Liquefaction of Lignocellulosics in Organic Solvents and Its Application

Chapter 10

Liquefaction of Lignocellulosics in Organic Solvents and Its Application Nobuo Shiraishi

Downloaded by UNIV MASSACHUSETTS AMHERST on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch010

Department of Wood Science and Technology, Kyoto University, Sakyo-ku, Kyoto 606, Japan

Liquefaction of lignocellulosics in organic solvents at temperatures of 240270 °C without catalysts and that at temperatures around 80-150 °C with acidic catalysts has been recently developed. These give very high yields of solvent solubles (around 90-95 % based on the lignocellulosic weight), and are quite different from the conventional liquefaction of lignocellulosics where one to several hours of treatments is required at 300-400 °C with or without catalysts. The latter often uses aqueous and/or organic solvents and usually results in quite low yields of 40-60 % because of the conversion of the lignocellulosics into gaseous compounds. Applications of the newly developed lignocellulosic liquids have also been conducted in the preparation of wood-based reactive adhesives, foams, moldings, fibers, and carbon fibers. The term 'liquefaction of lignocellulosics" has hitherto chiefly meant the procedures for the production of oil from biomass using very severe conditions of conversion (1-3). For example, Appel et ai tried to convert cellulosics to oil using homogeneous Na C0 catalyst in water and high boiling point solvent mixtures (anthracene oil, cresol, etc.) at pressures of 140-240 atm with synthesis gas, C O / H (3). Treatments of one hour at 300-350 °C result in 40-60 % yield of benzene solubles (oil) and a 95-99 % conversion of the starting materials. Thus, this type of liquefaction can be called oilification of lignocellulosics. In contrast, this review presents recent progress in lignocellulosic liquefaction done by milder treating conditions; that is, at temperatures of 240-270 °C without catalyst, or at temperatures of 80-150 °C with acidic catalysts. One special group of chemically modified woods can be dissolved in cresols even at room temperature as will be shown later. At any rate, the liquefaction or dissolution of chemically modified wood has been developed (4-8). Even the liquefaction of untreated wood was found to be possible as well (8-11). 2

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0097-6156/92A)476-0136$06.00/0 © 1992 American Chemical Society

In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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LIQUEFACTION OR DISSOLUTION OF WOOD


Chemically Modified Wood Chemically modified woods have been found to liquefy or dissolve in various neutral aqueous solvents, organic solvents, or organic solutions, depending on the characteristics of the modified wood (12-15). This work has been extended to the preparation of wood-based reactive adhesives, wood-based resins, etc., which will be discussed later. So far, three methods have been found for wood liquefaction. The first trial of liquefaction of wood was accomplished by using very severe dissolving conditions (16). A series of aliphatic acid esterified wood samples could be liquefied in benzyl ether, styrene oxide, phenol, resorcinol, benzaldehyde, aqueous phenols, chloroform-dioxane mixture, benzene-acetone mixture and so forth after treatment at 200-270 °C for 20-150 min. Carboxymethylated wood, allylated wood and hydroxyethylated wood have been found to be liquéfiable in phenol, resorcinol, or their aqueous solutions, formalin, etc. after standing or stirring at 170 °C for 30-60 min (6). Another method of liquefaction is to make use of solvolysis during the process (9, 17). By using conditions which allow phenolysis of a part of the lignin, especially in the presence of an appropriate catalyst, the liquefaction of chemically modified wood into phenols could be accomplished under milder conditions (at 80 C for 30 to 150 min). Allylated wood, methylated wood, ethylated wood, hydroxyethylated wood, acetylated wood and others have been found to dissolve in polyhydric alcohols, such as 1,6-hexanediol, 1,4-butanediol, 1,2-ethanediol, 1,2,3-propane triol (glycerol) and bisphenol A, by use of the liquefaction conditions described above. Each of them caused partial alcoholysis of lignin macromolecules (7). This means that several series of reactive solvents could be used in addition to the phenols to liquefy modified wood. The liquefaction processes give paste-like solutions with a considerably high concentration of wood solute (70 %). The solutions obtained with high concentration of wood can be used directly to prepare adhesives, foams and other molded products. This has opened a new field for utilizing wood materials. For the third method of liquefaction or dissolution, postchlorination, has been developed. Sakata and Morita (18) have recently found that when chemically modified woods are chlorinated, their solubility in solvents was enhanced tremendously. For example, at room temperature cyanoethylated wood can be dissolved only 9.25 % in o-cresol. However, once chlorinated, it can dissolve almost completely in the same solvent at room temperature. The chlorinatedcyanoethylated wood can also dissolve in resorcinol, phenol, and a LiCl-dimethylacetamide solution under heating. e

Untreated Wood So far, liquefaction or dissolution of chemically modified woods has been discussed. However, more recently, untreated wood was also found to be liquefied



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in several organic solvents (9, 10). This phenomenon was discovered during an investigation evaluating the effect of the degree of chemical modification of wood on liquefaction. For example, after treatment at 200-250 °C for 30-180 min, wood chips and wood meal were liquefied in phenols; bisphenols; alcohols such as benzyl alcohol; polyhydric alcohols such as 1,6-hexanediol, 1,4-butanediol; oxyethers such as methyl cellosolve, ethyl cellosolve, diethylene glycol, triethylene glycol, polyethylene glycol; 1,4-dioxane, cyclohexanone, diethyl ketone, ethyl n-propyl ketone and so forth. Liquefaction of untreated wood can also be achieved at a lower temperature of 150 °C and atmospheric pressure in the presence of acid catalysts (19). As the catalysts, phenolsulfonic acid and sulfuric acid can be used. The liquefied wood obtained is a paste-like solution with a highly concentrated wood solute, as high as 70 %. After liquefaction, wood components were found to be largely degraded, modified, and reactive. Thus, the wood solute can be used to prepare adhesives and other moldings. This also opened a new and practicalfieldfor utilizing wood materials.

APPLICATION OF T H E LIQUEFACTION OR DISSOLUTION OF WOOD Chemically Modified Wood There are many potential applications of the liquefaction or dissolution of chemically modified wood: fractionation of modified wood components (16, 18, 20), preparation of solvent-sensitive and/or reaction-sensitive wood-based adhesives (7, 15, 17, 21), preparation of resinified wood-based moldings such as foamtype moldings (7), and preparation of wood-basedfibersand their conversion to carbon fibers (22, 23), and so forth. Fractionation of modified wood components has been studied by at least three groups of investigators (16, 18, 20). For the fractionation, a dissolutionprecipitation technique has been successfully used. Sakata and Morita showed that quantitative precipitation of the wood components can be made by pouring the cresol solutions of chlorinated-cyanoethylated wood into an excess of ethanol, ethyl ether or other solvent (18). Since the recovered amounts of the precipitates were found to change with the type of the non-solvent, the fractionation of modified wood components was thought to be possible. Young et al. (20) also described the isolation of the main components of modified wood. The preparation of adhesives from chemically modified wood has been studied. Phenols, bisphenols, and polyhydric alcohols have been used as solvents for the modified wood. In these cases, the resin should contain meaningful amounts of modified wood (7, 15, 17, 21). Combined use of these reactive solvents with reactive agents, such as cross-linking agents and/or hardeners, if necessary, have given phenol-formaldehyde resins (such as resol resin), polyurethane



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resins, epoxy resins, etc. The chemically modified woods are designed not only to dissolve and disperse in thefinalresins, but also to chemically react and bond to the resins. Techniques have been used that make it possible for phenol, to react at the phenyl propane side chain of lignin through carbon-carbon bonds, or for polyhydric alcohols to react with lignin through ether bonds. This can be achieved by liquefaction or dissolution of the chemically modified wood into the reactive solvent using solvolysis techniques. In the case of epoxy resins, it can be also achieved by reacting various alcoholic hydroxyl groups remaining in the modified woods with epichlorohydrin, resulting in introduction of glycidyl groups. Cross-linking within and between wood components, especially between polysaccharide components during the last stage of resinification, by reaction with cross-linking agents can also be used. In order to prepare wood-based resins with meaningful amounts of the wood components, it is very important to liquefy or dissolve the chemically modified wood into the reactive solvents in high concentrations (more than 50 % of the chemically modified wood is preferable). When hydrophilic chemically modified woods, such as carboxymethylated wood, hydroxyethylated wood, or ethylated wood are used in wood-based adhesives, aqueous resol resin adhesives which maintain their solution state during the preparation are obtainable (15, 17, 21, 24, 25). When a phenol solution with a concentration more than 50 % is sought, the chemically modified wood powder can not be completely immersed in phenol, but can only be partly penetrated by the phenol during the first stage of liquefaction. When the heterogeneous mixture, however, is allowed to stand for about 30 min at 80 °C (without stirring in the presence of appropriate amounts of hydrochloric acid as the catalyst), a homogeneous paste can be obtained. Subsequent stirring of the paste for about 1-1.5 hr enhances the liquefaction. In this liquefaction process, a certain degree of phenolysis of wood components, especially that of lignin, takes place, which makes it easy to dissolve them in phenol. After neutralizing the paste with aqueous sodium hydroxide, a definite amount of formalin and sodium hydroxide are added and the mixture is resinified in accordance with the conventional procedure to prepare the resol resin adhesives. The appearance of the resin obtained is similar to that of the corresponding commercial phenol resin adhesive. The wood-based resol resin adhesives have superior glu ability and workability. The adhesives can be used with fillers, thickeners, and fortifiers such as wheat flour, coconut shell, walnut flour, and polymeric MDI (4,4'-diphenyl methane diisocyanate). The addition of appropriate fortifiers, especially cross-linking agents like polymeric MDI, into the wood-based adhesives enhances their dry-bond and water-proof gluabilities remarkably. For the preparation of wood-based polyurethane as well as epoxy resin adhesives, the above-mentioned hydrophilic chemically modified woods prepared by conventional methods are liquefied in polyhydric alcohols or bisphenol A in a manner similar to the liquefaction in phenol (7). Concentration of the modified wood is usually more than 50 %. Diluents such as ethanol or methanol are also



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often added to the liquefaction systems according as required. After liquefaction, the pastes are neutralized and the diluents distilled off. When the pastes are used in combination with suitable polyisocyanate compounds, they become wood-based polyurethane adhesives. When the pastes are further reacted with epichlorohydrin, glycidyl etherified resins are formed. These can be used with hardeners such as amines and acid anhydride, and they become wood-based epoxy resin adhesives. Generally, the wood-based epoxy resins tend to become very viscous or solid, depending on the conditions of preparation, and require dilution or dissolution with solvents such as ethyl acetate, acetone, etc. These resins make satisfactory adhesives which can be used in water-proof gluing. Molding materials such as foams or shaped moldings can also be obtained from chemically modified wood solutions (7). One of the examples is a wooden polyurethane foam. This can be prepared by adding an adequate amount of water as a foaming agent and a polyisocyanate compound as a hardener, to the 1,6-hexanediol solutions of allylated wood, mixing well and heating. When heated at 100 °C, foaming and resinification of the resins are initiated within 2 minutes and completed within several minutes. If promoters such as triethylamine are added, rapid reactions occur even at room temperature and foams can be obtained within several minutes. The foams thus obtained have low densities of around 0.04 g/cm , a substantial strength and elasticity in the compression deformation. In order to elucidate the role of the chemically modified wood within the foams, comparative experiments preparing the foams without the presence of the chemically modified wood have also been conducted. It was found that foaming actually occurs during the resinifying process, but immediately after that, a contraction in volume of the foam occurs, resulting in resin moldings with apparent densities around 0.2 g/cm with little foamed-cell structure remaining. This result reveals that the chemically modified wood plays a positive role in maintaining the shape of the foams during their formation. One other application of modified wood solutions is the formation of filaments or fibers. Tsujimoto et ai (22, 23) have prepared wood-based fibers from acetylated wood. After preparing the phenol solution of the acetylated wood, hexamethylene tetramine is added and the solution heated up to 150 °C to promote addition-condensation for a resinified solution with high spinnability. From the solution, filaments are spun and hardened in a heating oven at a definite heating rate. Maximum temperature for the hardening is 250 °C. By this way, continuousfilamentscan be easily obtained. Thesefilamentscan be carbonized to give carbonfilaments.Carbonization is carried out in an electrically-heated furnace at a maximum temperature of 900 °C with a heating rate of 5.5 °C/min. The strength of the carbon filaments was measured according to Japan Industrial Standard (JIS R7601) and tensile strengths up to 100 kgf/mm have been obtained. This strength is comparable to that of the pitch carbonfibersof general purpose grade. Further improvements of strength may be expected by improving the methods of spinning and carbonization. 2

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Fig. 1. Molding from lignocellulose after liquefaction in phenol, distillation of the free phenol, and curing by hexamethylene tetramine.

Untreated Wood Almost the same products have been prepared from wood solutions of untreated wood as those from chemically modified wood (26, 27). For example, resol-resin type adhesives could be prepared fromfiveparts of wood chips liquefied in two parts of phenol at 250 °C. The adhesives did not require severe adhesion conditions and were comparable to the corresponding commercial adhesives in their gluabilities. Acceptable water proof adhesion was attained for these adhesives. Resol-type phenol resin adhesives were also prepared from wood-phenol solutions liquefied at 150 °C with phenolsulfonic acid catalyst, and their gluabilities were examined (19). The results revealed that when these adhesives were used, it was easily possible to realize completely satisfactory water proof adhesion even under hot-press conditions at 120 °C, with a hot-pressing duration of 0.5 min per 1 mm thickness of plywood. The adhesion temperature of 120 °C was at least 15 C lower than ordinarily used adhesion temperature for resol resin adhesives. As a second example, foams can be prepared from untreated wood-polyethylene glycol solutions (28). Soft-type and hard-type foams can be prepared by changing the preparation conditions. The prepared foams were of density around 0.04 g/cm , having substantial strength and elasticity in the deformations. As a third example, novolak-resin type moldings can be prepared from untreated wood-phenol solutions (29, 30). After one part of wood meal was liquefied in two parts of phenol, unreacted phenol was distilled under reduced pressure. The wood-phenol liquefied and reacted powders obtained can be cured directly, when wood meal filler and hexamethylene tetramine are added and hot-pressed at 170-200 °C. The flexural strengths of the moldings were found to be comparable with those for commercial novolak moldings. An example of the moldings thus obtained is shown in Fig. 1. e

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The caxbon fibers described previously could also be prepared from untreated wood solutions. Tensile strengths up to 120 kgf/cm have been obtained so far. Further improvements of their physical properties may be also expected. 2


CONCLUDING REMARKS The present state of studies on wood liquefaction or dissolution has briefly been reviewed. We believe that this line of study is a new field for the chemical processing of wood with great future potential. To achieve progress in this field, more fundamental and critical studies should be made.

LITERATURE CITED 1. Vanasse, C.; Chornet, E.; Overend, R. P., Can. J. Chem. Eng., 1988, 66, 112. 2. Appel, H. R.; Wender, I.; Miller, R. D., "Conversion of Urban Refuse to Oil", U. S. Bureau of Mines, Technical Progress Report-25, 1969, 5. 3. Appel, H. R.; Fu, Y. C.; Illig, E. G.; Steffgen, F. W.; Miller, R.D., "Conversion of Cellulosics Wastes to Oil", U. S. Bureau of Mines, RI 8013, 1975, 27. 4. Shiraishi, N., Kobunshi Kako, 1982, 31, 500. 5. Shiraishi, N., Japan Patent, 1988, Sho 63-1992 (Appl. June 6, 1980). 6. Shiraishi, N.; Goda, K., Mokuzai Kogyo, 1984, 39, 329. 7. Shiraishi, N.; Onodera, S.; Ohtani, M.; Masumoto, T., Mokuzai Gakkaishi, 1985, 31, 418. 8. Shiraishi, N., Tappi Proceedings, 1987 International Dissolving Pulps Conference, 1987, 95. 9. Shiraishi, N.; Tsujimoto, N.; Pu, S., Japan Pat. (Open), 1986, Sho 61261358 (Submitted on May 14, 1985). 10. Pu, S.; Shiraishi, N.; Yokota, T., Abst. Papers Presented at 36th National Meeting, Japan Wood Res. Soc., Shizuoka, 1986, 179, 180. 11. Shiraishi, N., Mokuzai Kogyo, 1987, 42, 42. 12. Shiraishi, N., Sen-i To Kogyo, 1983, 39, P-95. 13. Shiraishi, N., "Chemistry of Wood Utilization" (H. Imamura, H. Okamoto, T. Goto, Y. Yasue, T. Yokota and T. Yosimoto Eds.), Kyoritsu Publ. Co., Tokyo, 1983, 294.


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14. Shiraishi, N., "Advanced Techniques and Future Approaches in Wood Chemicals", (J. Nakano and T. Haraguchi Eds.), CMC Inc., Tokyo, 1983, 271. 15. Shiraishi, N., Mokuzai Gakkaishi, 1986, 32, 755. 16. Shiraishi, N., Japan Pat. (Open), 1982, Sho 57-2301; Sho 57-2360.


17. Shiraishi, N.; Kishi, H., J. Appl. Polym. Sci., 1986, 32, 3189. 18. Morita, M.; Shigematsu, K.; Sakata, I., Abst. Papers Presented at 35th National Meeting, Japan Wood Res. Soc., Tokyo, 1985, 215, 216. 19. Tanihara, Y.; Kato, K.; Pu, S.; Saka, S.; Shiraishi, Ν., Abst. Papers Presented at 39th National Meeting, Japan Wood Res. Soc., Okinawa, 1989, 321. 20. Young, R. Α.; Achmadi, S.; Barkalow, D., Preprints of CELLUCON '84, Cartrefle-Wrexham, Wales, 1984, 65. 21. Kishi, H.; Shiraishi, N., Mokuzai Gakkaishi, 1986, 32, 520. 22. Tsujimoto, N., Preprints for l4th Symposium on Chemical Processing of Wood, Kyoto, 1984, 17. 23. Tsujimoto, N.; Yamakoshi, M.; Fukuchi, R., Preprints for Poster Presen tation, International Symposium on Wood and Pulping Chemistry, Van couver, 1985, 19. 24. Shiraishi, N.; Itoh, H.; Lonikar, S. V., J. Wood Chem. Technol., 1987, 7, 405. 25. Shiraishi, N., "Lignin Properties and Materials", A.C.S. Symp. Series, 397 (W. G. Glasser and S. Sarkanen Eds.), Amer. Chem. Soc, Washington D . C , 1989, 488. 26. Shiraishi, N.; Tamura, Y.; Tsujimoto, N., Mokuzai Kogyo, 1987, 42, 492. 27. Shiraishi, N.; Tamura, Y.; Tsujimoto, N., Mokuzai Kogyo, 1988, 43, 2. 28. Shiraishi, N., "Cellulosic Utilization; Research and Rewards in Cellulosics", Elsevier Appl. Sci., 1989, 97. 29. Kato, K.; Yoshioka, M.; Shiraishi, N., Abst. Papers Presented at 40th National Meeting Japan Wood Res. Soc, Tsukuba, 1990, 26. 30. Shiraishi, N.; Kato, K., Japan Pat., submitted (On May 30, 1989). RECEIVED May 2, 1991


Liquefaction of Lignocellulosics in Organic Solvents and Its Application

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