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254 X. Wang and B. Li have reduced the resistance to gas flow, thereby maximizing sorbent performance while minimizing energy consumption, and the highly stable interface (i.e., grafting copolymerization of amines onto fibers) between amines and the support [26]. Apart from the amine-modified fibrous adsorbents, porous carbon materials are well-known adsorbents for CO2 adsorption due to their highly developed porosity, extended surface area, flexible surface chemistry, and high thermal sta- bility. Recently, some new classes of carbon materials (e.g., carbon fiber, carbon molecular sieves) have emerged as adsorbents for gas separation and storage. Carbon fiber refers to fibers which are at least 90 wt% carbon in composition obtained by the controlled pyrolysis of an appropriate precursor material (e.g., pitch, PAN, rayon, nonheterocyclic aromatic polymers) [27]. Much research has been devoted to the preparation of PAN-based activated carbon fiber (PAN-ACF) from modified PAN, and some attention has been paid to the use of PAN-ACFs for CO2 removal. Hierarchical porous PAN-ACFs with a large Brunauer-Emmett-Teller (BET) surface area were made from PAN fibers through pre-oxidation and chemical activation. This type of material contains a large number of nitrogen-containing groups (N content>8.1 wt%) and consequently basic sites, leading to a faster adsorption rate and a higher CO2 adsorption capacity (2.4 mmol/g). Moreover, PAN- ACF adsorbents had stable CO2 adsorption/desorption performance under multiple cycling conditions [28]. 10.3 Why Electrospun Nanofibers? Due to the unique structure and mechanical properties of nanofibers, nanofiber- based sorbents are expected to have extremely low resistance for gas transport and thereby extremely fast kinetics. Due to the high surface area and low density of nanofibers, the developed nanofibrous sorbents are also expected to have high CO2 capture capacity. Various nanofibrous sorbents (e.g., nanofibrillated cellulose and graphite nanofibers or GNFs) have been developed for CO2 capture. For instance, Meng et al. [16] found that porous GNFs could be fabricated using a KOH etching method at temperatures in the 700–1,000 ıC range (Fig. 10.2a). The CO2 adsorption isotherms revealed that GNFs treated under 900 ıC had the highest BET surface area (567 m2/g) and the best CO2 adsorption capacity of 59.2 mg/g (Fig. 10.2b). Electrospinning is a simple and versatile technique to produce continuous nanofibers with a diameter down to nanometers. Due to the combination of a high fiber production rate and the simplicity of the setup, the electrospinning approach has the unique ability to produce nanofibers with most materials (organic, inorganic, or hybrid), easily providing various fibers with the desired composition and surface properties [15]. Their high surface-to-volume ratio, large porosity (up to over 80 %), and adjustable functionality make electrospun fibrous membranes useful for numerous applications in particulate gas separation [29, 30].PDF Image | Electrospun Nanofibrous Sorbents
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