Alkali metal ion exchanged ZSM-5 catalysts: on acidity and methanol-to-olefin performance

Abstract

The reactivity of partially alkali metal ion-exchanged X,H-ZSM-5 (X = Li, Na, Cs) catalysts was evaluated in methanol-to-olefin (MTO) conversion. ¹H MAS NMR spectroscopy using acetonitrile-d₃ and ammonia as probes revealed decreased Brønsted acid site density (ASD) and maintained acid site strength of alkali metal ion-exchanged X,H-ZSM-5 catalysts. Optimized ASD and increasing counter ion diameter increase the propene content and decrease the selectivity to alkanes and aromatics. Decreasing the ASD to 0.12 mmol g⁻¹ by exchange with Cs resulted in the best propene/ethene ratio of 5 and the lowest hydrogen transfer index (HTI) of 0.08. Smaller counter ions Na and Li yielded less alkene cycle products but more than those of the parent (ASD = 0.78 mmol g⁻¹). The X,H-ZSM-5 lifetime, conversion capacity, and coke amount correlate with the ASD. Their lifetime was outperformed by directly synthesized ZSM-5 catalysts. It is proposed that the ion size limits the space in the pores and introduces additional shape selectivity at the expense of lifetime.