Umesh P. Agarwal

FT-Raman Spectroscopy of Wood: Identifying Contributions of Lignin and Carbohydrate Polymers in the Spectrum of Black Spruce (Picea Mariana):

Good-quality Raman spectra of most wood species can now be obtained by using near-infrared Fourier transform Raman spectroscopy. To make effective use of such spectroscopic information, one needs to interpret the data in terms of contributions from various wood components and, for each component polymer, in terms of vibrational modes of its substructural units/groups. In the present work, Raman spectral features of black spruce (Picea mariana) wood were associated with lignin and/or carbohydrate polymers. Lignins spectral contributions were recognized in several ways. In addition to spectra of milled-wood and enzyme lignins, a spectrum of native lignin was obtained by subtracting the spectrum of acid chlorite delignified black spruce from the spectrum of an untreated wood sample. A comparison of lignin spectra indicated that the Raman features of the three lignins are very similar. Raman contributions of carbohydrate polymers, namely, those of cellulose and hemicellulose, were identified by using authentic and/or isolated samples and, in the case of cellulose, by using previously published spectra. Such an analysis showed that the hemicellulose present in black spruce did not give rise to any new, unique features that were not already present due to cellulose. Therefore, it was concluded that the hemicellulose contribution is broad and is hidden under the Raman contribution of cellulose. Also, peak positions of lignin contributions did not overlap with those of cellulose, and there were spectral regions where either lignin or cellulose contributed.

Raman microprobe evidence for lignin orientation in the cell walls of native woody tissue.

Raman microprobe spectra from the secondary wall of earlywood tissue from Picea mariana (black spruce) reveal evidence of the orientation of lignin relative to the plane of the cell wall. In most instances, the aromatic rings of the phenyl propane structural units are parallel to the plane of the cell-wall surface.

A comparative study of cellulose nanofibrils disintegrated via multiple processing approaches

Various cellulose nanofibrils (CNFs) created by refining and microfluidization, in combination with enzymatic or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized pretreatment were compared. The morphological properties, degree of polymerization, and crystallinity for the obtained nanofibrils, as well as physical and mechanical properties of the corresponding films were evaluated. Compared to refining, intense microfluidization contributed greater separation of nanofibril bundles, which led to an enhancement of mechanical strength and transparency for the resultant film. The selected enzymatic pre-treatments produced shortened fibers due to preferential hydrolysis of amorphous cellulose and, in combination with mechanical treatments, resulted in short and stiff cellulose nanocrystal (CNC)-like materials. Despite films from these CNC-like fibrils having inferior tensile strength, their tensile modulus and transparency were significantly improved compared to CNFs prepared without pre-treatment. The unique fiber morphology and high crystallinity potentially offer a green and ecologically friendly alternative for the preparation of CNCs and CNFs as part of an integrated biorefinery approach.

FT–Raman Investigation of Milled-Wood Lignins: Softwood, Hardwood, and Chemically Modified Black Spruce Lignins

Abstract Raman spectroscopy is being increasingly applied to study wood and other lignin-containing biomass/biomaterials. Lignins contribution to the Raman spectra of such materials needs to be understood in the context of various lignin structures, substructures, and functional groups so that lignin-specific features could be identified and the spectral information could be interpreted usefully. Additionally, to enhance the utility of Raman as a characterization tool, an understanding of chemical-treatment-induced changes to the lignin spectrum is important. In the present work, Raman spectra of four milled-wood lignins (MWLs)—black spruce, loblolly pine, aspen, and sweetgum—were compared, and using black spruce MWL, spectral changes brought about by alkaline hydrogen peroxide bleaching, hydrogenation, acetylation, and methylation reactions were analyzed. The band intensity changes depended upon the nature of the chemical treatments.

Estimation of cellulose crystallinity of lignocelluloses using near-ir ft-raman spectroscopy and comparison of the raman and segal-waxs methods

Of the recently developed univariate and multivariate near-IR FT-Raman methods for estimating cellulose crystallinity, the former method was applied to a variety of lignocelluloses: softwoods, hardwoods, wood pulps, and agricultural residues/fibers. The effect of autofluorescence on the crystallinity estimation was minimized by solvent extraction or chemical treatment or both. Additionally, when the roles of lignin and hemicellulose in the Raman crystallinity assessment were investigated, it was found that syringyl lignin containing lignocelluloses generated somewhat higher crystallinity, whereas the presence of hemicellulose reduced the crystallinity. Overall, when autofluorescence was minimized and corrections made for hemicellulose and syringyl lignin contributions, the univariate Raman method performed well and estimated cellulose crystallinity accurately. Moreover, when the Raman and Segal-WAXS methods were compared, we observed that in the absence of significant fluorescence, the Raman method was influenced mostly by hemicellulose and syringyl lignin, whereas the Segal-WAXS was affected by various types of lignin and hemicellulose. It was concluded that the near-IR FT-Raman method with corrections for influences of syringyl lignin and hemicellulose can be used to correctly estimate cellulose crystallinity.

ASSIGNMENT OF THE PHOTOYELLOWING-RELATED 1675 cm -1 RAMAN/IR BAND TO p-QUINONES AND ITS IMPLICATIONS TO THE MECHANISM OF COLOR REVERSION IN MECHANICAL PULPS

Abstract Using FT (Fourier-transform) Raman and FT infrared (IR) spectroscopies, a new band was detected at 1675 cm−1; this was most likely to have come from the yellow chromophores in photoexposed thermomechanical pulps (TMPs). On the basis of spectroscopic studies that involved both o- and p-quinone models and Fremys salt-oxidized TMP, the 1675 cm−1 band is assigned to the p-quinone functional group. Moreover, in the presence of known photoyellowing inhibitors, the photoyellowing behavior of methyl hydroquinone was similar to that of TMPs. Another important finding was that the molecular oxygen sensitivity of the laser-induced fluorescence (excited at 514.5 nm) of p-quinone and hydroquinone models was similar to what had been previously observed for yellowed and unyellowed TMPs. Taken together, these results provide strong support for a previously suggested yellowing hypothesis in which a hydroquinone/p-quinone couple was seen as an important leucochromophore/chromophore system in mechanical pulps.

Sequential Treatment of Mechanical and Chemimechanical Pulps with Light and Heat: A Raman Spectroscopic Study

Raman spectroscopy was used to study the effects of heat and light treatments on unbleached and peroxide-bleached mechanical and chemimechanical pulps. For bleached mechanical pulp, spectral changes were associated with the removal of coniferaldehyde structures in lignin. In contrast, chemimechanical pulping not only degraded coniferaldehyde units but also partially degraded coniferyl alcohol groups. Furthermore, spectral evidence supported formation of chromophores during chemimechanical pulping : bleaching removed chromophores from chemimechanical pulp. Investigation of unbleached and bleached chemimechanical pulps at 514.5 and 647.1 nm excitation wavelengths revealed a decline in intensity upon the longer wavelength excitation for certain bands, indicating the presence of residual chromophores and suggesting the presence of coniferaldehyde structures. Spectra of light-and heat-treated pulps displayed intensity changes at 1120, 1595, 1620, and 1654 cm -1 , which were found to be due to the involvement of coniferaldehyde and/or coniferyl alcohol structures in lignin. The most informative Raman band was at 1654 cm -1 . Although newly formed chemical groups/structures due to heat and light treatments could not be identified, new Raman contributions were detected in the lignin aromatic-stretch region. The effects of light or heat were compared in single and sequential treatments. In most cases. the second-stage treatment caused spectral changes that were significantly different from those resulting from direct treatment of pulp, indicating that the effect of the second stage depended on the chemical changes induced in the first stage. For unbleached mechanical and bleached chemimechanical pulp, the order of the single light and heat treatments was found to be important. The sequence of light followed by heat (light-heat) caused more decay in the intensity of the 1654 cm -1 band than did the opposite sequence (heat-light). In contrast, for bleached mechanical and unbleached chemimechanical pulp, similar changes were detected in the 1654 cm -1 band intensity upon sequential treatment. Raman information on treated pulps was correlated with the results of a previous UV-VIS reflectance study. In general, similarity of spectral changes (in the 370nm region) among various pulps and treatments did not necessarily indicate similarity between chemical changes in the pulps. The results seem to suggest significant variation at the molecular level among the responses of pulps for a given treatment and among the treatments for a given pulp.

Enzymatic hydrolysis of loblolly pine: effects of cellulose crystallinity and delignification

Abstract Hydrolysis experiments with commercial cellulases have been performed to understand the effects of cell wall crystallinity and lignin on the process. In the focus of the paper are loblolly pine wood samples, which were systematically delignified and partly ball-milled, and, for comparison, Whatman CC31 cellulose samples with different crystallinities. In pure cellulose samples, the percentage of cellulose hydrolysis was inversely proportional to the degree of crystallinity. For the loblolly pine samples, the extent of hydrolysis was low for the fraction with 74- to 149-μm particle size, but the ball-milled fraction was hydrolyzed easily. The impact of lignin removal was also influential as demonstrated on progressively delignified wood, i.e., the degree of saccharification increased with lignin removal. On the basis of data of 72 h hydrolysis time on materials with similar crystallinity, the cell wall was found to be eight times less hydrolyzable than Whatman CC31 cellulose. Taken together, cellulose crystallinity and composition are not as important as the ultrastructural changes caused by the disruption of the tightly packed regions of the cell wall that ensued upon acid chlorite delignification.

Determination of ethylenic residues in wood and TMP of spruce by FT-Raman spectroscopy

Abstract A method based on FT-Raman spectroscopy is proposed for determining in situ concentrations of ethylenic residues in softwood lignin. Raman contributions at 1133 and 1654 cm-1, representing coniferaldehyde and coniferyl alcohol structures, respectively, were used in quantifying these units in spruce wood with subsequent conversion to concentrations in lignin. For coniferaldehyde units, the intensity of the 1133 cm-1 peak was measured in the difference spectrum obtained by subtracting the bleached-wood spectrum from that of the unbleached. In the case of coniferyl alcohol residues, the intensity of the 1654 cm-1 band was calculated from the spectrum of extensively bleached wood. The concentrations of coniferaldehyde and coniferyl alcohol units in spruce lignin were found to be 3.8% and 3.4%, respectively, and were in good agreement with values determined by conventional techniques. This quantification of the ethylenic residues was based on the Raman intensities of 1% coniferaldehyde and 1% coniferyl alcohol in bleached softwood kraft pulp. Initially, as background for this work, a number of suitable lignin model compounds and a softwood lignin model polymer (G-DHP) were used to calibrate the Raman method and demonstrate that the Raman technique was well suited for quantification of ethylenic structures. Experimental results demonstrated that thermomechanical pulping reduced the concentrations of coniferaldehyde and coniferyl alcohol residues in comparison to wood by 28% and 24%, respectively.

Heterogeneity of lignin concentration in cell corner middle lamella of white birch and black spruce

SummaryRaman microprobe spectroscopy was used to study the concentration of lignocellulosics in the cell corner middle lamella. Spectra obtained from 1.6 μm regions, from 30 cell corner middle lamellae of both birch and spruce, showed the presence of lignin. However, the relative concentration of lignin to cellulose varied considerably. These results corroborate the view expressed in previous reports of the need for caution in using the lignin concentration values of cell corner middle lamella as a internal reference for studying the variation of lignin concentration in other morphological regions of the cell wall, such as secondary cell wall layers.