Abstract
From the variety of methods known for the depolymerization of organosolv lignin, a broad range of diversely substituted aromatic compounds are available today. In the present work, a novel two-step reaction sequence is reported, which is focused on the formation of phenols. While the first step of the depolymerization strategy comprises the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-catalyzed oxidation of organosolv lignin with nitrogen monoxide so that two waste materials are combined, cleavage to the phenolic target compounds is achieved in the second step employing hydrazine and potassium hydroxide under Wolff–Kishner-type conditions. Besides the fact that the novel strategy proceeds via an untypical form of oxidized organosolv lignin, the two-step sequence is further able to provide phenols as cleavage products, which bear no substituent at the 4-position.
Highlights
As a plant-derived biopolymer, lignin represents one of the major constituents of biomass besides cellulose and hemicellulose
We show that the two-step combination of nitrogen monoxide and hydrazine, when applied to organosolv lignin, does not lead to phenols 17 and pyrazoles
It has been shown that phenols can be obtained from organosolv lignin by a two-step sequence comprising DDQ-catalyzed oxidation with nitrogen monoxide followed by hydrazine-mediated cleavage and reduction
Summary
As a plant-derived biopolymer, lignin represents one of the major constituents of biomass besides cellulose and hemicellulose. In native lignin, which is largely colorless, the molecular weight of the oligomers and polymers may range from 1000 to 20 000 g/mol.[1] After acidic or alkaline treatment, which may be applied prior to a depolymerization process, the color of lignin typically changes to dark brown.[2] In 2014, the overall lignin market in the world amounted to around U.S $775 million and estimations foresee an increase to U.S $900 million by the year 2020.1 As the major side product in the pulp and paper industry,[3] lignin represents one of the most readily available natural polymers,[4] only around 2% of the 50 million tons of lignin were used for the production of chemicals in 2010. The remaining amount, and the vast majority, is transferred to power and heat generation.[5]
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