Furman, Gary S.

Lonsky, Werner F. W.

Scholarly Materials And Research @ Georgia Tech

©g,~O ©THE INSTITUTE OF PAPER CHEMISTRY, APPLETON, WISCONSINIPC TECHNICAL PAPER SERIESNUMBER 174CHARGE-TRANSFER COMPLEXES IN KRAFT LIGNIN.PART 2: CONTRIBUTION TO COLORG. S. FURMAN AND W. F. W. LONSKYMAY, 1986Property ofGEORGIA-PACIFIC CORPORATIONTechnical Information CenterAtlanta, GeorgiaDedicated to Prof. Dr. Karl Kratzl on the Occasion of his 70th BirthdayCHARGE-TRANSFER COMPLEXES IN KRAFT LIGNINPART 2: CONTRIBUTION TO COLORG. S. Furman* and W. F. W. LonskyThe Institute of Paper ChemistryAppleton, Wisconsin 54912ABSTRACTChelation and removal of transition metals from the kraft ligninemployed in this study did not reduce the lignin's visible absor-bance. Likewise, hydrogenation of carbon-carbon double bonds inthis lignin also had no effect on its visible spectrum. Thisindicated that extended conjugated systems were not a significantcontributor to the color of this lignin. Of the structuresinvestigated, quinones were found to be the major visible-lightabsorbing chromophores. Since a large component of the quinone'sabsorption results1from their participation in charge-transfercomplexes (CTC's), CTC's were a significant contributor to thecolor of this lignin.INTRODUCTIONIn the preceding publication, the existence of charge-transfercomplexes (CTC's) in kraft lignin was demonstrated. This paper willestablish the significance of the CTC's contribution to the colorof kraft lignin in comparison to other colored structures which havebeen proposed or identified in kraft lignin. These structures in-cluded quinones, transition metal complexes, and various unsaturatedstructures (carbon-carbon double bonds and carbonyls), which throughconjugation are thought to form extended conjugated systems.G. S. Furman's present address is: Kimberly-Clark Corporation,Neenah, WI 54956-1-RESULTS AND DISCUSSIONTransition Metal ComplexesVarious transition metals, iron being the most abundant, arefound in kraft lignin. In part they originate from the originalwood source, and in part they become associated with the ligninduring its contact with process waters and machinery. Some metalsare able to form colored complexes with phenolic and catecholic2 3structures in the lignin. 2,3 For example, complexes of ferricions with model lignin catechols have absorption maxima betweenapproximately 550 and 590 nm.The contribution of metal complexes to the visible absorptionspectrum of the present kraft lignin was investigated by the remov-al of these metals. This was accomplished by their chelationwith ethylenediaminetetraacetic acid (EDTA), followed by the extrac-tion of the metal chelates from the lignin via electrodialysis.The average contents of six transition metals found in thislignin are given in Table 1. Iron was the most prevalent metalfound, although its content was relatively low when compared toindustrially obtained kraft lignins. Also given in Table 1 arethe reduced levels of metals found after the lignin was treatedwith EDTA. The values shown were the lowest levels obtained foreach metal, compiled from several different experimental runs.Table 1. Metal content of kraft lignin.Quantity Found (ppm)Metal Original Kraft Lignina EDTA Chelated Kraft LigninbChromium 8.18 ± 1.64 3.2Manganese 0.55 ± 0.18 < 0.002Iron 37.67 ± 12.0 9.33Cobalt 0.70 ± 0.64 < 0.05Nickel 7.14 ± 1.71 2.0Copper 8.47 ± 0.81 2.85aAverage of three determinations.Best values.-2-Spectra of the lignin before and after metal removal are shownin Fig. 1. The particular lignin shown in Fig. 1 was treated withsodium borohydride and diimide (see below) prior to the EDTA treat-ment. Both of these treatments had no effect on the metal contentof the lignin. As is shown in Fig. 1, removal of the metals didnot result in a decrease in the lignin's absorbance above 500 nmand, in fact, a slight increase at shorter wavelengths wasobserved. The absorbance increase at shorter wavelengths wasattributed to the reoxidation of some of the borohydride reducedquinones during the lengthy electrodialysis procedure.[Fig.1 here]For comparison, an industrial kraft lignin (Indulin AT, West-vaco, S.C.) was also investigated. This lignin contained substan-tially higher levels of metals, as is shown in Table 2. Onceagain, the EDTA-electrodialysis treatment was able to signifi-cantly reduce these levels.Table 2. Metal content of Indulin AT.Quantity Found (ppm)Metal Before Removal After RemovalChromium 2.34 1.71Manganese 56.7 0.09Iron 176 8.28Cobalt 1.14 0.81Nickel 2.78 1.56Copper 1.26 0.72Visible spectra of the Indulin AT before and after the EDTAtreatment are given in Fig. 2. For this lignin, removal of themetals produced a small, but clearly evident decrease in the longwavelength region of the spectrum centered at approximately 520 nm(Fig. 3). The results obtained with the Indulin AT agreed well4with the earlier published data of Meshitsuka and Nakano, whofound EDTA treatment of a thiolignin produced a decrease in the-3-lignin's absorbance centered at 500 nm. The present absorbancedecrease is also close to the spectral region indicated fromstudies of ferric complexes with model catechols.[Fig. 2 and 3 here]The different behavior of the laboratory and industrial kraftlignins indicated the possibility of a threshold level of iron,below which, removal of additional iron would have no beneficialeffect in terms of color reduction. However, addition of iron(400 ppm) to the EDTA-treated laboratory kraft lignin did notresult in an increase in its absorbance. Also important to con-sider are the types of chelation sites available in each particularlignin preparation. In this respect, chelation sites within theIndulin AT appeared to be the more effective color producers.Extended Conjugated SystemsThe color contribution from extended conjugated systems, of5the type proposed by Marton, was examined by hydrogenating theirconnecting carbon-carbon double bonds, thereby interrupting theconjugation in these systems. The hydrogenation was accomplishedwith diimide. Diimide, N2H2, was chosen over catalytic agents inorder to avoid the problems of catalyst poisoning, steric restric-tions involved with approach to the catalyst surface, and non-specificity of reductions.The presence of sulfur groups in a molecule does not affectdiimide's ability to function as a reducing agent. Being a smallmolecule, diimide should be able to approach all carbon-carbondouble bonds within the lignin macromolecule. Finally, diimidereduces only nonpolar C-C and N-N multiple bonds; polar functionalgroups, including carbonyls, are inert. Reviews of diimide and itsmechanism of hydrogenation have been published by Miller and byHunig and coworkers.-4-For the hydrogenation of kraft lignin, diimide was generatedin situ from the cleavage of meta-nitrobenzenesulfonyl hydrazide.This cleavage was catalyzed thermally and by the addition of asmall amount of base, as shown in Fig. 4. The solvent used forthe hydrogenation was 2-methoxyethanol.[Fig. 4 here]Diimide hydrogenation of the kraft lignin was monitoredspectroscopically. Additional charges of meta-nitrobenzene-sulfonyl hydrazide were added to the lignin solution until nofurther decreases in the lignin's UV spectrum were detected. Thismethod is illustrated by the difference curves in Fig. 5; thethird diimide treatment did not produce any further reduction inabsorbance over the second treatment. The maximum reduction inabsorbance occurred at approximately 340 nm.[Fig. 5 here]The shapes of the curves in Fig. 5 suggested more than onecarbon-carbon double bond type was hydrogenated. Of the doublebonds found in lignin, those conjugated with an aromatic ringabsorb near 300 nm, phenylcoumarones absorb near 310 nm, andstilbenes near 330 nm. Evidence was obtained from differencespectra of ionized lignins which indicated stilbene bond typeswere the most numerous of those removed. Subtraction of anionized, sodium borohydride reduced, and diimide hydrogenatedlignin from an ionized, sodium borohydride reduced lignin yieldeda difference spectrum having a maximum at approximately 366 nm.The absorption bands of ionized hydroxy stilbenes have beenreported to occur between 370 and 380 nm.Remarkably, even though the diimide hydrogenation resulted inthe large decrease in absorbance centered at 340 nm, no decreasein absorbance was detected in the visible range of the spectrum.-5-This result is shown by the spectra in Fig. 6. These spectraindicated extended conjugated systems do not contribute to thecolor of kraft lignin. In fact, such extended conjugated systemsare probably not present in kraft lignin.[Fig. 6 here]In some hydrogenations, the kraft lignin was first reducedwith sodium borohydride prior to treatment with diimide. Theresults from these hydrogenations were similar to those justdescribed.The combined effect of borohydride reduction and diimidehydrogenation on the lignin's UV spectrum is shown in Fig. 7 and8. As can be seen, hydrogenation had a substantially greatereffect in reducing the lignin's absorbance. Overall, there waslittle absorbance left in this region of the UV spectrum, exceptfor the 280 nm phenolic maximum. The difference spectrum betweenthe original and the borohydride reduced and diimide hydrogenatedkraft lignins (Fig. 8) had a maximum at 328 nm. In the visibleregion of the spectrum, sodium borohydride had a much greatereffect than diimide hydrogenation in reducing the lignin's absor-bance. These results are discussed below.[Fig. 7 and 8 here]Quinones and CTC'sThe dual role quinones play as chromophores in kraft ligninwas pointed out in Part 1 of this paper. As was demonstrated,ortho-quinones acted as acceptor species in charge-transfercomplexes with free phenolic groups. The quinones, therefore,which are colored structures themselves, were also essential tothe formation of colored CTC's.Sodium borohydride reduction of the kraft lignin resulted indecreases of the lignin's absorption spectrum in both the UV and-6-visible regions. The decrease of absorbance in the UV region wasshown in Fig. 7. Difference spectroscopy between the original andreduced lignins revealed the maximum decrease occurred at approxi-mately 320 nm. Difference spectra of the ionized original andsodium borohydride reduced lignins revealed a maximum at 353 nm.8,9Both of these results agree with literature values for theabsorption band of a-carbonyl groups.In the visible region of the spectrum sodium borohydride reduc-tion produced the decrease in absorbance shown in Fig. 9. Thisdecrease was caused by the removal of both the quinone and CTCchromophores. As shown in Part 1, the major portion of thisdecrease (two-thirds at 430 nm) can be attributed to the CTC's.CTC's, therefore, were a significant contributor to the color ofthis kraft lignin.[Fig. 9 here]SUMMARYFor the kraft lignin produced in this laboratory, transitionmetal complexes did not contribute to the lignin's visible absorp-tion spectrum. On the other hand, these complexes were a signifi-cant contributor to the visible absorption spectrum of anindustrial kraft lignin. This contribution became increasinglymore significant at longer wavelengths and accounted for approxi-mately one-half of the lignin's absorbance at 700 nm.Extended conjugated systems also did not contribute to thevisible absorption spectrum of the laboratory-produced kraftlignin. Although hydrogenation of this lignin with diimide pro-duced large decreases in the lignin's UV absorbance, no decreasein the lignin's visible absorbance was detected.Quinones were the major visible-light absorbing chromophoresin the kraft lignin employed in this study. Importantly, thequinones' contribution to the visible spectrum was strongly-7-enhanced by their participation as accepting moieties in CTC's.Removal of these quinones by reduction resulted in a decreasedabsorbance throughout the visible region. This decreaseapproached 40% at 400 nm and gradually declined to about 25% at700 nm. Approximately two-thirds of the absorbance decrease at430 nm resulted from the disruption of CTC's.EXPERIMENTALThe lignin isolation and analysis, NaBH4 reduction, andelectronic absorption spectroscopic methods were described in thepreceding paper. Indulin AT was obtained from Westvaco, NorthCharleston, South Carolina.Metal Ion RemovalIn a typical experiment, the kraft lignin (0.78 g) wasdissolved in the minimum quantity of 2-methoxyethanol needed togive complete dissolution. This solution was poured into 150 mLof 0.05M EDTA, which precipitated the lignin in a swollen,accessible form. The precipitated lignin mixture was then centri-fuged and the supernatant decanted off. The isolated lignin wasredispersed in an additional 120 mL of fresh 0.05M EDTA.This lignin dispersion was loaded into the central compartmentof an electrodialysis cell (diagrammed in Fig. 10). The ligninwas retained within the cell by two cellulose acetate membranes(Sartorius Filters, Inc.; Hayward, C.A; filter no. SM11739).Impurities in the lignin sample, including EDTA-chelated metals,were electrodialyzed through the membranes and carried out of thecell by the water flowing through the two outer compartments.Additionally, two cation exchange columns (Metex Ionxchanger Model1; Illinois Water Treatment Co.; Rockford, ILL.) were placed inseries in the water line flushing the anodic side of the cell.-8-(Fig. 10 here)A voltage of 40-50 V was applied across the cell during thedialysis. The electrodialysis was continued until the currentmeasured across the cell had decreased to a constant value (3.65mA, at an applied voltage of 40 V). This required approximatelysix days. Following completion of the electrodialysis, thelignin-water suspension was removed from the cell and the ligninwas recovered by freeze-drying.Metal analysis was performed by VHG Labs, Inc., 140 HampsteadSt., Methuen, MA 01844, using inductively coupled plasma emissionspectroscopy.Diimide HydrogenationDiimide hydrogenations were performed on the original kraftlignin and on kraft lignin which had previously been reduced withNaBH4. In each case the procedure was the same. For example,NaBH4 reduced kraft lignin (20.5 g) was dissolved in 700 mL of2-methoxyethanol (Mallinckrodt AR) and placed in a 2000 mL,3-neck, round bottom flask. The flask was equipped with a gasinlet tube, a 250 mL dropping funnel, and a magnetic stir bar.The flask was placed in a 75°C oil bath and the lignin solutionwas adjusted to pH 8 by the addition of 10% KOH.meta-Nitrobenzenesulfonyl hydrazide (12.32 g), dissolved in100 mL of 2-methoxyethanol, was then added slowly through thedropping funnel over a period of approximately 30 minutes. Aftercompletion of this addition, the dropping funnel was replaced withan air condenser fitted with a CaCl2 tube. Gas evolution becameevident after approximately 15 minutes. A slight stream of N2 wasleft flowing over the lignin solution during the reaction.After 4 and 8 hours, new charges of meta-nitrobenzenesulfonylhydrazide (12.32 g in 100 mL of 2-methoxyethanol) were againslowly added to the reaction mixture. Throughout the course ofthe reaction, the pH of the lignin solution was kept slightlyalkaline by the occasional addition of 10% KOH.-9-After 12 hours, the reaction flask was removed from the oilbath and cooled to room temperature. In portions, the reactionsolution was transferred to a 2000 mL beaker. The beaker wasplaced in an ice bath, and the solution was then acidified (pH2-3) by the dropwise addition of 1N HC1. The hydrogenated ligninwas precipitated by the addition of distilled water. The precipi-tate was collected by centrifugation and washed twice withdistilled water. The isolated hydrogenated lignin was then driedand ground to a uniform powder.ACKNOWLEDGMENTSPortions of this work were used by G. Furman as partialfulfillment of the requirements for the Ph.D. degree at The Insti-tute of Paper Chemistry. Discussions with D. R. Dimmel and R. H.Atalla throughout the course of this work are gratefullyacknowledged.REFERENCES1. G. S. Furman, W. F. W. Lonsky, this journal.2. S. I. Falkehag, J. Marton, and E. Adler, Adv. in Chem. Ser.,59 75(1966).3. F. Imsgard, S. I. Falkehag, and K. P. Kringstad, Tappi, 54,1680(1971).4. G. Meshitsuka, and J. Nakano, Tappi, 56, 105(1973).5. J. Marton, In: "Lignins Occurrence, Formation, Structure,and Reactions," p. 679, K. V. Sarkanen and C. H. Ludwig(eds.), Wiley-Interscience, New York, 1971.6. C. E. Miller, J. Chem. Educ., 42, 254(1965).7. S. Hunig, H. R. Muller, and W. Thier, Angew. Chem. Int. Ed.Engl., 4, 271(1965).8. 0. Goldschmid, In: "Lignins Occurrence, Formation, Structure,and Reactions," p. 241, K. V. Sarkanen and C. H. Ludwig(eds.), Wiley-Interscience, New York, 1971.9. J. Marton, Tappi, 47, 713(1964).-10-500 550 600 650WAVELENGTH (nm)Visible absorption spectra of (a) NaBH4 reduced,N2H2 hydrogenated, and EDTA treated kraft lignin (9.33p2pm2Fe); and (b) NaBH4 reduced, N2H 2 hydrogenated kraftlignin; concentration, 7.5 mg/25ethanol..50.45aLU,0u{4coo.25.20.15.10.05.001-350Figure 1.700BeforeAfterI I I400 450 500I I I I550 600, 650 . 700WAVELENGTH (nm)Figure 2. Visible absorption spectra for Indulin AT before andafter metal removal; concentration, 7.6 mg/25 mL of2-methoxyethanol.1.0-.9-.8-LUUZm0CQ0C[fi.7-.6-.5-.4-.3-.2-.1-.0-350.040-UJ .035-Z .030-m(A .020-< .015-<- .010-LUo .005-.uuw T350I I I I I400 450 500 550 600WAVELENGTH (nm)Figure 3. Difference spectrum from Fig. 2.650650 7007000II-S-NH-NH 211KOH75°CHydrogenatedKraftLigninHydrogenation of kraft lignin.KraftLignin+Figure 4.3x4 hours2 hours1 hour36036OI380I400WAVELENGTH (nm)Difference spectra, for the times shown, between theoriginal and hydrogenated kraft lignins..14-.12-CUz0mco4nUJ0.10.08-.06-.04-.02-.00--. 02-260I28280 300300I320I340Figure 5.t1.9,.8'.7-LUo .5-zfm .4- \ .- Hydrogenated° 3.3-ff2 Original ----.2.0.0- I I I I _ I350 400 450 500 550 600 650 700WAVELENGTH (nm)Figure 6. Visible absorption spectra of original and hydrogenatedkraft lignins; concentration, 7.5 mg/25 mL DMF.IaI I I I I280 300 320 340 360 380WAVELENGTH (nm)I400Figure 7. UV spectra of (a) original kraft lignin, (b)NaBH reduced kraft lignin, and (c) NaBH4 reduced andN2H hydrogenated kraft lignin; concentration, 0.375mg/25 mL 2-methoxyethanol.'I.50-.45 -.40-Uz0co4e.35-.30 -.25-.20-.15-.10 -.05-o.0o0260I I TI . I280 300 320 340 360WAVELENGTH (nm)Figure 8.380Difference spectrum of original kraft lignin minusNaBH4 reduced and N2H2 hydrogenated kraft lignin..2001.175 -.150-LU0zo0(I.125-.100-.075-.050-.025-.000--. 025-260 400400----- Original----- Sodium Borohydride Reduced400 450 500 550 600 650 700WAVELENGTH (nm)Visible absorption spectra for original andNaBH4 reduced kraft lignins; concentration, 7.6 mg/25mL 2-methoxyethanol..8.7.6.5.4.3COoo=,m.1.0t350Figure 9.water inlet-cation exchangecolumns--- pressure relief3-wayanode - cathodecellulose acetatemembranesFigure 10. Electrodialysis cell with water supply.water holding tankIPC TECHNICAL PAPER SERIES #174AUTHORCharge-Transfer Complexes in KraftTITLE_Lignin. Part 2 Contribution toIPC TECHNICAL PAPER SERIES #174GEORGIA-PACIFICCORPORATIONATLANTA, GEORGIA-----------

Charge-transfer complexes in kraft lignin. Part 2, Contribution to color

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Charge-transfer complexes in kraft lignin. Part 2, Contribution to color

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