Exploring the impact of zeolite porous voids in liquid phase reactions: The case of glycerol etherification by tert-butyl alcohol

Abstract

The role of acidity (nature, concentration, strength) and textural properties in the etherification of glycerol with tert-butyl alcohol was studied for a wide range acid catalysts, such as Amberlyst® 15, silica, alumina, silica alumina and four type of zeolite, i.e. FAU, MOR, ∗BEA and MFI. The etherification of glycerol by tert-butyl alcohol is a thermodynamically limited reaction that occurs through a successive reaction sequence and follows an Eley-Rideal type mechanism. We found major evidence that glycerol etherification is not only a function of the amount of Brønsted acid sites, but that it further proceeds via a product shape selectivity mechanism. Indeed, the formation of di-substituted ethers appears at very low conversions for zeolites compared to meso- and macroporous acid catalysts. ∗BEA and MFI zeolites feature similar confining voids and resulting thus in similar intrinsic acid strengths (as proved by n-hexane cracking), but differ in the connectivity (4 vs. 6 channels) and access to these voids (0.54 vs. 0.67 nm), which leads to diffusion issues, notably for the MFI zeolite.