Selective Conversion of Lignin into Simple Aromatic Compounds
Lignin is a major component of non-edible biomass (15-30 percent by weight; 40 percent by energy). It is a cheap byproduct of pulp and biofuel production and is one of the few naturally occurring sources of high-volume aromatic compounds. Converting lignin’s complex biopolymer structure into simple organic chemicals attracts substantial interest.
For example, the world’s supply of artificial vanillin is produced by oxidizing spent sulfite liquor. However, this requires a chemical feedstock that arises from an expensive and polluting sulfite pulping process used in very few mills today. Other methods consume energy or are not suitable on an industrial scale. Also, reactions can be explosive.
All such methods suffer from safety and environmental concerns, as well as lack of structural specificity when starting from raw lignin. Clearly, better ways to chemically transform lignin are needed.Description
UW–Madison researchers have developed a metal-free, aerobic oxidation method that selectively transforms the benzylic alcohol in lignin to the corresponding ketone. The process uses a nitric acid (HNO3) catalyst combined with another Brønsted acid. The reaction leaves unchanged at least a portion of unprotected primary aliphatic alcohols in the lignin or lignin subunit.
The reaction may be carried out in any suitable polar solvent and in the presence of additional reagents including TEMPO and derivatives.Benefits
- Strong reactivity and selectivity
- No expensive catalysts
- Potential for high yield
First step in the conversion of lignin to valuable aromatic fine chemicals
Rahimi A., Azarpira A., Kim H., Ralph J. and Stahl S. S. 2013. Chemoselective Metal-Free Aerobic Alcohol Oxidation in Lignin. J. Am. Chem. Soc. 135, 6415–6418.Patents and Patent Applications
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SELECTIVE AEROBIC ALCOHOL OXIDATION METHOD FOR CONVERSION OF LIGNIN INTO SIMPLE AROMATIC COMPOUNDS
Described is a method to oxidize lignin or lignin sub-units. The method includes oxidation of secondary benzylic alcohol in the lignin or lignin sub-unit to a corresponding ketone in the presence of unprotected primarily aliphatic alcohol in the lignin or lignin sub-unit. The optimal catalyst system consists of HNO.sub.3 in combination with another Bronsted acid, in the absence of a metal-containing catalyst, thereby yielding a selectively oxidized lignin or lignin sub-unit. The method may be carried out in the presence or absence of additional reagents including TEMPO and TEMPO derivatives.
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