Nanoporous Carbon Membranes Research Articles

Influence of orientational ordering transition on diffusion of carbon dioxide in carbon nanopores

Simulation studies predicted an orientational ordering transition of the molecular axis of supercritical CO2, inside individual graphite pores, with size of the order of few molecular diameters. Study of the diffusion equation over a real porous medium, showed significant influence of the transition on the adsorbate diffusivity. Measured permeability of CO2 adsorbed in a nanoporous carbon membrane, confirmed the simulation results by exhibiting unusually high permeability values in the vicinity of the transition, and elucidated the significance of the thermodynamics in the study of the kinetic properties of the adsorbed phase.

Synthesis and characterization of catalytic nanoporous carbon membranes

AbstractA method is reported for the synthesis of nobel catalyic nanoporous carbon membranes. Defect‐free, nanoporous carbon films of 12.5−μm average thickness containing a dispersion of Ptx were synthesized on macroporous stainless‐steel supports. Ideal gas selectivities for the catalytic membranes were simliar to those of inert nanoporous carbon membranes with He–N2=58.6 and O2N2=4.9. The selective hydrogenation of monoolefins (propylen, l‐buiene, isobutylene) was used to probe the shape‐selective catalytic proerties nad the transport selectivities of the membranes. The results were modeled using the linear regime of coupled sdsorption, transport and rection in the membrane. Model regression yielded 14.9, 19.7. and 18.4 kJ/mol for the activation energies of permeation for propylene, isobutylene, and l‐butene, respectively. The system demonstrated selective reaction and transport favoring propane production with selectiveity ratios of 28.9:3.2.I for propane:n‐butane:isobutane at 125°C.

Synthesis and Characterization of Heteropolyacid Nanoporous Carbon Membranes

Heteropolyacid nanoporous carbon membranes were synthesized by re-precipitation of 12-tungstophosphoric acid on a prefabricated nanoporous carbon film. The Keggin structure is maintained on and within the 8 μm film as inferred from FTIR spectroscopy and XRD. Gas permeation reveals that separation performance of the carbon membrane is maintained after deposition although total membrane flux decreases by nearly 40% for N2 permeation. Semi-batch membrane reactor performance in the decomposition of methyl tert-butyl ether at 55°C shows significant enhancement of the conversion with 96.5% compared with 64.1% for a non-selective membrane analog under the same conditions.

On the preparation of supported nanoporous carbon membranes

New preparation techniques for synthesis of thin film supported nanoporous carbon membranes (SNPCM) using ultrasonic deposition (UD) have been examined. In contrast to other work we have published, in which membranes were prepared either by brush or spray coating of the polymer precursor onto the porous stainless steel support, the ultrasonic deposition method provides a greater degree of control over the deposition step. Permeation experiments on the UD-SNPCM with pure gases such as nitrogen (N 2) and oxygen (O 2) were used to benchmark properties and characteristics as a function of synthesis variables. The UD-SNPCM display ideal separation factors for O 2/N 2 ranging from 2 to 30 depending on synthesis conditions. With other gases ideal separation factors were as large as: H 2/CH 4=600, CO 2/CH 4=45, N 2O/N 2=17, and H 2/CO 2=14 at 295 K. Permeation rates ranged from 7.9×10 −13 to 4.1×10 −8 mol m −2 Pa −1 s −1 and were largely independent of pressure. Oxygen versus nitrogen selectivities were inversely proportional to oxygen flux.

Transport in nanoporous carbon membranes: Experiments and analysis

Single-component permeances of six gases were measured on three different supported nanoporous carbon membranes prepared by spray coating and pyrolysis of poly(furfuryl alcohol) on porous stainless-steel disks. Global activation energies were regressed from data collected as a function of temperature. Permeances and global activation energies were correlated to molecular size, assuming that entropic affects dominated the transport. The permeance was best correlated to the minimum projected area of the molecule computed from first principles. The free-energy barriers to transport within the membranes were derived from the temperature dependence of the permeance data, after accounting for porosity differences between the membranes and differences in molecular adsorption. Using transition-state theory and an entropic model derived, the free energy, enthalpy, and entropic barriers to transport within the membrane were examined as a function of molecular size. Computed on the basis of size, the entropic component of this barrier did not account for the large differences in the transition-state free energies. However, when these entropic barrier values were used to compute the enthalpic portion of the barrier free energies, the minimum projected area of each molecule correlated strongly. Furthermore, these enthalpic components of the barriers were fitted nicely by the Everett-Powl mean field potential, using only the pore size as the adjustable parameter. These results shed light on the underlying mechanism by which shape-selective transport takes place in the NPC membranes and small molecules are separated.

Characterization of Supported Nanoporous Carbon Membranes

Characterization of Supported Nanoporous Carbon Membranes

Characterization of Supported Nanoporous Carbon Membranes

A new method for the fabrication of supported nanoporous carbon membranes, involving ultrasonic deposition of the polymer precursor, is presented here. The method has been used to produce mechanically robust, continuous films of nanoporous carbon on tubular, macroporous stainless steel supports using polyfurfuryl alcohol as the polymer precursor. The films, which were characterized using microscopy and spectrometry, show unprecedented molecular sieving selectivities for very small molecules, such as oxygen, nitrogen, helium, and hydrogen.

Carbon membrane eyed for gas separation

Researchers have ultrasonically prepared a robust, nanoporous carbon membrane that allows oxygen molecules to pass through its pores 30 times faster than nitrogen molecules—an impressively high selectivity, according to one membrane expert. The researchers—chemical engineering professor Henry C. Foley of the University of Delaware, Newark, and research engineer Mark B. Shiflett of Du- Pont Central R&D in Wilmington, Del.-believe that such ceramic membranes, if developed further, could have potential for industrial gas separations and enrichment of protein broths. The membrane material might also have catalytic and fuel cell applications, according to Foley. In a paper published last week [ Science , 285, 1902 (1999)], Shiflett and Foley report that air passed through the carbon membrane was enriched by about 100%—changing from 21% oxygen to 44%. The enrichment would have been considerably greater if the membrane had performed ideally—that is, true to its ideal O 2 /N 2 selectivity of 30—but that hasn't ye...

Ultrasonic deposition of high-selectivity nanoporous carbon membranes

Ultrasonic deposition creates a thin film of polymer on a tubular, macroporous, stainless steel support. Using polyfurfuryl alcohol as the nanoporous carbon precursor and a pyrolysis temperature of 723 kelvin, a membrane was prepared with the following permeances, measured in moles per square meter per Pascal per second: nitrogen, 1.8 x 10(-12); oxygen, 5.6 x 10(-11); helium, 3.3 x 10(-10); and hydrogen, 6.1 x 10(-10). The ideal separation factors as compared to that for nitrogen are 30:1, 178:1, and 331:1 for oxygen, helium, and hydrogen, respectively.

Spray-coating of nanoporous carbon membranes for air separation

Nanoporous carbons (NPC) can be fabricated in the form of supported thin films with minimal defects and remarkably high size selectivity of oxygen over nitrogen. Spray coating of porous stainless steel disks with a solution of poly(furfuryl) alcohol (PFA) in acetone was used to synthesize nanoporous carbon membranes in a reproducible manner, and is the first reported case of this technique being used for supported carbon membrane synthesis. The disks were dried and then pyrolyzed in an inert atmosphere to a final temperature of 600°C. The resulting membranes were tested with binary oxygen/nitrogen mixtures. In experiments with a binary feed of oxygen and nitrogen, the membranes were found to have good oxygen over nitrogen selectivities (up to 4) and O 2 fluxes on the order of 10 −9 mol/m 2 s Pa. Attempts were made to modify membrane performance further by post-pyrolysis heating, additional coatings or by hydrocarbon pyrolysis. None of these post-treatments produced membranes superior to those derived directly from the initial spray coating and pyrolysis.

Separation of hydrogen sulfide–methane mixtures by selective surface flow membrane

Abstract The separation performance of hydrogen sulfide–methane mixture by a nanoporous carbon membrane called selective surface flow (SSF) membrane is described. The membrane selectively permeates H2S from CH4, producing a CH4 enriched gas at the feed pressure. Experimental data for the separation of H2S–CH4 mixtures using feed-gas H2S compositions of 10–50 mol.% and feed-gas pressures of 0.45–1.14 MPa are reported. The partial pressure of H2S in the feed gas is the key variable for establishing the separation performance of the SSF membrane. The H2S rejection–CH4 recovery data can be empirically correlated using a simple equation with a single adjustable parameter which is a function of feed-gas H2S partial pressure. The membrane separates H2S–CH4 mixtures very efficiently when the partial pressure of H2S is moderate (>0.2 MPa) even if the total feed-gas pressure is low (0.45 MPa).

Selective Surface Flow Membrane for Gas Separation

The selective surface flow membrane is a nanoporous carbon membrane which separates gas mixtures by a selective adsorption–surface diffusion–desorption mechanism. It selectively permeates the larger and the more polar components of a feed gas mixture. The separation characteristics of several different gas mixtures by the membrane are described. The membrane has been field-tested at a refinery site for separation of hydrocarbon–hydrogen mixtures.

Carbon nanotubule membranes for electrochemical energy storage and production

Ensembles of aligned and monodisperse tubules of graphitic carbon can be prepared by a templating method1,2,3,4 that involves the chemical-vapour deposition of carbon within the pores of alumina membranes5,6,7. Tubules with diameters as small as 20 nm have been prepared in this way7,8. The carbon comprising these tubules can be transformed from a disordered material to very highly ordered graphite5. Here we show that template-synthesized carbon tubules can be fabricated as free-standing nanoporous carbon membranes, and that narrower, highly ordered graphitic carbon nanotubes can be prepared within the membrane's tubules. Both the outer and the inner tubules are electrochemically active for intercalation of lithium ions, suggesting possible applications in lithium-ion batteries9,10. The membranes can also be filled with nanoparticles of electrocatalytic metals and alloys. Such catalyst-loaded membranes can be used to electrocatalyse O2 reduction and methanol oxidation, two reactions of importance to fuel-cell technology.

Separation of Bulk Carbon Dioxide-Hydrogen Mixtures by Selective Surface Flow Membrane

The separation performance of carbon dioxide-hydrogen mixtures by a nanoporous carbon membrane called selective surface flow membrane is described. The membrane selectively permeates CO2 from H2 and a H2 enriched gas is produced at the feed gas pressure. Extensive experimental data for the separation using feed gas pressures from 0.24 to 1.13 MPa and CO2 compositions from 5 to 75 (mol%) in H2 are reported. The data can be empirically correlated using a simple equation with a single adjustable-parameter. The adjustable parameter is found to be a linear function of the feed gas CO2 partial pressure.

Separation of bulk hydrogen sulfide–hydrogen mixtures by selective surface flow membrane

AbstractThe separation performance of binary hydrogen sulfide–hydrogen mixtures by a nanoporous carbon membrane called selective surface flow (SSF) membrane is described. The membrane selectively permeates H2S from H2, and a H2‐enriched stream is produced at the feed‐gas pressure. A two‐stage embodiment of the SSF membrane is described for production of high‐purity H2 with high H2 recovery from an equimolar H2S‐H2 feed gas. A novel protocol for operation of the two‐stage membrane is needed to achieve that separation goal.

Computer Simulation Studies on Gas Permeation through Nanoporous Carbon Membranes by Non-Equilibrium Molecular Dynamics.

The μVT-NEMD method, a combination of the μVT-MC and the boundary-driven non-equilibrium MD, has been applied for simulating permeation of pure- and mixed-gases through membranes with slit-like pores under the assumption of local adsorption equilibrium with the feed gas at the entrance of the membrane. The model system is a slit-like pore of graphite plane for the membrane and the methane and the ethane for permeating gases. To reach the stationary state in a simulation run, 50,000–60,000 time steps computation and 500–2,000 molecules in a cell are required for obtaining better statistics in the NEMD. Computer graphics animations clearly show that molecules having entered the pore gather together in monolayers on the surface to form downstream surface flow. Under a constant feed pressure condition, the mass fluxes of gases decrease significantly with increasing temperature because of the decrease in pore density in adsorption equilibrium with a feed gas. In the case of binary mixture permeation, enhanced selectivity of more adsorptive substance (ethane) was observed with an decrease in temperature, the reason for which is the significant decrease in pore density of methane, a weaker adsorptive, due to the competitive adsorption of ethane.

Performance and pore characterization of nanoporous carbon membranes for gas separation

The performance of a novel nanoporous carbon membrane for separation of hydrogen-hydrocarbon gas mixtures is described. The membrane selectively adsorbs hydrocarbons from hydrogen at the high pressure side and the adsorbed molecules then diffuse along the pore walls to the low pressure side. Pressure levels at thigh gh and low pressure sides of the membrane and the type and flow rate of the sweep gas at the low pressure side of the membrane were varied. The effects of these variables on the hydrogen recovery and hydrocarbon rejection by the membrane were investigated. Atomic force microscopy and scanning tunnelling microscopy scans of the membrane surface suggested that the membrane pore diameters were approximately 5 Å in diameter. Comparison between pure methane diffusivity through the membrane and those through nanoporous zeolites of various pore opening alsoo suggested that the membrane pores diameters were in the range of 5–6 Å. A sensitivity analysis indicated that the membranes contained a very narrow distribution of pore sizes.

Non-equilibrium molecular dynamics for simulating permeation of gas mixtures through nanoporous carbon membranes.

This paper deals with the computer modeling of permeation of a gas mixture through a nanoporous membrane by introducing a novel approach of non-equilibrium molecular dynamics (NEMD). A combination of the grand canonical ensemble (GCE or μVT) Monte Carlo (MC) technique and the boundary-driven NEMD method (Hafskjold et al., 1993; Ikeshoji and Hafskjold, 1994) is proposed for simulation of gas permeation based on the selective adsorption and surface diffusion mechanism

Activated carbon for gas separation and storage

Activated carbons offer a large spectrum of pore structures and surface chemistry for adsorption of gases, which are being used to design practical pressure swing and thermal swing adsorption processes for separation and purification of gas mixtures. The activated carbons are often preferred over the zeolitic adsorbents in a gas separation process because of their relatively moderate strengths of adsorption for gases, which facilitate the desorption process. Three commercial applications of activated carbons, (a) trace impurity removal from a contaminated gas, (b) production of hydrogen from a steam-methane reformer off gas, and (c) production of nitrogen from air, are reviewed. Four novel applications of activated carbons for gas separation and purification are also described. They include, (a) separation of hydrogen-hydrocarbon mixtures by selective surface flow of larger hydrocarbon molecules through a nanoporous carbon membrane produced by carbonization of a polymer matrix, (b) gas drying by pressure swing adsorption using a water selective microporous carbon adsorbent produced by surface oxidation of a hydrophobic carbon, (c) removal by selective adsorption and in-situ oxidation of trace volatile organic compounds from air by using a carbon adsorbent-catalyst composite, and (d) storage of compressed natural gas on high surface area carbons.

Nanoporous carbon membranes for separation of gas mixtures by selective surface flow

A novel nanoporous carbon membrane produced by carbonization of poly(vinylidene chloride) is described. The membrane separates gas mixtures by selective adsorption and surface diffusion of the more strongly adsorbed components. In particular, the membrane can very efficiently separate hydrocarbons from hydrogen from low-pressure gas streams.The method of preparation of the membrane and its characterization, pure and mixed gas permeabilities through the membrane, as well as an industrial application are reported.