Study On The Distribution Of Characteristic Acidic Groups Of Coal-based Fulvic Acid

In this paper,Hulunbuir lignite was used as the experimental coal to extract the coal-based fulvic acid and to realize the efficient utilization of lignite.Hydrogen peroxide combined with microwave to oxygenate lignite was used to extract fulvic acid,and high purity fulvic acid was obtained after purification by sulfuric acid-ethanol.Combined with chemical analysis and modern instrument analysis,some important structural information of the microstructures of fulvic acid can be obtained.In the experiment of extracting fulvic acid by hydrogen peroxide combined with microwave oxidation of lignite,the single factor and orthogonal test were used to optimize the reaction conditions.The optimal conditions for the extraction of fulvic acid from Hulunbuir lignite were as follows:the concentration of hydrogen peroxide was 18%,the reaction time was 10 min,microwave power was 300 W,the liquid-solid ratio was 8 and the yield was 17.64%.The liquid-solid ratio had the greatest effect on the experimental results.The crude fulvic acid was purified using the sulfuric acid-ethanol method to remove the inorganic salt components.Through single-factor and orthogonal test analysis,the best purification conditions were as follows:acid-to-material ratio was 0.10,purification temperature was 60°C,liquid-solid ratio was 10,purification time was 20 min,and yield was 90.42%.The ratio of acid and material had the greatest impact on the experimental results.After purification,the fulvic acid ash content decreased from 27.76%to 4.51%,and the purification effect was better.The grading yield of high-purity fulvic acid can reach 89.05%.Fulvic acid contains more polar molecules.Through elemental analysis,UV/Vis spectral analysis,infrared spectroscopy,thermogravimetric analysis,X-ray photoelectron spectroscopy,thermal analysis and¹³CNMR spectroscopy analysis of fulvic acid grading products,the results showed that:The content of characteristic acidic functional groups decreases with increasing relative molecular weight.The content of oxygen was significantly higher than that of lignite,and the E₄/E₆ values of grading products of fulvic acid were similar,indicating that they had similar aromatization degree and carbonyl conjugated degree.Fulvic acid contains more hydrophilic groups such as carboxyl groups and hydroxyl groups on the surface.During the pyrolysis process,fulvic acid has a large number of aromatic hydrocarbons and their substitutions,heterocyclic compounds,acid anhydrides formed by the decarboxylation of carboxyl groups,phenolic substances,carboxylic acid substances,and a small amount of sulfur-containing nitrogen compounds.A long straight chain structure is not contained in the detected substance,indicating that its molecular structure does not contain or contain less long-chain branched fatty structures.Based on experimental data,an average molecular model of fulvic acid grading products was constructed.The molecular formula was C₁₇H₁₉NO₁₀?Mr=397??C₃₃H₂₇NO₂₃?Mr=805??C₄₈H₃₇NO₃₆?Mr=1203?.From the optimized 2D and 3D molecular models,it can be seen that the molecular structure of fulvic acid contains a large number of characteristic acidic groups,of which the carboxyl content is about 2to 3 times the phenolic hydroxyl content,and the carboxyl groups are mostly distributed in the fatty chain.In the end,there is a small part directly linked to the benzene ring.The infrared spectrum obtained by Gaussian software simulation is in good agreement with the measured spectrum.Therefore,the molecular model constructed can reflect the molecular structure of fulvic acid to some extent.