Highly Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate Catalyzed by Mesoporous Vanadia-Titania

Article abstract of DOI:10.1021/acscatal.7b03843

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 VO_x/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.

Full text of DOI:10.1021/acscatal.7b03843

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Article
Highly selective oxidation of ethyl lactate to ethyl
pyruvate catalysed by mesoporous vanadia–titania
Wei Zhang, Giada Innocenti, Paula Oulego, Vitaly Gitis, Hai-Hong Wu,
Bernd Ensing, Fabrizio Cavani, Gadi Rothenberg, and N. Raveendran Shiju
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03843 • Publication Date (Web): 09 Jan 2018
Downloaded from http://pubs.acs.org on January 9, 2018
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ACS Catalysis
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Highly selective oxidation of ethyl lactate to ethyl
pyruvate catalysed by mesoporous vanadiaꢀtitania
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Wei Zhang,a Giada Innocenti,b Paula Oulego,c Vitaly Gitis,d Haihong Wu,e Bernd Ensing*,a
Fabrizio Cavani,b Gadi Rothenberg a and N. Raveendran Shiju*a
a
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, P.O. Box 94157,
1090GD Amsterdam, The Netherlands.
b Dipartimento di Chimica Industriale, ALMA MATER STUDIORUM Università di Bologna,
Viale Risorgimento 4, 40136 Bologna, Italy.
c
Department of Chemical and Environmental Engineering, University of Oviedo, C/ Julián
Clavería, s/n. Eꢀ33071, Oviedo, Spain.
d
Unit of Environmental Engineering, BenꢀGurion University of the Negev, PO Box 653, Beerꢀ
Sheva 84105, Israel.
e
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of
Chemistry, East China Normal University, 3663 North Zhongshan Road, Shanghai, 200062,
China.
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ABSTRACT: The direct oxidative dehydrogenation of lactates with molecular oxygen is a
‘greener’ alternative for producing pyruvates. Here we report a oneꢀpot synthesis of mesoporous
vanadiaꢀtitania (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 density of isolated vanadium species, which can expose the active sites and
facilitate the mass transport. Compared to homogeneous vanadium complexes and VOx/TiO2
prepared by impregnation, the mesoꢀVTN catalysts showed superior activity, selectivity and
stability in the aerobic oxidation of ethyl lactate to ethyl pyruvate. We studied also the effect of
various vanadium precursors which revealed that the vanadiumꢀinduced phase transition of
mesoꢀVTN 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 NH4VO3 derived VTN material,
yielding more V4+–O–Ti bonds in anataseꢀdominant 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 activating the deprotonation of
βꢀhydrogen. Molecular oxygen can replenish the surface oxygen to regenerate the V4+–O–Ti
bonds.
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KEYWORDS: biomass conversion • heterogeneous catalysis • mesoporous materials • in situ
DRIFTS • DFT
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ACS Catalysis
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INTRODUCTION
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Lignocellulosic biomass is attracting increased attention as a renewable carbon source for
commodity chemicals.1 Unlike with fuel applications, the high oxygen content and diversity of
biomassꢀderived ‘platform molecules’ make them suitable feedstocks for highꢀaddedꢀvalue
chemicals.2 Lactic acid (LA) and lactates are such platform molecules. They can be converted to
several commodity chemicals, including acrylic acid, pyruvic acid, lactide, 1,2ꢀpropanediol, and
acetaldehyde.3 The direct catalytic air oxidation of LA is a promising route to pyruvic acid,4 a
key intermediate in the pharmaceutical, agrochemical and food additives sectors.5 Pyruvic acid
occurs naturally as an intermediate product in carbohydrate and protein metabolisms in the body.
Since it can be converted to carbohydrates, to fatty acids or energy or to the amino acid alanine,
it is a key intermediate in several metabolic processes. Pyruvic acid salts and esters (pyruvates)
are used as dietary supplements. For example, calcium pyruvate is used as a fat burner in the
food industry. Pyruvic acid is used as a starting material for the synthesis of pharmaceuticals,
such as Lꢀtryptophan and Lꢀtyrosine and for the synthesis of amino acids such as alanine and
phenyl alanine. Pyruvic acid derivatives are also used to produce crop protection agents,
cosmetic agents and flavouring ingredients. It is also a reagent for regeneration of carbonyl
compounds from semicarbazones, phenylhydrazones and oximes. Currently, pyruvates are still
made via the energyꢀintensive pyrolysis of tartaric acid with stoichiometric KHSO4 as a
dehydrating agent. Ideally, this would be replaced by direct oxidative dehydrogenation using
molecular oxygen, giving water as the only byꢀproduct. But even though this reaction is
thermodynamically feasible, its selectivity is low, because LA is easily overꢀoxidised.6
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