Coordination Separation Of Benzene Carboxylic Acids Using Metal Ions From The Oxidative Depolymerization Products Of Lignite

Oxidative depolymerization of lignite to produce valuable organic acids such as benzene polycarboxylic acids（BPCAs）is a potential route to achieve non-energy and value-added utilization of lignite.However,due to the complexity of depolymerization products of lignite,the separation process becomes the bottleneck problem during the utilization of the depolymerization products,which limits seriously the development of this route.It is of great significance to explore the effective separation of high-value chemicals from lignite depolymerization products,and to promote the development of lignite oxygenated polymerization to prepare high-value additional chemicals and the clean utilization of lignite.In this study,alkali-oxygen oxidation method was used to depolymerize Shengli lignite to prepare depolymerized products,which contained BPCAs.Based on the coordination properties of carboxylic acids with metal ions,the coordination separation of BPCAs from depolymerization product mixture via the coordination of metal ions was proposed.The specific research contents and conclusions of this thesis are as follows:（1）The types and contents of BPCAs in the depolymerized products were determined by qualitative and quantitative analysis.Based on the structural characteristics of BPCAs,the feasibility of separation of BPCAs by the coordination between metal ions and BPCAs in the depolymerization solution was explored.Considering the complexity of depolymerized products,the feasibility of this route was verified in BPCAs simulated solution and then applied to real solution of lignite depolymerization products.The separation of BPCAs with different metal ions was studied,and the results showed that the separation route was feasible,and different metal ions showed different separation behaviors for BPCAs.Among them,rare earth metal ions(Ce³⁺,La³⁺,Pr³⁺,Sm³⁺and Yb³⁺)had relatively high separation yield for BPCAs in lignite depolymerization products,but the selectivity was low.Alkaline earth metal ions(Ca²⁺,Sr²⁺and Ba²⁺)had good selectivity for binding BPCAs in lignite depolymerization products,but the separation rate was low.（2）In order to improve the selectivity of rare earth metal ions,the effects of solution p H,amount of metal salts,reaction time and temperature on the separation of rare earth metal ions coordination BPCAs were studied.The results showed that the rare earth metal ions showed strong binding force with the eight BPCAs.The BPCAs in the lignite depolymerization products separated by the coordination of rare earth metal ions had relatively high separation rates.For example,under the conditions of solution p H=8,metal salt dosage 0.02 g/m L,reaction temperature 50°C and reaction time 2 h,the separation yield of different benzene carboxylic acids in the lignite depolymerization products of Yb³⁺could reach that of mellitic acid 75.1%,benzene pentacarbonic acid88.3%,pyromellitic acid 87.6%,prehnitic acid 87.6%,mellophanic acid 87.7%,trimellitic acid 77.1%,hemimellitic acid 77.3%and phthalic acid 45.5%.It was found that the solution p H,the amount of rare earth metal salts and the reaction time only affected the separation rate of BPCAs,but the reaction temperature did not.In addition,the separation effects of Yb³⁺coordination separation and butanone extraction of BPCAs were compared.The results showed that the two separation methods had similar separation effects,indicating that metal ion coordination separation of BPCAs was effective,and the use of organic solvents was not required for coordination separation.（3）In order to improve the separation effect of alkaline earth metal ions,the effects of solution p H,metal salt dosage,reaction time and temperature on the separation of alkaline earth metal ions BPCAs were studied.The results showed that Ca²⁺,Sr²⁺and Ba²⁺showed high selectivity for BPCAs with the number of carboxyl group≥4 at p H of 8(Ca²⁺)or 6(Sr²⁺and Ba²⁺).Under the optimal separation conditions:the solution p H=6,metal salt dosage 0.02 g/m L,reaction temperature 50°C and reaction time 2 h,the separation yield of different BPCAs in the lignite depolymerization products of Sr²⁺can reach mellitic acid35.2%,benzene pentacarbonic acid 3.0%and phthalic acid 5.0%.The separation yield of different BPCAs in the depolymerized products of Ba²⁺can reach mellitic acid 30.9%,benzene pentacarbonic acid 35.2%,pyromellitic acid 4.9%,prehnitic acid 25.4%,mellophanic acid 10.2%,trimellitic acid 5.0%and phthalic acid 2.6%.By comparing the effects of different factors,it was found that the solution p H had a significant effect on the selectivity and separation yield of BPCAs,while the amount of metal salt,reaction temperature and time mainly affected the separation yield of BPCAs,but had no obvious effect on selectivity.The precipitation structure of BPCAs intermediates formed by alkaline earth metal ions and BPCAs was analyzed.The results showed that BPCAs were separated from the mixture by the coordination between alkaline earth metal ions and carboxyl groups in BPCAs.In addition,alkaline earth metal ions(Ca²⁺)were recovered and recycled.The results showed that Ca²⁺could be reused for three times,and the separation efficiency of BPCAs was not significantly reduced.This paper established an effective route for separation of benzene carboxylic acid in lignite depolymerization products by metal ion coordination,which is expected to provide basic data and technical reference for separation of organic acids in lignite depolymerization products.