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Relevant academic research and scientific papers

Immobilized methyltrioxo rhenium (MTO)/H2O2 systems for the oxidation of lignin and lignin model compounds

A convenient and efficient application of heterogeneous methylrhenium trioxide (MTO) systems for the selective oxidation of lignin model compounds and lignins is reported. Environmental friendly and low-cost H2O2 was used as the oxygen atom donor. Overall, the data presented and discussed in this paper point toward the conclusion that the immobilized heterogeneous catalytic systems based on H2O2/and MTO catalysts are able to extensively oxidize both phenolic and non-phenolic, monomeric, and dimeric, lignin model compounds. Condensed diphenylmethane models were found also extensively oxidized. Technical lignins, such as hydrolytic sugar cane lignin (SCL) and red spruce kraft lignin (RSL), displayed oxidative activity with immobilized MTO catalytic systems. After oxidation, these lignins displayed the formation of more soluble lignin fragments with a high degree of degradation as indicated by the lower contents of aliphatic and condensed OH groups, and the higher amounts of carboxylic acid moieties. Our data indicate that immobilized MTO catalytic systems are significant potential candidates for the development of alternative totally chlorine-free delignification processes and environmental sustainable lignin selective modification reactions.

Methyltrioxorhenium: A new catalyst for the activation of hydrogen peroxide to the oxidation of lignin and lignin model compounds

The oxidative degradation of lignin under totally chlorine free conditions is one of the most relevant targets for the design of environmental friendly pulping and bleaching industrial processes. Methyltrioxorhenium was found a powerful and promising catalyst for the oxidation of both phenolic and non-phenolic lignin model compounds by use of hydrogen peroxide as primary oxidant. Three different technical lignins, hydrolytic sugar cane lignin (SCL), red spruce kraft lignin (RSL) and a hardwood organosolvent lignin (OSL), that are representative examples of widely diffused para-hydroxyphenyl-guaiacyl, guaiacyl and guaiacyl-syringyl lignins, were also extensively degraded under similar experimental conditions.

Photochemistry of methoxyhydroquinone and methoxy-p-benzoquinone in solution related to the photoyellowing of the lignocellulosics

The photoreactivity of methoxy-p-benzoquinone (MQ) and methoxyhydroquinone (MHQ) in dilute solution (10-4-10-3 M) was studied using continuous irradiation and laser flash photolysis (LFP). The quinone irradiated in degassed tetrahydrofuran (THF) gives MHQ and an adduct with the solvent. Only the formation of hydroquinone is observed in ethanol, and hydroxylation is evidenced in water, whereas the compound is stable in CCl4. The bis-quinone, 4,4′-dimethoxybiphenyl-2,5,2′,5′-bisquinone, and the dibenzofurane-quinone, 8-hydroxy-3,7-dimethoxydibenzofuran-1,4-quinone, are formed in the presence of MHQ, whereas the reactivity is low with ethylconiferyl alcohol. When MHQ is irradiated selectively in degassed THF, the formation of MQ and of the bis-hydroquinone, 4,4′-dimethoxy-2,5,2′,5′-tetrahydroxy-biphenyl, are observed. The dimer is oxidized photochemically or thermally into the mono- or bis-quinones, the process being accelerated in alkaline medium. The formation of the dimers is strongly favored by the contiguous presence of quinone and hydroquinone. When MHQ is selectively irradiated in the presence of trans-ethylconiferaldehyde (EtC), quinone formation and isomerization of EtC are observed. LFP experiments, performed with a selective excitation of MQ, indicate that the triplet state of the quinone is strongly quenched by MHQ to conduce to a semiquinone radical. The interaction between 3MQ* and MQ is mainly driven by an electron transfer process according to the similar value of the quenching rate constant found with another electron donor compound such as 1,4-dimethoxybenzene. By contrast, no strong quenching of 3EtC* by MHQ was observed. It is proposed that the photochemistry of the couple MQ/MHQ is governed by the formation of encounter complex between either 3MQ* and MHQ or 3MHQ* and MQ. Consequently, the fast part of the photoyellowing of lignocellulosics does not appear to involve the couple MHQ/MQ or MHQ/etherified coniferaldehyde, but more likely a combination of oxidation of the hydroquinone by ground-state oxygen and photohydration of the formed quinone from its triplet state, giving inter-alia more colored o-quinonoid type molecules.

ESTER LINKAGES BETWEEN LIGNIN AND GLUCURONIC ACID IN LIGNIN-CARBOHYDRATE COMPLEXES FROM FAGUS CRENATA

Conjugate acid oxidation of benzyl esters with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and trifluoroacetic acid (TFA) was applied to the binding site analysis of ester linkages between lignin and glucuronoxylan in Fagus crenata wood.Based on the conjugate acid DDQ-oxidation of a watersoluble lignin-carbohydrate complex (LCC-WE) from the beech wood, the frequency of the ester bonds between the lignin and glucuronic acid residue of glucuronoxylan was determined to be 1.6 per molecule of LCC-WE. - Key words: Fagus crenata; Fagaceae; lignin-carbohydrate complex (LCC); 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).

Incorporation of H2(18)O into the Cα but not the Cβ Position in Degradation of a β-O-4 Lignin Substructure Model by Phanerochaete chrysosporium

Degradation of a β-O-4 lignin substructure model dimer by a white rot fungus, Phanerochaete chrysosporium, was investigated using a culture containing H2(18)O, and the following conclusion were made. a) The direct hydrolysis at Cβ of the β-O-4