In: Proceedings of 1993 Pulping conference; 1993 November 1-3; Atlanta, GA. Atlanta, GA: TAPPI Press; 1993: 519-532. Book 2. FT RAMAN AND UV VISIBLE SPECTROSCOPIC In this study, NIR FT Raman spectroscopy is used in combination STUDIES OF A HIGHLY SELECTIVE with diffuse-reflectance infrared Fourier transform (DRIFT) POLYOXOMETALATE BLEACHING SYSTEM spectroscopy, transmission and reflectance UV visible spectroscopy, kappa numbers and brightness measurements to Ira A. Weinstock Umesh P. Agarwal study changes that occur in residual lignin during stages of the new Research Chemist Research Chemist kraft pulp bleaching process. The purpose of this study was James L. Minor Rajai H. Atalla threefold: (1) to compare FT Raman spectroscopy with more Research Chemist Supervisory Chemical Engineer traditional spectroscopic techniques; (2) to use FT Raman and UV visible spectroscopy to reveal chemical changes occurring in Richard S. Reiner residual lignin during bleaching; and (3) to demonstrate the Chemical Engineer effectiveness of FT Raman spectroscopy as a rapid, noninvasive USDA Forest Service USDA Forest Service technique for quantification of residual lignin. Forest Products Laboratory1 Forest Products Laboratory Madison, Wisconsin 53705 Madison, Wisconsin 53705 BACKGROUND U.S.A. U.S.A. Vibrational Spectroscopy ABSTRACT Raman and infrared techniques provide complimentary information. Vibrational modes that are weakly active in one Near-Infrared Fourier Transform (NIR FT) Raman spectroscopy technique are generally detected as strong bands in the other [1]. and ultra-violet (UV) visible spectroscopy were used to observe Nonetheless, traditional Raman spectroscopy, using visible laser chemical changes in residual lignin in softwood kraft pulp upon excitation, and DRIFT techniques are difficult to apply to the study exposure to a vanadium-substituted polyoxometalate, which is of kraft pulps. representative of a new class of bleaching agents currently under investigation in our laboratory. In conventional Raman In DRIFT, quantitative information is generally difficult to obtain spectroscopy, using visible laser excitation, considerable because of the heterogeneous nature of wood pulps [2]. Because fluorescence is normally excited when chromaphores are present. scattering coefficients depend on the wavelength of incident In IT Raman spectroscopy, however, using excitation in the NIR, radiation, light is absorbed in an irreproducible manner resulting in the magnitude of the fluorescence is significantly reduced. After unpredictable baseline fluctuations. This is particularly exposure of kraft pulp to solutions of the polyoxometalate α- troublesome in the quantification of residual lignin in kraft pulps Keggin-Kg[SiVW11O40], spectroscopic evidence for the oxidation where the concentration of absorbing species is low and the of phenols to quinones and a-hydroxyl (benzyl alcohol) moieties dominant lignin bands at 1,600 and 1,510 cm-1 are weak. In to α-ketones was obtained. The quantification of residual lignin by addition, these bands are partly obscured by the contributions of IT Raman spectroscopy of solid pulp samples and transmission -1 adsorbed water at 1,640 cm (OH2 bending mode) and a UV visible spectroscopy of dissolved pulp samples was neighboring polysaccharide band that rises sharply at 1,500 cm-1 demonstrated. (CH2 bending mode). INTRODUCTION In Raman spectroscopy, optical heterogeneity in the pulp sample and the presence of adsorbed water do not present problems. For At present, it is difficult to observe chemical transformations that example, Raman spectroscopy and Raman microspectroscopy occur in residual lignin and lignin-derived chromophores during using visible laser excitation and conventional scanning bleaching. This is a major obstacle to the development of new monochromator techniques have been used effectively in the study bleaching technologies. Most classical methods used in analysis of of plant cell walls, lignin orientation in native woody tissue, and lignocellulosic materials require separation and isolation of mechanical pulps [3-5]. However, when applied to unbleached and constituents in ways that disrupt and modify the structures of partially bleached kraft pulps, conventional Raman spectroscopy is interest. Furthermore, residual lignin in chemical pulps is often as not very successful [6]. The residual lignins in these materials little as 3% or less of the material. making its isolation or contain high concentrations of species that absorb visible light. As spectroscopic characterization particularly difficult. a result, laser excitation in the visible region gives rise to overwhelming fluorescence that completely swamps the Raman In support of an effort to develop new methods for bleaching signal. chemical wood pulps, we are exploring the use of new spectroscopic techniques for observing changes in residual lignin A new technique in Raman spectroscopy is NIR FT Raman [7]. In during bleaching. The goal of the bleaching program is to identify this technique, Raman scattering is generated by laser excitation in and develop technologies that meet two key criteria: lower levels the NIR region. For excitation in the NIR, a Nd:YAG laser with a of capital investment than current technologies and reduced impact lasing wavelength of 1,064 nm is most often used. As most on the environment. This approach has led us to identify materials do not absorb at NIR wavelengths, fluorescence is polyoxometalates as a promising class of delignifying agents. significantly reduced. Although the Raman signals resulting from Within the context of bleaching of kraft pulps, one class of NIR excitation are weaker than those observed in conventional polyoxometalates has the additional advantage that they can be Raman. this is more than compensated for by the use of Fourier regenerated with air or oxygen, thus substantially reducing energy transform techniques [7]. In fact, the time needed to acquire demand. In addition, this class of materials offers the possibility of detailed FT-Raman spectra is much shorter than that needed in a closed bleaching mill with complete mineralization of organic conventional Raman spectroscopy. Taken together, these effluent streams. advantages make the NIR FT Raman a technique well-suited to lignocellulosic research. We report here results obtained in an investigation of FT Raman spectroscopy as a tool for the study of wood pulps. Although some FT Raman studies of native woody 1The Forest Products Laboratory is maintained in cooperation with tissue are available, we are unaware of more detailed studies into the University of Wisconsin. This article was written and prepared wood products or pulps using the technique. by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. 1993 Pulping Conference / 519 Electronic Absorption Spectroscopy Although attractive as a noninvasive technique, complications arise because of sample heterogeneity. Here, as in DRIFT, the Various UV visible spectroscopy methods for the analytical dependence of Rayleigh scattering coefficients on wavelengths determination of lignin and chromophoric groups in lignocellulosic must be considered. More problematic, the theoretical treatment materials have been proposed [8]. The technique is an obvious for determination of K becomes less reliable in regions of high choice as the functionalized aromatic units of lignin and related light absorption. Below 350 nm, where lignin absorbs strongly, the structures containing extended conjugation absorb light in the near absorption coefficients become less reliable. Various postulates UV and visible range. Over the same region, the major component have been put forth 10 explain the reasons for the deviation from carbohydrates of lignocellulosic materials are transparent. Early theory, but a satisfactory theoretical treatment has not been studies using transmission techniques for the study of soluble advanced. Nonetheless, even for high lignin content samples such lignin preparations and model compounds have been particularly as mechanical pulps, reflectance spectroscopy can provide useful. However, difficulties remain in efforts to use UV visible chemical information in the near UV and visible regions of the spectroscopy to characterize and quantify the lignin components of spectrum where absorption is less intense. For example, using low native wood and wood products. basis weight handsheets (i.e., 10 g/m2) and wavelengths greater than 300 nm, it is possible to obtain measurements with high Transmittance UV Visible Spectroscopy reproducibility [19,20]. The greatest obstacle to the characterization and quantification of Polyoxometalate Bleaching wood and wood products by solution spectroscopy lies in their insolubility. Only a handful of solvents can dissolve these In delignification, as in many industrial and biochemical processes, materials. Of these, useful choices must be transparent over the UV the limitations apparently inherent in the use of oxygen and visible region and cause minimal chemical changes to the peroxides can be overcome by the introduction of appropriate structures of interest. The most commonly used quantitative soluble catalysts. This is what occurs in nature when wood-rotting method utilizes acetyl bromide to solubilize the wood or pulp fungi attack wood. These fungi use enzymes to catalyze the sample in acetic acid [9,10]. Acetyl bromide extensively degrades degradation of lignin by oxygen or hydrogen peroxide.