Residual Kraft Lignin Research Articles

Low-Energy Catalytic Electrolysis for Simultaneous Hydrogen Evolution and Lignin Depolymerization.

Here, a new proton-exchange-membrane electrolysis is presented, in which lignin was used as the hydrogen source at the anode for hydrogen production. Either polyoxometalate (POM) or FeCl3 was used as the catalyst and charge-transfer agent at the anode. Over 90 % Faraday efficiency was achieved. In a thermal-insulation reactor, the heat energy could be maintained at a very low level for continuous operation. Compared to the best alkaline-water electrolysis reported in literature, the electrical-energy consumption could be 40 % lower with lignin electrolysis. At the anode, the Kraft lignin (KL) was oxidized to aromatic chemicals by POM or FeCl3 , and reduced POM or Fe ions were regenerated during the electrolysis. Structure analysis of the residual KL indicated a reduction of the amount of hydroxyl groups and the cleavage of ether bonds. The results suggest that POM- or FeCl3 -mediated electrolysis can significantly reduce the electrolysis energy consumption in hydrogen production and, simultaneously, depolymerize lignin to low-molecular-weight value-added aromatic chemicals.

Demonstration of a Process for the Conversion of Kraft Lignin into Vanillin and Methyl Vanillate by Acidic Oxidation in Aqueous Methanol

In this work, a process for producing vanillin and methyl vanillate from an industrial kraft lignin by oxidation in aqueous methanol at acidic pH is proposed. A series of consecutive runs, comprising the steps of kraft lignin oxidation, monomer extraction, and filtration of residual lignin, were conducted. The depolymerization of kraft lignin (1 g per run) was carried out in a stirred batch reactor with 80 vol % methanol/water as the solvent, with H3PMo12O40 as a homogeneous catalyst, and with 10 bar of oxygen initially present. The monomeric products were recovered from the process by extraction with chloroform and quantified by gas chromatography/mass spectrometry (GC/MS). Residual kraft lignin was recovered from the reaction medium by filtration. After extraction and filtration, the reaction medium, containing the homogeneous catalyst, was reused in the next run. The sum of all quantified products (gas phase, extract, and solids), as well as the amount of consumed oxygen, reached constant levels after ...

Laccase mediator system: Oxidation of recalcitrant lignin model structures present in residual kraft lignin

Residual kraft lignin contains a greater amount of condensed structures and β-5 intermonomeric linkages with and without phenolic groups in comparison to native lignin. Some of the nonphenolic residual lignin substructures have been previously reported to be recalcitrant to degradation by laccase mediator systems (LMS). In this work, we report on the degradation of two lignin model compounds by LMS: a β-5 and a 5–5′ lignin substructure. According to our results, the oxidation by laccase of such compounds proceeded easily in the presence of phenolic groups, whereas the corresponding fully methylated nonphenolic compounds were difficult to oxidise even in the presence of the aromatic redox mediator (HBT, ABTS). On the other hand, the degradation was accomplished when surfactants were added to the system. The type of laccase, redox mediator and surfactant affected the extent of degradation.

Application of the NICA–Donnan approach to calculate equilibrium between proton and metal ions with lignocellulosic materials

The NICA (nonideal competitive adsorption)–Donnan model is employed to describe the interactions between Cu 2+, Pb 2+, Cd 2+, Mn 2+, and Fe 3+ ions and the lignins extracted from wheat bran (lignocellulosic substrate, LS) and from kraft pulp (residual kraft lignin, RKL), and between Cu 2+, Mn 2+, and Fe 3+ ions and wood fibers from kraft pulps. The charge of the LS and the fiber charge need to be obtained from potentiometric titration data for the LS, and by use of Donnan equilibrium, mass balance, and electroneutrality equations for the kraft fiber. The proton binding parameters for the LS and the kraft fiber, the total site densities ( Q max , 1 and Q max , 2 ), the median protonation constants ( K 1 and K 2 ), and nonideality–generic heterogeneity parameters ( m 1 and m 2 ) (subscripts 1 and 2 refer to the carboxyl and phenolic functional groups) are obtained by fitting these charge data. With the above proton parameters, the interactions between metal ions and the lignins (LS and RKL)/kraft fibers are calculated, and the metal binding parameters are obtained. These parameters are the binding constants of metal i ( K i , 1 and K i , 2 ), ion-specific nonideality parameters ( n i , 1 and n i , 2 ), and intrinsic heterogeneity parameters ( p 1 and p 2 ). p 1 and p 2 are the same for all metal ions binding to a specific sorbent. Here, p 1 and p 2 values obtained by fitting the binding data of a specific metal ion are used directly in binding calculations for other metal ions, and do not need to be fitted. By use of the above parameters for single metal ion binding, the binding relationship between a mixed metal ion and lignocellulosic substrate/kraft fiber can be predicted.

Alkaline oxidative degradation of diphenylmethane structures — Activation energy and computational analysis of the reaction mechanism

A diphenylmethane model compound (2,2'-methylenebis[6-methoxy-4-methylphenol]) and residual kraft lignin were treated with alkaline hydrogen peroxide. Kinetic data for the disappearance of the model and the diphenylmethane structures in the residual lignin was collected. The activation energies for the degradation were found to be similar (54 ± 11 kJ mol–1 for the model and 58 ± 5 kJ mol–1 for the residual lignin). A comparison of the activation energies with the data of a previous study on a biphenyl model compound (3,3'-dimethoxy-5,5'-dimethyl-[1,1'-biphenyl]-2,2'-diol) showed a substantially higher activation energy for the degradation of the latter. Pathways for the degradation of 2,2'-methylenebis[6-methoxy-4-methylphenol] were proposed and the intermediates subjected to computational analysis using a semiempirical method (PM3). The results suggest that initially a common pathway exists, resulting in 2-[2-hydroxy-3-methoxy-5-methyl-phenylmethyl]-4-methyl-2,4-hexadienedioic acid. Then the pathway branches into three, resulting in three major degradation products. The main driving force of the reactions is the formation of radical sites after reaction with hydroxyl radicals and subsequent radical coupling with perhydroxyl radicals to form peroxides. All the reactions on the pathways are exothermic except for the transformations of peroxides into dioxetanes. The dioxetanes cleave exothermically resulting in ring cleavage and fragmentation. The computed data permitted for the rationalization as to why the diphenylmethane structures appear to be more labile than biphenyl structures under alkaline oxidative conditions.Key words: activation energy, alkaline oxidative degradation, computational chemistry, lignin, reaction mechanism.

The reactivity of phenolic and non-phenolic residual kraft lignin model compounds with Mn(II)-peroxidase from Lentinula edodes

Three phenolic model compounds representing bonding patterns of residual kraft lignin were incubated with manganese peroxidase from Lentinula edodes. Extensive degradation of all the phenolic models, mainly occurring via side-chain benzylic oxidation, was observed. Among the tested model compounds the diphenylmethane α-5 phenolic model was found to be the most reactive, yielding several products showing oxidation and fragmentation at the bridging position. The non-phenolic 5-5′ biphenyl and 5-5′ diphenylmethane models were found unreactive.

Biomimetic degradation of lignin and lignin model compounds by synthetic anionic and cationic water soluble manganese and iron porphyrins

The biomimetic oxidation of 5-5′ condensed and diphenylmethane lignin model compounds with several water soluble anionic and cationic iron and manganese porphyrins in the presence of hydrogen peroxide is reported. The oxidative efficiency of manganese and iron meso-tetra(2,6-dichloro-3-sulphonatophenyl) porphyrin chloride (TDCSPPMnCl and TDCSPPFeCl, respectively), meso-tetra-3-sulphonatophenyl porphyrin chloride (TSPPMnCl) and manganese meso-tetra(N-methylpyridinio)porphyrin pentaacetate (TPyMePMn(CH3COO)5) was compared on the basis of the oxidation extent of the models tested. Manganese porphyrins were found more effective in degrading lignin substructures than iron ones. Among them the cationic TPyMePMn(CH3COO)5, never used before in lignin oxidation, showed to be the best catalyst. The catalytic activity of porphyrins in hydrogen peroxide oxidation of residual kraft lignin was also investigated. The use of quantitative 31P NMR allowed the focusing on the occurrence of different degradative pathways depending on the catalyst used. TPyMePMn(CH3COO)5 was able to perform the most extensive degradation of the lignin structure, as demonstrated by the decrease of aliphatic hydroxyl groups and carboxylic acids. Noteworthy, no significant condensation reactions occurred during manganese porphyrins catalyzed oxidations of residual kraft lignin, while in the presence of iron porphyrins a substantial increase of condensed substructures was detected.

SIZE EXCLUSION CHROMATOGRAPHY OF LIGNINS USING LITHIUM CHLORIDE/N,N-DIMETHYLACETAMIDE AS MOBILE PHASE. II. DISSOLVED AND RESIDUAL PINE KRAFT LIGNINS

Dissolved and residual pine kraft lignin samples were isolated from different delignification phases of kraft flow–through cooks. The molecular weight distributions (MWDs) were studied by size exclusion chromatography using lithium chloride/N,N-dimethylacetamide (LiCl/DMAc), not previously applied to isolated softwood lignin samples before. The MWDs were compared with results obtained using tetrahydrofuran (THF) or lithium chloride/N,N–dimethylformamide (LiCl/DMF) as mobile phase. Some important characteristics of the isolated lignin samples including the MWDs are reported and compared with a previous study on birch wood kraft lignins. The molecular weight of the lignins was changed in a similar way irrespective of mobile phase used. The molecular weight of the dissolved lignin increased during the cook. By contrast, the change in molecular weight of the residual lignin samples did not show a clear trend with respect to cooking time. One explanation for this irregular change may be the low efficiency of the acid dioxane extraction of the pine kraft pulp obtained early in the cook as this pulp contains a large amount of lignin. Apart from this sample, the size of the residual lignin sample decreases and resembles that of the dissolved lignin at the end of the cook. For all samples, the MWD was positioned at higher molecular weight range when LiCl/DMAc was used as the mobile phase compared with THF. The explanation for this behaviour seems to be that the polystyrene standards elutes later from polystyrene-divinylbenzene columns compared to the lignin samples when a mobile phase of higher polarity is used. The shapes of the distributions were also different between LiCl/DMAc and THF, whereas LiCl/DMAc and LiCl/DMF gave similar distribution profiles. These results emphasize the importance of using the same mobile phase and column packing material when comparing MWD and size of different lignins and also when comparing lignins and carbohydrate polymers in pulp.

Article

The complex interaction of softwood residual kraft lignin with oxygen at elevated temperatures, pressures, and pH's was investigated by isolating and systematically oxidizing the lignin, as a function of time and temperature. The isolation of the oxidized lignin was carried out using a multistep procedure developed specifically to recover all species. Detailed quantitative 31P NMR measurements provided, for the first time, three-dimensional plots describing the formation and (or) elimination of the various functional groups. Oxidative demethoxylation reactions were found to induce the formation of catechols, which are, most likely, reactive intermediates of the oxidation. The rate of carboxylic acid formation and guaiacyl phenol elimination was found to follow two distinct phases: a rapid initial phase followed by a slower phase. More specifically, the rate of carboxylic acid group formation was found to dramatically increase as the reaction temperature increased, analogous to the profiles obtained for the elimination of guaiacyl phenolic units. At reaction temperatures typical of conventional commercial oxygen delignification installations (80-100°C), only minor oxidation was found to occur within the lignin. At these temperatures, increasing the reaction time did not significantly aid the formation of carboxylic acid groups on lignin despite the idealized two-phase homogeneous conditions. The technological ramifications of these data imply that a three-phase oxygen delignification system, operating at temperatures below 100°C, causes only minor oxidative changes to the structure of the lignin in the fibre. It was also possible to isolate the effect of alkali and temperature on the structure of residual kraft lignin through experiments in which pressurized oxygen was replaced by nitrogen. These data revealed a concealed set of alkali-induced condensation and fragmentation reactions.Key words: bleaching, catechols, carboxylic acids, delignification, guaiacyl, kinetics, oxygen, phenols, phosphorus, nuclear magnetic resonance (nmr), residual kraft lignin.

The early oxidative biodegradation steps of residual kraft lignin models with laccase

A number of model compounds resembling the fundamental bonding patterns of residual kraft lignin, including a series of stilbenes, were incubated with laccase from Trametes versicolor in the presence and absence of delignification ‘mediators’ ABTS and HBT. The condensed kraft lignin model compounds seem to undergo initial degradation by laccase mainly via benzylic oxidation, demethylation and hydroxylation reactions. Phenolic 5-5′, diphenylmethane and α-5 lignin models were found to be degraded mainly via side-chain oxidation reactions. Among the models studied, a phenolic stilbene was found to be the most reactive, yielding several products showing side-chain oxidation/transposition, demethoxylation and hydroxylation reactions. Non-phenolic 5-5′, diphenylmethane and stilbene model compounds were found unreactive even in the presence of the laccase-mediator system.

Fundamentals of o×ygen delignification. Part II. Functional group formation/elimination in residual kraft lignin

The complex interaction of softwood residual kraft lignin with oxygen at elevated temperatures, pressures, and pH's was investigated by isolating and systematically oxidizing the lignin, as a function of time and temperature. The isolation of the oxidized lignin was carried out using a multistep procedure developed specifically to recover all species. Detailed quantitative 31 P NMR measurements provided, for the first time, three-dimensional plots describing the formation and (or) elimination of the various functional groups. Oxidative demethoxylation reactions were found to induce the formation of catechols, which are, most likely, reactive intermediates of the oxidation. The rate of carboxylic acid formation and guaiacyl phenol elimination was found to follow two distinct phases: a rapid initial phase followed by a slower phase. More specifically, the rate of carboxylic acid group formation was found to dramatically increase as the reaction temperature increased, analogous to the profiles obtained for the elimination of guaiacyl phenolic units. At reaction temperatures typical of conventional commercial oxygen delignification installations (80-100°C), only minor oxidation was found to occur within the lignin. At these temperatures, increasing the reaction time did not significantly aid the formation of carboxylic acid groups on lignin despite the idealized two- phase homogeneous conditions. The technological ramifications of these data imply that a three-phase oxygen delignification system, operating at temperatures below 100°C, causes only minor oxidative changes to the structure of the lignin in the fibre. It was also possible to isolate the effect of alkali and temperature on the structure of residual kraft lignin through experiments in which pressurized oxygen was replaced by nitrogen. These data revealed a concealed set of alkali-induced condensation and fragmentation reactions.

A Comparison of the Reactivity and Efficiency of Ozone, Chlorine Dioxide, Dimethyldioxirane and Hydrogen Peroxide with Residual Kraft Lignin

Residual lignin isolated from a conventional softwood (Picea mariana) kraft pulp was reacted in homogeneous solutions with varying charges of ozone, chlorine dioxide, dimethyldioxirane (DMD) and alkaline hydrogen peroxide, followed by quantitative 31 P NMR analyses of their functional groups. This effort supplied plots of relative reactivity of these reagents toward specific functional groups present in lignin, and it revealed salient features that control their reactivity and efficiency. The guaiacyl phenolic units, present on the lignin, were found to be the major sites of attack for all the surveyed oxidative treatments. At a given charge, ozone and chlorine dioxide were found to be most efficient in causing the formation of carboxylic acids on residual kraft lignin. Alternatively, low charges of ozone and DMD may act as activating stages prior to another bleaching stage i.e chlorine dioxide and/or alkaline hydrogen peroxide, since they were found to introduce free phenolic hydroxyl groups in residual lignin. Most of the condensed phenolic units present in residual kraft lignin react with the bleaching reagents surveyed. However, it is likely that certain condensed structures are resistant to attack. At high charges of alkaline hydrogen peroxide the extent of condensed phenolic unit elimination was similar to those observed for chlorine dioxide and ozone.