Oxidative Delignification Research Articles

Application of zeolite-encapsulated Cu(ii) [H4]salen derived from [H2]salen in oxidative delignification of pulp

Since the disclosure that salen complexes were potent low-temperature oxidation catalysts with exceptional selectivity, considerable efforts by our group have focussed on their development towards the efficient catalytic delignification of pulp, principally with environmentally benign oxygenic chemicals. These efforts have resulted in additional developments including the heterogenisation of catalytic systems as well as the hydrogenation for application in catalytic delignification of pulp. Herein, heterogeneous complexes of two ligands: [H2]salen and [H4]salen with copper as a transition metal were synthesized and encapsulated in zeolite cages by the impregnation (IM) and ship-in-a-bottle (SB) method. The resulting complexes were employed as catalysts for oxidative delignification of pulp, using peracetic acid (CH3COOOH) as an oxidant. The observed trends in catalytic activity showed that the encapsulation and hydrogenation of complexes played a key role in the selective delignification of pulp. By encapsulating and hydrogenating the complexes, their spatial separation increased which strongly enhanced the activity of complexes. The encapsulation and hydrogenation of complexes resulted in a significant increase in delignification and properties of pulp. The delignification was dependent on the nature of the complexes. The reasons were investigated by spectroscopic techniques.

New methods of heterogeneous catalysis for lignocellulosic biomass conversion to chemicals

This review deals with the use of solid catalysts for the enhancement of the efficiency and the development of a new generation of environmentally friendly, energy and resource efficient processes for the deep processing of lignocellulosic biomass to desired chemicals. The oxidative delignification of wood with hydrogen peroxide in the presence of the suspended TiO2 catalyst, the oxidation of wood with molecular oxygen in the presence of copper catalysts, the acidcatalyzed conversion of cellulose to glucose and levulinic acid, and the thermal conversion of lignin to fuel additives on solid acid catalysts are analyzed. New integrated processes based on the heterogeneous catalytic oxidation are suitable for the complex processing of lignocellulosic biomass to produce valuable chemicals and engine fuel components without the use of toxic and corrosion-active reagents.

Enzymatic bioconversion of kraft pulped and oxidative delignified sawdust from the Lagos Lagoon, Nigeria into fermentable sugars

The major obstacle during pretreatment of lignocellulosic biomass centers mostly on the insufficient separation of lignin from cellulose that could further be converted into bioenergy and other useful chemicals. The choice of an optimum biomass pretreatment process depends on the digestibility of lignocellulosic material, cost-effectiveness and overall impact on the environment. Kraft pulp samples from sawdust wastes obtained along the Lagos Lagoon in Nigeria were delignified with 30% hydrogen peroxide and successively saccharified with Trichoderma viridecellulase during different incubation periods. Oxidative delignification of all Kraft pulped sawdust samples resulted in an increased degree of bioconversion relative to the saccharification of Kraft pulped sawdust. The highest percentage increase in saccharification during bioconversion of delignified Kraft pulped sawdust relative to degradation of only Kraft pulped sawdust was as follows: Sawdust from R. heudelotii showed an increase of 727% after 30 min of degradation with a 447% increase obtained from the same sawdust after an incubation period of 1 h. An increase of 323% was calculated for sawdust from E. suaveolens and I. asarifoliaduring a degradation period of 3 h with a 238% increase in glucose production fromM. excelsa sawdust after a 6 h period of cellulase treatment. Key words: Bioenergy, sawdust, Trichoderma viride cellulase, delignification, saccharification.

Evidence for room temperature delignification of wood using hydrogen peroxide and manganese acetate as a catalyst

Manganese acetate was found to catalyze the oxidative delignification of wood with hydrogen peroxide at room temperature. The delignification reaction was monitored by optical and Raman microscopy, and liquid chromatography/mass spectrometry. When exposed to H2O2 and Mn(OAc)3 in aqueous solution, poplar wood sections were converted into a fine powder-like material which consisted of individual wood cells within 4days at room temperature and without agitation. Optical and Raman microscopy provided the spatial distribution of cellulose and lignin in the wood structure, and showed the preferential oxidation of lignin-rich middle lamellae. Raman spectra from the solid residue revealed a delignified and cellulose-rich material. Glucose yields following enzymatic hydrolysis were 20–40% higher in poplar sawdust pretreated with Mn(OAc)3 for 2, 4, and 7days at room temperature than those in sawdust exposed to water only for identical durations, suggesting the viability of this mild, inexpensive method for pretreatment of lignocellulosic biomass.

Oxidative delignification of wood in the supercritical carbon dioxide medium. 4. Acetylization of cellulose in the supercritical carbon dioxide medium

It is shown that cellulose can be acetylated with acetic anhydride in supercritical carbon dioxide in the absence of traditional catalysts, toxic solvents, and dilutants. As substrates, three cellulose samples were tested: sulfite and sulfate cellulose and cellulose obtained by low-temperature delignification in a supercritical carbon dioxide medium. The chemical composition of the cellulose diacetate product is characterized.

The oxidative delignification of wood in supercritical carbon dioxide: The functionalization of coniferous lignin

Changes in the content of lignin functional groups during low-temperature delignification under the action of hydrogen peroxide in supercritical (SC) CO2 in the preparation of fibrous cellulose half-finished product were considered. The action of the oxidizer caused changes in the molecular weight characteristics of lignins. The experimental data showed the occurrence of both oxidation and substitution processes. Oxidation in SC CO2 resulted in the formation of water- and alkali-soluble medium- and low-oxidized lignins because of decreased oxidizer consumption.

The oxidative delignification of wood in supercritical carbon dioxide: 3. The chemical composition of fibrous half-finished products

The influence of pressure on the parameters of low-temperature oxidative delignification of firry wood in supercritical carbon dioxide under the action of hydrogen peroxide is described. The chemical composition of the cellulose half-finished products is characterized.

Polyoxometalates in Oxidative Delignification of Chemical Pulps: Effect on Lignin

Chemical pulps are produced by chemical delignification of lignocelluloses such as wood or annual non-woody plants. After pulping (e.g., kraft pulping), the remaining lignin is removed by bleaching to produce a high quality, bright paper. The goal of bleaching is to remove lignin from the pulp without a negative effect on the cellulose; for this reason, delignification should be performed in a highly selective manner. New environmentally-friendly alternatives to conventional chlorine-based bleaching technologies (e.g., oxygen, ozone, or peroxide bleaching) have been suggested or implemented. In an attempt to find inorganic agents that mimic the action of highly selective lignin-degrading enzymes and that can be applicable in industrial conditions, the researchers have focused on polyoxometalates (POMs), used either as regenerable redox reagents (in anaerobic conditions) or as catalysts (in aerobic conditions) of oxidative delignification. The aim of this paper is to review the basic concepts of POM delignification in these two processes.

Influence of UV pretreatment on the abies wood catalytic delignification in the medium “acetic acid–hydrogen peroxide–TiO2”

Some regularities of abies-wood oxidative delignification by acetic acid–hydrogen peroxide mixture under the action of suspended TiO2 catalyst and UV pretreatment of wood pulp were studied. The combined action of TiO2 catalyst and of UV-pretreatment of abies-wood allow to produce at optimal conditions of the delignification process the chemically pure cellulose containing no residual lignin. The major characteristics of cellulose product obtained from abies-wood correspond to the characteristics of microcrystalline cellulose.

A new procedure for preparing microcrystalline cellulose

A procedure was developed for preparing microcrystalline cellulose, involving hydrolytic treatment of technical-grade (unbleached kraft) wood pulp, followed by oxidative delignification of the resulting lignocellulose powdered material with chlorine dioxide.

Enhancing exterior performance of clear coatings through photostabilization of wooden surfaces. Part 1: Treatment and characterization

The aim of this work was to produce a less photodegradation-prone substrate for clear coating by partially delignifying the surface cells of Pinus radiata boards to a depth of 2–3 mm while maintaining the integrity of the wood surface tissues in the delignified zone. To achieve this, several surface oxidative delignification treatments were trialled in the method development process and peracetic acid was chosen as the method for refinement and deployment. The treatment method was optimized to yield a significant degree of delignification compatible with the aim of producing a photostabilized yet intact wooden surface. A preweathering technique was also used as a second delignification method for producing photostabilized boards. Microscopic and chemical techniques were used to characterize the effects of chemical and preweathering treatments, which produced delignified surface envelopes 2–3 mm and 100 μm deep, respectively. Acetyl bromide lignin analyses, infrared spectroscopic analyses, and density changes of 30-μm-thick sections of peracetic-acid-treated samples, as a function of depth from the wood surface, suggested that a partial delignification had occurred that diminished with depth. Light and transmission electron microscopy provided evidence of delignification at the cellular level. In the surface layers of peracetic-acid-treated boards, all cell wall regions were delignified, with the middle lamella being the most severely affected. Lignin appeared to be completely removed from the cell corner middle lamella regions, but tracheids were still joined in other parts of the middle lamella. The S1, S2, and S3 walls were also delignified. In subsurface layers, cell walls were only partially delignified and the tissues held their integrity. In contrast, in the preweathered boards, cell walls in the outermost layers were completely separated at the middle lamella from photodegradation. Preferential lignin removal over that of hemicelluloses was achieved via oxidative treatment of solid wood. The outcome of this was the successful fulfillment of our aim to produce partially delignified wooden surfaces that retained sufficient strength and aesthetic appearance and were suitable for application in wooden structures, provided that the necessary protection measures were carried out. Due to the narrow surface zone produced, machining of such surfaces would not be recommended as the most delignified zone would be removed first. In Part 2 of this series, the photostability of treated boards, which had been clear coated and weathered, will be considered.

Hydrogen peroxide bleaching of cellulose pulps obtained from brewer’s spent grain

Brewer’s spent grain (BSG) was evaluated for bleached pulp production. Two cellulose pulps with different chemical compositions were produced by soda pulping: one from the original raw material and the other from material pretreated by dilute acid. Both of them were bleached by a totally chlorine-free sequence performed in three stages, using 5% hydrogen peroxide in the two initial, and a 0.25 N NaOH solution in the last one. Chemical composition, kappa number, viscosity, brightness and yield of bleached and unbleached pulps were evaluated. The high hemicellulose (28.4% w/w) and extractives (5.8% w/w) contents in original BSG affected the pulping and bleaching processes. However, soda pulping of acid pretreated BSG gave a cellulose-rich pulp (90.4% w/w) with low hemicellulose and extractives contents (7.9% w/w and <3.4% w/w, respectively), which was easily bleached achieving a kappa number of 11.21, viscosity of 3.12 cp, brightness of 71.3%, cellulose content of 95.7% w/w, and residual lignin of 3.4% w/w. Alkaline and oxidative delignification of acid pretreated BSG was found as an attractive approach for producing high-purity, chlorine-free cellulose pulp.

Mechanistic and steric issues in the oxidation of phenolic and non-phenolic compounds by laccase or laccase-mediator systems. The case of bifunctional substrates

Steric and redox issues of phenolic and non-phenolic substrates are investigated for a better insight of the reactivity features of the phenoloxidase laccase. Whenever a substrate is endowed with a redox potential too high for direct monoelectronic oxidation by the enzyme, or else is too much encumbered to access the enzymatic pocket, redox mediators overcome the problem, behaving as an interface between enzyme and substrate. For example, the small-sized mediator ABTS, once oxidised by laccase, fruitfully interacts with bulky substrates, 2,4,6-tri(But)-phenol providing a significant case. Other mediators, for example HBT, resort to a radical oxidation mechanism precluded to laccase, and can react with non-phenolic substrates which are impossible for the enzyme. The advantages provided by the mediators are discussed, and suitable phenolic compounds, as precursors of phenoxyl radical intermediates, emerge as a new proficient class. They could be the true natural mediators of laccase in the oxidative delignification. In fact, phenoxyl radical fragments generated by laccase from lignin, or from phenolic monomer residuals from the building up of lignin polymer or else deriving from lignin by oxidation with other ligninolytic enzymes, could oxidise non-phenolic residues of lignin thereby causing the breakdown of its alkyl network. The novel mechanistic probe 3,5-di(But)-4-OH-benzyl alcohol enables the decoupling of the reactivity channels of a phenolic vs. a benzylic alcohol moiety in the enzymatic oxidation of bifunctional substrates having structural features comparable to portions of lignin. Experimental support is thereby attained for the central role of laccase in biodelignification, in spite of the seemingly lower oxidation power of this enzyme with respect to other and stronger oxidising enzymes excreted by ligninolytic fungi.

Oxygen bleaching of kraft pulp with polyoxometalates and laccase applying a novel multi-stage process

A series of manganese substituted polyoxotungstates, [SiW 11Mn III(H 2O)O 39] 5− and [PW 11Mn III(H 2O)O 39] 4−, and [SiW 11V VO 40] 5− were applied as catalysts for the oxygen delignification of unbleached eucalypt kraft pulp with laccase of Trametes versicolor. Unlike to modest results obtained in the Laccase-Mediator System (LMS) at 45–60 °C (lignin oxidation and catalyst re-oxidation occurred at the same stage), a sustainable delignification with removal of about 50% of residual lignin was achieved with [SiW 11Mn III(H 2O)O 39] 5− and [SiW 11V VO 40] 5− when the kraft pulp treatment was carried out with polyoxometalate at 110 °C (lignin oxidation stage) and with laccase at 45 °C (catalyst re-oxidation stage) in separate stages. The use of [PW 11Mn III(H 2O)O 39] 4− in this multi-stage process was limited by the low re-oxidation rate with laccase. The best selectivity on the pulp delignification was found with polyoxoanion [SiW 11Mn III(H 2O)O 39] 5−, whereas [SiW 11V VO 40] 5− was the most effective in the oxidative delignification. The influence of process factors on the POMs re-oxidation, such as the amount of laccase, oxygen pressure and temperature is discussed. UV–vis and 51V NMR studies indicated that POMs maintained stable after redox turnovers during the pulp delignification.

Polychronous kinetics of oxidative delignification of coniferous kraft cellulose with hydrogen peroxide

The kinetics of oxidative delignification of coniferous kraft cellulose with hydrogen peroxide in the temperature range 60–100°C was studied.

Lignin reactions in oxygen delignification catalysed by Mn(II)-substituted molybdovanadophosphate polyanion

Abstract The lignin oxidation in the oxygen delignification/bleaching process catalysed by Mn(II) substituted heptamolybdopentavanadophosphate polyanion (HPA-5-MnII) was studied using monomeric lignin model compounds and Eucalyptus globulus dioxane lignin adsorbed on the bleached sulphite pulp. The analysis of reaction products after lignin models oxidation in the presence HPA-5-MnII and the parent heptamolybdopentavanadophosphate polyanion (HPA-5) showed similar qualitative and quantitative patterns, indicating the identical lignin oxidative delignification mechanisms with both catalysts. The higher delignification selectivity of the pulp in the reaction system HPA-5-MnII/O2 than with HPA-5/O2 was explained by the protective effect of MnII in the heteropolyanion towards oxidative degradation of polysaccharides. The study of lignin adsorbed on bleached sulphite pulp, in the HPA-5-MnII/O2 delignification system, showed the cleavage of β-O-4 linkages as the major contribution to the lignin depolymerisation. The formation of Cα-O-polysaccharide linkages in β-O-4 structures of lignin during the delignification was observed.

Peroxide-Ammonia Delignification of Pine Wood

Oxidative delignification of pine wood by hydrogen peroxide in aqueous ammonia was studied. A new method for producing cellulose and N-containing organic fertilizers was developed. It was demonstrated that a solution of hydrogen peroxide (3-9%) in aqueous ammonia delignified extensively wood to produce cellulose in yields up to 54.6% with a residual lignin content up to 1.1%. The process of pine-wood delignification by hydrogen peroxide in aqueous ammonia was described satisfactorily by a first-order kinetic equation.

Oxidative Delignification of Flax Fiber

Oxidative delignification of flax fiber at 98°C was studied. The intermediate material was produced in moderate yield with high degree of delignification. The optimum conditions of the process were determined.

Oxygen bleaching of kraft pulp catalysed by Mn(III)-substituted polyoxometalates

The catalytic response of Mn(III)-substituted polyoxometalates (MSP) of series [XW 11Mn III(H 2O)O 39] n− (X=B, Si, P) on the oxygen bleaching of eucalypt kraft pulp was investigated. All MSP tested showed catalytic activity in the delignification, which depended on MSP redox potentials and rates of their re-oxidation with dioxygen. The best delignification results were obtained with SiW 11Mn III, possessing relatively high redox potential ( E°=+0.65 V) and being partially re-oxidised by dioxygen during the pulp bleaching. Under the conditions of SiW 11Mn III catalysed bleaching (100 °C, 2 h, pH 4) the selectivity of delignification was higher than that in the conventional oxygen–alkaline bleaching. The delignification mechanism was elucidated by kinetic studies and by oxidation experiments with SiW 11Mn III using monomeric lignin model compounds (homovanillyl and homoveratryl alcohols). The delignification rate showed a pseudo first order with respect to lignin and a non-integer, lower than one, order for SiW 11Mn III and O 2. The effective activation energy of the delignification was about 76 kJ/mol. It was suggested that two consecutive one-electron oxidations of lignin substructures by SiW 11Mn III is the delignification rate limiting reaction step. The oxidation of phenolic lignin structures was much faster (more than 10 times) than non-phenolic units, i.e. the oxidative delignification is determined by reactivity of phenolic units. The delignification time profile in the reaction system SiW 11Mn III/O 2 showed a fast initial delignification period corresponding to 35–40% lignin removal followed by a slow delignification period. These features were explained by different reactivity of phenolic and non-phenolic lignin units as well as by the competitive coupling reactions of one-electron oxidised lignin substructures.

In vitro fermentation of rye carbohydrates including arabinoxylans of different structure

Freeze-dried ileal effluent from cannulated pigs fed rye bread diets based on either whole rye or one of three rye milling fractions enriched in pericarp/testa, aleurone and endosperm was incubated in an in vitro fermentation system with faecal inocula from pigs fed a whole grain rye diet. Three and 48 hours of in vitro degradation of arabinoxylans were comparable to the caecal and faecal degradation respectively. The productions of short-chain fatty acids were highest from the ileum samples of the diets high in fermentable carbohydrates (whole rye, aleurone and endosperm). However, fermentation of the pericarp/testa ileum sample produced half the amount of short-chain fatty acids resulting from the other ileum samples despite the fact that the pericarp/testa carbohydrates (CHO) were not fermented at all. Treating the pericarp/testa with alkali prior to the incubation increased fermentability considerably (from 0 to 49% of total arabinoxylans) and suggested that, at least in lignified tissue, alkali-labile cross-links were more important determinants of arabinoxylan (AX) degradability than AX structure itself. Oxidative delignification of the pericarp/testa ileal effluent increased fermentability too, but to a much lower degree, which indicates that the type and extent of links between polymers and lignin rather than the amount of lignin are important in determining degradability. © 2000 Society of Chemical Industry