Insight into Mechanism of Dihydroxyacetone Conversion into Lactic Acid over M-UiO-66 (M = Zr, Ti, Sn) Catalysts

Abstract

Based on experimental results and DFT-based approaches, the mechanism of cascade conversion of dihydroxyacetone into lactic acid in aqueous solution over heterogeneous catalysts based on M-UiO-66 (M = Zr, Sn, Ti) is proposed. The calculation results allow us to construct a potential energy surface of dihydroxyacetone conversion and determine the most energetically favorable pathway to lactic acid. The key reaction stage without the use of both alkali and acid additives as homogeneous co-catalyst is defined. The reaction is shown to proceed via the formation of pyruvaldehyde and its hydrated form, i.e. 1,3-dihydroxypropanone. The structure of active sites of hybrid catalysts based on M-UiO-66 (M = Zr, Sn, Ti) metal-organic frameworks is analyzed. The catalysts are shown to possess both Lewis acid sites in the form of coordinatively unsaturated transition metal cations and Brønsted acid sites in the form of terminal M-OH and bridging M-O(H)-M groups on the surface. The experimental and theoretical results show that the presence of M-UiO-66 based catalysts (M = Zr, Sn, Ti) decrease the energy barriers of the key reaction stage.The most efficient substituted Ti-UiO-66 catalyst with the optimal binding energy of the key substrates on the surface is established.