Raman Spectra of Delignified Plant Fibers: Exploring the Impact of Xylan’s Presence on the Spectral Features of Cellulose

Abstract

Wood and plants are made of fibers that contain, in addition to cellulose, lignin and hemicelluloses. Xylan and galactoglucomannan are the dominant secondary cell wall hemicelluloses. In modern times, fibers are important materials for the biorefinery industry and for developing biocomposites. For these and other applications, the structural analysis of fibers is important, and Raman spectroscopy is among the many analytical techniques used. However, given the structural similarity between hemicelluloses and cellulose, many of their Raman contributions overlap, and the extent to which the overlapping features of hemicellulose modify the spectrum of cellulose is not yet fully understood. The present investigation focuses on this aspect by examining xylan, one of the hemicelluloses. As a model system, samples with various mass ratios of cotton microcrystalline cellulose (MCC) and xylan (birch wood) were prepared and analyzed using FT-Raman spectroscopy. In most cases, the Raman intensities were sample-composition-dependent, and, when the selected band intensities were plotted against the xylan content, good linear correlations (with an R2 between 0.69 and 1.0) were obtained. The results indicated that with increased xylan content, the peak intensities increased at 1460, 898, and 494 cm−1 and declined at 1480, 1121, 1096, and 520 cm−1. Additionally, intensity changes (%) in the MCC bands with respect to MCC’s fractions in various mixture samples showed that, in most cases, the mixture intensities increased and were highly correlated with the xylan amounts in the mixtures (with an R2 between 0.75 and 0.97). These findings were applied to interpret Raman spectra of selected xylan-containing delignified plant fibers. It is hoped that the insights gained in this study will allow for better interpretation of the spectra of natural and treated plant materials.