ft raman and uv visible spectroscopic studies of a … 2. ft raman and uv visible spectroscopic...

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 STUDIES OF A HIGHLY SELECTIVE POLYOXOMETALATE BLEACHING SYSTEM

Ira A. Weinstock Umesh P. AgarwalResearch Chemist Research Chemist James L. Minor Rajai H. Atalla Research Chemist Supervisory Chemical Engineer Richard S. Reiner Chemical Engineer USDA Forest Service USDA Forest Service Forest Products Laboratory1 Forest Products LaboratoryMadison, Wisconsin 53705 Madison, Wisconsin 53705 U.S.A. U.S.A.

ABSTRACT

Near-Infrared Fourier Transform (NIR FT) Raman spectroscopyand ultra-violet (UV) visible spectroscopy were used to observe chemical changes in residual lignin in softwood kraft pulp upon exposure to a vanadium-substituted polyoxometalate, which is representative of a new class of bleaching agents currently under investigation in our laboratory. In conventional Raman spectroscopy, using visible laser excitation, considerable fluorescence is normally excited when chromaphores are present.In IT Raman spectroscopy, however, using excitation in the NIR, the magnitude of the fluorescence is significantly reduced. After exposure of kraft pulp to solutions of the polyoxometalate Ŭ-Keggin-Kg[SiVW11O40], spectroscopic evidence for the oxidation of phenols to quinones and a-hydroxyl (benzyl alcohol) moieties to Ŭ-ketones was obtained. The quantification of residual lignin byIT Raman spectroscopy of solid pulp samples and transmission UV visible spectroscopy of dissolved pulp samples was demonstrated.

INTRODUCTION

At present, it is difficult to observe chemical transformations that occur in residual lignin and lignin-derived chromophores during bleaching. This is a major obstacle to the development of new bleaching technologies. Most classical methods used in analysis of lignocellulosic materials require separation and isolation of constituents in ways that disrupt and modify the structures of interest. Furthermore, residual lignin in chemical pulps is often as little as 3% or less of the material. making its isolation or spectroscopic characterization particularly difficult.

In support of an effort to develop new methods for bleaching chemical wood pulps, we are exploring the use of new spectroscopic techniques for observing changes in residual lignin during bleaching. The goal of the bleaching program is to identifyand develop technologies that meet two key criteria: lower levels of capital investment than current technologies and reduced impact on the environment. This approach has led us to identifypolyoxometalates as a promising class of delignifying agents.Within the context of bleaching of kraft pulps, one class of polyoxometalates has the additional advantage that they can be regenerated with air or oxygen, thus substantially reducing energydemand. In addition, this class of materials offers the possibility of a closed bleaching mill with complete mineralization of organiceffluent streams.

1The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright.

In this study, NIR FT Raman spectroscopy is used in combination with diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy, transmission and reflectance UV visible spectroscopy, kappa numbers and brightness measurements to study changes that occur in residual lignin during stages of the new kraft pulp bleaching process. The purpose of this study was threefold: (1) to compare FT Raman spectroscopy with more traditional spectroscopic techniques; (2) to use FT Raman and UV visible spectroscopy to reveal chemical changes occurring in residual lignin during bleaching; and (3) to demonstrate the effectiveness of FT Raman spectroscopy as a rapid, noninvasive technique for quantification of residual lignin.

BACKGROUND

Vibrational Spectroscopy

Raman and infrared techniques provide complimentaryinformation. Vibrational modes that are weakly active in one technique are generally detected as strong bands in the other [1]. Nonetheless, traditional Raman spectroscopy, using visible laser excitation, and DRIFT techniques are difficult to apply to the study of kraft pulps.

In DRIFT, quantitative information is generally difficult to obtain because of the heterogeneous nature of wood pulps [2]. Because scattering coefficients depend on the wavelength of incident radiation, light is absorbed in an irreproducible manner resulting in unpredictable baseline fluctuations. This is particularlytroublesome in the quantification of residual lignin in kraft pulpswhere the concentration of absorbing species is low and the dominant lignin bands at 1,600 and 1,510 cm-1 are weak. In addition, these bands are partly obscured by the contributions of adsorbed water at 1,640 cm-1 (OH2 bending mode) and a neighboring polysaccharide band that rises sharply at 1,500 cm-1

(CH2 bending mode).

In Raman spectroscopy, optical heterogeneity in the pulp sampleand the presence of adsorbed water do not present problems. For example, Raman spectroscopy and Raman microspectroscopy using visible laser excitation and conventional scanningmonochromator techniques have been used effectively in the studyof plant cell walls, lignin orientation in native woody tissue, and mechanical pulps [3-5]. However, when applied to unbleached and partially bleached kraft pulps, conventional Raman spectroscopy is not very successful [6]. The residual lignins in these materials contain high concentrations of species that absorb visible light. As a result, laser excitation in the visible region gives rise to overwhelming fluorescence that completely swamps the Raman signal.

A new technique in Raman spectroscopy is NIR FT Raman [7]. In this technique, Raman scattering is generated by laser excitation in the NIR region. For excitation in the NIR, a Nd:YAG laser with a lasing wavelength of 1,064 nm is most often used. As most materials do not absorb at NIR wavelengths, fluorescence is significantly reduced. Although the Raman signals resulting from NIR excitation are weaker than those observed in conventional Raman. this is more than compensated for by the use of Fourier transform techniques [7]. In fact, the time needed to acquiredetailed FT-Raman spectra is much shorter than that needed in conventional Raman spectroscopy. Taken together, these 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 tissue are available, we are unaware of more detailed studies into wood products or pulps using the technique.

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Electronic Absorption Spectroscopy

Various UV visible spectroscopy methods for the analyticaldetermination of lignin and chromophoric groups in lignocellulosicmaterials have been proposed [8]. The technique is an obvious choice as the functionalized aromatic units of lignin and related structures containing extended conjugation absorb light in the near UV and visible range. Over the same region, the major componentcarbohydrates of lignocellulosic materials are transparent. Early studies using transmission techniques for the study of soluble lignin preparations and model compounds have been particularly useful. However, difficulties remain in efforts to use UV visible spectroscopy to characterize and quantify the lignin components of native wood and wood products.

Transmittance UV Visible Spectroscopy

The greatest obstacle to the characterization and quantification of wood and wood products by solution spectroscopy lies in their insolubility. Only a handful of solvents can dissolve these materials. Of these, useful choices must be transparent over the UV visible region and cause minimal chemical changes to the structures of interest. The most commonly used quantitativemethod utilizes acetyl bromide to solubilize the wood or pulpsample in acetic acid [9,10]. Acetyl bromide extensively degradesand acetylates lignin, but without destroying the aromaticity. Thus, the UV absorption at 280 nm, which is ascribed to the phenolic function, remains proportional to the quantity of original lignin.

Other wood and pulp solvents have been examined for their utility in analyzing original or modified lignin or chromophores by UV and visible light absorption. The cellulose solvents cadoxene [11], phosphoric acid [12], paraformaldehyde/dimethylsulfoxide(PF/DMSO) [13], and sulfur dioxide/diethylamine/dimethyl-sulfoxide (SO2/DENDMSO) [14] have all been investigated. One problem with cellulose solvents is that wood pulps generally become less soluble with increasing lignin content. Of these systems, phosphoric acid is the most convenient to prepare and use reproducibly and has good transparency over a wide range of visible and UV wavelengths. Pulps with substantial lignin content can be dissolved readily. Although dissolution of spruce wood has been claimed [12], in our experience, lignin does interfere with solubility. We have successfully dissolved pulps with kappa numbers greater than 70, but the time required for dissolution increased.

Reflectance UV Visible Spectroscopy

With any solvent system, the question of what effect the solvent might have on lignin or other pulp components remains unanswered. A noninvasive analytical method would be preferable.In principle, reflectance spectroscopy is such a method. Although most pulps contain components that are susceptible to photochemical transformation, these changes are unlikely to be significant on the time frame necessary for acquisition.

Reflectance spectroscopy has been extensively investigated for application to paper. The ratio of reflected to incident light is determined by the Rayleigh scattering and absorption of the paper under investigation. The light-scattering coefficient S and the absorption coefficient K are related to the reflectance of an infinitely thick stack of paper by the remission function of Kubelka and Munk [15,16]. The absorption coefficient, K, is the value that provides information about the absorbing characteristics of the material in the sample. In paper science, the Kubelka-Munk theoryis commonly used to determine S and K, either from two reflectance measurements or one reflectance and one transmission measwement [17]. In the present study, we used two reflectance measurements, the method of white and black backing [18,19].

Although attractive as a noninvasive technique, complications arise because of sample heterogeneity. Here, as in DRIFT, the dependence of Rayleigh scattering coefficients on wavelengths must be considered. More problematic, the theoretical treatment for determination of K becomes less reliable in regions of highlight absorption. Below 350 nm, where lignin absorbs strongly, the absorption coefficients become less reliable. Various postulates have been put forth 10 explain the reasons for the deviation from theory, but a satisfactory theoretical treatment has not been advanced. Nonetheless, even for high lignin content samples such as mechanical pulps, reflectance spectroscopy can providechemical information in the near UV and visible regions of the spectrum where absorption is less intense. For example, using low basis weight handsheets (i.e., 10 g/m2) and wavelengths greaterthan 300 nm, it is possible to obtain measurements with highreproducibility [19,20].

Polyoxometalate Bleaching

In delignification, as in many industrial and biochemical processes, the limitations apparently inherent in the use of oxygen and peroxides can be overcome by the introduction of appropriate soluble catalysts. This is what occurs in nature when wood-rotting fungi attack wood. These fungi use enzymes to catalyze the degradation of lignin by oxygen or hydrogen peroxide. The active sites of these enzymes are metalloporphyrins that have served as models for the preparation of metallo-organic (biomimetic) catalysts. Although effective, these synthetic metallo-organiccatalysts are expensive and susceptible to degradation during bleaching. For large-scale commercial bleaching, we sought more stable synthetic catalysts that could be prepared easily from inexpensive, nontoxic materials. The polyoxometalates are the synthetic catalysts that we have identified [21].

Polyoxometalate complexes, wherein the porphyrin ligand is replaced by an entirely inorganic polyoxometalate ion, are perhaps the most promising catalytic materials currently available for applications in bleaching [22-24]. They include a wide variety of water soluble, inorganic compounds, and provide transition metal ion coordination sites that are structurally similar to the coordination sites of natural and synthetic porphyrins. However, unlike metalloporphyrins, polyoxometalates are easily preparedfrom inexpensive, nontoxic mineral ores, and are remarkably stable to oxidizing conditions. We began working with several polyoxometalates early in 1992, in a close collaboration with Professor Craig Hill, Emory University, Atlanta. One class of polyoxometalates, the mixed addenda heteropolyoxometalates, could very well make it possible to replace chlorine compounds with the least expensive oxidant available: air. Other polyoxometalate complexes, designed to catalyze the activity of hydrogen peroxide or other peroxide compounds, are also under investigation. Still in the early stages of development, the polyoxometalate bleaching process used in our study appears to be as effective as a typical chlorine and extraction (CE) sequence in the delignification of softwood kraft pulp.

The polyoxometalate material used in this study was Ŭ-Keggin-K5[SiVW11O40] (1), a water soluble potassium salt of the monovanadium derivative of the tungstosillate Ŭ-Keggin-[SiW12O40]4- [25]. For effective bleaching, it is essential that the metals present in this material (tungsten and vanadium) be in their highest (do) electronic states. In the bleaching step. designated 'Vi

to represent the vanadium substituted polyoxometalate, mixtures of water, inorganic buffer, pulp, and the fully oxidized polyoxometalate are heated in a sealed vessel under nitrogen. During the reaction. the polyoxometalate is reversibly reduced as susceptible functional groups within the residual lignin in the pulp are oxidized. This leads to selective functionalization, 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 process concepts, will be published elsewhere.

EXPERIMENTAL

General Methods

The unbleached mixed-pine kraft pulp used in this study, provided by Consolidated Papers, Inc.,2 Wisconsin Rapids, Wisconsin, had a kappa number of 33.6 (approximately 5.6% lignin as kappa no. ÷ 6), an intrinsic viscosity of 34.2 mPa•s, and a brightness of 26.3. Microkappa numbers were obtained using Technical Association of the Pulp and Paper Industry (TAPPI) useful method um-246; handsheets were prepared by adaptation of TAPPI test method T218 om-83; pulp viscosities were obtained using TAPPI test method T230 om-89. A Technidyne Technibright TB-1 instrument was used to obtain reflectance measurements from single handsheets using both black and white backings. For each handsheet, black background reflectance values (Ro) and white background reflectance values (Rw) were obtained by averaging measurements from several spots. Brightnesses, reported as reflectance values (RÐ), were then calculated using the method of white and black backgrounds, as described later for reflectance spectroscopy samples. The backgrounds affected only the readingsof significantly brightened pulp samples.

Bleaching

The unbleached kraft pulp, as received from Consolidated Papers,Inc., was pretreated with a mild anaerobic alkaline extraction, designated Eo, to remove any easily solubilized lignin. To observe the progress of the bleaching reaction, subsequent polyoxometalate treatment was divided into three batch oxidations, V1, V2, and V3. A control sequence was done in parallel. Controls for the V stages were obtained by heating mixtures of pulp, water, and inorganic buffer with no polyoxometalate present. These heated control stages are designated by the symbol delta,. æ. Final brightening was achieved by treatment with hydrogen peroxide (P stage). The full bleaching and control sequences were designated EoV1V2V3EV4EP and Eoæ1æ2æ3Eæ4EP. Note: The designation Eo, should not be confused with its usual indication of an oxygenreinforced extraction. Individual pulp samples were identified with respect to V or æ stages (i.e., EoV1V2 is denoted 'V2'; EoV1V2V3E is denoted 'V3E'). The full bleaching sequence was performed three times and the control sequence once.

The preliminary and later E stages were performed using 1% NaOH and pulp consistencies (csc) of 1% to 2% for 2 h at 85°C under nitrogen. After each E stage, the pulps were collected in a Büchner funnel and washed once with 1% NaOH and three more times with water. The three subsequent V stages were performedunder 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 M solutions of Ŭ-Keggin-K5[SiVW11O40] (1) in 0.2 M pH 7 phosphate buffer. After purging with nitrogen, the reactor was heated to 125°C for 2 h. The reactor pressure was sustained with 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 yellowcolor of fully oxidized 1 to the dark purple of the reduced material. After each V stage, the pulps were collected in a Büchner funnel and washed three times with water. The fourth V stage was carried

2The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.

out as previously described with one exception. After 1 h at 125°C. 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. Final brightening was achieved using hydrogen peroxide. To remove trace metals, the pulps were soaked at room temperature in 0.5 weight percent solutions of sulfuric acid at 8% csc for 15 min and washed thoroughly with water. The bleaching mixtures consisted of 1.5% H2O2, 4% 41.5°B sodium silicate. 0.1% Mg2+

using the sulfate salt and 2.5% NaOH, all values being weight percentages on pulp. These mixtures were combined with pulps to a csc of 12% kneaded together in a polyethylene bag, and placed in an 80°C water bath for 2 h. The pulps were then soaked in a 0.1 weight percent solution of sodium bisulfite for 10 min and washed with water.


Spectra were recorded using a Perkin-Elmer Lambda 6 spectrophotometer. Phosphoric acid (83%) was prepared from reagent grade 85% acid. The absorbance of the diluted acid, obtained against water, should be less than 0.1 at 280 nm.

Pulps to be analyzed (about 10 mg) were thoroughly dried under vacuum at ambient temperature, weighed immediately, and mixed with 8 mL of 83% phosphoric acid in 25 mL Erlenmeyer flasks. The mixtures were stirred rapidly, but not so fast that stable foams were created. Complete pulp solution required from 2 to 16 h. The solutions were then quantitatively diluted with additional 83% phosphoric acid to a final volume of 10.0 mL. Spectra were obtained as soon as possible after dissolution of the pulp to minimize the slow formation of chromophores generated from reaction of the carbohydrates with phosphoric acid. Absolute absorbance values were converted to absorption coefficients, reported in units of liters per gram per cm (L/g•cm) based on pulp.Lignin concentrations were calculated using an absorptioncoefficient for kraft pulp residual lignin of 20 L/g•cm. This value was previously determined from kraft pulps containing varying amounts of lignin [13].


Small handsheets with basis weights of approximately 18 g/m2

were prepared in a Büchner funnel. Reflectance measurements were taken in a Perkin-Elmer Lambda 6 spectrophotometer fitted with a Labsphere RSA-PE-60 reflectance attachment. Measurements were taken over white and black backgrounds. The data was exported to a Lotus 1-2-3 spreadsheet for calculations. Values of RÐ, the reflectance of paper over a background of the same material of such thickness that the supporting background has no optical effect, were calculated using

where W is the reflectance of the white background alone. Ro is the reflectance over a black background, and Rw is the reflectance over a white background [19]. Scattering coefficients, S, were calculated using

where B is the basis weight of the sample. Kubelka-Munk absorption coefficients. K. were then calculated from the remission function K/S = (1-RÐ)2/2RÐ.

NIR FT Raman Spectroscopy

Pulp mats for study were prepared by gently compressing 20 mg portions of airdried pulp fibers in a KBr pellet press. Raman spectra were obtained using a Bruker IFS 66/FRA 106 system

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equipped with a 350 mW (1,064 nm) diode pumped laser. The pulp mats were sampled in the double sided, forward-backward scanning mode using the 180 degree Raman scattering geometry with a minor behind the sample for signal enhancement. Spectra were acquired at 300 mW of laser power and at 4 cm-1 spectralresolution, and corrected for instrument response and wavenumber dependence of the Raman scattering. Data acquisition time per spectrum was approximately 10-15min.

Lignin content was calculated by measuring changes in the 1,595 cm-1 band (1,671-1,545 cm-1), associated with the aromatic stretch of phenyl groups within the residual lignin. Spectra acquired in all but the final stages of the process included fluorescent backgrounds. Thus, for quantitative comparison, band areas were calculated as the peak above the baseline created by the fluorescence. For quantification, the band of interest must be compared to one that remains constant throughout the bleaching process. The cellulose band structure between 1,216-1,010 cm-1

was chosen for this purpose. Using these bands, changes in lignin content were quantified by two methods. In the first, integrated areas of the 1,595 cm-1 band were ratioed against integrated areas of the band structure between 1,216-1,010 cm-1. In the second, the peak heights of the 1,595 cm-1 band were ratioed against those of a cellulose band at 1,098 cm-1. Although spectra were acquired in the region 75-3,500cm-1, only the region from 800-1,800 cm-1 is shown in the figures.

DRIFT Spectroscopy

DRIFT spectra were obtained from handsheets, prepared byadaptation of TAPPl test method T218 om-83, sampled on a Mattson Galexy Series FTIR 5000. The carbohydrate band at 2,905 cm-1 was used as an internal reference. Because the amount of residual lignin was low (under 6% in all cases), only weak IR bands were observed. Where present, the usually strong lignin band at 1.57 10 cm-1 [26], arising from asymmetrical stretchingmodes of phenyl groups, was detected as a weak shoulder. Because this shoulder occurred on a steeply rising contribution from a neighboring IR band, quantification of the 1,510 cm-1 band was not possible. Instead, relative intensity changes were measured at 1,600 cm-1. Because this band overlapped with that arising from adsorbed water at 1,640 cm-1, quantitative information was obtained by ratio of the peak height at 1,600 cm-1 to that at 2,905 cm-1.

RESULTS AND DISCUSSION

Polyoxometalate Bleaching

Microkappa numbers, intrinsic viscosities, and Rm brightness measurements for the control and three repetitions of the polyoxometalate bleaching sequences are given in Table I. Microkappa numbers and intrinsic viscosity values are shown in Figure 1. Microkappa numbers were determined for the first five stages of the polyoxometalate sequences and for all stages of the control sequence. The following spectroscopic studies were carried out using pulps obtained from the control sequence and from trial 1 of the polyoxometalate bleaching sequences.

One notable feature of the polyoxometalate bleaching sequences was the high selectivity demonstrated in stages Eo through V3E. Kappa numbers and pulp viscosities were at least as good as those obtained in traditional CE sequences. A second feature of the polyoxometalate sequences was the decrease in brightness that occurred during the early stages of delignification (Fig. 2). After V1, the pulp was darker and possessed a reddish-orange hue not seen in the original brownstock. After a decrease in lignin content of more than 50% after V2, the pulp remained darker than it did after Eo or after the parallel control, æ2 (Table 1, entries Eo and æ2

and trial 1, V2). This feature, common to the present and other polyoxometalate bleaching systems, reflects the functionalization of susceptible structures within the residual lignin that occurs along the path towards lignin fragmentation and solubilization.

The FT Raman and solution UV visible spectra of pulp samplesexamined early in the bleaching process provided some detailed information regarding these chemical changes. Analysis of these chemical changes is described after the presentation of spectroscopic data that immediately follows. As the lignin content decreased and brightening began, less detailed chemical information was provided by FT Raman and solution UV visible spectra. Nonetheless, these techniques continue to providequantitative measures of lignin content. The spectroscopicquantification of residual lignin, and the complications that can arise from lignin functionalization during bleaching are addressed later in this section.


Solution UV visible spectra of pulps from the polyoxometalate sequence are presented in Figure 3. Because of the undefined nature of the pulps, the absorption coefficients 'a' were calculated on a per weight of pulp basis and reported in units of liters per gram per centimeter (L/g•cm). Lignin content was estimated from the aborption maximum at 280 nm. The largest decreases in lignin content occurred during the polyoxometalate stages V1, V2, V3, and V4. The final two stages V4E and P were essentially brightening treatments. In contrast, spectra of pulps from the control sequence (Fig. 4) demonstrated that the conditions of the bleaching experiment alone have little effect on the UV visible absorbing materials initially present in the unbleached brownstock (UB).

An interesting feature of the V1 treatment can be seen in Figure 5. For greater clarity, only the stages Eo through V3 are included. Absorbance at smaller wavelengths (210-240nm) decreased substantially with each polyoxometalate treatment. However, earlyin the process (Fig. 5, V1) absorbance at wavelengths greater than 250 nm increased as new chromophores were generated from substructures within the residual lignin. The increase in absorbance was particularly apparent in a comparison of V1 (microkappa number 23.1) with its parallel control, æ1 (microkappa number 31.1) (Fig. 6).


Results from reflectance UV visible spectra of selected stages, E,,, V1, V3, and P, of the polyoxometalate sequence are presented in Figure 7. Note that from 375-700nm, values of K calculated for V1 are greater than those of the preceding Eo stage. Over the same region, values of K for V2 (omitted for clarity) are closely coincident with that of Eo. These results were qualitatively similar to those obtained from transmission UV visible data, suggesting that the increases in absorbance seen in solutions of V1 and V2

were not attributable to reactions of partially oxidized lignin structures with phosphoric acid.

FT Raman Spectroscopy

The FT Raman spectra of UB, before and after mild alkaline pretreatment, Eo, are shown in Figure 8. Spectra of the full polyoxometalate and control sequences are presented in Figures 9 and 10 (trial 1, polyoxometalate sequence) and Figures 11 and 12 (control sequence). The band centered at 1,595 cm-1 (1,671-1,545 cm-1) is associated with symmetric aromatic stretching modes of phenyl groups within the residual lignin. The bands at less than 1.500 cm-1 were attributable primarily to cellulose. Little change in the 1.595 cm-1 band occurred upon mild alkaline pretreatment(Fig. 8. Eo). Upon initial polyoxometalate treatment (Fig. 9, V1),


the 1,595 cm-1 band broadened and shoulders were observed. The new contributions, although weak, were clearly evident at approximately 1,550 cm-1 and also in the region 1.650-1.680cm-1. These features, observed after the first V stage, are highlighted bycomparison of V1 with its parallel control, æ1 (Fig. 13).

Subsequent V stages (Fig. 9, V2 and V3) continued to remove lignin and attack the structures generated during the V1 stage. Upon extraction of the V3 pulp with alkali (V3E). further decline in the intensity of the 1,595 cm-1 band occurred. This decrease was most probably caused by the extraction of fragmented lignin from the carbohydrate matrix. After V4 (Fig. 10), the RÐ brightness of the pulp was 66.2 (Table I), and little evidence of aromatic structures remained. Although brightening the pulp, the final two stages, V4E and P, did not produce additional changes in the Raman. Throughout the control sequence (Figs. 11 and 12), little change in the 1,595 cm-1 band was observed.

DRIFT Spectroscopy

DRIFT spectra of the first six stages (Eo-V4) of the polyoxometalate (trial 1) and control sequences are presented in Figures 14 and 15. Because the amount of residual lignin was low (<6% in all cases), only weak IR bands were observed. The spectrum obtained after the Eo stage was essentially identical to that of the unbleached brownstock. and no further changes in either the polyoxometalate or control sequences were observed in the final stages, V4E or æ4E, and P. For this reason, spectra of the UB pulp and pulps recovered after the final two stages, V4E or C4E. and P are not shown.

The usually strong lignin band at 1,510 cm-1 [26], arising from asymmetrical stretching modes of phenyl groups. was detected as a weak shoulder in UB (not shown), Eo, and in all the control pulps (Figs. 14 and 15). As evident from visual comparison of Figures 14 and 15, phenyl groups were attacked during the V stages. This resulted in a rapid decline in the 1,510 cm-1 band. In contrast, the 1,600 cm-1 band, which represents phenyl and other related structures, appeared to decrease only after the second V stage (V2, Fig. 14). Although not evident from visual inspection, the peak height ratio of the 1,600 cm-1 band appeared to increase during the V1 stage (vide infra). However, as explained in the Background,the reliability of this quantitative measure was questionable. The rapid decline in the 1.510 cm-1 band during V1 and apparent initial increase in intensity of the 1.600 cm-1 band were consistent with the partial oxidation or functionalization of aromatic structures suggested by UV visible and FT raman. Decline in the 1,600 cm-1

band during the V2 stage, apparent by visual inspection, Suggestedfurther degradation of structures generated during V1. After the V4

stage, the intensity of the 1.5 10 cm-1 band was nearly zero. Residual intensity observed at 1,600 cm-1 was due to the decaying wing of the 1,640 cm-1 band (bending mode of water). DRIFT analysis, as in FT Raman analysis, provided no evidence of residual lignin after the V4 stage.

Chemical Changes in Residual Lignin During Polyoxometalate Bleaching

In native softwood lignin. approximately 1 in 10 substructural phenyl propane units contains phenolic groups and many ɓ-arylether structures posses a-hydroxyl moieties in the propyl side chain. Cleavage of aryl ether linkages during kraft pulping likelyleads to an increase in the frequency of phenolic groups. In addition, recent work has shown that a considerable amount of uncleaved ɓ-aryl ether and other structures found in native ligninsurvive the pulping process intact [27]. Many of these are likely to possess Ŭ-hydroxyl moieties in the propyl side chain.

The presence of phenolic and α-hydroxyl moieties was likely necessary for the fragmentation and solubilization of the residual kraft lignin observed in the present study. According to recentlypublished reports, polyoxometalates closely related to that used here are efficient catalysts for the aerobic oxidation of both phenols and benzylic alcohols. In organic solvents, the vanadium substituted molybdophosphate compound H5[PV2Mo10O40] (2) and its sodium salt Na5[PV2Mol0O40] oxidize activated phenols to quinones [28,29] and benzylic alcohols to Ŭ-carbonyls [30]. We observed analogous results with a few simple lignin models. In preliminary studies in water, simple phenolic lignin models were readily oxidized by 2. In addition, nonphenolic Ŭ-hydroxylcontaining models, such as veratryl alcohol (3,4-dimethoxybenzyl alcohol) [31] and 1-(3,4-dimethoxyphenyl)ethanol, were oxidized to the corresponding a-ketones as the major products [32]. To our knowledge, no study Concerning the oxidation of organic compounds by α-Keggin-K5[SiVW11O40] (1) has been reported.However, it is likely that the oxidation of phenols and benzylic alcohols by 1 will parallel that of 2, giving rise to quinones and a-ketones. Moreover, it is plausible that in the oxidation of polymericresidual kraft lignin by 1, intermediate single-electron steps similar to those suggested for the production of quinones and a-ketones by 2 [29], will lead to lignin fragmentation and solubilization.

Of the techniques previously discussed, FT Raman and solution UV visible spectroscopy provided the most detailed information concerning chemical changes in residual lignin. When used together, these techniques provided strong support for chemical changes suggested by the model studies just described.

The Raman scattering centered at 1,595 cm-1 represents symmetricstretching modes of phenyl moieties in the residual lignin. During the first V stage, this band broadened considerably and new shoulders were observed at approximately 1,550 cm-1, between 1,590-1,620 cm-1, and in the region 1,650-1,680 cm-1 (Fig. 13). Some of these new features can be identified by reference to the Raman scattering characteristics of known lignins and lignin model compounds.

Raman bands associated with lignin have recently been identified and the prominent bands assigned [5,33]. In our group, a largenumber of additional lignin models have been studied. Such studies have been used to identify lignin related bands in the spectra of lignocellulosics and to characterize the changes that occur when lignin-rich materials are subjected to various treatments [5,31,34,35]. Raman band assignments of lignin and models. selected from data obtained in our laboratory, are listed in Table II.

The Raman contribution at 1,550 cm-1 was likely due to the formation of ortho-quinones in the pulp. In the Raman spectrum of 3-methoxy-ortho-quinone, an intense Raman band was found to be present at this wavenumber (Table II) [6]. Data in Table II further suggest that the contributions in the region 1,650-1.680 cm-1 were likely due to the formation of α-carbonyl and para-quinone structures. As a result of unidentified shoulders between 1.590 cm-1 and 1,620 cm-1, a general broadening of the primaryband was observed. The band shifted to higher energy by approximately 5 cm-1 relative to the control (Fig. 13). Manydifferent phenyl units are present at this stage in bleaching. Functionalization of these units, or changes in their substituents, could give rise to increases in Raman scattering coefficients and/orsmall shifts in the frequencies of the symmetrical stretching modes usually observed at 1,595 cm-1.

Upon initial treatment with the polyoxometalate (V1), the pulpdarkened, took on a reddish-orange hue, and significant changes were observed over the entire UV visible region. As a tool for the characterization of chemical changes in residual lignin, UV visible


spectroscopy by itself is limited. Many chemical structures absorb in similar spectral regions, and vibronic coupling gives rise to broad overlapping bands. Nonetheless, lignin model studies and FT Raman data both point to the formation of quinones (most likelyorthoquinones from the oxidation and hydrolysis of phenolicguaiacyl units) and Ŭ-ketones. Structures of this type, which absorb strongly in the near UV and visible regions, might account for the increase in absorbance observed in spectra of V1 and V2 pulps.

Figure 16 shows difference spectra obtained from solution UV visible absorption coefficient data. The three plots were calculated by subtracting spectra of the UB, the alkaline pretreated (Eo), and parallel control (æ1) pulps from the spectrum of the V1 stage pulp.In all cases, the V1 pulp absorbed more strongly at wavelengths greater than 300 nm. New intensity was also observed at 260-270nm, a region near where one ortho-quinone model, 3-methoxy-ortho-quinone had a maximum in 83% phosphoric acid (Fig. 17); 3-methoxy-ortho-quinone also absorbed across the visible region. It had a dark red color in dilute solution. reminiscent of the reddish hue acquired by pulp during the V1 stage. The oxidation of Ŭ-hydroxyl lignin models to their corresponding Ŭ-ketones can result in an increase in absorbance in the near UV. Both acetovanillone and its methylated derivative dimethoxyacetophenone absorbed strongly in the near UV region with maxima between 300 and 350 nm in 83% phosphoric acid. Thus. the formation of α-ketones may have contributed to the local maximum observed at approximately 325 nm in V1-æ1 (Figs. 16 and 17).

Spectroscopic Quantification of Residual Lignin

Residual lignin in kraft pulps is generally measured titrametricallyand reported as kappa numbers. To evaluate if FT Raman might fill this need, Raman data was plotted against lignin content. Solution UV visible data were similarly evaluated and compared to the Raman results. The quantification of DRIFT and reflectance UV visible data are discussed in relation to FT Raman and transmittance UV visible techniques.

FT Raman and DRIFT spectroscopy

Quantification of Raman data is described in the Experimentalsection. Both peak height and integrated area ratios were calculated. It was anticipated that the use of peak ratios mightavoid Raman contributions arising from nonaromatic structures in the pulps. However, we found that the integrated area ratio method worked best. Quantification of the DRIFT results, by ratioing the peak height at 1,600 cm-1 to that of a carbohydrate band at 2,905 cm-1, was less successful.

Raman and IR peak height and area ratios calculated for trial 1 of the polyoxometalate and the control sequences are presented in Table III. Some variability in the Raman ratios calculated for the control sequence was observed. However, because only one control sequence was analyzed, it was not possible to determine whether this reflected experimental uncertainty or was caused by the conditions of the control stages. The same was true for the apparently elevated Raman peak ratio calculated for Eo. The 1,600 cm-1 IR band intensity values for samples from the control sequence included considerable variability, a substantial amount of which was likely inherent in the DRIFT technique. FT Raman data for the same pulps showed much less variability. A similar difference in variability between FT Raman and DRIFT data was seen in the polyoxometalate sequence ratios. Raman peak height ratios and integrated area ratios from Table III, against lignin 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,

and a correlation coefficient (R2) of 0.960. The data in Figure 19 give a slope of 8.03 x 10-3 ±5.21 x 10-4, a y-intercept of 5.81 x 10-3 ±1.15 x 10-2, and an R2 value of 0.983. In Figure 18, the peak height ratio for the V1 stage 1,595 cm-1 band appears noticeablyhigh. Although experimental uncertainty present in the Raman data has not been precisely determined, the elevated Raman intensity at V1 may reflect the production of substituted or functionalized phenyl groups with higher scattering coefficients. In the integrated area ratios in Figure 19. V1 does not appear markedly elevated. In addition, uncertainty in the slope of the integrated area ratio is 26.50%. while that for the slope of the peak height ratio is ±10.3%. From these results, it appears that integrated area ratios may better represent the concentration of phenyl groups.

Noteworthy in both Figures 18 and 19 is the low value of the y-intercept. This demonstrates that the lignin band at 1,595 cm-1 was made up exclusively of contributions from phenyl groups and closely related structures. As a result, the Raman technique can be used for rapid lignin quantification without the need for empirically determined zero-offset values.

Transmittance and Reflectance UV Visible Spectroscopy

Lignin content was estimated from solution UV visible data using an absorption coefficient of 20 L/g•cm at 280 nm (seeExperimental). In principle, the Kubelka-Munk absorption coefficients, K, were also proportional to the concentration of absorbing material after normalization to a constant basis weight. However, as discussed in the Background section, the proportionality does not hold in regions of high absorption (i.e., at 280 nm). Moreover, 18 g/m2 handsheets were used in the present work. As a result, at wave numbers less than 350 nm RÐ approached Ro and the absorption coefficients obtained were unreliable.

Lignin contents estimated from solution data for each stage of the polyoxometalate and control sequences, are reported as average percentage lignin in Table IV. Standard deviations are given for 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 kappa numbers of ±1.32. The high degree of reproducibility is typical of this technique.

Lignin estimates calculated for the control pulps match well with the lignin values determined from microkappa numbers. In contrast, estimates calculated for the trial 1 pulps were all quite high, clearly exceeding experimental uncertainty (vide infra). In addition, lignin content estimates for pulps from the final three stages, V4, V4E, and P, were nearly identical and did not decrease to zero as might be expected. These nonzero values likely reflected contributions arising from reactions of carbohydrates with phosphoric acid. If so. it is not clear why these same contributions did not appear as a systematic error in estimates of lignin content for the control pulps.

In the bleaching sequence, estimated lignin content decreased significantly during each of the four V stages. Figure 20 presentsestimated lignin (UV % lignin) against lignin calculated from microkappa numbers (microkappa % lignin).

The line in Figure 20 has a slope of 0.797 ±0.0971, a y-intercept of 1.23 ±0.359, and an R2 value of 0.944. As mentioned, the elevated lignin estimates for both the V1 and V2 pulps exceeded experimental uncertainty (Table IV). One explanation is that UV


and near UV bands of strongly absorbing materials generated earlyin the polyoxometalate treatment had significant tailings at 280 nm (Fig. 16). As a result, uncertainty in the slope of the UV (±0.0971 or 12.2%, Fig. 20) exceeded that calculated for Raman peak height ratios (10.3%, Fig. 18) or integrated area ratios (6.50%. Fig. 19).This observation points out the need to consider the impact of chemical changes on the spectroscopic quantification of residual lignin.

Unlike the Raman intensity (Figs. 18 and 19), the UV lignin has a nonzero y-intercept. In Figure 3, little decline in absorption at 280 nm is observed during the final three stages, V4, V4E, and P. This residual intensity accounts for the nonzero lignin estimates of 0.43%, 0.44%, and 0.41% reported for these samples in Table IV. 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%. Although still high, this agrees closely with results obtained in solution UV visible quantification of residual lignin in kraft pulpssubjected to more conventional chlorine extraction sequences [1 3,36].

CONCLUSIONS

NIR FT Raman, DRIFT, solution UV visible, and reflectance UV visible spectroscopy were used to observe chemical changes in residual kraft lignin during interrupted stages of a new polyoxometalate bleaching system. Used in combination, FT Raman and solution UV visible spectroscopy provided detailed information regarding some chemical changes possibly occurringin the residual lignin during bleaching. Evidence for the formation of ortho-quinones from phenols and Ŭ-ketones from Ŭ-hydroxyphenyl (benzylic alcohol) groups was detected by FT Raman spectroscopy and supported by reference to Raman spectraof model compounds, by lignin model oxidation studies, and by solution UV visible data. DRIFT spectroscopy, as a result of weak intensities of the major lignin bands and overlap of these bands with contributions from adsorbed water and carbohydrates,provided little detailed chemical information. Reflectance UV visible data, although qualitatively similar to transmittance UV visible data, failed to provide the detail observed using the solution method. Additional detailed information concerning lignin oxidation and fragmentation by polyoxometalates is the topic of planned lignin model oxidation studies.

FT Raman and solution UV data provide 3 means of quantifyingresidual lignin in kraft pulp. The UV method is highly reproducibleand provided us with accurate estimates of lignin in pulps containing 4% to 5% lignin. It has also been used effectively in the quantification of residual lignin during stages of a conventional chlorine extraction bleaching sequence. However, in the presentwork, the production of new UV and near UV absorbing materials during the early stages of bleaching complicated the quantification. In addition, accurate quantification requires the empirical determination of baseline offset values. This and other aspects of solution UV visible analysis are under investigation.

FT Raman spectra also reflected the production, early on in bleaching, of new functional groups. However, accurate quantification was still possible. Ratios of 1,595 cm-1 (phenyl)band peak heights and integrated areas against an internal carbohydrate reference band were calculated. Possible evidence for complications as a result of the formation of new structures with increased Raman scattering coefficients at 1,595 cm-1 was observed in the peak height ratio analysis. However, these complications were not observed using integrated area ratios, from which precise quantification was possible.

As a result of the nature of Raman scattering, pulp fibers can be analyzed without regard to optical inhomogeneity. In addition, the intensity of the lignin band at 1.595 cm-1. made up exclusively of contributions from phenyl groups and closely related structures, provides a direct measure of lignin content. As a result, the FT Raman technique can be used for rapid lignin quantification.

ACKNOWLEDGMENTS

We thank Nancy T. Kawai of Bruker Instruments, Inc. for acquiring the FT Raman spectra and Matthew A. Smith for obtaining the UV visible data. We also thank Sally A. Ralph for graphical assistance and Kolby Hirth for acquisition of the DRIFT Spectra.

REFERENCES

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5 Agarwal, U.P.. and Atalla. R.H., "Raman Spectroscopic Evidence for Coniferyl Alcohol Structures in Bleached and Sulfonated Mechanical Pulps," In: Photochemistry of Lignocellulosic Materials, Eds. C. Heimer and J.C. Scaiano, ACS Symposium Series 531, American Chemical Society, Washington DC, Chap. 2. 1993.

6. Agarwal, U.P., Unpublished results, 1992.

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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).

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33. Woitkovich, C.P., MS Thesis. "A Raman Spectroscopic Study of the Early Phase Acid-Chlorite Delignification of Loblolly Pine," Institute of Paper Chemistry, Appleton, Wl [Institute of Paper Science and Technology, Atlanta, GA], 1988.

34. Agarwal, U.P., Atalla, R.H., and Forsskahl. I., "Sequential Treatment of Mechanical and Chemimechanical Pulps with Light and Heat. Part 3- A Raman Spectroscopic Study," (in progress).

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Table I Bleaching results using Ŭ-Keggin- K5[SiVW11O40].a

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 æ3E) are shown.


Figure 2. RÐ 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 Eothrough V3 of the polyoxometalate bleaching sequence.

Figure 6. Comparison of solution UV visible spectra of V1 stage Figure 8. FT Raman spectra in the region 800-1,800.cm-1 ofand parallel control æ1 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 stagesEo, V1, V3, and P of the polyoxometalate bleaching Figure 9. FT Raman spectra in the region 800-1.800 cm-1 of sequence. pulps at stages V1 through V3E of the polyoxometalate

bleaching sequence.


Figure 10. FT Raman spectra region 800-1,800 cm-1 of pulps at the final stages V4 through P of the polyoxometalate bleaching sequence.

Figure 11. FT Raman spectra in the region 800-1,800 cm-1 of pulps at stages æ1 through æ3E of the control sequence.

Figure 12. FT Raman spectra in the region 800-1,800 cm-1 of pulps at the final stages A4 through P of the control sequence.

figure 13. Comparison of FT Raman spectra in the region 1,200-1,800 cm-1 of V1 stage and parallel control æ1 stagepulps.


Figure 14. DRIFT spectra in the region 1,450-3.150 cm-1 of the first six stages (Eo through V4) of the polyoxometalatebleaching sequence.

Figure 15. DRIFT spectra in the region 1.450-3,150 cm-1 of the first six stages (Eo through æ4) of the control sequence.

Figure 16. Difference spectra from solution UV visible absorption coefficient data. Calculations made by subtracting spectra of unbleached brownstock (UB), alkaline pre-mated pulp (Eo), and parallel control æ1 stage pulpfrom the spectrum of the V1 stage pulp.

Figure 17. Arbitrary absorbance units comparing the solution UV visible difference spectrum V1-æ1 with the spectrum of 3-methoxy-ortho-quinone dissolved in 83% phosphoric acid.

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Figure 18. 1,595 cm-1 Raman band intensities calculated as peak Figure 20. Lignin estimates from solution UV (280 nm) absorptionheight 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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