Ammonium salt-mediated antisolvent separation of valuable benzene pentacarboxylic and hexacarboxylic acids from the oxidative depolymerization product mixture of lignite

Abstract

Oxidative depolymerization of lignite into valuable chemicals such as benzene polycarboxylic acids (BPCAs) is a potential pathway for the high-value and non-energy utilization of lignite. Up to now, the selective separation of BPCAs from the complex depolymerization product mixtures remains a huge challenge and impedes the development of this route. BPA and BHA are key platform molecules for constructing high performance and functional materials. In this work, based on the differences in physical and chemical properties of BPCAs, ammonium salt-mediated antisolvent separation of BPCAs from the depolymerization products mixture of lignite was developed. The effects of different separation parameters on the separation efficiency were systematically studied. The results showed that the route could selectively separate benzene hexacarboxylic acid (BHA) and benzene pentacarboxylic acid (BPA) from both the simulated solution and the real lignite depolymerization products. For the real system, the separation yields of BHA and BPA were 76.0 % and 90.0 %, respectively. BHA and BPA accounted for 93.0 % among all BPCAs in the separated solution, indicating an enhanced purity compared to the initial depolymerized product mixture. The antisolvent methanol had high selectivity for BHA and BPA, and the separation selectivity could be tuned by optimizing the ammonia dosage, antisolvent methanol dosage, and the pH of the mother solution. As far as we know, this is the first report fulfilling the selective separation of the valuable BHA and BPA from the real complex depolymerized product mixture of lignite. This work contributes new separation route to promote the depolymerization utilization of lignite.