Acceleration effect of sodium halide on zeolite crystallization: ZSM-12 as a case study

In this work, the sodium halide , e.g., NaBr, NaCl, or NaF, was added into the synthesis system of ZSM-12 zeolite . The addition of sodium halide accelerated the crystallization of ZSM-12 zeolite, and the acceleration effect of different sodium halide increased in the order of NaBr < NaCl < NaF. To understand the acceleration mechanism of sodium halide, the crystallization process of ZSM-12 zeolite synthesized with and without sodium halide was studied comparatively by using XRD , SEM, TEM , TG and FTIR characterizations. Experimental results revealed that the accelerated crystallization of ZSM-12 zeolite arose from the combined effects of sodium cation and halide anion. The sodium cation induced the coagulation of colloidal particles of initial sol and avoided the formation of gel, resulting in the improved mass transfer. The halide anion promoted the combination of tetraethylammonium cation (as structure-directing agent) with the solid phase. A ZSM-12 zeolite with abundant mesopores was finally obtained from the synthesis system containing sodium halide. The Pt/ZSM-12 catalyst using the mesoporous zeolite as the support exhibited excellent activity and isomerization selectivity in the hydroisomerization of n -dodecane.