CHEMSUSCHEM MINIREVIEWS DOI: 10.1002/cssc.201300760
Polyols and Polyurethanes from the Liquefaction of Lignocellulosic Biomass Shengjun Hu, Xiaolan Luo, and Yebo Li*[a] Polyurethanes (PUs), produced from the condensation polymerizations between polyols and isocyanates, are one of the most versatile polymer families. Currently, both polyols and isocyanates are largely petroleum derived. Recently, there have been extensive research interests in developing bio-based polyols and PUs from renewable resources. As the world’s most abundant renewable biomass, lignocellulosic biomass is rich in hydroxyl groups and has potential as a feedstock to produce bio-based polyols and PUs. Lignocellulosic biomass
can be converted to liquid polyols for PU applications through acid- or base-catalyzed atmospheric liquefaction processes using polyhydric alcohols as liquefaction solvents. Biomass liquefaction-derived polyols can be used to prepare various PU products, such as foams, films and adhesives. The properties of biomass liquefaction-derived polyols and PUs depend on various factors, such as feedstock characteristics, liquefaction conditions, and PU formulations.
1. Introduction In general, polyols refer to compounds that contain two or more hydroxyl groups in one molecule. Depending on their molecular weights (Mw), polyols can be divided into monomeric and polymeric polyols. Monomeric polyols, such as glycerol and ethylene glycol (EG), are useful starting materials for the production of a broad spectrum of valuable chemicals through chemical reactions such as esterification,[1–3] dehydration,[4–6] and hydrogenation.[7–9] Polymeric polyols, which mainly consist of polyether and polyester polyols, are almost exclusively used to produce polyurethanes (PUs) through their condensation polymerization reactions with isocyanates (Scheme 1). To meet
Scheme 1. Polymerization between polyol and isocyanate to form PU.
different PU application needs, polyols with a wide range of chemical structures and properties have been produced commercially. Polyols commonly used for PU production have functionalities varying from 2 to 8 and Mw from 200 to 8000 g mol 1.[10] This broad range of polyol properties allows tunable design of PU structures, making them one of the most versatile families of polymeric materials.[10] A large number of PU products, such as foams, coatings, elastomers, and adhe[a] Dr. S. Hu, Dr. X. Luo, Prof. Y. Li Department of Food, Agricultural and Biological Engineering The Ohio State University/Ohio Agricultural Research and Development Center 1680 Madison Ave, Wooster, OH, 44691-4096 (USA) Fax: (+ 1) 330-263-3670 E-mail: [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
sives, have been commercially produced and used in many aspects of our daily lives. For example, flexible PU foams are widely used as cushion materials in applications such as car seats, mattresses, and upholstery, whereas rigid PU foams are common thermal insulation materials used by the refrigerator and construction industries. Currently, commercial polyols and PUs are mainly produced from petrochemical derivatives. Recently, concerns over the depletion of fossil resources have spurred extensive interest in developing bio-based polyols and PUs from renewable resources.[11] Vegetable oils and lignocellulosic biomass are two major types of biomass that have been extensively studied for their uses in producing bio-based polyols and PUs. Epoxidation/oxirane-ring opening, ozonolysis, hydroformylation, and transesterifcation/amidation are four major techniques used to convert vegetable oils to polyols.[11–13] Depending on the conversion techniques and vegetable oils used, vegetable oil-derived polyols exhibit different structures and material properties that are suitable for different applications. In recent years, several reviews on this topic have been published.[11–13] As another major feedstock studied for the production of bio-based polyols, lignocellulosic biomass materials, such as wood and agricultural crop residues, are considered to be the world’s most abundant renewable biomass. The main structural units of lignocellulosic biomass are cellulose (30–35 %), hemicellulose (15–35 %), and lignin (20–35 %),[14] all of which are highly functionalized materials rich in hydroxyl groups, making them promising feedstocks to produce bio-based polyols. However, lignocellulosic biomass materials are solid and need to be converted into liquid polyols before being used to produce PUs. Currently, two major technologies exist for this conversion: oxypropylation and liquefaction. The oxypropylation of biomass is a polymerization process that grafts oligo (propylene oxide) or propylene oxide homopolymer onto biomass ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS macromolecular structures.[15] Usually, this process is conducted under high pressure and high temperature (ca. 100–200 8C) in the presence of KOH as a catalyst, during which biomass is first activated and functionalized, followed by oxypropylation reactions with propylene oxide.[16, 17] Oxypropylation-derived polyols are generally mixtures of different compounds, including oxypropylated biomass, poly(propylene oxide), and some unreacted or partially oxypropylated biomass, which can be used directly for the preparation of PUs.[17] The production of polyols through the oxypropylation of various lignocellulosic biomass, such as sugar beet pulp,[18] cork,[15, 16] and lignin,[19–21] has been reported. Recently, a review on this topic was published.[17] Alternatively, lignocellulosic biomass can be converted into liquid polyols through an atmospheric liquefaction process. The liquefaction process is usually conducted at elevated temperatures (150–250 8C) under atmospheric pressure using polyhydric alcohols, such as polyethylene glycol (PEG) and glycerol, as liquefaction solvents.[22–26] The liquefaction can be either acid- or base-catalyzed with the former being more common. During the liquefaction process, biomass is degraded and decomposed into smaller molecules by polyhydric alcohols through solvolytic reactions. The polyols produced are largely a mixture of different compounds rich in hydroxyl groups and can be used directly to prepare various PUs such as foams,[25–28] adhesives,[29, 30] and films.[31–33] A large number of lignocellulosic biomass materials, such as wood[27, 34–37] and agricultural residues,[22, 23, 25, 26, 38–44] lignin,[45] and industrial and biorefinery byproducts,[24, 43, 46, 47] have been liquefied for the production of polyols and PUs. Though reviews on vegetable oiland oxypropylation-derived polyols and PUs were published previously, to the best of our knowledge, no reviews have been devoted to the discussion of polyols and PUs derived from the liquefaction of lignocellulosic biomass. This Review will focus on this topic, including the biomass liquefaction mechanism, factors that affect the efficiency of the liquefaction process, and general properties of biomass liquefaction-de-
Yebo Li, born in 1967 in Zibo, China, obtained his Ph.D. degree from China Agricultural University in 1993. He joined The Ohio State University (OSU) as assistant professor in 2007. He is currently an associate professor in the Department of Food, Agricultural, and Biological Engineering Department. He also holds an adjunct faculty position in OSU’s Department of Chemical and Biomolecular Engineering. His research focuses on the development of advanced technologies for the production of bioenergy and bioproducts from renewable sources. In 2011, he received OSU’s early career innovator of the year award and 2012 he was awarded American Society of Agricultural and Biological Engineering’s Rain Bird Engineering Concept of the Year Award.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org rived polyols and PUs. Finally, a summary and outlook of biomass liquefaction-derived polyols and PUs is provided.
2. Mechanism of Lignocellulosic Biomass Liquefaction by Polyhydric Alcohols The liquefaction of lignocellulosic biomass by polyhydric alcohols proceeds mainly through solvolytic reactions that cleave various chemical bonds (e.g., glycoside bonds) existing in biomass structures and produce smaller biomass-derived molecules or fragments.[48, 49] These small biomass derivatives are fairly reactive and can further react with either themselves or the liquefaction solvent, forming a large number of biomass and/or liquefaction solvent-derived compounds. Because of their different structures and morphologies, biomass components are usually liquefied at different stages of the liquefaction process. Usually, the liquefaction of hemicellulose, lignin, and amorphous cellulose occurs rapidly during the early stages of the liquefaction process because they have amorphous structures that are easily accessible to liquefaction solvents. In contrast, the liquefaction of crystalline cellulose is typically slower and continues until the later stages of the liquefaction process because it has a well-packed structure that is less accessible to the solvents.[22, 34, 44, 47] For this reason, cellulose liquefaction is commonly considered to be the rate-limiting step in the biomass liquefaction process.[50, 51] Previous research has suggested that the acid-catalyzed liquefaction of cellulose by different liquefaction solvents, including EG, PEG, and ethylene carbonate (EC), proceed through similar reaction pathways (Scheme 2).[48, 49] The cellulose is first decomposed by solvolytic reactions into glucose or other small
Scheme 2. Schematic representation of the reaction mechanism of acid-catalyzed liquefaction of cellulose in polyhydric alcohols.[48, 49]
cellulose derivatives that can react with the liquefaction solvent to form glycoside derivatives. Then, the produced glycoside derivatives can undergo further reactions to form levulinic acid and/or levulinates. The formation of glycoside and levulinic derivatives in an acid-catalyzed cellulose liquefaction process has also been confirmed by other studies.[50, 52] In a later effort, Zhang[53] conducted a qualitative analysis of the products formed during the acid-catalyzed liquefaction of bagasse in EG. The obtained liquefaction products were fractionated into ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS water-soluble, acetone-soluble, and insoluble residue fractions. Fractional analyses showed that the insoluble residue fraction mainly consisted of insoluble derivatives from cellulose and lignin; the acetone-soluble fraction was rich in high Mw derivatives from lignin; and the water-soluble fraction was mainly a mixture of EG, EG derivatives, diethylene glycol (DEG), saccharides, alcohols, aldehydes, ketones, phenols, and organic acids.[53] The presence of such a wide spectrum of different compounds in biomass liquefaction-derived polyols indicates the complex nature of the biomass liquefaction process: a large number of chemical reactions occur and compete against each other simultaneously. The reaction mechanism shown in Scheme 2 represents one of the major liquefaction reactions occurring during the acid-catalyzed liquefaction of cellulose. Although there have been studies on the mechanism of lignin liquefaction by phenol,[54–58] the mechanism/reaction pathway of lignin liquefaction by polyhydric alcohols has not been reported. Recondensation reactions among biomass derivatives and/or liquefaction solvents occur and compete against liquefaction reactions during the biomass liquefaction process. When dominant, these recondensation reactions can decrease the liquefaction efficiency by increasing the percent insoluble residues in biomass liquefaction-derived polyols. In fact, this phenomenon has been widely observed in acid-catalyzed biomass liquefaction processes.[22, 26, 34, 35] The negative effects of these recondensation reactions on biomass liquefaction can be largely mitigated or avoided by optimization of liquefaction parameters, such as liquefaction temperature and time, catalyst loading, and solvent-to-biomass weight ratio. The mechanism of recondensation reactions in acid-catalyzed biomass liquefaction processes was studied previously by liquefying cellulose, steamed lignin, alkali lignin, and their mixtures under the same liquefaction conditions.[50] No recondensation reactions occurred when cellulose or lignin was liquefied alone, even at a prolonged liquefaction time of 480 min. In contrast, when cellulose and lignin were liquefied together for 480 min, significant recondensations were observed, and percent insoluble residues reached 50 and 76 % for cellulose/steamed lignin and cellulose/alkali lignin mixtures, respectively.[50] Based on these observations, the authors suggested that recondensation reactions that occurred during acid-catalyzed biomass liquefaction processes were largely caused by reactions among depolymerized cellulose and degraded aromatic derivatives from lignin and/or by the nucleophilic displacement reaction of cellulose by phenoxide ion.[50]
3. Factors Affecting the Efficiency of the Liquefaction of Lignocellulosic Biomass 3.1. Lignocellulosic biomass The efficiency of biomass liquefaction varies with the compositions, structures, and morphologies of lignocellulosic biomass materials. A study on the liquefaction of seven different species of wood (hardwood and softwood) suggested that, although three hardwood species shared similar liquefaction be 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org haviors, significantly different liquefaction behaviors were observed for four softwood species.[35] Compared to hardwood, softwood exhibited faster liquefaction rates but earlier occurrences of unfavorable recondensation reactions. This was explained by the existence of large amounts of guaiacyl propane units in softwood that are more reactive than the syringyl propane units existing in hardwood.[35] Lee et al.[47] studied the liquefaction of three types of waste paper (i.e., box paper, newspaper, and business paper) and wood. The order of liquefaction rates, from highest to lowest, was found to be: wood, newspaper, business paper, and then box paper. The higher liquefaction rates of wood and newspaper were explained by their higher contents of lignin and hemicellulose, which can be liquefied rapidly at the early stages of the process.[47] Different liquefaction behaviors of agricultural crop residues have been observed through side by side comparisons, including cotton stalks and bagasse,[38] and corn stover, rice straw, and wheat straw.[40] More recently, Briones et al.[43] investigated the liquefaction behaviors of five types of agro-industrial residues (i.e., rapeseed cake, date seeds, olive stone, corncob, and apple pomace) and obtained liquefaction yields ranging from 84 to 97 %. 3.2. Liquefaction solvent Liquefaction solvents play paramount roles in the biomass liquefaction process. To produce polyols with suitable properties for PU applications, liquefaction solvents need not only to promote rapid and effective biomass liquefaction but also to possess suitable polyol properties for desired PU applications. The latter requirement is because most liquefaction processes are conducted at large liquefaction solvent-to-biomass weight ratios (approximately 3:1 to 5:1) to achieve high liquefaction efficiency. After liquefaction, the liquefaction solvent or its derivatives constitute a major portion of the biomass liquefaction-derived polyols and have significant effects on polyol properties. Thus, the choice of liquefaction solvent depends on both its liquefaction capability and its polyol properties (i.e., polyhydric alcohols, as liquefaction solvents, are also polyols). For example, a binary mixture of PEG400 (Mw: 400 g mol 1) and glycerol is most commonly used as a liquefaction solvent to produce polyols with properties suitable for rigid or semirigid PU foam applications.[22, 23, 28, 47] In contrast, PEG4000 (Mw: 4000 g mol 1) was used as a sole liquefaction solvent to produce polyols suitable for highly resilient PU foam applications.[59] Various polyhydric alcohols such as PEG, EG, glycerol, and EC have been used for biomass liquefaction.[27, 34, 36] The PEG400to-glycerol weight ratio at 4:1 showed high liquefaction efficiency and ability to dampen detrimental recondensation reactions.[22, 23, 26, 27, 34, 47] However, since lignocellulosic biomass materials are structurally heterogeneous, a 4:1 mixture of PEG400/ glycerol is not optimal for all types of biomass. Indeed, studies have suggested different optimal PEG400/glycerol weight ratios for different biomass materials, such as 9:1 for some wood species, bagasse, and cotton stalks[35, 38] and 2:3 for acidhydrolyzed residues of corncobs.[44] ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS Cyclic carbonates, such as EC and propylene carbonate (PC), are capable of rapidly liquefying cellulose and hardwood. Almost-complete liquefaction (i.e., percent solid residues less than 2 %) was achieved within 40 min at 150 8C for EC and PC, whereas incomplete liquefaction was observed even after 120 min for PEG400/glycerol (w/w: 4:1).[36] The high liquefaction efficiency of cyclic carbonates was explained by their high permittivity that leads to the high acid potential of liquefaction solvents.[36] However, in a later study, Wang et al.[60] liquefied corn stover (CS) using EC at different CS-to-EC weight ratios (0.2 to 0.4), and higher percent solid residues ranging from 2.0 to 11.6 % were observed after 90 min of liquefaction at 170 8C. These different observations might be caused by the different compositions and morphologies of biomass feedstocks used in the liquefaction processes. The liquefaction of lignocellulosic biomass usually requires large solvent-to-biomass weight ratios (i.e., 3:1 to 5:1) to obtain high liquefaction efficiency and minimize the detrimental effects of recondensation reactions.[27, 34, 38] For wood, high liquefaction efficiency (i.e., percent biomass residue less than 10 %) can be obtained at a solvent-to-biomass weight ratio as low as 3:1, whereas agricultural crop residues generally need a minimum solvent-to-biomass weight ratio of 5:1 to obtain comparable liquefaction efficiency.[23, 27, 34, 38] Yao et al.[34] studied the acid-catalyzed liquefaction of birch wood by PEG400/glycerol at different solvent/wood ratios (w/w: 3:1 to 11:9), and significant decreases in liquefaction efficiency were observed for ratios below 3:2. In a later study, Lee et al.[23] studied the liquefaction of corn bran under similar reaction conditions at solvent-to-biomass weight ratios of 5:1, 3:1, and 2:1, and decreases in biomass liquefaction efficiency were observed for ratios below 5:1. Similarly, Hassan and Shukry[38] studied the liquefaction of agricultural crop residues (i.e., bagasse and cotton stalks) and obtained high liquefaction efficiency at solvent-tobiomass weight ratios 5:1. Currently, almost all polyhydric alcohols used as liquefaction solvents are petroleum-derived and relatively expensive. To reduce the high cost and increase the renewability of the biomass liquefaction processes, Hu et al.[25] reported the use of crude glycerol, a byproduct from biodiesel production, as an alternative biomass liquefaction solvent for polyol production. Compared to conventional petroleum-derived solvents, crude glycerol is bio-based, renewable, and inexpensive, but contains impurities, such as FFAs, soap, and FAMEs.[61] For most valueadded conversion processes developed for crude glycerol so far, these impurities are unfavorable and detrimental and need to be removed beforehand. Hu et al.[25] investigated the liquefaction of soybean straw by 100 % unrefined crude glycerol under various reaction conditions. Despite the relatively low biomass liquefaction efficiency (around 70 % under optimal reaction conditions), the polyols and PU foams produced exhibited properties comparable to those obtained from conventional petrochemical solvent-based liquefaction processes. More importantly, this study also indicated that certain impurities in crude glycerol, such as FFAs, soap, and FAMEs, improved the properties of biomass liquefaction-derived polyols and PU foams as a result of their synergistic interactions with glycerol 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org and/or biomass components. These results indicated that unrefined crude glycerol can be directly used as a biomass liquefaction solvent for polyol production without expensive upgrading or refining treatments, which could potentially improve the economics and sustainability of lignocellulosic biomass liquefaction. 3.3. Catalyst The liquefaction of lignocellulosic biomass can be either acidor base-catalyzed, although the former is more commonly used. Among all acids investigated so far, concentrated (98 %) sulfuric acid has the highest catalytic ability in biomass liquefaction.[39] The effect of different sulfuric acid loadings on the liquefaction of lignocellulosic biomass has been extensively studied.[23, 38, 39, 44] Generally, significant improvements on biomass liquefaction efficiency were observed by increasing sulfuric acid loadings from 1 to 3 % (% wt., based on liquefaction solvent). For example, by increasing sulfuric acid loadings from 1 to 3 %, the percent biomass residue decreased from approximately 45 % to less than 20 % for the liquefaction of cotton stalks, wheat straw, and acid-hydrolyzed residues of corncobs.[38, 39, 44] Further increasing sulfuric acid loadings to over 3 % provided little improvement on biomass liquefaction efficiency, but increased the risk of accelerating detrimental recondensation reactions.[38, 44, 51] For most lignocellulosic biomass materials, sulfuric acid loadings of around 3–4 % provide a good balance between high liquefaction efficiency and effective retardation of detrimental recondensation reactions. Compared to the extensive studies on acid-catalyzed liquefaction processes, reports on base-catalyzed liquefaction processes are few. Generally, base-catalyzed liquefaction requires higher liquefaction temperatures (ca. 250 8C) to achieve liquefaction efficiencies comparable to those obtained from acidcatalyzed liquefaction.[62, 63] However, base-catalyzed liquefaction processes have the advantage of causing less corrosion to metal equipment used in the liquefaction process. 3.4. Liquefaction temperature and time The liquefaction temperature ranges widely from 130–250 8C, mainly depending on the types of catalysts used in liquefaction. Acid-catalyzed liquefaction is usually conducted at temperatures ranging from 150–170 8C, whereas base-catalyzed liquefaction usually needs higher temperatures of around 250 8C to achieve comparable liquefaction efficiencies. The temperature effect on the liquefaction of different types of lignocellulosic biomass, such as corn bran,[23] wheat straw,[22, 39] bagasse and cotton stalks,[38] corn stalk,[26] rapeseed cake residue,[46] and acid-hydrolyzed residues of corncobs,[44] has been evaluated. Among these studies, the liquefaction processes were all acidcatalyzed and conducted at temperatures ranging from 130 to 170 8C using binary mixtures of PEG 400/glycerol as liquefaction solvents. Significant improvements on biomass liquefaction efficiency were observed as temperatures increased from 130 to 150 8C, beyond which little improvement was observed. The liquefaction of lignocellulosic biomass usually featured ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS
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a rapid liquefaction stage in the first 15–30 min, after which liquefaction proceeded at a much slower rate.[22, 23, 44] As discussed previously, the first rapid liquefaction stage was largely because of the degradation of more accessible biomass components such as lignin, hemicellulose, and amorphous cellulose, whereas the slow stage of the process mainly featured the degradation of well-packed and less solvent-accessible crystalline cellulose.[22, 44, 47] In addition, it is worth mentioning that recondensation reactions among derivatives from biomass and/or liquefaction solvents always accompany and compete against the biomass liquefaction reactions. Under high liquefaction temperature and prolonged liquefaction time, these reactions may become significant enough to decrease biomass liquefaction efficiency. For most lignocellulosic biomass materials, acid-catalyzed liquefaction at 150 8C for 90 min provided high biomass liquefaction efficiency (> 90 % biomass conversion) without significant occurrence of detrimental recondensation reactions.[22, 23, 26, 34, 38, 39, 47] For base-catalyzed liquefaction processes, comparably high liquefaction efficiencies have been obtained at 240–250 8C for a liquefaction time of 60 min.[62, 63]
4. Biomass Liquefaction-Derived Polyols To assess their suitability in PU applications, polyols produced from the liquefaction of lignocellulosic biomass are typically characterized in terms of hydroxyl number, acid number, viscosity, and Mw, all of which change dynamically during biomass liquefaction. For example, the Mw of polyols usually increase at the early stages of the liquefaction process, as a result of the release of a large amount of macromolecules from biomass.[26, 50] As these macromolecules are gradually degraded, the Mw of polyols decrease accordingly but finally may increase again if significant recondensation reactions occur at the late stages of the process.[26, 50] The hydroxyl numbers of polyols decrease, whereas the acid numbers of polyols increase, with the progression of biomass liquefaction. The decreasing polyol hydroxyl numbers can be ascribed to the consumption of hydroxyl moieties by reactions such as oxidation and dehydration, whereas the increasing polyol acid numbers can be ex-
plained by the formation of organic acids from the decomposition of lignocellulosic biomass and/or the oxidation of polyhydric solvents.[23, 48, 49] The viscosities of polyols usually decrease as the liquefaction process proceeds.[22, 25, 38] In summary, for most lignocellulosic biomass liquefaction processes that use conventional pure polyhydric alcohols as liquefaction solvents, the hydroxyl numbers and viscosities of polyols decrease, whereas their acid numbers increase, as the extent of liquefaction increases.[22, 23] However, when crude glycerol was used as a liquefaction solvent, the acid numbers and viscosities of polyols decreased and increased, respectively, as the liquefaction temperature increased from 120 to 240 8C.[25] This contradictory observation was explained by the dominance of condensation reactions that occurred among crude glycerol components such as glycerol and fatty acids during liquefaction.[25] When liquefaction was conducted at 240 8C for prolonged reaction times, biomass liquefaction became dominant and the changes of polyol properties (i.e., hydroxyl and acid numbers, viscosity) with liquefaction time showed trends similar to those observed in conventional polyhydric alcohol-based liquefaction processes.[25] The properties of polyols produced from the liquefaction of a large number of ligncellulosic biomass materials, including woody and herbaceous plant biomass, have been reported, and a summary of them is shown in Table 1. Depending on the specific liquefaction parameters and biomass type, biomass liquefaction-derived polyols showed hydroxyl numbers ranging from approximately 100 to 600 mgKOH g 1, acid numbers from 0 to 40 mgKOH g 1, viscosities from 0.3 to 45 Pa s, and Mw from 250 to over 7000 g mol 1. Generally, biomass liquefaction-derived polyols are suitable for the production of rigid or semirigid PU foams, but their uses in the preparation of PU adhesives[29, 30] and films[31–33] have also been reported.
5. PUs Produced from Biomass LiquefactionDerived Polyols Biomass liquefaction-derived polyols are most commonly used to produce rigid or semirigid PU foams. Acid-catalyzed biomass
Table 1. General properties of polyols produced from the liquefaction of lignocellulosic biomass.[a] Lignocellulosic biomass
enzymatic hydrolysis lignin cellulose or waste paper agricultural crop residues[b] industrial or bio-refinery residues[c] wood[d] birch wood[e] soybean straw[f]
Biomass conversion [%]
Hydroxyl number [mg KOH g 1]
98 55–99 60–95 84–98 80–98 > 99 65–75
249 360–396 109–430 137–586 200–435 112–204 440–540
Polyol properties Acid number [mg KOH g 1] –[g] 19–30 15–30 28–34 12–38 24–41
Polyols and Polyurethanes from the Liquefaction of Lignocellulosic Biomass Shengjun Hu, Xiaolan Luo, and Yebo Li*[a] Polyurethanes (PUs), produced from the condensation polymerizations between polyols and isocyanates, are one of the most versatile polymer families. Currently, both polyols and isocyanates are largely petroleum derived. Recently, there have been extensive research interests in developing bio-based polyols and PUs from renewable resources. As the world’s most abundant renewable biomass, lignocellulosic biomass is rich in hydroxyl groups and has potential as a feedstock to produce bio-based polyols and PUs. Lignocellulosic biomass
can be converted to liquid polyols for PU applications through acid- or base-catalyzed atmospheric liquefaction processes using polyhydric alcohols as liquefaction solvents. Biomass liquefaction-derived polyols can be used to prepare various PU products, such as foams, films and adhesives. The properties of biomass liquefaction-derived polyols and PUs depend on various factors, such as feedstock characteristics, liquefaction conditions, and PU formulations.
1. Introduction In general, polyols refer to compounds that contain two or more hydroxyl groups in one molecule. Depending on their molecular weights (Mw), polyols can be divided into monomeric and polymeric polyols. Monomeric polyols, such as glycerol and ethylene glycol (EG), are useful starting materials for the production of a broad spectrum of valuable chemicals through chemical reactions such as esterification,[1–3] dehydration,[4–6] and hydrogenation.[7–9] Polymeric polyols, which mainly consist of polyether and polyester polyols, are almost exclusively used to produce polyurethanes (PUs) through their condensation polymerization reactions with isocyanates (Scheme 1). To meet
Scheme 1. Polymerization between polyol and isocyanate to form PU.
different PU application needs, polyols with a wide range of chemical structures and properties have been produced commercially. Polyols commonly used for PU production have functionalities varying from 2 to 8 and Mw from 200 to 8000 g mol 1.[10] This broad range of polyol properties allows tunable design of PU structures, making them one of the most versatile families of polymeric materials.[10] A large number of PU products, such as foams, coatings, elastomers, and adhe[a] Dr. S. Hu, Dr. X. Luo, Prof. Y. Li Department of Food, Agricultural and Biological Engineering The Ohio State University/Ohio Agricultural Research and Development Center 1680 Madison Ave, Wooster, OH, 44691-4096 (USA) Fax: (+ 1) 330-263-3670 E-mail: [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
sives, have been commercially produced and used in many aspects of our daily lives. For example, flexible PU foams are widely used as cushion materials in applications such as car seats, mattresses, and upholstery, whereas rigid PU foams are common thermal insulation materials used by the refrigerator and construction industries. Currently, commercial polyols and PUs are mainly produced from petrochemical derivatives. Recently, concerns over the depletion of fossil resources have spurred extensive interest in developing bio-based polyols and PUs from renewable resources.[11] Vegetable oils and lignocellulosic biomass are two major types of biomass that have been extensively studied for their uses in producing bio-based polyols and PUs. Epoxidation/oxirane-ring opening, ozonolysis, hydroformylation, and transesterifcation/amidation are four major techniques used to convert vegetable oils to polyols.[11–13] Depending on the conversion techniques and vegetable oils used, vegetable oil-derived polyols exhibit different structures and material properties that are suitable for different applications. In recent years, several reviews on this topic have been published.[11–13] As another major feedstock studied for the production of bio-based polyols, lignocellulosic biomass materials, such as wood and agricultural crop residues, are considered to be the world’s most abundant renewable biomass. The main structural units of lignocellulosic biomass are cellulose (30–35 %), hemicellulose (15–35 %), and lignin (20–35 %),[14] all of which are highly functionalized materials rich in hydroxyl groups, making them promising feedstocks to produce bio-based polyols. However, lignocellulosic biomass materials are solid and need to be converted into liquid polyols before being used to produce PUs. Currently, two major technologies exist for this conversion: oxypropylation and liquefaction. The oxypropylation of biomass is a polymerization process that grafts oligo (propylene oxide) or propylene oxide homopolymer onto biomass ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS macromolecular structures.[15] Usually, this process is conducted under high pressure and high temperature (ca. 100–200 8C) in the presence of KOH as a catalyst, during which biomass is first activated and functionalized, followed by oxypropylation reactions with propylene oxide.[16, 17] Oxypropylation-derived polyols are generally mixtures of different compounds, including oxypropylated biomass, poly(propylene oxide), and some unreacted or partially oxypropylated biomass, which can be used directly for the preparation of PUs.[17] The production of polyols through the oxypropylation of various lignocellulosic biomass, such as sugar beet pulp,[18] cork,[15, 16] and lignin,[19–21] has been reported. Recently, a review on this topic was published.[17] Alternatively, lignocellulosic biomass can be converted into liquid polyols through an atmospheric liquefaction process. The liquefaction process is usually conducted at elevated temperatures (150–250 8C) under atmospheric pressure using polyhydric alcohols, such as polyethylene glycol (PEG) and glycerol, as liquefaction solvents.[22–26] The liquefaction can be either acid- or base-catalyzed with the former being more common. During the liquefaction process, biomass is degraded and decomposed into smaller molecules by polyhydric alcohols through solvolytic reactions. The polyols produced are largely a mixture of different compounds rich in hydroxyl groups and can be used directly to prepare various PUs such as foams,[25–28] adhesives,[29, 30] and films.[31–33] A large number of lignocellulosic biomass materials, such as wood[27, 34–37] and agricultural residues,[22, 23, 25, 26, 38–44] lignin,[45] and industrial and biorefinery byproducts,[24, 43, 46, 47] have been liquefied for the production of polyols and PUs. Though reviews on vegetable oiland oxypropylation-derived polyols and PUs were published previously, to the best of our knowledge, no reviews have been devoted to the discussion of polyols and PUs derived from the liquefaction of lignocellulosic biomass. This Review will focus on this topic, including the biomass liquefaction mechanism, factors that affect the efficiency of the liquefaction process, and general properties of biomass liquefaction-de-
Yebo Li, born in 1967 in Zibo, China, obtained his Ph.D. degree from China Agricultural University in 1993. He joined The Ohio State University (OSU) as assistant professor in 2007. He is currently an associate professor in the Department of Food, Agricultural, and Biological Engineering Department. He also holds an adjunct faculty position in OSU’s Department of Chemical and Biomolecular Engineering. His research focuses on the development of advanced technologies for the production of bioenergy and bioproducts from renewable sources. In 2011, he received OSU’s early career innovator of the year award and 2012 he was awarded American Society of Agricultural and Biological Engineering’s Rain Bird Engineering Concept of the Year Award.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org rived polyols and PUs. Finally, a summary and outlook of biomass liquefaction-derived polyols and PUs is provided.
2. Mechanism of Lignocellulosic Biomass Liquefaction by Polyhydric Alcohols The liquefaction of lignocellulosic biomass by polyhydric alcohols proceeds mainly through solvolytic reactions that cleave various chemical bonds (e.g., glycoside bonds) existing in biomass structures and produce smaller biomass-derived molecules or fragments.[48, 49] These small biomass derivatives are fairly reactive and can further react with either themselves or the liquefaction solvent, forming a large number of biomass and/or liquefaction solvent-derived compounds. Because of their different structures and morphologies, biomass components are usually liquefied at different stages of the liquefaction process. Usually, the liquefaction of hemicellulose, lignin, and amorphous cellulose occurs rapidly during the early stages of the liquefaction process because they have amorphous structures that are easily accessible to liquefaction solvents. In contrast, the liquefaction of crystalline cellulose is typically slower and continues until the later stages of the liquefaction process because it has a well-packed structure that is less accessible to the solvents.[22, 34, 44, 47] For this reason, cellulose liquefaction is commonly considered to be the rate-limiting step in the biomass liquefaction process.[50, 51] Previous research has suggested that the acid-catalyzed liquefaction of cellulose by different liquefaction solvents, including EG, PEG, and ethylene carbonate (EC), proceed through similar reaction pathways (Scheme 2).[48, 49] The cellulose is first decomposed by solvolytic reactions into glucose or other small
Scheme 2. Schematic representation of the reaction mechanism of acid-catalyzed liquefaction of cellulose in polyhydric alcohols.[48, 49]
cellulose derivatives that can react with the liquefaction solvent to form glycoside derivatives. Then, the produced glycoside derivatives can undergo further reactions to form levulinic acid and/or levulinates. The formation of glycoside and levulinic derivatives in an acid-catalyzed cellulose liquefaction process has also been confirmed by other studies.[50, 52] In a later effort, Zhang[53] conducted a qualitative analysis of the products formed during the acid-catalyzed liquefaction of bagasse in EG. The obtained liquefaction products were fractionated into ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS water-soluble, acetone-soluble, and insoluble residue fractions. Fractional analyses showed that the insoluble residue fraction mainly consisted of insoluble derivatives from cellulose and lignin; the acetone-soluble fraction was rich in high Mw derivatives from lignin; and the water-soluble fraction was mainly a mixture of EG, EG derivatives, diethylene glycol (DEG), saccharides, alcohols, aldehydes, ketones, phenols, and organic acids.[53] The presence of such a wide spectrum of different compounds in biomass liquefaction-derived polyols indicates the complex nature of the biomass liquefaction process: a large number of chemical reactions occur and compete against each other simultaneously. The reaction mechanism shown in Scheme 2 represents one of the major liquefaction reactions occurring during the acid-catalyzed liquefaction of cellulose. Although there have been studies on the mechanism of lignin liquefaction by phenol,[54–58] the mechanism/reaction pathway of lignin liquefaction by polyhydric alcohols has not been reported. Recondensation reactions among biomass derivatives and/or liquefaction solvents occur and compete against liquefaction reactions during the biomass liquefaction process. When dominant, these recondensation reactions can decrease the liquefaction efficiency by increasing the percent insoluble residues in biomass liquefaction-derived polyols. In fact, this phenomenon has been widely observed in acid-catalyzed biomass liquefaction processes.[22, 26, 34, 35] The negative effects of these recondensation reactions on biomass liquefaction can be largely mitigated or avoided by optimization of liquefaction parameters, such as liquefaction temperature and time, catalyst loading, and solvent-to-biomass weight ratio. The mechanism of recondensation reactions in acid-catalyzed biomass liquefaction processes was studied previously by liquefying cellulose, steamed lignin, alkali lignin, and their mixtures under the same liquefaction conditions.[50] No recondensation reactions occurred when cellulose or lignin was liquefied alone, even at a prolonged liquefaction time of 480 min. In contrast, when cellulose and lignin were liquefied together for 480 min, significant recondensations were observed, and percent insoluble residues reached 50 and 76 % for cellulose/steamed lignin and cellulose/alkali lignin mixtures, respectively.[50] Based on these observations, the authors suggested that recondensation reactions that occurred during acid-catalyzed biomass liquefaction processes were largely caused by reactions among depolymerized cellulose and degraded aromatic derivatives from lignin and/or by the nucleophilic displacement reaction of cellulose by phenoxide ion.[50]
3. Factors Affecting the Efficiency of the Liquefaction of Lignocellulosic Biomass 3.1. Lignocellulosic biomass The efficiency of biomass liquefaction varies with the compositions, structures, and morphologies of lignocellulosic biomass materials. A study on the liquefaction of seven different species of wood (hardwood and softwood) suggested that, although three hardwood species shared similar liquefaction be 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org haviors, significantly different liquefaction behaviors were observed for four softwood species.[35] Compared to hardwood, softwood exhibited faster liquefaction rates but earlier occurrences of unfavorable recondensation reactions. This was explained by the existence of large amounts of guaiacyl propane units in softwood that are more reactive than the syringyl propane units existing in hardwood.[35] Lee et al.[47] studied the liquefaction of three types of waste paper (i.e., box paper, newspaper, and business paper) and wood. The order of liquefaction rates, from highest to lowest, was found to be: wood, newspaper, business paper, and then box paper. The higher liquefaction rates of wood and newspaper were explained by their higher contents of lignin and hemicellulose, which can be liquefied rapidly at the early stages of the process.[47] Different liquefaction behaviors of agricultural crop residues have been observed through side by side comparisons, including cotton stalks and bagasse,[38] and corn stover, rice straw, and wheat straw.[40] More recently, Briones et al.[43] investigated the liquefaction behaviors of five types of agro-industrial residues (i.e., rapeseed cake, date seeds, olive stone, corncob, and apple pomace) and obtained liquefaction yields ranging from 84 to 97 %. 3.2. Liquefaction solvent Liquefaction solvents play paramount roles in the biomass liquefaction process. To produce polyols with suitable properties for PU applications, liquefaction solvents need not only to promote rapid and effective biomass liquefaction but also to possess suitable polyol properties for desired PU applications. The latter requirement is because most liquefaction processes are conducted at large liquefaction solvent-to-biomass weight ratios (approximately 3:1 to 5:1) to achieve high liquefaction efficiency. After liquefaction, the liquefaction solvent or its derivatives constitute a major portion of the biomass liquefaction-derived polyols and have significant effects on polyol properties. Thus, the choice of liquefaction solvent depends on both its liquefaction capability and its polyol properties (i.e., polyhydric alcohols, as liquefaction solvents, are also polyols). For example, a binary mixture of PEG400 (Mw: 400 g mol 1) and glycerol is most commonly used as a liquefaction solvent to produce polyols with properties suitable for rigid or semirigid PU foam applications.[22, 23, 28, 47] In contrast, PEG4000 (Mw: 4000 g mol 1) was used as a sole liquefaction solvent to produce polyols suitable for highly resilient PU foam applications.[59] Various polyhydric alcohols such as PEG, EG, glycerol, and EC have been used for biomass liquefaction.[27, 34, 36] The PEG400to-glycerol weight ratio at 4:1 showed high liquefaction efficiency and ability to dampen detrimental recondensation reactions.[22, 23, 26, 27, 34, 47] However, since lignocellulosic biomass materials are structurally heterogeneous, a 4:1 mixture of PEG400/ glycerol is not optimal for all types of biomass. Indeed, studies have suggested different optimal PEG400/glycerol weight ratios for different biomass materials, such as 9:1 for some wood species, bagasse, and cotton stalks[35, 38] and 2:3 for acidhydrolyzed residues of corncobs.[44] ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS Cyclic carbonates, such as EC and propylene carbonate (PC), are capable of rapidly liquefying cellulose and hardwood. Almost-complete liquefaction (i.e., percent solid residues less than 2 %) was achieved within 40 min at 150 8C for EC and PC, whereas incomplete liquefaction was observed even after 120 min for PEG400/glycerol (w/w: 4:1).[36] The high liquefaction efficiency of cyclic carbonates was explained by their high permittivity that leads to the high acid potential of liquefaction solvents.[36] However, in a later study, Wang et al.[60] liquefied corn stover (CS) using EC at different CS-to-EC weight ratios (0.2 to 0.4), and higher percent solid residues ranging from 2.0 to 11.6 % were observed after 90 min of liquefaction at 170 8C. These different observations might be caused by the different compositions and morphologies of biomass feedstocks used in the liquefaction processes. The liquefaction of lignocellulosic biomass usually requires large solvent-to-biomass weight ratios (i.e., 3:1 to 5:1) to obtain high liquefaction efficiency and minimize the detrimental effects of recondensation reactions.[27, 34, 38] For wood, high liquefaction efficiency (i.e., percent biomass residue less than 10 %) can be obtained at a solvent-to-biomass weight ratio as low as 3:1, whereas agricultural crop residues generally need a minimum solvent-to-biomass weight ratio of 5:1 to obtain comparable liquefaction efficiency.[23, 27, 34, 38] Yao et al.[34] studied the acid-catalyzed liquefaction of birch wood by PEG400/glycerol at different solvent/wood ratios (w/w: 3:1 to 11:9), and significant decreases in liquefaction efficiency were observed for ratios below 3:2. In a later study, Lee et al.[23] studied the liquefaction of corn bran under similar reaction conditions at solvent-to-biomass weight ratios of 5:1, 3:1, and 2:1, and decreases in biomass liquefaction efficiency were observed for ratios below 5:1. Similarly, Hassan and Shukry[38] studied the liquefaction of agricultural crop residues (i.e., bagasse and cotton stalks) and obtained high liquefaction efficiency at solvent-tobiomass weight ratios 5:1. Currently, almost all polyhydric alcohols used as liquefaction solvents are petroleum-derived and relatively expensive. To reduce the high cost and increase the renewability of the biomass liquefaction processes, Hu et al.[25] reported the use of crude glycerol, a byproduct from biodiesel production, as an alternative biomass liquefaction solvent for polyol production. Compared to conventional petroleum-derived solvents, crude glycerol is bio-based, renewable, and inexpensive, but contains impurities, such as FFAs, soap, and FAMEs.[61] For most valueadded conversion processes developed for crude glycerol so far, these impurities are unfavorable and detrimental and need to be removed beforehand. Hu et al.[25] investigated the liquefaction of soybean straw by 100 % unrefined crude glycerol under various reaction conditions. Despite the relatively low biomass liquefaction efficiency (around 70 % under optimal reaction conditions), the polyols and PU foams produced exhibited properties comparable to those obtained from conventional petrochemical solvent-based liquefaction processes. More importantly, this study also indicated that certain impurities in crude glycerol, such as FFAs, soap, and FAMEs, improved the properties of biomass liquefaction-derived polyols and PU foams as a result of their synergistic interactions with glycerol 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org and/or biomass components. These results indicated that unrefined crude glycerol can be directly used as a biomass liquefaction solvent for polyol production without expensive upgrading or refining treatments, which could potentially improve the economics and sustainability of lignocellulosic biomass liquefaction. 3.3. Catalyst The liquefaction of lignocellulosic biomass can be either acidor base-catalyzed, although the former is more commonly used. Among all acids investigated so far, concentrated (98 %) sulfuric acid has the highest catalytic ability in biomass liquefaction.[39] The effect of different sulfuric acid loadings on the liquefaction of lignocellulosic biomass has been extensively studied.[23, 38, 39, 44] Generally, significant improvements on biomass liquefaction efficiency were observed by increasing sulfuric acid loadings from 1 to 3 % (% wt., based on liquefaction solvent). For example, by increasing sulfuric acid loadings from 1 to 3 %, the percent biomass residue decreased from approximately 45 % to less than 20 % for the liquefaction of cotton stalks, wheat straw, and acid-hydrolyzed residues of corncobs.[38, 39, 44] Further increasing sulfuric acid loadings to over 3 % provided little improvement on biomass liquefaction efficiency, but increased the risk of accelerating detrimental recondensation reactions.[38, 44, 51] For most lignocellulosic biomass materials, sulfuric acid loadings of around 3–4 % provide a good balance between high liquefaction efficiency and effective retardation of detrimental recondensation reactions. Compared to the extensive studies on acid-catalyzed liquefaction processes, reports on base-catalyzed liquefaction processes are few. Generally, base-catalyzed liquefaction requires higher liquefaction temperatures (ca. 250 8C) to achieve liquefaction efficiencies comparable to those obtained from acidcatalyzed liquefaction.[62, 63] However, base-catalyzed liquefaction processes have the advantage of causing less corrosion to metal equipment used in the liquefaction process. 3.4. Liquefaction temperature and time The liquefaction temperature ranges widely from 130–250 8C, mainly depending on the types of catalysts used in liquefaction. Acid-catalyzed liquefaction is usually conducted at temperatures ranging from 150–170 8C, whereas base-catalyzed liquefaction usually needs higher temperatures of around 250 8C to achieve comparable liquefaction efficiencies. The temperature effect on the liquefaction of different types of lignocellulosic biomass, such as corn bran,[23] wheat straw,[22, 39] bagasse and cotton stalks,[38] corn stalk,[26] rapeseed cake residue,[46] and acid-hydrolyzed residues of corncobs,[44] has been evaluated. Among these studies, the liquefaction processes were all acidcatalyzed and conducted at temperatures ranging from 130 to 170 8C using binary mixtures of PEG 400/glycerol as liquefaction solvents. Significant improvements on biomass liquefaction efficiency were observed as temperatures increased from 130 to 150 8C, beyond which little improvement was observed. The liquefaction of lignocellulosic biomass usually featured ChemSusChem 2014, 7, 66 – 72
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CHEMSUSCHEM MINIREVIEWS
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a rapid liquefaction stage in the first 15–30 min, after which liquefaction proceeded at a much slower rate.[22, 23, 44] As discussed previously, the first rapid liquefaction stage was largely because of the degradation of more accessible biomass components such as lignin, hemicellulose, and amorphous cellulose, whereas the slow stage of the process mainly featured the degradation of well-packed and less solvent-accessible crystalline cellulose.[22, 44, 47] In addition, it is worth mentioning that recondensation reactions among derivatives from biomass and/or liquefaction solvents always accompany and compete against the biomass liquefaction reactions. Under high liquefaction temperature and prolonged liquefaction time, these reactions may become significant enough to decrease biomass liquefaction efficiency. For most lignocellulosic biomass materials, acid-catalyzed liquefaction at 150 8C for 90 min provided high biomass liquefaction efficiency (> 90 % biomass conversion) without significant occurrence of detrimental recondensation reactions.[22, 23, 26, 34, 38, 39, 47] For base-catalyzed liquefaction processes, comparably high liquefaction efficiencies have been obtained at 240–250 8C for a liquefaction time of 60 min.[62, 63]
4. Biomass Liquefaction-Derived Polyols To assess their suitability in PU applications, polyols produced from the liquefaction of lignocellulosic biomass are typically characterized in terms of hydroxyl number, acid number, viscosity, and Mw, all of which change dynamically during biomass liquefaction. For example, the Mw of polyols usually increase at the early stages of the liquefaction process, as a result of the release of a large amount of macromolecules from biomass.[26, 50] As these macromolecules are gradually degraded, the Mw of polyols decrease accordingly but finally may increase again if significant recondensation reactions occur at the late stages of the process.[26, 50] The hydroxyl numbers of polyols decrease, whereas the acid numbers of polyols increase, with the progression of biomass liquefaction. The decreasing polyol hydroxyl numbers can be ascribed to the consumption of hydroxyl moieties by reactions such as oxidation and dehydration, whereas the increasing polyol acid numbers can be ex-
plained by the formation of organic acids from the decomposition of lignocellulosic biomass and/or the oxidation of polyhydric solvents.[23, 48, 49] The viscosities of polyols usually decrease as the liquefaction process proceeds.[22, 25, 38] In summary, for most lignocellulosic biomass liquefaction processes that use conventional pure polyhydric alcohols as liquefaction solvents, the hydroxyl numbers and viscosities of polyols decrease, whereas their acid numbers increase, as the extent of liquefaction increases.[22, 23] However, when crude glycerol was used as a liquefaction solvent, the acid numbers and viscosities of polyols decreased and increased, respectively, as the liquefaction temperature increased from 120 to 240 8C.[25] This contradictory observation was explained by the dominance of condensation reactions that occurred among crude glycerol components such as glycerol and fatty acids during liquefaction.[25] When liquefaction was conducted at 240 8C for prolonged reaction times, biomass liquefaction became dominant and the changes of polyol properties (i.e., hydroxyl and acid numbers, viscosity) with liquefaction time showed trends similar to those observed in conventional polyhydric alcohol-based liquefaction processes.[25] The properties of polyols produced from the liquefaction of a large number of ligncellulosic biomass materials, including woody and herbaceous plant biomass, have been reported, and a summary of them is shown in Table 1. Depending on the specific liquefaction parameters and biomass type, biomass liquefaction-derived polyols showed hydroxyl numbers ranging from approximately 100 to 600 mgKOH g 1, acid numbers from 0 to 40 mgKOH g 1, viscosities from 0.3 to 45 Pa s, and Mw from 250 to over 7000 g mol 1. Generally, biomass liquefaction-derived polyols are suitable for the production of rigid or semirigid PU foams, but their uses in the preparation of PU adhesives[29, 30] and films[31–33] have also been reported.
5. PUs Produced from Biomass LiquefactionDerived Polyols Biomass liquefaction-derived polyols are most commonly used to produce rigid or semirigid PU foams. Acid-catalyzed biomass
Table 1. General properties of polyols produced from the liquefaction of lignocellulosic biomass.[a] Lignocellulosic biomass
enzymatic hydrolysis lignin cellulose or waste paper agricultural crop residues[b] industrial or bio-refinery residues[c] wood[d] birch wood[e] soybean straw[f]
Biomass conversion [%]
Hydroxyl number [mg KOH g 1]
98 55–99 60–95 84–98 80–98 > 99 65–75
249 360–396 109–430 137–586 200–435 112–204 440–540
Polyol properties Acid number [mg KOH g 1] –[g] 19–30 15–30 28–34 12–38 24–41
