Metal ion-induced separation of valuable organic acids from a depolymerized mixture of lignite without using organic solvents

Ruthenium ion catalytic oxidation depolymerization of lignite under ultra-low dosage of RuCl3 catalyst and separation of the organic products with inorganic salts

Depolymerization of lignite into valuable chemicals via ruthenium ion catalytic oxidation (RICO) is a potential route for the non-energy utilization of lignite. However, the high cost of the Ru catalyst during depolymerization and the high content of inorganic salts in the product solution limit the development of this route. In this work, RICO depolymerization of lignite was conducted under an ultra-low dosage of RuCl₃ catalyst to decrease the usage of the catalyst during the RICO process. Different approaches were attempted to fulfill the separation of benzene polycarboxylic acids (BPCAs) products with the inorganic salts derived from the oxidant NaIO₄, including butanone extraction and desalting via crystallization under different temperatures. The results show that lignite can be efficiently depolymerized under the mass ratio of RuCl₃/lignite as low as 1/1000 by prolonging the reaction time without decreasing the depolymerization degree and BPCAs yields compared to the commonly used mass ratio of 1/10. Butanone can extract ca. 91% of the total BPCAs in the product solution, and the inorganic salts content (mainly NaIO₃) in the extraction solution was as low as 0.19 mg mL^-1. A new strategy of first acidification of depolymerization aqueous solution by HCl and then extraction by butanone is proved to be efficient for the separation of BPCAs with inorganic salts. Salting out via crystallization under lower temperature can remove ca. half content of the salts, and the efficiency is inferior to butanone extraction. The low usage of RuCl₃ can efficiently decrease the catalyst cost of the RICO process, and butanone extraction can fulfill the enrichment of BPCAs and the separation of BPCAs with inorganic salts. This work is meaningful for the potential application of RCIO depolymerization of lignite for the production of valuable chemicals.

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

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.

A General Route of Using Lignite Depolymerized Derivatives for Catalyst Construction: Insights into the Effects of the Derivative Structures and Solvents

Depolymerization is an emerging and promising route for the value-added utilization of low-rank coal (LRC) resources, and how to use the complex depolymerized mixtures efficiently is of great importance for this route. In this work, we designed the rational route of using depolymerized mixtures from lignite via ruthenium ion-catalyzed oxidation (RICO) depolymerization directly without complex separation to construct a Zr-based hydrogenation catalyst. The prepared catalyst was applied into the catalytic transfer hydrogenation of biomass-derived carbonyl compounds. Meanwhile, a copper-based oxidation catalyst was also constructed via a similar route to investigate the universality of the proposed route. Special insights were given into how the depolymerized components with different structures influenced the performances of the catalysts. The effects of the solvents used during the catalyst preparation (H2O and DMF) were also studied. The results showed that the proposed route using the depolymerized mixtures from lignite via RICO to construct catalysts was feasible for both Zr-based and Cu-based catalysts. The two catalysts prepared gave high efficiency for their corresponding reaction, i.e., the Zr-based catalyst for catalytic transfer hydrogenation of biomass-derived carbonyl compounds and the Cu-based catalyst for selective oxidation of alcohols into aldehydes. Different depolymerized components contributed differently to the activity of the catalyst, and the solvents during the preparation process could also influence the activity of the catalyst. The depolymerized components and the solvents influenced the activities of the Zr-based catalyst mainly via changing the Zr contents, the microenvironment of Zr4+, and the specific areas of the catalyst.