The
direct oxidative dehydrogenation of lactates with molecular
oxygen is a “greener” alternative for producing pyruvates.
Here we report a one-pot synthesis of mesoporous vanadia–titania
(VTN), acting as highly efficient and recyclable catalysts for the
conversion of ethyl lactate to ethyl pyruvate. These VTN materials
feature high surface areas, large pore volumes, and high densities
of isolated vanadium species, which can expose the active sites and
facilitate the mass transport. In comparison to homogeneous vanadium
complexes and VOx/TiO2 prepared
by impregnation, the meso-VTN catalysts showed superior activity,
selectivity, and stability in the aerobic oxidation of ethyl lactate
to ethyl pyruvate. We also studied the effect of various vanadium
precursors, which revealed that the vanadium-induced phase transition
of meso-VTN from anatase to rutile depends strongly on the vanadium
precursor. NH4VO3 was found to be the optimal
vanadium precursor, forming more monomeric vanadium species. V4+ as the major valence state was incorporated into the lattice
of the NH4VO3-derived VTN material, yielding
more V4+–O–Ti bonds in the anatase-dominant
structure. In situ DRIFT spectroscopy and density functional theory
calculations show that V4+–O–Ti bonds are
responsible for the dissociation of ethyl lactate over VTN catalysts
and for further activation of the deprotonation of β-hydrogen.
Molecular oxygen can replenish the surface oxygen to regenerate the
V4+–O–Ti bonds.
Aqueous‐phase conversion of glyceraldehyde to lactic acid was investigated over Nb2O5, TiO2, ZrO2 and SnO2 in a fixed‐bed up‐flow reactor. Special attention was given to the catalysts acidity regarding the type, amount, strength and tolerance to water of surface acid sites. These sites were assessed by infrared spectroscopy of pyridine adsorbed on dehydrated and hydrated catalysts as well as by isopropanol decomposition. It was found that Nb2O5 and TiO2 have the highest fraction of water‐tolerant Lewis acid sites (40 and 47 %), while only 6 % was estimated for ZrO2. No relevant Lewis acidity was observed on SnO2, but it was noticed the presence of strong base sites. The transformation of glyceraldehyde into lactic acid proceeded via a cascade reaction in which glyceraldehyde is firstly dehydrated to pyruvaldehyde, followed by its rearrangement to lactic acid with the addition of a water molecule. The dehydration step occurs on Brønsted acid sites and/or on water‐tolerant Lewis acid sites. These latter sites also determine the selectivity to lactic acid. Strong base sites promote glyceraldehyde fragmentation leading to formaldehyde with high selectivity.
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