CO2 separation of membranes consisting of Mxene/ILs with X: A perspective from molecular dynamics simulation
Graphical abstract
Introduction
Carbon dioxide capture and storage technology essentially controls excessive emissions of flue gas emitted from fossil fuels [1], [2], [3], [4]. The supported liquid membrane (SLM), one of carbon capture technologies, is prepared by embedding the CO2-philic solvents into the pores of the solid porous framework to achieve a considerable CO2 permeation speed and separation factor. Separation technologies by exploiting a supported liquid membrane have highly energy efficiency and a relatively low investment cost [5], [6], [7], [8], which have become a promising solution for carbon capture [9], [10]. The consistency of the selected porous framework carrier with the internal liquid agent is recognized as the basis of SLMs design. The first vital feature of liquid media refers to the physical and chemical stability of the absorbent.
Ionic liquids (ILs) comprise organic cations and organic/inorganic anions undergoing unlimited structural variations [5], [6]. ILs possesses unique properties such as high thermal stability and nonvolatility [10], [11], [12]. Coulombic interactions and van der Waals interactions between cations and anions tremendously impact the physiochemical properties exhibited by ILs [13]. Thus, various task-specific ILs under a range of cation–anion combinations have been developed to build supported ionic liquid membranes (SILM) for CO2 separation [14], [15]. Besides the stability of the solvent, the compatibility exhibited by the solvent and the porous support should be considered as well. Over the past few years, the emerging material MXenes has been experimentally adopted to synthesize 2D layered membranes, which exhibit several advantages, such as ultra-fast water permeability [16], precise ion screening [17], and most importantly, unprecedented gas separation performance [18].
Gogotsi et al. [19] initially proposed the MXenes chemical embedding and large-scale layering method. Ding et al. [20] made a major breakthrough in the application of 2D MXene layered film in 2018 and effectively synthesized Ti3C2Tx Mxene 2D layered membrane with highly ordered nanochannels. The free spacing between adjacent MXene nanosheets was nearly 0.35 nm, so the synthesized membrane could act as a molecular sieve to separate gases. This work is critical to gas separation and would propose a vital method for hydrogen production and carbon dioxide capture. In addition, the MXenes nanolayer has exhibited great potential in molecular selection by chemically and structurally adjusting interlayer properties [21], [22]. The modified MXenes surface could significantly alter the characteristics in variable applications, while supporting continuous optimization characteristic. Oxygen/fluorine-rich functional groups exhibit excellent compatibility and affinity to ILs, and they are capable of building hydrogen bonds with ILs. Thus, these liquid molecules can be immobilized on the solid surface as if they are anchored, thereby significantly impacting the structure and properties exhibited by the confined ILs and ensuring the stability and durability exhibited by the entire system [23], [24], [25], [26].
The combination of IL and MXenes can widen the contact area between gas and ILs and address numerous limitations of conventional adsorption and absorption processes. However, in the existing macroscopic studies on separation systems, fluids and solids have generally acted as the continuous phases, and the details of molecular interactions attributed to heterogeneity have been ignored. Moreover, non-bonding interactions significantly impact the liquid/solid interaction and transport properties exhibited by microfluidics [27], [28], [29]. Next, as impacted by the complexity of the microstructure of MXenes and the cumbersome experimental operation, the experimental methods independently cannot systematically and effectively study materials such as MXenes. Molecular simulation is capable of directly providing molecular-level information that cannot be directly acquired experimentally, and it has been employed in several gas transport studies in layered membranes [30].
In the present study, through MD simulations, the dynamic separation behavior of CO2 and N2 in SILMs was examined. With this introduction, the building details of separation models are presented in Section 2. In Section 3, the major work on exploring the behavioral mechanism of gas separation in SILMs is elucidated. To reveal the characteristics of interaction in absorption and separation systems, different functional groups were applied for comparative analysis. Subsequently, width of nanoslits and additive was adopted to explore the effect of variable response on the separation result. In Section 4, the conclusion of this study is drawn.
Section snippets
Models
By considering the calculation scale of molecular simulation and a more accurate description of the microscopic separation as a vital part of the gas separation [31], [32], the gas–liquid interface system was selected as shown in Fig. 1. All the systems exhibiting the dimensions (xyz) of 24.6 × 60 × 200 Å were built by Materials Studio (MS) [33], including the modified MXene (Ti3C2) samples. Three types of functional groups, including F, OH and O, were considered in the calculation here [34].
Results and discussion
The number of separated and absorbed molecules of the gas phase in liquid membranes was extracted separately, as shown in Fig. 2. The area of separation and absorption was defined by complying with the Gibbs dividing surface (GDS), thereby providing the specific location of the gas phase space and the liquid phase space [43], [44]. Given the simulated results, the corresponding selectivity was also determined, and the difference between the separated and absorbed molecules could be
Conclusions
The behavioral mechanism of gas separation and transfer of gas molecules in IL-Mxene membranes was investigated through the molecular dynamic simulations. Micro-mechanisms cannot be overall revealed by rigorous experimental methods, and the mechanism is also easy to be overlooked in molecular simulation research. In this study, given the structural characteristics, the IL- Mxene membrane with different functional groups was examined. First, during the separation, the CO2 permeability of
CRediT authorship contribution statement
Timing Fang: Writing – original draft. Xiangshuai Meng: Writing – review & editing. Guohui Zhou: Resources. Kun Jiang: Funding acquisition. Xiaomin Liu: Visualization, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is financially supported by the Taishan Scholars Program of Shandong Province (tsqn201909091), the National Natural Science Foundation of China (U1704251), and the High-Grade Talents Plan of Qingdao University.
References (47)
- et al.
Separation of a Binary Gas Mixture by Thermal Diffusion in a Two-Dimensional Cascade of Many Small Cavities
Int. J. Heat Mass Tran.
(2020) - et al.
New Solvent Blends for Post-Combustion CO2 Capture, Green
Energy Environ.
(2019) - et al.
Insight into the Behavior at the Hygroscopicity and Interface of the Hydrophobic Imidazolium-Based Ionic Liquids, Chinese
J Chem. Eng.
(2021) - et al.
Electron Paramagnetic Resonance Studies of the Chelate-Based Ionic Liquid in Different Solvents, Green
Energy Environ.
(2020) - et al.
Hand-Holding and Releasing between the Anion and Cation to Change their Macroscopic Behavior in Water, Green
Energy Environ.
(2019) - et al.
Corrosion Inhibiting Performance and Mechanism of Protic Ionic Liquids as Green Brass Inhibitors in Nitric Acid, Green
Energy Environ.
(2020) - et al.
Recent Advances in Mxenes Supported Semiconductors Based Photocatalysts: Properties, Synthesis and Photocatalytic Applications
J. Ind. Eng. Chem.
(2020) - et al.
Effects of functional groups and anion size on the charging mechanisms in layered electrode materials
Energy Storage Mater.
(2020) - et al.
Nanoconfined Deep Eutectic Solvent in Laminated Mxene for Efficient CO2 Separation
Chem. Eng. J.
(2021) - et al.
Insight into the Properties and Structures of Vapor-liquid Interface for Imidazolium-based Ionic Liquids by Molecular Dynamics Simulations
J. Mol. Liq.
(2021)