One-pot synthesis of hierarchical mesoporous nano-ZSM-5 zeolite and its catalytic performance in the conversion of methanol to hydrocarbons

Conversion of Methanol to hydrocarbons (MTH) using zeolite catalysts is a promising process based on the non-oil based sources [1]. Despite the excellent properties of zeolites as heterogeneous catalysts in the process of MTH, the purely microporous character of zeolites results in difficult to access active sites. These micropores could cause severe diffusion limitations in the zeolite that often result in coking thus affecting catalytic activity [2]. Developing nanocrystal zeolites with hierarchical mesoporous structure is more effective to prolong the catalyst life [3]. In this study, First organic template-free hydrothermal synthesis of ZSM-5 was performed. Then, nano-sized ZSM-5 zeolite with intercrystalline mesopores in an easy one-pot procedure by the use of dodecyltrimethyl ammonium bromide as template in the zeolite synthesis gel was prepared. The textural and acid properties of catalysts were characterized using FTIR, X-ray diffraction, SEM, TEM, TGA, NH3-TPD and BET analysis. Characterizations proved smaller crystal size, more specific surface area, lower surface acidity and more uniform dispersion for the surfactant-assisted synthesized sample. The hierarchical nano-zeolite catalyst displayed 228 m2g-1 of BET surface area, in contrast to the 192 m2g-1 of the conventional zeolite. The dehydration of methanol over synthesis catalysts carried out in a fixed-bed reactor at 390 °C and WHSV of 4.5 h-1 under ambient pressure. The mesoporous nanocatalyst exhibited great improvements in catalytic life time, selectivity to C‏5+ hydrocarbons and yield of liquid hydrocarbon products. Also Mesoporous nano-ZSM-5 catalyst converted more methanol than the conventional one and its catalytic lifetime increased about 20% relative to conventional catalyst ( 90% for about 58 h). The high activity and stability of the mesoporous nano-ZSM-5 in the MTH process attributed to the reduced diffusion limitations due to the widened pore size distribution, reduced crystal size and lowered surface acidity of mesoporous nano-ZSM-5.