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Bromides in zeolite synthesis / zeolites in bromide synthesis and conversion

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DOI: 10.1016/s0926-9614(05)80021-8

Abstract

Abstract Quaternary ammonium bromides and hydroxides (quats) are applied as templates in the synthesis of zeolites with relatively high Si/Al ratio. Examples will be given of the use of mon-, di-, poly- and associated quats as templates in zeolite growth. Templated zeolites of the MFI-type can be grown in a lateral or in an axial way onto metal supports, providing promising composite systems, for separation and catalysis, respectively. Upon calcination the template is removed and the zeolite's well-defined pores are available for adsorption and catalysis. Particularly challenging is the field of electrophilic aromatic substitution. Here often non-regenerable metal chlorides serve as the catalyst in present industrial practice. Zeolites are about to take over the job and in fact are doing so for aromatic alkylation. Aromatic brominations have been mainly carried out using X, Y and L zeolites. Improved para/ortho ratios have been observed upon brominating halobenzenes, benzyl halides, and biphenyl. The side product HBr leads to decreased activity and selectivity. This problem has been addressed by adding scavengers, by working in the gas phase, and by applying oxidative bromination. Silver(I)-loaded zeolites and silica-alumina can be used in carbohydrate coupling. The Ag(I) activates the 1-Br-substituent in this nucleophilic substitution. Because of the size of the reactants only the outer surface of the zeolites is active. Cu-exchanged zeolites have been examined in the nucleophilic substitution of halobenzenes towards aminated and oxygenated systems. Selectivities are dependent on the zeolite's pore sizes. Upon passing bromobenzene and hydrogen over zeolite Pt-H-beta dehydrobromination followed by hydrogenation and isomerization takes place. In this way undesired aromatic bromides can be recycled.

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