Hierarchical porous zeolite ZSM-58 derived by desilication and desilication re-assembly

Abstract

Hierarchical porosity in zeolite ZSM-58 was achieved by desilication with pure solutions of NaOH and mixtures of NaOH and cetyltrimethylammonium bromide (CTAB) in a desilication and desilication re-assembly process. The resulting ZSM-58 samples were investigated concerning their chemical and textural structure by means of transmission and scanning electron microscopy, X-ray diffraction, nitrogen and argon physisorption, elemental analyses, 27Al and 29Si solid-state magic angle spinning nuclear magnetic resonance spectroscopy, and temperature programmed desorption of ammonia. In the case of applying pure NaOH solutions on Si/Al ratios of Si/Al=50 rather macroporous zeolite ZSM-58 with broad pore size distribution was obtained and pore volume depended on the sodium hydroxide concentration. Higher Si/Al ratios did not result in any additional porosity. Adding CTAB to the desilication solution lead to hierarchical ZSM-58 materials with narrow additional pores in the range of 3–4nm resulting from an amorphous layer on the surface of the ZSM-58 crystals that was long range ordered for intermediate NaOH concentrations independent from the initially deployed Si/Al ratio. The total specific surface area of such materials was as high as 868m2g−1 with high values of external specific surface area and simultaneously the desilication yield could be increased due to the re-assembly of dissolved silica species. Therefore, high values for the hierarchy factor and desilication efficiency could be achieved. The amount of strong acid sites of the desilicated samples decreased with increasing NaOH concentration because of the generation of silanol groups, pentahedral, and octahedral coordinated extra framework aluminum.