# Polymeric and Crystalline Materials for Effective and Sustainable CO\({}_{2}\) Capture
## 2 Background
### CO\({}_{2}\) Capture Using Adsorbents and Membranes  
Adsorption is the process by which molecules adhere to the surface of a solid material (adsorbent) [31; 32; 33]. In CO\({}_{2}\) capture using adsorbents, porous materials with high surface areas and/or carbon dioxide-specific functional groups are utilized to selectively adsorb CO\({}_{2}\) molecules from gas streams. CO\({}_{2}\) capture via adsorption is a widely used method for separating and concentrating carbon dioxide from gas mixtures, such as flue gas or ambient air [34]. The gas stream, with most of the CO\({}_{2}\) removed, is then emitted to the atmosphere or utilized for a different purpose. The solid is then purified in stages using differences in pressure, temperature, or other stimuli to remove the CO\({}_{2}\) and further compress it for storage. The advantages of this process are numerous, including the high selectivity, flexibility of the process, scalability, and regenerability. Nevertheless, certain limitations can accompany the process, including the high energy cost, low adsorbent stability, and the spaciousness of the equipment. The membranous process can serve as an attractive alternative, offering a complementary approach, where the selective permeation of CO\({}_{2}\) through a membrane material takes place, while other gases are retained [35]. The resulting separation is based on differences in molecular size, solubility, and/or diffusivity. Membranes for such systems consist of thin, porous layers through which the gas mixture is passed. The captured CO\({}_{2}\) can then be collected on the permeate side of the membrane for further processing or storage. Such a gas separation technique is currently widely used for air separation, hydrogen production, natural gas purification, and post- and pre-combustion carbon dioxide capture. An advantage of this process over adsorbent-based separations is that it is continuous, without any alterations in temperature or pressure. Upstream pressure is the only energy input required to induce the separation. This way, the challenges associated with energy efficiency, the spaciousness of the design, and modularity can be easily solved, while the selectivity of the separation and membranous degradation problems must be addressed. While the aforementioned technologies offer the same results via two complementary approaches, the appropriate method must be carefully selected depending on the process requirements, scale, and specifications. More precisely, membrane separation offers alternative energy- and carbon-efficient approaches to the current gas separation technologies, such as pressure swing adsorption (PSA), temperature swing adsorption (TSA), and cryogenic distillation [36]. Their advantages are numerous: membranes do not require thermal or pressure-driven phase changes, offering economic competitiveness, with the market steadily growing since 1977. Furthermore, the emergence of MOFs and their successful formation into crystalline films, followed by the success in mixed matrix membrane synthesis, has enabled the development of new materials with unique and outstanding properties [37]. To be considered for post-combustion flue gas treatment and to be economically competitive with amine absorption technology, membranes should demonstrate a high CO\({}_{2}\) permeance of 1000-5000 GPU and a CO\({}_{2}\) selectivity of 30-50 [37; 38; 39]. A flue gas produced by a typical 550 MW power plant contains 12-14 % by volume of CO\({}_{2}\)necessitating an enormous CO\({}_{2}\) permeance relative to traditional polymer membrane systems. For the pre-combustion capture process, the requirements are slightly easier, with a CO\({}_{2}\) permeance greater than 200 GPU and a CO\({}_{2}\)/H\({}_{2}\) selectivity greater than 20 desired to be competitive with typical PSA processes [37; 40].