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

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 Si/Al 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 FT-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 Si/Al 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.