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. The active and acetylates lignin, but without destroying the aromaticity. Thus, sites of these enzymes are metalloporphyrins that have served as the UV absorption at 280 nm, which is ascribed to the phenolic models for the preparation of metallo-organic (biomimetic) function, remains proportional to the quantity of original lignin. catalysts. Although effective, these synthetic metallo-organic catalysts are expensive and susceptible to degradation during Other wood and pulp solvents have been examined for their utility bleaching. For large-scale commercial bleaching, we sought more in analyzing original or modified lignin or chromophores by UV stable synthetic catalysts that could be prepared easily from and visible light absorption. The cellulose solvents cadoxene [11], inexpensive, nontoxic materials. The polyoxometalates are the phosphoric acid [12], paraformaldehyde/dimethylsulfoxide synthetic catalysts that we have identified [21]. (PF/DMSO) [13], and sulfur dioxide/diethylamine/dimethyl- sulfoxide (SO2/DENDMSO) [14] have all been investigated. One Polyoxometalate complexes, wherein the porphyrin ligand is problem with cellulose solvents is that wood pulps generally replaced by an entirely inorganic polyoxometalate ion, are perhaps become less soluble with increasing lignin content. Of these the most promising catalytic materials currently available for systems, phosphoric acid is the most convenient to prepare and use applications in bleaching [22-24]. They include a wide variety of reproducibly and has good transparency over a wide range of water soluble, inorganic compounds, and provide transition metal visible and UV wavelengths. Pulps with substantial lignin content ion coordination sites that are structurally similar to the can be dissolved readily. Although dissolution of spruce wood has coordination sites of natural and synthetic porphyrins. However, been claimed [12], in our experience, lignin does interfere with unlike metalloporphyrins, polyoxometalates are easily prepared solubility. We have successfully dissolved pulps with kappa from inexpensive, nontoxic mineral ores, and are remarkably numbers greater than 70, but the time required for dissolution stable to oxidizing conditions. We began working with several increased. polyoxometalates early in 1992, in a close collaboration with Professor Craig Hill, Emory University, Atlanta. One class of Reflectance UV Visible Spectroscopy polyoxometalates, the mixed addenda heteropolyoxometalates, could very well make it possible to replace chlorine compounds With any solvent system, the question of what effect the solvent with the least expensive oxidant available: air. Other might have on lignin or other pulp components remains polyoxometalate complexes, designed to catalyze the activity of unanswered. A noninvasive analytical method would be preferable. hydrogen peroxide or other peroxide compounds, are also under In principle, reflectance spectroscopy is such a method. Although investigation. Still in the early stages of development, the most pulps contain components that are susceptible to polyoxometalate bleaching process used in our study appears to be photochemical transformation, these changes are unlikely to be as effective as a typical chlorine and extraction (CE) sequence in significant on the time frame necessary for acquisition. the delignification of softwood kraft pulp.

Reflectance spectroscopy has been extensively investigated for The polyoxometalate material used in this study was α-Keggin-

application to paper. The ratio of reflected to incident light is K5[SiVW11O40] (1), a water soluble potassium salt of the determined by the Rayleigh scattering and absorption of the paper monovanadium derivative of the tungstosillate α-Keggin- under investigation. The light-scattering coefficient S and the 4- [SiW12O40] [25]. For effective bleaching, it is essential that the absorption coefficient K are related to the reflectance of an metals present in this material (tungsten and vanadium) be in their infinitely thick stack of paper by the remission function of Kubelka highest (do) electronic states. In the bleaching step. designated 'Vi and Munk [15,16]. The absorption coefficient, K, is the value that to represent the vanadium substituted polyoxometalate, mixtures of provides information about the absorbing characteristics of the water, inorganic buffer, pulp, and the fully oxidized material in the sample. In paper science, the Kubelka-Munk theory polyoxometalate are heated in a sealed vessel under nitrogen. is commonly used to determine S and K, either from two During the reaction. the polyoxometalate is reversibly reduced as reflectance measurements or one reflectance and one transmission susceptible functional groups within the residual lignin in the pulp measwement [17]. In the present study, we used two reflectance are oxidized. This leads to selective functionalization, measurements, the method of white and black backing [18,19]. fragmentation, and eventual solubilization of the residual lignin in the pulp. Complete details concerning polyoxometalate chemistry

520 / TAPPI Proceedings and bleaching, along with polyoxometalate regeneration and other out as previously described with one exception. After 1 h at 125°C. process concepts, will be published elsewhere. the temperature was increased to 150°C (taking about 30 min) and maintained at that temperature, at a pressure of 585 kPa, for 0.5 h. EXPERIMENTAL Final brightening was achieved using hydrogen peroxide. To remove trace metals, the pulps were soaked at room temperature in General Methods 0.5 weight percent solutions of sulfuric acid at 8% csc for 15 min and washed thoroughly with water. The bleaching mixtures 2+ The unbleached mixed-pine kraft pulp used in this study, provided consisted of 1.5% H2O2, 4% 41.5°B sodium silicate. 0.1% Mg by Consolidated Papers, Inc.,2 Wisconsin Rapids, Wisconsin, had a using the sulfate salt and 2.5% NaOH, all values being weight kappa number of 33.6 (approximately 5.6% lignin as kappa percentages on pulp. These mixtures were combined with pulps to no. ÷ 6), an intrinsic viscosity of 34.2 mPa•s, and a brightness of a csc of 12% kneaded together in a polyethylene bag, and placed 26.3. Microkappa numbers were obtained using Technical in an 80°C water bath for 2 h. The pulps were then soaked in a 0.1 Association of the Pulp and Paper Industry (TAPPI) useful method weight percent solution of sodium bisulfite for 10 min and washed um-246; handsheets were prepared by adaptation of TAPPI test with water. method T218 om-83; pulp viscosities were obtained using TAPPI test method T230 om-89. A Technidyne Technibright TB-1 Transmittance UV Visible Spectroscopy instrument was used to obtain reflectance measurements from single handsheets using both black and white backings. For each Spectra were recorded using a Perkin-Elmer Lambda 6 handsheet, black background reflectance values (Ro) and white spectrophotometer. Phosphoric acid (83%) was prepared from background reflectance values (Rw) were obtained by averaging reagent grade 85% acid. The absorbance of the diluted acid, measurements from several spots. Brightnesses, reported as obtained against water, should be less than 0.1 at 280 nm. reflectance values (R∞), were then calculated using the method of white and black backgrounds, as described later for reflectance Pulps to be analyzed (about 10 mg) were thoroughly dried under spectroscopy samples. The backgrounds affected only the readings vacuum at ambient temperature, weighed immediately, and mixed of significantly brightened pulp samples. with 8 mL of 83% phosphoric acid in 25 mL Erlenmeyer flasks. The mixtures were stirred rapidly, but not so fast that stable foams Bleaching were created. Complete pulp solution required from 2 to 16 h. The solutions were then quantitatively diluted with additional 83% The unbleached kraft pulp, as received from Consolidated Papers, phosphoric acid to a final volume of 10.0 mL. Spectra were Inc., was pretreated with a mild anaerobic alkaline extraction, obtained as soon as possible after dissolution of the pulp to designated Eo, to remove any easily solubilized lignin. To observe minimize the slow formation of chromophores generated from the progress of the bleaching reaction, subsequent polyoxometalate reaction of the carbohydrates with phosphoric acid. Absolute treatment was divided into three batch oxidations, V1, V2, and V3. absorbance values were converted to absorption coefficients, A control sequence was done in parallel. Controls for the V stages reported in units of liters per gram per cm (L/g•cm) based on pulp. were obtained by heating mixtures of pulp, water, and inorganic Lignin concentrations were calculated using an absorption buffer with no polyoxometalate present. These heated control coefficient for kraft pulp residual lignin of 20 L/g•cm. This value stages are designated by the symbol delta,. ∆. Final brightening was was previously determined from kraft pulps containing varying achieved by treatment with hydrogen peroxide (P stage). The full amounts of lignin [13]. bleaching and control sequences were designated Reflectance UV Visible Spectroscopy EoV1V2V3EV4EP and Eo∆1∆2∆3E∆4EP. Note: The designation Eo, should not be confused with its usual indication of an oxygen reinforced extraction. Individual pulp samples were identified with Small handsheets with basis weights of approximately 18 g/m2 respect to V or ∆ stages (i.e., EoV1V2 is denoted 'V2'; EoV1V2V3E were prepared in a Büchner funnel. Reflectance measurements is denoted 'V3E'). The full bleaching sequence was performed three were taken in a Perkin-Elmer Lambda 6 spectrophotometer fitted times and the control sequence once. with a Labsphere RSA-PE-60 reflectance attachment. Measurements were taken over white and black backgrounds. The The preliminary and later E stages were performed using 1% data was exported to a Lotus 1-2-3 spreadsheet for calculations. NaOH and pulp consistencies (csc) of 1% to 2% for 2 h at 85°C Values of R∞, the reflectance of paper over a background of the under nitrogen. After each E stage, the pulps were collected in a same material of such thickness that the supporting background has Büchner funnel and washed once with 1% NaOH and three more no optical effect, were calculated using times with water. The three subsequent V stages were performed under identical conditions. The reactions were carried out in a stirred, high-pressure Parr reaction vessel with a glass liner. The pulps were reacted under nitrogen at 3% csc in bright yellow 0.05 where W is the reflectance of the white background alone. Ro is M solutions of α-Keggin-K5[SiVW11O40] (1) in 0.2 M pH 7 the reflectance over a black background, and Rw is the reflectance phosphate buffer. After purging with nitrogen, the reactor was over a white background [19]. Scattering coefficients, S, were heated to 125°C for 2 h. The reactor pressure was sustained with calculated using nitrogen at about 340 kPa. Small aliquots were taken periodically to monitor solution pH and consumption (reversible single-electron reduction) of 1. During bleaching, the pH slowly decreased from seven to no less than six. The liquor changed from a bright yellow color of fully oxidized 1 to the dark purple of the reduced material. where B is the basis weight of the sample. Kubelka-Munk After each V stage, the pulps were collected in a Büchner funnel absorption coefficients. K. were then calculated from the remission and washed three times with water. The fourth V stage was carried function K/S = (1-R∞)2/2R∞. NIR FT Raman Spectroscopy

Pulp mats for study were prepared by gently compressing 20 mg 2The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. portions of airdried pulp fibers in a KBr pellet press. Raman spectra were obtained using a Bruker IFS 66/FRA 106 system Department of Agriculture of any product or service.

1993 pulping Conference / 521 equipped with a 350 mW (1,064 nm) diode pumped laser. The pulp and trial 1, V2). This feature, common to the present and other mats were sampled in the double sided, forward-backward polyoxometalate bleaching systems, reflects the functionalization scanning mode using the 180 degree Raman scattering geometry of susceptible structures within the residual lignin that occurs along with a minor behind the sample for signal enhancement. Spectra the path towards lignin fragmentation and solubilization. were acquired at 300 mW of laser power and at 4 cm-1 spectral resolution, and corrected for instrument response and wavenumber The FT Raman and solution UV visible spectra of pulp samples dependence of the Raman scattering. Data acquisition time per examined early in the bleaching process provided some detailed spectrum was approximately 10-15min. information regarding these chemical changes. Analysis of these chemical changes is described after the presentation of Lignin content was calculated by measuring changes in the spectroscopic data that immediately follows. As the lignin content 1,595 cm-1 band (1,671-1,545 cm-1), associated with the aromatic decreased and brightening began, less detailed chemical stretch of phenyl groups within the residual lignin. Spectra information was provided by FT Raman and solution UV visible acquired in all but the final stages of the process included spectra. Nonetheless, these techniques continue to provide fluorescent backgrounds. Thus, for quantitative comparison, band quantitative measures of lignin content. The spectroscopic areas were calculated as the peak above the baseline created by the quantification of residual lignin, and the complications that can fluorescence. For quantification, the band of interest must be arise from lignin functionalization during bleaching are addressed compared to one that remains constant throughout the bleaching later in this section. -1 process. The cellulose band structure between 1,216-1,010 cm Transmittance UV Visible Spectroscopy was chosen for this purpose. Using these bands, changes in lignin content were quantified by two methods. In the first, integrated -1 Solution UV visible spectra of pulps from the polyoxometalate areas of the 1,595 cm band were ratioed against integrated areas sequence are presented in Figure 3. Because of the undefined of the band structure between 1,216-1,010 cm-1. In the second, the nature of the pulps, the absorption coefficients 'a' were calculated peak heights of the 1,595 cm-1 band were ratioed against those of a on a per weight of pulp basis and reported in units of liters per cellulose band at 1,098 cm-1. Although spectra were acquired in gram per centimeter (L/g•cm). Lignin content was estimated from the region 75-3,500cm-1, only the region from 800-1,800 cm-1 is the aborption maximum at 280 nm. The largest decreases in lignin shown in the figures. content occurred during the polyoxometalate stages V1, V2, V3, and V4. The final two stages V4E and P were essentially DRIFT Spectroscopy brightening treatments. In contrast, spectra of pulps from the control sequence (Fig. 4) demonstrated that the conditions of the DRIFT spectra were obtained from handsheets, prepared by bleaching experiment alone have little effect on the UV visible adaptation of TAPPl test method T218 om-83, sampled on a absorbing materials initially present in the unbleached brownstock Mattson Galexy Series FTIR 5000. The carbohydrate band at 2,905 (UB). cm-1 was used as an internal reference. Because the amount of residual lignin was low (under 6% in all cases), only weak IR An interesting feature of the V1 treatment can be seen in Figure 5. bands were observed. Where present, the usually strong lignin For greater clarity, only the stages Eo through V3 are included. band at 1.57 10 cm-1 [26], arising from asymmetrical stretching Absorbance at smaller wavelengths (210-240nm) decreased modes of phenyl groups, was detected as a weak shoulder. Because substantially with each polyoxometalate treatment. However, early this shoulder occurred on a steeply rising contribution from a in the process (Fig. 5, V1) absorbance at wavelengths greater than neighboring IR band, quantification of the 1,510 cm-1 band was 250 nm increased as new chromophores were generated from not possible. Instead, relative intensity changes were measured at substructures within the residual lignin. The increase in absorbance 1,600 cm-1. Because this band overlapped with that arising from was particularly apparent in a comparison of V1 (microkappa adsorbed water at 1,640 cm-1, quantitative information was number 23.1) with its parallel control, ∆1 (microkappa number obtained by ratio of the peak height at 1,600 cm-1 to that at 31.1) (Fig. 6). 2,905 cm-1. Reflectance UV Visible Spectroscopy RESULTS AND DISCUSSION Results from reflectance UV visible spectra of selected stages, E,,, Polyoxometalate Bleaching V1, V3, and P, of the polyoxometalate sequence are presented in Figure 7. Note that from 375-700nm, values of K calculated for Microkappa numbers, intrinsic viscosities, and Rm brightness V1 are greater than those of the preceding Eo stage. Over the same measurements for the control and three repetitions of the region, values of K for V2 (omitted for clarity) are closely polyoxometalate bleaching sequences are given in Table I. coincident with that of Eo. These results were qualitatively similar Microkappa numbers and intrinsic viscosity values are shown in to those obtained from transmission UV visible data, suggesting Figure 1. Microkappa numbers were determined for the first five that the increases in absorbance seen in solutions of V1 and V2 stages of the polyoxometalate sequences and for all stages of the were not attributable to reactions of partially oxidized lignin control sequence. The following spectroscopic studies were carried structures with phosphoric acid. out using pulps obtained from the control sequence and from trial 1 of the polyoxometalate bleaching sequences. FT Raman Spectroscopy

One notable feature of the polyoxometalate bleaching sequences The FT Raman spectra of UB, before and after mild alkaline was the high selectivity demonstrated in stages Eo through V3E. pretreatment, Eo, are shown in Figure 8. Spectra of the full Kappa numbers and pulp viscosities were at least as good as those polyoxometalate and control sequences are presented in Figures 9 obtained in traditional CE sequences. A second feature of the and 10 (trial 1, polyoxometalate sequence) and Figures 11 and 12 polyoxometalate sequences was the decrease in brightness that (control sequence). The band centered at 1,595 cm-1 (1,671-1,545 occurred during the early stages of delignification (Fig. 2). After cm-1) is associated with symmetric aromatic stretching modes of V1, the pulp was darker and possessed a reddish-orange hue not phenyl groups within the residual lignin. The bands at less than seen in the original brownstock. After a decrease in lignin content 1.500 cm-1 were attributable primarily to cellulose. Little change in -1 of more than 50% after V2, the pulp remained darker than it did the 1.595 cm band occurred upon mild alkaline pretreatment after Eo or after the parallel control, ∆2 (Table 1, entries Eo and ∆2 (Fig. 8. Eo). Upon initial polyoxometalate treatment (Fig. 9, V1),

522 / TAPPI Proceedings the 1,595 cm-1 band broadened and shoulders were observed. The The presence of phenolic and α-hydroxyl moieties was likely new contributions, although weak, were clearly evident at necessary for the fragmentation and solubilization of the residual approximately 1,550 cm-1 and also in the region 1.650-1.680cm-1. kraft lignin observed in the present study. According to recently These features, observed after the first V stage, are highlighted by published reports, polyoxometalates closely related to that used comparison of V1 with its parallel control, ∆1 (Fig. 13). here are efficient catalysts for the aerobic oxidation of both phenols and benzylic alcohols. In organic solvents, the vanadium Subsequent V stages (Fig. 9, V2 and V3) continued to remove substituted molybdophosphate compound H5[PV2Mo10O40] (2) lignin and attack the structures generated during the V1 stage. and its sodium salt Na5[PV2Mol0O40] oxidize activated phenols to Upon extraction of the V3 pulp with alkali (V3E). further decline in quinones [28,29] and benzylic alcohols to α-carbonyls [30]. We the intensity of the 1,595 cm-1 band occurred. This decrease was observed analogous results with a few simple lignin models. In most probably caused by the extraction of fragmented lignin from preliminary studies in water, simple phenolic lignin models were the carbohydrate matrix. After V4 (Fig. 10), the R∞ brightness of readily oxidized by 2. In addition, nonphenolic α-hydroxyl the pulp was 66.2 (Table I), and little evidence of aromatic containing models, such as veratryl alcohol (3,4-dimethoxybenzyl structures remained. Although brightening the pulp, the final two alcohol) [31] and 1-(3,4-dimethoxyphenyl)ethanol, were oxidized stages, V4E and P, did not produce additional changes in the to the corresponding a-ketones as the major products [32]. To our Raman. Throughout the control sequence (Figs. 11 and 12), little knowledge, no study Concerning the oxidation of organic change in the 1,595 cm-1 band was observed. compounds by α-Keggin-K5[SiVW11O40] (1) has been reported. However, it is likely that the oxidation of phenols and benzylic DRIFT Spectroscopy alcohols by 1 will parallel that of 2, giving rise to quinones and a- ketones. Moreover, it is plausible that in the oxidation of polymeric residual kraft lignin by 1, intermediate single-electron steps similar DRIFT spectra of the first six stages (Eo-V4) of the polyoxometalate (trial 1) and control sequences are presented in to those suggested for the production of quinones and a-ketones by Figures 14 and 15. Because the amount of residual lignin was low 2 [29], will lead to lignin fragmentation and solubilization. (<6% in all cases), only weak IR bands were observed. The Of the techniques previously discussed, FT Raman and solution spectrum obtained after the Eo stage was essentially identical to UV visible spectroscopy provided the most detailed information that of the unbleached brownstock. and no further changes in either concerning chemical changes in residual lignin. When used the polyoxometalate or control sequences were observed in the together, these techniques provided strong support for chemical final stages, V4E or ∆4E, and P. For this reason, spectra of the UB changes suggested by the model studies just described. pulp and pulps recovered after the final two stages, V4E or C4E. and P are not shown. The Raman scattering centered at 1,595 cm-1 represents symmetric

The usually strong lignin band at 1,510 cm-1 [26], arising from stretching modes of phenyl moieties in the residual lignin. During asymmetrical stretching modes of phenyl groups. was detected as a the first V stage, this band broadened considerably and new -1 weak shoulder in UB (not shown), Eo, and in all the control pulps shoulders were observed at approximately 1,550 cm , between (Figs. 14 and 15). As evident from visual comparison of Figures 14 1,590-1,620 cm-1, and in the region 1,650-1,680 cm-1 (Fig. 13). and 15, phenyl groups were attacked during the V stages. This Some of these new features can be identified by reference to the resulted in a rapid decline in the 1,510 cm-1 band. In contrast, the Raman scattering characteristics of known lignins and lignin model

1,600 cm-1 band, which represents phenyl and other related compounds. structures, appeared to decrease only after the second V stage (V2, Fig. 14). Although not evident from visual inspection, the peak Raman bands associated with lignin have recently been identified -1 and the prominent bands assigned [5,33]. In our group, a large height ratio of the 1,600 cm band appeared to increase during the number of additional lignin models have been studied. Such V1 stage (vide infra). However, as explained in the Background, studies have been used to identify lignin related bands in the the reliability of this quantitative measure was questionable. The spectra of lignocellulosics and to characterize the changes that -1 rapid decline in the 1.510 cm band during V1 and apparent initial occur when lignin-rich materials are subjected to various increase in intensity of the 1.600 cm-1 band were consistent with treatments [5,31,34,35]. Raman band assignments of lignin and the partial oxidation or functionalization of aromatic structures models. selected from data obtained in our laboratory, are listed in suggested by UV visible and FT raman. Decline in the 1,600 cm-1 Table II. band during the V2 stage, apparent by visual inspection, Suggested -1 further degradation of structures generated during V1. After the V4 The Raman contribution at 1,550 cm was likely due to the stage, the intensity of the 1.5 10 cm-1 band was nearly zero. formation of ortho-quinones in the pulp. In the Raman spectrum of

Residual intensity observed at 1,600 cm-1 was due to the decaying 3-methoxy-ortho-quinone, an intense Raman band was found to be present at this wavenumber (Table II) [6]. Data in Table II further wing of the 1,640 cm-1 band (bending mode of water). DRIFT suggest that the contributions in the region 1,650-1.680 cm-1 were analysis, as in FT Raman analysis, provided no evidence of likely due to the formation of α-carbonyl and para-quinone residual lignin after the V4 stage. structures. As a result of unidentified shoulders between 1.590 cm-1 and 1,620 cm-1, a general broadening of the primary Chemical Changes in Residual Lignin band was observed. The band shifted to higher energy by During Polyoxometalate Bleaching approximately 5 cm-1 relative to the control (Fig. 13). Many different phenyl units are present at this stage in bleaching. In native softwood lignin. approximately 1 in 10 substructural Functionalization of these units, or changes in their substituents, phenyl propane units contains phenolic groups and many β-aryl could give rise to increases in Raman scattering coefficients and/or ether structures posses a-hydroxyl moieties in the propyl side small shifts in the frequencies of the symmetrical stretching modes chain. Cleavage of aryl ether linkages during kraft pulping likely usually observed at 1,595 cm-1. leads to an increase in the frequency of phenolic groups. In addition, recent work has shown that a considerable amount of Upon initial treatment with the polyoxometalate (V1), the pulp uncleaved β-aryl ether and other structures found in native lignin darkened, took on a reddish-orange hue, and significant changes survive the pulping process intact [27]. Many of these are likely to were observed over the entire UV visible region. As a tool for the possess α-hydroxyl moieties in the propyl side chain. characterization of chemical changes in residual lignin, UV visible

1993 Pulping Conference / 523 spectroscopy by itself is limited. Many chemical structures absorb and a correlation coefficient (R2) of 0.960. The data in Figure 19 in similar spectral regions, and vibronic coupling gives rise to give a slope of 8.03 x 10-3 ±5.21 x 10-4, a y-intercept of 5.81 x broad overlapping bands. Nonetheless, lignin model studies and FT 10-3 ±1.15 x 10-2, and an R2 value of 0.983. In Figure 18, the peak Raman data both point to the formation of quinones (most likely height ratio for the V1 stage 1,595 cm-1 band appears noticeably orthoquinones from the oxidation and hydrolysis of phenolic high. Although experimental uncertainty present in the Raman data guaiacyl units) and α-ketones. Structures of this type, which absorb has not been precisely determined, the elevated Raman intensity at strongly in the near UV and visible regions, might account for the V1 may reflect the production of substituted or functionalized increase in absorbance observed in spectra of V1 and V2 pulps. phenyl groups with higher scattering coefficients. In the integrated area ratios in Figure 19. V1 does not appear markedly elevated. In Figure 16 shows difference spectra obtained from solution UV addition, uncertainty in the slope of the integrated area ratio is visible absorption coefficient data. The three plots were calculated 26.50%. while that for the slope of the peak height ratio is ±10.3%. by subtracting spectra of the UB, the alkaline pretreated (Eo), and From these results, it appears that integrated area ratios may better parallel control (∆1) pulps from the spectrum of the V1 stage pulp. represent the concentration of phenyl groups. In all cases, the V1 pulp absorbed more strongly at wavelengths greater than 300 nm. New intensity was also observed at Noteworthy in both Figures 18 and 19 is the low value of the y- 260-270nm, a region near where one ortho-quinone model, 3- intercept. This demonstrates that the lignin band at 1,595 cm-1 was methoxy-ortho-quinone had a maximum in 83% phosphoric acid made up exclusively of contributions from phenyl groups and (Fig. 17); 3-methoxy-ortho-quinone also absorbed across the closely related structures. As a result, the Raman technique can be visible region. It had a dark red color in dilute solution. reminiscent used for rapid lignin quantification without the need for of the reddish hue acquired by pulp during the V1 stage. The empirically determined zero-offset values. oxidation of α-hydroxyl lignin models to their corresponding α- ketones can result in an increase in absorbance in the near UV. Transmittance and Reflectance UV Visible Spectroscopy Both acetovanillone and its methylated derivative dimethoxyacetophenone absorbed strongly in the near UV region Lignin content was estimated from solution UV visible data using with maxima between 300 and 350 nm in 83% phosphoric acid. an absorption coefficient of 20 L/g•cm at 280 nm (see Thus. the formation of α-ketones may have contributed to the local Experimental). In principle, the Kubelka-Munk absorption maximum observed at approximately 325 nm in V1-∆1 (Figs. 16 coefficients, K, were also proportional to the concentration of and 17). absorbing material after normalization to a constant basis weight. However, as discussed in the Background section, the Spectroscopic Quantification of Residual Lignin proportionality does not hold in regions of high absorption (i.e., at 280 nm). Moreover, 18 g/m2 handsheets were used in the present Residual lignin in kraft pulps is generally measured titrametrically work. As a result, at wave numbers less than 350 nm R∞ and reported as kappa numbers. To evaluate if FT Raman might fill approached Ro and the absorption coefficients obtained were this need, Raman data was plotted against lignin content. Solution unreliable. UV visible data were similarly evaluated and compared to the Raman results. The quantification of DRIFT and reflectance UV Lignin contents estimated from solution data for each stage of the visible data are discussed in relation to FT Raman and polyoxometalate and control sequences, are reported as average transmittance UV visible techniques. percentage lignin in Table IV. Standard deviations are given for FT Raman and DRIFT spectroscopy triplicate runs at each stage of the new bleaching sequence and for duplicate runs of the controls. In all but one case, standard deviations are less than 0.22, corresponding to an uncertainty in Quantification of Raman data is described in the Experimental kappa numbers of ±1.32. The high degree of reproducibility is section. Both peak height and integrated area ratios were typical of this technique. calculated. It was anticipated that the use of peak ratios might avoid Raman contributions arising from nonaromatic structures in Lignin estimates calculated for the control pulps match well with the pulps. However, we found that the integrated area ratio method the lignin values determined from microkappa numbers. In worked best. Quantification of the DRIFT results, by ratioing the contrast, estimates calculated for the trial 1 pulps were all quite peak height at 1,600 cm-1 to that of a carbohydrate band at high, clearly exceeding experimental uncertainty (vide infra). In 2,905 cm-1, was less successful. addition, lignin content estimates for pulps from the final three stages, V4, V4E, and P, were nearly identical and did not decrease Raman and IR peak height and area ratios calculated for trial 1 of to zero as might be expected. These nonzero values likely reflected the polyoxometalate and the control sequences are presented in contributions arising from reactions of carbohydrates with Table III. Some variability in the Raman ratios calculated for the phosphoric acid. If so. it is not clear why these same contributions control sequence was observed. However, because only one did not appear as a systematic error in estimates of lignin content control sequence was analyzed, it was not possible to determine for the control pulps. whether this reflected experimental uncertainty or was caused by the conditions of the control stages. The same was true for the In the bleaching sequence, estimated lignin content decreased apparently elevated Raman peak ratio calculated for Eo. The significantly during each of the four V stages. Figure 20 presents 1,600 cm-1 IR band intensity values for samples from the control estimated lignin (UV % lignin) against lignin calculated from sequence included considerable variability, a substantial amount of microkappa numbers (microkappa % lignin). which was likely inherent in the DRIFT technique. FT Raman data for the same pulps showed much less variability. A similar The line in Figure 20 has a slope of 0.797 ±0.0971, a y-intercept of difference in variability between FT Raman and DRIFT data was 1.23 ±0.359, and an R2 value of 0.944. As mentioned, the elevated seen in the polyoxometalate sequence ratios. Raman peak height lignin estimates for both the V1 and V2 pulps exceeded ratios and integrated area ratios from Table III, against lignin experimental uncertainty (Table IV). One explanation is that UV content (microkappa numbers) of polyoxometalate treated pulps, are presented in Figures 18 and 19.

By least squares analysis, the curve in Figure 18 has a slope of 1.40 x 10-2 ±1.44 x 10-3, a y-intercept of 1.10 x 10-3 ±3.18 x 10-2,

524 / TAPPI Proceedings and near UV bands of strongly absorbing materials generated early As a result of the nature of Raman scattering, pulp fibers can be in the polyoxometalate treatment had significant tailings at 280 nm analyzed without regard to optical inhomogeneity. In addition, the (Fig. 16). As a result, uncertainty in the slope of the UV (±0.0971 intensity of the lignin band at 1.595 cm-1. made up exclusively of or 12.2%, Fig. 20) exceeded that calculated for Raman peak height contributions from phenyl groups and closely related structures, ratios (10.3%, Fig. 18) or integrated area ratios (6.50%. Fig. 19). provides a direct measure of lignin content. As a result, the FT This observation points out the need to consider the impact of Raman technique can be used for rapid lignin quantification. chemical changes on the spectroscopic quantification of residual lignin. ACKNOWLEDGMENTS

Unlike the Raman intensity (Figs. 18 and 19), the UV lignin has a We thank Nancy T. Kawai of Bruker Instruments, Inc. for nonzero y-intercept. In Figure 3, little decline in absorption at 280 acquiring the FT Raman spectra and Matthew A. Smith for nm is observed during the final three stages, V4, V4E, and P. This obtaining the UV visible data. We also thank Sally A. Ralph for residual intensity accounts for the nonzero lignin estimates of graphical assistance and Kolby Hirth for acquisition of the DRIFT 0.43%, 0.44%, and 0.41% reported for these samples in Table IV. Spectra. An average offset value of 0.43%. combined with an uncertainty in the y-intercept of 0.36%. might reduce the y-intercept to 0.44%. REFERENCES Although still high, this agrees closely with results obtained in solution UV visible quantification of residual lignin in kraft pulps 1. Lin-Vien. D.. Colthup. N.B., Fatelry, W.G., and Grasselli, J.G., subjected to more conventional chlorine extraction sequences eds. The Handbook of Infrared and Raman Characteristic Frequencies [1 3,36]. of Organic Molecules, Academic Press, San Diego CA. 1991. 2. Fraser, D.J.J., and Griffiths. P. R.. Applied Spectroscopy, "Effect CONCLUSIONS of Scattering Coefficient on Diffuse Reflectance Infrared Spectroscopy," 44: 193 (1990). NIR FT Raman, DRIFT, solution UV visible, and reflectance UV visible spectroscopy were used to observe chemical changes in 3. Agarwal, U.P., and Atalla. R.H., Planta, "In-situ Raman residual kraft lignin during interrupted stages of a new Microprobe Studies of Plant Cell Walls: Macromolecular Organization polyoxometalate bleaching system. Used in combination, FT and Compositional Variability in the Secondary Wall of Picea Mariana Raman and solution UV visible spectroscopy provided detailed (Mill.) B.S.P., 169: 325 (1986). information regarding some chemical changes possibly occurring in the residual lignin during bleaching. Evidence for the formation 4. Atalla. R.H., Agarwal, U.P., and Bond, J,S., "Raman of ortho-quinones from phenols and α-ketones from α- Spectroscopy," In: Methods in Lignin Chemistry. Eds. S.Y. Lin and hydroxyphenyl (benzylic alcohol) groups was detected by FT C.W. Dence. pp. 162-176.Springer-Verlag, 1992. Raman spectroscopy and supported by reference to Raman spectra of model compounds, by lignin model oxidation studies, and by 5 Agarwal, U.P.. and Atalla. R.H., "Raman Spectroscopic Evidence solution UV visible data. DRIFT spectroscopy, as a result of weak for Coniferyl Alcohol Structures in Bleached and Sulfonated intensities of the major lignin bands and overlap of these bands Mechanical Pulps," In: Photochemistry of Lignocellulosic Materials, with contributions from adsorbed water and carbohydrates, Eds. C. Heimer and J.C. Scaiano, ACS Symposium Series 531, provided little detailed chemical information. Reflectance UV American Chemical Society, Washington DC, Chap. 2. 1993. visible data, although qualitatively similar to transmittance UV visible data, failed to provide the detail observed using the 6. Agarwal, U.P., Unpublished results, 1992. solution method. Additional detailed information concerning lignin oxidation and fragmentation by polyoxometalates is the topic of 7. Hendra. P., Jones, C.. and Warnes. G., eds., Fourier Transform planned lignin model oxidation studies. Rman Spectroscopy- Instrumentation and Chemical Applications, Ellis Horwood, Chichester UK, 1991. FT Raman and solution UV data provide 3 means of quantifying residual lignin in kraft pulp. The UV method is highly reproducible 8. Goldschmid, O., In:Lignins - Occurrence, Formation, Structure and provided us with accurate estimates of lignin in pulps and Reactions. Sarkanen, K. V., and Luwig, C. H., eds., Wiley- containing 4% to 5% lignin. It has also been used effectively in the Interscience. New York, Chap. 6, p. 241 (1971). quantification of residual lignin during stages of a conventional chlorine extraction bleaching sequence. However, in the present 9. Johnson, D. B.. Moore. W. E., and Zank. L. C., Tappi, "The work, the production of new UV and near UV absorbing materials Spectrophotometric Determination of Lignin in Small Wood Samples," during the early stages of bleaching complicated the quantification. 44(11): 793(1961). In addition, accurate quantification requires the empirical determination of baseline offset values. This and other aspects of 10. Iiyama, K., and Wallis, A. F. A., Wood Sci. and solution UV visible analysis are under investigation. Technol., "An Improved Acetyl Bromide Procedure for Determining Lignin in Wood and Pulp." 22(3): 271 (1988). FT Raman spectra also reflected the production, early on in bleaching, of new functional groups. However, accurate 11. Sjostrom, E., and Enstrom, B., Svensk Papperstid. quantification was still possible. Ratios of 1,595 cm-1 (phenyl) "Spectrophotometric Determination of the Residual Lignin in band peak heights and integrated areas against an internal Pulp after Dissolution in Cadoxen," 69(15): 469 (1966). carbohydrate reference band were calculated. Possible evidence for complications as a result of the formation of new structures with 12. Bethge, P. O., Gran, G., and Ohlsson, K., Svensk Papperstidn., increased Raman scattering coefficients at 1,595 cm-1 was "Determination of Lignin in Chemical Wood Pulp observed in the peak height ratio analysis. However, these I. Principles and Methods," 55: 44( 1952). complications were not observed using integrated area ratios, from which precise quantification was possible. 13. Minor J. L., and Stauffacher, K. A., Unpublished results, 1991.

1993 Pulping Conference / 525 14. Minor, J. L., and Isogai. A., Unpublished results, 1990. 32. Weinstock. I. A.. and Hammel, K. E., Unpublished result.

15. Kubelka, P.. and Munk, F., Zh. (Tech. Phys.) 12(11a): 593 33. Woitkovich, C.P., MS Thesis. "A Raman Spectroscopic Study (1931). of the Early Phase Acid-Chlorite Delignification of Loblolly Pine," 16. Kubelka. P., J. Opt. Soc. Amer., "New Contributions to the Institute of Paper Chemistry, Appleton, Wl [Institute of Paper optics of Intensely Light-scattering Materials, Part I.," Science and Technology, Atlanta, GA], 1988. 38(5):448 (1948). 34. Agarwal, U.P., Atalla, R.H., and Forsskahl. I., "Sequential 17. Kortum, G., Reflectance Spectroscopy-Principles, Methods, Applications, Springer-Verlag, New York. 1969. Treatment of Mechanical and Chemimechanical Pulps with Light and Heat. Part 3- A Raman Spectroscopic Study," (in progress). 18. Polcin, J., and Rapson, W. H., Tappi, "Spectrophotometric Study of Wood Chromophores in Situ II. Determination of the 35. Agarwal, U.P., and Atalla, R.H., J. Wood Chem. Technolo. Absorption Spectrum of Lignin from Reflectance and "Raman Spectral Features Associated with Chromophores in High- Reflectivity Measurements," 52(10): 1965 (1969). Yield Pulps." (accepted).

19. Schmidt, J. A., and Heitner, C., Tappi J., "Use of UV- 36. Minor, J. L., and Smith, M., Unpublished results. visible diffuse reflectance spectroscopy for chromophore research on wood fibers: a review," 76(2): 117 (1993). a Table I Bleaching results using α-Keggin- K5[SiVW11O40]. 20. Forsskåhl, I., and Janson, J., Nordic Pulp and Paper Research Journal, "Sequential treatment of mechanical and chemimechanical pulps with light and heat." 7(2): 48 (1992).

21. Weinstock. I. A., and Hill, C. L., U.S. Patent Pending.

22. Hill, C. L.. and Brown, R. B. Jr., JACS, "Sustained epoxidation of olefins by oxygen donors catalyzed by transition metal substituted polyoxometalates, oxidatively resistant inorganic analogues of metalloporphyrins," 108(3): 536 (1986). 23. Lyon, D. K., Miller, W. K., Novet. T., Domaille, P. J., Evitt, E., Johnson, D. C., and Finke, R. G., JACS, "Highly oxidative resistant inorganic-porphyrin analogue polyoxometalate oxidation catalysts," 113(19): 7209 (1991).

24. Pope, M. T., and Muller, A., Angew. Chem. Int. Ed. Engl., "Polyoxometalate chemistry: An old field with new dimensions in several disciplines," 30: 34 (1991).

25. Altenau, J. J., Pope, M. T., Prados, R. A., and So, H., Inorganic Chemistry, "Models for heteropoly blues. Degrees of valence trapping in vanadium(IV)- and molybdenum(V)- substituted Keggin anions," 14(2): 417 (1975).

26. Faix, O., "Raman Spectroscopy," In: Methods in Lignin Chemistry, Eds. S.Y. Lin and C.W. Dence, Subchapters 4.1 and 5.2, Springer-Verlag, 1992.

27. Gellerstedt. G., Lindfors, E. L., Lapierre, C., and Robert, D., Forst-Holzwirtsch., "Reactivity of lignin in Birch kraft cooking," (168): 224 (1991).

28. Lissel, M., Jansen in de Wal, H.. and Neumann; R. Tet. Lett., 33: 1795-1798 (1992).

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31. Weinstock, I.A., Atalla, R.H., Agarwal, U.P., Minor, J., and Petty, C., Spectrochimica Acta, "Fourier Transform Raman spectroscopic Studies of a Novel Wood Pulp Bleaching System," 49A, 819 (1993).

526 / TA PPI Proceedings Table II Raman band positions and their assignments in the Table IV Lignin Content of Pulps by Solution UV Analysis. spectral region 1,500-1,800 cm-1.a

Table III Raman and IR Intensity Data.

Figure 1. Microkappa numbers and intrinsic viscosities for three trials of the polyoxometalate bleaching sequence and for a control sequence in which the polyoxometalate was omitted. Data for the first five stages Eo through V3E (or ∆3E) are shown.

1993 Pulping Conference / 527 Figure 2. R∞ brightnesses determined for pulps from stages Eo Figure 4. Solution UV visible spectra of pulps from each stage of through V3E of the polyoxometalate bleaching the full control sequence. sequence.

Figure 3 Solution UV visible spectra of pulps at each stage of the full polyoxometalate bleaching sequence.

Figure 5. Solution UV visible spectra of pulps at stages Eo through V3 of the polyoxometalate bleaching sequence.

528 / TAPPI Proceedings Figure 6. Comparison of solution UV visible spectra of V1 stage Figure 8. FT Raman spectra in the region 800-1,800.cm-1 of and parallel control ∆1 stage pulps. unbleached kraft brownstock (UB), before and after the mild alkaline pretreatment stage, Eo.

Figure 7. Comparison of absorption coefficients. K, and wavelengths from 250-700 nm for pulps from stages -1 Eo, V1, V3, and P of the polyoxometalate bleaching Figure 9. FT Raman spectra in the region 800-1.800 cm of sequence. pulps at stages V1 through V3E of the polyoxometalate bleaching sequence.

1993 Pulping Conference / 529 Figure 10. FT Raman spectra region 800-1,800 cm-1 of pulps at Figure 12. FT Raman spectra in the region 800-1,800 cm-1 of the final stages V4 through P of the polyoxometalate pulps at the final stages A4 through P of the control bleaching sequence. sequence.

Figure 11. FT Raman spectra in the region 800-1,800 cm-1 of figure 13. Comparison of FT Raman spectra in the region 1,200- -1 pulps at stages ∆1 through ∆3E of the control sequence. 1,800 cm of V1 stage and parallel control ∆1 stage pulps.

530 / TAPPI Proceedings Figure 14. DRIFT spectra in the region 1,450-3.150 cm-1 of the Figure 16. Difference spectra from solution UV visible absorption first six stages (Eo through V4) of the polyoxometalate coefficient data. Calculations made by subtracting bleaching sequence. spectra of unbleached brownstock (UB), alkaline pre- mated pulp (Eo), and parallel control ∆1 stage pulp from the spectrum of the V1 stage pulp.

Figure 15. DRIFT spectra in the region 1.450-3,150 cm-1 of the first six stages (Eo through ∆4) of the control sequence.

Figure 17. Arbitrary absorbance units comparing the solution UV visible difference spectrum V1-∆1 with the spectrum of 3-methoxy-ortho-quinone dissolved in 83% phosphoric acid.

I993 Pulping Conference / 531 Figure 18. 1,595 cm-1 Raman band intensities calculated as peak Figure 20. Lignin estimates from solution UV (280 nm) absorption height ratios against microkappa numbers of coefficients against lignin values derived from polyoxometalate treated pulps. microkappa numbers of polyoxometalate treated pulps.

Figure 19. 1,595 cm-1 Raman band intensities calculated as integrated am ratios against microkappa numbers of polyoxometalate treated pulps.

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