Two-dimensional nanochannel membranes for molecular and ionic separations

Two-dimensional (2D) nanosheets have emerged as promising functional materials owing to their atomic thickness and unique physical/chemical properties. By using 2D nanosheets as building blocks, diverse kinds of two-dimensional nanochannel membranes (2DNCMs) are being actively explored, in which mass transport occurs in the through-plane and interlayer channels of 2D nanosheets. The rational construction and physical/chemical microenvironment regulation of nanochannels are of vital significance for translating these 2D nanosheets into molecular separation membranes and ionic separation membranes. Focusing on the recent advances of 2DNCMs, in this review, various porous/nonporous 2D nanosheets and their derived nanochannels are first briefly introduced. Then we discuss the emerging top-down and bottom-up methods to synthesize high-quality 2D nanosheets and to prepare high-performance 2DNCMs. As the major part of this review, we focus on three types of nanochannels, which are based on nonporous nanosheets, intrinsically porous nanosheets and perforated nanosheets. The strategies for regulating the physical and chemical microenvironments in the nanochannels are emphasized. The representative applications of 2DNCMs in molecular separations (gas separation, liquid separation) and ionic separations are presented. Finally, the current challenges and future perspectives are highlighted.

Metal Soap Membranes for Gas Separation

Kumar Varoon Agrawal, Deepu Joseph Babu, Davide Campi, Jian Hao, Qi Liu

Metal soaps or metal alkanoates are metal-organic complexes held together with metal cations and the functional groups of hydrocarbon chains. They can be synthesized at a high yield by simply mixing the metal and organic sources, forming crystalline frameworks with diverse topology, and have been studied in the past because of their rich polymorphism-like liquid crystals. Their ability to melt while retaining the crystalline properties upon cooling is unique among nanoporous materials and is especially attractive for membrane fabrication. Herein, metal soaps as a new class of material for molecular separation are reported. Three metal soaps, Ca(SO4C12H25)(2), Zn(COOC6H13)(2), and Cu(COOC9H19)(2), hosting lamellar structure with molecular-sized channels are synthesized. They are processed in thin, intergrown, polycrystalline films on porous substrates by two scalable methods, interfacial crystallization and melting with an extremely small processing time (a minute to an hour). The resulting crystalline films are oriented with the alkyl chains perpendicular to the porous substrate which favors molecular transport. The prepared membranes demonstrate attractive gas separation behavior, e.g., 300-nm-thick Ca(SO4C12H25)(2)membrane prepared in a minute using interfacial crystallization yields H(2)permeance of 6.1 x 10(-7)mol m(-2)s(-1)Pa(-1)with H-2/CO(2)selectivity of 10.5.

WILEY-V C H VERLAG GMBH2020

Crystal Engineering of Metal-Organic Framework Thin Films for Gas Separations

Kumar Varoon Agrawal, Deepu Joseph Babu, Guangwei He, Luis Francisco Villalobos Vazquez de la Parra

Metal organic frameworks or MOFs have witnessed a phenomenal rise owing to a highly tunable synthetic chemistry allowing flexibility in the selection of its constituents, namely metal nodes and linkers. Combined with their superior adsorption and diffusion properties, MOFs have become one of the most promising nanoporous materials for the fabrication of high-performance membranes. Polycrystalline MOF membranes have yielded one of the best gas separation performances, and are expected to replace or partially substitute thermally driven separation processes. In this respect, we present our perspective on the crystal engineering of MOF films that offers control over nucleation and growth of MOFs, film morphology, lattice defects, and therefore the separation performance of the resulting MOF films.

AMER CHEMICAL SOC2019

Centimeter-scale gas-sieving nanoporous single-layer graphene membrane

Kumar Varoon Agrawal, Mostapha Dakhchoune, Guangwei He, Shiqi Huang, Wan-Chi Lee

High-permeance, molecular-sieving, nanoporous single-layer graphene (NSLG) membranes are highly promising for gas separation. However, the formation of cracks during the transfer of NSLG to a low-cost porous support is difficult to avoid. These cracks are detrimental to gas selectivity, and therefore, make the scale-up of the gas-sieving NSLG membranes challenging. To mitigate the crack formation on low-cost macroporous supports, herein, we demonstrate mechanical reinforcement of the graphene film with a two-layer composite carbon film. The bottom layer of the composite film is a 100-nm-thick block-copolymer film derived nanoporous carbon (NPC) film with a pore size of 20–30 nm. This layer makes an intimate contact with NSLG and prevents generation of crack. However, the NPC film by itself is not robust enough to cover the rough surface of low-cost macroporous supports and tends to generate occasional cracks. This is prevented by spin-coating a 500-nm-thick multi-walled carbon nanotube (MWNT) film, hosting pore size of 200–300 nm, on top of the NPC film. This imparts enough mechanical strength to NSLG/NPC film to be successfully suspended on a low-cost, macroporous, nonwoven metal wire mesh on a centimeter-scale while completely avoiding cracks. As a result, H2/CH4 and H2/CO2 selectivities of 11–23 and 5–8, respectively, higher than the corresponding Knudsen selectivities of 2.8 and 4.7, respectively, are obtained from the centimeter-scale NSLG membranes. The reinforced membranes are mechanically robust and can successfully withstand transmembrane pressure difference of 4 bar. When the MWNT film is directly coated on NSLG without using the intermediate NPC layer, the gas sieving behavior is not observed, likely due to the development of nanoscale cracks. This underlines the crucial role of the hierarchical pore structure in the composite carbon film in realizing the gas-sieving graphene membranes.

2020

Centimeter-scale gas-sieving nanoporous single-layer graphene membrane

Kumar Varoon Agrawal, Mostapha Dakhchoune, Guangwei He, Shiqi Huang, Wan-Chi Lee

2020