parameters and other properties. One approach is to confine the semiconductor inside a template, for instance the cuboctahedral cages of the sodalite framework or other zeolite structures, which have been characterized by multinuclear NMR methods [345-347], including the mesoporous channel material MCM-41 [341, 348]. 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

This table shows that it is diflicult, even in a model system, to present a simple view of the nature of the adsorption site because of the number of different parameters involved in the stabilization of OJ. For zeolites the problem is apparently more diflicult than for oxides, since not only do the framework ions and the exchanged cations form two distinct types of adsorption sites but the latter can migrate within the zeolite structure. It is difficult to obtain a full description of the coordination of the exchanged cations and so far there has been no systematic study on this point. 

Not every parameter used in the various force fields will be described in great detail, as this is not a review concerned with the simulation of aspects of zeolite structure. Instead, we aim to present the essential features of the various parameters that are used and to group force fields into certain families that essentially originate from one of a handful of key references. 

Zeolite structures pose unconventional problems for crystal structure refinement. These problems arise from positional disorder pseudo-symmetry, twinning, high mobility of some atoms, and (sometimes) the inaccessibility of single-crystal data. Methods are discussed for investigating split atoms, Si-Al distribution, pseudo-symmetry, and for dealing with parameter correlation and limited data sets. Some additional techniques which have not been applied to zeolite structures are mentioned. 

Structural Parameters of Some Zeolites and the Chemical Shift for the Si(4Si) Resonances ... 

Comparison of Structural Parameters Determined by Density Functional Theory and by EXAFS Spectroscopy Characterizing Samples Made from Chemisorption of [Rh(CO)2(acac)] on Dealuminated Y Zeolite (Goellner, Gates et al., 2000) ... 

Demarquay and Fraissard have shown that the intercept, o0, can be interpreted in terms of a "mean free path" of xenon in the zeolite and have related this NMR-derived mean free path to structural parameters such as channel diameter by a Monte Carlo simulation procedure. They empirically determined the intercept to depend on mean free path as... 

X-ray powder diffraction has been the primary tool used in zeolite structure research. With new high-flux sources, the size requirement of useful single-crystals for structure determination studies has decreased significantly. In addition, refinements of atomic coordinates of known structures using Rietveld powder techniques have become common (24). The solution of a dozen or more new zeolite structure types within the last several years has added to our knowledge base for looking at unknowns (for examples, see references 25-31), and has made us better able to characterize catalyst materials and to correlate synthesis, sorptive, catalytic, and process parameters to their structures (32,33). 

De Vos Burchart et al. have recently developed a force field for modeling zeolites.21 The model originally was intended for all-silica zeolites but was quickly extended to aluminum-containing zeolites. The parameters were derived from several sources. Standard bond dissociation energies were used, and the force constants were refined to fit the structure of ZSM-5, the structure and frequencies of a-quartz, in addition to unit cell dimensions of other zeolites. With the all-silica model, the authors were able to calculate heats of formation... 

A full comparison of the two discussed zeolite structures and the adsorption properties based on parameters influencing the zeolite pore dynamics are given in Table 4 Table 4 Specifications, parameters of relevance to the deformation of framework, framework dynamics and adsorption properties of zeolite-types MFI and RHO ... 

Both the rigid ion and shell potential models have been used in energy minimization studies of dense and microporous silica and molecular sieves. Kramer and coworkers reported parameters of the rigid ion potential model for silicates, aluminosilicates, and aluminophosphates. The model also includes parameters for extra-framework cations such as Na and Cl . Both the rigid ion and shell models were used by Catlow and coworkers in modeling silicate and zeolite structures. ... 

Figure 2.2 The structure of zeolite N, a cubic framework with a lattice parameter of 36.9A. The framework consists of two different, interpenetrating zeolite structures, sodalite (yellow) and ZK5 (blue) structure.

It appears then that it should be of interest, at least for some reactions catalyzed by zeolites, to use the concept of hardness and softness of the acid sites [20,24]. In our case, we have chosen the energy of the LUMO as a parameter to evaluate the "hardness" of the zeolites, and clusters of different sizes were used to simulate the zeolite structure and composition [25,26]. The values were obtained by semiempirical and ab initio calculations. Semiempirical calculations were done with the MOPAC-6.0 program [27] using the PM3... 

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