Gas-Phase Oxidation of Glycerol to Dihydroxyacetone over Tailored Iron Zeolites

A novel chemocatalytic technology for the valorization of glycerol (GLY) is presented, where the continuous gas-phase oxidation to dihydroxyacetone (DHA) is accomplished over iron-containing zeolites in the presence of molecular oxygen. The catalyst design elucidated the impact of acidic properties and iron speciation on the performance. For this purpose, a series of catalysts displaying a zeolitic or amorphous structure, different Bronsted and Lewis acid site concentrations and strengths, and distinct iron species were synthesized, characterized by a multitechnique approach, and evaluated in a fixed-bed reactor. Fe-silicalite prepared by isomorphous substitution of iron in the all-silica framework followed by steam activation at 873 K exhibits very mild acidity and highly dispersed iron species in the form of isolated cations or small FeOx clusters in extra framework positions, leading to a stable DHA yield of ca. 90%. In contrast, impregnated or hydrothermally prepared and steamed aluminum-containing catalysts feature strong acidity and/or a high iron clustering degree, promoting competitive dehydration or oxidation reactions and resulting in poor DHA yields. The optimal catalytic system identified largely outperforms the expensive noble-metal-based catalysts reported to date and sets the basis for a process converting waste glycerol into DHA, a platform for the manufacture of polymers and fine chemicals. Interestingly, our study also uncovers the first oxidation reaction over iron zeolites in which molecular oxygen is a suitable oxidant and the typically applied N2O is ineffective.