Influence of template on crystallization of ZSM-5 zeolites

Pentasil zeolites of ZSM-5 type are synthesised hydrothermally using triethyl-n-proplyammonium bromide (TEPA-Br) and triethyl-n-butylammonium bromide (TEBA-Br). The crystallization kinetics, followed by XRD, SEM and thermal analysis, clearly demonstrate the influence of size and molecular weight of the templating quaternary ammonium cation (QAC) species on the rates of nucleation and crystallization. The values of the apparent activation energies for nucleation and crystal growth indicate that both nucleation and crystal growth are faster when TEPA-Br rather than TEBA-Br is used as a template. The quantitative identification of intergrown phases characterizes both the phases to be ZSM-5 zeolite. Thermoanalytical curves for both these zeolites in as-synthesised forms exhibit two-step oxidative decomposition of the occluded organic species. This suggests that the quaternary ammonium cation may be located at two energetically different sites within the zeolite channels. The equilibrium sorption capacity, however, is found to increase in the order of size and molecular weight of the templating species in both the zeolites. The nature of acid site distribution, obtained from the temperature programmed desorption of ammonia is found to be independent of the templating species used during the synthesis.