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J. Cent. South Univ. Technol. (2010) 17: 45−49 DOI: 10.1007/s11771−010−0009−3

Preparation of lactic from in ionic system

HUANG Jia-ruo(黄嘉若), LI Wen-sheng(李文生), ZHOU Xiao-ping(周小平)

College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: A new reaction system was designed to economically convert glucose to lactic acid environment-friendly. Hydrophobic ionic were chosen as solvent that can promote the decomposition reaction of glucose, and the catalytic performance of the solid bases was evaluated. Both the reaction temperature and time can affect the yield of lactic acid. A high yield (97%) of lactic acid was achieved under the optimal reaction condition. The 1H NMR spectra and HPLC-MS were used to identify the formation of the lactic acid and variations of ionic liquid. It is found that ionic-liquids have a unique solvent effect for glucose and bases. can be used as solvent to extract calcium lactate. This shows a great potential of hydrophobic ionic liquids in the solid bases catalyzed reaction that is limited by the weak of solid bases in organic and water solution.

Key words: glucose; lactic acid; ionic liquid; solid ; catalysis

100 ℃. However, in order to obtain lactic acid with a 1 Introduction high yield of 57%, the excess amount of hydroxide must be used. Excessive sodium hydroxide A sustainable future for the chemical industry leads to a number of problems. It is particularly difficult requires feedbacks based on the renewable rather than to separate sodium lactate from sodium hydroxide. JIN steadily depleting resources. Inability to effectively et al [13] reported an improvement for this process transform five or six-carbon carbohydrates into building through adding the to oxidize all blocks derived from the nature is the major barrier organic to at a yield of 75%. towards this challenging goal. Both and glucose, ZHAO et al [14] suggested that ionic liquid can containing six carbon atoms, are potential feedbacks for dissolve glucose and enhance the acid-catalyzed this purpose, and recently efforts have focused on decomposition of glucose. According to their experiment, converting them to lactic acid, a chemical commodity. the ionic liquid [AMIM]Cl showed higher excellent Some researchers reported similar processes such as solubility to salts and glucose. In this work, the ionic converting glucose into 5- (HMF) liquid [OMIM]Cl was used as solvent to enhance the in a two-phase reaction system [1]. It is well known in yield of lactic acid for the glucose decomposition sugar chemistry that the lactic acid is a typical base reaction. catalyzed product from carbohydrates [2−7]. Lactic acid and its derivatives could potentially replace 2 Experimental voluminously consumed petroleum that is currently used to make plastic and fine chemicals [8−9]. A process to Ionic liquids were prepared according to the produce pure lactic acid with a high yield from the procedures described in Refs.[15−16]. Glucose (＞99%, abundant and renewable carbohydrates at low energy mass fraction) was used as a test material because it was cost must be developed before a biorefinery platform can a primary intermediate compound during the conversion be built on the basis of this substrate. Currently lactic of carbohydrates. 1.0 mol/L standardized solution lactic acid is produced by glucose fermentation, and the acid (Alfa Aesar Company, China) was also used for the drawbacks for enzyme catalysis reaction are long quantitative analysis of lactic acid. NaOH (99%) and reaction time and very strict reaction condition [10]. Ca(OH)2 (99%) were selected as alkaline catalysts ONDA et al [11−12] reported a reaction to prepare lactic obtained from China National Medicine Corporation Ltd. acid by catalytic decomposition of glucose with sodium All experiments were carried out in a bath micro- hydroxide. The reaction could be carried out below reactor made up of stainless steel with an internal volume

Foundation item: Project(2006BAE02B05) supported by the Key Projects in the National Science and Technology Pillar Program During the 11th Five-year Plan Period; Project(2005CB221406) supported by the National Basic Research Program of China Received date: 2009−03−18; Accepted date: 2009−06−17 Corresponding author: ZHOU Xiao-ping, Professor; Tel: +86−731−88821017; E-mail: [email protected] 46 J. Cent. South Univ. Technol. (2010) 17: 45−49 of 5 mL. The typical experimental procedures were as adding water as extracting agent. So in next experiments, follows: the desired amount of glucose, and the desired C22H43ClN2 was chosen as standard solvent to evaluate solid base catalysts with quantitative ionic liquid the reaction system. prepared in advance were put into a batch heated with a temperature programming oven. After a desired reaction 3.2 Effect of reaction time on preparation of lactic time, the reaction time was defined as the time that acid reactor was kept in the oven. 0.400 g test material was In order to find the best condition for preparation of used in all the experiments. lactic acid, three samples were chosen to evaluate the After the reaction was finished, the samples were influence of reaction time. The reactions were carried out diluted to about 10 mL with de-ionized water. The in a stainless steel bath. The amount of glucose was fixed solution was extracted and filtered, adding hydrochloride at 0.400 g, that of calcium hydroxide was 0.150 g and ＜ acid to adjust pH 7. The samples were injected in that of the ionic liquid C22H43ClN2 was 1.000 g. Because HPLC. HPLC analysis was performed with a Agilent the for the ionic liquid is about 353 K the LC−1200 system equipped with a UV detector (214 nm). samples covered the range from 353 to 373 K. Fig.1 A C−18 column was used to separate the samples for the shows that the sample reacted at 353 K has obviously analysis of lactic acid, with volume ratio of water to different results compared with results of other samples. as mobile phase. The yield of lactic acid is near 5% at first, and as reaction 5׃acetonitrile of 95 1 The product was characterized by H NMR spectra time increases, it does not show a distinct improvement. and LC−MS. The lactic acid yield was defined After 60 min, the yield still preserves at near 5% and as the percentage of lactic acid mass to initial glucose keeps at this value for further increment of time. For the mass on the carbon-atom basis. experiments at 363 K and 373 K, the lactic acid yields initially increase until 60 min and then decrease for 3 Results and discussion subsequent rise in time. Both of these data show reaction time of 60 min is an advantageous time for obtaining a 3.1 Characteristic of ionic liquid in base-catalyzed high yield of lactic acid. The highest yield at 363 K for glucose decomposition reaction 60 min is about 55%, which is larger than 45% at 373 K. Five kinds of ionic liquids were chosen to evaluate the influences of the ionic liquids. The reactions were carried out at 363 K for 60 min, adding the same amount of glucose (0.400 g), calcium hydroxide (0.150 g), and ionic liquid fixed at 1.000 g. Table 1 demonstrates that all samples have similar yields of lactic acid. The results indicate that ionic liquids act as solvent and have similar solubility for glucose and calcium hydroxide. For entry 4 and entry 5, ionic liquids have a long chain which makes them hydrophobic. But they still show good solubility for glucose and calcium hydroxide. Entry 4 and entry 5, have the same alkyl , but different anions chloride for entry 4 and bromide for entry 5. According to the yields of lactic acid, the effect of anion ion can be excluded. In ionic liquid system, glucose reacts with calcium Fig.1 Influence of reaction time on yield of lactic acid hydroxide to form calcium lactate which is easy to (Reaction condition: 0.400 g glucose, 1.00 g ionic liquid, dissolve in water. Hydrophobic ionic liquid and calcium 0.150 g calcium hydroxide for every samples) hydroxide are facile to separate with calcium lactate by In order to further confirm the influence of

temperature on yield of lactic acid and to find the Table 1 Influence of ionic liquids characteristic on yield of advantageous condition for lactic acid production, a lactic acid series of experiments were conducted. In these Test No. Ionic liquid Base Yield/% experiments, the reactions were carried out in the same 1 C H ClN Ca(OH) 49 6 9 2 2 conditions with different reaction temperatures. To 2 C8H15ClN2 Ca(OH)2 51 evaluate the influence of temperatures more clearly 3 C20H39ClN2 Ca(OH)2 55 333 K was chosen as the lowest point, and 464 K was the 4 C22H43ClN2 Ca(OH)2 53 highest for this investigation. At the stage of low 5 C22H43BrN2 Ca(OH)2 51 temperature such as 343 K, calcium hydroxide does not

J. Cent. South Univ. Technol. (2010) 17: 45−49 47 show activity. This may imply that ionic liquids are generate HMF, levulinic acid and formic acid. Lactic unable to dissolve both glucose and calcium hydroxide acid is an organic acid which is capable of initiating an under the solid condition. If the temperature is higher acid-catalyzed glucose reaction. So, excessive supply of than their melting point, the yield of lactic acid increases calcium hydroxide may be used to neutralize lactic acid. rapidly. Fig.2 shows that from 353 to 373K, the yield The effect of ionic liquid supply on the preparation of jumps from 5% to 55%. But the further increment of lactic acid is similar with that of the solid base (calcium temperature such as between 373 and 400 K may lead to hydroxide). In entry 8, a yield of lactic acid (about 100%) a drop of the yield to 20% at 400 K. If the temperature is is achieved using more ionic liquid as solvent. 1H NMR higher than 400 K, the yield does not dramatically spectra confirm that only the peaks which belong to decline and keep at 10%. The results demonstrate that lactic acid exist in the spectra. both time and temperature have important influences upon the production of lactic acid. Consequently, the Table 2 Influence of mole ratio of base to solvent on yield of highest lactic acid yield of 55% occurs at 363 K for 60 lactic acid min. Test m(Ca(OH)2)/ m(H2O)/ n(Ca(OH)2)/ Yield/ No. g g n(H2O) % 1 0.154 1 0.78 41 2 0.185 1 0.94 47 3 0.229 1 1.16 54 4 0.397 1 2.01 58 5 0.154 2 0.39 48 6 0.185 2 0.47 57 7 0.228 2 0.58 68 8 0.400 2 1.01 97 Reaction condition: 0.400 g glucose, reaction temperature 363 K, reaction time 60 min, calcium hydroxide as catalyst, and [OMIM]Cl as reaction solvent.

Fig.2 Influence of reaction temperature on yield of lactic acid The technology may good enough to be (Reaction condition: 0.400 g glucose, 60 min, calcium commercialized with a recycled ionic liquid reaction hydroxide as catalyst, [OMIM]Cl as reaction solvent) system. For this reaction, it has the same reaction mechanism with that in water. But the distribution of The above results have advantages compared with products is in favor of lactic acid, which may be caused the results in water solution using sodium hydroxide as by glucose and calcium hydroxide existing as another catalyst with the yield of 41%. As we know, in form in the solution of ionic liquid. water/sodium hydroxide reaction system, glucose would decompose to generate formic acid and as 3.4 Effect of different bases on yields of lactic acid by-products. But the HPLC results show that formic acid As mentioned previously, a good reaction condition and acetic acid are not observed in ionic liquid reaction for preparing lactic acid was identified. In this system. This is probably due to a low movement ability investigation, the catalyst was always set as calcium of both hydroxide anion and lactic acid in a pure ionic hydroxide. A series of experiments were carried out to liquid solvent system. evaluate the influence of different alkaline and alkaline oxides. The results are shown in Table 3. From Table 3, it 3.3 Effect of solid base and ionic liquid for can be seen that most of the catalysts show activity for preparation of lactic acid the reaction, especially some oxides such as MgO and

At the beginning, the amount of calcium hydroxide hydroxides such as Ba(OH)2. In general, saturated increased from 100% (entry 1) to 270% (entry 4), based solubility of the catalysis is very low in water, which on the mole ratio of hydroxide ion to glucose, fixing the causes them to have weak performance in the production mass of C22H43ClN2 at 1.000 g. Table 2 shows that the of lactic acid. For example, the yields in Ca(OH)2 and increment of calcium hydroxide can enhance the yield Ba(OH)2 conditions are not more than 5%, and MgO effectively. Especially, the excessive supply of calcium even does not show catalytic effect for the any formation hydroxide has an obvious effect. The results are in of lactic acid because the saturated solubility of MgO is accordance with those using sodium hydroxide in the only 0.84 mg per 100 g water. Generally speaking, it is water system. In the acid condition, glucose is inclined to disadvantageous for catalytic efficiency of solid alkaline

48 J. Cent. South Univ. Technol. (2010) 17: 45−49 oxides and hydroxides compared with homogenous 5% based on mass for every time in this recycle program. catalysts because homogenous reactions provide a nice After all, ionic liquid essentially has a surfactant contact among reactive reagents. According to the results structure and a certain water solubility. of these insoluble catalysts (entry 1, 3, 4, 5 and 7), ionic liquids provide a good solubility for these catalysts. In Table 4 Effect of recycle of hydrophobic liquids on yield of this reactive system, the alkaline-catalyzed reactions in lactic acid the solution of ionic liquids are homogeneous processes. Recycle times m(C22H43ClN2)/g Yield/% After the completion of reaction, water is introduced into 1 2.001 55 stainless steel autoclave to make the system 2 1.973 53 heterogeneous. The design is an advanced technology 3 1.952 51 that solves the problem of low catalytic effect of 4 1.931 52 heterogeneous catalysts and the difficulty of separation 5 1.903 49 of homogeneous catalysts.

3.6 1H NMR spectra of ionic-liquids comparison Table 3 Influence of different bases on yield of lactic acid Fig.3 shows 1H NMR spectrum the fresh ionic- Test No. Base m(Base)/g Yield/% liquid. Fig.4 shows 1H NMR spectrum of the products

1 Ca(OH)2 0.162 55.0 washed by de-ionic water three times. This process was

2 Sr(OH)2 0.530 32.9 used to extract lactate and glucose which both could

3 Ba(OH)2 0. 630 22.3 solve in water. Because the decomposition products of 4 MgO 0.080 7.0 C22H43ClN2 have a long carbon chain that is unsolvable in water, they can form new peaks in 1H NMR spectrum. 5 CaO 0.112 31.0 Fig.3 and Fig.4 have peaks that belong to ionic liquid 6 SrO 0.206 14.0 chemical shift of 1.130. After reaction, they do not form 7 BaO 0.106 33.0 8 LiOH 0.096 32.0 9 NaOH 0.160 51.0 Reaction condition: 0.400 g glucose, reaction temperature 363 K, reaction time 60 min, and [OMIM]Cl as reaction solvent.

According to the results in Table 3, both oxides and hydroxides can catalyze the reaction, so the oxygen atom may be the active part of the catalysts. Calcium hydroxide (55.0%) and sodium hydroxide (51.0%) show the highest catalytic efficiency for production of lactic acid because both of them have the strongest basic capacity.

Fig.3 1H NMR spectrum for ionic liquid (C H ClN ) as 3.5 Recycle of hydrophobic ionic liquids 22 43 2 standard sample before reaction As the investigations above mentioned, a series of conditions were explored, but the most important for these reactive systems is the recycle capability of hydrophobic ionic liquids. C22H43ClN2 is evaluated. The result shows that it is available for the production of lactic acid. So it is chosen as solvent for the recycle test. The results are shown in Table 4. Every test was carried out in the same reactive conditions: 0.400 g glucose, 0.150 g calcium hydroxide, and 363 K for 60 min. After reaction, water was used for three times to extract calcium lactate and other products that can solve in water, and hydrochloric acid was used to wash calcium hydroxide that is insoluble in water. The remnant solid is the ionic liquid and is heated at 393 K for 12 h to 1 evaporate water. Table 4 demonstrates that the yield of Fig.4 H NMR spectrum of used ionic liquid (C22H43ClN2) lactic acid can keep at a stable level, the loss is less than washed by water after reaction completion

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