Oxidation Of Lignite Research Articles

Study on the Characteristics of Combustible Mixed Gas Production during Lignite Oxidation Process

CO, H2, and other combustible gases will be produced during coal oxidation in coal mines, which will increase the risk of explosion when mixed with methane. Therefore, it is very important to understand the production characteristics of combustible gas during coal oxidation. In this paper, a programmed temperature gas test system is built to study the impact of lignite on the production of gases at different particle sizes and temperatures, and the release characteristics of gases are also analyzed. The result shows that the production of combustible gas is influenced by the coal particle size significantly when the temperature is above 200 °C, and it decreases as the particle size increases. CO is the main gas during the early stage of coal spontaneous combustion, and the release of CH4 and H2 increases after 300 °C. The fitted equations of gas generation and temperature are consistent with the experimental results. The research results are helpful in understanding the hazards of coal spontaneous combustion and have a certain guiding significance for coal mine monitoring and prevention of coal spontaneous combustion.

Synergistic effects of Paenibacillus mucilaginosus and Penicillium pimiteouiense on the extraction of humic substances from lignite

Based on the common step of alkaline extraction-acid precipitation, lignite treatment techniques have been developed to improve extraction efficiency and yield of humic acid (HA). This study explored the combination of bacterium Paenibacillus mucilaginosus (P.m) LT1906 and fungus Penicillium pimiteouiense (P.p) LL2205 strains for pre- and post-treatment of lignite collected from eastern Inner Mongol to enhance HA and fulvic acid (FA) yield and plant growth-promoting activity. The lignite was subjected to P.m followed by HNO3 treatment, HNO3 followed by P.p treatment, and a combination of P.m, HNO3 and P.p treatments, then performed by alkaline extraction-acid precipitation technique. The results indicated that P.m pretreatment increased HA yield by 12.47 %, while P.p posttreatment significantly enhanced FA yield by 13.37 %. Combined bacterial and fungal treatments notably improved the total yields of HA and FA by 19.12 %. Structural analysis showed that P.m pretreatment caused the deformation on the lignite surface and the complete lignite oxidation and leaching reactions, thereby increasing the production of HA, yet had no impact on the elemental composition or chemical structure of HA. P.p posttreatment introduced oxygencontaining functional groups and reduced aromatic components through oxidative depolymerization of lignite, leading to increased FA production and HA seed germination-promoting activity.

A combined experimental and theoretical study on the mechanism of catalytic oxidation of lignite to carboxylic acids by iron

The oxygen-oxidation of lignite to carboxylic acids (CAs) using environmentally friendly iron catalyst represents an efficient and clean technology for lignite utilization. However, the mechanism remains unclear due to the difficulty in observing the cleavage of chemical bonds and the challenge in capturing transient intermediates and radicals. In this work, the microscopic reaction mechanism of iron-catalytic oxidation of lignite was investigated by combining experimental and theoretical methods. Catalytic oxidation experiments revealed that water-soluble intermediates generated by solid-state lignite decomposition underwent a continuous carbon–carbon bond cleavage oxidation (including oxidative cleavage of aliphatic moieties and oxidative ring-opening of aromatic moieties) to form CAs with aldehyde moieties as the direct precursor. Based on molecular dynamics simulations and density functional theory, a reaction network from lignite to CAs was proposed. The iron catalyst was present in the form of iron-aqua complexes in the liquid phase and iron-phenyl complexes in the solid phase. These iron-aqua/iron-phenyl complexes could reduce the energies of oxidation reactions, promoting the cleavage of bridge bonds, the formation of o-quinones and the oxidation of aromatic moieties, thus accelerating elementary reactions for the formation of CAs. These findings provide a theoretical support for the oxidation of lignite and an in-depth insight into iron-catalytic mechanism.

Catalytic oxidation of lignite by Pt/TiO2 can enhance cadmium adsorption capacity

Addressing global warming necessitates innovative strategies in fossil fuel management. This study evaluates lignite, a low-rank coal with limited calorific value, exploring applications beyond its use as fuel. Utilizing Pt/TiO2 catalytic oxidation, the research aims to enhance the cadmium adsorption capacity of lignite in wastewater. Lignite, treated with 0.5% Pt/TiO2 at 125 °C for 2 h, demonstrated a threefold increase in cadmium adsorption capacity. Characterization using TGA-DSC confirmed the modification process as exothermic and self-sustainable. Spectroscopic analysis and Boehm titration revealed significant alterations in pore structure, surface area, and oxygen-containing functional groups, emphasizing the effectiveness of catalytic oxidation. Adsorption mechanisms such as complexation, cation exchange, and cation-π interactions were identified, enhancing Cd adsorption. Techniques, including the d-band model, H2-TPR, and O2-TPD, indicated that dissociative adsorption of molecular O2 and the subsequent generation of reactive oxygen species introduced additional oxygen-containing functional groups on the lignite surface. These findings provide essential strategies for the alternative use of lignite in environmental remediation, promoting sustainable resource utilization and enhancing cost-effectiveness in remediation processes. Environmental implicationThis study innovates in using lignite to reduce cadmium (Cd) contamination in wastewater. Employing Pt/TiO2 catalytic oxidation, lignite is transformed, enhancing its cadmium adsorption capacity. This process, being exothermic, contributes to decreased energy consumption. The approach not only mitigates the hazardous impacts of cadmium but also aligns with sustainability by reducing greenhouse gas emissions and energy use, showcasing a multifaceted environmental advancement.

Controllable selectivity of carboxylic acids produced by H3+n[PMo12‐nVnO40]‐catalyzed oxidation of lignite and its oxidative mechanism analysis

AbstractAlthough the oxidation of lignite has been extensively studied, the factors affecting the selectivity of carboxylic acids (CAs) are unclear. This attractive work examines the changes in CAs selectivity when lignite is oxidized by H3+n[PMo12‐nVnO40] (HPA‐n, n=1‐5) and discloses the catalytic mechanism of HPA. The results show that the selectivity of benzene polycarboxylic acids and acetic acid (AA) increases with increasing vanadium content of HPA‐n, such as mellitic acid and benzenepentacarboxylic acid from 0.8 % and 3.0 % to 5.4 % and 16.2 %, respectively. Consequently, the differences in the concentration of HPA or H2SO4 used to obtain optimal formic acid and AA yields confirm that the intrinsic properties of CAs calculated by Gaussian 09 can also affect their selectivity. HPA can not only activate lignite through electron transfer, but also further activate oxygen, leading to the generation of active substance ⋅OH in water. Consequentially, ⋅OH as the key reactive substance can promote the breakage of bridge bonds, side chains and condensed aromatic rings during HPA‐catalyzed oxidation of lignite. Thus, the oxidation pathway of lignite over HPA is proposed. This work constructs a relationship between oxidation capacity and CAs selectivity and discloses the lignite oxidation pathway at the molecular level, which is a significant progress in HPA‐catalyzed conversion of lignite.

A novel method for producing benzene polycarboxylic acids by electrochemical oxidation of Zhaotong lignite in aqueous NaCl solution

The development of a coal-based synthetic route to produce benzene polycarboxylic acids (BPCAs) is of great importance for the highly efficient utilization of lignites. In this paper, aqueous NaCl electrolytic system was used to oxidize Zhaotong lignite to prepare BPCAs. The electrochemical oxidation of lignite in aqueous NaCl solution could produce more BPCAs than that in aqueous NaOH solution. The aqueous NaCl electrolytic system could in-suit produce a stable OCl−, which was synthesized by the combination reaction between Cl2 and OH− generated in the anode or cathode, respectively. The in-suit produced OCl− would degrade the organic structures of the lignite dispersing in the electrolyte to generate BPCAs. The formation of BPCAs could be greatly affected by current density, electrolysis time and the addition amount of NaCl in the electrolytic system, which resulted from that the factors played an important role in the generation of OCl−. The coal related model compounds including anthracene and phenanthrene were used to investigate the electrochemical oxidation mechanism of the lignite. The results indicated that the aromatic ring structures in the lignite were attacked by O2− from the OCl− to afford BPCAs.

Systematic separation of carboxylic acids from H8[PMo7V5O40]-catalyzed lignite oxidation products via deep eutectic solvent

To separate the valuable carboxylic acids (CAs) from the complicated oxidized products (OPs) of H8[PMo7V5O40]-catalyzed lignite, systematic processes including distillation, ether extraction, acetone dissolution and separation by deep eutectic solvent (DES) were first constructed and sequentially performed. First, formic acid and acetic acid are separated in vacuum at 56 °C with the separation efficiency of 95% and 82%, respectively. Subsequently, sequential treatments of OPs by ether extraction and acetone/water dissolution are implemented to reduce the low-polarity compounds contents and fully dissolve CAs from concentrated residue. Separation results indicate that choline chloride (ChCl) shows a better separation performance compared to acetylcholine chloride and gives optimal separation efficiencies of 78.19% and 94.45% for benzenepentacarboxylic acid (BPA) and mellitic acid (MLA) within 2 h at ambient temperature, respectively. The hydrogen-bonded binding energies (BEs) formed in DES are calculated based on density functional theory, and results show that total BEs of MLA and ChCl is as high as −31.09 kcal/mol, and the next is −26.79 kcal/mol for BPA, leading to their quick and high separation efficiencies within 2 h. As a result, only MLA and BPA show the high correlation with pseudo-second-order kinetic model, and the rate constant of MLA is as high as 0.3367 g/(mg·h), which is far more than that of BPA. Additionally, the effect of hydrogen bonds on the separation of CAs follows the order of Cl∙∙∙H-O > O-H∙∙∙O-C > O-H∙∙∙O = C. Overall, this attractive strategy provides ideas for efficient separation of CAs, which will pave the way for utilization of lignite.

Crucial role of sulfonated ionic liquids as promoter for efficient production of valuable carboxylic acids by H5PMo10V2O40-catalyzed aerobic oxidation of lignite

To explore the effect of adding acidic ionic liquids (ILs) to the heteropoly acid/H2SO4 catalytic system on the aerobic oxidation of lignite, acidic ILs functionalized with -SO3H were synthesized accordingly and mixed with H5PMo10V2O40 (HPA-2) in various molar ratios. The optimum combination of HPA-2 and 1-methyl-3-(4-sulfobutyl) imidazolium (MIMBS) with molar ratio of 0.3 showed excellent performance for the catalytic oxidation of lignite, with the yields of carboxylic acids (CAs) (including benzene carboxylic acids, formic acid and acetic acid) reaching 326.21 ± 12.86 mg/g lignite, which considerably exceeded the 136.08 ± 6.23 mg/g lignite of pure HPA-2. Highest occupied molecular orbital energy of CAs calculated by Gaussian 09 on 6-31G(d) basis set and Hammett function analysis confirmed that the oxidative activity and acidity of the catalytic system could be moderately modulated by sulfonated ILs, thereby increasing the yield of CAs. In addition, the effect of H2SO4 concentration on the oxidation of lignite was investigated via the HPA-2/H2SO4/MIMBS system with different molar ratios, and the results showed that CAs yield increased first and then decreased with increasing H2SO4 concentration, but the concentration of H2SO4 at the maximum yield of CAs was significantly reduced from 0.28 wt% to 0.14 wt% once MIMBS was added. A possible process for the catalytic oxidation of lignite by sulfonated ILs/HPA-2/H2SO4 in aerobic aqueous solution is proposed, which generally involves the reactions of hydrolysis, oxidation and re-oxidation. Due to the good reproducibility, the robust homogeneous catalytic system of HPA-2/sulphonated ILs has the advantages of including increased yields of CAs and reduced H2SO4 dosage, which will pave the way for the efficient utilization of lignite.

Study on the mechanism of lignite oxidation inhibition by antioxidant resveratrol

Study on the mechanism of lignite oxidation inhibition by antioxidant resveratrol

Physicochemical properties, combustion kinetics and thermodynamics of oxidized lignite

Lignite oxidizes easily during storage and transportation. Furthermore, the surface characteristics of lignite change during oxidation, affecting its thermal conversion process. Herein, the surface functional groups and pore structures of lignite oxidized at temperatures below 190 °C were investigated. Moreover, the kinetics and thermodynamics of the combustion of lignite were analyzed using thermogravimetric analysis. The relationship between the surface structure and combustion performance of oxidized lignite was established, and the effect of oxidation on the characteristics of the combustion process was evaluated. The results revealed that the value of CH3/CH2 increased considerably upon increasing oxidation temperature from 70 to 150 °C. The number of –OH groups decreased with increasing in oxidation temperature, whereas the number of CO groups decreased gradually, then increased significantly, and lastly decreased with increasing oxidation temperature. The mesoporous structure of lignite was severely damaged during oxidation, the specific surface area and pore volume of oxidized lignite were lower than those of raw coal, and the average pore diameter of oxidized lignite was significantly higher than that of raw lignite. The poor combustion performance of oxidized lignite was attributed to its low mesopore specific surface area and low pore volume. The activation energy (Ea) of lignite during combustion was calculated to be 35.29 kJ/mol using the Ginstling-Broushtein equation, and the Ea of oxidized lignite ranged between 40.53 and 45.93 kJ/mol. These results indicate that a large amount of energy was required to burn oxidized lignite. Therefore, the oxidation of lignite should be avoided during the storage process to damage the pore structure, thus affecting the combustion performance; the pore structure of oxidized lignite can be adjusted to enhance its combustion performance. This will effectively improve the utilization efficiency and economic value of lignite.

Co-Thermal Oxidation of Lignite and Rice Straw for Synthetization of Composite Humic Substances: Parametric Optimization via Response Surface Methodology

In this study, Baoqing lignite (BL) and rice straw (RS), which were the representatives of low-rank coal and biomass, were co-thermally oxidized to produce composite humic substances (HS), including humic acid (HA) and fulvic acid (FA). Taking HS content as the output response, the co-thermally oxidizing conditions were optimized through single factor experiment and response surface methodology (RSM). The structures of HA and FA prepared under optimized conditions were analyzed by SEM, UV, and FTIR. Results showed that HS content was clearly influenced by the material ratio, oxidation time, and oxidation temperature, as well as their interactions. The optimized co-thermal oxidization condition was as follows: BL and RS pretreated with a material ratio of 0.53, oxidation time of 59.50 min, and oxidation temperature of 75.63 °C. Through verification, the experimental value (62.37%) had a small relative error compared to the predicted value (62.27%), which indicated that the developed models were fit and accurate. The obtained HA had a tightly packed block structure; FA had a loosely spherical shape. The molecular weight of FA was 2487 Da and HA was 20,904 Da; both had a smaller molecular weight than that reported in other literature. FA showed strong bands at 1720 cm-1, thus confirming the presence of more oxygen-containing functional groups. The appearance of double peaks at 2900~2980 cm-1 indicated that HA contains more aliphatic chains. The co-thermal oxidation of BL and RS gives a new method for the synthesis of HS, and the optimization of co-thermal oxidation conditions will provide fundamental information for the industrialization of composite HS.

Experimental and quantum chemical study on the inhibition characteristics of triphenyl phosphite to lignite oxidation at low temperature

In this study, the inhibition characteristics of triphenyl phosphite (TPPI) to the lignite oxidation at low temperature was studied. The experimental methods were applied to analyze the inhibition effect of TPPI. The results show that TPPI has a significant inhibition effect on the lignite oxidation at low temperature and the higher the concentration, the more obvious the inhibition effect. Simultaneously, the strong radical scavenging ability of TPPI leads to a significant change in raw coal. The quantum chemical calculations indicate that the inhibition mechanism of TPPI is to inert the highly reactive ROO· and RO· in the coal, while reducing the content of the oxidation precursor R·. The activation energy of each reaction is small and can occur at normal temperature and pressure. This study may provide a reference for the development and utilization of inhibitors.

Effect of Oxygen on Radical Reaction during Oxidation of Different-rank Coals

ABSTRACT The spontaneous combustion disaster caused by low-temperature oxidation of coal often leads to mine fire which poses a serious threat to the safe mining of underground coal resources. Due to the complexity of coal and the variability of gas atmosphere, it is difficult to prevent the spontaneous combustion of residual coal in goaf. In this paper, the radical characteristics during oxidation of lignite, bitumite, and anthracite in anaerobic, anoxic, and normal oxygen atmospheres were measured by preparing mixed gases with different oxygen concentrations. On this basis, the spontaneous combustion characteristics of different-rank coals under different oxygen concentrations were explored. The experimental results show that the primary radical concentrations of different-rank coals are quite different in the initial stage of oxidation. Among the three types of coals, anthracite has the highest primary radical concentration while lignite has the lowest primary radical concentration. As the oxidation proceeds, the radical concentration of lignite grows at a significantly higher rate than those of bitumite and anthracite. In addition, the g factor values of the three types of coal all increase slightly with the rise of temperature, and the g factor value of lignite is always greater than those of bitumite and anthracite. During coal oxidation, the ΔH values of lignite, bitumite and anthracite vary in the range of 0.78–0.70, 0.6–0.48, and 0.70–0.50, respectively. The ΔH value of lignite is larger than those of bitumite and anthracite, but it experiences a smaller decrease than bitumite and anthracite. With the increase of oxygen concentration, the radical concentration and g factor of coal increase while the line width decreases. Through the analysis, the growth rate of radical concentration and g factor, instead of radical concentration, were used to evaluate the oxidation capacity of coal. The results reveal that the radical reaction in lignite is more sensitive to oxygen than those in bituminous and anthracite.

Study on gas characterization and fluorescence characteristics of intermediates in biogenic gas production from lignite by ultrasound assisted hydrogen peroxide pretreatment

In order to improve the utilization efficiency of coal by microorganisms and increase gas production, ultrasound assisted hydrogen peroxide oxidation of lignite was studied. The changes of accumulated gas, methane content, carbon dioxide content, and fluorescence characteristics of intermediate products in lignite and oxidized lignite system were studied by drainage gas gathering method, gas chromatography (GC) and fluorescence spectrophotometer. It was found that the cumulative gas could be divided into three stages: rapid gas production, slow gas production, and basic stable stage. The methane production rate of raw coal and lignite treated by ultrasound assisted hydrogen peroxide reached 987.23 μmol/g and 1290.22 μmol/g, respectively. The methane yield for oxidized lignite was 30.69% higher than that of raw lignite. The ultrasonic-assisted hydrogen peroxide pretreatment for lignite could effectively increase methane production. The analysis results of the fluorescence characteristics of the intermediates indicated that the total fluorescence intensity (FI) integral of intermediates decreased with the increase of time. The fluorescence peak positions mainly changed in tyrosine aromatic proteins and soluble microbial metabolites. Ultrasound-assisted hydrogen peroxide pretreatment lignite changed the amount of tyrosine-based aromatic proteins, hydrophobic organic acid fulvic acid, humic acids, and soluble microbial metabolites in the slow and stable stages of lignite biogas.

Effect of pyrolysis on Zhaotong lignite oxidation with aqueous sodium hypochlorite

The pyrolysis treatment of lignites could remove thermal-unstable aliphatic structures but enrich aromatic structures in resulting pyrolysis residues, which would facilitate producing benzene polycarboxylic acids (BPCAs). Herein, the demineralized Zhaotong lignite (DZL) was pyrolyzed at 250–550 °C to afford the corresponding pyrolysis residues. Subsequently, DZL and its pyrolysis residues were subjected to oxidation with aqueous sodium hypochlorite. By XRD, FTIR, and element analyses, it was found that the content and polycondensation degree of aromatic structures simultaneously increased with increasing pyrolysis temperature. Furthermore, the yield and selectivity of BPCAs both increased along with raising pyrolysis temperature in resulting products from fully oxidation of DZL and its pyrolysis residues. Based on the distribution of BPCAs, peri-condensed aromatic structures were dominant in DZL and its DPRs, and obviously increased with increasing pyrolysis temperature. More interestingly, good correlation was found between the yield of BPCAs and aromaticity parameters. Meanwhile, the yield of benzenepentacrboxylic acid was well associated with polycondensation degree parameters.

Influence of metal nitrates on the process of coal oxidation of various metamorphism stages

The process of oxidation of lignite and bituminous coal modified with metal nitrates (Cu, Ce, Na, Co, Fe) in the amount of 5 wt.% is studied. Application of additives to the coal samples is carried out by the method of incipient wetness impregnation with preliminary dissolution of the additive in an aqueous-alcohol solution with a volume ratio of C2H5OH/H2O=50/50. Reactivity of the samples is studied by thermogravimetric analysis in the oxidation mode at a heating rate of 2.5 °C/min in the temperature range 25–600°C at atmospheric pressure. It has been found that application of activating additives contributes to reduction of the initial temperature of intensive oxidation (Ati=5–97°C) and to a shift of the process to the low-temperature region as a whole. According to the results obtained, the greatest activity is specific for addition of Cu(NO3)2. It is manifested in a decrease in the temperature of the beginning of intensive oxidation ti and an increase in the average reaction rate wav. The Coats-Redfern method is used to calculate activation energy of the oxidation process in the presence of activating additives. Maximum decrease in Eais observed for a copper nitrate-modified coal sample.

Extraction of Fulvic Acid from Lignite and Characterization of Its Functional Groups.

Fulvic acid (FA) is a complex organic mixture composed of small molecules. The structure and composition of FA vary greatly because of the different raw materials used for preparing FA. In this work, FA was extracted from shallow low-rank lignite by hydrogen peroxide (H2O2) in a microwave field, and the functional groups of FA were characterized. The optimal extraction process was determined, with the H2O2 concentration being the key factor affecting the yield of FA. Thermogravimetric analysis showed that FA was mainly composed of low molecular weight and readily pyrolyzed compounds. As shown by Fourier transform infrared spectroscopy, in the process of FA extraction by H2O2 oxidation of lignite, the content of −COOH increased, long-chain aliphatic compounds decreased, stretching vibrations of aromatic ring skeletons disappeared, and aromatic ring substitution became mainly tri- or disubstitution. Fluorescence spectroscopy indicated that FA had a low degree of aromaticity. X-ray photoelectron spectroscopy qualitatively and quantitatively revealed that the main modes of carbon–oxygen bonding in FA were C–O–, COO–, and C=O. Thus, this study not only lays a foundation for studying the composition and structure of coal-based FA but also opens a new avenue for a clean and efficient utilization of lignite.

Influence of the Heating Rate on the Activation of Coal and Lignite Oxidation by Copper Nitrate

Adding copper nitrate Cu(NO3)2 is known to activate the oxidation of coal and lignite. In the present work, the change in its activating properties with increase in the heating rate is studied. The Cu(NO3)2 is first dissolved in a 50/50 (by volume) mixture of ethyl alcohol and water. Then it is applied to the fuel by steeping, to a content of 5 wt %. Activated oxidation is studied by thermal analysis, with different heating rates (2.5, 10, 20, and 40°C/min) in the temperature range 25–1000°C, at atmospheric pressure. With increase in heating rate, the catalytic effect of the additive is intensified: the initial (30–115°C) and final (85–180°C) oxidation temperatures fall, with increase in the maximum reaction rate. Mass-spectrometric analysis of the gaseous oxidation products shows that, in the presence of copper nitrate, increase in the heating rate leads to greater CO2 emission, with shorter oxidation time. The dependence of the activation energy on the degree of fuel conversion is determined by the Friedman method. The decrease in the mean activation energy when using Cu(NO3)2 is ~14 kJ/mol for lignite and ~35 kJ/mol for coal.

Intensification of bituminous coal and lignite oxidation by copper-based activating additives

The oxidation of lignite and bituminous coal samples modified by 5 wt% (in terms of dry salt) addition of copper salts Cu(NO3)2, CuSO4, and Cu(CH3COO)2 was studied. The samples’ reactivity was studied by thermogravimetry within a temperature range of 45–600 °C at a heating rate of 2.5 °C/min in an oxidizing environment. The introduction of activating additives has resulted in a significant decrease in the temperature of intense oxidation onset (ΔTi = 20/94 °C), in a reduction in the sample residence time in the volatile matter release region (Δte = 2/22 min) and the total duration of the coal combustible mass oxidation (Δtf = 8/14 min). The Friedman method was used to calculate the activation energy values for the oxidation process of the modified samples. The maximum change in activation energy values was observed for the bituminous coal sample. The possible mechanism behind the action of the copper-salt additives, which activate the oxidation of lignite and bituminous coal, is discussed. According to the data of mass spectrometric analysis, the concentration of NOx in the reaction products decreases as the temperature of the activated oxidation process is shifted towards the low-temperature region.

Alkali-oxygen oxidation mechanism for producing benzene carboxylic acids from lignite: Experimental and molecular modelling studies

Alkali-oxygen oxidation of lignite is a potential route to produce high-value carboxylic acids, especially benzene carboxylic acids. However, this method leads to an unfavorably large consumption of inorganic alkali and a loss of more than 50% of the carbon in lignite as byproduct CO2. A combined theoretical and experimental study was applied to investigate the lignite alkali-oxygen oxidation mechanism at a molecular level. By analyzing lignite model simulations and the experimental results of model compounds, we obtained the formation pathways of carboxyl moieties and the alkali catalytic effect. The hydroxy radical is the major oxidant for dehydrogenation and oxygenation. The carboxyl group formation followed a chain-reaction pathway, which included side-chain/bridge-bond oxidation, aromatic carbon ring-opening oxidation and decarboxylation. Active sites for carboxyl group formation were clarified and confirmed by model compound experiments. Density functional theory calculations showed that the added alkali could promote side-chain/bridge-bond oxidation and the ring-opening reaction of polycyclic moieties by decreasing their reaction energies.