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Ind. Eng. Chem. Res. 2009, 48, 8920–8925
KINETICS, CATALYSIS, AND REACTION ENGINEERING Effect of Alkaline Catalysts on Hydrothermal Conversion of Glycerin into Lactic Acid Zheng Shen,† Fangming Jin,*,† Yalei Zhang,† Bing Wu,† Atsushi Kishita,‡ Kazuyuki Tohji,‡ and Hisanori Kishida§ State Key Laboratory of Pollution Control and Resources Reuse, College of EnVironmental Science and Engineering, Tongji UniVersity, Shanghai 200092, China, Graduate School of EnVironmental Studies, Tohoku UniVersity, Sendai 980-8579, Japan, and EnVironmental Systems and Plant Headquarters, Hitachi Zosen Corporation, Kyoto 625-8501, Japan
Hydrothermal treatment of glycerin was carried out at 300 °C by using eight alkaline catalysts, including hydroxides of alkali metals, alkaline-earth metals, and aluminum. All alkaline catalysts promoted the formation of lactic acid or lactate salts from glycerin, except for Al(OH)3. The alkali-metal hydroxides were more effective than alkaline-earth-metal hydroxides on the catalysis of hydrothermal reactions. On the hydrothermal conversion of glycerin into lactic acid, the catalytic effectiveness followed the sequence of KOH > NaOH > LiOH for alkali-metal hydroxides, and Ba(OH)2 > Sr(OH)2 > Ca(OH)2 > Mg(OH)2 for alkaline-earth hydroxides. An excellent lactic acid yield of 90% was attained on hydrothermal conversion of glycerin at 300 °C with KOH or NaOH as a catalyst. KOH was superior to NaOH as a catalyst since it worked at a lower concentration or within a shorter reaction time to obtain the same lactic acid yield. The hydrothermal conversion of glycerin depended not only on the hydroxide ion concentration but also on the metal ions of catalysts. 1. Introduction More recently, it has become more important to use biodiesel as an alternative energy source because of global warming and a shortage of fossil fuel. Global biodiesel production increased from 2.1 × 106 in 2004 to 3.9 × 106 m3 in 2005, and the potential market for biodiesel is estimated to be 20 × 1018 J by 2050.1 Compared with fossil fuel, biodiesel has been considered to be more environmentally friendly for its lower emission feature. In general, about 10 wt % of crude glycerin is generated during the biodiesel-manufacture processes as a byproduct. Moreover, the typical glycerin produced by a biodiesel plant contains 50% or less glycerin and impurities such as water, alcohol, salt, and so on.2 The high cost of glycerin purification and overcapacity force small and medium biodiesel producers to pay for the disposal rather than to utilize the crude glycerin. To increase the market values of the biodiesel byproduct, it is necessary to convert glycerin into other chemicals. A hydrothermal process is one of the most promising processes for treating crude glycerin, because water at high temperature and pressure behaves as a reaction medium with outstanding properties.3,4 More importantly, the hydrothermal process may directly convert crude glycerin into high-value-added products without a dewatering pretreatment. Many research groups have reported the application of hydrothermal processes in material syntheses,5-8 waste destruction,9-12 plastic recycling,13,14 coal liquefaction,15 and biomass processing.16-21 The hydrothermal conversion of glycerin into acrolein under acidic conditions has been reported.22,23 On the other hand, few studies have been carried out to convert glycerin into lactic acid by hydrothermal * To whom correspondence should be addressed. Tel.: (86) 6598 5792. Fax: (86) 6598 5792. E-mail:
[email protected]. † Tongji University. ‡ Tohoku University. § Hitachi Zosen Corp.
treatment with alkaline catalysts. Essentially all of the current commercial biodiesel producers use base-catalyzed reactions because alkaline catalysts exhibit a higher rate of transesterification under moderate reaction conditions.2 Although a number of potential applications of glycerin, such as synthesis gas and methanol production, have already been reported,24,25 to our knowledge, there is no other research reported for converting glycerin into lactic acid under alkaline-catalyzed hydrothermal conditions except the following one. In our previous study, we have found that glycerin could be effectively converted into lactic acid at 300 °C with NaOH as an alkaline catalyst.26 Lactic acid is attracting more attention to be a material for producing biodegradable plastics. Since lactic acid yield from glycerin was considerably high (about 90%), and the reactions were conducted at a moderate temperature of 300 °C, the industrialization of lactic acid production from glycerin should have high potential. However, many factors affecting the reactions have not been studied; for example, the effect of different alkaline catalysts on the conversion of glycerin into lactic acid is still not clear. Therefore, in this study, we investigated the influence of alkaline catalysts on the hydrothermal conversion of glycerin by using various hydroxides of alkali metals, alkaline-earth metals, and aluminum. The results of this study could provide a feasible hydrothermal route and some fundamental data for the industrialization of lactic acid production from glycerin in the future. 2. Experimental Section 2.1. Materials and Experimental Procedure. Glycerin (99%) was used as a starting material. Alkaline catalysts used in this study were as follows: LiOH · H2O (98%), NaOH (96%), KOH (85%), Mg(OH)2 (97%), Ca(OH)2 (96%), Sr(OH)2 · 8H2O (98%), Ba(OH)2 · 8H2O (98%), and Al(OH)3 (98%). All reagents were obtained from Wako Pure Chemical Industries, Osaka,
10.1021/ie900937d CCC: $40.75 2009 American Chemical Society Published on Web 09/10/2009
Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009
Japan. In all experiments, deionized water was used and the reactor was purged with nitrogen to remove the dissolved oxygen prior to the reactions. Most experiments were performed in a batch-type reactor made of stainless steel SUS 316 tubing (12.7 mm o.d., 1 mm wall thickness, and 111 mm length) with two-end fittings, providing an inner volume of 10 mL, except the two for studying the effect of reactor materials. The typical procedure of hydrothermal reactions is described as follows: A 4 mL water mixture with 0.33 M glycerin and 0.25-2.5 M initial hydroxide ion concentration of hydroxides was added to the batch reactor, and then the reactor was put into a salt bath preheated to a desired temperature. In the salt bath, the reactor was shaken while being kept horizontal, to mix well and enhance heat transfer. After a desired reaction time, the reactor was removed from the salt bath and put into a cold-water bath to quench the reaction. The reaction time was defined as the period during which the reactor was kept in the salt bath. The real reaction time is shorter than the apparent reaction time, because the heat-up time to raise the temperature of the reaction media from 20 to 300 °C was about 15 s. The temperature of the salt bath was taken as the reaction temperature. After cooling, samples of the liquid phase in the reactor were collected for analysis. In all experiments, we fixed the temperature at 300 °C and the water filling rate at 40%. So, the reaction pressure was about 9 MPa, which could be estimated from the water saturation pressure at 300 °C.27 To investigate the effect of the reactor materials on the hydrothermal reactions, a batch reactor with a Teflon inner wall having an inner volume of 20 mL was used, which had been described elsewhere.28 The typical reaction procedure by using this reactor is as follows. An 8 mL water mixture with 0.33 M glycerin and 1.25 M NaOH was put into the reactor. After being sealed, the reactor was placed in an electric furnace that had been preheated to 300 °C. After the desired reaction time, the reactor was removed from the electric furnace for cooling at room temperature (25 °C). Then, liquid samples were collected for analyzing by high-pressure liquid chromatography (HPLC). 2.2. Sample Analysis. The liquid samples were filtered through a 0.45 mm filter and then were adjusted with sulfuric acid until the solution reached pH values of 2-3. After that, the liquid samples were analyzed by HPLC and GC/MS. HPLC analysis was performed with Waters HPLC system equipped with a tunable absorbance detector (UV detector, Waters 486) and a differential-refraction meter (RI detector, Waters 410), controlled with a Millennium-600 workstation. In the HPLC analysis, two columns (Shodex KC811) were used in series, and the solvent was 1 mM HClO4 with a flow rate of 1.0 mL/ min. In GC/MS analysis, a Hewlett-Packard model 5890 Series II gas chromatograph equipped with a model 5890B mass selective detector was used. Details on the conditions for GC/ MS and HPLC analyses were available elsewhere.29 Glycerin was analyzed quantitatively by GC/MS, while organic acid was analyzed by HPLC. 3. Results and Discussion 3.1. Effect of Alkaline Catalysts on Hydrothermal Conversion of Glycerin into Lactic Acid. To clarify the effect of alkaline catalysts on hydrothermal conversion of glycerin into lactic acid, a series of experiments were conducted at 300 °C with various amounts of hydroxides of alkali metals, alkalineearth metals, and aluminum. The reason to set the reaction temperature at 300 °C was that, in our previous experiments
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Figure 1. Effect of alkaline catalysts on production of lactic acid from glycerin (temperature, 300 °C; reaction time, 90 min).
using NaOH as a catalyst, it was found to be an optimal temperature for hydrothermal conversion of glycerin into lactic acid.26 The effect of alkaline catalysts on the hydrothermal conversion of glycerin is illustrated in Figure 1 by plotting the lactic acid yield and the glycerin conversion versus the initial concentration of hydroxide ion for the different catalysts used. It should be noted that in the above cases the real products were lactate salts, rather than lactic acid, due to the alkaline conditions. But, for simplicity and easy comparison among different lactates, we used lactic acid as the target product for the yield calculation and discussion. The yield of products was defined as the percentage of mole of products to that of the initial glycerin, while the glycerin conversion was the percentage of mole of converted glycerin to that of the initial one. The initial hydroxide ion concentration was determined on the basis of the assumption that all hydroxides could be completely dissolved and converted into free hydroxide and metal ions in water. From Figure 1, it can be seen that the increase of the initial hydroxide ion concentration led to a remarkable increase in the lactic acid yield and in the glycerin conversion for all the hydroxides, except for Al(OH)3. These observations suggest that the conversion of glycerin into lactic acid is strongly dependent on the hydroxide ion concentration. It is most likely that the conversion of glycerin into lactic acid was a base-catalyzed reaction as reported in our past research.26 Therefore, the excellent catalytic performance of alkali-metal hydroxides is not surprising since they have stronger alkalinity and higher solubility in water than alkaline-earth-metal hydroxides at low temperatures (see solubility in Table 1). This result also indicates that a stronger base is necessary for converting glycerin into lactic acid. However, it should be noted that when the initial concentration of hydroxide ion increased to be more than 1.25 M, no significant change in the yield of lactic acid was observed
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Table 1. Yields of Products, the Remaining Glycerin, and Final pH after Hydrothermal Reactions of Glycerin with Different Hydroxides as Catalystsa
solubility (g, 293 K) lactic acid (%) formic acid (%) acetic acid (%) acrylic acid (%) remaining glycerin (%) initial pH final pH a
KOH
NaOH
LiOH
Ba(OH)2
Sr(OH)2
Ca(OH)2
Mg(OH)2
122 90.0 Ca(OH)2 > Mg(OH)2 for alkaline-earth hydroxides. An excellent lactic acid yield of 90% was reached in the hydrothermal conversion of glycerin at 300 °C with KOH or NaOH as a catalyst. KOH was superior to NaOH as a catalyst since it worked at a lower concentration or within a shorter reaction time to obtain the same lactic acid yield. The hydrothermal conversion of glycerin depended not only on the hydroxideion concentration but on the metal ions of catalysts.
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ReceiVed for reView June 08, 2009 ReVised manuscript receiVed August 27, 2009 Accepted August 30, 2009 IE900937D
