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

Abstract

The separation of gaseous mixtures is essential in the chemical industry, including hydrogen separation in ammonia or petrochemical plants, nitrogen separation from air, CO 2 separation in natural gas processing, [7] olefin/ paraffin separation, [1] and H 2 S separation from sour gas. [8] Similar to separation processes in general, thermal-based gasseparation methods, such as cryogenic distillation, amine adsorption, and vapor condensation, consume a high amount of energy, which can be significantly reduced using gas-separation membrane units. [5,8] A membrane that can separate gases allows certain components in a mixture to permeate at higher rates than others. The economic competitiveness of a gas-separation membrane highly depends on its gas permeance K and its selectivity S. The permeance K i of gas species i (in SI units of mol m −2 s −1 Pa −1 ) is defined as K i = F i /Δp i , where F i (in SI units of mol m −2 s −1 ) is the flux of gas i through the membrane, and Δp i is the partial pressure difference (the driving force) of gas i between the feed side and the permeate side. For relatively thick conventional membranes, whose crossmembrane transport resistance is dominated by the bulk interior, the permeance K i is inversely proportional to the membrane thickness d. Correspondingly, the permeability P i of gas i (in SI units of mol m −1 s −1 Pa −1 ) is defined aswhich is an intrinsic property of a material. The selectivity S ij between gases i and j is defined as S ij = K i /K j = P i /P j .Polymers have been the most widely used materials for gasseparation membranes for decades because of their relatively low cost and low manufacturing difficulty. [8,9] However, membrane separations using state-of-the-art polymeric membranes cannot outcompete the thermal-based separation methods economically. [7] One of the limitations of the polymeric membranes is the trade-off between permeability and selectivity, originally investigated by Robeson. [10,11] The best combination of permeability and selectivity for a binary gas pair is referred to as the polymer upper bound. This trade-off originates from the interplay between the free volume spacing in the polymer matrices and the size of the gas molecules. [12] Smaller polymeric spacing increases the selectivity but sacrifices the permeability due to the lower gas 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 ar...

Realizing high performance gas filters through nano-particle deposition

“…The last mechanism is called barrier activated transport, which happens when the pore diameter has almost the same dimension as that of the gas molecule. An excellent review of these regimes and the simulations done in these regimes has been presented by Yuan et al 42 As we will show our work straddles all these regimes, due to the broad range pore size distribution. In such systems, it is difficult to use numerical techniques that are specialized for any one category of pore size.…”

In Situ Nucleation‐Decoupled and Site‐Specific Incorporation of Å‐Scale Pores in Graphene Via Epoxidation

“…The zero-dimensional and tunable nature of lattice vacancy defects in graphene has led to new regimes in several applications including isotope separation, [1] the realization of an ultrafast molecular transport, [2][3][4][5][6][7][8] and an improved resolution in the single-molecule analysis. [9][10][11][12] Improving control over the the graphene lattice and form energy-minimizing clusters.…”

“…However, so far, only limited control over the vacancy defect formation has been achieved because carbon removal from an existing pore (expansion step) is coupled with the creation of a new pore (nucleation step). [8] Selective expansion of premade pores is only possible with techniques that do not nucleate new pores such as electronbeams below the energy threshold to remove sp 2 carbon from a pristine graphene lattice, [24] and CO 2 etching at moderately high temperatures (e.g., 750-1000 °C). [27] As a result, it is challenging to limit the unwanted expansion of pores when a high density of pores is desired, e.g., for molecular separation application.…”

Photoswitchable Nanoporous Metal–Organic Framework Monolayer Film for Light-Gated Ion Nanochannel

“…In recent years, 2D monolayer films have attracted significant attention in various applications such as separations, 1 proton conduction, 2 energy storage and conversion, 3 solar cells, 4 and electrochemical devices. 5,6 In general, 2D monolayer films have the following advantages: (i) high specific surface area and abundant active sites, 7 (ii) specific edge effect capable of enhancing chemical activity, 8 and (iii) notably shortened mass transfer path and significantly increased separation flux due to their porosity and nanoscale thickness.…”

“…In recent years, 2D monolayer films have attracted significant attention in various applications such as separations, proton conduction, energy storage and conversion, solar cells, and electrochemical devices. , In general, 2D monolayer films have the following advantages: (i) high specific surface area and abundant active sites, (ii) specific edge effect capable of enhancing chemical activity, and (iii) notably shortened mass transfer path and significantly increased separation flux due to their porosity and nanoscale thickness. , Typically, graphene as a kind of 2D monolayer films exhibits many excellent properties, including single atom thickness, high crystallinity, and outstanding electrical properties, enabling a broad range of unprecedented applications. , Other 2D monolayer films with similar properties to graphene but with different compositions, such as metal oxides, transition metal dichalcogenides, , hexagonal boron nitride, and graphitic carbon nitrides, have also achieved rapid development.…”