Studies on Selective Oxidation of Lignite to Carboxyl-contained Chemicals and Coal Structures
Supervisor:liu zhen yu wu wei ze
Coal is the main source of energy and chemicals in China,and it is a mixture of complicated structures.The structures of coals determine their properties and further determine their applications.In this work,we investigated the selective oxidation of lignite to carboxyl-contained chemicals and the structures of coal.The main contents and conclusions are shown as follows.(1)In the previous alkali-oxygen oxidation of lignite,large amounts of inorganic alkali and acids are consumed,and the reaction temperatures are high.In this work,we developed three catalysts(H5PV2Mo10O40,NaV03,FeCl3)and proposed a new method to produce carboxyl-contained chemicals from lignite by catalytic oxidation with 02.The reaction pathway and mechanism were also investigated.The results indicate that this method has high efficiency to produce carboxyl-contained chemicals from lignite.Compared with alkali-oxygen oxidation,the method decreases the use of inorganic alkali and acids in a large amount,and lower the reaction temperature,and the catalysts have good reusability.In the process of catalytic oxidation,lignite is first converted into water-soluble intermediates,which are then converted into carboxylic acids.The second step is the rate controlling step.It has been found that in the catalytic system,catalyst promotes both the conversion of lignite and the generation of carboxylic acids.Sulfuric acid added in the catalytic system not only promotes the degradation of lignite,but also changes the activity of the catalyst.In the oxidation process,oxidized catalytic active species can be reduced by lignite and water-soluble intermediates to form reduced catalytic active species,which can be reoxidized by 02 to complete a redox cycle,processing the oxidation continuously.(2)Catalytic oxidation of lignite to carboxyl-contained chemicals by 02 is a high-efficiency method.However,in the process of catalytic oxidation of lignite,a large amount of CO2 was produced.In this work,we proposed a new method to suppress the generation of CO2 and improve the utilization of carbon in lignite,and we also investigated the mechanism of suppressing the formation of CO2 with methanol.The studies show that introducing methanol into NaV03-H2SO4 can effectively suppress the formation of C02 and promote the generation of oxygen-contained chemicals from lignite.So the utilization of carbon in lignite is greatly improved.The use of methanol decreased the catalytic activity of NaVO3 in aqueous solution and reduced the lignite conversion.A large number of parent structures of benzene carboxylic acids(BCAs)are preserved in the residues and are not converted into BCAs,so the yield of BCAs decreases.In the process of lignite oxidation,aldehyde and carboxyl groups contained products are generated as the intermediates.When methanol is introduced into the aqueous solution,aldehyde groups are converted into methylals by acetalization,and carboxyl groups are converted into esters by esterification.Hence,the application of methanol can protect aldehyde groups against being converted to carboxyl groups by acetalization,and carboxyl groups against being converted into C02 by esterification.(3)At present,the proposed structural models of lignite cannot explain the generation and the distributions of BCAs from lignite via oxidation.Therefore,we proposed a new method to construct the structural model of lignite,which can reflect the generation and distributions of BCAs.In this work,based on the distributions of BCAs from lignite via oxidation and combined with the ultimate analysis of lignite,the molecular formula and molecular weight of lignite structural model was determined firstly.Then,combined with the result of 13C NMR,a structural model of the aromatic clusters of lignite was constructed.At last,combined with all the results of characterization,a structural model of Huolinhe lignite was constructed.The proposed structural model can not only meet the results of various characterizations,but also can well explain the generation of BCAs in the process of lignite oxidation and their distributions.Through the construction of lignite structural model,we proposed a new way to study the structural model of coal,which can be used to study structures and structural models of other materials containing aromatic structures.(4)The change in structural characteristics of lignite during oxidation and the generation of BCAs distributions from lignite via oxidation were studied for insights into its structure and application.The result indicates that in Xiaolongtan lignite,naphthalene and benzene are the dominant aromatic rings,and around these aromatic rings,a large number of alkyl side-chains are connected.These structural characteristics determine that the yields of benzene pentacarboxylic acid,benzene tetracarboxylic acids and benzene tricarboxylic acids are more than those of other BCAs.In the process of alkali-oxygen oxidation,lignite is first converted into humic acids,then converted into water-soluble acids,and last to carboxylic acids.The last conversion step is the rate-controlling step.In the lignite oxidation,C-O bonds are easily cleaved.The aromatic structures(except condensed aromatic ring)are easily depolymerized from lignite,while the condensed aromatic rings and long alkyl chains are difficult to be oxidized and preserved in the residues.Humic acids from lignite via oxidation contained large numbers of aromatic structures and carboxyl groups,and the bridge chain lengths between two aromatic clusters are shorter than those of lignite.(5)The relationship of aromatic structures of coal and coal rank was revealed.In this work,we investigated the difference of BCAs distributions from 8 coals with different ranks via oxidation and the change of coal structures in the process of coalification.The result indicates that with the increase of coal rank,the total yield of BCAs increases,the structure of coal becomes more and more difficult to be degraded and the optimal reaction time increases.In the process of coalification(from lignite to anthracite),the aromatic cluster size and the degree of condensed aromatic rings are both increases,but the substituted degree of aromatic rings decreases.In lignite,aromatic rings are mainly benzene,naphthalene,and anthracene,and aromatic rings are arranged in a liner catenation manner.There is a large amount of alkyl side-chains around aromatic rings in lignite,and these aromatic rings in lignite tend to be connected with long aliphatic chains.The above structural characteristics determine that the yields of benzene pentacarboxylic acid,benzene tetracarboxylic acids and benzene tricarboxylic acids are more than those of other BCAs from lignite via oxidation.When the carbon content of coal is more than 87%,aromatic rings in bituminous coal are arranged in a circular catenation manner,and aliphatic chain length between aromatic clusters decreases.Aromatic rings in anthracites are mainly arranged in a circular catenation manner,more and more aromatic clusters tend to be directly connected to others.Compared with lignite,the yields of benzene tricarboxylic acids and benzene dicarboxylic acids are increases from bituminous coal and anthracite via oxidation.