Direct Synthesis of Hierarchical ZSM-5 Zeolite and Its Performance in Catalyzing Methanol to Gasoline Conversion

Abstract

A simple method was developed and tested to synthesize hierarchical ZSM-5 zeolites using tetrapropylammonium hydroxide as a sole structure directing agent to generate intercrystalline mesopores. XRD analysis confirmed the formation of ZSM-5 zeolites. SEM and TEM imaging analyses revealed the presence of hierarchical structure, formed by self-assembly of highly crystalline, nanosized primary particles. Solid-state MAS NMR analysis showed the vast majority of Al atoms were tetrahedrally incorporated with Si atoms in the zeolite framework. N2 physisorption studies indicated that the mesopores of an average pore size of 30 nm were created in the hierarchical ZSM-5 zeolites with an abundance of micropores. The performance of these zeolites in catalyzing methanol to gasoline conversion was evaluated using a fixed-bed reactor operating at 350 °C with ca. 16.5% methanol in nitrogen at a total pressure of 1.1 MPa and WHSV of 1.2 h–1. The hierarchical ZSM-5 zeolites exhibited almost 100% methanol conversion and 59% selectivity toward the gasoline range hydrocarbons. The coke formation on the used catalyst was studied using a N2 physisorption technique and a thermogravimetric analyzer. The hierarchical ZSM-5 zeolite showed less than 2% coke formation and deposition over 24 h on stream, much less compared to the conventional ZSM-5 zeolite with nearly 8% carbon deposition under the same reaction conditions.