Defining the Photostabilization Succor Properties of Acetylated Lignin. Art J. Ragauskas Yunqiao Pu, Lucian A. Lucia

Transcription

1 Defining the Photostabilization Succor Properties of Acetylated Lignin Art J. Ragauskas Yunqiao Pu, Lucian A. Lucia

2 Succor: something that furnishes relief Seminar utline: Reversion background Reversion mechanism verview of acetylation photostabilization process Research Goal Experimental Design Results Conclusions

3 Brightness Reversion Background All mechanical pulps exhibit photoreversion properties λ: nm Attributed to lignin Influenced by 2, metals, moisture, ph % Reflectance 457nm Photoreversion of kraft and HW bleached chemithermomechanical pulp (BCTMP) test sheets Period Irradiation/days 100% BCTMP Kraft 1:1 - BCTMP:Kraft

4 Brightness Reversion Background ne of the most studied wood chemistry problems Brightness Reversion Citations C.F. Cross: Described the rapid discoloration of ground wood paper. J. Royal Society of Arts, 45, 684(1897). P. Klemm: Attributed groundwood discoloration, in part, to light. Paper Makers Monthly J., 41:4(1903).

5 Brightness Reversion: Mechanism Redox Cycle Involved in Photoyellowing. hυ, Ο 2 Lignin Lignin RH -R H Lignin 2 H hυ, Ο 2 H -H 2 Lignin -H 2 Phenols & 2 play a key role

6 Brightness Reversion Background Free Radical Ketyl Re action H H 3 C H R or R H H 3 C H Yellowed Pulp Ph H CH 3 CH 3 CH 3 Generates reactive phenoxy radicals

7 Brightness Reversion Background Pathways Involved In Photoyellowing of Mechanical Pulp. Phenacyl Pathway Phenoxy Pathway H 3 C H 3 C H hν Lignin H H + PhH CH 3 CH 3 CH 3 + H CH 3 H. CH 3 Yellowed Pulp Yellowed Pulp Singlet xygen * + 2 (singlet) Yellowed Pulp 2 H 3 C CH 3 Phenols & 2 play a key role H

8 Photostabilization Technologies Antioxidants Ascorbic Acid/Sulfite, Thiols, Thiosulfinates Note: once consumed photoyellowing continues UV-Absorbers Benzophenones, Benzotriazole, Fluorescent Whitening Agent Note: typically require large applications to be very effective Synergistic Combinations Antioxidants/UV-Absorbers Improved performance with reduced additive dosage

9 Photostabilization Technologies Fiber Modification (> block reactive phenoxy sites) Hydrogenation (C. Li: remove all aromatic rings) Alkylation (block reactive phenoxy groups) Esterification: Acetylation* (block reactive phenoxy groups) Immerse mechanical pulp sheet in acetic anhydride (80 o -100 o C) for min. Paulsson et al., Nordic Pulp Paper Res., 232(1994)

10 Photostabilization: Acetylation TAPPI Bright. ffice Lighting Aspen CTMP % Acetylation 10.3 & TAPPI Bright. Xe Irradiation time (h) Paulsson and Ragauskas Nordic Pulp Paper Res., J. 1998

11 Acetylation Photostabilization Mechanism Model Compound Studies

12 Acetylation Photostabilization Mechanism Model Compounds H H CH 3 Ac 2 /H + Rate of Acetylation PhH > H >> α H H CH 3 Blocking PhH retards photoyellowing chemistry Paulsson et al. Nordic Pulp Paper Res., 11:2 109(1996)

13 Acetylation Photostabilization Mechanism Model Compounds R Ac 2 /H + R Ac R R Ac Ac Thiele-Winter reaction R CH 3 Ac 2 /H + Paulsson et al. Nordic Pulp Paper Res., 11:4: 220(1996) No Reaction Removes chromophores & hinders proposed redox photoyellowing cycle

14 Research Goal: Characterize the fundamental changes in lignin structure upon acetylation and their contribution to photostabilizing acetylated BCTMP

15 Acetylation Induced Reactions of Lignin: Experimental Design SW BCTMP Acetone Extracted 0.01 N HCl Dioxane: Water "White" Residual Lignin Acetylation* Ac2() 105C for 10, 30, 60, 150 min Characterize H/C/P NMR Characterize H/C/P NMR *p-dioxane used as co-solvent to aid solubility of lignin

16 Acetylation Induced Reactions of Lignin: 13 C NMR Primary Aliphatic Acetates Secondary Aliphatic Acetates Ac/Aromatic Ring Ac/Aromatic ring Acetylation Time/min Acetylation Time/min Ac/Aromatic Ring Aromatic Acetates Acetylation is accompanied by a fast & slow phase Acetylation Time/min

17 Acetylation Induced Reactions of Lignin: 13 C NMR Ratio of substituted:unsubstituted Ar H CH 3 Ar-R:Ar-H Slight increase in condensed lignin Acetylation Time/min NMR data also suggests a slight decrease in methoxyl group content

18 Acetylation Induced Reactions of Lignin: 13 C NMR H 1.2 Content of B--aryl ether/aromatic ring lignin H Me B--aryl ether/ar ring H Me Slight decrease in - -aryl ether units Acetylation Time/min

19 Acetylation Induced Reactions of Lignin: 1 H NMR Aromatic H Aromatic Hydrogen Content mmol/gr lignin Acetylation Time/min Slight increase in condensed lignin & decrease in methoxyl group Me Me Content mmol/gr lignin Acetylation Time/min

20 Acetylation Induced Reactions of Lignin: 31 P NMR H Cl P P CH 3 Aliphatic RH (mmol/g) H P CH Rxn t/(min) Acid (mmol/g) Acetylation follows 13 C NMR analysis Rxn t/(min)

21 Acetylation Induced Reactions of Lignin: 31 P NMR- RH C5 noncondensed PhH (mmol/g) Rxn T/(min) C5 condensed PhH (mmol/g) Rxn T/(min) C5-condensed PhH exhibited reduced reactivity to acetylation

22 Acetylation Induced Reactions of Lignin: 31 P NMR -Quinone P(CH 3)3 P(CH 3)3 P CH3 CH3 CH3 I II III H 2 R R p-quinones yield comparable hydrolyzed adduct R=P(CH3)(H) or H

23 Acetylation Induced Reactions of Lignin: 31 P NMR Quinone P Quinone Adduct Internal Standard ppm

24 Acetylation Induced Reactions of Lignin: 31 P NMR Quinone Quinone Content (mmol/gr) ne of the first documented results demonstrating the loss of quinones in lignin by acidic acetylation Rxn T/(min) Residual quinone content (> 30 min) may be due, in part, to p-quinones that Paulsson found no reaction with Ac 2 /H +

25 Acetylation Induced Reactions Conclusions No major structural changes in lignin Slight increase in condensed structures Most C5-noncondensed PhH are acetylated Partial acetylation of C5-condensed PhHs Limited reactivity of condensed PhHs could explain why Paulsson reported that not all PhH are acetylated even after extended reaction conditions Acetylated PhH would certainly retard phenoxy based photoyellowing pathways

26 Acetylation Induced Reactions Conclusions Acetylation removes >60% of quinoidal structures in lignin Retard redox photo-cycle photoyellowing pathway hυ, Ο 2 Lignin Lignin RH -R H Lignin 2 H hυ, Ο 2 H -H 2 Lignin -H 2

27 Acetylation Induced Reactions Conclusions Acetylation procedure slightly decreases --aryl ether structures Retard free radical ketyl photoyellowing pathway H H 3 C H R or R H H 3 C H Yellowed Pulp Ph H CH 3 CH 3 CH 3

28 Acknowledgments Member Companies of Institute of Paper Science and Technology US Department of Agriculture, Improved Utilization of Wood Grant #: Special Thanks to T.Edler