Metal Confined in 2D Membranes for Molecular Recognition and Sieving towards Ethylene/Ethane Separation

Abstract

lenge for 2D-material membranes to separate mixtures of molecules with extremely close physical properties and kinetic diameters owing to the lack of size-sieving properties and specific affinity of the 2D nanochannels. For instance, towards ethylene/ethane separation, the current distillation technology is very energy-consuming and concurrently increases carbon dioxide emissions. [8] It is considered as one of the seven chemical separations that if improved can reap great global benefits. [9] The most widely investigated membrane material for ethylene/ethane separation is a carbon molecular sieve (CMS), which possesses ultramicropores with molecular sieving properties. [10] However, the relatively low mechanical strength and aging problem might restrict its development. An alternative is developing ethylene or ethane-selective adsorbent, [11] which is based on designing specific interactions towards targeted molecules. Until now, there is no reported membrane material having both molecular sieving and specific interaction properties for efficient ethylene/ethane separation. Molecular recognition is some of the most significant features of chemical and biological systems. [12] The recognition effect stems from specific noncovalent interactions, including hydrogen bonding, metal coordination, van der Waals forces, and π-π interactions. The most common recognition motif is between antibodies and aptamers, [13] where the key lies in a unique structure formed by a folded and constrained bioheteropolymer that creates a binding pocket/interface able to recognize a specific molecule. Such exquisite molecular complementarity between antibodies and aptamers inspires us to mimic their structure and feature to construct 2D channels with optimized geometry and chemistry configurations for gaining improved molecular discrimination properties.Herein, we design novel metal-confining 2D sub-nanometer channels with molecular recognition and precise size-sieving for discriminating ethylene over ethane (Scheme 1). Graphene oxide (GO) nanosheets are assembled into 2D channels with precisely controlled sub-nanometer interlayer height to discriminate planar ethylene (4.84 Å × 4.18 Å × 3.28 Å) over stereoscopic ethane (4.82 Å × 4.08 Å × 3.81 Å) by the size difference of 0.53 Å along the c axis (Figure S1 and Table S1, Supporting Membranes with nanochannels have exhibited great potential in molecular separations, while it remains a great challenge to separate molecules with very close physical properties and kinetic diameters (e.g., ethylene/ethane) owing to the lack of size-sieving property and specific affinity. Herein, a metal confined 2D sub-nanometer channel is reported to successfully discriminate ethylene over ethane via molecular recognition and sieving. Transition metal cations are paired with polyelectrolyte anions to achieve high dissociation activity, forming reversible complexation with ethylene. Aberration-corrected transmission electron microscopy observes that the metals with size of ≈2 nm are uniformly confined in graph...

Insights into High Li+/Mg2+ Separation Performance Using a PEI-Grafted Graphene Oxide Membrane

“…Surprisingly, the interlayer spacings of the GO–PEI membrane were limited to 10.80 Å in the wet state, far smaller than 11.59 Å of the GO membrane. The thickness of the GO monolayer (a) is 3.35 Å, as reported . Thus, the effective size of the GO–PEI nanochannel is 7.45 Å, which is very close to the hydrated ion diameter of Li + , whereas much smaller than that of Mg 2+ .…”

“…The thickness of the GO monolayer (a) is 3.35 Å, as reported. 29 Thus, the effective size of the GO−PEI nanochannel is 7.45 Å, which is very close to the hydrated ion diameter of Li + , whereas much smaller than that of Mg 2+ . These results could be attributed to the electrostatic and crosslinking effects between the PEI molecular chain and GO nanosheets in the lamellar structure of the GO−PEI membrane, which prevented the swelling between pristine GO lamellas.…”

“…The effective total length of GO membranes was calculated by L d × h / d , where L d , h , and d represented the GO flake size, thickness, and interlayer spacing of the membrane, respectively (see Figure S16 in the Supporting Information). When the membrane thickness and interlayer spacing were fixed, the length of ion transport for GO–PEI membranes is proportional to the GO flake size. Thus, when the lateral GO flake size reduced, more ion transport channels of GO–PEI membranes were obtained, achieving enhanced Li + permeation rate.…”

“…The sieving performance of the 2D membranes is mainly determined by size sieving and electrostatic repulsion. − For example, Lu et al have obtained a 2D laminar membrane by stacking MXene nanosheets with the introduction of poly­(sodium 4-styrene sulfonate), and the membrane showed a high selectivity of 28 for Li + /Mg 2+ separation with 0.08 mol m –1 h –1 Li + permeation rate . Graphene oxide (GO), as a typical 2D material, has been widely used for molecular/ion sieving. − However, due to the large number of oxygen-containing functional groups on the surface of GO nanosheets, the interlayer spacing of the membrane is easy to expand in an aqueous solution. , On the other hand, these attached functional groups are negatively charged and have a cationic attraction rather than the repulsive effect, resulting in the poor performance of the GO membrane in ion sieving. , Xi et al used a controllable chemical reduction method to regulate the interlayer spacings of the GO membrane for ion sieving. Even though the selectivity of monovalent ions (i.e., K + , Na + , and Li + ) with multivalent ions is significantly improved compared with the pristine GO membrane, the collapsed interlayer channels led to an extremely low permeation rate of Li + (<0.01 mol m –2 h –1 ) .…”

Electrically Modulated Nanofiltration Membrane Based on an Arch-Bridged Graphene Structure for Multicomponent Molecular Separation

“…Overcoming the limitations of polymeric membranes, separation membranes constructed by two-dimensional (2D) materials have exhibited unique attributes in precise molecular sieving owing to their unique nacre-like structures and ultrafast mass transport. − As typical 2D materials, graphene-based membranes consisting of an atom-thick 2D carbon lattice and oxygen functional groups have emerged as candidates for nanofiltration due to unimpeded water permeation, excellent chemical resistance, easy solution processability, and mechanical strength. − Functionalized 2D membranes with advanced performances have achieved great progress and shown potential in many applications. − Recently, several pioneering works have focused on developing polymer-decorated 2D membranes to modulate the permeation of water and ion molecules by external stimuli such as pH, gas, light, and temperature. − However, most previous attempts were based on modulating the molecular configuration or surface hydrophilicity of graphene-based membranes. ,− Moreover, similar to the polymer-based membranes, the response of polymer-decorated graphene-based membranes often takes tens of minutes (>30 min) to respond to external factors and restore to the original state, owing to the diffusion of signal chemicals and the relaxation of polymer chains. − Therefore, fast, reversible, and remote control of graphene-based membranes are highly desired in practical applications. − Zhou et al developed a fast-response graphene oxide (GO) based membrane to electrically control water permeation by introducing conductive filaments via electrical breakdown and ionizing the water cluster within the 2D capillary . Meanwhile, Li et al demonstrated a nanoporous graphene-based membrane to accelerate ion diffusion by modulating the interfacial electrical double layer under an external electrostatic field .…”