Reusability Investigation of a Ruthenium Catalyst during Ruthenium Ion-Catalyzed Oxidative Depolymerization of Lignite for the Production of Valuable Organic Acids.

Abstract

A clean and efficient conversion process is essential for the utilization of low-rank coals. Lignite, a typical representative of the low-rank coal family, has huge potential for the production of valuable chemicals via the oxidative depolymerization reaction. Ruthenium ion-catalyzed oxidation (RICO) is an effective route for lignite depolymerization under mild conditions, but the high cost of precious Ru limits the potential large-scale application of RICO. How to recycle and reuse Ru is critical to promote the application of RICO. In this work, a novel and efficient approach for reusing Ru through recycling the solvent mixture containing Ru was established for RICO. First, the influence of different reaction parameters on the depolymerization degree of lignite and benzene polycarboxylic acid (BPCA) yields was investigated. Second, the distribution of Ru in the organic phase (OP), aqueous phase (AP), and residual solid phase (RSP) was analyzed after the RICO reaction. Finally, based on the distribution of Ru in different phases, a novel route of recycling Ru by reusing the Ru-containing solvents was proposed. The results showed that the dosage of RuCl3 and NaIO4 had a significant influence on both the depolymerization degree of lignite and BPCA yields. The distribution of Ru had a close relationship with the depolymerization degree of lignite and the dosage of NaIO4. After the depolymerization reaction, the CCl4 phase containing Ru was reused directly as the solvent for the next run, which could fulfill the reuse of both CCl4 and Ru. The results proved that the Ru-containing CCl4 phase could maintain catalytic performance for 5 runs. This work provides an efficient route to reuse Ru for the RICO depolymerization of lignite into valuable organic acids. As far as we know, this is the first report concerning the recycling and reuse of Ru during the RICO of lignite. This work is important for the application of RICO in lignite depolymerization.