Membranes carbon

Since the pioneering paper of Koresh and Soffer on carbon molecular sieve membranes in 1983 [300] much research has been carried out in the field of carbon-based gas-separation membranes. Selectivities and permeabilities far above the performance of the best polymers have been obtained for carbon molecular sieve membranes by many researchers. One example is a recent publica- 

As can be seen from the table the pure gas selectivities of the nanoporous carbon membrane are quite low, e.g. 1.19 for butane/hydrogen. However, for the mixture given in Tab. 7.6 the butane/hydrogen selectivity increases to 94. The reason for this is that the butane is selectively absorbed over hydrogen at the carbon pore wall and because the pores are so small the pathway for hydrogen is blocked. This effect of selective surface flow and pore blocking was first observed by Barrer et al. [303]. Due to its unmatched selectivity the nanoporous 

Cas Pure gas permeability (Barrer) Mixed gas permeability Mixed gas selectivity 

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Because carbon has a natural affinity for adsorption of heavy hydrocarbon species and polar molecules, CMS membranes need to be used at a sufficiently high temperature to eliminate contribution/interference of the adsorption. In contrast, strong adsorption of heavier molecules may be used to separate those species by adsorption as discussed earlier by the SSF mechanism (Rao and Sircar, 1993b). The SSF carbon membranes typically have pore dimensions much greater than those needed for CMS membranes since the separation is based on the adsorbed species effectively blocking permeation of other components (Fuertes, 2000). Carbon membranes are resistant to contaminants such as H2S and are thermally stable and can be used at higher temperatures compared to the polymeric membranes. For the synthesis gas environment, the hydrothermal stability of carbon in the presence of steam will be a concern limiting its operation temperature. 

Fuertes, A.B., Adsorption-selective carbon membrane for gas separation, ]. Membr. Sci., 177, 9, 2000. 

Hatori, H., H. Takagi, and Y. Yamada, Gas separation properties of molecular sieving carbon membranes with nanopore channels, Carbon, 42, 1169-1173, 2004. 

Koresh, J.E. and A. Sofer, Study of molecular sieve carbon membranes, Part 1. Pore structure, gradual pore opening, and mechanism of molecular sieving, /. Chem. Soc., Faraday Trans. I, 76,2457,1980. 

Okamoto, K.,S. Kawamura, M. Yoshino, H. Kita, Y. Hirayama, N. Tanihara, and Y. Kusuki, Olefin/ paraffin separation through carbonized membranes derived from an asymmetric polyimide hollow fiber membrane, Ind. Eng. Chem. Res., 38, 4424,1999. 

Saufi, S.M. and A.F. Ismail, Fabrication of carbon membranes for gas separation—a review, Carbon, 42,241-259,2004. 

In addition to the particulate adsorbents listed in Table 16-5, some adsorbents are available in structured form for specific applications. Monoliths, papers, and paint formulations have been developed for zeolites, with these driven by the development of wheels (Fig. 16-60), adsorptive refrigeration, etc. Carbon monoliths are also available as are activated carbon fibers, created from polymeric materials, and sold in the forms of fabrics, mats, felts, and papers for use in various applications including in pleated form in filters. Zeolitic and carbon membranes are also available, with the latter developed for separation by selective surface flow [Rao and Sircar, J. Membrane Sci., 85, 253 (1993)]. 

T. Page, R. Johnson, J. Hormes, S. Noding, and B. Rambabu, A study of methanol electro-oxidation reactions in carbon membrane electrodes and structural properties of Pt alloy electro-catalysts by EXAES, J. Electroanal. Chem. 485, 34-41 (2000). 

In this section a short introduction will be given on the synthesis of porous ceramic membranes by sol-gel techniques and anodization, carbon membranes, glass membranes and track-etch membranes. An extensive discussion will be given in Sections 2.3-2.S. 

In gas separation applications, polymeric hollow fibers (diameter X 100 fim) are used (e.g. PAN) with a dense skin. In the skin the micropores develop during pyrolyzation. This is also the case in the macroporous material but is not of great importance from gas permeability considerations. Depending on the pyrolysis temperature, the carbon membrane top layer (skin) may or may not be permeable for small molecules. Such a membrane system is activated by oxidation at temperatures of 400-450 C. The process parameters in this step determine the suitability of the asymmetric carbon membrane in a given application (Table 2.8). 

This study also reported that films deposited on carbon membranes at temperatures >80°C were of hexagonal (wurtzite) structure, with a high density of planar defects, in contrast to the zincblende obtained from both hydroxide and ion-by-ion mechanisms at lower temperatures and to the epitaxial films on InP at all temperatures. 

Despite these failures, microporous carbon membranes continue to be a subject of research by a number of groups [67-70], The selectivities obtained are often very good, even for simple gas mixtures such as oxygen/nitrogen or carbon dioxide/methane. However long-term, it is difficult to imagine carbon membranes... 

R. Ash, R.M. Barrer and P. Sharma, Sorption and Flow of Carbon Dioxide and Some Hydrocarbons in a Microporous Carbon Membrane, J. Membr. Sci. 1, 17 (1976). 

The transport rates of carbon dioxide and hydrogen sulfide through these carbonate membranes can be significantly increased by adding catalysts to increase the rates of the slow reactions of Equations (11.21) and (11.22). A variety of materials can be used, but the anions of the weak acids such as arsenite, selenite and hypochlorite have been found to be the most effective. Small concentrations of these components increase permeation rates three- to five-fold. 

Maid et al. [40] developed a nickel dispersed carbon membrane catalyst about 100 pm thick. Between the membrane plates, the gas flow occurred in gaps of various thickness between 200 and 1 500 pm. The plates were mounted into a stack-like testing device for methanol decomposition to carbon monoxide and hydrogen. 

The problem with use of polymeric membranes in this application is plasticization, leading to much lower selectivities with gas mixtures than the simple ratio of pure-gas permeabilities would suggest. For this type of separation, a Robeson plot based on the ratio of pure-gas permeabilities has no predictive value. Although membranes with pure-gas propylene/propane selectivities of 20 or more have been reported [43, 44], only a handful of membranes have been able to achieve selectivities of 5 to 10 under realistic operating conditions, and these membranes have low permeances of 10 gpu or less for the fast component (propylene). This may be one of the few gas-separation applications where ceramic or carbon membranes have an industrial future. 

Conventional polymeric hydrogen separation membranes yield hydrogen at low pressure. Air Products and Chemicals has demonstrated a carbon membrane on an alumina support that removes hydrocarbons from hydrogen/hydrocarbon mixtures and leaves the hydrogen at high pressure40. 

With respect to carbon membranes, the molecular sieving carbon membranes, produced as unsupported flat, capillary tubes, or hollow fibers membranes, and supported membranes on a macropo-rous material are good in terms of separation properties as well as reasonable flux and stabilities, but are not yet commercially available at a sufficiently large scale, because of brittleness and cost among other drawbacks [3,6],... 

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. 

The present work is the first attempt to develop the methods of preparation of carbon membrane catalysts with metal nanoparticles in a carbon membrane matrix. 

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