ABSTRACT
Nanostructured materials such as microporous polymers, 2D materials like graphene, and crystalline metal-organic frameworks (MOFs) experienced fast development in the last few years. Such materials are of interest for different applications, such as gas storage and heterogeneous catalysis. In addition, membranes based on nanostructured materials displayed outstanding properties when considered for molecular separations. This chapter focuses on microporous polymers with intrinsic microporosity (PIMs), which are gaining increasing interest in developing membranes for large-scale gas separations in commercial and environmental applications. The chapter presents the strategies pursued in order to tailor the transport properties of these ultrapermeable materials.
7.1 IntroductionMembrane gas separation rapidly became a competitive separation technology since the serial production of polymeric membranes in late 1970. In general, membrane processes give several benefits than many conventional separation techniques (e.g., distillation, extraction, absorption, and adsorption). Membrane systems do not require a phase change, thus resulting in a low energy requirement. They do not involve expensive adsorbents and/or solvents. Membrane systems are fully automated, modular, with small footprint and weight, and thus, very attractive for offshore gas processing platforms. The absence of moving parts makes gas separation systems particularly suited for remote locations where reliability is critical. All these features contribute to implementing the logic of Process Intensification in various industrial contexts. Considering gas and vapor separations, today there are different industrial applications for membrane plants, ranging from hydrogen recovery from off-gases in refinery operations, to nitrogen generation from air and to the upgrading of natural gas and biogas (i.e., predominantly removing CO2 from CH4) (Bernardo et al., 2009). Engineering and material science both contributed to the development of these applications. The commercially available membranes are based on a few polymers. Indeed, polymeric materials are cheap and can be easily processed in thin films required for cost-effective applications. However, for many potential large-scale applications, such as the capture of CO2 from power station flue gases, there is a need for new materials that offer high permeability combined with good selectivity. The performance of a membrane material depends on productivity and separation efficiency, which can be expressed in terms of a permeability coefficient and a selectivity, respectively. Typically, in polymeric materials, the permeability P is the product of a diffusion coefficient D and a solubility coefficient S, i.e., P = DS. The ideal selectivity for two gases (i and j) is obtained as the ratio of the individual permeability, ai/j = Pi/Pj. In 1991, Robeson demonstrated a limitation of polymeric membranes for gas separation applications as a trade-off between permeability and selectivity (Robeson, 1991). Polymers recently developed allowed a shift in the upper bounds toward more
