Synthesis, characterization and catalytic activity of ZSM-5 zeolites having variable silicon-to-aluminum ratios

Hydrogel prepared from sodium silicate and aluminum sulfate with silicon-to-aluminum molar ratios (Si/Al) in the range 15-100 were readily transformed into ZSM-5 zeolite when subjected to heating by a rapid crystallization method using tetrapropylammonium bromide (TPA(.)Br) template at pH 10. The crystallization of gel having SUM molar ratio of 10 was achieved with difficulty and represented the lower limit of Si/Al ratio, while the gel with Si/Al molar ratio of 9 could not be crystallized even after several attempts. Calcination of the synthesized zeolites was carried out at 873 K and ion-exchange was achieved using aqueous ammonium nitrate solution. All synthesized zeolites were found to be ZSM-5 as confirmed by XRD and supported by FF-IR characterization. The synthesized zeolites were found stable up to 1323 K, and possess surface properties and pore sizes comparable to those of ZSM-5 zeolites. The catalytic activity of the synthesized zeolites was evaluated at 343, 353, 363 and 373 K in a flow reactor. For the reaction of methanol and isobutene to produce methyl tertiary butyl ether (MTBE), the activity increased with decreasing SUM molar ratio of the zeolites and with increasing reaction temperature. These zeolites were found to have methyl tertiary butyl ether selectivity in the range of 90-98% that decreased with increase in reaction temperature. (C) 2003 Elsevier B.V. All rights reserved.