Development and characterization of mesoporosity in ZSM-12 by desilication

The generation of mesopores in the range of 15–20 nm by alkaline treatment of a unidimensional high silica content zeolite, namely ZSM-12 was achieved. The factors that affect the generation of mesopores were thoroughly investigated. For the ZSM-12 with Si/Al ratio of 58, additional mesopores with volume in the range 0.05–0.61 cm3 g−1 were generated without destroying the microporous framework of ZSM-12 by varying the NaOH solution concentration, operation temperature and time. The concentration of NaOH solution was found to be the most dominant factor affecting the desilication process. Meanwhile, parent ZSM-12s with different Si/Al ratios in the range of 31–500 were alkali treated in order to investigate the influence of Si/Al ratios on the desilication of ZSM-12. It was found that the content of tetrahedrally coordinated aluminum in the zeolite frame work also controls the formation of mesopores by facilitating desilication process in alkaline medium. For the ZSM-12 with relatively low Si/Al ratios ranging from 31 to 58, relatively high NaOH concentration was favorable for the generation of mesopores while preserving the microporous structure and acidic property of ZSM-12. However, for the samples with higher Si/Al ratios, lower NaOH concentrations favored the creation of mesopores. In terms of uniform mesopore generation, ZSM-12s with Si/Al ratios in the range of 31–58 were more appropriate. Desilicating the as synthesized ZSM-12 samples (prior to calcination) is much more difficult than desilicating calcined ZSM-12 samples of comparable Si/Al ratios. The diffusivity of long chain hydrocarbons through the framework of alkaline treated ZSM-12 samples improved compared to the untreated ones. The ease to control the mesoporous network developed in ZSM-12 by alkaline treatment offers a new and economic path for the improvement of diffusion in the zeolite framework.