DEGRADATION OF LIGNIN BY OZONE by Reginald A.D. Mbachu, B.Sc. (Hons) @ (University of Nigeria, Nsukka) A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfilment of the requirements for the degree of Doctor of Philosophy Department of Chemistry, April 1979 McGill University, Montreal, P.Q. i ABSTRACT A physicochemical study of the mechanism of lignin degradation by ozone has been made using spruce periodate and cuoxam lignins and spruce wood protolignin. The ozonization was carried out'in 45% aqueous acetic acid. In all cases, degradation was found to follow first order kinetics indicating a similar reaction mechanism and a single rate controlling process. From the rate c~4stants obtained, it was concluded that the carbohyd­ rate moieties in wood do not affect the rate of protolignin degradation and dissolution. Spectroscopic studies of the alkali­ soluble degradation products of lignin showed that the chemical mechanism is probably electrophilic, involving the attack on phenol ether bonds, methoxyl groups, aromatic and other un­ saturated structures and the formation of carboxyl groups. De­ carboxylation occurs in the later stages of the reaction. From a consideration of the possible reactions that may degrade the lignin network, it was concluded that the observed phenol ether bond cleavage is the principal network-degrading reaction during ozonization. A tentative mechanism has, therefore, been proposed for the cleavage of the phenol ether bonds in lignin. Molecular weights and molecular weight distributions of the alkali-soluble degradation products, as studied by gel permeation chromatography and ultracentrifugation, indicate that a random heterolytic degradation mechanism is involved. The significance of the various 0· results in relation to the lignin architecture is discussed. ii Finally, a study was made of the effect of ozone on cellulose and hemicelluloses during the delignification of wood. The results are discussed in the light of a potential pulping method using ozone. iii RESUME Une etude physicoch:imi.que du rrecani.sn:e de degradation des lignines / / periodate et cuoxame de 1' epinette et de la protolignine du l:ois d 'epinette a ete' ' effectuee.I L •ozonisation fut acccmplie en solution aqueuse d' acide acetique a 45%. Une cinetique de premier order lors de la degradation fut obtenue dans les trois cas, indiquant un m9can.isrre de reaction similaire et que la degradation e5t controlee par une seuie etape. A partir des constantes de vitesse abtenues, il fut conclu que les hydrates de carbone dans le bois n' influencaient r::as le taux de degradation et de dissolution de la lignine. Des etudes spectroscopiques sur les produits de de9-radation de la lignine / / qui sont solubles en milieu alcalin, ont rrontre que le m=canisrne de la reaction est fort probablem:mt de type e'lectrophilique 1 CattpJrtant une attaque des liens ether ~lique, des groupes rcethoxyles, arorratiques et autres structures insaturees, ainsi que la forma.tion de groupes carboxyl­ iques. La decarroxylation se produit a des stades ulterieurs de la reaction. / / / / En considerant les reactions qui peuvent possiblernent degrader le reseau de la lignine, il fut conclu que . la scission du lien ether phenolique est la reaction principale de degradation du reseau ~t 1 'ozonisation. Un rrecanisrre possible pour la scission du lien ether phenolique de la \ / / lignine fut des lors propose. Les poids rroleculaires ainsi que les dis- tributions de poids :rroleculaires des produits de de9radation solubles en milieu basique, furent etudies par chrarnatograrru.e en perm:i'ation de gel et / / / / _, par ultracentrifugation. Les resultats on indique un mecanisme heteroly- tique de degradation de la lignine s 'effectuant au hasard. L' i.rrq;x::Jrtance de / / ces divers resultats sur 1' architecture de la lignine et consideree. Finale:- / rrent, une etude fut faite sur l'effet de l'ozone sur la cellulose et sur iv / . ,• ,. 1 'hemicellulose pendant la delignification du bois. I.Bs resultats sont discute's a la lumiere d 1 une rre"t..i']ode fOSSible de reduction en pate qui utiliserant l'ozone. V FOREWORD The use of ozone as a delignifying agent has recently attracted attention owing to its potential as an answer to the increasing pollution problems encountered in the pulp and paper industry. A detailed understanding of the lignin-ozone reaction is expected to facilitate efforts to optimize ozone delignification processes. The work described in this thesis is aimed at elucidating the mechanism of lignin degradation by ozone. The work was carried out in the physical chemistry division of the Pulp and Paper Re­ search Institute of Canada at McGill University, under the super­ vision of Dr. R.St. John Manley. The thesis comprises five chapters, four appendices and two sections in which claims to original research and suggestions for further research are outlined. Four of the chapters are written in the form of scientific papers and may be submitted for public­ ation with minor alterations. The scheme of the thesis is as follows: CHAPTER 1 - a general introduction in which topics relevant to the investigation are discussed. The topics include the chemistry of wood, the structure and architecture of lignin, the theory of gelation, the nature of the reaction between lignin and ozone, the use of ozone in pulping and bleaching and the,degradation of carbohydrates during ozone delignification of wood. CHAPTER 2 - a study of the kinetics of lignin degradation by ozone. CHAPTER 3 - a structural investigation of the alkali-soluble 0 degradation products of lignin obtained during ozonization. CHAPTER 4 - a study of the molecular weights and molecular weight vi distributions of the alkali-soluble degradation products ob- tained during the ozonization of lignin. CHAPTER 5 - an investigation of the fate of the carbohydrates during the degradation of the protolignin in wood by ozone. APPENDICES - supplementary material such as detailed experimental procedure, data, calculations and theory of the techniques employed in the investigation. The thesis closes with claims to original research and suggestions for further work. A glossary of symbols is also included at the end of the thesis. R.A.D. Mbachu April, 1979. vii ACKNOWLEDGEMENTS The author wishes to express his sincere gratitude to all those who have contributed to the completion of this work. He is particularly indebted to: Dr. R. St. John Manley, for his able guidance, patience and assistance throughout the investigation and in the preparation of the thesis; Dr. H.I. Bolker for valuable critical comments and very useful suggestionsi Drs. D.A.I. Goring, D.G. Gray and A.S. Perlin for helpful discussions: Dr. G.K. Hamer, for proton magnetic resonance spectroscopy; Mr. W.Q. Yean, for the molecular weight determinations; Mr. V. Berzins, for chemical analysis; Mr. G. Suranyi, for general assistance in the laboratory; Mrs. c. Ewan and s. Shaieb for typing this thesis; Fellow graduate students for their friendship and helpful discussions; McGill University, for a Demonstratorship, 1975-1976; The Pulp and Paper Research Institute of Canada, for a Studentship, 1976-1979, laboratory accommodation and the use of their facilities and services. The Federal Nigerian Government for a Scholarship in 1976. viii Last but not least, I wish to express my indebtedness 0 to Kate for her understanding, moral support and encouragement throughout this work. ix TABLE OF CONTENTS CHAPTER 1 GENERAL INTRODUCTION . 1 THE CHEMISTRY OF WOOD ••••••••••••••••••••••••••••••• 4 STRUCTURE AND ARCHITECTURE OF LIGNIN................ 7 THEORY OF GELATION . • . • . • • • . • . • . • • . • • • . 17 Critical condition for gelation in cross-linked systems . 20 Weight fraction of sol in a gelled polymer ...... 23 Degree of polymerization and weight fraction of sol . 2 6 Molecular size distribution in infinite networks. 29 THE NATURE OF THE LIGNIN-OZONE REACTION............. 34 Ozone • • • • • • • . • • . • • • . • • • • • . • • . • • • • . • • • • . • • • . • 34 Ozonolysis of lignin .•................•..•...... 36 THE USE OF OZONE IN BLEACHING AND PULPING •.•..•.•.•. 41 DEGRADATION OF CARBOHYDRATES BY OZONE ....•....•....• 43 Cellulose . .. 4 3 Hemicelluloses . 44 RATES OF SOLUBILIZATION OF WOOD COMPONENTS.......... 44 SCOPE AND AIMS OF THE THESIS • . • . • . 4 5 REFERENCES . 48 CHAPTER 2 DEGRADATION OF LIGNIN BY OZONE (I) THE KINETICS OF LIGNIN DEGRADATION BY OZONE ........ 56 X 0 ABSTRACT ••••••••••••.•••••••.••••••••••.••.•••••••• 57 IN'rRODUCTION . 58 EXPERIMENTAL . .. .. 61 Ozonization of periodate, cuoxam lignin and spruce protolignin ..•..••...•....•.•....•.••.•. 61 Recovery of the alkali-soluble degradation products . 6 4 Estimation of lignin in treated wood samples... 64 RESULTS AND DISCUSSION . • • • . • . • • • . 6 6 Effects of stirring on yield of lignin .......•. 66 Rate of degradation of the isolated lignins .... 69 Consumption of ozone by lignin ....••.•.•.••.••. 72 Rate of degradation of spruce protolignin .•...• 75 Comparison of rate constants of degradation.... 80 CONCLUSION 81 REFERENCES 83 CHAPTER 3 DEGRADATION OF LIGNIN BY OZONE (II) SPECTROSCOPIC STUDIES OF THE ALKALI-SOLUBLE DEGRAD- AT ION PRODUCTS . • . • . • . • • . • . • • • • • . • • . • 8 5 ABSTRACT • • • • • • • • • • • • • • • • • • • • • . • • • • • • • • • • • • • • • • • • • • • 8 6 INTRODUCTION 88 EXPERIMENTAL . 91 Infra-red analysis . 91 Proton magnetic resonance spectroscopy .•...•... 91 Ultra-violet analysis . • . • . • . • . • . • • . • .