High-temperature efficient isomorphous substitution of boron into ZSM-5 nanostructure for selective and stable production of ethylene and propylene from methanol

Abstract

The present work demonstrates that using high temperature hydrothermal method is an effective route to achieve the efficient isomorphous substitution of boron into MFI lattice structure. Thus, the nanostructured boron-incorporated ZSM-5 zeolites with short crystallization times were synthesized hydrothermally by different high temperatures (250, 300 and 350 °C). The physicochemical properties of BZSM-5 samples were characterized by XRD, FESEM, EDX, BET-BJH, FTIR, TPD-NH3 and TG/DTG analyses. It was found that high temperature and short synthesis time increased the crystallinity and improved the distribution of Si and B atoms in the crystalline framework of ZSM-5 zeolite. Moreover, TPD-NH3 results indicated that the weak acidity was strengthened and strong acidity was weakened for BZSM-5 catalyst synthesized at 350 °C and 0.5 h. The prepared catalysts were tested for production of ethylene and propylene from methanol in a fixed bed reactor at atmospheric pressure, reaction temperatures of 400–500 °C and gaseous space velocity of 10500 cm3/gcat.h. It can be emphasized that the catalytic performance of BZSM-5 materials was greatly influenced by crystallite size and as well as the number of active species. Therefore, it was observed that an increase in the concentration of weak acid sites leads to an increase in the catalytic activity. In this case, BZSM-5(350, 0.5) catalyst had maximum olefins production, about ethylene selectivity (10%) and propylene selectivity (82%), after 35 h time on stream.