Bifunctional (Al)TS-1 Prepared by Sequential Insertion of Active Sites for Ethylene to Glycol

Abstract

Zeolites containing Ti and Al possess both oxidation and acid catalytic activities. This paper reports the synthesis of bifunctional (Al)TS-1 through sequentially inserting Al into a parent TS-1 via hydrothermal post-treatment. This effectively avoids the competitive insertion between Al and Ti into the MFI framework that often occurs during co-crystallization synthesis of Al-TS-1. By controlling specific compositions of the post-treatment solution containing aluminum isopropoxide, (NH4)2CO3 and/or tetrapropylammonium hydroxide (TPAOH), (Al)TS-1 of different morphology and oxidative/acidic characteristics are produced. Their catalytic activities toward oxidative hydration of ethylene to ethylene glycol (EG) are evaluated. This study reveals that (NH4)2CO3 treatment increases the content of octahedrally coordinated Ti species (Ti-VI) in (Al)TS-1, which improves its oxidative activity. On the other hand, TPAOH treatment facilitates Al insertion into the framework, enhancing acid strength and acid amount in (Al)TS-1. Moreover, treatment in mixed (NH4)2CO3 and TPAOH solution further increases the content of Ti-VI and enriches Al on the surface of (Al)TS-1. Owing to the high contents of active Ti species (Ti-IV + Ti-VI) and framework Al achieved by this sequential approach, (Al)TS-1 exhibits a high catalytic activity for ethylene to EG, achieving 97.6% EG selectivity at a H2O2 conversion of 66.7%. Furthermore, TPAOH treatment also produces internal cavities within (Al)TS-1 particles. This creates a microenvironment within the (Al)TS-1 catalyst that is beneficial for the targeted reaction.