Environ. Sci. Technol. 2005, 39, 1893-1902
Controlling Hydrothermal Reaction Pathways To Improve Acetic Acid Production from Carbohydrate Biomass F A N G M I N G J I N , * ,† Z H O U Y U Z H O U , † TAKEHIKO MORIYA,‡ HISANORI KISHIDA,§ HISAO HIGASHIJIMA,| AND HEIJI ENOMOTO† Department of Environmental Science and Technology, Graduate School of Environmental Studies, Tohoku University, Aoba-ku, Sendai 980-8579, Japan, Research and Development Center, Tohoku Electric Power Co., Inc., Sendai 981-0952, Japan, Environmental Systems & Plant Headquarters, Hitachi Zosen Corporation, Kyoto 625-8501, Japan, and Water & Sludge Engineering Center, Environmental Systems & Plant Headquarters, Hitachi Zosen Corporation, Tokyo, 100-8121, Japan
A two-step hydrothermal process to improve the production of acetic acid was discussed. The first step was to accelerate the formation of 5-hydroxymethyl-2-furaldehyde (HMF), 2-furaldehyde (2-FA), and lactic acid (LA), and the second step was to further convert the furans (HMF, 2-FA) and LA produced in the first step to acetic acid by oxidation with newly supplied oxygen. The acetic acid obtained by the two-step process had not only a high yield but also better purity. The contribution of two pathways via furans and LA in the two-step process to convert carbohydrates into acetic acid was roughly estimated as 8590%, and the ratio of the contributions of furans and LA to yield acetic acid was estimated as 2:1. The fact that WO of carbohydrates is not capable of producing a large amount of acetic acid, while the two-step process can enhance the acetic acid yield, can be explained because formic acid is a basic product of direct oxidation of carbohydrate, and acetic acid in WO of carbohydrates may come from the oxidation of dehydration products of aldose.
Introduction As the amount of organic waste increases and environmental problems become serious, the development of an effective and acceptable organic waste treatment process is becoming increasingly important. Supercritical water oxidation (SCWO) and/or wet oxidation (WO) are attractive alternative tools as an organic waste treatment method, particularly when incineration is not suitable for waste with a high water content as an example. Therefore, the oxidation of various organics in supercritical water has been examined in a number of * Corresponding author phone: +81-22-217-7385; fax: +81-22217-7385; e-mail:
[email protected]. † Tohoku University. ‡ Tohoku Electric Power Co., Inc. § Environmental Systems & Plant Headquarters, Hitachi Zosen Corp. | Water & Sludge Engineering Center, Hitachi Zosen Corp. 10.1021/es048867a CCC: $30.25 Published on Web 02/01/2005
2005 American Chemical Society
investigators. Our research group also carried out a study on the oxidation of kitchen garbage in supercritical water. Our previous results showed that SCWO was effective for treatment of garbage (1-3), but the effectiveness of WO was limited by the rate of oxidation of acetic acid for all of the selected garbage. Because acetic acid is a stable intermediate product, a high reaction temperature, catalyst, and/or excess oxygen are required for its complete decomposition. The oxidation of other waste material in supercritical water or WO also showed the same results. Acetic acid is an important organic chemical. One important industrial use of acetic acid is to produce calcium magnesium acetate (CMA) as a noncorrosive road deicer for cold areas in winter. CMA has been proposed as a substitute for chloride salt deicers, because it has acceptable ice-melting properties and is also relatively benign to the environment, noncorrosive to automobiles, and is biodegradable (4-6). The cost, however, of manufacturing CMA is a major drawback that limits its widespread application as a roadway deicer. If the residual acetic acid in partial SCWO or WO of organic wastes can be utilized to produce acetate rather than being decomposed, then it should reduce the cost of manufacturing CMA. Hereafter, the term WO is used to include partial oxidation. Although traditional biochemical processes, such as fermentation, can also convert biomass to acetic acid, their reaction rate is generally low, while WO can be completed within minutes. Further, cellulose and hemicellulose in lignocellulosic materials are not directly used for bioconversion because of their intimate association with lignin; generally specific pretreatment processes are necessary to easily treat these carbohydrates by biochemical process. With this in mind, a new method was studied using various organic wastes, such as various garbage, rice hulls, and straw, with calcium/magnesium sources such as oyster shells and industrial wastes including MgO and Mg(OH)2, to produce an acetate deicer (7-11). These results indicate that acetic acid can be obtained and can be increased by optimizing the reaction condition of WO of organic waste, but there is a limit to acetic acid production; that is, only 11-13% of acetic acid yield at most on the carbon basis was obtained for all test materials selected. More precisely, the acetic acid yield from carbohydrate biomass by conventional WO (direct oxidation) is hardly further improved. So, to enhance the production of acetic acid from carbohydrate biomass, a new two-step process was proposed (12). The two-step process consists of both a hydrothermal reaction without a supply of oxygen (the first-step reaction) and an oxidation reaction (the second-step reaction). The first step is to accelerate the formation of 5-hydroxymethyl-2-furaldehyde (HMF) and 2-furaldehyde (2-FA), because these furans can produce a large amount of acetic acid by their oxidation, and the second step is to further convert the furans produced in the first step to acetic acid by oxidation with newly supplied oxygen. It was shown that the acetic acid yield was greatly increased by the two-step process. Previous work (12), however, only specifically presented a possible way of controlling the reaction to accelerate an acetic acid production with some mechanistic explanation. The objective of this work is to make clear the whole view of the two-step reaction treatment, by (1) examining if there are pathways other than that via furans, (2) evaluating the contribution via respective pathways to acetic acid production, and (3) discussing the mechanism of reactions involved. VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Composition of Rice Hulls (wt % db) (Results Are the Average of Three Separate Sample Analyses) cellulosea
hemicellulosea
ligninb
SiO2c
soluble extractsd
37.7
18.8
14.1
22.5
6.9
a
Determined by using NaClO2 (13). b Determined by the Klason method. c Obtained by combustion at a temperature of 600 °C for 20 min. d Determined by a solvent of a mixture of benzene/ethanol at a ratio of 2:1 (v/v).
the second-step reaction was loaded into a batch reactor, which was made of stainless steel 316 tubing (3/8 in., 1-mm wall thickness and 120-mm long) with two end fittings, providing an inner volume of 5.7 cm3. A loading of a mixture of deionized water and hydrogen peroxide was set at 60% in all experiments. After the loading procedure was completed, the reactor was immersed in a salt bath that had been preheated to the desired temperature. In the salt bath, the reactor was shaken, keeping it horizontally, to enhance mixing and heat transfer. After the desired reaction time, the reactor was taken out of the salt bath and immediately put into a cold-water bath to quench the reaction. The reaction time was defined as the time that the reactor was kept in the salt bath. The real reaction time is shorter than the apparent reaction time, because the time required to raise the temperature of reaction medium from 20 to 300 °C was about 15 s. Reaction pressures with and without H2O2 supply were about 17 and 9 MPa, respectively. All experiments were performed with degassed water and by purging the reactor with nitrogen. In a two-step process, the two-step procedure was taken: one is a hydrothermal reaction without a supply of H2O2 (the first-step reaction) and the other WO (the second-step reaction). That is, for the first-step reaction, only the test material and water were added to the reactor for the reaction. After the first-step experiment was completed, H2O2 was added to the cooled reactor, and then the oxidation reaction took place in the second step.
FIGURE 1. Schematic of batch reactor system.
Experimental Section Materials and Oxygen Supply. In our previous study (10) concerning acetic acid production by WO of cellulosic biomass, rice hulls were chosen as a raw material because they represented an abundant renewable and widely available resource, especially in those areas where it was an important waste product of the agricultural industry. Rice hulls used in this study were obtained from a local harvest, whose components are shown in Table 1. The rice hulls were ground to pass a 100-mesh size screen in a cross-beater mill (Retsch GmbH & Co.KG (Germany)). Starch (potato) and glucose as representatives of the main components of food wastes were also used as test materials. Cellulose used in this study is a filter paper powder (under 200-mesh, Toyo Roshi Kaisha, Ltd.). Starch and glucose were of reagent grade. Additionally, other chemicals, such as HMF, 2-FA, and lactic acid, were also used as test materials. Lactic acid was in the form of an 85% solution; HMF and 2-FA were of analytical or reagent grade. All chemicals used were obtained from Wako Pure Chemicals Industries Ltd. (Osaka). A 30% solution of hydrogen peroxide was used as an oxidant for experimental convenience. The oxygen supply was defined as the ratio of the amount of H2O2 supplied to the stoichiometric demand for complete oxidation of carbohydrates to carbon dioxide and water, assuming 1 mol of H2O2 gives 1/2 mol of O2. Experimental Procedure. All experiments were conducted in a batch reactor system shown in Figure 1. It was described previously (1, 14), but basically the reactor was placed horizontally into a salt bath for the reaction. A detailed description of the experimental procedure is given elsewhere (1, 14). A brief summary is given below. The desired amount of test material and deionized water for the first-step reaction or a mixture of deionized water and hydrogen peroxide for 1894
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Product Analysis. After the reaction, samples were collected for analysis. In this study, only liquid samples were analyzed. Solid samples were not specifically analyzed, because no measurable solid products were observed except experiments with cellulose in the absence of H2O2, where little solid-phase product (about 10 carbon %) was observed. For GC/MS analyses, a Hewlett-Packard model 5890 Series II gas chromatograph equipped with a model 5890B Mass Selective Detector was used. The samples were separated on a HP-INNOWAX capillary column (cross-linked polyethylene glycol) for water-soluble compounds with polar functional group (-OH, -CdO, -COOH) using helium as the carrier. Identification of intermediate products was made with total and selected ion chromatograms with the aid of a computer library, as well as a comparison of GC retention times of products and authentic compounds. HPLC analysis was performed with a Waters HPLC system equipped with a tunable absorbance detector (UV/vis detector) (Waters 486) and a differential refractometer (RI detector) (Waters 410), controlled with a Millenium 600 workstation. One of two kinds of column was used to separate more intermediate products. One was RSpak KC-811 (Shodex) for organic acids analysis with a UV detector, to identify polar compounds with a polar functional group (-OH, -CdO, -COOH), such as carboxylic acids, alcohols, and ketones, and another was SUGAR SH1011 (Shodex) for saccharide analysis with a RI detector, to identify sugars. When RSpak KC-811 was used, samples were separated through a series of two identical columns to obtain a better separation. Peak identification was accomplished by comparison of sample peak retention times with those of standard solutions of pure compounds. Details on the conditions for GC/MS and HPLC analyses are available elsewhere (1, 14). Acetic acid was obtained quantitatively by GC/MS, and lactic acid, HMF, and 2-FA were obtained by HPLC. The GC/MS and HPLC chromatographs were calibrated daily prior to and during the sample analysis. All quantitative data reported in this study were the average of 3-5 samples analyses. The total residual organic carbon concentration (TOC) in liquid samples was also measured with a TOC analyzer (Shimadzu TOC 5000A).
FIGURE 2. Proposed two-step process for enhancing acetic acid yield.
Results and Discussion Reaction Pathway via Lactic Acid to Acetic Acid. In a previous publication (12), it was explained that the increase of acetic acid yield in the two-step reaction process was attributed to the increase of the conversion rate to HMF and 2-FA. However, our later study (15) revealed that besides HMF and 2-FA, lactic acid was also formed. Therefore, we postulated that water in the subcritical region acted as an effective acidbase catalyst (15), because it was generally known in carbohydrate chemistry that HMF and 2-FA were products of the facile, acid-catalyzed dehydration of carbohydrates, and lactic acid was a product of alkaline degradation of carbohydrates. Further, we also found that the conversion mechanism of cellulosic biomass to lactic acid in subcritical water without adding alkaline catalyst appeared to follow the same pathways elucidated in the conversion of sugar to lactic acid in an alkaline solution (15). Recently, many researchers have also reported that water in the near-critical region acts as an effective acid and/or base catalyst by confirming some model acid/base-catalyzed reactions, such as ether reactions, hydrolyses of esters, dehydration of alcohol, and alkylation reaction (16-22). Because near-critical water may act not only as an effective acid catalyst but also as base catalyst, it is quite possible that lactic acid, like HMF and 2-FA, could also be formed in a certain quantity without adding a base catalyst. If the oxidation of lactic acid can also produce a large amount of acetic acid, this may suggest that there is another pathway, via lactic acid, contributing to the increase of acetic acid yield in the two-step reaction process. An oxidation experiment with lactic acid was performed under the conditions of temperature of 300 °C, with reaction time of 1 min and oxygen supply of 70%, to test if lactic acid can produce a large amount of acetic acid. As a result, the yield of acetic acid is very high, reaching about 40% on the carbon base. Therefore, the schematic expression of the twostep reaction process may be revised as shown in Figure 2. HMF, 2-FA, and Lactic Acid Production in the First Step. To attain a high acetic acid yield by the two-step process, first, HMF, 2-FA, and lactic acid production in the first step must be high. Experiments with cellulose and starch were performed over a wide range of conditions, with temperatures varying from 250 to 350 °C and reaction time from 30 s to 6 min, to determine the experimental conditions leading to favorable yields of HMF, 2-FA, and lactic acid from carbohydrates in the first step. All yields were reported in carbon percent values on carbon of the initial reactant. As mentioned before, the formation of HMF, 2-FA, and lactic acid in hydrothermal treatment without adding an acid or alkaline catalyst may be attributed to the action of acidbase catalyst of water in a subcritical water region. Considering that the ionization constant, which is directly connected with the acid- and base-catalyzed reaction, reaches a maximum near 300 °C at the saturation pressure (23), first, a series of experiments was conducted at 300 °C to study the effect of reaction time. As shown in Figure 3A, the yields of HMF, 2-FA, and lactic acid all increased with an increase in the reaction time for both cases of cellulose and starch. All of HMF, 2-FA, and lactic acid decrease as the reaction time increases beyond 2 min for cellulose and 1 min for starch. The fact that starch gave the highest yields of HMF, 2-FA,
and lactic acid at a shorter reaction time of 1 min is probably because hydrolysis of starch is easier than cellulose. However, for both cellulose and starch, the difference of yields of HMF, 2-FA, and lactic acid at a reaction time between 1 and 2 min was not critical. Moreover, in Figure 3A, it can be seen that the yields of HMF, 2-FA, and lactic acid decrease after 2 min for cellulose and 1 min for starch. This trend for HMF is distinct from the other two. This observation may suggest that HMF is unstable and easier to decompose than 2-FA and lactic acid. Nevertheless, the HMF yield always exceeded 2-FA and the lactic acid yields for both cellulose and starch at all conditions. A higher yield of HMF than 2-FA may be attributed to the fact that hexoses are generally easily converted to HMF, and a higher yield of HMF than lactic acid indicates that the selectivity of carbohydrates to form HMF is higher than the formation of lactic acid under our experimental conditions. Subsequently, the second series of experiments were conducted for a 2 min reaction time for cellulose and 1 min for starch to investigate the effect of reaction temperature. As shown in Figure 3B, yields of HMF, 2-FA, and lactic acid initially increased then decreased as the reaction temperature increased. As expected, a temperature of 300 °C gave the highest yield for HMF, 2-FA, and lactic acid in both cases of cellulose and starch. This result gives further support that water in a subcritical water region may act as an effective acid-base catalyst. Moreover, comparing the effect of reaction temperature and reaction time, it can be seen that the effect of reaction temperature is more sensitive than that of reaction time. Similarly, the decrease after reaching 300 °C was significant for HMF, but not for 2-FA and lactic acid. In summary, the highest yield of HMF, 2-FA, and lactic acid in the first-step reaction is obtained at the condition where a reaction temperature is 300 °C and a reaction time of 1-2 min. The coincidence of the condition for obtaining the highest yields of HMF, 2-FA, and lactic acid indicates that it is easy to control the reaction temperature and the reaction time to obtain the highest yield of HMF, 2-FA, and lactic acid. Acetic Acid Production by Oxidation of HMF, 2-FA, and Lactic Acid in the Second Step. The oxidation experiments using HMF, 2-FA, and lactic acid as test materials were performed at a temperature between 280 and 330 °C, at a reaction time varying from 0.5 to 2 min, and with oxygen supply varying from 50 to 100%, to obtain the optimum condition to produce acetic acid. As shown in Figure 4A, results of the effect of temperature show that increasing the temperature from 280 to 300 °C leads to an increase in acetic acid yield, while a further increase in temperature decreases the yield for all of HMF, 2-FA, and lactic acid. For the influence of reaction time, Figure 4B shows that the yield of acetic acid increased and then decreased as the reaction time increased for all of HMF, 2-FA, and lactic acid. The highest acetic acid yield was obtained at a reaction time of 1 min. The influence of oxygen supply on the acetic acid concentration (Figure 4C) is such that the acetic acid yield increases when the oxygen supply increases from 50% to 70%, but then decreases as the oxygen supply increases further. Therefore, the condition for obtaining the highest acetic acid yield is the same condition for all of HMF, 2-FA, and lactic acid, that is, reaction temperature of 300 °C, reaction time of 1 min, and oxygen supply of 70%. Moreover, from Figure 4, it can also be seen that an outstanding yield of acetic acid can be obtained from lactic acid. This may imply that the acetic acid yield in the two-step process could be higher if it is possible to make the lactic acid yield increase by controlling the reaction condition in such a way as by adding an alkaline catalyst. Acetic Acid Production by the Two-Step Process. Under the respective optimum conditions in the first-step and the VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Effect of reaction time and temperature on the production of HMF, 2-FA, and lactic acid from cellulose and starch in the first step.
TABLE 2. Comparison of Acetic Acid Yieldsa and Purityb by a Conventional Wet Oxidation (WO) and a Two-Step Process conventional WO yield, purity, color and % % shape rice hulls 11.7
33.4
cellulose
9.0
26.6
starch
9.6
28.2
temp, °C time, min O2, %
brown, char brown, char brown, char
two-step process yield, purity, % %
color and shape
21.7
75.5
light yellow
16.3
68.5
white
17.5
70.0
white
Reaction Condition 300 1 70
300,c 300c 2, 1 0, 70
a The percentage of TOC of acetic acid and starting material. b The percentage of TOC of acetic acid and the other residuals remaining in the liquid after reaction. c Italic and bold numbers correspond to conditions in the first and second steps, respectively.
FIGURE 4. Effect of reaction conditions on acetic acid production from the oxidation of HMF, 2-FA, and lactic acid. second-step processes obtained above, cellulose and starch as well as rice hulls were treated in the two-step reaction process. Results are shown in Table 2 with those obtained by the conventional WO (12) for comparison. The acetic acid yield obtained by the two-step process is up to 21.7% for rice hulls, and 16.3% and 17.5% for cellulose and starch, respectively. These are approximately twice as much as those obtained by the conventional WO procedure. Furthermore, 1896
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when utilizing an acetic acid, both a high yield of acetic acid and a high purity of acetic acid are desired. The purity of acetic acid is defined here as the percentage of organic carbon of acetic acid in a liquid sample against the TOC of the liquid sample after reaction. As shown in Table 2, the purity of acetic acid was improved from about 30% in the conventional WO to about 70% in the two-step reaction process. It should be noted that other than acetic acid, most of the other component is formic acid, and trace amounts of other low molecular weight carboxylic acids, such as maleic acid, malonic acid, and propionic acid. As mentioned before, our purpose in generating acetic acid is to produce calcium magnesium acetate (CMA) as a noncorrosive road deicer. Because calcium formate Ca(HCOO)2, as well as calcium compounds of other low molecular weight carboxylic acids,
FIGURE 5. Two competing pathways from carbohydrate to acetic acid (YP-F and YP-L are respective yields of furans and lactic acid from carbohydrate, and YF-A and YL-A are the respective acetic acid yields from furans and lactic acid by WO).
FIGURE 6. GC/MS total ion chromatogram after oxidation of cellulose at 300 °C, 1 min. can also be used as a noncorrosive deicer, formic acid and other carboxylic acids may be also useful products. In this case, the total purity of all carboxylic acids would be over 99%, indicating that a separation of acetic acid and formic acid from other residual products in the solution after the two-step reaction process would not be necessary. Contribution via Furans and Lactic Acid To Produce Acetic Acid in the Two-Step Process. As discussed above, HMF, 2-FA (furans), and lactic acid are the main intermediate products, which make a great contribution to acetic acid production in the two-step process. That is, as shown in Figure 5, polysaccharides are converted to acetic acid in the two-step process mainly through two competing pathways via furans and lactic acid. On the basis of the pathways shown in Figure 5, the contribution of furans and lactic acid to form acetic acid may be expressed as follows:
CF ) YP-F × YF-A/YC
(1)
CL ) YP-L × YL-A/YC where CF and CL are the contribution of furans (HMF and 2-FA) and lactic acid to acetic acid, respectively, YP-F and YP-L are the respective yields of furans and lactic acid from carbohydrate in the first-step reaction, YF-A and YL-A are the respective acetic acid yields from furans and lactic acid by WO, and YC is the total acetic acid yield in the two-step reaction process. When substituting YP-F and YP-L by the experimental data of the highest yield of furans (HMF and 2-FA) and lactic acid from cellulose in the first-step reaction described in Figure 3A, and substituting YF-A and YL-A by the highest yields of acetic acid from furans and lactic acid in the second-step reaction described in Figure 4, the results obtained showed that the contribution of furans was roughly 40%, while that of lactic acid was about 20%. That is, the ratio of the contributions of furans and lactic acid to yield acetic acid was roughly 2:1. Possible Pathways Other Than Those via Furans and Lactic Acid. As described above, 40% and 20% of the total acetic acid yield in the two-step process comes from furans and lactic acid, respectively. This, in turn, means that the origin of the other 40% is unknown. In other words, besides
pathways via furans and lactic acid, there is still a possibility of the existence of other undiscardable pathways contributing to the acetic acid production. To find the missing origins or examine whether there are other pathways, intermediate products in the first-step reaction for cellulose were analyzed in detail. Figure 6 displays a GC/MS total ion chromatogram in a 12-40 min region for a sample after the reaction of cellulose at 300 °C for 1 min. Other regions were eliminated because no clear, meaningful peak appeared. It can be seen that many compounds were formed including HMF and 2-FA. Among these compounds, the peaks labeled with 1-7 represent the compounds that were identified as shown in Figure 6. The identification of these compounds, except 5-methyl-2furaldehyde, was done by matching both the mass spectrum and the GC retention time for each compound with those of the authentic compounds. For 5-methyl-2-furaldehyde, its identification was performed only by matching the mass spectrum, because the authentic standard of 5-methyl-2furaldehyde was not commercially available. Subsequently, further identification was performed by HPLC analysis. As shown in Figure 7, many compounds other than HMF, 2-FA, and lactic acid appear. Most of these compounds, which could not be identified by GC/MS, were identified here, as shown in Figure 7. These compounds may be divided into three groups: (1) low molecular weight dicarboxylic acids with 2-4 carbon atoms (peaks labeled D1-D3), (2) three-carbon compounds of glyceraldehyde, pyruvaldehyde, dihydroxyacetone, and propenoic acid (peaks labeled TC1-TC5), and (3) 1,2,4-benzenetriol (peak labeled B1). Identification of HMF, 2-FA, B1 (1,2,4-benzentriol), A (acetic acid), and D1 (maleic acid) was easily performed by only comparing the GC retention time for each compound with that of the authentic compound. However, as shown in Figure 7, D2 (malonic acid) and TC1 (glyceraldehyde) have very similar retention times, and a group of D3 (malonic acid), TC2 (pyruvaldehyde), and TC3 (lactic acid) and a group of TC4 (dihydroxyacetone) and F (formic acid) also have very similar retention times in each group, so that their identification was hardly performed by only comparing the retention times. Adding an authentic compound to a sample and then comparing the change of peak before and after the addition was done for confirmation. For the result obtained with a column of SUGAR SH1011, as shown in Figure 8, glucose, fructose, lactic acid, and some C3 compounds such as glyceraldehyde, dihydroxyacetone, and pyruvaldehyde were found. Identification of glucose, fructose, and lactic acid was easily obtained by only comparing the LC retention time for each compound with that of the authentic compound. However, identification of glyceraldehyde, pyruvaldehyde, and dihydroxyacetone was performed by not only comparing the retention times but also comparing the change of peak after adding an authentic compound to a sample, because these three compounds had adjacent compounds that had very similar retention times. After identifying the intermediate products, the formation mechanism of these intermediate products is discussed as follows. Levulinic and formic acids may be considered to be the rehydration products of HMF, because levulinic and formic acids are typically formed from the ionic and acidcatalyzed hydrolysis of HMF (24). A supplementary decomposition experiment of HMF at 300 °C for 2 min confirmed this. This result further may suggest that the reaction known as an acid-catalyzed reaction under ambient conditions can be done in subcritical water probably because of the acidcatalytic role of subcritical water (15-22), although Tester et al. (25) reported that the acid-catalyzed hydrolysis of HMF was not an operative pathway in supercritical water. Moreover, a considerable amount of 1,2,4-benzenetriol was also found in the experiments with HMF; thus, 1,2,4-benzenetriol VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. HPLC chromatogram after reaction of cellulose at 300 °C, 1 min (UV detection 210 nm, column: KC811 × 2).
FIGURE 8. HPLC chromatograms after reaction of cellulose at 300 °C and 1 min (A) and 2 min (B) (column: SUGAR SH1011, RI detection). was also considered to be a degradation product of HMF. Similar results were obtained by Luijikx et al. (26) in a hydrothermal experiment of HMF at 300 °C, and a possible reaction pathway from HMF to 1,2,4-benzenetriol was explained: first, the reaction was initiated by hydrolysis of the furan ring, and the intermediate could be tetrahydroxycyclohexadiene; finally, 1,2,4-benzenetriol was produced by dehydration of the cyclohexadiene derivative. Glyceraldehyde, pyruvaldehyde, and dihydroxyacetone should be the precursors involved in the formation reactions of lactic acid in hydrothermal degradation of glucose (15). Further, to find the degradation products of lactic acid, supplementary decomposition experiments of lactic acid were conducted at 300 °C and for 1 and 2 min. As a result, propenoic (acrylic) acid, propionic acid, and acetic acid were identified. Because lactic acid (hydroxyl carboxylic acid) bears a hydroxyl and a carboxyl group, it displays reaction properties that are characteristic of both functionalities. One of the characteristic reactions of the hydroxyl group (alcohols) is dehydration to give alkenes in the presence of an acid and the application of heat (27). It is known that hydroxy carboxylic acids can also undergo dehydration on heating to give lactides or R,β-unsaturated acids or lactose ring depending on the position of the hydroxyl group to the -COOH group (28). These discussions suggest to us to consider that acrylic acid is most likely a dehydration product of lactic acid. Mok et al. (29) and Lira et al. (30) also reported that the treatment of lactic acid in supercritical and subcritical water could produce acrylic acid by dehydration mechanism. In addition, it is known that R-hydroxy carboxylic acids can undergo decarbonylation to give acetic acid via acetaldehyde in the presence of a strong acid such as H2SO4 (31). Taking 1898
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into account that high-temperature water in a subcritical region has the action of an acid-catalyst as mentioned above, acetic acid is likely to be a decarbonylation product of lactic acid via acetaldehyde, although acetaldehyde was not detected. This could probably be due to its instability. Mok et al. (29) reported that acetic acid can be formed by not only the decarbonylation of lactic acid, but also decarboxylation in a free-radical mechanism in their study on the formation of acrylic acid from lactic acid in supercritical water. However, acetic acid formed from lactic acid should likely be a product of acid-catalyzed mechanism in our experimental conditions, because the possibility of a free-radical reaction is low near 300 °C. Moreover, the amount of acetic acid among degradation products of lactic acid was higher than propenoic acid, suggesting that the decarbonylation of lactic acid occurred more easily than dehydration under our experimental conditions. Finally, propionic acid may be a product of the hydrogenation of propenoic acid. However, the amount of propionic acid was quite low, suggesting that the hydrogenation reaction occurred to a very low extent under our experimental conditions. This is probably because the source of hydrogen may be the high-temperature water, while the water near 300 °C may not produce a large amount of hydrogen. Additionally, as shown in Figure 7, some low molecular weight dicarboxylic acids such as maleic acid, malonic acid, and succinic acid were also formed, but quantitative analyses showed that the concentration of these dicarboxylic acids was quite low, below 10 ppm. The discussion above shows that identified intermediate products shown in Table 3, except HMF, 2-FA, and lactic acid, in hydrothermal treatment of cellulose can be roughly classed into three groups: (1) the degradation products of HMF, (2) the precursor and degradation products of lactic acid, and (3) other products. Other products include low molecular weight dicarboxylic acids and products of thermal degradation of carbohydrates such as 1-hydroxy-2-propanone. As mentioned above, the concentration of dicarboxylic acids was quite low. Although the concentration of 1-hydroxy-2-propanone was not very high, a supplementary experiment for conversion of 1-hydroxy-2-propanone into acetic acid showed that the conversion rate was fairly high, so that the contribution of the pathway via 1-hydroxy-2propanone into acetic acid was estimated to be 5% out of 100% at most. Furthermore, the material balance for carbon of all identified residual intermediate products against the TOC of solution sample was approximately 90%. These observations may allow us to conclude that the major intermediate products, other than furan and lactic acid, would
TABLE 3. Grouping of Identified Intermediate Products
be the formation and degradation products of furans and lactic acid, and the contribution of two pathways via furans and lactic acid in the two-step process to convert carbohydrates into acetic acid would be roughly estimated as 8590%, with about a 5% contribution of a thermal process via 1-hydroxy-2-propanone. Mechanistic Implications for WO and the Two-Step Reaction of Carbohydrate. Although there is extensive literature concerning the WO of a carbohydrate biomass, few studies examined the WO mechanism of carbohydrates. Many of these studies were concerned with proving the ability of WO to treat carbohydrate waste with high efficiency. Also, some of these studies are directed toward the utilization of biomass through its conversion to chemical feedstocks, so, efforts mainly focused on maximizing product yields by changing conditions. The WO of organics is generally thought to proceed through a free radical mechanism. On the basis of the oxidation mechanism of high molecular weight carboxylic acids in supercritical water (14), the oxidation mechanism of secondary alcohols in a liquid phase (32, 33), and experimental results that the oxidation of all of cellulose, starch, and glucose yielded a considerably high quantity of formic acid (see Figure 9), a reaction scheme which might best appear to explain the oxidation pathways leading to the formation of formic acid is assumed, as shown in Figure 10. At first, polysaccharides such as cellulose and starch undergo hydrolysis to form hexoses (mainly glucose), and then the oxidation of hexoses (glucose) takes place. It is known that, in aldose, the most susceptible group is the -CHO group.
FIGURE 9. GC/MS total ion chromatograms for cellulose, starch, and glucose after oxidation at 300 °C, 2 min (0.5 min for glucose), and 70% H2O2 supply. Mcginnis et al. (34) in their studies on wet oxidation of model carbohydrate compounds also reported that the -CHO group at C-1 of monosaccharide plays an important role in initiating the WO. So, one of the most probable oxidation sites is at VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 10. Assumed oxidation pathways (direct oxidation) of carbohydrates.
FIGURE 11. Mechanism of two-step process for carbohydrates. the -CHO group, giving the corresponding aldonic acid (gluconic acid). It is followed by rupture of C-1-C-2 (R-scission) to give a formic acid and the next lower aldonic acid. The lower aldose may then repeat the process until aldose is entirely degraded stepwise to formic acid. The R-scission may occur in two ways. One may be a direct rupture at the R-position to -COOH group because R-hydroxy acid is relatively unstable. The other may be oxidation splitting by abstraction of R-hydrogen of a hydroxyl radical because the hydrogen of C-1 (R-hydrogen to the -COOH group) is relatively active. According to the mechanism of R-position scission, one molecule of glucose may yield six molecules of formic acid. 1900
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Rupture of C-2-C-3 (β-scission) should also occur by β-hydrogen abstraction of hydroxyl radical, yielding an oxalic acid and aldonic acid with two less carbon atoms. The aldonic acid with four carbons may further undergo β-oxidation splitting to yield two molecules of oxalic acid. Consequently, the oxidation splitting at the β-position may give rise to the formation of three molecules of oxalic acid. For the oxidation of oxalic acid, it is generally considered that it is easily oxidized directly to form CO2 and H2O. However, our further oxidation experiments with oxalic acid at 300 °C, 60 s, and 70% of oxygen supply showed that the oxidation of oxalic acid yielded a considerable amount of formic acid. The formation of formic acid is probably due to the decarboxylation of oxalic
acid. Mantzavinos et al. (35) also reported that the oxidation of oxalic acid formed formic acid by the decarboxylation mechanism. Therefore, the oxalic acid produced by the β-position scission can further undergo degradation to lead to the formation of H2O and CO2 (oxidation) and formic acid (decarboxylation). Further, besides oxidation of the -CHO group, the -CH2OH group could also be oxidized, because the treatment of aldose with a more vigorous oxidizing agent brings about the oxidation of not only the -CHO group but also the -CH2OH group. The oxidation at two ends should give the corresponding aldaric acids (dicarboxylic acids). Subsequently, the oxidation of aldaric acids may proceed in a way similar to that of gluconic acid oxidation. That is, R-scission may directly yield formic acid, and β-scission may give formic acid via oxalic acid. Yet, because the oxidation of aldaric acids may occur at two ends simultaneously, the degradation rate could be much faster than aldonic acid. In our experimental conditions, because of a high temperature of 300 °C and a strong oxidant of H2O2 used, it is possible that oxidation of two ends occurred preferentially. It can be understood from the above discussion that formic acid should be a basic product in the WO of carbohydrates in the case of either oxidation at only the -CHO group or the simultaneous oxidation at the -CHO and the -CH2OH groups, or in the case of either R-scission or β-scission. This may be the reason WO (direct oxidation) of carbohydrates produces little acetic acid. A possible source of acetic acid in the WO of carbohydrate may be intermediates in other reactions such as dehydration products of aldose, because dehydration of aldose occurs easily. These suggestions imply that if aldose does not eliminate the -OH group, little acetic acid is formed. The two-step process shown in Figure 11does express such a mechanism. That is, first direct dehydration makes hexoses form HMF or first hexoses are cleaved at a middle (C-3 and C-4), and then there is dehydration and rearrangement to form lactic acid. A reason HMF and lactic acid can yield a large amount of acetic acid is probably due to the elimination of three water molecules from glucose for HMF and two for lactic acid. In summary, this two-step process takes advantage of acid-base catalyst of near-critical water to improve the formation of some dehydration products, which can produce a large amount acetic acid by their oxidation. The two-step process is not complex and has the following advantages: (1) the control of the process is very easy with only a hydrothermal reaction before the usual WO without adding a catalyst; (2) it can reduce the cost of manufacturing acetic acid or CMA deicer as compared to the usual WO, because the cost is most sensitive to the yield or concentration of acetic acid. On the contrary, from a viewpoint of complete decomposition of carbohydrate waste, the results discussed above may give us some suggestion of how to easily decompose carbohydrates to CO2 and H2O. That is, the dehydration of aldose should be inhibited, because the effectiveness of WO is limited by the rate of oxidation of acetic acid and hydration of aldose would produce acetic acid. To avoid dehydration or the formation of acetic acid, an excess supply of oxygen and avoiding treatment near 300 °C may be useful, because an excess supply of oxygen may increase the selectivity for direct oxidation, and the dehydration occurs easily near 300 °C.
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Received for review July 21, 2004. Revised manuscript received November 18, 2004. Accepted December 16, 2004. ES048867A
