Hydrothermal synthesis of H-ZSM-5 catalysts employing the mixed template method and their application in the conversion of methanol to light olefins

Abstract

A hydrothermal procedure was utilized to synthesize high silica H-ZSM-5 catalysts for application in methanol to olefin (MTO) reaction by partial and total substitution of tetrapropyl ammonium hydroxide (TPAOH) with equivalent amount of the less expensive tetrapropyl ammonium bromide (TPABr) template. Five H-ZSM-5 zeolites with a similar Si/Al ratio (200) were synthesized using various proportions of TPAOH and TPABr. The TPA+ concentration (0.04 M) and initial gel pH (10.5) were constant. The catalysts were characterized by TG–DTA, XRD, ICP, FE-SEM, BET, FT-IR and NH3-TPD techniques. The samples showed cubic morphology, but their particle sizes and shapes were different. The highest crystallinity were obtained by single template synthesis for MFI structure whereas using mixed template synthesis led to lower crystallinity and larger crystallite sizes. Increasing molar ratio of TPAOH in initial gel from 0.33 to 3 decreased the purity of H-ZSM-5, and also affected its crystallinity and particle size. The NH3-TPD results indicated that both TPAOH and TPABr templates exhibited almost the same effect on the acidity of the as-synthesized catalysts, whereas mixing the two templates increased the total acidity and decreased the content of strong acid site. The catalytic performance of all samples was evaluated in MTO reaction in a fixed-bed reactor at 480 °C and atmospheric pressure using feed containing a mixture of 50 wt% methanol in water with methanol WHSV of 0.9 h−1. All the catalysts exhibited approximately similar methanol conversion though their propylene selectivities and products distributions were greatly dependent on the template composition. This work verified that despite providing the same TPA+ and OH− concentrations in the initial gel, the H-ZSM5 samples with different properties could form using diverse template combinations.