Ethanol-assisted synthesis of titanium-rich TS-1 zeolite: a new hexa-coordinated Ti site for efficient propylene epoxidation

Abstract

TS-1 (MFI) zeolites are widely utilized in various green oxidation reactions, such as alkene epoxidation and alcohol oxidation. To elevate their catalytic efficacy, strategies such as increasing Ti content or constructing highly active Ti species within zeolite are commonly employed. Here, we synthesized a titanium-rich TS-1 zeolite (Si/Ti ratio of 25.6) with an unprecedented hexa-coordinated Ti species, demonstrating high activity in propylene epoxidation, achieved by utilizing ethanol as a crystal growth modifier through a two-step crystallization strategy. The introduced ethanol regulated the crystallization pathway from the classical to the non-classical route, and enhanced the incorporation of Ti within the TS-1 framework by balancing the insertion of Ti and the crystal growth. The hexa-coordinated Ti species promoted by using ethanol as a crystal growth modifier was identified as Ti(OH₂)(OSi)₃(OSiOH)₂ by means of ultraviolet-resonance Raman spectroscopy, X-ray absorption spectroscopy and theoretical calculations, differing from the conventional Ti(H₂O)₂(OH)₂(OSi)₂ in TS-1. The as-prepared TS-1 zeolite was applied in the propylene epoxidation reaction, showing exceptional catalytic activity (H₂O₂ conv. 35.6%), high 1,2-propylene oxide selectivity (93.3%), and excellent stability. This work introduces an ethanol-assisted synthesis strategy that facilitates the incorporation of a new hexa-coordinated Ti species into TS-1 zeolite that is highly active in oxidation reactions.