Polyacrylonitrile porous

Fig. 4.25 Adsorption isotherms showing low-pressure hysteresis, (a) Carbon tetrachloride at 20°C on unactivated polyacrylonitrile carbon Curves A and B are the desorption branches of the isotherms of the sample after heat treatment at 900°C and 2700°C respectively Curve C is the common adsorption branch (b) water at 22°C on stannic oxide gel heated to SOO C (c) krypton at 77-4 K on exfoliated graphite (d) ethyl chloride at 6°C on porous glass. (Redrawn from the diagrams in the original papers, with omission of experimental points.)...

SPAN module. It was mentioned at the beginning that the special polyacrylonitrile fibers of SPAN have a wall thickness of 30 gm, which is considerably thicker than the 8 gm wall thickness of the SMC modules [19]. As a consequence, the presence of stronger capillary effects from the special porous fiber material of the SPAN module would be a reasonable conclusion. Furthermore, the texture of the special polyacrylonitrile fibers is expected to have better surface properties, supporting the permeation of molecules as compared with synthetically modified cellulose. In conclusion, both convection and diffusion effectively contribute to the filtration efficiency in a SPAN module, whereas for the SMC membrane, diffusion is the driving force for molecular exchange, the efficiency of which is also considerable and benefits from the large surface-to-volume ratio. 

In practical application, it was reported that the platinum particles dispersed in highly porous carbonized polyacrylonitrile (PAN) microcellular foam used as fuel-cell electrocatalyst160 have the partially active property. The fractal dimension of the platinum particles was determined to be smaller than 2.0 by using the potentiostatic current transient technique in oxygen-saturated solutions, and it was considered to be a reaction dimension, indicating that not all of the platinum particle surface sites are accessible to the incoming oxygen molecules. 

The carbon materials attract the increasing interest of membrane scientists because of their high selectivity and permeability, high hydrophobicity and stability in corrosive and high-temperature operations. Recently many papers were published considering last achievements in the field of carbon membranes for gas separation [2-5]. In particular, such membranes can be produced by pyrolyzing a polymeric precursor in a controlled condition. The one of most usable polymer for this goal is polyacrylonitrile (PAN) [6], Some types of carbon membranes were obtained as a thin film on a porous material by the carbonization of polymeric predecessors [7]. Publications about carbon membrane catalysts are not found up to now. 

However, it is also possible to cycle CM made from pyrolyzed polyacrylonitrile in aqueous electrolytes, according to Beck and Zahedi [378]. Figure 30 shows relatively flat redox peaks around the quinone/hydroquinone center (f/s — 0 V, about 0.7 V vs. SHE). Protons are the counterions in this case. A polyquinonimine structure is concluded from (electro)chemical and FTIR data (cf. Fig. 34). These acceptor-type compounds have relatively high specific capacities of about 300 Ah/kg in the steady state. The initial capacities are even higher. It should be mentioned that graphite nanotubules were synthesized in the nanopores of a porous AI2O3 matrix at 250/ 600 °C [433]. 

In cross-flow flltration, the wastewater flows under pressure at a fairly high velocity tangentially or across the filter medium. A thin layer of solids form on the surface of the medium, but the high liquid velocity keeps the layer from building up. At the same time, the liquid permeates the membrane producing a clear filtrate. Filter media may be ceramic, metal (e.g., sintered stainless steel or porous alumina), or a polymer membrane (cellulose acetate, polyamide, and polyacrylonitrile) with pores small enough to exclude most suspended particles. Examples of cross filtration are microfiltration with pore sizes ranging from 0.1 to 5 pm and ultrafiltration with pore sizes from 1 pm down to about 0,001 pm. 

A membrane designated "Solrox" made by Sumitomo Chemical Company is closely related to the above plasma polymerized composite membranes. A 1980 report by T. Sano described the Sumitomo process (31). A support film was cast from a polyacrylonitrile copolymer containing at least 40 mole percent acrylonitrile. The support film was dried and exposed to a helium or hydrogen plasma to form a tight cross-linked surface skin on the porous polyacrylonitrile support film. Data in a U.S. Patent issued in 1979 to Sano et al showed that the unmodified support film had a water flux of 87 gfd (145 L/ sq m/hr) at 142 psi (10 kg/sq cm). After the plasma treatment a reverse osmosis test using 0.55 percent NaCl at 710 psi (4895 kPa) showed 10.5 gfd (17.5 L/sq m/hr) flux at 98.3 percent salt rejection (32). This membrane appears to fall between a conventional asymmetric membrane and a composite membrane. If the surface skin is only cross-linked, one might call it a modified asymmetric membrane. However, if the surface skin is substantially modified chemically to make it distinct from the bulk of the membrane it could be considered as a composite type. 

Porous membranes can be made of polymers (polysulfones, polyacrylonitrile, polypropylene, silicones, perfluoropolymers, polyimides, polyamides, etc.), ceramics (alumina, silica, titania, zirconia, zeolites, etc.) or microporous carbons. Dense organic membranes are commonly used for molecular-scale separations involving gas and vapor mixtures, whereas the mean pore sizes of porous membranes is chosen considering the size of the species to be separated. Current membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), gas and vapor separation (GS), and pervaporation (PV). Figure 1 indicates the types and sizes of species typically separated by these different separation processes. 

For membrane solvent extraction, Schlosser et al. [114] provided a comprehensive review for organic acid separation and recovery. Large-scale utilization of membrane solvent extraction [107,115,116] modules in pharmaceutical/chemical plants is used. Lopez and Matson [115] demonstrated enzymatic resolution of diltiazem precursor using hydrophilic polyacrylonitrile hollow fibers and an aqueous-organic interface on the outside surface of these fibers. Klaassen and Jansen [116] and Porebski et al. [107] illustrated the use/ performances of modules of porous hydrophobic PP hollow fibers in chemical plants achieving aqueous-to-organic and organic-to-aqueous solvent extraction, respectively. 

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