ARTICLE pubs.acs.org/IECR
Preparation of Pure Methyl Esters from Corresponding Alkali Metal Salts of Carboxylic Acids Using Carbon Dioxide and Methanol Prashant P. Barve,† Sanjay P. Kamble,† Jyeshtharaj B. Joshi,‡,§ Milind Y. Gupte,† and Bhaskar D. Kulkarni*,† †
Chemical Engineering and Process Development Division, National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India ‡ Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai-400019, India § Homi Bhabha National Institute (HBNI), Anushaktinagar, Mumbai-400094, India ABSTRACT: In the present article, for the first time, we report the synthesis of alkyl esters by direct esterification of alkali metal salts of carboxylic acids using carbon dioxide and alcohol. Methyl acetate, methyl benzoate, methyl salicylate, and methyl lactate have been synthesized by esterification of sodium acetate, sodium benzoate, sodium salicylate, and calcium lactate, respectively. The esterification reaction was carried out in a batch as well as in a semicontinuous mode of operation. A detailed study on the esterification of calcium lactate using methanol and carbon dioxide was carried out to record the effects of various operating parameters (like CO2 pressure, reaction temperature, moisture content in the calcium lactate, and initial concentrations of calcium lactate) on the esterification reaction. This synthesis route produces highly pure methyl lactate by direct esterification of calcium lactate with calcium carbonate as byproduct. The byproduct calcium carbonate was characterized for its crystallinity, surface area, and pore volume. The process route has the advantage that the synthesized byproduct can be recycled into the fermenter to make corresponding alkali metal lactate or the finely precipitated calcium carbonate can be used for various other applications. Thus, the recovery and recycle of alkali metal is possible providing a pollution free process for synthesis of pure methyl lactate.
1. INTRODUCTION The chemical process industries, all over the world, have been facing the challenges of developing innovative products and processes in the wake of eroding profit margins amidst highly globalized trade competition and fast growing environmental constraints.1 Biorenewable chemicals are at the center stage, because of rising global crude oil prices and a growing desire to reduce dependence on petroleum. Biobased chemicals typically are environmentally friendly, possess low toxicity, and have favorable biodegradability, making them prime candidates for replacements of petroleum-based products. The esters of biobased organic acids like lactic acid, citric acid, etc. fall into the category of benign or green solvents and are promising replacements for halogenated petroleum-based solvents in a wide variety of applications.2,3 Alkyl esters can be used as additives in a variety of products, including paints, grease removers, packaging, and cleansers. Low-cost salt esters can potentially be used to produce other chemicals such as copolymers of biodegradable plastics, acrylates, glycol, and other specialty chemicals.4 Highly pure s-(-)-methyl lactate is one of the important esters of biobased organic acid product having interesting applications at an industrial level. It can be used in pharmaceuticals and in the production of high purity lactic acid. The lactic acid so produced finds applications in dairy products, as an acidulating agent in the alimentary field, as an intermediate for the production of plasticizer agents, adhesives, pharmaceutical products, as a mordant in wool dying, and many others. Similarly, the high purity lactic acid prepared from high purity methyl lactate has considerable prospects in industrial development and production of biocompatible and biodegradable polymers which are useful for manufacturing bags, application films, in the field of sanitary and medical applications, etc.4,5 r 2011 American Chemical Society
Development of environmentally benign processes based on the utilization of greenhouse gases such as carbon dioxide has therefore gained considerable attention in recent years.6 To the best of our knowledge, the preparation of alkyl esters using carbon dioxide gas and alcohol and its great potential in synthesis have not yet been realized. In the present article methyl acetate, methyl benzoate, methyl salicylate, and methyl lactate have been synthesized by esterification of sodium acetate, sodium benzoate, sodium salicylate, and calcium lactate. The esterification reaction was carried out in batch as well as semicontinuous mode of operation. The detailed study pertaining on the esterification of calcium lactate using methanol and carbon dioxide has been performed. The effect of various operating parameters like CO2 pressure, reaction temperature, moisture content in the calcium lactate, and initial concentrations of calcium lactate on esterification reaction were studied in detail.
2. EXPERIMENTAL SECTION 2.1. Materials. All the reagents used for experimental studies were of analytical reagent grade. Calcium lactate (purity >99%) was obtained from V. P. Chemicals, Pune, India. Methyl benzoate, methyl acetate, methyl salicylate, sodium benzoate, sodium acetate, sodium salicylate, dried methanol, and CaCO3 standard of analytical grade were procured from E. Merck India Ltd., Mumbai. Special Issue: Nigam Issue Received: March 29, 2011 Accepted: September 15, 2011 Revised: September 12, 2011 Published: September 15, 2011 1498
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Figure 1. Schematic experimental setup for esterification of alkali metal salts of carboxylic acid.
Carbon dioxide gas (purity >99.9999%) was procured from Deluxe Gas Limited, Pune, India. 2.2. Characterization. The synthesized byproduct calcium carbonate sample was characterized for its crystallinity, surface area and pore volume. The X-ray diffraction pattern of the calcium carbonate was examined using XRD. Powder X-ray diffraction studies were carried on Phillips analytical diffractometer with monochromated CuKα radiation (λ- 1.54 Å). The scanning range of 2θ was set between 1° and 10°. BET surface area, pore volume, and average pore diameter of the calcium carbonate samples were determined by N2 adsorption desorption technique (Quantachrome Autosorb NOVA-1200). 2.3. Apparatus. Figure 1 shows the schematic diagram for the esterification of alkali metal salts of carboxylic acids using methanol and carbon dioxide gas. The esterification reaction was performed in a 5 L stainless steel high pressure autoclave (Amar Equipments Ltd.,
Mumbai) equipped with temperature sensor, pressure regulator, agitator speed monitor, solenoid valve, condenser, and a sampling port. The agitator consists of two impellers viz. the pitched blade impeller and the other gas inducing impeller. The latter is located at the bottom of the agitator in order to introduce the carbon dioxide gas at the bottom of the reaction mixture. The temperature, pressure, and speed of the agitator were monitored through the PID control panel. 2.4. Experimental Procedure. 2.4.1. Dehydration of Calcium Lactate Powder. Calcium lactate powder was dried under vacuum (50 mbar) and at temperature 90 95 °C using vacuum dryer for 24 h. The moisture in the calcium lactate was reduced from 23% to 10%, 6% and 1.5% in order to understand the effect of moisture content on the equilibrium formation of methyl lactate. The calcium lactate powder with definite moisture content was used in the subsequent experiments. 1499
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Table 1. Esterification of Sodium Acetate, Sodium Benzoate, Sodium Salicylate, and Calcium Lactate Using Carbon Dioxide and Methanol sr. no
type of carboxylic
moles salts of
acid salt used
carboxylic acid
moles of methanol
CO2 pressure Kg/cm2
reaction temperature °C
product compositions
yield (%)
1
sodium acetate
1.00
37.50
40.07
170
methyl acetate sodium acetate
81.00 19.00
2
sodium benzoate
1.74
46.88
59.8
170
methyl benzoate
59.10
sodium benzoate
40.90
3
sodium benzoate
4.00
23.30
atmospheric pressure under
170
CO2 flow 4
5
sodium salicylate
calcium lactate
0.58
1.38
46.88
28.13
63.3
170
54.2
2.4.2. Direct Esterification of Alkali Metal Salts of Carboxylic Acids Using Methanol and Carbon Dioxide: Batch Mode of Operation. A known quantity of dry alkali metal salt of carboxylic acid and pure methanol was charged in the stainless steel high pressure reactor and continuously stirred at 13.6 rps using agitator for 15 min at ambient temperature in order to get homogeneous solution. Initially the reactor was flushed twice with 1.5 kg/cm2 N2 gas for removal of air from the high pressure reactor. Then a known amount of carbon dioxide gas at the desired pressure was taken into the reactor through carbon dioxide gas cylinder via CO2 regulator. Then the reaction mixture was heated in order to reach the desired temperature. When the desired reaction conditions (reaction temperature and pressure) were reached, then it was considered as the zero reaction time, and the sample was withdrawn through the sample tube as a zero time sample. Subsequently samples were withdrawn at specific time intervals through the sample tube connected to a cooled metal tube and immediately transferred to an ice bath in order to ensure that no further reaction occurs. The experiment was continued until equilibrium formation of alkyl ester was obtained. During the experiment, reaction pressure, temperature, and agitator speed were continuously monitored using the PID control panel. 2.4.3. Direct Esterification of Sodium Benzoate Using Methanol and Carbon Dioxide: Semicontinuous Mode of Operation. Semicontinuous esterification of alkali metal salt of carboxylic acid namely sodium benzoate was carried out in the same stainless steel high pressure autoclave of 5 L capacity. The dehydrated sodium benzoate along with a trace amount of toluene was charged a high pressure reactor vessel along with 800 g of diphynyl oxide and continuously stirred at 15 rps using agitator. Then reaction mixture was heated until 170 °C and was maintained at this temperature throughout the reaction. Pure methanol was passed through a sparger at the bottom of reactor at the rate of 200 g/h. Similarly, carbon dioxide gas was also passed simultaneously from the bottom of the reactor through a separate gas sparger at the rate of 40 L/h. The vapors of unreacted methanol and methyl ester formed, and unreacted carbon dioxide gas rose through the reactor; methyl ester and menthol were condensed in the condenser and collected in the receiver as distillate. This operation was continued for 5 h. The distillate collected was analyzed for its methyl benzoate. 2.4.4. Analysis. In the case of the esterification reaction, samples were centrifuged and filtered through a membrane filter to
175
methyl benzoate
35.80
sodium benzoate
64.20
methyl salicylate
47.
phenol
7.30
sodium salicylate methyl lactate
45.10 81.2
lactic acid
9.34
calcium lactate
9.39
separate particulate matters. The concentration of alkyl esters and methanol were analyzed using Shimadzu made GC-MS Model-QP5000. The concentration of methyl lactate and lactic acid was also measured by an HPLC (Dionex make) equipped with a C18 Hypersil BDS column, Dionex Summit P 680A pump, and Dionex ASI 100 autosampler. The mobile phase consisted of 0.01 M potassium dihydrogen phosphate (pH 2.5) with 7% acetonitrile as an organic modifier. The chromatographic peaks were monitored using a UV detector at a detection wavelength of 205 nm. The moisture content in the calcium lactate, methanol, and reaction samples were analyzed by the Karl Fisher titration method using automatic Karl Fischer instrument supplied by M/s Lab India Ltd., Mumbai. In the case of esterification of calcium lactate, after completion of the reaction, the resultant mass was allowed to cool until 30 °C, and the reaction mixture was filtered on the basket centrifuge at 50 rps. The wet cake obtained from the centrifuge consisted of calcium carbonate and some traces of unreacted calcium lactate. The unreacted calcium lactate was removed by addition of excess methanol, and the resulting solid mass was allowed to dry in the oven at 100 110 °C. The dry weight of calcium carbonate was measured. The material balance of the reaction was found to be within (5%.
3. RESULTS AND DISCUSSION 3.1. Esterification of Sodium Benzoate, Sodium Acetate, Sodium Salicylate, and Calcium Lactate Using Carbon Dioxide and Methanol. The preparation of pure alkyl esters such as
methyl benzoate, methyl acetate, methyl salicylate, and methyl lactate from the corresponding salts of carboxylic acid sodium benzoate, sodium acetate, sodium salicylate, and calcium lactate using carbon dioxide and alcohol like methanol has been attempted. The esterification reaction was performed at different CO2 pressures as well as atmospheric pressure and reaction temperature 170 °C. Table 1 shows the yield of methyl acetate, methyl benzoate, methyl salicylate, and methyl lactate under these experimental conditions. From Table 1 it can be concluded that the alkyl esters can be synthesized from the alkali metal salts of carboxylic acid using an alcohol like methanol and carbon dioxide. In view of the importance of methyl lactate, further experiments were carried out on esterification of calcium lactate using carbon dioxide gas and methanol. 3.2. Characterizations Calcium Carbonate Reaction Byproduct. Figure 2 shows the XRD pattern of standard calcium 1500
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Table 2. BET Surface Area, Pore Size, and Pore Volume of Byproduct Calcium Carbonate sr.
source of calcium
surface area
pore volume
avg. pore size
no
carbonate
(m2/g)
(cm3/g)
(Å)
1 2
standard synthesized in the
16.10 65.21
0.0618 0.308
153.5 189.1
present work
Figure 2. Comparison of X-ray diffraction pattern of the synthesized calcium carbonate with standard calcium carbonate.
carbonate sample and one prepared by the esterification of calcium lactate using CO2 and methanol. It can be seen that the phase of the synthesized calcium carbonate was well-crystallized calcite with a hexagonal structure (JCPDS 83-0577 and 83-1762) and match with the standard calcium carbonate sample. The results of BET surface area and pore volumes obtained by a multipoint adsorption method are presented in Table 2 and show that the calcium carbonate prepared in the present process gives a higher specific surface area than the standard calcium carbonate sample indicating that particle size of synthesized calcium carbonate in the present study is much smaller than that of standard calcium carbonate. The CaCO3 synthesized also shows higher pore volume as well as average pore size as compared to the CaCO3 standard. 3.3. Reaction Scheme for the Esterification of Calcium Lactate Using CO2 and Methanol. The schematic representation of the esterification reaction is shown in Figure 3. Eq (1) shows the overall reaction, while eqs (2), (3), and (4) shows steps involved in the reaction mechanism. The details of each reaction steps are discussed as follows: 1) In the first step, the formation of carbonic acid takes place via carbon dioxide reaction with water. This is followed by dissociation of carbonic acid to give protons (eq 2). 2) In the second step, calcium lactate reacts with carbonic acid giving calcium carbonate and lactic acid. This reaction proceeds with abstraction of proton by lone pair electron of oxygen of calcium lactate and creates carbocation which is neutralized by releasing a calcium atom. These calcium ions then combine with carbon dioxide giving calcium carbonate and lactic acid. This reaction appears to be instantaneous (eq 3). 3) In the third step lactic acid undergoes an esterification reaction with methanol and produces methyl lactate as a final product (eq 4). In this reaction, a certain amount of water is very essential to produce lactic acid and subsequently methyl lactate. In order to understand the effect of moisture content in the calcium lactate further experimental results are discussed in Section 3.4.4. 3.4. Effect of Operating Parameters on Esterification of Calcium Lactate. Several batch experiments were carried out to study the effects of various operating parameters like reaction temperature, CO2 pressure, moisture content in the calcium lactate, and initial concentration of calcium lactate on esterification of calcium lactate in methanol.
3.4.1. Effect of Temperature. The effect of increasing the reaction temperature from 150 to 185 °C on the esterification of calcium lactate in methanol [at moisture content 6%, CO2 pressure 20 kg/cm2, 25% calcium lactate wt% and impeller speed 13.6 rps] was studied in the present work. It can be seen from Figure 4 that the equilibrium formation of methyl lactate (MLA) increases with an increase in the reaction temperature, while the equilibrium concentration of lactic acid (LA) decreases as the temperature increases. This indicates that, at higher temperature, most of the lactic acid formed is immediately converted into methyl lactate. During the course of the reaction a small amount of water (2 10%) is formed which helps the dissociation of calcium lactate to lactic acid. Figure 5 shows typical profiles of reaction products of the direct esterification of calcium lactate using CO2 and methanol. As can be seen the conversion of calcium lactate into methyl lactate increases with an increase in reaction time, while lactic acid concentration decreases during the course of the reaction. From Figure 5 it is also clear that moisture content initially increases and after about 4 h it becomes nearly constant. The general method of preparation of methyl lactate reported in the literature involves acidification of alkali metal lactate (calcium lactate) with mineral acid like sulfuric acid to generate crude lactic acid and alkali metal sulfate (CaSO4) as the byproduct.6,7 The esterification of crude lactic acid at higher temperature gives methyl lactate as product with water as byproduct. This process is associated with a number of drawbacks like (i) due to high temperature esterification, substantial quantities of undesired byproducts like hydroxyl methyl furfural, 2-pentene-1-ol, etc., are formed; (ii) the isolation of methyl lactate in pure anhydrous form from the mixture of alcohol, water, and methyl lactate is difficult, because methyl lactate forms an azeotrope with water and also reacts with water; (iii) the crude lactic acid at high temperature and at low pH is corrosive in nature and hence requires expensive material of construction during handling; and (iv) alkali metal sulfate (CaSO4) is produced as a byproduct and cannot be recycled to the fermentator resulting in a continuous demand of fresh calcium salt for fermentation.5,7,8 The advantage of this synthesis route produces highly pure methyl lactate by direct esterification of calcium lactate with calcium carbonate as byproduct. The merit of this route is that i) in this reaction dehydrated calcium lactate was used and during the reaction a small amount water is formed which is immediately consumed in the formation of carbonic acid (the detailed reaction scheme is shown in Figure 3) and hence the water content in the reaction mixture is very negligible. Therefore separation of methyl lactate from lactic acid and a byproduct (calcium carbonate) is quite easy. ii) In this route calcium carbonate is formed as a byproduct which can be recycled into the fermenter to make the corresponding alkali metal lactate or the finely precipitated calcium carbonate can be used for various other applications. Thus, the recovery and recycle of alkali metal is possible providing a pollution free process for synthesis of pure methyl lactate by avoiding the formation of calcium sulfate. 1501
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Figure 3. Detailed reaction mechanism of esterification of calcium lactate using CO2 and methanol.
3.4.2. Effect of CO2 Pressure. The effect of CO2 pressure in the range of 5 to 20 kg/cm2 on esterification of calcium lactate using methanol [at initial moisture content 6%, reaction temperature 175 °C, 25% calcium lactate wt% and impeller speed 13.6 rps] was examined, and the results are shown in Figure 6. As can be seen the equilibrium formation of methyl lactate substantially increases with an increase in CO2 pressure, while the lactic acid concentration decreases with an increase in CO2 pressure. This is because the solubility of CO2 gas in methanol increases with an increase in pressure.
3.4.3. Effect of Initial Concentration of Calcium Lactate. The effect of initial concentration of calcium lactate on esterification was studied at fixed initial moisture content 6%, reaction temperature 175 °C, impeller speed 13.6 rps, and CO2 pressure 10 kg/cm2 and is shown in Figure 7. The rate of methyl lactate formation increases with an increase in reaction time up to 4 h, and then it becomes nearly constant. It was also found that at 5%, 15%, and 25% initial concentration of calcium lactate, the conversion of calcium lactate into methyl lactate is high; however at 35% initial 1502
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Figure 4. Effect of reaction temperature on esterification of calcium lactate using CO2 and methanol [moisture content 6%, CO2 pressure 20 kg/cm2, 25% calcium lactate wt%, and impeller speed 13.6 rps]: • moles of methyl lactate (gmol/kg) and 2 moles of lactic acid (gmol/kg).
Figure 5. Typical pattern of reaction products profile during the esterification of calcium lactate using CO2 and methanol [reaction temperature160 °C, moisture content 6%, CO2 pressure 20 kg/cm2, 25% calcium lactate wt%, and impeller speed 13.6 rps]: • moles of lactic acid (gmol/kg), 9 moles of methyl lactate (gmol/kg), and 2 moisture content (%w/w).
concentration of calcium lactate, the conversion of calcium lactate to methyl lactate decreases. At initial 35% concentration of calcium lactate, the moisture content in the reaction solution is high and hence backward reaction rate becomes dominant. Another possible reason may be at that lower concentration of calcium lactate (initial concentration of calcium
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Figure 6. Effect of CO2 pressure on esterification of calcium lactate in methanol [moisture content 6%, reaction temperature 175 °C, 25% calcium lactate wt%, and impeller speed 13.6 rps]: • moles of methyl lactate (gmol/kg) and 2 moles of lactic acid (gmol/kg).
Figure 7. Effect of initial calcium lactate concentration on esterification of calcium lactate using CO2 and methanol [moisture content 6%, reaction temperature 175 °C, CO2 pressure 10 kg/cm2, and impeller speed 13.6 rps]: ( 5% wt calcium lactate concentration, 9 15% wt calcium lactate concentration, 2 25% wt calcium lactate concentration, and 35% wt calcium lactate concentration.
lactate 5 and 15 w/w%), the ratio of methanol to lactic acid is high, and hence the rate of formation of methyl lactate is higher and vice versa. 1503
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Figure 8. Concentration profile of lactic acid during the esterification of calcium lactate at different initial concentration of calcium lactate [moisture content 6%, reaction temperature 175 °C, CO2 pressure 10 kg/cm2, and impeller speed 13.6 rps]: • 5% wt calcium lactate concentration, ) 15% wt calcium lactate concentration, 2 25% wt calcium lactate concentration, and 35% wt calcium lactate concentration.
Figure 8 also shows the concentration of lactic acid at different initial concentration of calcium lactate. Figure 8 show the concentration of lactic acid at 35% concentration of calcium lactate is very high as compared to 5, 10, and 25 w/w% of initial concentration of calcium lactate. It was observed that the lactic acid concentration was nearly zero at reaction time of 5 h, while at higher initial concentration of calcium lactate of 25 and 35%w/w, the lactic acid concentration was found to be around 0.3 and 0.7 g mol/kg respectively. This also indicates that the backward reaction rate (rate of formation of lactic acid) is higher at 35% initial concentration of calcium lactate than 5%, 15%, and 25% initial concentration of calcium lactate. 3.4.4. Effect of Initial Moisture Content. The effect of varying the moisture content in the calcium lactate on its esterification in methanol [at fixed pressure 20 kg/cm2, reaction temperature 175 °C, 25% calcium lactate wt%] was examined and shown in Figure 9. The moisture content in the calcium lactate was varied from 1.5% to 23%. Generally calcium lactate contents 23% moisture at room temperature. It was observed that the moisture content in the calcium lactate has a significant effect on esterification of calcium lactate. From Figure 9 it can be observed that, with an increase in the initial moisture content, the equilibrium formation of methyl lactate decreases. In the presence of water, the formation of lactic acid reaction becomes dominant during the initial reaction period; because of ionization of calcium lactate and subsequently the rate of esterification reaction decreases (a higher concentration of water hampers the rate of esterification reaction). In the absence of water the reaction rate was found to be very sluggish. Hence an optimum amount of water (1.5 to 6%) is essential for the direct esterification of calcium lactate to methyl lactate using CO2 and methanol.
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Figure 9. Effect of initial moisture content in the calcium lactate on esterification of calcium lactate using CO2 and methanol [reaction temperature 175 °C, 25% calcium lactate wt%, CO2 pressure 20 kg/cm2, and impeller speed 13.6 rps]: • moles of methyl lactate (gmol/kg).
4. CONCLUSIONS In this article a novel eco-friendly route for the preparation of pure alkyl esters has been presented using alkali metal salts of carboxylic acid, carbon dioxide, and methanol. This synthesis route produces highly pure methyl lactate by direct esterification of calcium lactate with calcium carbonate as byproduct. The synthesized byproduct namely calcium carbonate was found to be a well-crystallized calcite form with a hexagonal structure. The synthesized calcium carbonates prepared (CaCO3synthesized) in the present process gives a higher specific surface area as compared to the standard calcium carbonate CaCO3. From the experimental results it was observed that, with an increase in the CO2 pressure and temperature, the equilibrium formation of methyl lactate increases. However, with an increase in moisture content in the calcium lactate in methanol, the equilibrium formation of methyl lactate decreases. The process route has the advantage that the synthesized byproduct can be recycled into the fermenter to make the corresponding alkali metal lactate or the finely precipitated calcium carbonate can be used for various other applications. Thus, the recovery and recycle of alkali metal is possible providing a pollution free process for synthesis of pure methyl lactate. ’ AUTHOR INFORMATION Corresponding Author
*Phone: 020 25902150. Fax: 020 25902612. E-mail: bd.kulkarni@ ncl.res.in.
’ ACKNOWLEDGMENT Authors thankful to Dr. V. B. Chavan, Mr. P. A. Bhujang, Mr. Ravi Mawale, and Mr. Kailas Barange for kind help during experimental work and analysis of reaction samples. 1504
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’ REFERENCES (1) Pal, P.; Sikder, J.; Roy, S.; Giorno, L. Process intensification in lactic acid production: A review of membrane based processes. Chem. Eng. Process. 2009, 48, 1549. (2) John, R. P.; Nampoothiri, K. M.; Pandey, A. Fermentative production of lactic acid from biomass: an overview on process developments and future perspectives. Appl. Microbiol. Biotechnol. 2007, 74, 524. (3) Kolah, A.; Asthana, N. S.; Vu, D. T.; Lira, C. T.; Miller, D. J. Reaction kinetics of the catalytic esterification of citric acid with ethanol. Ind. Eng. Chem. Res. 2007, 46, 3180. (4) Vijayakumar, J.; Aravindan, R.; Viruthagiri, T. Recent trends in the production, purification and application of lactic acid. Chem. Biochem. Eng. 2008, (Q) 22 (2), 245. (5) Barve, P. P.; Kulkarni, B. D.; Nene, S. N.; Shinde, R. W.; Gupte, M. Y.; Kamble, S. P. Novel processes for the preparation of pure alkyl esters from alkali metal salts of carboxylic acids using carbon dioxide and alcohols. Provisional Patent Application No NF-15/2009, 1842/DEL/ 2009. (6) Spivey, J. J.; Wilcox, E. M.; Roberts, G. W. Direct utilization of carbon dioxide in chemical synthesis: Vinyl acetate via methane carboxylation. Catal. Comm. 2008, 9, 685. (7) Joglekar, H. G.; Rahman, I.; Babu, S.; Kulkarni, B. D.; Joshi, A. Comparative assessment of downstream processing options for lactic acid. Sep. Purif. Technol. 2006, 52, 1. (8) Barve, P. P.; Kulkarni, B. D.; Shinde, R. W.; Gupte, M. Y.; Joshi, C. N.; Thite, G. A.; Chavan, V. B.; Deshpande, T. R. Process for preparing L(+) lactic acid. Publication No WO/2007/010548 A1. (9) Barve, P. P.; Rahman, I.; Kulkarni, B. D. Pilot plant study of recovery of lactic acid from ethyl lactate. Org. Process Res. Dev. 2009, 13, 573.
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