Water Clusters in Lignite and Desorption Energy Calculation by Density Functional Theory.

The interaction of water and hydrophilic sites with hydroxyl, carboxyl, and multiple oxygen-containing functional groups (OFGs) in lignite molecules was studied by density functional theory. The adsorption of water molecules on the lignite surface initially resulted in the formation of hydrogen bond-driven stable rings by three to four water molecules, followed by the formation of three-dimensional water clusters like a ″patchwork″. Aqueous layer thickness obtained from the water cluster size was 0.4-0.6 nm, which was consistent with the experimental data. Thus, pore-filling water beyond this range was less affected by the OFGs on the surface. Calculation of the adsorption energy predicts that the water clusters were primarily formed in the hydrophilic sites with three OFGs (site 1, including a carbonyl group, an alcoholic hydroxyl group and an etheroxy group in tetrahydropyran), then in COOH, and in O-H. For isolated hydroxyl groups, the interaction between the hydroxyl group and water molecules was weaker than that between the water molecules. When the water cluster was located at the hydrophilic sites with two or more OFGs, the adsorption energy of lignite-water interaction was higher than that of water-water interaction. Investigating the thermodynamics of the adsorption process at a molecular scale will help in understanding both drying and resorption process of dried lignite during industrial production.

Introduction

Lignite has a high moisture content of 30–70 wt % on a wet basis.[1] The moisture content in lignite is classified in various ways,[2,3] and the widely accepted classification is based on parameters such as monolayer-adsorbed water, multilayer-adsorbed water, capillary water, and bulk water.[4−6] These classifications were based on the outcomes of adsorption experiments and solid-state nuclear magnetic resonance (1H NMR) spectroscopy. The occurrence of water was related to its local physicochemical environment and affected the desorption energy consumption during drying or pyrolysis process.

Scientific and rational molecular structure is an important foundation for molecular-level scientific research. Mathews and Chaffee[7] provided a comprehensive review of the molecular structure of coal. The first lignite model was published by Wender in 1976 (Figure a)[8] and is still the most widely accepted model because of its small molecular weight, rich oxygen-containing functional groups (OFG), and good representation. Molecular simulation of the adsorption of some water molecules into porous carbon materials[9−11] can be found in literature. Feng et al.[12−15] used carbon nanotubes, graphene oxide, and small molecules with different OFGs as lignite monomers to investigate the interaction between OFG and water (nH2O@OFG) and found that the interaction energy between water and carboxyl molecules (nH2O@carboxyl) was the largest, followed by the interaction with phenolic hydroxyl, carbonyl, and benzene, which was similar to the results obtained by Wu et al.[16] (interaction of 1H2O@OFG carboxyl > phenolic hydroxyl > carbonyl > alcoholic hydroxyl > ether). Wu et al. also accurately demonstrated the hydrogen bonds (HB), van der Waals interactions, and steric repulsion by reduced density gradient isosurface. Vu et al.[17,18] used lignin molecule to investigate the lignite–water interactions and found a significantly reduced mobility of water molecules in local to the lignin hydroxyls owing to hydrogen bond (HB) formation. Xiao et al.[19] studied the HB interactions between the organic oxygen/nitrogen monomers of lignite and water molecules; water cage clusters and water film clusters were formed around the OFGs and nitrogen-containing heterocyclic plane in the organic oxygen and nitrogen monomers, respectively.

Figure 1

Structure of lignite molecule and its electrostatic potential (ESP), colored on a blue–green–red scale according to the values of ESP ranging from −6.2 e–2 to 6.2 e–2. Colored balls represent C atom in dark gray, H atom in light gray, and O atom in red; the corresponding isovalue is 0.1 au. (For an interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The water cluster size is dependent on the physicochemical structure of the materials.[20−23] Do et al.[20,21] suggested that water molecules are adsorbed around the OFGs in the form of clusters, and water is adsorbed into the micropores when the concentration of this water cluster is high enough. An ab initio investigation of water cluster suggested that the smaller clusters with n = 4, 5, or 8 could be used as the basic building blocks for generating larger clusters.[23] In our previous dynamic vapor sorption experiment of lignites,[22] we fitted our adsorption/desorption isotherms with the D.D model[20,21] and found that the critical size of water clusters penetrating and residing in the lignite micropores was six to seven water molecules.

In this study, hydrophilic sites on the lignite surface with 1, 2, and 3 OFGs were chosen, and the water clusters were developed local to the hydrophilic sites as the occurrence of water in the experiments. Based on the experimental results, the cluster size was estimated at seven water molecules. Compared with OFGs, nonpolar functional groups, such as alkanes and benzene rings, have much weaker interactions with water[24] and only acted as a “chemical environment” for nH2O@lignite in this study.

Results and Discussion

Electrostatic Potential Distribution in the Molecular Model of Lignite

The electrostatic potential (ESP) distribution, which is used to predict the reactive sites of electrophilic and nucleophilic reactions, is shown in Figure . The ESP varies from −6.2 e–2 to 6.2 e–2 on moving from blue to red. The ESP around the oxygen atom was low, while that around the hydrogen atom connected to the oxygen atom was high. The HB donor and acceptor in the lignite molecule were distributed on the OFGs, and the lignite molecule could simultaneously behave as a HB donor and acceptor during the HB formation. The choice of hydrophilic sites of lignite was based on the analysis of ESP distribution.

Formation of Water Clusters on the Hydrophilic Sites of Lignite

The adsorption of water on lignite is essentially based on the weak interaction between the molecules, including HBs and van der Waals interactions. During the formation of a HB, the oxygen atom in both OFGs and water can serve as HB acceptors, while the hydrogen atoms connected to the oxygen atom in OFGs and water molecules can act as donors. The hydroxyl groups on the lignite surface would be dissolved by the nonfreezing water nearby, and there might be a rapid proton–proton exchange that would increase the mobility of the hydroxyl groups. A similarity between the protons in hydroxyl groups and those in nonfreezing water could be observed in an NMR analysis,[25] which meant that OFGs and water molecules played similar roles in the formation of HBs.

The most important OFGs in lignite are the hydroxyl and carboxyl groups. Thus, single hydroxyl and carboxyl group, as well as multiple OFGs were selected to explore the formation of water clusters on the hydrophilic sites on the lignite surface.

Water Clusters near the Hydroxyl Group (O–H) in Lignite Molecular Model

In lignite with low moisture content, the water molecules around the hydrophilic sites are not evenly distributed on the surface of the coal molecule but in the form of water clusters, like a “patchwork”,[26,27] patched together at the active site. Therefore, the configuration of the water molecules in this study was mainly in the form of water clusters.

First, we chose a hydroxyl group that was spatially away from other OFGs (labeled O–H; Figure ). Water molecules were added one by one near O–H to optimize the structure. The size of the water cluster was six to seven water molecules, as obtained by fitting the experimentally obtained lignite moisture desorption isotherm.[28] The optimized structure of nH2O@O–H (n = 1–7) is shown in Figure .

Figure 2

Formation of water cluster on the hydrophilic sites of O–H, labeled as nH2O@O–H (a to g correspond to n = 1–7).

The first water molecule was connected to a hydrogen atom from the hydroxyl group, and the hydroxyl group acted as an electron donor. Because of the chainlike connection, the HB length was 1.88 Å. The oxygen atom from the hydroxyl group of the second water molecule formed a HB, and the hydroxyl group of the second water molecule acted as an electron acceptor. During this, the hydroxyl group acted both as an electron donor and an acceptor and directly adsorbed two water molecules to form two HBs. In the commonly accepted adsorption mechanism,[6,20] water molecules directly adsorbed with OFGs are considered as the monolayer water (primary adsorption), and the monolayer water adsorbs further water molecules to form the multilayer water (secondary adsorption). Our study reveals that the number of the monolayer water molecules directly adsorbed with an OFG was more than one water molecule, especially for the carboxyl group. A HB was also formed between the two water molecules; thus, the addition of the second water molecule introduced two HBs, and finally the three oxygen atoms formed a cyclic trimer. The lengths of the HBs were between 1.81 and 1.86 Å, which was lower than that of the HB of 1H2O@O–H. When a third water molecule was added, the four oxygen atoms formed a cyclic tetramer; the length of the HB was further reduced to 1.70–1.75 Å. Since the strength of the HB is related to its length for the same type of HBs, the interaction should be stronger with the water molecule added in it.

When the fourth water molecule was added, the hydroxyl oxygen acted as an HB acceptor of two HBs, resulting in an increase in the length of the original HB to 2.04 Å. There were also HBs between the two donors, and the lengths of the HBs between them were close to that of a cyclic trimer. However, the length of the HB of the hydroxyl group (HBO–HD) as an electron donor and the HB close to it reduced to 1.69 Å, which meant that these two were the strongest HBs in this water cluster. Although the configuration of the oxygen atoms was not destroyed, the water cluster transformed to a three-dimensional (3D) structure and the length of different HBs also changed significantly.

After the addition of the fifth and sixth water molecules, a relatively regular triangular prism-shaped cavity was obtained. On the upper and lower surfaces of the triangular prism, the lengths of the two HBs were close to each other, ensuring the symmetry of the structure. Both the hydroxyl groups and water molecules had 3 HBs connected to other molecules, and they served as both HB donors and acceptors. The strongest HB was HBO–HD, as mentioned above, and the HB parallel to HBO–HD.

The addition of the sixth water molecule transformed the triangle on the bottom of the original triangular prism to a quadrilateral, while the triangle on the upper bottom surface was retained. However, the HB in the upper position corresponding to the insertion position extended to 2.53 Å, thus weakening the interaction. The length of the HBO–HD further decreased to 1.65 Å.

In summary, when the hydroxyl group acted as a hydrophilic site, with the increasing number of water molecules, the HBs were affected by the newly added water molecules; the length of the original HBs changed first, and then some water molecules were rearranged, resulting in the destruction of the original HBs. Therefore, the configurational optimization of water cluster resulted in the global (and not local) optimization of the entire water cluster, which was also an important reason for the nonlinear increase in the adsorption energy.

Isomers were involved in the adsorption of water molecules on OFGs in lignite, including the hydroxyl group, which has HB donor and acceptor. Each water molecule could act as HB donor and acceptor, too. There could be various types of isomers involved in the adsorption of water molecules, especially multiple water molecules with OFGs, to form larger water clusters. In this study, there were some empirical choices in the process of water cluster construction. When adding water molecules, the maximum possible number of HBs were formed to increase the adsorption energy and obtain a more stable configuration. At the same time, isomers were explored, and the lowest-energy structures among the isomers are shown in this work.

Water Clusters near Carboxyl Groups in the Molecular Model of Lignite

Water clusters near the carboxyl groups in the molecular model of lignite are shown in Figure . The parameters of hydrogen bond length are given in Figures S1–S3. When a water molecule was present near the carboxyl group, the carbonyl oxygen and hydrogen atoms on the carboxyl group acted as HB donors and acceptors, respectively, and formed a cyclic trimer with the water molecules. Similar to the adsorption of water molecules near the hydroxyl group, the addition of the second water molecule formed a cyclic tetramer and the addition of the third water molecule formed a 3D cluster. The difference lay in the two oxygen atoms of the carboxyl group, and the carbonyl group also acts as a HB acceptor. Unexpectedly, when six water molecules were adsorbed, the size of the water cluster was larger, and it interacted with another OFG close to it. Therefore, the size and configuration of the water clusters were related to the local density of sites and their relative location, and not the overall site density.[22]

Figure 3

Formation of water cluster on the hydrophilic sites of COOH, labeled as nH2O@COOH (a to g corresponds to n = 1–7).

Water Clusters Located in the Hydrophilic Sites with Two or More OFGs

The water cluster located in the hydrophilic sites with multiple OFGs was studied. The hydrophilic site with three OFGs (site 1, containing a carbonyl group, an alcoholic hydroxyl group, and an etheroxy group in tetrahydropyran) in the initial configuration of lignite molecules was selected. When water clusters were adsorbed near a hydroxyl group (different from the hydroxyl group discussed in Figure ), the change in the configuration of the lignite molecules was similar to ″folding″, and the water molecules interacted with the two other OFGs. Thus, the water clusters interacted with multiple OFGs. In fact, the first added water molecule already had three HBs. The formation of water clusters in site 1 is shown in Figure . When the water cluster reaches a certain size after being adsorbed near the carboxyl group, it also interacted with the surrounding OFGs (Figure f).

Figure 4

Formation of water cluster on the hydrophilic sites of COOH, labeled as nH2O@site 1 ((a–g) corresponds to n = 1–7).

Thermodynamic Calculation of the Adsorption Energy of Water Clusters on Lignite

The desorption energy (ΔEs) for desorbing water as isolated water molecules is expressed aswhere the molecular energies of lignite and single water molecule are expressed as El and EH, respectively. The energy of lignite and n water molecules after adsorption is expressed as El+. EBSSE is the corrected energy for basis set superposition error, and the EBSSE given by the basis sets presented here is performed using the counterpoise method. Generally, EBSSE could reduce the bond energy, even though the value is small, making the calculated value closer to the experimental value. Moreover, it had been mentioned in several papers that the BSSE must be corrected for calculations on van der Waals molecules.[29]

The desorption energy of water molecules near O–H is shown in Figure . Both the total energy of all the water molecules and the average energy of every water molecule are given in Figure a. The total adsorption energy of water on the lignite molecules increased from 4.51 to 62.43 kcal/mol with increasing number of adsorbed water molecules. The average adsorption energy of each water molecule was in the range 4.51–9.93 kcal/mol; in the water cluster with three to seven water molecules, the average adsorption energy was higher (8.86–9.93 kcal/mol).

Figure 5

Adsorption energy of water cluster near O–H site with (a) the number of the water molecules and (b) the number of HB.

The HB of the first two water molecules adsorbed is directly formed by the lignite molecules, and the subsequent HBs gradually increased with HBs between the water molecules. The number of HBs between the water and lignite molecules was between one to three throughout the process. The increase in the average adsorption energy of each water molecule indicated that the interaction between lignite and water was weaker than that between the water molecules when water molecules were adsorbed near O–H. This could be because each water molecule had two hydroxyl groups, which could form further HBs, and the increase in the HBs directly determined the adsorption energy between the water clusters and lignite molecules. Figure b shows the total and average adsorption energies with the number of total HBs formed in the lignite–water molecular structure, and the adsorption energy of one HB was in the range 4.45–6.65 kcal/mol.

The average adsorption energy of each water molecule adsorbed near COOH at the early stage was also higher (7.68–10.59 kcal/mol), as shown in Figure . This could be attributed to the fact that COOH itself contained more HB acceptors and donors. This result also confirmed that the interaction of nH2O@COOH was stronger than of nH2O@OH during adsorption.

Figure 6

Adsorption energy of water cluster besides the COOH site with (a) the number of the water molecules and (b) the number of HB.

The adsorption energy of water clusters located in site 1 is shown in Figure . The adsorption energy of 1H2O@site 1 was higher than that of nH2O@site 1 (n > 2) and that of nH2O@COOH (n < 3) was also higher than that of nH2O@COOH (n > 3), which meant that the lignite–water interaction is stronger than the water–water interaction during the adsorption at COOH and site 1. This is consistent with the literature.[30] However, for O–H, the water–water interaction was stronger than the lignite–water interaction. Most OFGs formed one HB with the water molecules, and the secondary water molecules affected the primary adsorbed water molecules. However, the energy of the primary adsorbed water molecule was not necessarily the highest and was dependent on the length of the HB and configuration of the water cluster.

Figure 7

Adsorption energy of water cluster in the hydrophilic region with (a) the number of the water molecules and (b) the number of HB.

The average adsorption energy of 1H2O@site 1 was the highest (10.96 kcal/mol) and much higher than those of 1H2O@OH (4.51 kcal/mol) and 1H2O@COOH (9.36 kcal/mol). However, the total adsorption energy of nH2O@site 1 (n > 2) was not obviously higher than those of nH2O@OH and nH2O@COOH. Thus, the most significant superiority of site 1 was at the start of the adsorption process. The average adsorption energy of each HB was slightly lower than that of nH2O@COOH, because of which the total adsorption energy was determined by the number of HBs.

Comparing of Model Calculation and Experimental Result

The density functional theory (DFT) calculations revealed that the size of the clusters was 0.4–0.7 nm. For example, the farthest distance between the atoms in hydroxyl group and water cluster was 0.48 nm, and the longest distance in the water cluster was 0.59 nm. The NMR analysis reveals that the nonfreezable water forms mono- or bilayers, providing a shield of thickness in the range of 0.3–0.6 nm.[25] Hence, the water cluster size in this study was reliable.

The experimental adsorption energy was in the range 10.85–12.35 kcal/mol for hard lignite (as determined in our previous study)[31] and 10.4–14.0 kcal/mol for soft lignite.[32] Site 1 and COOH had a higher primary adsorption energy, and the highest average adsorption energy of nH2O@OFGs was 10.96 kcal/mol, which was close but lower than the experimentally obtained value. This might be because the actual adsorption processes are much more complex than the simulated adsorption on a single hydrophilic site.

The desorption results showed that the local density of hydrophilic sites had the most significant effect on water desorption and resorption process. Hence, it was better to reduce the density of hydrophilic sites when drying; reducing the carboxyl groups could also be effective. During the decomposition of OFGs in the thermal treatment[33,34] (Figure ), the OFGs had an obvious reduction at 240 °C for 20 min and 320 °C for 10 min, and the reduction of carboxyl group at 320 °C for 10 min was larger than that at 240 °C for 20 min. When drying, both the residue water content and OFGs should be taken into account to reduce the resorption of the drying products. Combining with the drying kinetics and pyrolysis analysis from our previous study,[35,36] it is clear that drying at 300 °C could reduce the water content and OFGs and prevent heat loss from lignite pyrolysis, and thus, may be a good choice.

Figure 8

Reduction of OFGs in the thermal-treated process.[33,34] Temperatures of different OFGs that start to decompose are also provided.

Conclusions

The interaction between lignite and water molecules was explored by DFT and compared with our previous experimental results. Different hydrophilic sites with single OFG (hydroxyl and carboxyl) or multiple OFGs in the lignite molecular model were selected for water adsorption. During the growth of the water cluster, the configurational optimization of the water cluster was the global (and not local) optimization of the entire water cluster, which was also an important reason for the nonlinear increase in the adsorption energy. The interaction between lignite and water was weaker than that between water molecules when water molecules were adsorbed near O–H. When water clusters were around COOH and site 1, more HBs between water molecules and lignite molecule were formed, and the interaction between lignite and water was stronger than that between the water molecules. Estimation of the adsorption energy reveals that the water clusters are primarily formed in site 1, then in COOH, and lastly in O–H.

Methods

Lignite Molecular Model and Calculation Methods Selections and Properties of Lignite Molecular Model

The first lignite model was published by Wender in 1976 (Figure a).[8] This model has the following advantages. (1) The molecular weight of this model is small, with the formula of C47H54O13, which can greatly improve the calculation efficiency. (2) It is rich in OFGs, i.e., alcoholic hydroxyl groups, phenolic hydroxyl groups, carboxyl groups, carbonyl groups, ether bonds, and benzofuran, which cover all the common functional groups with just over a hundred atoms, are present. (3) The oxygen content of this model is about 20%, which is close to that obtained experimentally (23–25%). (4) This model has good representation. With low degree of condensation, abundant OFGs, various fat side chains, etc., this model is consistent with the basic characteristics of lignite molecules and is widely accepted (studies based on this model have been cited nearly 150 times).

The comparison of the Fourier transform infrared (FTIR) spectra of the model and experiment is shown in Figure S4. The peak position of oxygen-containing functional groups (wavenumber < 2500 cm–1) from FTIR of this model is in agreement with that of experiment, but the composition of coal in experiment is always complex with different organics and minerals. The most obvious differences were located in the range of wavenumber of 2500–4000 cm–1, and the wavenumber of 3000–3600 cm–1 is assigned to –OH from quartz in the sample. But this model is consistent with the basic characteristics of lignite molecules and is widely accepted; therefore, this model was used to study the adsorption of water by lignite molecules in this study.

Calculation Software and Calculation Method

The molecular thermodynamic calculations were performed using the quantum chemical software Gaussian 09. The hybrid DFT method, B3LYP, and the basis set 6-31G(d) were used. The desorption energy obtained were corrected by zero point and basis set superposition error. Lower stabilization energy of water clusters might be obtained by more accurate methods and higher level of basis set, but the computation time for obtaining a stable structure will be much longer (months for a single calculation); it was quite difficult to obtain stable 7H2O·OFGs structures. Considering that the overall trends in the energetics remain unaltered with the change in the basis set or the level of theory,[23] this basis set was chosen for this study.