Carbon Alloys part 15 docx

474 Chapter 29 -;lo-8- 10-7 4- ? 10-9 9 E E -10-10 8 i! k 10-11 10-12 Q - - 0 C 0 a However, solid porous supports are not always necessary for asymmetric hollow carbon fibers [ 14,23,24,30,31,33,34,47]. The precursor hollow fiber developed by Kusuki et al. [23] was spun from polyimide and had an O.D. of 0.40 mm and an I.D. of 0.12 mm. It was then heat-treated in air at 673 Kfor 30 min and pyrolyzed at 873-1273 K for 3.6 min. The hollow fiber shrunk to 0.35-0.28 mm O.D. and 0.11-0.09 mm I.D. depending on the carbonization temperature. The fractured face of the membrane was composed of a skin layer and a macroporous bulk layer. Such a structure confers flexibility on the carbon fiber. L k$: . I . I . - - c2Hg : C3H8 7 r i Permeation temperature i . =308K I.,. 3 Permeances of Molecular Sieving Carbon Membranes 3.1 Unary Gases Gas Separations with Carbon Membranes 475 10-12 0.2 0.3 0.4 0.5 Kinetic diameter [ nm] Fig. 6. Effect of kinetic diameters of permeates on their permeances at 298 K 1331. 0 MembraneA a Membrane B A Membrane C 0 v) c - ._ % 10-15 IIIIIIII~ 5 10 15 Projected area x IO2 [nm2] - Fig. 7. Relationship between intrinsic permeances at 294 K and minimum projection area of permeating molecules [40]. temperature of 873 IC After correction for the adsorption effect, intrinsic permeances to single-component gases was correlated with the projected areas of the molecules. As shown in Fig. 7 [40], the intrinsic permeance to carbon dioxide was much lower than that to hydrogen. 3.2 Binary and Ternary Mixtures Based on the permeation modes shown in Table 1, mixtures of carbon dioxide and nitrogen fall into regime 11. The permeance to the less adsorptive component 476 500 - I 1, 4oo w I P 0" 300 0, 9 * 200 d c 0 L c 5 Q 100 Q) v) 0 - IIIIIII 10-7 -Temp. = 298K 'm r 4. 9 ? E = 10-8 - r - Feed pressure = 16OkPa - - - L - - Ideal selectivity i2 a 8 - - E a - W 10-9 3 17' - - - - - - - - - - - - - - - - - 0 IIIIIIIII Chapter 29 Fig. 8. Effect of carbon dioxide concentration in the feed on CO$CH, separation factor and permeance of carbon dioxide [34]. (nitrogen) decreases with increasing fraction of the more adsorptive component (carbon dioxide) on the feed side. Figure 8 [34] shows typical data for this category. The total pressure is not a major factor in the separation of dry gases [30]. When gases are condensed in pores, however, a different permeation mode appears. As shown in Fig. 9, the permeance to carbon dioxide reached a maximum near its critical pressure and then decreases at higher pressures [47]. The maximum was not observed at a permeation temperature of 333 K which is higher than the critical temperature. Carbon membranes are effective for the separation of alkanes and alkenes. Hayashi et al. [18] found that a carbonized membrane prepared using a BPDA-pp'ODA polyimide procedure gave higher C,HdC,H, and C,H4/C2H, perm- selectivities than those of the corresponding polyimide membrane. The C,HdC,H, selectivity was approximately 30 at a permeability coefficient of 50 Barrer, which is 4 I I I I I - - - - - - - - 0123456 Pressure [MPa] Fig. 9. Effect of total pressure on permeances [47]. Gas Separations with Carbon Membranes 477 I 40 I 111 I I I I I '''I II & ID Permeation temperature = 373K - Carbonization temp. m 4 Fig. 10. Relationship between C,HJC,H, separation factor and C,H, permeance for single-component systems [31]. equivalent to a permeance of 3 x lo-' mol m-' s-I Pa-', when the membrane thickness, 6, is 6 pm. This suggests that the carbonized membranes possess a micropore structure which is capable of differentiating between alkane and alkene molecules. These high separation factors are explained in terms of the minimum size of the molecules. The minimum size of C,H, is 0.40 nm, which is smaller than that of C,H,, 0.43 nm [18]. Okamoto et al. [31] reported similar results, as shown in Fig. 10. The C,HdC,H, separation factor was 10-20, while the GH, permeance was larger than lo4 mol m-'s-' Pa-'. The separation factor for ternary component systems is complicated. Figure 11 shows the separation factors for C01-CH4-Hz systems [53], The mole fractions of carbon dioxide and hydrogen were kept equal with a varying mole fraction of CH,. The HJCH, separation factor decreased with an increase in the mole fraction of CH,. This is in accord with the findings relative to a binary H,-CH, system. However, the CO,/CH, separation factor increased at higher CH, mole fractions. 120 100 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 CH4 mole fraction [-] Fig. 11. Effect of feed composition on separation factor for C02-CH,-H2 systems. Permeation temperature = 293 K, pressure = 207 kPa [53]. 478 Chapter 29 4 Oxidation of Molecular Sieving Carbon Membranes Heat-treatment in an oxidizing atmosphere improves permeances [21,22,28]. Kusakabe et al. [28] carbonized BPDA-pp’ODA membranes in an inert atmosphere at 973 K, followed by oxidization using a mixture of 0,-N, (02 fraction = 0.1) at 573 K for 3 h. The oxidation decreased the H/C ratio and increased the O/C ratio, suggesting that peripheral alkyl groups had undergone decomposition and that oxygen had been incorporated into the membrane. Figure 12 shows permeances for carbon mem- branes at permeation temperatures of 338 K and 373 K [21]. The oxidation resulted in an increase in permeance without greatly altering the permselectivities. Thus, oxidation at 573 K for 3 h significantly increased the micropore volume while the pore size distribution remained relatively unchanged. However, treatments in an oxidative atmosphere need to be more extensively studied because results often are not reproducible. In order to examine the long-term stability, Hayashi et al. [21] exposed BPDA-pp’ODA-based carbon membranes, which had been carbonized at 973 K, to air at 373 Kfor one month. As shown in Fig. 13, this exposure caused no change to the O/C ratio and only a limited decrease in the H/C ratio following decomposition of peripheral alkyl groups. The N/C and O/C ratios remained constant during the exposure. After the long-term oxidation, the membrane was heat-treated in nitrogen at 873 K for 4 h. Figure 13 also shows that the mass of the membrane decreased to 84% of its original value as a result of the long-term oxidation. After the heat treatment, it further decreased to 76% of its original value, while the H/C, O/C and N/C ratios remained unchanged within experimental error. Figure 14 shows data on 0.2 0.3 0.4 0.5 0.6 Kinetic diameter [nm] Fig. 12. Effect of oxidation at 673 K for 3 h on permeances of membranes carbonized at 973 K. Permeation temperature = 338 K; (0) = as-formed, (0) = oxidized in 0,. Permeation temperature = 373 K, (m) = as-formed, (A) = oxizided in 0,-N, mixture (0, fraction = O.l), (0) = oxidized in 0, [28]. Gas Separations with Carbon Membranes Heat-treatment period [h] 479 2.; iI Heat-treatment in Np at 073K Oxrdation period [day] Fig. 13. Changes in mass and elemental distribution in membranes during the stability test at 373 Kin air. The mass of the initial membrane is assumed to be unity [21]. Heat-treatment period [h] 01234 .7 IO - 7 .8 g 10 ty .9 E 10 8 IO s E! 11 k IO 7 9 - I 10 P .12 IO 0 10 20 30 Oxidation time [day] Fig. 14. Effect of long-term exposure to air at 373 K and post heat-treatment in nitrogen at 873 Kfor 4 h on permeances at 338 K [21]. permeances, for the long-term oxidized and heat-treated carbon membrane, to a single-component gas at the permeation temperature of 338 K. The permeances of the membrane decreased with increasing exposure time and recovered approximately to original values after heat-treatment. The membrane showed a COJCH, perm- 480 Chapter 29 selectivity of 80 at 338 K. Thus, oxidation in air at 373 K does not appear to have a great effect on the chemical structure of the membranes. Surface oxides, introduced by oxidation at 373 K, may reduce the aperture of the micropores and decrease permeances. Most of the surface oxides are decomposed by post heat-treatment at 873 K. The separation properties of carbon membranes are sensitive to steam, which is strongly adsorptive. Jones and Koros [15] reported that permeances to oxygen and nitrogen for an asymmetric hollow fiber carbon membrane, carbonized at 773-823 K, decreased to 0.4-0.5 of the initial value after the membrane was exposed to air at relative humidities of 2345% at ambient temperature. The stability of the carbon membrane was improved by coating the membrane with perfluoro-2,2'-dimethyl-1,3- dioxole or tetrafluoroethylene [16]. The oxygen flux, after exposure to humidity, decreased to 11% as a result of the coating procedure. 5 Separation Based on Surface Flow Fuertes [50] prepared carbon membranes by carbonizing cured phenolic resin films at 973 K in vacuum, followed by oxidizing the membranes with air at 537-673 K for 30 min. The oxidation decreased the separation factors for single component gases. For combinations of adsorptive hydrocarbon and nonadsorptive nitrogen, however, the permeance to nitrogen decreased as shown in Figure 15, and, as a result, the separa- tion factor for hydrocarbon increased. This effect is pronounced for hydrocarbons, which have high adsorptivity and small molecular size. Carbon membranes with 0.4-1.5 nm diameter pores were also developed and applied to separation of hydrocarbons from hydrogen [44,46,58,59]. An enriched hydrogen stream was produced on the high-pressure side (Le., the feed side) of the membrane. Carbon membranes of this type have also been applied to the separation of hydrogen sulfide from hydrogen [60] and methane [61]. t \ Gas rrixtures: PertmanUN2 = 50/50 50 90 130 170 210 250 290 Critiwl volume of permeant [dml] Fig. 15. Relationship of nitrogen permeances at 293 K and critical volumes of co-existing gas [50]. Gas Separations with Carbon Membranes 481 6 Conclusions Molecular sieving carbon membranes, from a variety of polymeric precursors, are made by carbonizing precursor membranes. When carbonization conditions, such as temperature and time, are properly selected, the permeation properties and stabili- ties of the carbon membranes are greatly improved. Permeation through carbon membranes is dependent on molecular size relative to pore size, adsorptivity, diffusiv- ity and pore size distribution. Controlled oxidation at elevated temperatures increases permeances of the carbon membranes. However, permeances may be decreased by exposing the membranes to an oxidative or humidified atmosphere as the result of formation or the adsorption of oxygen-containing functionalities in the pores. Permeances can be recovered by heat-treating the membranes in an inert atmosphere without damage to selectivity. Carbon membranes with pores of 0.4-1.5 nm are useful for producing hydrogen at the exit of the feed side by transferring hydrocarbons or hydrogen sulfide to the permeate side via surface flow through the pores. References I. S.A. McKelvey, D.T. Clausi and W.J. Koros, A guide to establishing hollow fiber macro- scopic properties for membrane allocations. J. Memb. Sci., 124: 223-232, 1997. 2. C.M. Zimmerman, A. Singh and W.J. Koros, Tailoring mixed matrix composite mem- branes for gas separations. J. Memb. Sci., 137: 145-154,1997. 3. W.J. Koros and R. 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[...]... the electronic structure of F-ACFs The carbon nanotubes, MWNTs, synthesized by the template carbonization technique, behave as ACFs in regard to fluorination 3 The Chemistry of Carbon Nanotubes with Fluorine and Carbon Alloying by Fluorination 3.1 Side Wall Fluorination of Single-wall Carbon Nanotubes Nanometer-sized carbon tubes (nanotubes), a novel form of carbon, have currently attracted considerable... fluorination is an effective way to prepare carbon alloys with new functionalities Chapter 30 486 Of the many types of carbon materials, the chemistry of carbon nanotubes is of considerable interest in terms of alloying by heteroatom-doping or intercalation and of side-wall chemical functionality Both single-wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs) are amphoteric and form... opportunities to prepare carbon alloys with new functionalities Carbon alloyed nanotubes were prepared by selective fluorinationof hidden surfaces of multi-wall carbon nanotubes using a template carbonization technique Carbon- fluorine bonds are formed on the hidden surfaces of the tubes while internal surfaces retained their sp*-hybridization Fluorination (as a form of carbon- alloying) significantly... the walls were perturbed 3.3 Temphte-synthesizedCarbon Nanotubes Initial preparation methods for SWNTs or MWNTs include carbon- arc synthesis, laser vaporization of graphite and catalyticdecomposition of hydrocarbons Synthesis of carbon nanotubes by a new template method has recently been developed [13 -151 Kyotani et al [13,14] developed the template carbonization technique based on anodic aluminum... structural forms of carbons such as activated carbon, carbon fibers, and carbon nanotubes Of these, extents of carbon s-p hybridization vary resulting in diverse electronic structures affecting their electrical conductivity, electron ionization potential, electron affinity, etc Physical properties are affected with, for example, electrical conductivity of fluorine -carbon materials changing widely from 2x... Technol., 15: 121-129,1999 485 Chapter 30 Property Control of Carbon Materials by Fluorination Hidekazu Touhara Department of Materials Chemis*, Faculy of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan Abstract: Fluorination is effective in chemically modifying and controlling physicochemical properties of carbon materials over a wide range so creating opportunities to prepare carbon. .. binding energies for C-F bonds Fig 1 XPS binding energies (eV) of fluorinated activated carbon fibers (F-ACFs), hidden surface fluorinated multi-wall carbon nanotubes (F-MWNTs) and other fluorinated carbon materials Propeq Control of Carbon Materials by Fluorination 487 linked cyclohexane-type chairs with sp3-hybridizedcarbons, and are, consequently, electrical insulators Films of (CF),, and (C,F), from... of Lilcarbon nanotube rechargeable cells in an aprotic medium (with reversible lithium insertion in hidden surfaces) as studied by galvanostaticdischarge-chargeexperiments Keywords :Carbon nanotubes, Activated carbon fibers, Fluorination, Hidden surface, Electrochemical lithium insertion, Adsorption properties 1 Introduction The interaction of fluorine with carbon materials provides fluorine -carbon. .. properties [l] carbon and fluorine varies from covalent, through semi-ionic, to ionic, with van der Waals interactions also having a role To understand the wide range of interactions between carbon and fluorine, the diversity of structure of carbon materials has to be taken into account There are three allotropes of carbon, viz., graphite, diamond, and fullerenes, and other structural forms of carbons such... microstructures of the carbon material deposited on the C-A120,films [7] In the Raman spectra of fluorinated samples, two bands was observed at -1340 and -159 0 cm-' being features of Raman peaks of less-ordered carbon materials The spectra confirm that internal carbon atoms, below the outermost layer, still retain their $-hybridization after fluorination Property Control of Carbon Materials by Fluorination . and fullerenes, and other structural forms of carbons such as activated carbon, carbon fibers, and carbon nanotubes. Of these, extents of carbon s-p hybridization vary resulting in diverse. controlling physicochemical properties of carbon materials over a wide range so creating opportunities to prepare carbon alloys with new functionalities. Carbon alloyed nanotubes were prepared. fluorination is an effective way to prepare carbon alloys with new functionalities. 486 Chapter 30 Of the many types of carbon materials, the chemistry of carbon nanotubes is of considerable