Avninder Singh, S. P. Mirajkar, Sahida Sharma
Dec 28, 1999
Citations
1
Influential Citations
26
Citations
Journal
Journal of Molecular Catalysis A-chemical
Abstract
Abstract 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 investigated over various zeolites using both NBS and liquid Br 2 as brominating agents. A comparison under similar reaction conditions with H 2 SO 4 , in the absence of catalyst and FeCl 3 (in the case of toluene) is also investigated for each reaction. Zeolite H-beta is found to be selective compared to the conventional catalysts and other zeolites in the bromination of chlorobenzene and toluene whereas selectivity for 4-bromo- o -xylene (4-BOX/3-BOX) over H-beta and H 2 SO 4 was found nearly comparable in the bromination of o -xylene. In the bromination of toluene, acidic H-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, œ -bromotoluene, is enhanced. The conversion of NBS, rate of NBS conversion (mmol g −1 h −1 ) and selectivity for products 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 zeolite. Thus, the generated Br + attacks the aromatic ring resulting in the formation of brominated nuclear products.