Furan and benzene ring formation in cellulose char: the roles of 5-HMF and reducing ends.

The authors previously proposed that 5-hydroxymethylfurfural (5-HMF) can be produced from the reducing ends of cellulose as a key intermediate during carbonization. The present work investigated the mechanisms by which furan and benzene rings are formed in cellulose char based on carbonization at 280 °C using 13C-labeled 5-HMF together with 13C-labeled glucose (as a model for the cellulose reducing ends). Glycerol was added to the 5-HMF to prevent the formation of stable glassy polymers. The resulting char was subjected to pyrolysis gas chromatography/mass spectrometry (764 °C, 5 s hold time) and the incorporation of 13C in furan-, phenol-, benzofuran- and benzene-type fragments was assessed. The apparent formation mechanisms include a direct rearrangement of the six carbons of 5-HMF to phenols, Diels-Alder reactions of furan rings with double bonds to give benzofurans and a more random process involving reactive fragments producing benzene rings. On the basis of these results, the roles of 5-HMF and reducing ends during cellulose carbonization are discussed herein.

Microwave Effect in Hydrolysis of Levoglucosan with a Solid Acid Catalyst for Pyrolysis-Based Cellulose Saccharification.

Pyrolysis-based saccharification consisting of fast pyrolysis followed by hydrolysis of the resulting anhydrosugars such as levoglucosan is a promising method for converting cellulosic biomass into glucose that can be used for producing biofuels and biochemicals. In the present study, hydrolysis of levoglucosan was evaluated in water with a polystyrene sulfonic acid resin (a solid acid catalyst) by heating under microwave irradiation or in an oil bath at 95 °C-120 °C. When the equilibrium temperature of the solution was the same, the conversion rate of levoglucosan was greater under microwave irradiation than in an oil bath. Model experiments indicate that the sulfonyl groups of the solid acid catalyst were selectively heated by microwave irradiation. The temperature of the reaction solution in the vicinity of the catalyst was locally higher than the equilibrium temperature of the solution, which enabled hydrolysis to proceed efficiently.

Influence of Proteins on the Lignin Decomposition Behavior of Japanese Cedar (Cryptomeria japonica) Wood by Supercritical Methanol Treatment.

The effect of adding protein on the decomposition behavior of lignin in Japanese cedar under supercritical methanol conditions (270 °C/27 MPa) was studied. The Klason method was used to detect the lignin content in the insoluble residue following to a 30 min treatment. Adding either an animal (bovine serum albumin) or plant (soy) protein enhanced delignification from 50 to 65% of the lignin-based wt %. This result was attributed to enhanced lignin depolymerization owing to inhibited lignin recondensation and/or the suppressed formation of polysaccharide-derived char via reactions between the protein and polysaccharides. Although the solubilization of lignin was promoted and the yield of lignin-derived low-molecular-weight compounds increased, the selectivity of major monomers such as coniferyl alcohol (CA) and γ-methylated CA decreased. The addition of proteins has a substantial impact on the decomposition behavior of cell wall components under supercritical methanol conditions. This information provides insights into the use of protein-rich lignocelluloses.

Topochemistry of the Delignification of Japanese Beech (Fagus crenata) Wood by Supercritical Methanol Treatment.

The topochemistry of Japanese beech (Fagus crenata) wood delignification was evaluated in this study following a supercritical methanol treatment (270 °C, 27 MPa). Ultraviolet microscopic analysis of the insoluble residue revealed that the lignin in the secondary wall was easily decomposed and removed because of the preferential cleavage of ether-type linkages. In contrast, the middle lamella lignin was initially resistant to supercritical methanol but eventually decomposed and was removed. In addition, UV-absorbing secondary products formed selectively inside the parenchyma cells. Results from the supercritical methanol treatment of demineralized beech wood indicated that inorganic substances in the lumen of parenchyma affected the formation of these secondary products, thus leading to an overestimation of the residual lignin. Therefore, the topochemistry of delignification was more precisely evaluated when using demineralized beech wood.

Hydroxymethylfurfural as an Intermediate of Cellulose Carbonization.

Hydrogen bond donor solvents such as aromatic solvents inhibit the secondary degradation of cellulose-derived primary pyrolysis products. In a previous study, we found that the formation of solid carbonized products was completely inhibited during cellulose pyrolysis in aromatic solvents, with 5-hydroxymethylfurfural (5-HMF) recovered in certain yields instead. This indicated that 5-HMF is an intermediate in cellulose carbonization. To confirm this hypothesis, the thermal reactivity of 5-HMF was investigated. At 280 °C, pure 5-HMF polymerized into a hard glassy substance through OH group elimination, but further conversion was slow. When pyrolyzed in the presence of glycerol, a model of coexisting primary pyrolysis products from cellulose, a coupling reaction proceeded. Reactions characteristic of cellulose carbonization then occurred, including the formation of acidic groups and benzene-type structures in the solid products. These results confirmed the above hypothesis. The molecular mechanism of cellulose carbonization is discussed, focusing on the crystalline nature.

Hydroxymethylfurfural as an Intermediate of Cellulose Carbonization.

Invited for this month's cover is the group of Haruo Kawamoto at Kyoto University (Japan). The cover picture shows the carbonization pathway of cellulose via 5-hydroxymethylfurfural as an important intermediate. Thermochemical conversion is one of the promising technologies to convert cellulose, which is the main component of lignocellulosic biomass, into biofuels and biochemicals. Understanding the molecular mechanism of thermal degradation of cellulose is important for developing efficient conversion techniques by controlling the reaction. Read the full text of their Full Paper at 10.1002/open.202000314.

Carbonization of cellulose cell wall evaluated with ultraviolet microscopy.

This is the first study of cellulose carbonization in the interior of cell walls. Cotton cellulose was pyrolyzed under nitrogen or in aromatic solvents (benzophenone, diphenyl sulfide, and 1,3-diphenoxybenzene) at 280 °C, and cross sections of the cell walls were examined using ultraviolet (UV) microscopy. After pyrolysis under nitrogen, UV absorption caused by carbonization appeared inside the cell walls. The absorptivity of the cell interiors was homogeneous and slightly lower than that of the cell surfaces. The UV spectra had maximal absorption at ca. 250 nm. The spectra of model compounds and Py-GC/MS analysis data suggested that furanic and polycyclic aromatic structures were present in the carbonized products. The use of aromatic solvents decreased the yields of solid carbonized products and the UV absorptivity, which remained homogeneous throughout the cross sections. The mechanism of cellulose carbonization in cell walls is discussed along with the influence of aromatic solvents.

Electrocapillarity and zero-frequency differential capacitance at the interface between mercury and ionic liquids measured using the pendant drop method.

The structure of ionic liquids (ILs) at the electrochemical IL|Hg interface has been studied using the pendant drop method. From the electrocapillarity (potential dependence of interfacial tension) differential capacitance (Cd) at zero frequency (in other words, static differential capacitance or differential capacitance in equilibrium) has been evaluated. The potential dependence of zero-frequency Cd at the IL|Hg interface exhibits one or two local maxima near the potential of zero charge (Epzc), depending on the cation of the ILs. For 1-ethyl-3-methylimidazolium tetrafluoroborate, an IL with the cation having a short alkyl chain, the Cdvs. potential curve has one local maximum whereas another IL, 1-octyl-3-methylimidazolium tetrafluoroborate, with the cation having a long alkyl chain, shows two maxima. These behaviors of zero-frequency Cd agree with prediction by recent theoretical and simulation studies for the electrical double layer in ILs. At negative and positive potentials far from Epzc, the zero-frequency Cd increases for both the ILs studied. The increase in zero-frequency Cd is attributable to the densification of ionic layers in the electrical double layer.

Optical circulator for an in-line-type compact lidar.

The application of an optical circulator is demonstrated for an in-line-type lidar. The lidar's transmitter and receiver are installed in a telescope. The optical circulator of interest here can separate the transmitting laser beam and the echo lights on the same optical axis. It can also divide the echo lights simultaneously into orthogonally polarized components. An insertion loss of 2.2 dB and isolation of >60 dB for the developed optical circulator are obtained in a laser-transmitting situation. This optical circulator makes it possible to measure the polarization ratio caused by cloud phases with a narrow field of view in an in-line-type lidar operation.