Gas Separations using Nanoporous Atomically Thin Membranes: Recent Theoretical, Simulation, and Experimental Advances

Porous graphene and other atomically thin 2D materials are regarded as highly promising membrane materials for high-performance gas separations due to their atomic thickness, large-scale synthesizability, excellent mechanical strength, and chemical stability. When these atomically thin materials contain a high areal density of gas-sieving nanoscale pores, they can exhibit both high gas permeances and high selectivities, which is beneficial for reducing the cost of gas-separation processes. Here, recent modeling and experimental advances in nanoporous atomically thin membranes for gas separations is discussed. The major challenges involved, including controlling pore size distributions, scaling up the membrane area, and matching theory with experimental results, are also highlighted. Finally, important future directions are proposed for real gas-separation applications of nanoporous atomically thin membranes.

Surface coverage of alcohols on carbon nanomembranes under ambient conditions

Molecular adsorption on 2D membranes plays a key role in surface-mediated permeation offering selectivity benefits for chemical separation. As many vaporous compounds are demonstrated to pass 2D membranes faster than...

Theoretical description of molecular permeation via surface diffusion through graphene nanopores

We establish a theoretical model to describe the surface molecular permeation through two-dimensional graphene nanopores based on the surface diffusion equation and Fick's law. The model is established by considering molecular adsorption and desorption from the surface adsorption layer and the molecular diffusion and concentration gradient on the graphene surface. By comparing with the surface flux obtained from molecular dynamics simulations, it is shown that the model can predict well the overall permeation flux especially for strongly adsorbed molecules (i.e. CO₂ and H₂S) on graphene surfaces. Although good agreement between the theoretical and simulated density distribution is hard to achieve owing to the large uncertainty in the calculation of surface diffusion coefficients based on the Einstein equation, the model itself is very competent to describe the surface molecular permeation both from the aspects of the overall permeation flux and detailed density distribution. This model is believed to supplement the theoretical description of molecular permeation through graphene nanopores and provide a good reference for the description of mass transport through two-dimensional porous materials.

Hidden porous boron nitride as a high-efficiency membrane for hydrogen purification

Nanoporous atom-thick two-dimensional materials with uniform pore size distribution and excellent mechanical strength have been considered as the ideal membranes for hydrogen purification. Here, our first-principles structure search has unravelled four porous boron nitride monolayers (m-BN, t-BN, h'-BN and h''-BN) that are metastable relative to h-BN. Especially, h'-BN consisting of B6N6 rings exhibits outstanding selectivity and permeability for hydrogen purification, higher than those of common membranes. Importantly, h'-BN possesses the mechanical strength to sustain a stress of 48 GPa, which is two orders of magnitude higher than that (0.38 GPa) of a recently reported graphene-nanomesh/single-walled carbon nanotube network hybrid membrane. The excellent selectivity, permeability and mechanical strength make h'-BN an ideal candidate for hydrogen purification.

Molecular transport in ionic liquid/nanomembrane hybrids

Ionic liquids and nanoscale membranes are both considered as promising functional components to design next-generation gas separation technologies. Herein, we combine free-standing carbon nanomembranes (CNMs) with [bmim][Tf2N] ionic liquid having affinity to carbon dioxide, and explore molecular permeation through such a composite membrane. Gas transport measurements reveal an increase in the transmembrane flux of carbon dioxide as compared to that of bare CNMs, whereas passage of helium is found to be suppressed in accordance with the solubility constants. Upon exposure to water vapor, the behavior of the hybrid membrane appears to differ strikingly as hydrophilic properties of CNMs are camouflaged by the hydrophobic nature of the ionic liquid. Kinetic simulations are conducted to account for the change in permeation mechanism, and the results agree with the experimental data obtained. Our study confirms that molecular transport in two-dimensional membranes can be tailored by imparting chemical functionalities, but at the same time highlights practical challenges in surface modification.

Scandium Decoration of Boron Doped Porous Graphene for High-Capacity Hydrogen Storage

The hydrogen storage properties of the Scandium (Sc) atom modified Boron (B) doped porous graphene (PG) system were studied based on the density functional theory (DFT). For a single Sc atom, the most stable adsorption position on B-PG is the boron-carbon hexagon center after doping with the B atom. The corresponding adsorption energy of Sc atoms was -4.004 eV. Meanwhile, five H₂ molecules could be adsorbed around a Sc atom with the average adsorption energy of -0.515 eV/H₂. Analyzing the density of states (DOS) and the charge population of the system, the adsorption of H₂ molecules in Sc-B/PG system is mainly attributed to an orbital interaction between H and Sc atoms. For the H₂ adsorption, the Coulomb attraction between H₂ molecules (negatively charged) and Sc atoms (positively charged) also played a critical role. The largest hydrogen storage capacity structure was two Sc atoms located at two sides of the boron-carbon hexagon center in the Sc-B/PG system. Notably, the theoretical hydrogen storage capacity was 9.13 wt.% with an average adsorption energy of -0.225 eV/H₂. B doped PG prevents the Sc atom aggregating and improves the hydrogen storage effectively because it can increase the adsorption energy of the Sc atom and H₂ molecule.

Porous Graphene Oxide/Porous Organic Polymer Hybrid Nanosheets Functionalized Mixed Matrix Membrane for Efficient CO2 Capture

The computational simulation of porous graphene oxide (PGO) indicated that it has great potential for the preparation of gas separation membranes. However, scaling up the manufacture of multilayer, defect-free porous graphene oxide membrane with consistently sized nanopores is extremely challenging. Here, we prepared layer-by-layer CO₂-philic Pebax^@1657 membranes that were functionalized by o-hydroxyazo-hierarchical porous organic polymers (o-POPs) and PGO. The d-spacing of pristine PGO could be finely regulated through CO₂-philic o-POPs to facilitate the permeability of CO₂. In addition, the o-POPs exhibit "N₂-phobic, CO₂-philic" properties with the phenolic hydroxyl and the azo group. The best of the POP-PGO membrane exhibits that the CO₂ permeability and ideal selectivity of CO₂/N₂ are 232.7 Barrer and 80.7, respectively, and it has surpassed the Robeson's upper bound (2008).

Separation selectivity and structural flexibility of graphene-like 2-dimensional membranes

Single-layer membranes of porous graphene, graphyne derivatives (α/α2/β-graphyne), and porous boron nitride (BN) with similar pore sizes (approximately 8 × 6 Å) have been evaluated for separating pentane isomers using first-principles calculations. In spite of their slightly bigger pore sizes, graphyne derivatives only allow linear pentane molecules to go through their native pores, while in contrast porous graphene and BN membranes only block dibranched isomers, i.e., neopentane molecules. Analyses of the geometric changes reveal that the structural flexibility of the membrane determines the penetration barrier. During the penetration of molecules, rigid membranes like graphyne derivatives may exhibit similar distortion to flexible porous graphene and BN; however the energy increase for the former is twice that for the latter. More importantly, the passing molecules experience about two times geometry distortion and four times energy barrier increase when going through rigid membranes compared with flexible membranes. The more deformed passing molecule and the less deformed rigid membrane can result in a much higher penetration barrier. Our results show that the flexibility of porous BN is comparable to that of porous graphene while graphyne derivatives are much more rigid.

Nanoporous MoS2 monolayer as a promising membrane for purifying hydrogen and enriching methane

We present a theoretical prediction of a highly efficient membrane for hydrogen purification and natural gas upgrading, i.e. laminar MoS₂ material with triangular sulfur-edged nanopores. We calculated from first principles the diffusion barriers of H₂ and CO₂ across monolayer MoS₂ to be, respectively, 0.07 eV and 0.17 eV, which are low enough to warrant their great permeability. The permeance values for H₂ and CO₂ far exceed the industrially accepted standard. Meanwhile, such a porous MoS₂ membrane shows excellent selectivity in terms of H₂/CO, H₂/N₂, H₂/CH₄, and CO₂/CH₄ separation (>10³, >  10³, >  10⁶, and  >  10⁴, respectively) at room temperature. We expect that the findings in this work will expedite theoretical or experimental exploration on gas separation membranes based on transition metal dichalcogenides.

Sc-Decorated Porous Graphene for High-Capacity Hydrogen Storage: First-Principles Calculations

The generalized gradient approximation (GGA) function based on density functional theory is adopted to investigate the optimized geometrical structure, electron structure and hydrogen storage performance of Sc modified porous graphene (PG). It is found that the carbon ring center is the most stable adsorbed position for a single Sc atom on PG, and the maximum number of adsorbed H₂ molecules is four with the average adsorption energy of −0.429 eV/H₂. By adding a second Sc atom on the other side of the system, the hydrogen storage capacity of the system can be improved effectively. Two Sc atoms located on opposite sides of the PG carbon ring center hole is the most suitable hydrogen storage structure, and the hydrogen storage capacity reach a maximum 9.09 wt % at the average adsorption energy of −0.296 eV/H₂. The adsorption of H₂ molecules in the PG system is mainly attributed to orbital hybridization among H, Sc, and C atoms, and Coulomb attraction between negatively charged H₂ molecules and positively charged Sc atoms.

Fundamental transport mechanisms, fabrication and potential applications of nanoporous atomically thin membranes

Graphene and other two-dimensional materials offer a new approach to controlling mass transport at the nanoscale. These materials can sustain nanoscale pores in their rigid lattices and due to their minimum possible material thickness, high mechanical strength and chemical robustness, they could be used to address persistent challenges in membrane separations. Here we discuss theoretical and experimental developments in the emerging field of nanoporous atomically thin membranes, focusing on the fundamental mechanisms of gas- and liquid-phase transport, membrane fabrication techniques and advances towards practical application. We highlight potential functional characteristics of the membranes and discuss applications where they are expected to offer advantages. Finally, we outline the major scientific questions and technological challenges that need to be addressed to bridge the gap from theoretical simulations and proof-of-concept experiments to real-world applications.

Inhibition effect of a non-permeating component on gas permeability of nanoporous graphene membranes

The inhibition effect of a non-permeating component on gas permeability of nanoporous graphene membranes is identified using molecular dynamics simulations.