Exploring the multifunctionality and accessibility of vanadosilicates to produce acrylic acid in one-pot glycerol oxydehydration

Abstract

Acrylic acid is one of the most attractive products directly produced from glycerol, and many efforts are still made to thoroughly understand the role of different catalytic sites in the reaction. In this work, we prepared Al-free vanadosilicates presenting structures analog to ferrierite and ITQ-6 zeolites (2D and 3D structures). The materials were efficient in catalyzing the one-pot conversion of glycerol to acrylic acid. The different vanadium species in the catalysts had specific roles in each step of the reaction. By exposing samples to humid conditions, dissociative adsorption of water produced hydroxylated sites (O3V-OH-Si) that acted as extrinsic Brønsted acid sites. The deprotonation energy of these sites was estimated by DFT calculations and found to be close to deprotonation energy of intrinsic Brønsted acidity of aluminosilicates with the same zeolitic structure, indicating the ability of the active site to dehydrate glycerol to acrolein. The formation of these sites seems to effectively block potential Lewis acidity of the vanadosilicates since acetol, a dehydration side product, was not formed. Spectroscopic data showed changes in oxidation states of vanadium in these sites after the reaction, presenting V5+ and V4+ states, indicating the activity of these sites on the oxidation step during oxidation of acrolein to acrylic acid. By decreasing vanadium content during synthesis, delamination to ITQ-6 was more effective, increasing accessibility and, consequently, the productivity of sites.