Zeolite-catalysed Alkylation of Polynuclear Aromatics

Shape-selective reactions occur by differentiating reactants, products, and/or reaction intermediates according to their shape and size in sterically restricted environments of the pore structures of microporous crystals16. If all of the catalytic sites are located inside a pore that is small enough to accommodate both the reactants and products, the fate of the reactant and the probability of forming the product are determined by molecular size and configuration of the pore as well as by the characteristics of its catalytic center, i.e., only a reactant molecule whose dimension is less than a critical size can enter into the pore and react at the catalytic site. Furthermore, only product molecule that can diffuse out through the pore will appear in the product. 

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 

Recently, the synthesis of symmetrically substituted dialkylpolynuclear aromatic hydrocarbons, such as 2,6-diisopropylnaphthalene and 4,4 -diisopropyl-biphenyl has been studied because they are superior candidates of components for advanced materials.4,7 Polynuclear aromatics require larger space for the transition state intermediate composed of reactants and acid sites inside the pores than do mononuclear reactants. For these reasons, twelve-membered ring zeolites, especially HM, are suitable for the formation of the smallest products although the selectivity varies with reactants and zeolites. In this paper, we review the shape-selective alkylation of polynuclear aromatics catalysed by zeolites. 

Although a clear distinction among these mechanisms is difficult, there is an important difference between product selectivity and restricted transition-state selectivity mechanisms. In the former mechanism, the product composition inside the pores should either be close to equilibrium, or the selectivity for the products inside the pores should be lower than that for bulk products. However, the selectivity for the narrowest isomer of the encapsulated products should be as high as that of bulk products in the latter mechanism. 

Derouane and his co-workers14,15 proposed that hilly environments offered by pore openings, cut channels, and/or cavities at the external surface of zeolites will preferentially adsorb and shape reactant molecules depending on their stereochemistry and their ability to optimize their van der Waals interaction with the framework, i.e. their capacity to nest . Adsorption will be favored for molecules (or intermediates) which can easily adapt their geometry. 

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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. 

The product distribution in the zeolite-catalysed alkylation of polynuclear aromatics depends on the structure of zeolite pores. High regioselectivities were observed in the HM catalysed isopropylation of polynuclear aromatics, such as biphenyl, naphthalene, p-terphenyl, and dibenzofuran, to yield predominantly the least bulky products e.g., 4,4 -DIPB for biphenyl, and 2,6-DIPN for naphthalene. These reactions are controlled by steric restriction at the transition state inside the pores and by the entrance of intermediate products molecules into the pores. On the other hand, the catalysts with large-pore HY and HL zeolite are controlled at low temperature by the electron density of the reactant molecule and at higher temperature by the stability of the product molecules because their pores have enough space for a transition state, which allow the formation of all corresponding isomers. 

Drs. Yoshihiro Sugi and Yoshihiro Kubota next present a review of the zeolite-catalysed alkylation of polynuclear aromatics. Their chapter gives an especially useful and general approach to understanding deactivation in these materials. 

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