In recent years, due to the oversupply of glycerol globally, its price has dropped dramatically or become valueless. The aim of this study is to convert glycerol into high value-added compounds such as glycolic acid and lactic acid in one-pot electrochemical cell. The electrochemical process was carried out over platinum (as anode electrode) and activated carbon composite (as cathode electrode), with amberlyst-15 as reaction catalyst. The results obtained have proven that this simple method is applicable to produce glycolic acid and lactic acid in one step electrochemical process with a total product yield above 70 %. Finally, the overview reaction mechanism to the formation of these products was proposed.
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Ashri Wan Daud, Wan Mohd

Cognet, Patrick

Kheireddine Aroua, Mohamed

Lee, Ching Shya

Pérѐs, Yolande

English

Engineering Conferences International

PRODUCTION OF LACTIC ACID AND GLYCOLIC ACID IN ONE-POT ELECTROCHEMICAL CELL  Ching Shya Lee, University of Toulouse, France and University of Malaya, Malaysia leecs@um.edu.my Mohamed Kheireddine Aroua, University of Malaya, Malaysia Wan Mohd Ashri Wan Daud, University of Malaya, Malaysia Patrick Cognet, University of Toulouse, France Yolande Pérѐs, University of Toulouse, France   Key Words: Bioglycerol, Amberlyst-15, Electro-oxidation, Electro-reduction, Platinum  In recent years, due to the oversupply of glycerol globally, its price has dropped dramatically or become valueless. The aim of this study is to convert glycerol into high value-added compounds such as glycolic acid and lactic acid in one-pot electrochemical cell. The electrochemical process was carried out over platinum (as anode electrode) and activated carbon composite (as cathode electrode), with amberlyst-15 as reaction catalyst. The results obtained have proven that this simple method is applicable to produce glycolic acid and lactic acid in one step electrochemical process with a total product yield above 70 %. Finally, the overview reaction mechanism to the formation of these products was proposed.  Introduction  The rapid growth of world biodiesel production has caused a surplus of glycerol, as glycerol is a by-product generated during the biodiesel production. Therefore, extensive research is done by worldwide in order to transform glycerol via chemical or biological pathway into various high added-value chemicals such as glyceric acid, glycolic acid, 1,3-propanediol, 1,2-propanediol, acrolein, glycerol carbonate, dihydroxyacetone, lactic acid, etc [1-3]. However, these processes have many drawbacks, such as high running production cost as high pressure and high temperature are required [4, 5]. In addition, non-environmental friendly solvent such as sulfolane, 1-methyl-pyrrolidinone, 1,3-dimethyl-2-imidazolidinone are widely used in the chemical process [4, 6-9]. Those drawbacks may have negative impacts to the environment and make these processes economically not practical.  In order to address the limitations from these existing methods, electrochemical method can be considered as an environmental friendly process in which the chemical reagents are substituted by the electron. From the previous reported works, glyceric acid, glycolic acid, glyceraldehyde, formic acid, etc have been obtained as the main products during the electro-oxidation via platinum, gold and palladium electrode [10-13]. The selectivity towards the production of these oxidized compounds is highly dependant on pH and applied potential.  Among these oxidation compounds, glycolic acid is one of the chemicals that find broad application in food and pharmaceutical industry. Another useful chemical that is greatly produced from glycerol is lactic acid. This chemical also finds wide utilization in food, personal care and pharmaceutical industry. So far, the production of lactic acid is preferably based on fermentation of carbohydrates; however calcium lactate was generated through this process [14]. Lux et. al. introduced a new approach by combining the electro-oxidation of glycerol with chemical catalytic pathway. In their study, the selectivity towards lactic acid is high but it involved of complicated reaction set-up [15].     In this study, a simple electrochemical method is presented, to convert glycerol into lactic acid and glycolic acid with solid acid catalyst amberlyst-15 in one-pot electrochemical cell over platinum (Pt) and activated carbon composite (ACC) electrode. Lactic acid is reported for the first time to be produced via one step electrochemical process without involving complicated reaction pathway. In this paper, both electro-oxidation and electro-reduction of glycerol will be presented and discussed in details. The reaction mechanism is proposed.   Experimental  Electrode preparation  Activated carbon composite electrode (ACC) with diameter 1.5 cm was prepared by mixing 80 % activated carbon (99.5 % purity with average particle size of 100 μm and 950 m2/g specific surface areas, Sigma Aldrich) with 20 % carbon black (99 % purity with average particle size of 13 nm and 550 m2/g specific surface areas, Alfa-chemicals). The morphological appearance of the ACC electrode was studied by scanning electron miscroscopy (SEM) (Hitachi SU-8000, Japan).  Electrochemical conversion of glycerol Electrochemical reaction of 0.15 M glycerol (0.10 L) with 3.0 g amberlyst-15 (with 0.014 mole of free active ion H+) was carried out in a one-pot electrochemical cell as shown in Figure 1. Pt electrode (with surface area of 44.0 cm2) and ACC electrode (with active surface area of 34.7 cm2) were used as the anode and cathode electrode, respectively. The reaction temperature was controlled by a heating jacket at 80 ⁰C temperature; a magnetic bar was used to stir the mixture at 800 rpm. 1.5 g of sodium sulfate salt (Na2SO4) was added into the solution in order to improve the conductivity. External electricity was supplied by a regulated DC power supply at a 1.0 A constant current.    Figure 1: Electrochemical reaction set-up  Analysis  The chemical compounds that were produced from the electrochemical processes were identified by gas chromatography-mass spectroscopy (GC-MS) (Agilent, Model 7890). The glycerol conversion and percentage of yields was quantified by gas chromatography (GC) (Model 6890, Agilent). The GC-MS and GC analysis conditions are shown in Tables 1 and 2.    water out  Anode electrode (platinum) Cathode electrode (activated carbon composite) DC power supply Magnetic stirrer water in water out condenser Reactant (Glycerol + Amberlyst-15) water in Table 1: Gas chromatography-mass spectroscopy analysis conditions Column DB-23 MS (30m x 0.25mm x 0.25 μm) Detector  MS Carrier Gas He Injector Temperature 240°C Flow Rate 0.8 ml/min Injection volume 1 µl Oven Temperature 45°C (5 min)  10°C/min  240°C (5 min)  Ionisation Electron impact (70eV)  Table 2: Gas chromatography analysis conditions Column ZB-Wax (30m x 0.25mm x 0.25 μm) Detector  FID Carrier Gas He  Injector Temperature 240°C Detector  240°C Flow Rate 2 ml/min Injection volume 1 µl Oven Temperature 45°C (5 min)  10°C/min  240°C (5 min)   Results and Discussions  Scanning electron microscopy analysis (SEM)  SEM was used to investigate the surface morphology of ACC electrode before (virgin) the electrochemical reaction. The scans were performed at high magnification (x 50,000 times) with image resolution at 1.00 μm. The SEM image for virgin ACC electrode (Figure 2) clearly showed that ACC had high porosity with average pore sizes range from 90 to 170 nm. This porous surface could play an important role in the electrochemical reaction, which will be described in details in the following section.  (b)  Figure 2: SEM images of virgin ACC electrode  Electrochemical conversion of glycerol to lactic acid   Under acidic condition, glycolic acid and lactic acid were the main products obtained through the electrochemical process. In Figure 3, it is clearly shown that the amount of glycerol was slowly converted to glycolic acid and lactic acid, passing a maximum level at 5-hour, given the yield of 52.0 % and 27.0 % with correspondent selectivity of 62.0 % and 31.9 %, respectively. Other minor products such as acetaldehyde, formic acid, acetol, ethylene glycol, glyceraldehyde, acetic acid, 1,2-propanediol and 1,3-propanediol were also obtained from this reaction.    Figure 3: Effect of reaction time on glycerol conversion and percentage of yield for glycolic acid and lactic acid  Results analysis shows that glycerol C-C bond was broken, leading to the formation of a 2-C compound (glycolic acid). Nevertheless, glycerol was also able to be electro-oxidised to dihydroxyacetone (DHA) [2]. DHA was unstable in acidic conditions and it further oxidised to pyruvic acid in anodic region. In this case study, the electrolysis process was carried out in a one-pot electrochemical cell. Therefore, pyruvic acid was further reduced to lactic acid at the cathodic region. As described earlier using SEM analysis, ACC with high porosity could help to hold the reaction intermediate from oxidation, thus facilitating the electro-reduction process [16]. The reaction mechanism is shown in Scheme 1.     010203040506070809010001020304050607080901000 1 2 3 4 5 6 7 8Percentage of Yield (%, in mole C)Glycerol conversion (%, in mole C)Time (hour)Glycerol conversionLactic acidGlycolic acid Scheme 1: Proposed reaction mechanism of the electrochemical conversion of glycerol in amberlyst-15 medium on ACC cathode electrode and Pt anode electrode. Conclusions The electrochemical study of glycerol under acidic condition (amberlyst-15) over Pt anode electrode and ACC cathode electrode was investigated. This novel electrochemical method successfully converted glycerol to lactic acid and glycolic acid in one-step process without using complicated reaction set-up. In this case study, porous surface of ACC electrode was reported to give an added advantage to the electrochemical reduction process. In addition, amberlyst-15 has improved the product yields, as it may have worked as a redox catalyst during this electrosynthesis process, given a total product yields as high as 70 %. Continuation product separation is the main challenge for this work.   References [1] S. Bagheri, N.M. Julkapli, W.A. Yehye, Catalytic conversion of biodiesel derived raw glycerol to value added products, Renewable and Sustainable Energy Reviews, 41 (2015) 113-127. [2] M. Simões, S. Baranton, C. Coutanceau, Electrochemical Valorisation of Glycerol, ChemSusChem, 5 (2012) 2106-2124. [3] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. Della Pina, From Glycerol to Value-Added Products, Angewandte Chemie International Edition, 46 (2007) 4434-4440. [4] T. Kurosaka, H. Maruyama, I. Naribayashi, Y. Sasaki, Production of 1,3-propanediol by hydrogenolysis of glycerol catalyzed by Pt/WO3/ZrO2, Catalysis Communications, 9 (2008) 1360-1363. [5] C.S. Lee, M.K. Aroua, W.M.A.W. Daud, P. Cognet, Y. 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Production of lactic acid and glycolic acid in one-pot electrochemical cell

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Production of lactic acid and glycolic acid in one-pot electrochemical cell

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