Carbon coated alumina membranes

Carbon, which has stability in aqueous as well as non-aqueous solutions, is also a candidate for porous inorganic membranes. At present, a few manufacturers have commercialized carbon UF membranes, the pore sizes of which are larger than 10 nm. No carbon nanofiltration membranes have been reported, while carbon gas separation membranes, which have pore sizes less than 1 nm, have been reported by several groups [7]. Nomura et al. [40] coated poly (vinylidene chloride) on a-alumina supports and pyrolyzed them at 825° C... 

We examined the field emission properties 29) of the carbon nanotubes prepared in the pores of an alumina membrane whose pore diameter was 230 nm. The wall thickness and the length of the carbon tubes obtained were 15 nm and 60 pm, respectively. The emitting ends of the tubes were open, while the other ends were connected to the same carbonized film formed on the surface of the membrane. As shown in Figure 12, we utilized the gold-coated indium / tin oxide (ITO) as the anode and the carbon tubes as cathode. The gap between the tube ends and the anode was 130 pm. According to the measured results given in Figure 12, the turn on applied electric field is 3 V / pm, which is comparable to... 

Sotowa et al. prepared zeolite Y membranes ion-exchanged with platinum and different silica-coated/rhodium loaded yalumina membranes for the preferential oxidation of carbon monoxide [552]. The membranes were coated onto a-alumina tubes. While the defect-free thickness of the zeolite membranes was around 3 pm, the combined silica/y-alumina membrane was of 200- and 700-nm thickness. 

Other methods to prepare porous membranes include pyrolysis for carbon membranes, heat treatment and leaching for mesoporous glass membranes, and anodization for alumina membranes. The microporous carbon membranes are prepared by coating a polymeric precursor such as polyfurfuril alcohol and polycarbosilane on porous substrates, followed by controlled pyrolysis under N2 atmosphere [15]. The carbon membrane structure is determined by the fabrication variables, including the polymeric solution concentration, solvent extraction, heating rate, and pyrolysis temperature [16]. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

Different methods have been used to deposit microporous thin films, including solgel, pyrolysis, and deposition techniques [20], Porous inorganic membranes are made of alumina, silica, carbon, zeolites, and other materials [8], They are generally prepared by the slip coating method, the ceramic technique, or the solgel method (Section 3.7). In addition, dense membranes are prepared with metals, oxides, and other materials (Chapter 2). 

The fact that ALD is based on a self-terminating gas-solid reaction yielding excellent deposition conformality aliows to coat high aspect ratio nanostructures, including colloidal arrays, anodized alumina and track etched poly(carbonate) membranes [19, 20]. 

Rao and Sircar [5-7] introduced nanoporous supported carbon membranes which were prepared by pyrolysis of PVDC layer coated on a macroporous graphite disk support. The diameter of the macropores of the dried polymer film was reduced to the order of nanometer as a result of a heat treatment at 1,000°C for 3 h. These membranes with mesopores could be used to separate hydrogen-hydrocarbon mixtures by the surface diffusion mechanism, in which gas molecules were selectively adsorbed on the pore wall. This transport mechanism is different from the molecular sieving mechanism. Therefore, these membranes were named as selective sitrface flow (SSF ) membranes. It consists of a thin (2-5 pm) layer of nanoporous carbon (effective pore diameter in the range of 5-6 A) supported on a mesoporous inert support such as graphite or alumina (effective pore diameter in the range of 0.3-1.0 pm). The procedures for making the selective surface flow membranes were described in [5, 7]. In particular, the requirements to produce a surface diffusion membrane were shown clearly in [7]. 

They also formed the condensed polynuclear aromatic (COPNA) resin film on a porous a-alumina support tube. Next, a pinhole-free CMSM was produced by carbonization at 400-1,000°C [29], The mesopores of the COPNA-based caibon membranes did not penetrate through the total thickness of each membrane and served as channels which increased permeances by linking the micropores. CMSMs produced using COPNA and BPDA-pp ODA polyimdes showed similar permeation properties even though they had different pore stractures. This suggests that the micropores are responsible for the permselectivities of the carbonized membrane. Besides that, Fuertes [30] used phenohc resin in conjunction with the dip coating technique to prepare adsorption-selective carbon membrane supported on ceramic tubular membranes. 

Examples and Applications An example of the above approaches has been demonstrated for hydrogen separation. An SSF membrane (Sircar et al., 1999) was formed by coating a thin layer of polyvinylidene chloride-acrylate terpolymer latex containing 0.1 -0.14 mm polymer beads in an aqueous emulsion on the bore side of a macroporous alumina tube (0.56 cm internal diameter, 0.16 cm wall thickness). The latex was dried at 50°C under N2 and subsequently heated to 600°C followed by passivation by heating to 200-300°C in an oxidizing atmosphere. The resulting carbon membrane has a pore size of 5-7 A and a thickness of 2-3 p.m. 

Surface-selective flow membranes made of nanoporous carbon, which is a variation of molecular sieving membranes, were developed by Rao et al. (1992) and Rao and Sircar (1993). The membrane can be produced by coating poly(vinylidene chloride) on the inside of a macroporous alumina tube followed by carbonization to form a thin membrane layer. The mechanism of separation is by adsorption-surface-diffusion-desorption. Certain gas components in the feed are selectively adsorbed, permeated through the membrane by surface diffusion, and desorbed at the low-pressure side of the membrane. This type of membrane was used to separate H2 from a mixture of H2 and CO2 (Sircar and Rao, 2000), and their main advantage is that the product hydrogen is at the high-pressure side eliminating the need for recompression. The membrane, however, is not industrially viable because of its low overall separation selectivity. In addition, since the separation mechanism involves physical adsorption, operation at low temperatures is required. 

New classes of MWCNTs/carbon nanocomposite thin films were introduced by Tseng et al. [17]. These were prepared by incorporating MWCNTs into polyimide (PI) precursor solution. The carbon films were obtained in only one coating step by spin-coating on a microporus alumina substrate and carbonization at 773 K. The MWCNTs/carbon nanocomposite thin film exhibited an ideal CO2 flux that was 8656.6 Barter, and a separation factor for CO2/N2 of 4.1 at room temperature and 1 atm feed pressure was achieved. It was 2-4 times higher than that of pure carbon membrane prepared by the same procedure and conditions. 

Instead of the PA precursors, Hayashi et al. deposited a polyimide film on the outer surface of a porous alumina tube by dip-coating three times. After imidization and pyrolyzation at 973-1073 K, the carbon membranes were fabricated on porous alumina tube. The enhancement of the volume of micropores accessible to smaller molecules has been observed. Hayashi et al. obtained an optimal pyrolysis temperature of 973 K and maximum permeance was achieved. In order to improve selectivity, a carbon layer was further deposited on the resultant supported carbon membrane by chemical vapor deposition (CVD) of propylene at 923 K.The CVD process favors the deposition of carbon in micropores, which explains the increase of the selectivity of CO2/N2 from 47 to 73. 

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