Shape selective reactions pore size effect

A preparation of designed catalyst is one of the interest subjects to understand the catalysis. Efforts have been paid for the development of unique preparation method[1] those are metal cluster catalysts derived from metal carbonyls, tailored metal catalysts through organometallic processor and ultra-fine metal particle catalysts prepared by metal alkoxides, etc. These preparation methods are mainly concentrated to design the active sites on support surfaces. However, the property of support itself is also a dominant factor in order to conduct smoothly the catalytic reaction. It is known that some supports are valuable for the improvement of selectivity. For example, zeolites are often used as catalysts and supports for their regular pore structures which act effectively for the shape selective reaction[2]. In order to understand the property of support, the following factors can be pointed out besides the pore structure structure, shape, surface area, pore size, acidity, defect, etc. Since these are strongly correlated to the preparation procedure, lots of preparation techniques, therefore, have been proposed, too. Studies have been still continued to discover the preparation method of novel materials as well as zeolites[3]. 

Conducting reactions in nanospace where the dimensions of the reaction vessel are comparable to those of the reactants provides a new tool that can be used to control the selectivity of chemical transformations.1 This dimensional aspect of nano-vessels has been referred to as shape selectivity.2 The effect of spatial confinement can potentially be exerted at all points on the reaction surface but its influence on three stationary points along the reaction coordinate (reactants, transition states, and products) deserve special attention.3,4 (1) Molecular sieving of the reactants, excluding substrates of the incorrect dimension from the reaction site can occur (reactant selectivity). (2) Enzyme-like size selection or shape stabilization of transition states can dramatically influence reaction pathways (transition state selectivity). (3) Finally, products can be selectively retained that are too large to be removed via the nano-vessel openings/pores (product selectivity). 

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

Martens et a/. 3S showed that HZSM-22 synthesized in a pure form with controlled crystal size is a promising catalyst for the oligomerization of propene to Q-C12 olefins. It was found that the reaction occurs at or near the outer surface and the products formed are mainly dimers. In addition, an increase of the linearity of the oligomers as compared with HZSM-5 or solid phosphoric acid was found. It was tentatively proposed that active sites located at the pore mouths are responsible for this shape selective effect. 

Our work on the alkylation of meta-diisopropylbenzene with propene over the acid form of various 12-member ring zeolites and molecular sieves shows that these catalysts can reveal shape selective behavior (39). As the effective size of the voids increases, the ratio of the formed 1,3,5- to 1,2,4-triisopropylbenzene increases e.g., mordenite and zeolite Y give 1.1 and 2.5, respectively. Additionally, an amorphous Si02/Al203 catalyst yields a ratio of 3.5. Thus, the smaller 12-ring materials show shape selective behavior. Based on these results, extra-large pore materials such as VPI-5 may show some shape selectivity for this reaction, if acid sites can be incorporated into the material. 

While the intrinsic activity and selectivity of a catalyst establish its performance in the absence of mass transfer effects, it is well known that the placement of the active components and access to these components by reactants can play a major role in the performance of practical catalysts. One of the challenges for reaction engineers is to develop models for predicting the distribution of active components in a catalyst and the effects of this distribution, together with the pore size distribution and particle size and shape, on the performance of a catalyst. 

Anisole acetylation, which was one of the main reactions investigated, was first shown to be catalysed by zeolite ten years ago by Bayer (13), which was confirmed by Harvey et al. (14), then by Rhodia (15). Large pore zeolites and especially those with a tridimensional pore structure such as HBEA and HFAU were found to be the most active at 80°C, in a batch reactor with an anisole/acetic anhydride molar ratio of 5 and after 6 hours reaction, the yield in methoxyacetophenone (MAP) was close to 70% with HBEA and HFAU zeolites, to 30% with HMOR and 12% with HMFI. With all the zeolites and also with clays and heteropolyacids, the selectivity to the para-isomer was greater than 98%, which indicates that this high selectivity is not due to shape selective effects but rather to the reaction mechanism (electrophilic substitution). The lower conversion observed with HMOR can be related to the monodimensional pore system of this zeolite which is very sensitive to blockage by heavy secondary products. Furthermore, limitations in the desorption of methoxyacetophenone from the narrow pores of HMFI are probably responsible for the low activity of this intermediate pore size zeolite. 

Protonic zeolites find industrial applications as acid catalysts in several hydrocarbon conversion reactions. The excellent activity of these materials is due to two main properties a strong Bronsted acidity of bridging Si—(OH)-Al sites (Scheme 3.4, right) generated by the presence of aluminum inside the silicate framework and shape selectivity effects due to the molecular sieving properties associated with the well defined crystal pore sizes, where at least some of the catalytically active sites are located. 

An issue of debate is the relative roles of internal and external sites in the catalytic process. The effects of shape selectivity, clearly present in product distribution, seem to indicate a predominance of intra-porous hydroxylation. However, the different catechol/hydroquinone ratio in methanol (0.5) and acetone (1.3), could indicate a significant contribution of sites located on the outer surface of the crystals, particularly for crystallite sizes <0.3 xm. Tuel and others, studying the time course of the reaction and the solubility of tarry deposits, went further and concluded that catechol and hydroquinone were produced on different sites, external and internal respectively [49]. The effect of acetone and methanol simply reflected their ability to maintain external sites clean from tar deposits, which are soluble in the former and insoluble in the latter. On the other hand, Wilkenhoner and others concluded, with the support of kinetic constants estimated independently for internal and external sites, that catechol was also produced in the pores over the entire reaction profile, albeit at a lower rate [47]. The contribution of the outer surface for crystal sizes close to 0.1 (xm ranged from 46% in methanol to 69% in acetone. 

The most important aspect concerning the catalytic application of zeolites is not this range of potential acid-base properties since that is also available with the amorphous aluminosilicates. Instead it is the presence in these crystalline materials of molecular sized cavities and pores that make the zeolites effective as shape selective catalysts for a wide range of reactions. >53-59... 

The CLD modification of zeolites can also effectively enhance the shape selectivity of catalytic reactions. Gao and coworkers used this method to adjust the pore size of HZSM-5 and effectively enhanced the shape selectivity of the zeolite for toluene-disproportionation reaction (see Table 6.18).[67] It is seen that as the SiC>2 deposition amount is increased, the pore size of the zeolite decreases gradually, and the toluene conversion reduces slightly but the p-X/XX value increases from 0.28 to 0.41, that is, the para-selectivity is enhanced by 46% the p-X concentration exceeds the thermodynamic equilibrium value. 

The aim of the present work is to examine the catalytic activity and shape selectivity of the medium pore molecular sieve SAPO-31 (pore size 5.4 A) in the isomerization of m-xylene. The effect of Pt impregnation on SAPO-31 in the m-xylene transformation is also reported. The activity of SAPO-31 is compared with that of the medium pore SAPO-11 and the large pore SAPO-5 in the above reaction. 

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