Selective Oxidation Of Lignite For The Preparation Of Carboxyli Chemicals And Inhibition Of CO₂ Production

Lignite is rich in reserves and it has a high oxygen content,which can be used to prepare carboxylic chemicals by oxidation methods.Carboxylic chemicals are important chemical basic materials,among which the production of benzene carboxylic acids（BCAs）is closely related with the structure of lignite.At present,the structural model of lignite can be inferred based on the yield distribution of BCAs.Therefore,whether high（or close to theoretical）yield of BCAs can be obtained by oxidation methods is a critical issue for efficient utilization of lignite and accuracy of lignite structural model.The complex structure of lignite makes it impossible to calculate the theoretical yield of BCAs from the lignite,but the difference between the yield of BCAs obtained by the used oxidation method and the theoretical yield can be explored indirectly through model compounds.In this work,lignite and its model compounds were oxidized by three methods:alkali-oxygen oxidation,alkaline potassium permanganate oxidation and catalytic oxygen oxidation.The effects of reaction conditions on the yields of carboxylic acids（especially BCAs）were investigated.The regularity of the oxidation of model compounds was explored.The differences in the inferred aromatic ring structures of lignite were compared between the two oxidation methods,alkali-oxygen oxidation and alkaline potassium permanganate oxidation.A large amount of CO₂ is generated during the oxidation of lignite.Methanol is added to the Na VO₃-catalyzed oxidation system and can inhibit CO₂ generation from lignite.However,the conversion mechanism is not clear.In this work,model compounds of lignite were used to be oxidized in Na VO₃-catalyzed oxidation system to reveal the mechanism.The main conclusions are shown as follows.（1）The molar yields of products obtained from model compounds by alkali-oxygen oxidation were 20%～60%of theoretical yields;the molar yields of products by alkaline potassium permanganate oxidation were all above 70%of theoretical yields.So,the oxidation of lignite by potassium permanganate could yield benzene carboxylic acids close to theoretical yields.（2）The oxidation of Xiaolongtan（XLT）lignite by alkali-oxygen oxidation could produce BCAs and small molecular acids（SMAs）with mass yields of20.5%and 38.2%,and the oxidation by alkaline potassium permanganate yielded BCAs and SMAs with mass yields of 31.4%and 28.5%.The differences in yield distribution of BCAs obtained by the two oxidation methods were due to the peroxidation of BCAs under the alkali conditions.（3）The aromatic ring structure of XLT lignite is mainly composed of monocyclic and bicyclic rings.The content of aromatic ring structures in XLT lignite that can be converted to BCAs has been quantified as 1.067 mmol/g based on the yield of benzene carboxylic acids.The main pathway of generation of benzene carboxylic acids from lignite was proposed.（4）The regularity of Na VO₃-catalyzed oxygen oxidation of lignite model compounds to carboxylic acids was investigated.Benzene carboxylic acids were generated from aromatic ring structures without oxygen atoms attached to at least one benzene ring.To reveal that the addition of methanol can inhibit CO₂ production during the catalytic oxidation of lignite,model compounds of lignite were also used to be oxidized in the Na VO₃ catalytic system.The regularity of inhibiting CO₂ production by the addition of methanol in the catalytic system was investigated.The results show that the addition of methanol to the Na VO₃ catalytic system can effectively reduce the formation of CO₂ in the oxidation process for the structures without oxygen attached on the benzene ring and for the structures with benzene carboxylic acids and one or none hydroxyl group on the benzene ring.Small molecule acids that are relatively stable in the reaction system（e.g.,fumaric acid）can be esterified with methanol,thus protecting it from excessive oxidation.