Factors limiting oxygen delignification of kraft pulp

Abstract

Softwood kraft pulp was subjected to a laboratory one- and three-stage oxygen delignification process. Pulp and liquor samples were collected at different stages of the process with particular attention being paid to the early and late stages. A novel residual lignin isolation method extracted about 65% of the oxidized residual lignins at a purity exceeding 90%. Using this methodology coupled to quantitative 31P NMR, 13C NMR, 2D heteronuclear (HMQC) NMR spectroscopic and analytical pyrolysis techniques allowed a thorough characterization of the residual and solubilized lignin fractions at the various stages of the process. Our conclusions do not point to a single factor as being responsible for the limits confronting oxygen delignification. Amongst the major factors impeding the effectiveness of oxygen delignification are: (i) the accumulation of relatively inert 5,5' biphenyl structures originally present in kraft lignin from dibenzodioxocin ring opening reactions; and (ii) the accumulation of considerably less reactive p-hydroxyphenyl structures. Detailed quantitative information was obtained and discussed in relation to the presence and role of these unreactive phenolic fractions on the residual oxidized lignins. As such we arrived at important conclusions as to why and how these structures remain and accumulate on the fiber. In addition, issues related to the profiles of the total phenolic hydroxyl content of the residual lignins and the remaining arylglycerol β-O-4 structures are discussed.Key words: analytical pyrolysis, biphenyl structures, HMQC, NMR, oxidation mechanism, oxygen delignification, p-hydroxyphenyl moieties, quantitative 13C NMR, quantitative 31P NMR, residual lignin, solubilized lignin, total phenolic hydroxyls.

Wet explosion pretreatment of loblolly pine leads to an increase in methoxylation of the lignin

“…The significant observation from the reported results is that the S-type compounds increase with an increase in the severity of the wet explosion conditions. This was found to contradict the previous observations made by studying the effect of steam explosion and wet oxidation separately on lignocellulosic biomass [30][31][32][33][34] since these studies indicated that S units after pretreatment are cleaved to produce more G units resulting in a reduction in the S/G ratio and an increase in the lignin surface condensation. However, from both the NMR and the GC-MS analysis, wet explosion pretreatment showed an increase in the S units and decrease of H and G, which could best be explained through a selective methoxylation of H and G lignins during the pretreatment.…”

“…The implication of this study is significant since it is generally accepted that the action of oxygen on softwood lignin usually enriched its H units [33]. These studies have attributed this increase in H units with its high recalcitrance during oxygen delignification.…”

Structural modification of eucalypt pulp lignin in a totally chlorine-free bleaching sequence including a laccase-mediator stage

“…This compound interferes in protein estimation based on N content (1-2% N can be found in most residual lignins, but up to 5% N after laccase mediator treatment). The enrichment in lignin H units during pulp delignification reported in other studies (Akim et al 2001;Tamminen et al 2003) is most probably due to contaminating protein, as tyrosine residues gives rise to the same Py-GC/MS products and HSQC correlations (Choi and Faix 1998).…”

Chemical Pretreatments of Wood Chips Prior to Alkaline Pulping - A Review of Pretreatment Alternatives, Chemical Aspects of the Resulting Liquors, and Pulping Outcomes

“…Under alkaline conditions, oxygen is an efficient oxidizing agent for organic compounds, resulting in the formation of various reactive oxygen intermediates, such as hydroperoxyl and hydroxyl radicals, hydrogen peroxide, and hydroperoxide anions. The complicated oxidation process of oxygen delignification involves several radical chain reactions, which are combined with various organic compounds mainly originating from lignin, carbohydrates, and some extractives (Starnes Jr. 1980;McDonough 1996;Alén 2000;Akim et al 2001;Kalliola et al 2011). From lignin, oxidative degradation leads to the formation of various aliphatic carboxylic acids via different intermediates, such as catechol, quinone, and muconic acids Rovio et al 2011).…”