Hierarchical ZSM-5 zeolite synthesized via dry gel conversion-steam assisted crystallization process and its application in aromatization of methanol

Abstract

Hierarchical ZSM-5 zeolites were prepared by “Dry Gel Conversion-Steam Assisted Crystallization” (DGC-SAC) procedure without the involvement of additional template. Moreover, the formation mechanism and feather of mesopores were summarized. Nanoscale Kirkendall effect was existed in the growth of zeolite crystals under high SiO2/TPA+ ratio, which was responsible for the production of intra-crystal mesopores. However, small ZSM-5 nano-crystallites, with the average diameter of <50 nm, were successfully synthesized under low SiO2/TPA+ ratio. The random assembly of these nano-crystallites gave rise to abundant inter-crystal mesopores within these nano-crystallites zeolites. Benefiting from the significantly higher concentration of TPA+ at the surface of amorphous aluminosilicate during the DGC-SAC process, the DGC-SAC procedure exhibited higher usage efficiency of template (TPA+) than that of conventional “Hydrothermal Crystallization” (HC) route, and the ZSM-5 zeolites with a low consumption of TPA+ (SiO2/TPA+ = 53.7) could be successfully synthesized by DGC-SAC method. Compared with the conventional ZSM-5 zeolites prepared by HC routes, the hierarchical ZSM-5 zeolites prepared by DGC-SAC procedure exhibited significantly higher catalytic lifetime and selectivity of light aromatics (benzene (B), toluene (T) and xylene (X)) in aromatization of methanol under same low addition of TPA+ (SiO2/TPA+ = 13.5). Moreover, the produced hierarchical ZSM-5 zeolites (SiO2/TPA+ = 13.5) also exhibited superior catalyst stability after being further hydrothermally treated at 160 °C for 72 h. TG analysis of all used catalysts confirmed that the produced hierarchical ZSM-5 zeolites had higher coke capability and lower average rate of coke formation.