Facile creation of hierarchical nano-sized ZSM-5 with large external surface via desilication-recrystallization of silicalite-1 for conversion of methanol to hydrocarbons
Diffusion limitation caused by micropores of ZSM-5 results in its deactivation in methanol to hydrocarbons reaction. Introduction of mesopores leads to the decrease in length of the micropores and increase in the external surface area, which could reduce the coke formation in micropores and increase the coke capacity. In this work, mesopores and acidity were fabricated synchronously for ZSM-5 though the desilication-recrystallization process of silicalite-1 and a series of nano-sized ZSM-5 with large external surface area of over 200 m2 g-1 were obtained, which were much higher than previously reported ones. Based on the precise regulation of hydrothermal time, it was found that desilication and recrystallization occurred simultaneously and their different levels determined the pore structure and acidity. The 27Al MAS NMR results revealed that five-coordinated Al and octahedral coordinated extra-framework Al convert into tetrahedral coordinated framework Al during desilication-recrystallization process. The catalytic studies indicated that the lifetime and product selectivity were dependent on the pore structure and the acidic properties of the catalysts. The external surface area of the ZSM-5 prepared by 24 h treatment was 206 m2 g-1 and the acidity was 0.58 mmol g-1 which resulted in longer lifetime of 164 h and high aromatic selectivity of 74.5% were achieved. Interestingly, the regenerated catalyst exhibited 2.5 fold longer lifetime as compared to the fresh catalyst, which was ascribed to the maintained large external surface area and the decreased acid sites after reaction and regeneration. This result of excellent regeneration performance was hardly ever reported. This work would provide excellent example for preparation of highly stable ZSM-5 based catalysts for some diffusion-limited reactions.