Liquid phase bromination of aromatics over zeolite H-beta catalyst

The liquid phase bromination of chlorobenzene, toluene and xylenes (o-, m-, p-) is catalyzed using zeolite as catalyst and N-bromosuccinimide (NBS) as the brominating agent. In addition, the bromination of toluene has been in vestigated over various zeolites using both NBS and liquid Br-2 as brominat ing agents. A comparison under similar reaction conditions with H2SO4, in t he absence of catalyst and FeCl3 (in the case of toluene) is also investiga ted for each reaction. Zeolite II-beta is found to be selective compared to the conventional catalysts and other zeolites in the bromination of chloro benzene and toluene whereas selectivity for 4-bromo-o-xylene (4-BOX/3-BOX) over H-beta and H2SO4 was found nearly comparable in the bromination of o-x ylene. In the bromination of toluene, acidic II-beta favours the formation of nuclear products whereas in the absence of any catalyst, in the presence of weakly acidic H-Y and potassium exchanged zeolites K-beta and K-L, the concentration of side-chain product, oe-bromotoluene, is enhanced. The conv ersion of NBS, rate of NBS conversion (mmol g-l h-L) and selectivity for pr oducts are strongly influenced by the reaction parameters. As the reaction time, catalyst amount, reaction temperature and molar ratios of NBS/toluene are increased, an increase in the conversion of NBS is noticed. Presumably , the catalytic bromination of aromatics proceeds by an electrophile (Br+) which is generated by the heterolytic cleavage of NBS/Br-2 by an acidic zeo lite. Thus, the generated Br+ attacks the aromatic ring resulting in the fo rmation of brominated nuclear products. (C) 1999 Elsevier Science B.V. All rights reserved.