Study of the adsorption mechanisms of NH₃, H₂S and SO₂ on sepiolite using molecular dynamics simulations

Abstract

The adsorption mechanisms of hazardous gas molecules such as NH₃, H₂S and SO₂ on sepiolite have not yet been elucidated. Therefore, molecular dynamics (MD) simulations were employed to investigate the adsorption behaviour of sepiolite towards NH₃, H₂S and SO₂. A calculation model for sepiolite containing structural and zeolitic water molecules was constructed in this study. The adsorption sites and molecular configurations of the hazardous gases in the sepiolite channels were studied. The radial distribution function was employed to evaluate the interactions between the gas molecules and sepiolite. The results show that the order of adsorption capacity of sepiolite for the gases is as follows: SO₂ > H₂S > NH₃. These three types of gas molecules absorbed in the channel nanopores of sepiolite exhibit different atomic configurations. The diffusion coefficients of the gas molecules in the channels decreased in the following order: NH₃ > H₂S > SO₂. In addition, the diffusion coefficients were affected significantly by the ratio of the number of gas/water molecules. This study provides new perspectives for understanding the molecular processes responsible for the adsorption properties of sepiolite.

As a natural mineral, sepiolite has many industrial applications related specifically to its physical properties (Zheng et al., 2010; Deng et al., 2019; Jiang et al., 2020, 2021; Hamid et al., 2021). Due to their inherently large surface areas (~320 m² g^–1) and great adsorption capacities, sepiolite-based materials have shown great potential for applications in the fields of environmental clean-up and protection (Alcântara et al., 2012; Fayazi et al., 2019; Bashir et al., 2020). Sepiolite is a microcrystalline hydrated magnesium silicate hydrate that belongs to the phyllosilicate family. It has a unit cell formula of Si₁₂O₃₀Mg₈(OH)₄(H₂O)₄⋅8H₂O. This silicate is composed of continuous silica tetrahedral chains with the apical oxygen inverted periodically. Furthermore, discontinuous octahedral sheets are constructed in the [010] direction, thereby forming channels parallel to the c-axis of the crystal (García-Romero & Suárez, 2013, 2018; Lu & Wang, 2022). The channel structure provides sepiolite with a very large specific surface area, potentially diversifying its advanced applications (Qiu et al., 2021; Erdoğan & Esenli, 2022). In addition, due to its nanochannels with a size range of 0.6–0.9 nm, sepiolite has been classified as a molecular sieve material (Alver, 2018; Cecilia et al., 2018). Therefore, sepiolite could be used as an active component for the adsorption of various small polar molecules (Delgado et al., 2007; Yuan et al., 2018). Experimental results have indicated the dependence of the gas adsorption of sepiolite on molecule size as well as on the interactions between the molecules and the channel walls.

Acid treatment has been used widely to improve the adsorption properties of sepiolite (Yebra-Rodríguez et al., 2003). Acid-activated sepiolite exhibited weak acidity and small ζ-potential values, indicating the dominance of Lewis acid centres on its surface (Sabah et al., 2007). The special affinity of sepiolite towards NH₃ indicated the occurrence of specific interactions with the acid groups on its surface. Although the adsorption behaviour of sepiolite has been studied widely, the molecular mechanism underlying the impacts of hazardous gases such as NH₃, H₂S and SO₂ on the adsorption behaviour of sepiolite has not yet been resolved. Due to the limitations of experimental technologies, the interactions between these hazardous gas molecules and sepiolite cannot be measured directly. However, molecular dynamics (MD) simulations can provide more precise microscopic information on sepiolites compared to experimental methods.

The distribution and dynamics of the confined water in the channel nanopores of sepiolite have been investigated using MD simulations (Zhou et al., 2016). Based on these results, a new zeolitic water model was proposed to study the water mobility in these channels. Herein, the MD simulation method was employed to investigate the adsorption mechanism of NH₃, H₂S and SO₂ on sepiolite. The adsorption sites and molecular configurations of the hazardous gases in the sepiolite channels were studied. This study provides new research insights and methods for studying the application of sepiolite materials.

Experimental

Simulation structure of sepiolite

The molecular models of sepiolite shown in Fig. 1 were constructed according to previous studies (Muniz-Miranda et al., 2016; Zhou et al., 2016). The calculation model was based on the molecular formula Mg₈(Si₁₂O₃₀)(OH)₄(OH₂)₄⋅nH₂O, where n indicates the number of zeolitic water molecules in the calculated system (Kitayama & Hayakawa, 1992; Suárez & García-Romero, 2013). The H₂O molecules in sepiolite can be classified into two groups after structure optimization: (1) H₂O molecules that are bonded strongly to Mg²⁺ ions located at the edges of octahedral sheets, corresponding to the structural water reported in previous studies (Balci, 1999; Fitaroni et al., 2019); and (2) H₂O molecules that form hydrogen bonds between themselves, called ‘zeolitic water molecules’ (Cornejo & Hermosin, 1988; Balci, 1999). The calculated system, containing zeolitic water molecules, is shown in Fig. 1c. Water molecules were located in the channels inside sepiolite.

Fig. 1. Computational models of sepiolite. The Si, O, Mg and H atoms are represented in yellow, red, green and grey, respectively. The pink and blue atomic clusters indicate structural water and zeolitic water, respectively. (a) The atomic configuration of the cell before simulation. (b) The atomic configuration of the cell after a relaxation process. (c) The atomic configuration of the supercell in the MD simulation.

The adsorption of gas molecules into sepiolite channels was simulated by constructing a computational model. The computational model contained 160 structural water molecules and 320 zeolitic water molecules. Owing to the high stability of the structural water, these molecules were maintained in the channels throughout the simulation process. Following this, a portion of the zeolitic water molecules was substituted artificially with the gas molecules. The ratios of the number of gas/water molecules (χ_i) are listed in Table 1.

Table 1. Ratios of the numbers of gas/water molecules (χ_i) in the channels of the computational model.

Forcefield setup and MD parameters

The MD simulation was performed using the Forcite module in BIOVIA Material Studio 2017 R2. All simulations were performed by using the fully atomistic COMPASS II forcefield (Sun et al., 2014; He et al., 2020). Equilibration was performed in a canonical ensemble at 300 K with a time constant of 1 × 10^–15 s. Pre-equilibration procedures (100 × 10^–20 s) were performed to ensure that the system reached the equilibrium state. The simulations were performed for ~3 × 10^–12 s with a time step of 2 × 10^–15 s. The atom coordinates were saved every 1 × 10^–15 s.

Results and discussion

The distributions of adsorbed gas molecules in the channels of sepiolite are shown in Fig. 2. The coordinate origin was placed at the centre of the channel. At various values of χ_i, sepiolite showed similar adsorption behaviour towards NH₃ in the x- and y-directions (Fig. 2a,d). These results indicate that the number of zeolitic water molecules had a minor effect on the adsorption of NH₃ molecules. The distributions of the NH₃ molecules in the channels were symmetrical in the x- and y-directions. The adsorption peaks were located at 0.5 and 1.4 Å from the centre of the channels in the x-direction and at 2.5 and 4.5 Å from the centre of the channels in the y-direction. For H₂S (Fig. 2b,e), the molecular distribution changed with increasing χ_i values. In the x-direction, the adsorption peak was located at the centre of the channel at χ₁ = 0.2 before disappearing at χ₁ = 0.5. Two adsorption peaks were observed at 1.5 Å from the centre of the channel. Four adsorption peaks were observed at χ₃ = 1 and χ₄ = 2. In the y-direction, the molecular distribution of H₂S exhibited a similar shape at various χ_i values. For SO₂ (Fig. 2c,f), the molecular distribution was affected by the value of χ_i in the x- and y-directions. The results obtained indicated that the molecular configuration of the gases in the channels was determined by both χ_i and the interactions between the gas molecules and sepiolite. The adsorbed gas molecules in the channels were distributed symmetrically.

Fig. 2. Distribution of the adsorbed gas molecules in the channels. (a,d) Distribution of NH₃ in the x- and y-directions. (b,e) Distribution of H₂S in the x- and y-directions. (c,f) Distribution of SO₂ in the x- and y-directions.

The radial distribution function (RDF) was employed to evaluate the interaction between gas molecules and sepiolite (Abbaspour et al., 2018; Lamichhane & Ghimire, 2021). For NH₃ adsorption, the N–N, N–O_w, N–Si, Mg–O_w, Si–O_w and N–O_s distances are presented in Fig. 3, where O_w = O atoms in zeolitic water molecules and O_s = O atoms in structural water molecules. The RDFs of N–N and N–O_s in Fig. 3a,f show a similar pattern. The peaks were located at ~3.2 Å, indicating that NH₃ molecules exhibited the same level of affinity towards the nearest N and Si atoms. The RDF in Fig. 3b demonstrates a stronger N–O_w interaction compared to N–N and N–O_s interactions. The NH₃ molecules exhibited a weak interaction with the Si atoms. Therefore, the NH₃ adsorption of sepiolite can be increased via the interaction between NH₃ and zeolitic water molecules. The complicated adsorption process is associated with the number of zeolitic water molecules. However, the structural water molecules were bonded tightly to the Mg²⁺ in the sepiolite, producing only a small decrease in NH₃ adsorption. The results in Fig. 3 indicate that the N–N, N–O_w, N–Si, Mg–O_w, Si–O_w and N–O_s interactions were not affected significantly by the change in χ_i. For H₂S (Fig. S1), the S–O_w distance was very similar to the S–O_s distance, indicating that the H₂S molecules interacted identically with the O atoms in both zeolitic and structural water molecules. In comparison, the SO₂ molecules (Fig. S2) exhibited a high affinity to the O atoms in the structural water but not in the zeolitic water. The interactions among the SO₂ molecules increased with the decreasing number of zeolitic molecules. These results demonstrate that the SO₂ molecules tended to adsorb onto the channel walls, indicating the stability of SO₂ molecules adsorbed on sepiolite.

Fig. 3. RDF patterns of NH₃ adsorption with various values of χ_i, wherein N indicates the N atoms in NH₃, O_w is the O atoms in zeolitic water molecules, Si and Mg indicate the Si and Mg atoms in sepiolite and O_s indicates the O atoms in structural water molecules. (a) N–N; (b) N–O_w; (c) N–Si; (d) Mg–O_w; (e) Si–O_w; and (f) N–O_s. Z = distance between the atoms.

In the absence of zeolitic water in the channel, the structural water molecules were bound tightly to the Mg²⁺ during the simulation process for both H₂S and SO₂ (Fig. 4). However, during the adsorption of NH₃, some of the bonds between the structural water molecules and Mg²⁺ were broken, and so the water molecules interacted with the NH₃ molecules. In addition, the adsorbed gas molecules in the channels exhibited various configurations. The H₂S and SO₂ molecules exhibited ring-like structures in the channels.

Fig. 4. Snapshoots of the molecular configurations of the gases in the channels after relaxation at χ₄ = 2.0, where N is represented by the blue balls, S is represented by the yellow balls, O is represented by the red balls, H is represented by the grey balls and Mg is represented by the green balls. (a) NH₃ model; (b) H₂S model; and (c) SO₂ model.

The stability of the adsorption was evaluated using the diffusion coefficients of the gas molecules, and the results are shown in Table 2 (Zhou et al., 2016; Largo et al., 2020). For NH₃, the diffusion coefficient of the molecules in the channel was 0.53 × 10^–9 m² s^–1 at χ₁ = 0.2, which decreased to 0.17 × 10^–9 m² s^–1 at χ₂ = 0.5. When χ₃ increased continuously to 1.0, the diffusion coefficient increased slightly and then declined to 0.17 × 10^–9 m² s^–1 at χ₄ = 2.0. The relatively high diffusion coefficient indicates a weak interaction between NH₃ molecules and sepiolite. The change in the H₂S diffusion coefficient was similar to that observed for NH₃, which indicates that the adsorption of H₂S molecules is related to the number of zeolitic water molecules. At χ₁ = 2.0, the diffusion coefficient of H₂S was 0.50 × 10^–12 m² s^–1, which was three orders of magnitude lower than that at χ₁ = 0.2. This indicates that sepiolite shows a great adsorption ability towards H₂S. In comparison, the diffusion coefficients of SO₂ and H₂O decreased gradually with increasing χ_i, which could be ascribed to the strong interaction between SO₂ and the walls of the channel. In addition, the diffusion coefficients of SO₂ were an order of magnitude lower than those of H₂S, suggesting a greater adsorption ability for SO₂ in sepiolite. At χ₄ = 2.0, the diffusion coefficients of structural water molecules for both H₂S and SO₂ were also similar, which is in agreement with the molecular configurations shown in Fig. 4.

Table 2. Diffusion coefficients of the gas molecules in the channels of the computational model.

Conclusions

MD simulations have been employed successfully to study the adsorption mechanism of hazardous gases such as NH₃, H₂S and SO₂ on sepiolite. Upon calculation of the distribution and diffusion coefficients of the gas molecules in the channels of sepiolite, it was found that the adsorption ability of sepiolite for the three gases followed the order SO₂ > H₂S > NH₃. While the NH₃ molecules in the channels exhibited great affinity for the water molecules, and H₂S molecules showed strong interactions with the water molecules and the walls of the channels. The SO₂ molecules exhibited strong interactions with both the Si atoms in sepiolite and the O atoms in structural water. Thus, it was found that the atomic configuration of the gas molecules in the channels are affected by both χ_i and the interactions between the gas molecules and sepiolite. Additionally, ring-like structures are observed in the molecular configurations of H₂S and SO₂. Future experiments will be required to validate these findings.