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China’s oil imports are continuing to rise, with its external dependence exceeding 40%. Coupled with the volatile international oil market, this has led to supply shortages, price fluctuations, and energy security issues. China is relatively rich in coal resources, and converting coal into oil has become an important part of its energy security strategy; it represents a practical option for energy substitution in the country. However, the current direct coal liquefaction process is carried out under hydrogen-rich, high-temperature, and high-pressure conditions, which imposes strict requirements on the performance of equipment and the engineering setup. Therefore, applying certain pretreatments based on the chemical structure of coal can improve its reactivity in liquefaction, thereby addressing issues such as low conversion rates in coal liquefaction and the tendency of coal’s macromolecules to coking during the liquefaction process. The theoretical basis for the swelling effect of coal solvents is the macromolecular two-phase structure of coal. This theory views the organic macromolecular structure of coal as consisting of a macromolecular phase with a three-dimensional cross-linked network structure, and a small-molecule phase dissolved within the network; these two phases interact through weak bonds such as hydrogen bonds, charge transfer bonds, or van der Waals forces to form an organic entity. By using polar organic solvents for swelling treatment under certain conditions, the hydrogen bonds between the small-molecule phase and the network structure in coal can be broken, the weak bonds within the coal structure can be severed, and the degree of cross-linking in the coal structure can be reduced, thereby allowing the small-molecule phase in coal to become fully exposed and achieving the goal of increasing the reactivity of coal. 1 Experimental Section 1.1 Preparation of coal samples The coal samples were obtained from the coal used for coal liquefaction at the Shenhua Group’s Coal Liquefaction Demonstration Plant (hereinafter referred to as Shenhua coal); the results of their industrial analysis and elemental analysis are shown in Table 1. The solvents toluene and pyridine used for swelling pretreatment are of analytical grade. The coal swelling pretreatment uses a coal-to-solvent ratio of 2:1 (mass ratio of solvent to coal). The coal sample was ground to 200 mesh and dried under vacuum at 85°C for 24 hours before use. The swelling rate of coal was determined using the method described in the literature, and the coal extraction rate was calculated in the same manner as reported in the literature. http://www.nmtech.com.cn/jishuwang/upload1/0808191513109315.jpg 1.2 Hydrogenation liquefaction experiment of coal samples: The liquefaction experiment of coal is carried out in a domestically produced 1L-GCF type high-pressure reactor. The steps involve accurately weighing a certain amount of coal sample along with 3% catalyst FeS; after mixing them evenly, the mixture is placed in a high-pressure reactor. Solvent **naphthalene** is added in a ratio of 3:1 (solvent/coal by mass). The reactor is purged with N2 4–5 times, then with H2 5–6 times, and finally pressurized to 7 MPa. The mixture is then stirred and heated to 420°C; after maintaining this temperature for 60 minutes, it is allowed to cool naturally, after which the product is obtained. The separation of reaction products and the calculation of yield are as described in the literature. 2 Results and Discussion 2.1 Effect of swelling treatment on swelling degree Table 2 presents the experimental results for the determination of the swelling degree. As can be seen from the data in the table, the degree of swelling of the coal after swelling treatment in methanol solvent is higher than that of the raw coal. This is because the swelling degree of coal is generally controlled by the permeability of the solvent and the plasticity of the coal’s macromolecular structure. The swelling treatment with toluene and pyridine destroys weak covalent bonds such as hydrogen bonds and ether bonds within the coal’s macromolecular structure (as shown by IR spectroscopy results), reducing the cross-linking density of the coal’s molecular structure and thereby increasing the permeability of methanol into the coal structure as well as its diffusion capacity. The swelling degree test results show that swelling pretreatment can cause changes in the molecular structure of coal, and pyridine has a stronger swelling effect than toluene; this effect may improve the performance of coal in direct liquefaction. http://www.nmtech.com.cn/jishuwang/upload1/0808191513458539.jpg 2.2 Effect of swelling treatment on the extraction rate Solvent extraction of coal is a process in which small-molecule components within coal are released using appropriate solvents that possess electron-donating and electron-accepting capabilities. Through extraction, information on the reactivity of coal can be obtained, as the amount of small-molecule phase in the extract represents to a certain extent the degree of coal reactivity. Table 3 lists the extraction rates of swollen coal and raw coal after **furan (THF) extraction for 24 hours. Due to the low content of small molecular compounds bound to the coal by non-covalent bonds in Shenhua coal, the solvent extraction rate of the raw coal is low. After swelling treatment with toluene and pyridine, the weak covalent bonds such as hydrogen bonds and ether bonds in the macromolecular structure of coal are disrupted, resulting in a looser network structure and an increased content of the small-molecule phase. This leads to a significant increase in the extraction efficiency of coal, which is consistent with the effect of swelling treatment on the degree of swelling. http://www.nmtech.com.cn/jishuwang/upload1/0808191514575263.jpg 2.3 IR spectroscopic analysis of raw coal and swelled coal: The Shenhua coal from Inner Mongolia, China, belongs to the long-flame coal category; its main structural component is a macromolecular network formed by covalent bonds, with low levels of small molecular compounds bound to the coal molecules through non-covalent forces. Hydroxyl groups are primarily present within the macromolecular framework. After swelling treatment with toluene and pyridine, the changes in the macromolecular structure of coal were further confirmed through the IR spectrum of the swollen coal, as shown in Figure 1. As shown in Figure 1, after swelling, the peak intensities of the active groups in the macromolecular framework structure of Shenhua coal underwent significant changes: the peak shape of the hydroxyl group absorption peak at 3420 cm⁻¹ was relatively weaker and blunter compared to that of the raw coal, while the intensity of the bending vibration absorption peaks associated with aromatic ether and phenol structures at 1610 cm⁻¹ and 1160 cm⁻¹, as well as those of the aromatic ring vibration peaks at 758 cm⁻¹ and 680 cm⁻¹, was relatively reduced compared to that of the raw coal. This indicates that the swelling pretreatment alters the distribution of hydrogen bonds in the coal structure, and to a certain extent disrupts the hydrogen bonding between hydroxyl groups and ether oxygens, as well as weak covalent bonds such as ether bonds and aromatic carbon-carbon bonds, causing them to break to some extent. This in turn loosens the macromolecular structure of the coal, increasing the mobility of its molecular structure. Furthermore, the intensity of the various characteristic peaks in pyridine-swollen coal is slightly lower than that in toluene-swollen coal (see Figure 2), indicating that the swelling effect of pyridine is stronger than that of toluene. http://www.nmtech.com.cn/jishuwang/upload1/0808191515439323.jpg 2.4 Thermogravimetric analysis of raw coal and swelled coal. Thermogravimetric analysis of coal allows it to be determined how the weight loss of the coal sample changes as temperature rises, thereby providing information regarding the pyrolysis of the coal sample. Comparing the thermogravimetric results of different coal samples allows one to understand their structural differences. The thermogravimetric analysis results of raw coal, pyridine-swollen coal, and toluene-swollen coal are shown in Figure 3. As can be seen from Figure 3 in http://www.nmtech.com.cn/jishuwang/upload1/0808191516108898.jpg, there are significant differences in the pyrolysis properties between coal swelled with toluene (line b in the figure) and pyridine (line c in the figure) and raw coal: the onset weight loss temperature for the pyrolysis of raw coal is approximately 370℃ ； Coal subjected to swelling pretreatment experiences significant weight loss at low temperatures, and by the time the pyrolysis temperature reaches around 300°C, the weight loss has become considerable. This is because the swelling treatment causes the macromolecular structure of coal to become looser; the small molecular components are displaced from their original positions by the solvent, resulting in a rearrangement of the coal’s macromolecular structure. This reduces the degree of cross-linking in the coal’s molecular structure, thereby making it more susceptible to pyrolysis. Thermogravimetric experiment data show that at a pyrolysis temperature of 450°C, the pyrolytic weight loss of toluene- and pyridine-swollen coal reached 13.4% and 16.8%, respectively, both of which are higher than the 7.2% weight loss of the raw coal. 2.5 Temperature changes during the heating process of swollen coal liquefaction The heating process of coal liquefaction is divided into two stages: physical heating and physico-chemical heating. The first stage is the endothermic process of coal, during which no hydrogenation liquefaction reaction occurs in the coal ； The second stage is the hydrogenation reaction stage of coal; this stage is a process that involves both heat absorption and heat release, as small molecules within the coal’s network structure gradually undergo hydrogenation reactions. The lower the starting temperature at which the boundary between the two stages occurs, the better the liquefaction activity of the coal, and the earlier direct hydrogenation liquefaction begins. http://www.nmtech.com.cn/jishuwang/upload1/0808191516375015.jpg As can be seen from Figure 4, raw coal is in a physical heating phase before 150 ℃, with a relatively slow rate of temperature increase ； As the temperature rises further, the catalytic hydrogenation reaction gradually occurs; heat is released during this reaction, and the temperature rises more rapidly as a result. As indicated by IR and TG analyses, the macromolecular network structure of the swollen coal becomes looser, which increases the number of radical fragments in the coal structure and causes the small-molecule phase to detach from its original position. This enhances the coal’s ability to undergo hydrogenation and to supply hydrogen, as well as its liquefaction activity; therefore, the pretreated swollen coal undergoes hydrogenation reactions at around 120°C. Furthermore, since polymers are converted into low-molecular-weight substances during this process, the reaction time for the liquefaction process is relatively prolonged. 2.6 Effect of swelling treatment on coal liquefaction properties. Liquefaction experiments on swollen coal can reveal the effect of the swelling agent on coal swelling, directly indicating the properties of the coal after swelling treatment in terms of direct hydrogenation liquefaction. The results of the direct hydrogenation liquefaction experiments on raw coal, toluene-swollen coal, and pyridine-swollen coal are listed in Table 4. As shown in Table 4, the liquefaction conversion rate and oil and gas yield of pyridine-swollen coal increased by 14.21% and 16.61%, respectively, compared to raw coal, while the yield of precursor aromatics decreased by 2.09%. The results of the direct hydrogenation liquefaction experiments on swollen coal further indicate that the swelling treatment alters the coal structure by reducing the degree of association in the macromolecular framework network within the coal, thereby expanding the spatial volume of this framework structure. This facilitates greater contact between the interior of the coal structure and the hydrogen-supplying solvent, reduces the occurrence of reverse reactions during liquefaction, and thus improves the conditions for the hydrogenation liquefaction of coal. http://www.nmtech.com.cn/jishuwang/upload1/0808191517116820.jpg 3 Conclusion (1) The swelling treatment of Shenhua coal using toluene and pyridine destroys weak covalent bonds such as hydroxyl hydrogen bonds, ether bonds, and aromatic carbon-carbon bonds within the coal macromolecular structure. This process expands the spatial structure of the coal macromolecules and alters the degree of intermolecular cross-linking in the coal structure, resulting in an increased swelling degree of the treated coal in methanol solvent, a higher extraction rate in furan solvent, a lower pyrolysis temperature, and an increased degree of thermal weight loss. (2) Swelling treatment with toluene and pyridine improved the reaction performance of coal catalytic hydrogenation for direct liquefaction. Under the experimental conditions, the liquefaction conversion rates of Shenhua coal treated with toluene and pyridine increased by 7.86% and 14.21%, respectively, while the oil and gas yields increased by 8.82% and 16.61%, respectively. (3) Both toluene and pyridine achieved certain effects on the swelling capacity of Shenhua coal and its reactivity in liquefaction after swelling, and the swelling treatment with pyridine caused greater damage to the structure of Shenhua coal than toluene.