Efficient Oxidative Depolymerization of Xilinguole Lignite to Produce Humic Acids with Little CO₂ Production

Abstract—At present, non-fuel utilization as alternative chemical feedstocks becomes very important and significant for the efficient conversion of lignite. To reduce CO₂ production, an oxy-cracking technique in weak alkaline aqueous was conducted to selectively depolymerize Xilinguole lignite (XL) into humic acid (HA) under mild condition in this paper. The results indicated that XL has high reactivity, and can be depolymerized into HA with very little CO₂. Elevated temperature is favorable for cracking of XL and converting HA into small molecular water-soluble acids and CO₂. KOH mainly catalyzes the oxidative depolymerization of XL macromolecular structure. Lower temperature can reduce the formation of CO₂ and the usage of alkali. Under 80°C, KOH/XL = 0.33 and 2 h, HA weight yield was the highest, reaching 81%, as well as no CO₂ detected. Further, the obtained HA was separated and characterized by FTIR, CPMAS ¹³C-NMR, TG/DTG and element analyses. Compared to XL and oxidized residue, HA contains more OH and COOH groups and aromatic rings, and less aliphatic groups. Due to low alkali usage and negligible CO₂, the oxidative depolymerization of XL to produce HA shows high utilization efficiency of carbon, and is a promising clean and efficient utilization approach.