CO2 separation of membranes consisting of Mxene/ILs with X: A perspective from molecular dynamics simulation

https://doi.org/10.1016/j.molliq.2021.118099Get rights and content

Highlights

  • Absorption/separation systems with functional groups were studied.

  • Two-step effect of Mxene on gas separation was revealed.

  • Increase in width weakened effects of Mxene on gas molecules.

  • Complex structure inside liquid membranes helps the gas to exist stably.

Abstract

Based on the molecular dynamic simulation, this study examined the behavioral mechanism of gas separation and transfer in SILMs. For the gas separation, the critical factor was set as the response to functional groups. In this study, the [Bmim][Tf2N]/Mxene-OH, -O and -F systems were applied for a comparative analysis, while the effects of width and additives on nanoslits were examined. As reported from this study, the functional groups indirectly affected the existence and selectivity of CO2 in separation systems, whereas they controlled the distribution of IL. The increase in the nanoslits width reduced the effect of functional groups on the migration of gas molecules. Moreover, the complex structure within the liquid membrane contributed to the stable existence of CO2 by adding graphene nanosheets. The present study highlighted the effect of flexibility on gas separation at a molecular level, which might help optimize the development of 2D-materials.

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.

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