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10 Electrospun Nanofibrous Sorbents and Membranes for Carbon Dioxide Capture 253 membranes are typically classified as inorganic (e.g., ceramic, metal oxide, metallic, molecular sieves, and MOFs) or organic (e.g., cellulose acetate, polysulfone, polyamide, and polyimide) [19]. A membrane separates species by selectively permeating certain components of a mixture faster than others through a thin barrier in response to an external driving force, such as a concentration, partial pressure, or, more generally, a chemical potential gradient. A variety of mechanisms influence the separation of gases by a membrane, with the most important being solution diffusion and molecular sieving. Membranes offer a number of inherent advantages over other technologies for separating gases, including (1) simple, passive operation with no moving parts; (2) environmentally benign separation without the use of hazardous chemicals; (3) small footprint, which is critically important in some applications (e.g., aboard aircraft, spacecraft, or on offshore natural gas platforms); and (4) lower energy use because they can separate species without a phase change. Clearly, membranes represent a promising technology for gas separation; how- ever, they suffer a number of drawbacks, particularly with regard to CO2 capture from flue gas. In this case, the low CO2 partial pressure provides a minimal driving force for gas separation, which creates an energy penalty due to the need for compression of the feed gas. Membrane materials also suffer from a decrease in permeability over time due to particulate deposition on the surface [19]. 10.2.4 Fibrous Sorbents Solid amine adsorbents using a fibrous structure instead of particles as the matrix are expected to offer amazing benefits (e.g., fast kinetics and high CO2 capture capacity) for the adsorption of CO2 because of their high external surface area and porosity, low pressure drops, and flexibility of the matrix fibers. Recently, Li et al. [24] developed a novel fibrous adsorbent for CO2 capture by coating polyethylenimine (PEI) on a glass fiber matrix using epoxy resin as a cross-linking agent. They found that a maximum adsorption capacity of 6.3 mmol CO2/(g of PEI) was obtained at a PEI/epoxy resin ratio of 10:1. Then, they replaced epoxy resin (MW D 370) with a lower molecular weight cross-linking agent (i.e., epichlorohydrin, MW D 92.5) and developed adsorbents of PEI modified glass fibers [23]. The resultant fibrous sorbent improved adsorption performance (e.g., higher CO2 capacity, faster kinetics, and better regenerability) [25]. The maximum CO2 adsorption capacity of 13.08 mmol CO2/(g of PEI) was achieved at a coating weight of 45 wt%. Polymer supports for amines have attracted considerable attention since they are light in weight, flexible, and easy to handle [6]. Yang et al. [26] demonstrated that solid amine-containing fibrous adsorbent could be prepared by pre-irradiation grafting copolymerization of allylamine onto polyacrylonitrile fiber (PAN-AF). The higher grafting degree resulted in higher CO2 uptake, and the adsorption capacity of PAN-AF reached 6.22 mmol CO2/(g of PAN-AF) at the grafting degree of 60 wt%. They attributed the good performance of this fibrous adsorbent to the fibrous structure, which mightPDF Image | Electrospun Nanofibrous Sorbents
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