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Literature Cited Bolton, J. R., Ed. "Solar Power and Fuels"; Academic Press: New York, 1977. Duffle, J. A,; Beckmann, W. A. "Solar Energy Thermal processes"; wiley: New York, 1974; Chapter 7. Hautala, R. R.; Little, J.; Sweet, E. Sokr Energy 1977, 19, 503. Hautala, R. R.; King, R. B.; K ~ I c., , Ed, " s E ~ ~ Chemical ~ ~~ conver~ ~ sion. and Storaae"; The Humana Press: Clifton. NJ. 1979: DD 333-369. Jones, G., 11; Chh-ng, S. H.; Xuan, P. T. J. Photochem. 1979,'iO, 1. Jones, G., 11; Ramachandran, B. R. J. Org. Chem. 1976, 41, 78s. Kabakoff, D. S.; Bunzii, G. J.: Oth, J.; Hammond, W.; Berson, J. J. Am. Chem. SOC.1975, 9 7 , 1510. Laird, T. Chem. Ind. 1978, 6 , 186.
1983,22, 633-636
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Lane, J. E.; Mau, A. W. H.; Pompe, A.; Sassa, W. H. F.; Spurling, T. H. "Chemlcal Storage of Solar Energy"; CSIRO Aust. Div. Appl. Org. Chem., Techn. Paper No. 4, 1977; Vol. 4, p 1. Lichtin, N., Ed. "The Current State Of Knowledge of Photochemical Formation of Fuel"; N.S.F./RANN Report P.B. 246229 (1974), National Science Foundation: Washington, DC. ~Scharf,, H. D.; Flelschhauer, J.; Leismann, H.; Ressler, I.; Schleker, W.; Weitz, R. Angew. Chem., Int. Ed. Engl. 1977, 18, 652.
Received for review October 6 , 1982 Revised manuscript received April 7, 1983 Accepted April 21, 1983
Conversion of Biomass into Chemicals with High-Temperature Wet Oxidation Gary D. McGinnls,*+Wilber W. Wilson,$ Shawn E. Prince,+and Chyl-Cheng Chenx Forest Products Laboratory, Mississippi State University, Mississippi State, Mississippi 39762, Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, and Biochemistry Department, Purdue University, West Lafayette, Indiana 47907
High-temperature (172-277 "C) wet oxidation of woods leads to the production of a mixture of organic acids. The organic compounds produced Include formic acid, acetic acid, methanol, and a mixture of hydroxylated organic acids. In this study, the effects of oxygen pressure, temperature, the type of wood, and the addition of ferric sulfate on the yield of the organic acids were determined.
Introduction The potential for using waste biomass as a source of fuel and chemicals to partially replace petroleum requirements has been reviewed (Goldstein, 1981). This potential can be realized only if processes that are both economical and efficient are developed for converting the biomass. In an earlier paper, our laboratory has reported some initial results of a biomass pretreatment process which uses water and high-pressure oxygen (McGinnis et al., 1983). The major reaction of biomass under relatively mild wet-oxidation conditions (120-172 "C) was solubilization of the hemicellulose and partial solubilization of the lignin. Thus, the wet-oxidation products are a liquid fraction containing dissolved hemicelluloses and lignin and a solid fraction enriched in cellulose but also containing some lignin. The wet-oxidation pretreatment not only fractionated the biomass sample, but also rendered the cellulose more susceptible for acid hydrolysis to glucose, an important result if fermentation to ethanol is the desired end-product for the biomass conversion. At higher wet-oxidation temperatures (172-227 "C),the reaction became increasingly more oxidative and caused a considerable amount of fragmentation and oxidation of the biomass, ultimately leading to the formation of a series of organic acids plus smaller amounts of neutral organic compounds (Schaleger and Brink, 1977,1978). Schaleger and Brink identified formic, acetic, glycolic, oxalic, levulinic, and succinic acids as the major products formed by the wet oxidation of wood. They also identified a series of neutral compounds, including glucose, mannose, xylose,
Forest Products Laboratory. Department of Chemistry. Purdue University.
Table I. Wet-Oxidation Conditions temperature time oxygen pressure
wood water reactor volume a
171-227 "C 30 min 240-480 psia 15 g 150 mL 600 mL
Initial oxygen pressure in the reactor at 25 "C.
galactose, arabinose, and methanol. This study was done with air at pressures at 45-100 psi and temperatures between 160 and 220 O C . The study described here was undertaken to identify more fully the products formed by high-temperature wet oxidation and to determine how various reaction parameters, such as oxygen pressure, temperature, wood species, and the presence of metal salts, affect the yield of these products. Experimental Materials and Methods Three types of biomass from the southeastern United States were used; loblolly pine (Pinus taeda L.), black oak (Quercus uelutina Lam.), and a sample of mixed hardwood materials obtained by whole tree chipping. Bark from the pine and oak was removed; the wood was dried and then ground to pass a 2-mm screen in a Wiley mill. The mixed hardwood material, containing approximately 15% bark and twigs, was used directly after grinding. The wet-oxidation conditions used in this study are Iisted in Table I. The ground wood and water were mixed in a weight ratio of 1:lO and placed in a 600-mL reactor (Parr Model 4521). A t the conclusion of the wet-oxidation reaction, the solids and liquids were separated by filtration. The solid material was air dried, weighed, and stored in an air-tight container. A small sample was removed, weighed accurately, and
0196-4321/83/1222-0633$01.50/00 1983 American Chemical Society
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Figure 1. The yield of formic acid from the wet oxidation of loblolly pine (*I, black oak (u), and mixed hardwoods ( 0 )at an oxygen pressure of 240 psi with no catalyst.
placed in a 110 "C oven for 12 h in order to determine the moisture content. This value was then used to calculate the yield of solid product. The acids were isolated from the liquid fraction using reverse-phase chromatography (Waters C-18 Sep-pak No. 51910). Samples of the wet-oxidation product were passed through the reverse-phase columns using water as the eluent. The carboxylic acids were not retained on the column while other less polar organic compounds were held by the column. After this purification step, the liquid sample containing the acids was analyzed by high-performance liquid chromatography by use of a cation-exchange column (Bio-Rad HPX-87). Volatile products, such as methanol, acetaldehyde, acetone, and ethanol, were determined quantitatively by gas chromatography with a Porapak Q column (100-120 mesh, 10 ft X 1/8 in.), and a flame-ionization detector. The volatile carboxylic acid and neutral products were identified by gas chromatography-mass spectrometry, while the nonvolatile hydroxylated acids were separated by gas chromatography and identified by mass spectrometry after conversion into their trimethylsilyl derivatives. The total acid content was determined by titration using dilute sodium hydroxide. Results and Discussion Formic Acid. The organic acid that was produced in the greatest yield by the high-temperature wet oxidation was formic acid. The relationship between the yield of this acid (based on the starting weight of the dried wood sample) and the wet-oxidation temperature is shown in Figure 1. For these reaction conditions (240 psi oxygen pressure, no catalyst), the type of starting material had only a small effect on the temperature profile; all three woody materials gave a maximum yield of 14-16 wt% in the temperature range 190-205 "C. It should be noted that acids are formed even at low temperatures, but their concentrations become appreciable only a t higher temperatures. Many metal salts have been found that catalyze the formation of acids from wood by wet oxidation (McGinnis et al., 1982), including ferric sulfate, cupric sulfate, and
Figure 2. The yield of formic acid from the wet oxidation of loblolly pine (*), black oak (w), and mixed hardwoods ( 0 )catalyzed with 0.67% ferric sulfate at an oxygen pressure of 240 psi.
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aluminum sulfate. The effect of adding 0.67% ferric sulfate on the wet oxidation is shown in Figure 2. Yields for oak were increased from 14 to 21%; for mixed hardwoods, from 16 to 24%;and for pine, from 14 to 29%. This catalyst was not only effective in improving the yields of formic acid but also in shifting the temperature where maximum yield occurred to lower values by about 10 O C , except for the mixed hardwoods. No explanation for the observed effects of the ferric sulfate was apparent to the authors at this time. The effect of increased oxygen pressure during wet oxidation on the yield of formic acid was also investigated.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 635
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Figure 4. The yield of acetic acid from the uncatalyzed wet oxidation of mixed hardwoods at pressures of 240 psi (01,360 psi (D), and 480 psi (*I.
Figure 6. The yield of acetic acid from the wet oxidation of different wood species: mixed hardwoods (e),black oak (D), and loblolly pine (*) at an oxygen pressure of 240 psi.
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Figure 7. The yield of methanol from the wet oxidation of different wood species: mixed hardwoods (O),black oak (D), and loblolly pine (A)at an oxygen pressure of 240 psi.
TEMPERATURE (T)
Figure 5. The yield of acetic acid as a function of oxygen pressure from the wet oxidation of mixed hardwoods catalyzed with 0.67% ferric sulfate at pressures of 240 psi (e),360 psi (D), and 480 (*).
Figure 3 shows the yield data for the mixed hardwoods at 240, 360, and 480 psi oxygen pressure, with no catalyst. Under these conditions, the maximum yield of formic acid (21%)was obtained at about 190 "C and a pressure of 480 psi. At the lower oxygen pressures (360 and 240 psi), the maximum yield occurred at approximately the same temperature, but yields decreased to 17 and 15%, respectively. Acetic Acid. The yield of acetic acid from the mixed hardwoods is shown at three different oxygen pressures in Figure 4. As the oxidative conditions became more severe, the yield of acetic acid continued to increase, at least over the ranges of temperature and oxygen pressure
used in this study. In contrast to the result observed for formic acid, the addition of ferric sulfate to the reaction mixture actually decreased the yield of acetic acid (Figure 5). In all experiments, the amount of acetic acid produced was much greater than that which could be accounted for by the presence of acetyl groups found in hardwoods (3-5%). However, the hardwood species did consistently yield more acetic acid than loblolly pine (Figure 6). Although acetyl groups in wood probably form acetic acid which initiates reactions in wet oxidation, they do not account for all of the acetic acid that was produced. Under the conditions of high-temperature wet oxidation, formic and acetic acids were the major acids produced, accounting for 80-95% of the total acidity. If milder wet-oxidationconditions were employed,the hydroxylated
636 Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
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Figure 8. The chromatogram (total ion current) of the silylated nonvolatile products from the wet oxidation of loblolly pine wood.
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ydrates of wood during the wet-oxidation reaction. These products can be separated by gas chromatography and identfied by mass spectrometry, after conversion into their trimethylsilyl derivatives. The products include glucose, arabino-1,4-lactone, and erythronic, threonic, succinic, glyceric, and glycolic acids (Figure 8). The yield of these products varied depending on the temperature, oxygen pressure, and the absence or presence of a catalyst. At the lower temperature, these acids made up an appreciable fraction of the total acid fraction, but at higher temperatures, higher oxygen pressures, or in the presence of ferric sulfate, acetic and formic acid made up over 95% of the total acid fraction (Figure 9). The other major products formed during wet oxidation are carbon dioxide, traces of carbon monoxide, and an insoluble material which consists of ash and other organic products formed during the reaction. The yield of the insoluble portion varies depending on the reaction condition (Figure 10). In general it decreases with increased oxygen pressure; it also decreases at the lower temperature with the addition of ferric sulfate. In summary, the high-temperature, wet-oxidation reaction of wood produces high yields of acetic and formic acid, an insoluble product, carbon dioxide, and traces of methanol. Based on the weight of starting wood, the optimum yield of formic and acetic acid was 28% and 14%, respectively.
: I UNCATALYZED
170
190
210
230
Registry No. Glucose, 50-99-7; arabino-1,4-lactone, 13280-76-7; erythronic acid, 13752-84-6; threonic acid, 3909-12-4;succinic acid, 110-15-6; glyceric acid, 473-81-4; glycolic acid, 79-14-1; ferric sulfate, 10028-22-5; formic acid, 64-18-6; acetic acid, 64-19-7; methanol, 67-56-1.
Literature Cited Goldstein, I. S. "Biomass Availability and Utility for Chemicals", "Organic Chemicals from Biomass"; CRC Press: Boca Raton, FL, 1981. McGinnis. G. D.: Wilson. W. W.: Prince. S. Ind. €nu. Cbem. prod. Res. D e v . 1983. in press. Schaleger, L. L.; Brink, D. L. "Chemical Production by Oxidative Hydrolysis of Lignocellulose"; Tappi Conference, Forest Bioiogy/Wood Chemistry, Madison WI. 1977. Schaieger,'L. .; Brink, D. L. TAPPI, 1978, 67(4), 65.
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Received for review November 22, 1982 Accepted April 13, 1983
