Reactant shape selectivity , zeolite catalysis

Examples of shape selective catalysis for each dass are well illustrated with the addic zeolites, but there are far fewer examples of such catalysis with metal containing zeolites. Reactant shape selectivity is illustrated by the early work of... 

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Catalysis of 12-membered zeolites, H-mordenlte (HM), HY, and HL was studied In the alkylation of biphenyl. The para-selectlvltles were up to 70% for Isopropylblphenyl (IPBP), and 80% for dllsopropylblphenyl (DIBP) In HM catalyzed Isopropylatlon. Catalysis of HY and HL zeolites was nonselectlve. These differences depend on differences In pore structure of zeolites. Catalysis of HM to give the least bulky Isomer Is controlled shape-selectlvely by sterlc restriction of the transition state and by the entrance of IPBP Isomers. Alkylation with HY and HL Is controlled by the electron density of reactant molecule and by the stability of product molecules because these zeolites have enough space for the transition state to allow all IPBP and DIBP isomers. Dealuminatlon of HM decreased coke deposition to enhance shape selective alkylation of biphenyl. 

Reglospeclflc functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [1-5]. Catalysis using zeolites is the most promising way to prepare sterlcally small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Sterlc restrictions by zeolites Increase the formation of preferred products and prevent the formation of undesirable products [6]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL In the alkylation of biphenyl. 

Three types of shape-selective catalysis are distinguished depending on whether pore size limits the entrance of reactant molecules, the departure of product molecules, or the formation of certain transition states [6]. The suitability of zeolite structure for the catalysis is essential for high shape-selectivity. Alkylation of biphenyl is also explained by sterlc control by pore size and shape of zeolite. HY, HL and HM have different pore structures... 

Shape-selective catalysis in zeolite requires that the reactants diffuse inwards to the active sites located at the intracrystalline volume (pores), and that products counterdif-fuse after the reaction. At the active sites, presence of a high local electric field may direct the reaction according to steric requirements to yield specific products. Thus, shape-selectivity may be achieved by virtue of geometric factors, Coulombic field at the active sites and/or difference in diffusion rates. Accordingly, three different kinds of shape-selectivity are distinguished (Dwyer, 1984). If the geometric factors are such that... 

Zeolites are the most promising microporous materials for achieving highly shape-selective catalysis because their pores are uniformly distributed and have dimensions allowing both the organic reactants and products to enter, to react, and to leave.1... 

Zeolite catalysed alkylation of polynuclear aromatics is considered to be simultaneously governed by several mechanisms. To achieve highly shape-selective catalysis, it is essential that the pore size precisely corresponds to the molecular dimensions of reactants and products, and to the transition state of the reaction intermediates. 

We also mentioned stereospecificity of metal-catalyzed reactions inside zeolite cavities. In acid catalysis by zeolites it is well known that shape selectivity can be imposed by (1) selective admission of reactants fitting into zeolite pores, (2) selective release of products able to diffuse through zeolite channels, while larger molecules are retained, and (3) transition state selectivity, favoring, e.g., a monomolecular transition state over a bimolecular state in a narrow cavity. New tools that have conceptually been added to this arsenal include the collimation of molecules diffusing through well-defined pores, which then hit an active site preferentially via one particular atom or group. 

Shape selective catalysis differentiates between reactants, products, or reaction intermediates according to their shape and size. If almost all catalytic sites are confined within the pore structure of a zeolite and if the pores are small, the fate of reactant molecules and the probability of forming product molecules are determined by molecular dimensions and configurations as well as by the types of catalytically active sites present. Only molecules whose dimensions are less than a critical size can enter the pores, have access to internal catalytic sites, and react there. Furthermore, only molecules that can leave the pores, appear in the final product. 

Stereoselective catalysis in zeolites is still one of the ultimate goals in zeolite science. Earlier work in this field was summarized recently [4]. More recently, Mahrwald et al. [95] reported that the addition of aluminophosphate molecular sieves in the liquid phase alkylation of a-chiral benzaldehydes by butyllithium results in an increased proportion of the so-called Cram product in the diastereomeric mixture. It is argued that in this Grignard type reaction the adsorption of the reactants on the molecular sieves favors the attack at the sterically less hindered position of the molecule. This shape selectivity effect is even observed when the reactant is adsorbed at the outer crystal surface, as demonstrated for the case of the small-pore AIPO4-I7. 

Protic zeolites may also offer a new type of spacial constraint and the attendant shape selectivity, in acid catalysis conducted at low-enough temperatures when the reactive intermediate is an adduct complex formed between an olefin and the protonated oxide ion ( C NMR evidence). With a reaction intermediate "spirming top" linked to fiamewoik oxygen, a new type of spacial constraint is imposed on the reactant molecule. 

However, a very specific sort of catalytic behaviour peculiar to clays and zeolites is shape-selective catalysis. Essentially, owing to the fixed nature of the framework channels and cavities, only molecules of the right geometry can pass through. As only certain molecules will have the right geometry to pass into the zeolite, only these molecules will reach the active sites. The selectivity can occur either in reactant or product, or even the transition state. 

The concept of molecular shape-selective catalysis is based on the action of catalytically active sites internal to the zeolitic framework, to diffusivity resistance either to reactant molecules or to product molecules or to both and to void limitation to reaction intermediates.This implies an intimate interaction between the shape, size and configuration of the molecules and the dimension, geometry and tortuosity of the channels and cages of the zeolite. Several types of effects exist ... 

This can be taken further when two molecular reactants are small enough to enter the pores of a zeolite. If they undergo a reaction, promoted by the acidic sites (see below) within a cavity, when they reach it together, the final products of that reaction are dictated by the size of the zeolite cavity and the diameter of the pores. Even if more than one product is possible, if only one of them is of a suitable size or shape to escape from the interior of the zeolite, that will be the preferred product. This has enormous implications for reactions using zeolites as catalysts, and is known as shape-selective catalysis. 

ABSTRACT. The amount of published work on molecular shape-selective catalysis with zeolites is vast. In this paper, a brief overview of the general principles involved in molecular shape-selectivity is provided. The recently proposed distinction between primary and secondary shape-selectivity is discussed. Whereas primary shape-selectivity is the result of the interaction of a reactant with a micropore system, secondary shape-selectivity is caused by mutual interactions of reactant molecules in micropores. The potential of diffusion/reaction kinetic analysis and molecular graphics for rationalizing molecular shape-selectivity is illustrated, and an alternative explanation for the cage and window effect in cracking and hydrocracking is proposed. Pore mouth catalysis is a speculative mechanism advanced for some systems (a combination of a specific zeolite and a reactant), which exhibit peculiar selectivities and for which the intracrystalline diffusion rates of reactants are very low. 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

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