Effect of -O- on Water Molecule Adsorption and Adsorption Mechanism of Lignite and Coke

-e high moisture content of lignite restricts its extensive and efficient use. Furthermore, the reabsorption of lignite is also a factor that affects lignite spontaneous combustion. -erefore, it is of great importance to study the process and mechanism of water molecule desorption and adsorption on lignite and coke (25–950°C) to achieve the clean and efficient utilization of lignite and environmental protection. Proton nuclear magnetic resonance (H-NMR), thermogravimetric analysis, and other techniques were used in this study to explore the water molecule absorption and desorption processes of lignite pyrolysis at different temperatures (25–950°C) and the special contributions of ether bonds to water molecule adsorption. A mechanism of lignite water molecule adsorption was proposed.-e results showed that ether bonds played a special role in the water molecule adsorption by pyrolyzed lignite.-e ether bond content was greater in the coal samples at 300 and 950°C, which changed the trend of lignite water molecule absorption and the distribution of water (T2) detected in the H-NMR experiments and delayed the escape of water molecules during moisture desorption. -e total amount of adsorbed water decreased first and then increased in the coal samples as the pyrolysis temperature increased. However, the maximum adsorption interactions of each coal sample increased first and then decreased. -is was the result of the interactions between the pores and the oxygen-containing functional groups. Based on the above analysis, water molecule adsorption mechanism models of lignite and coke were constructed. -is study offers a new approach for investigating the water molecule adsorption and adsorption mechanisms of lignite and coke.


Introduction
e energy crisis is becoming more and more serious around the world. Lignite is an important coal reserve. However, owing to its high moisture content, low heat value, easy weathering, and spontaneous combustion, the utilization efficiency of lignite is relatively low [1]. At present, the common method to improve the utilization efficiency is lignite upgrading, but the current technologies fail to solve the problems of moisture adsorption and spontaneous combustion of lignite [2]. e microstructural and adsorption characteristics of lignite have been the focus of many scholars. Tao et al. [3] compared the material composition, pore structure, and adsorption capacity of lignite and candle coal and found that lignite has a higher inertinite content, larger pore volume, better connectivity, and greater specific surface area (SSA) than candle coal. In another article, a continuous distribution model of pore space was constructed for coal reservoirs based on the measured data of mercury intrusion porosimetry (MIP) and low-temperature nitrogen adsorption (LTNA) experiments.
is model can obtain the complete pore size distribution from nanopores to microfractures [4]. Series and parallel seepage simulations of coal matrix pore samples, natural fracture samples, and artificial fracture samples with three permeability ranges were designed and performed [5]. Another analysis showed that after water immersion, the content of hydroxyl and aromatic hydrocarbons in the coal increased significantly, and the temperature at which fat-based and oxygen-containing functional groups participated in the reaction decreased [6]. e monolayer water in soft brown coal comprises only about one twentieth of the total water present, and it is attached to coal by hydrogen bonds in suitable polar functional groups [7]. Gutierrez-Rodriguez et al. [8] estimated the hydrophobicity of coal. In recent years, the research on this subject has focused more on the adsorption effect of the functional groups on water. Liu et al. [9] suggested that the potential to form more hydrogen bonds is the key factor influencing the interaction energy between model compounds and water molecules. e behavior of water in coal depends on many factors, such as intermolecular hydrogen bonding [10]. Water molecules interact with the oxygen-containing functional groups on the surface of coal by hydrogen bonds [11][12][13][14].
e research on the moisture adsorption of lignite has mainly focused on the water occurrence state and the influence of -OH, C�O, and other functional groups [7,[15][16][17]. However, there are few studies on the effect of a single functional group, such as -O-, on water molecule adsorption in the existing literature.
In this study, the pore distribution, the effect of oxygencontaining functional groups on water molecule adsorption, and the relationship between the oxygen-containing functional groups, pores, and water molecule adsorption were examined [18]. Based on previous studies, the influence of ether bonds on the water molecule adsorption in lignite and coke (<950°C) was investigated through proton nuclear magnetic resonance ( 1 H-NMR) and water molecule absorption and desorption experiments to lay a foundation for the inhibition of lignite reabsorption and spontaneous combustion.

Method
All of the data obtained in this experiment were collected through three parallel experiments, and their average values were calculated and reported as the final values.

Sample Preparation.
Lignite samples were collected from the Shengli Mine of China. e coal samples were crushed and screened, and those with particle sizes of 200-400 mesh were stored for future use. Raw coal was stored directly at room temperature (25°C), and other coal samples were extracted by pyrolysis in a fixed-bed reactor. Coal samples were selected and heated to 50, 75, 105, 200, 300, 500, 700, and 950°C at 5°C/min under a nitrogen atmosphere for 1 h. e obtained sample was labeled with the corresponding temperature, and the temperature label of the raw coal (untreated coal sample) was 25°C. Table 1 displays the results of the proximate and ultimate analyses of all coal samples obtained by an industrial analyzer (5E-MAG6700), an element analyzer (5E-CHN2000), and an infrared sulfur meter (5E-IRS II).

Moisture Adsorption and Desorption.
e moisture adsorption experiments were performed in an SHBY-40B standard curing box (manufactured by Jiangsu Wuxi Southern China Experimental Instrument Inc.). e coal samples were spread on a 35 mm culture dish, which was then placed into a constant-temperature (30°C) and constant-humidity (85%) box. e first 6 h was divided into three phases, with 2 h for each phase and an interval of 30 min. e samples were weighed every 1 or 2 h. Equilibrium moisture adsorption was reached if the difference between the two adjacent weights was smaller than 0.01 g. e adsorption of water and the equilibrium adsorption ratio were calculated as follows: where m 0 is the mass of the lignite sample and m t is the mass of the lignite sample after water absorption. Lignite and coke samples after the adsorption of water molecules were desorbed with a temperature rise rate of 2°C/ min and a temperature range of 25-200°C. A thermogravimetric analyzer was used to obtain the thermogravimetry (TG), derivative thermogravimetry (DTG), and differential thermal analysis (DTA) curves of the desorbed lignite after adsorption.

Proton Nuclear Magnetic Resonance ( 1 H-NMR).
1 H-NMR (VTMR20-010V-T; Shanghai Niumag) was performed to test the water contents of the samples. e testing parameters were a resonance frequency of 21.306 MHz, magnetic intensity of 0.5 T, coil diameter of 10 mm, and a magnetic temperature of 35.00°C. e sample signal values were collected using NMR analysis software, and Carr-Purcell-Meiboom-Gill (CPMG) sequences were obtained. On this basis, the T 2 spectrum was obtained through inversion with the simultaneous iterative reconstruction technique (SIRT). e coal samples (1-1.25 g) were placed into a detector oven, and the detection limit of water was 10 mg. e coal samples were immersed in water for 24 h under standard conditions to fill all the pores with water such that they became saturated water samples. e coal samples were placed in an environment with a specified humidity, and water was adsorbed to the surface of the lignite. is kind coal sample is called an adsorbed water sample. e interaction strength between the lignite and water molecules is expressed as a transverse relaxation time T 2 (ms), as follows [19]: where M xy is the component of the macroscopic magnetization vector on the x-y axis, M 0 is the initial magnetization vector, and T 2 is the transverse relaxation time. e interaction strength M xy is related to the relaxation time t by equation (2).

Oxygen-Containing Functional
Groups. Oxygen-containing functional groups were tested using Fourier-transform infrared spectroscopy (FTIR, NEXUS670; American Nicolet Corporation). e KBr method was performed to prepare the samples. e samples and KBr were mixed at a ratio of 1 :120. Infrared spectra were obtained within the range of 400-4000 cm −1 , and the scanning time was set at 25 s. e resolving power and wavenumber accuracy were 0.125 and 0.001 cm −1 , respectively. e calculation of the content of oxygen-containing functional groups is shown in Supplementary Materials S1.

Microstructure of Lignite Coke.
e specific surface areas and pore distributions of the coal samples were determined by a low-temperature nitrogen adsorption method using a 3H-2000PS2 tester from the Baystar Instrument Technology (Beijing) Co., Ltd. e Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to calculate the specific surface area, pore volume, and pore distribution for each sample. e surface morphologies and particle sizes of the coal samples were observed using scanning electron microscopy (SEM, s-3400N, Hitachi company). e accelerating voltage was 20 kV, and backscattered electron imaging was performed.

Moisture Content of Coal Samples after Equilibrium
Adsorption in Difference Method. In this study, we found that under conditions with a constant temperature of 30°C and humidity of 85% and after the coal samples absorb water to achieve equilibrium adsorption, the moisture content of coal samples decreased first and then increased with the increase in treatment temperature ( Figure 1). e moisture content of raw coal at 25°C is the highest, reaching 26.44%. e moisture content of heat-treated coal sample at 105°C was higher than that at 75°C, which may be due to the exposure of functional groups after the escape of water molecules with strong binding force. Comparing Figure 1 with Table 1, we can see that the coal sample heat treated at 105°C achieved a maximum adsorption rate of 22.80%. At higher heat-treatment temperatures, the adsorption rate decreased. In the 200-300°C range, the coal sample equilibrium adsorption rate was relatively low, and at 700°C, the heat-treated coal sample equilibrium adsorption rate was the lowest, with a value of 12.66%, and the moisture content was 13.28%. At 950°C, the equilibrium adsorption rates of the coal samples increased up to 22.82%, and the moisture content was 23.29%. e moisture adsorption capacities of the coke at 300 and 950°C were abnormal as well ( Figure 1: A 300 and B 950 ). Table 2 shows the T 2 distribution and spectral intensity of the equilibrium adsorbed water. e total water content (A total ) decreased with the increase in the temperature below 700°C. e water content of the coal sample increased at 950°C (Figure 2).

Moisture Content of Coal Samples after Equilibrium Adsorption in 1 H-NMR Analysis.
When the heat-treatment temperature of the coal sample was lower than 105°C, the total amount of adsorbed water (A total ) of the coal sample decreased with the increase in the heat-treatment temperature (Table 2 and Figure 2). is is similar to the trend in Figure 1. However, the coal sample heated at 105°C showed its particularity, which may be caused by its strong binding force with water molecules after the exposure of the functional group, which cannot be detected by  H-NMR. When the heat-treatment temperature was 105-700°C, the amount of adsorbed water decreased, the rate of decrease of the amount of adsorbed water of the coal sample heated at 300°C slowed ( Figure 2: A 300 ), and the amount of adsorbed water of the coal sample heated at 950°C increased sharply ( Figure 2: B 950 ). With the increase in the heat-treatment temperature, the coal samples for heat treatment at 300 and 950°C showed abnormal moisture adsorption.

Water Desorption.
e TG curves of the water desorption from lignite after adsorption were obtained by desorption experiments (Figure 3). e amount of desorbed water of raw coal (25°C) after water absorption was the largest, approaching 20%. As the pyrolysis temperature of lignite increased, the amount of desorbed water of lignite decreased gradually. e amounts of desorbed water of the coal samples treated at 50, 75, and 105°C were almost the same, and those of the coal samples treated at 300 and 950°C were abnormal. e amount of desorbed water of the coal sample treated at 300°C was greater than that of coal sample treated at 200°C, and the amount of desorbed water of the coal sample treated at 950°C was greater than that of coal sample treated at 500°C, which was consistent with the results of the adsorption process described above (Figures 1 and 2). e corresponding DTA ( Figure 4) and DTG ( Figure 5) data were further analyzed. Most of the adsorbed water was removed at 80°C. Small amounts of water in the coal samples at 300 and 950°C needed more energy to release from the sample surfaces, and the removal temperature was around 100°C. Strong binding interactions occurred in the coal samples heated at 300°C, similar to that in the coal samples heated at 950°C.
In the moisture absorption and desorption experiments of lignite coke, with the increase in the heat-treatment temperature, the equilibrium amount of adsorbed water of the coal coke increased first (<105°C), then decreased (105-700°C), and then increased dramatically (700-950°C). At the same time, the coal samples for heat treatment at 300 and 950°C showed abnormal moisture adsorption behaviors. Water desorption experiments showed that some of the water in the coal samples for heat treatment at 300 and 950°C was difficult to remove, and the water had relatively stronger bonds.

State of Adsorbed Water
Occurrence. 1 H-NMR was used to investigate the differences in the state of adsorbed water under saturated and unsaturated conditions. e primary moisture adsorbed on the surfaces of the lignite and coke was detected by 1 H-NMR, and the peak shifts of the adsorption curves were used to show the main binding interaction. Figure 6 shows the water molecule adsorption capacities of the coke samples with different degrees of pyrolysis. When the treatment temperature was below 500°C, the relaxation time (T 2 ) moved to the left as the temperature increased (curve a-b in Figure 6). T 2 showed a decreasing trend, but the H 2 O adsorption capacity was increasing. T 2 reflects the degrees of freedom of the protons in the binding interactions between coal and water molecules, which was gradually increasing. e value of T 2 at 700 and 950°C shifted to the right (curve b-c in Figure 6), and the binding interactions of coal samples with adsorbed water molecules decreased. In the pyrolysis process, the oxygencontaining functional groups with weak binding interactions escaped first as the temperature increased. e residual binding interactions in coal were strong, and the absorption of water molecules was strong. However, the total amount of adsorption was reduced. erefore, the maximum adsorption interactions of each coal sample increased first and then decreased, and this process was closely related to the evolution of oxygen-containing functional groups [20]. e distribution T 2 in the saturated water lignite samples was investigated (Figure 7). e T 2 values of the coal samples at 700 and 950°C significantly shifted to the right, indicating that the water molecule binding interactions in the coal samples were weak, and the relaxation times were long.
Meanwhile, the T 2 of the coal sample heated at 950°C shifted to the left relative to that of the coal sample heated at 700°C, and there were significant peaks at relaxation times  between 0.2 and 0.5 ms (Figure 7(a): B 950 ). is indicated that the adsorption capacity of the coal sample heated at 950°C was different from that of the coal sample heated at 700°C. e T 2 distribution in the adsorbed of lignite was investigated (Figure 7(b)). e peak of the coal sample heated at 950°C was significantly different from that of the coal sample heated at 700°C for relaxation times between 0.2 and 0.5 ms (Figure 7(b): B 950 ). Based on these results and the 1 H-NMR results described above, there were strong binding interactions in the coal sample heated at 950°C, similar to that in the coal sample heated at 300°C (Figure 7). e results of the 1 H-NMR experiment and lignite coke absorption and desorption experiments had good consistency. With the increase in the heat-treatment temperature, the structure of the lignite coke changed at around 300 and 950°C, which led to an enhanced binding effect of the lignite coke with some of the water molecules, a rise in the desorption temperature, and a relative increase in the amount of moisture adsorbed.  Journal of Chemistry calculation of the content of oxygen-containing functional groups is shown in Supplementary Materials S1. e oxygen-containing functional group content in the lignite increased first and then decreased (Figure 8), and the change of the water molecule adsorption quantity (Figures 1 and 2) was in good agreement. In particular, the ether bonds at 300 and 950°C were abnormal. In the pyrolysis process, the content of ether bonds generally showed a downward trend, but at 300 and 950°C, the downward trend slowed and even changed to an upward trend (Figure 8(c): A 300 , B 950 ), which was consistent with previous experimental results [22]. e moisture adsorption capacity of the 300 and 950°C lignite coke (Figure 1:  A 300 , B 950 ; Figure 2: A 300 , B 950 ) was consistent with the changes of the ether bond content. is indicated that the increase in the ether bond contents in the coal samples at 300 and 950°C was an important reason for the increase of the adsorbed moisture in the coal coke.

Physical and Chemical Structure Changes during Lignite
Meanwhile, the 1 H-NMR data showed that a small amount of moisture with strong adsorption was present in the lignite coke at 300 and 950°C (Figure 7), and there was also a small amount of moisture present during the process of water desorption, whose removal temperature was higher than that of the other water (Figures 3, 5, and 6). Based on the above analysis, the generation of ether bonds in the lignite coke at 300 and 950°C enhanced the binding of the lignite coke surface to some of the moisture.

Pore Size Distribution.
e pore distributions of coal samples were determined by 1 H-NMR, and the pore size distribution is shown in Figure 9 and Table 3. e microstructural evolution of the heat-treated lignite was further investigated using SEM (Figure 10). e lignite coke below the heat-treatment temperature of 200°C exhibited no chemical differences from the raw coal. With the removal of water in the pores, more pores were exposed, resulting in a significant increase in the pore content. When the heat-treatment temperature was higher than 200°C, the heat-treatment process caused significant weight loss and gas escape [18]. In particular, condensation polymerization occurred at about 300°C, resulting in ether bonds and pore contraction ( Figure 10). Under 500°C heat treatment, the number of lignite coke pores increased (Table 3 and Figures 9 and 10). At this point, the bridge bond fracture reached its maximum value. e porosity of lignite coke treated at 700°C continued to increase (Table 3 and Figures 9 and 10), and it increased significantly, which was caused by condensation polymerization [23]. When the heat-treatment temperature was 950°C, the proportion of pores below 10 nm was significantly reduced, and the pore size distribution moved toward the direction of large pores, which also indicated that condensation polymerization and pore collapse occurred at this stage.
Based on the above changes of the amount of water molecule adsorption and the water binding state in the lignite coke, as well as the physical and chemical changes that occurred in the lignite coke heat-treatment process, it was inferred that the abnormal changes of the water molecule adsorption on the lignite coke heated at 300 and 950°C were caused by the formation of ether bonds and pore changes.

H 2 O Adsorption Mechanism of Lignite and Coke.
According to above analysis and previous research [18], the water molecule adsorption mechanisms of lignite and coke were determined, and they are shown in Figure 11.
When the heat-treatment temperature was below 105°C, with the increase in temperature, moisture escaped and oxygen-containing functional groups were exposed in the lignite. e number of active sites or oxygen-containing functional groups was the highest [18]. e moisture was absorbed on the active sites, which led to increased water molecule absorption in lignite in this temperature range. However, most of the absorbed water molecules were single layered. is was associated with a strong absorption force.  Journal of Chemistry e relaxation time (T 2 ) decreased, and the pre−105°C heat treatment of the coal sample moisture relaxation time decreased ( Figure 6). When the heat-treatment temperature was 105-500°C, the content of oxygen functional groups -OH and C�O continued to decrease and the adsorbed moisture decreased with the increase in temperature. However, the bonding interactions of water molecules were relatively strong, with relaxation time decreasing (Figure 6). e increase in the ether bonds in the coal samples at 300°C slowed the decreasing trend of water molecule absorption   1 and 2). e oxygen-containing functional groups (OH and -O-) of the coal samples heated at 500 and 700°C continued to decrease, and C�O disappeared ( Figure 8). As a result, an insufficient number of water molecules were adsorbed to form the first layer water. Significant multilayer water appears in 500°C coal samples ( Figure 6). e amount of water absorption in the 500°C and 700°C coal samples was relatively small for all the coal samples, as the content of oxygen functional groups in those two samples was small (Figures 1 and 2). However, the T 2 of the lignite coke heated at 500°C was the shortest, and the adsorption force was the strongest (Figure 6). e coal sample heated at 950°C contained a certain number of ether bonds, and the water molecule absorption increased significantly (Figures 1 and  2). However, the maximum adsorption force was the weakest (Figure 6), because the functional groups were almost gone (Figure 8).
rough comprehensive consideration of the pore changes [18], it was found that the cracks were larger (Figures 7(a) and 9) and the multilayer water was not easy to retain. e moisture state of the coal sample heated at 700°C changed, and the amount of multilayer adsorption increased (Figure 7(b)). ere should be a critical point between 500 and 700°C at which the moisture adsorption changes from a single layer to multiple layers ( Figure 6). Although there was little -OH in 950°C coal samples, the -O-content increased ( Figure 8). Meanwhile, large-aperture crannies disappeared (Figures 7(a) and 9), and the apertures (20-1000 nm) were uniformly distributed (Figures 7(a) and 9). As the numbers of -OH and -O-groups were relatively small (1000 and 63, respectively), there was less single-layer water, and most of the adsorbed water was multilayer. At the same time, owing to the well-distributed apertures compared to those of the coal sample treated at 700°C, the multilayer water could be retained. As a result, the number of adsorbed water molecules of the coal sample heated at 950°C surpassed those of the samples heated at 700 and 500°C (Figures 1 and 2). e uniform distribution of pore diameters allowed the multilayer water to be retained, which was the body of the absorbed water (Figure 7(a)).

Conclusion
In summary, in this study, a new 1 H-NMR method for studying the water molecule adsorption of lignite and coke was proposed. is method was used to explore the water absorption and desorption processes of lignite pyrolysis at different temperatures (25-950°C) and the special contributions of ether bonds to the water adsorption.
e adsorption was demonstrated to be dependent on the presence of "ether bonds" in the lignite pyrolysis process, which could significantly improve the adsorption effect of water. e increase in the ether bond content (at 300 and 950°C) resulted in a significant increase in the water adsorption capacity and a significant decrease in the desorption capacity. e reason for this phenomenon was the strong adsorption capacities of the ether bonds. e ether bonds could cause water molecules to form stable adsorption layers in a monolayer water state. In the pyrolyzed coal samples at 950°C, the adsorption capacities of the ether bonds remained high after the monolayer of water was saturated, and the water molecules underwent multilayer adsorption. is study offers a new approach for investigating the water molecule adsorption and the adsorption mechanism of lignite and coke.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no direct financial relationship with the commercial entities mentioned in this study that might lead to conflicts of interest. Journal of Chemistry 9