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For better understanding of the roles of cellulose reducing ends during thermal degradation of cellulose and wood, cellulose samples impregnated with methyl-β-D-glucopyranoside (GlcβOMe), a simple non-reducing sugar model, were pyrolyzed under N2 at relatively low temperatures of 200-280 °C. By the impregnation, cellulose was rather stabilized against discoloration and weight-loss through converting the reducing ends into the glycosides with GlcβOMe. Alternatively, polymerization and discoloration of GlcβOMe were accelerated in the presence of cellulose. A mechanism via reducing sugars as reactive intermediates formed through hydrolysis is proposed to explain these phenomena. These information would be useful to understand the interactions between cellulose and hemicellulose in wood cell wall as well as the role of the reducing ends in cellulose thermal degradation., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The gas-phase pyrolysis of levoglucosan (LG), the major intermediate species during cellulose gasification, was studied experimentally over the temperature range of 400-900 °C. Gaseous LG did not produce any dehydration products, which include coke, furans, and aromatic substances, although these are characteristic products of the pyrolysis of molten LG. Alternatively, at >500 °C, gaseous LG produced only fragmentation products, such as noncondensable gases and condensable C1 -C3 fragments, as intermediates during noncondensable gas formation. Therefore, it was determined that secondary reactions of gaseous LG can result in the clean (tar- and coke-free) gasification of cellulose. Cooling of the remaining LG in the gas phase caused coke formation by the transition of the LG to the molten state. The molecular mechanisms that govern the gas- and molten-phase reactions of LG are discussed in terms of the acid catalyst effect of intermolecular hydrogen bonding to promote the molten-phase dehydration reactions., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Non-catalytic liquefaction of Japanese beech (Fagus crenata) wood in subcritical phenol was investigated using a batch-type reaction vessel. After samples were treated at 160 °C/0.9 MPa-350 °C/4.2 MPa for 3-30 min, they were fractionated into a phenol-soluble portion and phenol-insoluble residues. These residues were then analyzed for their chemical composition. Based on the obtained data, the kinetics for liquefaction was modeled using first-order reaction rate law. Subsequently, the liquefaction rate constants of the major cell wall components including cellulose, hemicellulose, and lignin were determined. The different kinetic mechanisms were found to exist for lignin and cellulose at two different temperature ranges, lower 160-290 °C and higher 310-350 °C, whereas for hemicellulose, it was only liquefied in the lower temperature range. Thus, the liquefaction behaviors of these major cell wall components highlighted hemicellulose to be the most susceptible to liquefaction, followed by lignin and cellulose., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Thermal glycosylation and degradation reactions of cellulose (Avicel PH-101) were studied in the presence or absence of alcohols (glycerol, mannitol, 1,2,6-hexanetriol, 3-phenoxy-1,2-propanediol, and 1-tetradecanol) under N(2) at 60-280°C. In the presence of glycerol (heating time, 10 min), the reducing ends were converted into glycosides when the temperature of the glycerol was >140°C without the addition of any catalysts. A temperature of 140°C is close to that required for the initiation of thermal polymerization (glycosylation). Although the conversion was only around 20% in the range of 140-180°C, the reactivity increased above 200-240°C where the thermal expansion of cellulose crystals is reported to become significant. Finally, all reducing ends were converted into glycosides at 260°C. Such heterogeneous reactivity likely arose from the lower reactivities of the reducing ends in the crystalline region due to their lower accessibility to glycerol, although the reactivity in the non-crystalline region was similar to that of glucose. Alcohols that have a lower OH/C ratio did not react with the reducing ends, suggesting that the hydrophilicity of the alcohol was critical for the glycosylation reaction to proceed. The glycosylated cellulose samples were found to be significantly stabilized against pyrolytic coloration. The results of neat cellulose pyrolysis indicated that two competitive reactions, thermal glycosylation and degradation, formed a dark-colored substance at the reducing ends while the internal glucose units in the cellulose were comparatively stable., (2010 Elsevier Ltd. All rights reserved.)

Boric acid inhibited the acid-catalyzed depolymerization of cellulose in sulfolane, a non-aqueous medium, at high temperature. Formation of the dehydration products such as levoglucosenone, furfural, and 5-hydroxymethyl furfural were also effectively inhibited. Similar inhibition was observed for cellooligosaccharides and starch, although the glucosidic bonds in methyl glucopyranosides and methyl cellobioside were cleaved to form alpha-d-glucofuranose cyclic 1,2:3,5-bisborate.

Solid-state hydrolysis proceeded with cellulose and methyl alpha- and beta-D-glucopyranosides in the presence of hydrated magnesium chloride. This reaction was effective even at >100 degrees C since the hydrated water, which is held by MgCl(2) up to >200 degrees C, is utilized as a nucleophile. Excess water made this reaction ineffective due to the competition between water and sugar oxygen atoms in coordinating with Mg(2+), a Lewis acid. Consequently, this hydrolysis reaction is characteristic of solid-state reactions.

The water-soluble portion and precipitates obtained by supercritical (SC) water treatment of microcrystalline cellulose (Avicel) were enzymatically hydrolyzed. Glucose could be produced easily from both substrates, compared with the Avicel. Therefore, SC water treatment was found to be effective for enhancing the productivity of glucose from cellulose by the enzymatic hydrolysis. It is also found that alkaline treatment or wood charcoal treatment reduced inhibitory effects by various decomposed compounds of cellulose on the enzymatic hydrolysis to achieve higher glucose yields. Furthermore, glucose obtained by SC water treatment followed by the enzymatic hydrolysis of cellulose could be converted to ethanol by fermentation without any inhibition.

Japanese beech (Fagus crenata Blume), its cell wall components, and model compounds were treated by supercritical water (380 degrees C, 100 MPa) for 5 s using a batch-type reactor to investigate the production behavior of low molecular weight organic acids. It was found that cellulose and hemicellulose were decomposed to formic acid, pyruvic acid, glycolic acid, acetic acid, and lactic acid, whereas lignin was barely decomposed to such organic acids under the given conditions. However, after prolonged treatment (380 degrees C, 100 MPa, 4 min) of lignin, some organic acids were recovered owing perhaps to the decomposition of the propyl side chain of lignin. It was additionally revealed that the predominant organic acid recovered was acetic acid, which might be derived from the acetyl group of hemicellulose in Japanese beech.

The water-soluble (WS) portion obtained by supercritical water treatment of lignocellulosics was studied for its fermentability to ethanol. A fermentation test of the WS portion showed it was not fermented to ethanol. Therefore, a wood charcoal treatment was applied to the WS portion to remove furan and phenolic compounds, which are thought to be the inhibitors to sugar fermentability. It was found that treatment with wood charcoal can be effective at removing these inhibitors and improving the fermentability of the WS portion without reducing the levels of fermentable sugars.

Abstract: In sulfolane (tetramethylene sulfone), which is a good solvent for the primary product, levoglucosan, cellulose is pyrolyzed completely into soluble products without forming any char. Residues during pyrolysis in sulfolane at 200, 240 and 330°C were obtained always as colorless non-carbonized substances. From the change in the crystallinity and crystallite size as compared with the ordinary pyrolysis, a heterogeneous mechanism is indicated for cellulose pyrolysis, starting from a molecule which is less stabilized due to lack of some intermolecular interactions. [Copyright &y& Elsevier]

Abstract: The influence of alkali metal chlorides (NaCl, KCl) and alkaline earth metal chlorides (MgCl2, CaCl2) on cellulose pyrolysis was studied with thermal analysis and isothermal pyrolysis in N2 at 150–400°C. Alkali and alkaline earth metal chlorides affected the cellulose pyrolysis in different way. Although alkali metal chlorides did not change the weight loss temperature of bulk cellulose (main part of cellulose) so much, alkaline earth metal chlorides substantially reduced the temperature. Furthermore, the influence of alkaline earth metal chlorides on the weight loss behavior (<400°C) was very dependent on the amount of loading bellow 0.30mol/mol of the glucose-unit of cellulose, while the influences of the alkali metal chlorides were almost independent of the amount. Both metal chlorides significantly changes the low MW product composition even at a very low level of addition. [Copyright &y& Elsevier]

Abstract: Cellulose–hemicellulose and cellulose–lignin interactions during pyrolysis at gasification temperature (800°C) were investigated with various cellulose samples mixed with hemicellulose (glucomannan or xylan) or milled wood lignin. Significant interactions were observed in cellulose–lignin pyrolysis; lignin inhibited the thermal polymerization of levoglucosan formed from cellulose and enhanced the formation of the low molecular weight products from cellulose with reduced yield of char fraction; cellulose reduced the secondary char formation from lignin and enhanced the formation of some lignin-derived products including guaiacol, 4-methyl-guaiacol and 4-vinyl-guaiacol. Comparatively weak interactions were also observed in cellulose–hemicellulose pyrolysis. Finally, factors influencing the wood pyrolysis at gasification temperature are discussed. [Copyright &y& Elsevier]

Abstract: Pyrolysis behavior of wood at gasification temperature (800°C) was investigated focusing on the behaviors of the wood constituent polymers [cellulose, hemicellulose (glucomannan and xylan) and lignin (milled wood lignin)]. Tar compositions (iso-propanol-soluble and water-soluble tar fractions), which were characterized with GPC, GC–MS, GC-FID (oxime-TMS analysis), capillary electrophoresis and 1H NMR analysis, were quite different between wood polysaccharides and lignin. Furthermore, comparison of the tar- and secondary char-forming behaviors indicated that comparatively stable primary tar from wood polysaccharides undergo secondary reactions including carbonization after condensation at the reactor wall with lower temperature than their boiling points, while that primary tar from lignin is more reactive to give the vapor phase carbonization products during volatilization. [Copyright &y& Elsevier]

Abstract: Oxime-trimethylsilylation (TMS) method was applied to the analysis of wood pyrolysate. Quantitative determination of hydroxycarbonyls such as glycolaldehyde, which are important pyrolysis products of wood polysaccharide, is difficult as indicated by the gas chromatography–mass spectrometry (GC–MS) and 1H NMR analysis of glycolaldehyde. Glycolaldehyde decomposed into formic acid and the unknown compound with molecular weight of 72 at the injector in its GC analysis. 1H NMR analysis of glycolaldehyde in acetone-d 6 indicated the complex mixture with its dimerization products. Glycolaldehyde was quantified precisely as oxime-TMS derivative (E/Z-mixture) of the monomer by GC after oximation with hydroxylamine hydrochloride and the following trimethylsilylation with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA). With this method, the other carbonyls such as furfural, 5-hydroxymethylfurfural and hydroxyacetone could also be determined as their oxime-trimethylsilylated derivatives. Furthermore, anhydrosugars such as levoglucosan and levomannosan in wood pyrolysate were also determined, simultaneously, as their TMS derivatives. Finally, oxime-TMS method is proposed as a quantification method of the pyrolysis products derived from wood polysaccharide. [Copyright &y& Elsevier]

Pyrolysis of cellulose in sulfolane, an aprotic polar solvent, was conducted at the temperature between 200 and 330 °C. Sulfolane was used as a good solvent for levoglucosan, the major anhydromonosaccharide formed from cellulose pyrolysis, for prevention of the polymerization reaction. Cellulose was observed completely decomposed into soluble products in sulfolane within 3, 10, 60 and 480 min at 330, 280, 240 and 200 °C, respectively. The soluble products had molecular weights less than 500 after acetylation (GPC analysis) and similar product composition to that from cellulose pyrolysis under nitrogen (levoglucosan, levoglucosenone, furfural and 5-hydroxymethylfurfural by HPLC analysis). Pyrolysis of cellulose in polar solvent, which can solubilize anhydromonosaccharides, is proposed as a method for selective formation of levoglucosan and other low molecular-weight (MW) substances. As well, the cellulose pyrolysis in sulfolane did not suffer from carbonization reactions (microscopic and IR spectroscopic analysis) as did cellulose pyrolysis under nitrogen or in dioctyl phthalate (a poor solvent for levoglucosan) which gave brown/black solids. The residues obtained from the pyrolysis in sulfolane were colorless and gave similar IR spectra to that of the original cellulose. Based on these results, a ‘surface-peeling mechanism’ is proposed, and the role of the solvent in the mechanism is discussed. [Copyright &y& Elsevier]