High-temperature and short-time hydrothermal fabrication of nanostructured ZSM-5 catalyst with suitable pore geometry and strong intrinsic acidity used in methanol to light olefins conversion

Abstract

High temperature hydrothermal synthesis method was developed to preparation of nanostructured ZSM-5 molecular sieves at short crystallization time. A series of catalysts were synthesized at various temperatures and crystallization times for achievement of pure ZSM-5 phase with MFI structure. The synthesized catalysts were investigated with XRD, FESEM, EDX, BET-BJH, FTIR and TPD-NH3 techniques. The results revealed that hydrothermal synthesis conditions generally affected the nucleation rate, particle size, textural properties and acidic nature of ZSM-5 catalysts. It was found that pure ZSM-5 materials with high crystallinity could be obtained at specific crystallization conditions of about 300 °C for 1.5 h and also 350 °C for 0.5 h. Increasing the hydrothermal temperature to 350 °C and decreasing the crystallization time to 0.5 h led to the formation of small particles with high specific surface area of 392 m2/g. Furthermore, ammonia TPD spectra showed that ZSM-5(300-1.5) catalyst contained higher amount of acid sites and less acid strength compared to ZSM-5(350-0.5) catalyst. The catalytic performance of samples was studied for conversion of methanol to light olefins under different reaction conditions. Interestingly, the proper pore geometry along with the strong intrinsic acidity resulted in a tendency for excessive production of light olefins for ZSM-5(350-0.5) catalyst. The selectivity of light olefins over this catalyst was increased about 94% in the long time on stream (2100 min). Also, the possible reaction pathway for ZSM-5 synthesis at high temperatures was discussed in details.