Abstract
Being the major renewable source of bio-aromatics, lignin possesses considerable potential for the chemical industry as raw material. Kraft lignin is a couple product of paper industry with an annual production of 55,000,000 ton/y and is considered the largest share of available lignin. Here we report a facile approach of Kraft lignin depolymerization to defined oligomeric units with yields of up to 70 wt.%. The process implies utilization of an aqueous base in combination with a metal containing catalyst and an alcohol under non-oxidative atmosphere at 300 °C. An advantage of the developed approach is the facile separation of the oligomer product that precipitates from the reaction mixture. In addition, the process proceeds without char formation; both factors make it attractive for industrialization. The suppression of the repolymerization processes that lead to char formation is possible when the combination of metal containing catalyst in the presence of an alcohol is used. It was found that the oligomer units have structural features found in phenol-acetaldehyde resins. These features result from the base catalyzed condensation of lignin fragments with in situ formed aldehydes. Catalytic dehydrogenation of the alcohol provides the latter. This reaction pathway is confirmed by the presence condensation products of Guerbet type reactions.
Highlights
In the context of increasing greenhouse gas emissions, the search for renewable resources that can be integrated into petrochemical values chains becomes more and more pressing [1]
In order to isolate maximum amount of lignin oligomers products that tend to stick to the internal parts of the autoclave, the autoclave and stirrer were washed with ethanol and this solution was further used for washing of retentate collected on the filter
It was found that presence of catalyst, alcohol, base, and non-oxidative gas are all required for precipitation of lignin oligomers out of solution
Summary
In the context of increasing greenhouse gas emissions, the search for renewable resources that can be integrated into petrochemical values chains becomes more and more pressing [1]. Later Huang et al studied the effect of supercritical ethanol on lignin depolymerization with Cu-doped MgAl mixed oxide (CuMgAlOx) as a catalyst at 300 ◦C for 4–8 h In this case the high yield of deoxygenated monoaromatics was obtained without char formation. Watanabe and coworkers showed that the acid-catalyzed degradation of lignin in a hydrophobic solvent like toluene, containing a small amount of methanol under mild temperatures 140–170 ◦C, helps to suppress char formation. Obtained in this case were monoaromatics existing in the form of dimethyl acetal derivatives due to reaction of in situ formed enol ether with alcohol [55]. The use of a hydrogenation/dehydrogenation catalyst in the presence of an alcohol was found to minimize char formation, while an inorganic base serves as depolymerization catalyst
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