Layered graphene nanomaterials for gas separation membrane applications

Membrane-based gas separation technologies have attracted great interest because of their potential to improve separation performance, lower energy and capital costs, reduce the equipment size, minimize the environmental footprint, and provide easier operation. The performance of gas separation membranes, which can be characterized in terms of permeability and selectivity, is primarily dependent on the physical and chemical properties of the membrane materials. The performance of membrane based separation technologies is limited by several factors, including the minimum thickness of the membrane (to maximize the flux), the maximum achievable selectivity, and their robustness at higher temperatures and pressures or under corrosive and reactive conditions. Among the emerging membrane materials, graphene oxide (GO) is a promising material that can dramatically enhance the gas separation performance of the membrane technology beyond the limits of conventional membrane materials in terms of both permeability and selectivity. This performance enhancement is due to the ultimate thinness, superior mechanical strength, large surface area, and unique two-dimensional layered structure of GO. Monolayer GO can be engineered in several ways to form ultrathin layered GO membranes with a narrow pore size distribution. Graphene oxide membranes can allow extremely high fluxes because of their ultimate thinness and unique layered structure. In addition, the high selectivity is due to the molecular sieving or diffusion effect resulting from their narrow pore size distribution or their unique surface chemistry. Graphene oxide membranes can be prepared in several forms: as supported, self-standing, and nanocomposite materials. Self-standing GO membranes show promising selectivity with higher permeability because of the ultimate thinness of the membrane. However, supported GO membranes are more mechanically robust and may be better options for practical separation conditions. In this research, we have developed a facile preparation method of fabricating supported ultra-thin GO membrane and thin self-standing GO membrane. First, to achieve a better exfoliation of the graphene-oxide, we have adopted several approaches, such as using surfactants with GO solution, optimizing sonication and centrifugation parameters, etc. We have observed that the size of the GO particle (lateral dimension of exfoliated GO flake) greatly influences the formation of the membrane and hence impacts the gas separation characteristics of the GO membrane. Furthermore, we have developed a facile preparation of ultra-thin supported GO-PES membrane, which can block the transport of almost all the gas particle through the membrane. To achieve selectivity toward target gas, selective pores are developed in the completely stacked graphene-oxide chain by vacuum-drying, heating, or partial reduction of the graphene-oxide. The effect of moisture content in the structural integrity of the membrane is also realized with several controlled drying approaches of the ultra-thin membranes. Finally, the hydrocarbon (methane, propane, and butane) mixture separation performance of the GO membranes is evaluated by analyzing the gas mixture composition in both feed and permeate side using gas chromatography. We have characterized the gas separation performance for thin GO-PES, self-standing GO, ultra-thin GO-PES, reduced GO-PES membranes. Moreover, aside from understanding the transport mechanism through the interlayer spaces of GO membrane, we have analyzed the compared performances of different types of GO membranes to separate a hydrocarbon gas mixture of methane, propane, and butane. In all the membranes, two different mechanisms of hydrocarbon gas separation are observed, adsorption-desorption based and molecular sieving based, depending on the pressure difference between the feed and permeate side of the GO membrane. Because of its thick support material, very thin GO membrane layer, and controllable interlayer spacing, the ultra-thin GO-PES gas separation membranes are simultaneously robust, highly permeable, and highly selective