Highly selective/enantioselective Pt-ReOx/C catalyst for hydrogenation of L-malic acid at mild conditions

The catalyst preparation strategy was based on a strict introduction sequence of rhenium and platinum precursors and their strong interaction with carbon support resulted in the formation of 0.5 nm Pt-ReOx species of atomic dispersion, where platinum is metallic, while monolayer rhenium is partially oxidized (Re2+). The reaction kinetics was studied taking into account the process of L-malic acid association leading to the formation of inactive cyclic oligomeric species. High TOFs (ca. 50 h(-1)), selectivities (ca. 99%) and stability of Pt-ReOx/C catalysts in aqueous-phase hydrogenation of L-malic acid, which are close to those of the homogeneous pincer type complexes, were revealed at mild conditions (T = 90-130 degrees C). Taking into account that (i) hydrogenation reaction occurred 2-3 orders of magnitude faster than its racemization and (ii) association of L-malic acid dominates at low temperatures and in a concentrated solution, special reaction conditions that allow obtaining chemically and optically (ee > 99%) pure (S)-3-hydroxy-gamma-butyrolactone and (S)-1,2,4-butanetriol were found. Basing on HAADF-STEM, EDX, XPS, and kinetic studies, the structure of active species and basic reaction pathways are proposed. (C) 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.