Method for hydrothermal treatment to regulate phenolic hydroxyl groups in coal

Abstract

The application discloses a kind of methods for hydrothermal treatment regulation and control of phenolic hydroxyl in coal, it is related to the field of coal processing and utilization, the method comprises the following steps: S1, vacuum drying treatment is carried out to coal sample, and pretreated coal sample is obtained;S2, the pretreated coal sample is mixed with reaction medium and sealed, and a mixture is obtained;S3, the mixture is heated for hydrothermal reaction, and after heat preservation and pressure preservation, cooling, pressure relief, the solid product is taken out and washed to neutral, vacuum drying treatment is carried out, and the regulated and controlled coal sample is obtained.The process flow of the application is simple, the requirement for equipment is low, easy to enlarge implementation, only needs conventional high-pressure reaction kettle, without special atmosphere protection or complex separation device, can be directly applied on existing coal chemical industry hydrothermal treatment equipment, has better industrial adaptability, and then can be popularized to different coal kinds and scale production line, provides universal technical scheme for directional regulation and control of functional groups in coal, thereby powerfully promotes the process of fine processing and utilization of coal.

Description

Technical Field

[0001] This invention relates to the field of coal processing and utilization technology, and in particular to a method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment. Background Technology

[0002] Coal, as an important energy and chemical raw material, has oxygen-containing functional groups in its molecular structure that significantly influence its physicochemical properties and processing pathways. Among these, the phenolic hydroxyl group, as a key active functional group, directly determines the coal's reactivity, flotation performance, agglomeration properties, and derivative preparation potential. In coal liquefaction, gasification, coking, and new coal chemical processes, excessively high or low concentrations of phenolic hydroxyl groups can have adverse effects. For example, in coal liquefaction, excessively high phenolic hydroxyl group concentrations can easily lead to catalyst poisoning; in gasification, excessively low concentrations can affect the gasification reaction rate. Therefore, achieving precise control of the phenolic hydroxyl group concentration in coal is of great significance for optimizing coal conversion processes and improving the yield of target products.

[0003] Currently, existing methods for controlling the concentration of phenolic hydroxyl groups in coal mainly include chemical oxidation, chemical reduction, microwave treatment, and pyrolysis. While chemical oxidation can increase the phenolic hydroxyl content to some extent, it poses a high risk of environmental pollution and is costly. Chemical reduction relies on reducing agents such as hydrogen and sodium borohydride, and requires stringent reaction conditions, typically under high pressure and high temperature. Microwave treatment and pyrolysis are relatively simple to operate, but their precision in controlling phenolic hydroxyl groups is low, making targeted control difficult. Hydrothermal technology, as a green and efficient material modification method, offers advantages such as mild reaction conditions, environmental friendliness, and strong controllability. It can utilize the unique physicochemical properties of water under high temperature and pressure to achieve targeted control of phenolic hydroxyl groups in the coal molecular structure.

[0004] However, a systematic and stable hydrothermal method for adjusting phenolic hydroxyl groups in coal has not yet been developed in the existing technology. Therefore, developing an efficient, environmentally friendly, and controllable method for adjusting phenolic hydroxyl groups in coal is of great significance for promoting the clean and efficient utilization of coal. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides a method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The present invention provides a method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment, comprising the following steps:

[0007] S1. Vacuum dry the coal sample with a particle size of 60-200 mesh to obtain a pretreated coal sample;

[0008] S2. The pretreated coal sample is mixed with the reaction medium and then sealed to obtain a mixture; the reaction medium is an aqueous solution containing a free radical scavenger;

[0009] S3. Heat the mixture to 250-260 ℃ for hydrothermal reaction, keep it at the temperature and pressure, cool and depressurize, take out the solid product, wash it until neutral, and vacuum dry it to obtain the controlled coal sample.

[0010] In a preferred embodiment of the present invention, in step S1, the coal sample is one of bituminous coal, lignite, or anthracite.

[0011] In a preferred embodiment of the present invention, in step S1, the temperature of the vacuum drying process is 80-100 °C, the vacuum degree is -0.08 to -0.1 MPa, and the drying time is 4-6 h.

[0012] In a preferred embodiment of the present invention, in step S2, the free radical scavenger is a composite free radical scavenger, composed of a primary antioxidant and an inclusion stabilizer.

[0013] In a preferred embodiment of the present invention, in step S2, the primary antioxidant is 2,6-di-tert-butyl-p-cresol, and the inclusion stabilizer is hydroxypropyl-β-cyclodextrin.

[0014] In a preferred embodiment of the present invention, in step S2, the mass ratio of 2,6-di-tert-butyl-p-cresol to hydroxypropyl-β-cyclodextrin is 1:1.8-2.4.

[0015] In a preferred embodiment of the present invention, in step S2, the amount of free radical scavenger added is 3-5% of the coal sample mass; the liquid-solid ratio of the pretreated coal sample to the reaction medium is 5-15:1 mL / g.

[0016] In a preferred embodiment of the present invention, in step S3, the heating rate is 3-4 °C / min.

[0017] In a preferred embodiment of the present invention, in step S3, the pressure of the hydrothermal reaction is 3-6 MPa; and the heat preservation and pressure holding time is 6-8 h.

[0018] In a preferred embodiment of the present invention, in step S3, the temperature of the vacuum drying process is 80-100 °C, the vacuum degree is -0.08 to -0.1 MPa, and the drying time is 4-6 h.

[0019] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0020] (1) This invention provides a method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment. The process of this invention is simple, has low equipment requirements, and is easy to scale up. It only requires a conventional high-pressure reactor and does not require special atmosphere protection or complex separation devices. It can be directly applied to the hydrothermal treatment equipment of existing coal chemical enterprises. Compared with the limitations of microwave treatment and pyrolysis methods, which require special generating devices and are difficult to achieve uniform treatment, this invention has better industrial adaptability and can be extended to production lines of different coal types and scales. It provides a universal technical solution for the directional regulation of functional groups in coal, thereby powerfully promoting the process of refined coal processing and utilization.

[0021] (2) This invention combines a composite free radical scavenger with hydrothermal technology. In a mildly treated subcritical water environment, the phenolic hydroxyl groups in coal that are weakly conjugated with the aromatic ring preferentially undergo cleavage and removal, generating oxygen-containing free radicals and small molecule phenolic free radicals. The main antioxidant rapidly captures free radicals by virtue of its phenolic hydroxyl structure, forming stable free radical adducts. Hydroxypropyl β-cyclodextrin uses its hydrophobic cavity to encapsulate the captured free radicals and small molecule phenolic products, preventing them from undergoing secondary condensation reaction with the coal matrix. This allows the reaction to continue at a relatively mild temperature. Compared with the high-temperature deep phenol removal conditions of existing hydrothermal technologies, this invention achieves the same or even better removal effect under mild conditions, thereby avoiding drastic changes in coal structure caused by high temperature. This not only reduces energy consumption but also preserves the skeletal value of coal as a raw material for subsequent processing, thus creating favorable conditions for the clean and efficient conversion of coal.

[0022] (3) Through the mechanism of free radical capture and inclusion stabilization, the present invention can maintain the main chemical structure of coal to the maximum extent. The ratio of aliphatic hydrogen to aromatic hydrogen in the coal sample after reaction is minimal compared with that of the original coal, and the maximum weight loss temperature point is almost unshifted. The integrity of the main structure of coal can be well maintained. Compared with the high structural damage caused by existing methods at the same dephenolization depth, the present invention can control the structural loss to a very low level, significantly improve the quality of the coal sample after dephenolization, and thus maintain stable reaction behavior in the subsequent gasification and liquefaction process, avoiding catalyst poisoning or uncontrolled product distribution caused by structural damage, thereby expanding the utilization pathways of coal.

[0023] (4) This invention uses water as the only reaction medium and adds only a trace amount of recyclable composite free radical scavenger. No toxic or harmful substances are involved in the whole process. The hydroxypropyl β-cyclodextrin in the scavenger comes from renewable resources. The inclusion complex formed by it and phenols can be recovered by cooling and precipitation after the reaction. The recovered product can be used as an antioxidant or fine chemical raw material. As a result, the washing wastewater after the reaction contains only trace amounts of organic matter and can be recycled after simple treatment. This effectively avoids the pollution of acidic and heavy metal waste liquid generated by traditional chemical oxidation-reduction methods. It can realize the greening of the reaction medium and the resource utilization of by-products, thereby reducing the cost of wastewater treatment and improving the overall economic efficiency and environmental friendliness of the process. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic flowchart of a hydrothermal treatment method for regulating phenolic hydroxyl groups in coal according to the present invention.

[0026] Figure 2 This is a graph showing the phenolic hydroxyl content of coal samples at different hydrothermal temperatures according to the present invention. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0029] Application Overview:

[0030] Hydrothermal technology refers to the use of water as a reaction medium in a closed reactor. Heating creates a high-temperature, high-pressure environment, causing water to enter a subcritical or supercritical state, thereby acquiring unique solubility, mass transfer properties, and reactivity. In subcritical water, the ionization constant increases, and the ion product constant increases by several orders of magnitude compared to room temperature, giving it both acid and base catalytic functions. Simultaneously, the dielectric constant of water decreases, significantly enhancing its ability to dissolve organic matter. These characteristics make hydrothermal technology an ideal means of coal structure control, enabling targeted modification of specific functional groups in coal without introducing external chemical reagents.

[0031] However, the applicant found that when existing hydrothermal technologies are applied to the regulation of phenolic hydroxyl groups in coal, those skilled in the art generally believe that a high temperature of >280 ℃ is an important condition for achieving deep removal of phenolic hydroxyl groups. Therefore, existing research has mostly focused on high-temperature hydrothermal treatment above 300 ℃.

[0032] However, while high-temperature hydrothermal treatment can achieve deep removal of phenolic hydroxyl groups, it also leads to the severe breakage of weak covalent bonds (such as bridging bonds and aliphatic side chains) in coal, resulting in a large number of small-molecule hydrocarbons entering the aqueous phase. This not only causes coal pyrolysis and reduced yield, but also leads to the formation of new, structurally uncertain oxygen-containing functional groups due to the rearrangement and condensation of small-molecule fragments, interfering with the precise control of phenolic hydroxyl groups. Moreover, the damage to the main structure of coal caused by high-temperature treatment is irreversible, severely affecting the skeletal value of coal as a feedstock for subsequent gasification or liquefaction, thus greatly limiting the practical application of dephenolized coal samples.

[0033] To address the aforementioned problems, this invention proposes a method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment. By introducing a composite free radical scavenger composed of a primary antioxidant and an inclusion stabilizer into the hydrothermal reaction medium, deep removal of phenolic hydroxyl groups from coal can be achieved under mild treatment conditions, while preserving the integrity of the coal's main structure to the maximum extent. This effectively solves the problem of coal structural damage caused by traditional high-temperature hydrothermal treatment, thereby achieving targeted and precise control of phenolic hydroxyl group concentration and providing a new technical approach for the clean and efficient utilization of coal.

[0034] It should be noted that the raw materials, equipment and reagents used in this invention can all be purchased from the market or obtained through existing preparation methods.

[0035] like Figure 1 As shown, a method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment includes:

[0036] S1. Vacuum dry the coal sample with a particle size of 60-200 mesh to obtain a pretreated coal sample;

[0037] S2. After mixing the pretreated coal sample with the reaction medium, seal the mixture to obtain a mixture; the reaction medium is an aqueous solution containing a free radical scavenger;

[0038] S3. Heat the mixture to 250-260 ℃ for hydrothermal reaction, keep it at the temperature and pressure, cool and depressurize, take out the solid product, wash it until neutral, and vacuum dry it to obtain the controlled coal sample.

[0039] It should be noted that although traditional high-temperature hydrothermal treatment at >280 ℃ can remove phenolic hydroxyl groups, the high temperature also causes the weak covalent bonds such as bridging bonds and aliphatic side chains in coal to break violently, generating a large number of small molecule hydrocarbon fragments. After these fragments enter the aqueous phase, they not only cause a decrease in coal yield, but also form new oxygen-containing functional groups with uncertain structures through rearrangement and condensation. Under mild conditions, the present invention can block the propagation of chain scission reactions because free radicals are captured and stably included in time, thus preserving the macromolecular skeleton of coal.

[0040] In some specific implementations, in step S1, the coal sample is one of bituminous coal, lignite, or anthracite.

[0041] In some specific embodiments, in step S1, the temperature of the vacuum drying process is 80-100 °C, the vacuum degree is -0.08 to -0.1 MPa, and the drying time is 4-6 h.

[0042] In some specific embodiments, in step S2, the free radical scavenger is a composite free radical scavenger, consisting of a primary antioxidant and an inclusion stabilizer.

[0043] In some specific embodiments, in step S2, the primary antioxidant is 2,6-di-tert-butyl-p-cresol, and the inclusion stabilizer is hydroxypropyl-β-cyclodextrin.

[0044] In some specific embodiments, in step S2, the mass ratio of 2,6-di-tert-butyl-p-cresol to hydroxypropyl-β-cyclodextrin is 1:1.8-2.4.

[0045] In some specific implementations, in step S2, the amount of free radical scavenger added is 3-5% of the coal sample mass; the liquid-solid ratio of the pretreated coal sample to the reaction medium is 5-15:1 mL / g.

[0046] In some specific implementations, the heating rate in step S3 is 3-4 °C / min.

[0047] In some specific implementations, in step S3, the pressure of the hydrothermal reaction is 3-6 MPa; the holding time is 6-8 h.

[0048] In some specific embodiments, in step S3, the temperature of the vacuum drying process is 80-100 °C, the vacuum degree is -0.08 to -0.1 MPa, and the drying time is 4-6 h.

[0049] To further simplify and make the present invention achieve its objectives and effects, the present invention will be further illustrated in conjunction with the following specific embodiments and comparative examples, but the present invention is not limited to the scope of the embodiments described herein.

[0050] It should be noted that in the examples and comparative examples, the coal samples were lignite samples with a particle size of 80 mesh and an initial phenolic hydroxyl concentration of 2.94 mmol / g.

[0051] Example 1:

[0052] S1. Place a lignite sample with a particle size of 80 mesh in a vacuum drying oven and dry it for 5 h at a temperature of 90 ℃ and a vacuum degree of -0.09 MPa to remove moisture and impurities, and obtain a pretreated coal sample.

[0053] S2. 2,6-Di-tert-butyl-p-cresol and hydroxypropyl-β-cyclodextrin, in a mass ratio of 1:2, were dissolved in water to prepare a reaction medium. The amount of free radical scavenger added was 4% of the coal sample mass, and the liquid-solid ratio of the reaction medium to the pretreated coal sample was 1:10 mL / g. The pretreated coal sample and the reaction medium were added to a high-pressure reactor, stirred for 5 min to mix thoroughly, and then sealed to obtain a mixture.

[0054] S3. The sealed high-pressure reactor is heated to 256 ℃ at a heating rate of 4 ℃ / min, with an internal pressure of 4 MPa. The reaction is maintained at this temperature and pressure for 7 h. The reactor is then allowed to cool naturally to room temperature. The pressure is released to atmospheric pressure, and the coal sample is removed. The sample is washed repeatedly with water until the pH of the washing solution is 7. The sample is then placed in a vacuum drying oven and dried for 5 h at a temperature of 90 ℃ and a vacuum of -0.09 MPa to obtain the regulated lignite sample.

[0055] Example 2:

[0056] This embodiment is basically the same as Embodiment 1, except that the amount of the components in the free radical scavenger is different. The specific steps of S2 are as follows: 2,6-di-tert-butyl-p-cresol and hydroxypropyl-β-cyclodextrin in a mass ratio of 1:1.8 are dissolved in water to prepare a reaction medium. The amount of free radical scavenger added is 4% of the mass of the coal sample, and the liquid-solid ratio of the reaction medium to the pretreated coal sample is 1:10 mL / g. The pretreated coal sample and the reaction medium are added to a high-pressure reactor, stirred for 5 minutes to mix thoroughly, and then sealed to obtain a mixture.

[0057] Example 3:

[0058] This embodiment is basically the same as Embodiment 1, except that the amount of the components in the free radical scavenger is different. The specific steps of S2 are as follows: 2,6-di-tert-butyl-p-cresol and hydroxypropyl-β-cyclodextrin in a mass ratio of 1:2.4 are dissolved in water to prepare a reaction medium. The amount of free radical scavenger added is 4% of the mass of the coal sample, and the liquid-solid ratio of the reaction medium to the pretreated coal sample is 1:10 mL / g. The pretreated coal sample and the reaction medium are added to a high-pressure reactor, stirred for 5 minutes to mix thoroughly, and then sealed to obtain a mixture.

[0059] Example 4:

[0060] This embodiment is basically the same as Embodiment 1, except that the hydrothermal reaction temperature is different. The specific steps of S3 are as follows: the sealed high-pressure reactor is heated to 250 ℃ at a heating rate of 4 ℃ / min, the pressure inside the reactor is 4 MPa, and the reaction is maintained at the same temperature and pressure for 7 h. The reactor is then allowed to cool naturally to room temperature, the pressure is released to atmospheric pressure, the coal sample is taken out, and it is repeatedly washed with water until the pH value of the washing liquid is 7. Then it is placed in a vacuum drying oven and dried for 5 h at a temperature of 90 ℃ and a vacuum degree of -0.09 MPa to obtain the regulated lignite sample.

[0061] Example 5:

[0062] This embodiment is basically the same as embodiment 1, except that the hydrothermal reaction temperature is different. The specific steps of S3 are as follows: the sealed high-pressure reactor is heated to 260 ℃ at a heating rate of 4 ℃ / min, the pressure inside the reactor is 4 MPa, and the reaction is maintained at the same temperature and pressure for 7 h. The reactor is then allowed to cool naturally to room temperature, the pressure is released to atmospheric pressure, the coal sample is taken out, and it is repeatedly washed with water until the pH value of the washing liquid is 7. Then it is placed in a vacuum drying oven and dried for 5 h at a temperature of 90 ℃ and a vacuum degree of -0.09 MPa to obtain the regulated lignite sample.

[0063] Comparative Example 1:

[0064] This comparative example is basically the same as Example 1, except that: a traditional high-temperature hydrothermal reaction is used. The specific steps of S2 are: the pretreated coal sample and water are added to the high-pressure reactor at a liquid-solid ratio of 1:10 mL / g, stirred for 5 min to mix thoroughly, and then sealed to obtain a mixture.

[0065] The specific steps of S3 are as follows: The sealed high-pressure reactor is heated to 300 ℃ at a heating rate of 4 ℃ / min, the pressure inside the reactor is 7 MPa, and the reaction is maintained at the same temperature and pressure for 7 h. The reactor is then allowed to cool naturally to room temperature, the pressure is released to atmospheric pressure, the coal sample is taken out, and it is repeatedly washed with water until the pH of the washing solution is 7. Then, it is placed in a vacuum drying oven and dried for 5 h at a temperature of 90 ℃ and a vacuum of -0.09 MPa to obtain the regulated lignite sample.

[0066] Comparative Example 2:

[0067] This comparative example is basically the same as Example 1, except that no free radical scavenger was added. The specific steps of S2 are as follows: the pretreated coal sample and water are added to the high-pressure reactor at a liquid-solid ratio of 1:10 mL / g, stirred for 5 min to mix thoroughly, and then sealed to obtain the mixture.

[0068] Comparative Example 3:

[0069] This comparative example is basically the same as Example 1, except that the components of the free radical scavenger are different. The specific steps of S2 are as follows: 2,6-di-tert-butyl-p-cresol is dissolved in water as a free radical scavenger to prepare a reaction medium. The amount of free radical scavenger added is 4% of the mass of the coal sample, and the liquid-solid ratio of the reaction medium to the pretreated coal sample is 1:10 mL / g. The pretreated coal sample and the reaction medium are added to a high-pressure reactor, stirred for 5 min to mix thoroughly, and then sealed to obtain a mixture.

[0070] Comparative Example 4:

[0071] This comparative example is basically the same as Example 1, except that the components of the free radical scavenger are different. The specific steps of S2 are as follows: hydroxypropyl-β-cyclodextrin is dissolved in water as a free radical scavenger to prepare a reaction medium. The amount of free radical scavenger added is 4% of the mass of the coal sample, and the liquid-solid ratio of the reaction medium to the pretreated coal sample is 1:10 mL / g. The pretreated coal sample and the reaction medium are added to a high-pressure reactor, stirred for 5 min to mix thoroughly, and then sealed to obtain a mixture.

[0072] Comparative Example 5:

[0073] This comparative example is basically the same as Example 1, except that the amount of the components in the free radical scavenger is different. The specific steps of S2 are as follows: 2,6-di-tert-butyl-p-cresol and hydroxypropyl-β-cyclodextrin in a mass ratio of 1:1.5 are dissolved in water to prepare a reaction medium. The amount of free radical scavenger added is 4% of the mass of the coal sample, and the liquid-solid ratio of the reaction medium to the pretreated coal sample is 1:10 mL / g. The pretreated coal sample and the reaction medium are added to a high-pressure reactor, stirred for 5 minutes to mix thoroughly, and then sealed to obtain a mixture.

[0074] Comparative Example 6:

[0075] This comparative example is basically the same as Example 1, except that the amount of the components in the free radical scavenger is different. The specific steps of S2 are as follows: 2,6-di-tert-butyl-p-cresol and hydroxypropyl-β-cyclodextrin in a mass ratio of 1:2.8 are dissolved in water to prepare a reaction medium. The amount of free radical scavenger added is 4% of the mass of the coal sample, and the liquid-solid ratio of the reaction medium to the pretreated coal sample is 1:10 mL / g. The pretreated coal sample and the reaction medium are added to a high-pressure reactor, stirred for 5 minutes to mix thoroughly, and then sealed to obtain a mixture.

[0076] Comparative Example 7:

[0077] This comparative example is basically the same as Example 1, except that the hydrothermal reaction temperature is different. The specific steps in S3 are as follows: the sealed high-pressure reactor is heated to 245 ℃ at a heating rate of 4 ℃ / min, the pressure inside the reactor is 4 MPa, and the reaction is maintained at the same temperature and pressure for 7 h. The reactor is then allowed to cool naturally to room temperature, the pressure is released to atmospheric pressure, the coal sample is taken out, and it is repeatedly washed with water until the pH of the washing liquid is 7. Then it is placed in a vacuum drying oven and dried for 5 h at a temperature of 90 ℃ and a vacuum of -0.09 MPa to obtain the regulated lignite sample.

[0078] Comparative Example 8:

[0079] This comparative example is basically the same as Example 1, except that the hydrothermal reaction temperature is different. The specific steps in S3 are as follows: the sealed high-pressure reactor is heated to 265 ℃ at a heating rate of 4 ℃ / min, the pressure inside the reactor is 4 MPa, and the reaction is maintained at the same temperature and pressure for 7 h. The reactor is then allowed to cool naturally to room temperature, the pressure is released to atmospheric pressure, the coal sample is taken out, and it is repeatedly washed with water until the pH of the washing liquid is 7. Then it is placed in a vacuum drying oven and dried for 5 h at a temperature of 90 ℃ and a vacuum of -0.09 MPa to obtain the regulated lignite sample.

[0080] Performance testing: The phenolic hydroxyl concentration of the regulated lignite samples obtained in Examples 1-5 and Comparative Examples 1-6 was measured sequentially, and the performance of the loss of aliphatic hydrogen / aromatic hydrogen ratio compared with the initial coal sample was tested. The results are shown in Table 1.

[0081] Determination of phenolic hydroxyl concentration: Weigh 1 g of the coal sample dried to constant weight and place it in a 250 mL dry three-necked flask. Purge the flask with nitrogen for 5 min to remove air, then add 100 mL of dehydrated anhydrous pyridine. Extract at room temperature for 24 h with magnetic stirring to ensure complete dissolution of the phenolic hydroxyl groups in the coal. Then transfer the three-necked flask to an oil bath apparatus, connect the reflux condenser to the nitrogen protection gas line, heat to 80 °C and maintain constant temperature with stirring for 2 h to ensure complete release of the phenolic hydroxyl groups in the extract. After the reaction, transfer the mixture to a centrifuge tube and centrifuge at 4000 r / min for 15 min. Filter the supernatant through a 0.45 μm organic phase filter membrane to obtain the filtrate for testing. Transfer 20 mL of the filtrate to an Erlenmeyer flask, add 2-3 drops of thymolphthalein indicator, and titrate with a 0.05 mol / L potassium hydroxide-anhydrous ethanol standard solution (standardized with potassium hydrogen phthalate) until the solution turns light blue and does not fade for 30 s. Record the volume of standard solution consumed. At the same time, perform a blank experiment to deduct the influence of solvent, calculate the concentration of phenolic hydroxyl groups in the coal sample, and perform three parallel determinations for each sample and take the average value.

[0082] Determination of the aliphatic hydrogen / aromatic hydrogen ratio loss rate: 2 mg of dried coal sample and 200 mg of spectroscopically pure potassium bromide were placed in an agate mortar and ground thoroughly under infrared lamp irradiation until the particle size was less than 2.5 μm. The mixed powder was transferred to a tableting mold and pressed into a transparent sheet under a pressure of 10 MPa for 1 min. The sheet was placed in the sample chamber of a Fourier transform infrared spectrometer, and scanning was performed against a background of pure potassium bromide tablets. The scanning range was set to 4000-400 cm⁻¹. -1 4 cm resolution -1 The infrared spectrum of the sample was obtained by scanning 32 times. After baseline correction, the spectrum was integrated at 2800-3000 cm⁻¹. -1 The peak area of ​​aliphatic CH stretching vibration (corresponding to aliphatic hydrogen content) within the range of 3000-3100 cm⁻¹ -1 The area of ​​the aromatic CH stretching vibration peak (corresponding to the aromatic hydrogen content) within the range was calculated to obtain the aliphatic hydrogen / aromatic hydrogen ratio. The Hal / Har values ​​of the raw coal sample and the regulated coal sample were measured separately. The ratio loss rate was calculated according to the formula "loss rate = (raw coal Hal / Har - regulated coal Hal / Har) / raw coal Hal / Har × 100%". Three tablets were prepared in parallel for each sample, and the average value was taken.

[0083] Table 1:

[0084] Performance testing Phenolic hydroxyl group concentration (mmol / g) Loss rate of aliphatic hydrogen / aromatic hydrogen ratio (%) Example 1 0.31 4.2 Example 2 0.37 3.8 Example 3 0.28 4.5 Example 4 0.42 3.5 Example 5 0.25 5.8 Comparative Example 1 0.33 18.5 Comparative Example 2 1.85 6.5 Comparative Example 3 0.78 10.2 Comparative Example 4 0.96 9.5 Comparative Example 5 0.51 7.8 Comparative Example 6 0.48 8.5 Comparative Example 7 1.20 3.2 Comparative Example 8 0.18 12.0

[0085] As shown in Table 1:

[0086] A comparison between Example 1 and Comparative Example 1 reveals that: Comparative Example 1, without the addition of a composite free radical scavenger, employed a conventional high-temperature hydrothermal reaction, raising the temperature to 300 °C. Although it could reduce the phenolic hydroxyl concentration to 0.33 mmol / g through high-temperature thermal decomposition, close to the 0.31 mmol / g of Example 1, its aliphatic hydrogen / aromatic hydrogen ratio loss rate was as high as 18.5%, far exceeding the 4.2% of Example 1. This is because in subcritical water at 300 °C, the attack of water molecules and thermal energy is indiscriminate. Besides the removal of the target phenolic hydroxyl groups, numerous weak covalent bonds in the coal macromolecular structure, such as bridging bonds connecting aromatic rings and aliphatic side chains, also underwent violent homolytic cleavage, generating a large number of alkyl and oxygen-containing free radicals. These free radicals can trigger chain cleavage reactions, leading to severe disintegration of the coal skeleton structure. Meanwhile, the small molecular fragments generated by the fracture are prone to rearrangement and condensation at high temperatures, generating new polycyclic aromatic hydrocarbons with uncertain structures. This not only interferes with the precise control of phenolic hydroxyl groups, but also undermines the skeletal value of coal as a raw material for subsequent processing.

[0087] A comparison of Example 1 and Comparative Example 2 reveals that, under the same mild temperature, without the addition of any free radical scavenger, the concentration of phenolic hydroxyl groups only decreased from the initial 2.94 mmol / g to 1.85 mmol / g, indicating poor removal efficiency. This suggests that under mild conditions of 250-260 °C, although the cleavage and removal reaction of some phenolic hydroxyl groups can be initiated, generating corresponding oxygen-containing free radicals and small molecule phenolic free radicals, the lack of an effective capture and stabilization mechanism causes the nascent active species to re-attack the coal matrix or undergo side reactions such as coupling and disproportionation, leading to the rapid termination of the free radical reaction chain. This prevents the reaction from progressing to a deeper level through a continuous, unidirectional cleavage-capture cycle, as is possible with the presence of a scavenger.

[0088] A comparison of Example 1 and Comparative Examples 3-4 reveals that when the composite free radical scavenger uses only a single component, whether it is the primary antioxidant 2,6-di-tert-butyl-p-cresol or the inclusion stabilizer hydroxypropyl-β-cyclodextrin, the removal efficiency of the phenolic hydroxyl groups (0.78 mmol / g and 0.96 mmol / g, respectively) and the retention of coal structure (loss rates of 10.2% and 9.5%, respectively) are significantly lower than in Example 1 using the composite component. In Comparative Example 3, although 2,6-di-tert-butyl-p-cresol can effectively scavenge free radicals and generate stable free radical adducts, these adducts and the small molecule phenolic products generated in the reaction remain free in the reaction system. They may be adsorbed back onto the coal matrix surface or undergo secondary decomposition and condensation at high temperatures, introducing new uncertainties and interfering with the reaction. In Comparative Example 4, although hydroxypropyl-β-cyclodextrin has inclusion capacity, it does not have efficient free radical scavenging function and cannot terminate the free radical chain reaction in time. The coal pyrolysis reaction will continue, resulting in significant structural damage.

[0089] A comparison of Examples 1-3 and Comparative Examples 5-6 reveals an optimal range for the mass ratio of 2,6-di-tert-butyl-p-cresol to hydroxypropyl-β-cyclodextrin in the composite free radical scavenger. Examples 1-3 all achieved excellent phenolic hydroxyl group removal and extremely low structural damage. However, when the mass ratio in Comparative Example 5 was reduced to 1:1.5, indicating a relatively insufficient amount of hydroxypropyl-β-cyclodextrin, its hydrophobic cavity could not completely encapsulate the adducts formed after the main antioxidant scavenges free radicals, as well as the small molecule phenols generated in the system. The unencapsulated products underwent secondary reactions, leading to a decrease in removal efficiency and an increase in structural damage. Conversely, when the mass ratio of Comparative Example 6 increased to 1:2.8, that is, the amount of hydroxypropyl-β-cyclodextrin was relatively excessive, the excessive cyclodextrin may affect the interfacial properties between coal and reaction medium through its hydrophilic shell, or form a physical coating layer on the coal surface, which to some extent hinders the diffusion of the main antioxidant to the reactive sites, reduces its efficiency in capturing free radicals, and also leads to poorer dephenolization effect and increased structural damage.

[0090] A comparison of Examples 1, 4-5 and Comparative Examples 7-8 shows that the hydrothermal reaction temperature is a key parameter for achieving precise control. Figure 2 The content of phenolic hydroxyl groups in coal samples at different hydrothermal temperatures is shown. Examples 1, 4, and 5 all achieved good overall results. When the temperature of Comparative Example 7 was reduced to 245 °C, the thermodynamic driving force was insufficient, and the cleavage and removal reaction of phenolic hydroxyl groups was difficult to initiate effectively. Even with the presence of a scavenger, the reaction was incomplete, resulting in a still high residual concentration of phenolic hydroxyl groups (1.20 mmol / g). When the temperature of Comparative Example 8 was increased to 265 °C, although the thermodynamic driving force was enhanced and the removal of phenolic hydroxyl groups was more thorough (0.18 mmol / g), the excessively high temperature caused some originally stable covalent bonds to break, generating additional free radicals. Although a scavenger was present, its scavenging ability was relatively insufficient when faced with a sudden increase in the number of free radicals, and it could not completely suppress the resulting damage to the coal skeleton structure. This indicates that within the 250-260 °C window, it is possible to selectively remove highly reactive phenolic hydroxyl groups while maximally protecting the main structure of the coal.

[0091] The above description is based on the preferred embodiments of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description, and all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0092] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment, characterized in that, Includes the following steps: S1. Vacuum dry the coal sample with a particle size of 60-200 mesh to obtain a pretreated coal sample; S2. The pretreated coal sample is mixed with the reaction medium and then sealed to obtain a mixture; the reaction medium is an aqueous solution containing a free radical scavenger; S3. Heat the mixture to 250-260 ℃ for hydrothermal reaction, keep it at the temperature and pressure, cool and depressurize, take out the solid product, wash it until neutral, and vacuum dry it to obtain the controlled coal sample.

2. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 1, characterized in that: In step S1, the coal sample is one of bituminous coal, lignite, or anthracite.

3. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 1, characterized in that: In step S1, the temperature of the vacuum drying process is 80-100 °C, the vacuum degree is -0.08 to -0.1 MPa, and the drying time is 4-6 h.

4. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 1, characterized in that: In step S2, the free radical scavenger is a composite free radical scavenger, consisting of a primary antioxidant and an inclusion stabilizer.

5. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 4, characterized in that: In step S2, the primary antioxidant is 2,6-di-tert-butyl-p-cresol, and the inclusion stabilizer is hydroxypropyl-β-cyclodextrin.

6. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 5, characterized in that: In step S2, the mass ratio of 2,6-di-tert-butyl-p-cresol to hydroxypropyl-β-cyclodextrin is 1:1.8-2.

4.

7. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 1, characterized in that: In step S2, the amount of free radical scavenger added is 3-5% of the coal sample mass; the liquid-solid ratio of the pretreated coal sample to the reaction medium is 5-15:1 mL / g.

8. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 1, characterized in that: In step S3, the heating rate is 3-4 °C / min.

9. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 1, characterized in that: In step S3, the pressure of the hydrothermal reaction is 3-6 MPa; the time for heat preservation and pressure maintenance is 6-8 h.

10. The method for regulating phenolic hydroxyl groups in coal through hydrothermal treatment according to claim 1, characterized in that: In step S3, the vacuum drying process is performed at a temperature of 80-100 °C, a vacuum degree of -0.08 to -0.1 MPa, and a drying time of 4-6 h.

Images