MXenes are emerging rapidly as a new family of multifunctional nanomaterials with prospective applications rivaling that of graphenes. Herein, a timely account of the design and performance evaluation of MXene‐based membranes is provided. First, the preparation and physicochemical characteristics of MXenes are outlined, with a focus on exfoliation, dispersion stability, and processability, which are crucial factors for membrane fabrication. Then, different formats of MXene‐based membranes in the literature are introduced, comprising pristine or intercalated nanolaminates and polymer‐based nanocomposites. Next, the major membrane processes so far pursued by MXenes are evaluated, covering gas separation, wastewater treatment, desalination, and organic solvent purification. The potential utility of MXenes in phase inversion and interfacial polymerization, as well as layer‐by‐layer assembly for the preparation of nanocomposite membranes, is also critically discussed. Looking forward, exploiting the high electrical conductivity and catalytic activity of certain MXenes is put into perspective for niche applications that are not easily achievable by other nanomaterials. Furthermore, the benefits of simulation/modeling approaches for designing MXene‐based membranes are exemplified. Overall, critical insights are provided for materials science and membrane communities to navigate better while exploring the potential of MXenes for developing advanced separation membranes.

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The diffusion process of methane in a silicalite single-crystal membrane has been investigated using the Dual Control Volume-Grand Canonical Molecular Dynamics method. Simulations of full-membrane transport and the three individual contributions that comprise the overall process (entrance to the pores, intra-crystalline diffusion, and exit from the pores) show that the contribution of surface resistance to the overall transport resistance in zeolite membranes is larger and longer range than one might expect. A model is proposed on the basis of the additivity of these contributions. From the individual simulations of exit and entrance zones, it is shown that the adsorption and desorption resistances approach an asymptote with increasing crystal thickness. However, the asymptotic trend has not been observed in full membrane simulations within the thickness limit of this work, possibly because of the coupling between the entrance and exit effects. Since the surface resistance is limited to less than 1 µm and the single-crystal membrane comprises 100 µm, the surface resistance still represents a relatively small contribution to the overall resistance. Therefore, the diffusion process through the single-crystal membrane is dominated by the internal transport of the sorbate molecules along the principal (z-) axis of the crystal.

In this third article of the series, a new anisotropic united atoms (AUA) intermolecular potential parameter set has been proposed for the carbon force centers connecting the aromatic rings of polyaromatic hydrocarbons to predict thermodynamic properties using both the Gibbs ensemble and NPT Monte Carlo simulations. The model uses the same parameters as previous AUA models used for the aromatic CH force centers. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data of naphthalene at 400 and 550 K. The new model has been evaluated on a series of polyaromatic and naphthenoaromatic hydrocarbons over a wide range of temperatures up to near-critical conditions. Vaporization enthalpy, liquid density, and normal boiling temperature are reproduced with good accuracy. The new potential parameters have also been tested successfully on toluene, 1,3,5-trimethylbenzene, styrene, m-xylene, n-hexylbenzene, and n-dodecylbenzene to demonstrate their transferability to alkylbenzenes.

The variation of surface resistances to diffusion of molecules through the silicalite single-crystal membranes as a function of permeants has been investigated using the Dual Control Volume-Grand Canonical Molecular Dynamics method. For this purpose three spherical molecules, CH4, Ar, and CF4, have been selected. This selection enabled the study of a range of molecular diameters and interaction energies. Simulation results showed that the magnitude of surface resistance in zeolite membranes depends on the permeant-crystal interaction size and energy. Furthermore, the range of the surface resistance, defined as the distance from the surface beyond which the surface resistance becomes constant, is primarily a function of molecular size:  For smaller molecules the range of surface resistances is shorter while its magnitude is lower. Variations in mass-transfer resistances and diffusivities were studied in further detail with a parametric sensitivity analysis by varying permeant-crystal interaction size and well depth, as well as molecular weight in the manner of a factorial design. This procedure allowed checking for the significance of these factors and their cross-interactions during adsorption from the gas phase into the silicalite. The parametric study showed that the Lennard-Jones gas-crystal size interaction dominates the surface resistance of molecules that penetrate silicalite crystals, but interaction energy is also significant. Although, different sets of parameters yield similar equilibrium concentration values in adsorption studies, the surface resistance varies drastically with variations in these parameters.

An optimization has been performed for the parameters of an Anisotropic United Atoms (AUA) intermolecular potential for thermodynamic property prediction using both Gibbs ensemble and NPT Monte Carlo simulations. The model uses the same parameters as previous AUA models for the aromatic CH, alkyl CH2, and methyl CH3 groups as well as the CH and CH2 groups for olefins. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data of p-xylene at 311 and 491 K. The model has been evaluated on a series of alkylbenzenes and styrene including toluene, o- and m-xylene, trimethylbenzene, n-propylbenzene, n-hexylbenzene, and n-dodecylbenzene from ambient temperature to near-critical temperature. Vaporization enthalpy, liquid density, and normal boiling temperature are reproduced with an average error of 2% or lower. Although the proposed AUA model is very simple, as it does not include any electrostatic charges, it accounts fairly well for the influence of the alkyl substituents over a large range of temperature and carbon number.

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The effect of strong and weak hydrophilic sites, Al atoms with associated extraframework Na cations and silanol nests, respectively, in high-silica MFI zeolites on water adsorption was investigated using Monte Carlo simulations. For this purpose, a new empirical model to represent potential energy interactions between water molecules and the MFI framework was developed, which reproduced the hydrophobic characteristics of a siliceous MFI-type zeolite, silicalite-1, with both the vapor-phase adsorption isotherm and heats of adsorption at 298 K being in good agreement with experimental data. The proposed model is also compatible with previous hydrocarbon potential models and can be used in the adsorption simulations of VOC-water mixtures. Adsorption simulations revealed that strongly hydrophilic Al sites in Na-ZSM-5 zeolites coordinate two water molecules per site at low coverage, which promotes water clustering in the vicinity of these sites. However, weakly hydrophilic silanol nests in silicalite-1 are in coordination with a single water molecule per site, which does not affect the adsorption capacity significantly as expected. However, even in the presence of 0.125 silanol nest per unit cell, the increase in the heat of adsorption at low coverage is drastic.