Selective oxidation of lignite to carboxyl chemicals

Abstract

Selective oxidation of lignite to carboxyl chemicals is an important way for the utilization of lignite in a clean and efficient manner. At present, the methods of lignite oxidation to carboxyl chemicals include nitric acid oxidation, sodium hypochlorite oxidation, hydrogen peroxide oxidation, ruthenium-ion-catalyzed oxidation, alkali-oxygen oxidation, catalytic oxygen oxidation and other oxidation. Different methods of lignite oxidation may result in different carboxyl chemicals in compositions and yields. However, the oxidation product is a mixture including several carboxyl chemicals and needs to be separated and purified for further utilization. Therefore, researchers have also studied the separation of carboxylic acids using different ways. In this review, first, different methods of lignite oxidation and their mechanism of lignite oxidation are introduced, and the properties and the problems of the oxidation methods are analyzed. Second, the extraction of carboxylic acids in reaction solutions and the separation of carboxylic acid mixtures and the existing problems are also reviewed and discussed. Although many efforts have been paid to the selective oxidation of lignite to carboxyl chemicals, many problems still exist in the ways to industrial applications. Therefore, finally, the challenges and possible future developments in selective oxidation of lignite to carboxyl chemicals are analyzed and outlined.