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Highly Efficient Oxidation of Ethyl Lactate to Ethyl Pyruvate Catalysed by TS-1 Under Mild Conditions Tianliang Lu, Junpeng Zou, Yuzhong Zhan, Xiaomei Yang, Yiqiang Wen, Xiangyu Wang, Lipeng Zhou, and Jie Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03558 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018
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ACS Catalysis
Highly Efficient Oxidation of Ethyl Lactate to Ethyl Pyruvate Catalysed by TS-1 Under Mild Conditions Tianliang Lua,b, Junpeng Zoua, Yuzhong Zhana, Xiaomei Yangb, Yiqiang Wenb, Xiangyu Wangb, Lipeng Zhoub,*, Jie Xuc a
School of Chemical Engineering and Energy, Zhengzhou University, 100 Kexue Road,
Zhengzhou 450001, People’s Republic of China b
College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Kexue Road,
Zhengzhou 450001, People’s Republic of China c
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China
ABSTRACT: Highly efficient oxidation of ethyl lactate (ELA) to ethyl pyruvate (EP) was realized over TS-1 in the presence of aqueous H2O2 (30 wt%) at low temperature without other solvent. 100% ELA conversion and 97.8% EP yield were obtained at 50 oC after 9 h. Conversion rate of ELA is sensitive to the reaction temperature. High reaction temperature (70 oC) leads to the increase of conversion rate of ELA, but causes the fast hydrolysis and decarboxylation of EP to form by-products of acetic acid and CO2. Based on the characterization (pyridine-FTIR and UV-vis) and reaction results, active species of Ti(OOH) were proven. Also, a non-radical
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mechanism for conversion of ELA to EP for this catalytic system was proposed. By kinetic analysis, the formation of Ti(OOH) is confirmed as the rate-determining step. Apparent activation energy (103.4 kJ mol−1) was also obtained. Furthermore, TS-1 was highly stable for the oxidation of ethyl lactate. There were almost no change in ELA conversion and EP yield throughout 10 reaction runs.
KEYWORDS: Ethyl lactate; Ethyl pyruvate; TS-1; Catalytic oxidation; Hydrogen peroxide INTRODUCTION Catalytic conversion of biomass to valuable chemicals has attracted much attention recently due to the environmental benign and sustainable character. Pyruvic acid and its esters (pyruvate) are important raw materials and chemical intermediates which are widely applied in food, plastic, pharmaceutical, and pesticidal industries.1-4 They can also be used as the precursors in the synthesis of bioactive substances.5,6 Nowadays, the commercial production of pyruvate is realized by the dehydrative decarboxylation of tartaric acid. Excess KHSO4 as dehydrating agent is required in this process, leading to the low atom efficiency and environmental pollution.7 Pyruvate can also be produced by microbial fermentation of carbohydrates.8-10 However, precise regulation of the reaction conditions including pH value, temperature and so on, low space-time yield, high cost of product separation and purification limit the industrialization of this method. Thus, development of a green, efficient, alternative route for the production of pyruvate is necessary. Lactic acid and esters (lactates) which can be produced readily from renewable biomass, such as cellulose, starch, sugars and glycerol, are highly functionalized (hydroxyl and carboxyl groups) bio-based feedstocks with great appeal.11-17 Molecular structure of lactates is similar to that of
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pyruvates. Valuable pyruvates can be produced from lactates by dehydrogenation method. Considering the excellent atom efficiency of this route, catalytic oxidative dehydrogenation strategies including vapour and liquid phase have been developed.18-28 In vapour phase strategy, various metal catalysts including Fe2O3-MoO3,18 TeO2-MoO3,18,19 Ni-Nb-O,20 MoVNbOx,21 iron phosphate,22 and MoO3-TiO223 were investigated. For this strategy, high reaction temperature favors the activation of lactates and O2 (oxidant) on surface of catalysts. However, high reaction temperature (≥200 oC) also leads to many side reactions, particularly decarboxylation of both lactates and pyruvates. Furthermore, heavy energy-consuming in this process would increase the production cost. Thus, selective synthesis of pyruvates from lactates under lower temperature in liquid phase is desired from the viewpoint of green chemistry. Transition metal oxides and noble metals were usually used as catalysts for the conversion of lactates to pyruvates in liquid phase. Haysshi’s group tested catalytic activity of various metal oxides on the conversion of lactates to pyruvates in liquid phase at 130 oC.24 TiO2, ZrO2 and SnO2 were proven to be highly selective catalysts for producing pyruvate. However, high selectivity can only be kept at a low lactate conversion. With the increase of reaction time and lactate conversion, pyruvate selectivity decreased dramatically. So, in this catalysis system, pyruvate yield is low (
