Zeolite catalyst pore size

A specially formulated catalyst for maximizing the yield of light olefins can be tailored in the light of the feed properties and target products by optimizing the composition of a mixture of different natural zeolites. The pore size distribution of the matrix should be sueh as to allow access to the large molecules of the feedstock, whilst incorporation of large pore Y zeolite favors intermediate molecule formation and mesoporous ZSM-5 favors production... 

The important property of ZSM-5 and similar zeolites is the intercrystalline catalyst sites, which allow one type of reactant molecule to diffuse, while denying diffusion to others. This property, which is based on the shape and size of the reactant molecules as well as the pore sizes of the catalyst, is called shape selectivity. Chen and Garwood document investigations regarding the various aspects of ZSM-5 shape selectivity in relation to its intercrystalline and pore structure. 

Electrochemical studies, in combination with EPR measurements, of the analogous non-chiral occluded (salen)Mn complex in Y zeoUte showed that only a small proportion of the complex, i.e., that located on the outer part of the support, is accessible and takes part in the catalytic process [26]. Only this proportion (about 20%) is finally oxidized to Mn and hence the amount of catalyst is much lower than expected. This phenomenon explains the low catalytic activity of this system. We have considered other attempts at this approach using zeolites with larger pore sizes as examples of cationic exchange and these have been included in Sect. 3.2.3. 

Although the natural zeolites are widely used (around 4 million tpa) they are not particularly valuable as commercial catalysts. This is due to a number of factors including natural variations in crystal size and porosity as well as the actual small pore size, which limits their synthetic usefulness. Natural zeolites do, however, find widespread use in applications such as removal of heavy metals from water, odour removal and building materials e.g. cavity grouting and sprayed concrete). 

For most catalysts, mesopores are dominant, whereas for materials derived from zeolites or active carbons, micropores are the most important. Determination of the pore size distribution is indispensable in catalysis research. 

However, for the heavier resides, zeolite pore structure may preclude their use in HCK. We have introduce the effect of the pore size and distribution on the conversion and coke formation of asphaltene containing feeds (Section 5.2.1), but we should also point out that they also affect the dispersion of the hydrogenation metals on the catalyst surface. A poor dispersion will also lead to poor hydrogenation and indirectly favor coke formation. 

The zeolite structure also plays a large role in RC product distribution. Weitkamp et al.62 conducted experiments with Pt/HZSM-5 catalysts, which have very narrow pore sizes when compared with other zeolites, such as USY or SAPO. They found that c is I trans-1,3-dime thylcyclopentane was formed, while 1,1 and 1,2-DMCP were not. This indicates that the more oval shaped 1,3-DMCP was able to diffuse through the pores, while the more bulky and spherical isomers were not, and thus not seen in the product distribution. In short, when compared with dealkylation to cyclohexane, ring contraction of MCH is a more effective pathway to yield higher ON products. However, in order to further improve the ON, ring-opening of the RC isomers may be necessary, as shown below. 

Until the recent discovery of UTD-1 and CIT-5, the largest pore zeolites known were composed of pore structures having 12-MRs or less. Many of these materials such as zeolite Y have enjoyed immense commercial success as catalysts (2). There is some evidence from catalytic cracking data that suggests the inverse selectivity found with the 12-MR pore ( 7.5 A) structure such as for SSZ-24 (Chevron) might be used to enhance octane values of fuel (3). However, small increases in pore size as well as variations in pore shape and dimensionality could further improve the catalysts. Pores with greater than a 12-MR structure might allow the conversion of... 

A.B. "Shape Selective Conversion Over Intermediate Pore Size Zeolite Catalysts" Am. Inst, of Chem. Eng. 72,... 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

The reason for the high selectivity of zeohte catalysts is the fact that the catalytic reaction typically takes place inside the pore systems of the zeohtes. The selectivity in zeohte catalysis is therefore closely associated to the unique pore properties of zeohtes. Their micropores have a defined pore diameter, which is different from all other porous materials showing generally a more or less broad pore size distribution. Therefore, minute differences in the sizes of molecules are sufficient to exclude one molecule and allow access of another one that is just a little smaller to the pore system. The high selectivity of zeolite catalysts can be explained by three major effects [14] reactant selectivity, product selectivity, and selectivity owing to restricted size of a transition state (see Figure 4.11). 

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