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This chapter covers a body of material that deals with movement of mass along gradients and between phases. It examines the commonalities and differences between linear driving forces, net rates of adsorption, and permeation. Each has the common feature that reaction is not involved but does involve transport between apparently well-defined regions. It focuses on chemically reactive systems in anticipation of eventually analyzing problems that involve mass transfer and reaction. Adsorption is a fundamental process of separation that is practiced for different purposes; removal of volatile organics contaminants (VOCs) from air is one, removal of water vapor from nitrogen is another. Adsorption is the process by which, molecules in the fluid phase in contact with a solid, move to the solid surface and interact with it. Once at the solid surface, these molecules may be reversibly or irreversibly adsorbed, that is, they may come back off the surface to the fluid phase with their full molecular integrity intact, or they may be so strongly bound that the rate of removal is for all purposes close enough to zero to be considered zero. True adsorption is a mass action process rather than a mass transfer process. What this means is that it will occur even in the absence of a concentration gradient between the bulk gas and the surface. It comes about due to the rapid and chaotic motion of the fluid phase molecules, and their impingement on the surface. From the elementary kinetic theory of an ideal gas one can compute the number of molecules impinging upon a surface per unit time per unit area at a given temperature and pressure.

Sixty years ago, cupric acetate was proposed as one of the first homogeneous, heterolytic hydrogen activation catalysts. Later on, it was demonstrated that dihydrogen complexation is unequivocally the first step preceding H H bond cleavage. In this study, using a home-made, high-pressure adsorption setup working at different temperatures, we examined the carbon-nanoconfined counterparts of the former in order to elucidate the nature of H 2 -metal complex interaction inside the nanosized space provided by the porous support. In this regard, we found that by lowering temperature from 100 °C to room temperature, a marked alteration would take place in the nature of interaction. By doing so, we moved from what appeared to be (H 2 )CuH 2 , a dihydride-dihydrogen complex, to a Kubas compound releasing H 2 in a fully reversible manner. The nanoconfined metal complex discussed here may serve either as a prototypical, room-temperature hydrogen storage medium with optimum heat of interaction, or as a solid-phase hydrogen activation catalyst operating under mild conditions.

Spherical micron-sized carbon powders were synthesized from feedstock furfuryl alcohol and tested as anodes in sodium ion batteries (SIBs). A long cycle life of 1000 cycles is achievable with this carbon at C rate (3–4 mg cm−2 loading and i = 200 mA g−1) yielding a steady capacity of ca. 115 mA h g−1. The results from solid-state 23Na MAS NMR analyses of cycled electrodes indicate no correlation in voltage profiles with sodium site nature (graphene or nanopores), which is a new observation in SIB carbon anodes.

Nanoporous carbons were synthesized and treated with nitric acid after which the change in their propensities for ammonia adsorption was determined. The adsorbents had mean pore sizes ranging from 0.5 to 12 nm and they included polyfurfuryl alcohol (PFA)-derived carbons, commercial activated carbons, and both soft and hard-templated mesoporous carbons. The carbons were treated with concentrated nitric acid at elevated temperatures (e.g. 90 °C) and for time spans between 15 and 240 min. The textural properties of carbons, before and after nitric acid treatment, were determined using CO2 adsorption at 0 °C. Ammonia adsorption uptakes were measured at 25 °C and at pressures up to 9.5 bar. The highest total uptake of ammonia on the native carbons, that is prior to nitric acid treatment, was ∼10 mmol/g at 1 bar and 25 °C; this was obtained on a microporous carbon derived from the pyrolysis of polyfurfuryl alcohol and polyethylene glycol blends followed by CO2 oxidation. Nitric acid treatment of this carbon significantly increased its total uptake of ammonia to 17 mmol/g. This sample provided a reversible uptake of 14 mmol/g after it was outgassed at 160 °C and 10−5 bar under dynamic vacuum. This is 2 mmol/g higher than state-of-the-art ammonia adsorbents such as COF-10. An x-ray photoelectron spectrum (XPS) showed that the oxygen content of the carbon increased from 3 at.% to 16.4 at.% after the nitric acid treatment. Heats of adsorption profiles, calculated from adsorption isotherms, started from 165 kJ/mol and quickly dropped to 40 kJ/mol with ammonia loading. This showed that the isosteric heat of adsorption on the remaining surface was still higher than the native carbon sample resulting in high reversible adsorption uptake, even after excluding the irreversible adsorption on the very high energy sites.

In this article, we report the results of careful, room-temperature, high-pressure adsorption studies, in which simple complexes of copper and silver reversibly adsorb hydrogen when dispersed within nanoporous carbon. Whereas these complexes alone did not adsorb hydrogen, when they were nanoconfined within carbon, they adsorbed 1–3 mol of H2 per metal center. As a figure of merit, nanoconfined cupric formate adsorbed ∼2.6 H2/Cu at 100 bar and 20 °C, much larger than any reported metal-containing adsorption medium. On carbon alone, the heat of hydrogen adsorption decreased with an increase in adsorption extent, limiting to insufficient levels of uptake. By contrast, when these metal salts were dispersed within the carbon, the heats of adsorption increased markedly in a linear manner, meaning that the thermodynamics has moved in the right direction and is not self-limiting. Such thermodynamic behavior is associated with side-on dihydrogen binding onto a metal center, the so-called Kubas binding. Such intera...

The effect of boron substituted carbon (BC x , x ∼ 3–5) coatings on the hydrogen adsorption properties of porous carbons was investigated via spectroscopic characterization and hydrogen adsorption measurements. Thin films of BC x were synthesized by reacting BCl 3 and benzene in a chemical vapor and then depositing the product as a solid (CVD). The BC x deposits were collected on microporous carbon substrates and mesoporous silica aerogel. High-resolution solid-state boron NMR, 11 B HR-NMR, detected boron atoms in a symmetric chemical environment with trigonal coplanar coordination. Results from analyses based on NMR, X-ray photoelectron spectroscopy (XPS), and d-spacings, calculated from electron diffraction and X-ray diffraction (XRD) patterns, indicated that 17 at.% boron was substitutionally incorporated into the carbon framework. Thus the material has an empirical formula of BC 4.9 . Hydrogen adsorption data were collected at 100 bar and 25 °C. The BC x coatings reduced the surface area of the porous substrates by 30–50%. However, at the same time, they increased the heat of adsorption and the adsorption capacities per unit area by as much as a factor of five. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) revealed wagging energies at 1190 cm −1 attributable to hydrogen interactions with C–B–C bonds.

Carbon materials with bimodal porosity have shown enhanced performance in a wide variety of applications including catalysis, energy storage and fluid separation. Presence of mesoporosity is essential to lower the mass transfer limitation imposed by the microporous nature of the carbons. The synthesis approaches used to prepare bimodal carbons with controlled micro/mesopore size and narrow pore size distribution, usually involve multi step processes and the use of harsh chemicals and solvents. Herein, we present a simple one step method that can be used to synthesize carbon with bimodal pore size distribution. Simultaneous polymerization of furfuryl alcohol and phloroglucinol-formaldehyde in the presence of a structure-directing agent (Pluronic F-127) was carried out and the resultant polymer was pyrolyzed to yield the bimodal carbon. Effect of polymerization conditions such as concentrations of monomer, initiator and surfactant on the bimodal pore size distribution of the carbon was studied in detail. Pyrolyzed precursors form carbons with narrow mean micropore size of 0.5 nm and mean mesopores ranging from 3.5 to 6 nm. The range of the mesopore size could be altered by varying the polymerization parameters (acid and surfactant concentration) as well as selective oxidation using CO 2 gas.

In heterogeneous catalysis, pore size exerts an influence on reaction pathway, selectivity, equilibrium and adsorption constants. This effect can in principle cause noticeable changes in selectivity. Here, we present an analysis of a diffusion–reaction process in the pores of a catalyst comprised of platinum nanoparticles embedded within a molecular sieving carbon. When the alkenes are hydrogenated over this catalyst, the reaction takes place within the ultramicropores of the carbon. Experimental data for the liquid phase hydrogenation of different alkenes over platinum supported on the carbon versus platinum embedded within the same carbon were collected. From these data kinetic parameters and diffusion coefficients for reactions were evaluated. The forward rate constant for 2-methyl-1-pentene hydrogenation was found to be almost one order of magnitude larger within the embedded platinum catalyst versus the supported platinum catalyst. The variation in pore size and reactant molecule dimension, were also found to affect the adsorption equilibrium constants and diffusion coefficients. For 2-methyl-1-pentene molecule with the highest steric hindrance, K increased to 500 g/mol in embedded catalyst compared to 100 g/mol on supported catalyst. At the same time the diffusion coefficient for 2-methyl-1-pentene was one order of magnitude smaller than 1-hexene.

Reactant shape selective catalysis occurs when substrates of different sizes and shapes are consumed at different rates over catalysts that combine molecular sieving transport processes with reaction. By contrast the same substrates react at nearly equivalent rates over catalysts that have large, open pores that do not induce any form of molecular sieving. Here we describe the design and synthesis of reactant shape selective catalysts for liquid phase hydrogenation reactions. Using an emulsion polymerization of furfuryl alcohol, we have made catalysts that consist of microporous carbon nanospheres within which are embedded platinum nanoparticles. The porosity of the carbon spheres was found to be a key parameter affecting catalyst activity and selectivity; porosity was varied by adding pore forming agents, such as polyethylene glycol with different molecular weights, during synthesis, or by mild oxidation of the as-synthesized catalyst using carbon dioxide. In addition to increasing porosity to reduce mass transfer limitations, a synthesis of smaller carbon spheres (

We present a simple method to synthesize shape selective carbon catalysts for large alkene hydrogenation reactions by tailoring porosity of platinum embedded in polyfurfuryl alcohol derived carbons. A small amount of mesoporosity, in addition to the intrinsic microporous nature of the carbon, shortens the diffusion length for the reactant molecule, enabling these materials to be used for catalysis in the liquid phase. A systematic study of hydrogenation reactions of liquid phase alkenes is reported. The molecular sieving effect of the catalyst was examined by varying molecular length, size, double bond position, stereoregularity and the number of double bonds in the alkenes.

Carbon spheres were synthesized by emulsion polymerization and pyrolysis of polyfurfuryl alcohol. Pluronic F-127 was used as the structure-directing agent to synthesize polymer spheres that after pyrolysis led to carbon spheres with average sizes from 50 nm to few micrometers in diameter depending upon the conditions of polymerization. As-synthesized carbon spheres possess high surface areas of around 480 m2/g with an average mean pore size of 0.5 nm. These spheres can be activated using carbon dioxide to create much higher surface areas (>1500 m2/g). Different compositional regions of the pseudo-ternary phase diagram of surfactant/monomer/solvent were explored in order to determine the effects of changes in the emulsion polymerization variables on the kinds of carbon morphologies that could be derived from polyfurfuryl alcohol after pyrolysis. The diameter of the carbon spheres was found to be sensitive to monomer and surfactant concentrations, acid molarity and solvent composition. In general, the diameter of the spheres grew with increasing furfuryl alcohol concentration and decreasing surfactant concentration, respectively. By varying the acid concentration and solvent composition, a minimum diameter for spheres was found. The formation and size of the spheres are strongly influenced both by micelle growth and the polymerization mechanism.

In the present work, design and operation of a high pressure gas adsorption apparatus at room temperature and at pressures up to 100 bar are discussed. A theoretical and experimental error analysis is done to determine accuracy and robustness of the measurements. For this study, activated carbon was selected as the adsorbent and hydrogen as the adsorbate gas. A sensitivity analysis was done by taking into account the effects of temperature, pressure, volume and weight of the sample. The analysis shows that the volumes of the sample and reference cells as determined by helium-free space measurements have significant effect on the accuracy of the adsorption uptake measurement. For instance, a 0.1% error in the measurement of either volume led to approximately a 3% error in hydrogen uptake measurement at 298 K and 100 bar.

Boron/nitrogen substituted carbons were synthesized by co-pyrolysis of polyborazylene/coal tar pitch blends to yield a carbon with a boron and nitrogen content of 14 at% and 10 at%, respectively. The presence of heteroatoms in these carbons shifted the hydrogen evolution overpotential to −1.4 V vs Ag/AgCl in aqueous electrolytes, providing a large electrochemical potential window (∼2.4 V) as well as a specific capacitance of 0.6 F/m2. An asymmetric capacitor was fabricated using the as-prepared low surface area carbon as the negative electrode along with a redox active manganese dioxide as the positive electrode. The energy density of the capacitor exceeded 10 Wh/kg at a power density of 1 kW/kg and had a cycle life greater than 1000 cycles.

Asymmetric aqueous electrochemical capacitors with energy densities as high as 22 Wh kg −1 , power densities of 11 kW kg −1 and a cell voltage of 2 V were fabricated using cost effective, high surface carbon derived from coal tar pitch and manganese dioxide. The narrow pore size distribution of the activated carbon (mean pore size ∼0.8 nm) resulted in strong electroadsorption of protons making them suitable for use as negative electrodes. Amorphous manganese dioxide anodes were synthesized by chemical precipitation method with high specific capacitance (300 F g −1 ) in aqueous electrolytes containing bivalent cations. The fabricated capacitors demonstrated excellent cyclability with no signs of capacitance fading even after 1000 cycles.

Without any catalyst or pressure, bulk non-graphitizing carbon does not exhibit any change in its atomic order (or graphitize) even at 3000 °C. We propose a route for significant graphitization in polymer-derived disordered carbon materials at moderate temperatures by pre-aligning the precursor polymer chains. Accordingly, we employ a template-based fabrication process, where the capillary forces align the polyfurfuryl alcohol chains in the form of nanowires with diameter from 100–200 nm. The nanowires are pyrolyzed at temperatures ranging from 600 °C to 2000 °C and characterized with transmission electron and Raman microscopy as well as electrical conductivity measurements. The observed significant change in atomic rearrangement is explained by the alignment of precursor chains during the templated synthesis.

Nanofibers of poly(ethyl 2-cyanoacrylate) were directly grown via a template-less vapor phase polymerization technique directly on surface modified glass substrates. Several commercially available glass slides were investigated for polymer nanofiber deposition. In addition, glass substrates were also modified in the laboratory using silanes with different functional groups. The growth of nanofibers at different relative humidities was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was found that nanofiber formation is favored when the polymerization occurs at relative humidities greater than 68%. The diameter and the number density of the nanofibers were examined in terms of the wettability of cyanoacrylate monomer on the modified glass substrates.

The type of anionic initiator used to polymerize ethyl 2-cyanoacrylate was found to influence the morphology of the polymer formed via vapor phase polymerization. Depending upon the type of initiator, polymerization of ethyl 2-cyanoacrylate resulted in either the formation of neat polymer nanofibers (∼200 nm in diameter) or thin films. Based on the classification of anions using Hard Soft Acid Base principles, we found that harder anions favored polymer film formation while softer ones favored polymer nanofibers. Infrared (IR) spectroscopy, scanning electron microscopy (SEM) and gel permeation chromatography (GPC) were used to characterize the structure, morphology and molecular weight of the synthesized polymers, respectively. Finally, a mechanism of formation of different polymer morphologies is proposed.

A new pathway to synthesize a carbon with both nanoporosity and pre-graphitic structures has been discovered by annealing at 2000 °C a CO 2 activated, non-graphitizing, nanoporous carbon originally derived from polyfurfuryl alcohol. The activation process with CO 2 overcomes the barrier to graphitization normally present in this carbon even when treated at high temperature. Gas adsorption analysis, skeletal density measurements, X-ray diffraction, and transmission electron microscopy are utilized to probe the structure of both the non-activated and the activated carbons at 800, 1200, 1800, and 2000 °C. The influence of activation time is also examined. Prior to activation the nanopore walls are comprised of several layers of disordered graphenes. Activation eliminates the barrier to graphitization by reducing the number of layers below the limit of detection and by removing carbon material highly susceptible to oxidation. Annealing at 2000 °C of the carbon activated to 84% burnoff induces the formation of pre-graphitic domains amongst the nanoporous carbon. The (0 0 2) bands corresponding to 2 θ = 24.3°, 26°, and 26.5° are identified and assigned to amorphous, turbostratic, and graphitic morphologies. A pore volume of 0.50 cm 3 g −1 localized in pores below 2 nm in size is preserved after annealing.

Porous carbons with tunable porosity and electrical conductivity have received considerable attention due to their potential application as anodes in lithium ion batteries, electrocatalysts in fuel cells, and high surface area electrodes in electrical double layer capacitors [1,2]. High porosities and surface areas are critical to the performances of these materials. Significant porosity and surface area are easily created by activation of non-graphitizing carbon, yet the material lacks in conductivity [3–5]. Whereas graphitic carbon possesses high conductivity, it strongly resists activation by traditional methods [5]. Recently graphitic carbons with controlled pore sizes have been obtained by pyrolysis of graphitizing carbon precursors deposited in mesoporous silica templates [2,6,7]. However, the preparation of carbon with nanopores, width less than 2 nm, is unattainable by this method, thus limiting its ultimate surface area, porosity, and performance. Graphitization of non-graphitizing carbon occurs in the presence of a variety metals, but the product is contaminated with the catalyst [8–11]. Stress graphitization of pure carbon has been reported at temperatures in excess of 2000 C [12,13]. Porosity is not preserved. Herein we describe the discovery of a new route to the synthesis of a pure nanoporous carbon containing pre-graphitic structures with both high surface area and high nanoand mesoporosity from the readily available polymer precursor polyfurfuryl alcohol. Nanoporous carbon (NPC) is synthesized by pyrolysis of polymers, such as polyfurfuryl alcohol (PFA). Due to its strong resistance to transformation to graphite even when annealed at temperature above 2000 C, it is classified as non-graphitizing, which was defined by Franklin [14]. This resistance is attributed to the presence of extensive cross-linking in the precursor, which creates kinetically frozen disorder through a chaotic misalignment of the graphenes in the carbon that result during pyrolysis [15– 17]. Disorder in NPC gives rise to porosity and the majority of pores are narrowly distributed in the range from 0.4 to 0.5 nm width [16,18,19]. The pore volume and surface area of NPC can be further developed by simple activation with CO2 [20,21]. We have found that the pre-graphitic ordering of PFA-derived NPC can be accomplished after CO2 treatment at 900 C, which generates activated NPC (a-NPC). The a-NPC provides an unexpected pathway to pre-graphitic carbon with nanoporosity by subsequent annealing at 2000 C. First, p-toluenesulfonic acid monohydrate (0.048 gm, Sigma–Aldrich) was dissolved in 5 ml of Triton X-100 (Sigma–Aldrich) by heating mildly. Then to this solution, 5 ml of furfuryl alcohol (99% Sigma–Aldrich) was added. The reaction mixture was stirred magnetically at 10 C. After polymerization for 48 h the product was transferred to a quartz boat and pyrolyzed under flowing argon in a quartz tube furnace. The sample was heated at a rate of 10 C min 1 to 800 C and held for 1 h. The carbon product was pure, without inorganic contaminants. It was ground and sieved to a particle size of < 38 lm. Activated NPC was prepared in a quartz tube furnace. 0.5 gm of carbon was heated to 900 C over 1 h in flowing argon. After 1 h of soak time the gas was switched to CO2 and soaked for an additional 3.5 h. The sample was cooled back to room temperature under argon. High temperature

The preparation of porous stainless steel supports was found to have a significant impact on the properties of nanoporous carbon membranes fabricated upon them. Nanofillers were incorporated into porous stainless steel supports to modify the pore structure by reducing the average pore size and porosity. Carbon membrane properties were examined as a function of support variables such as filler content, shape, size and nature of the particles. Optimum performances, in terms of the ideal selectivity ratio for oxygen to nitrogen permeances ( S O 2 / N 2 ∼ 3 – 6 ) and the oxygen permeance (10−8 mol m−2 s−1 Pa−1), were obtained when the filler completely saturated the support. This represents about a two order of magnitude improvement in oxygen permeance when compared to carbon membranes prepared on unmodified porous stainless steel supports. The origin of the improvement in the permeance is due to the formation of carbon membranes which are on average two orders of magnitude thinner than those formed on unmodified supports, i.e., the carbon membranes exists as very thin layers around and between the silica nanoparticles. A simple geometric model based on the packing of silica particles inside the porous stainless steel support is proposed to visualize and quantify this effect. The generality of the support modification concept is also demonstrated by the ability to employ different types of nanofillers and support geometries to obtain carbon membranes with high flux. Air separation experiments show that these membranes can produce both oxygen rich streams enriched to as much as 48% by volume and nitrogen rich streams enriched to over 90% by volume at reasonable operating conditions.

The pyrolysis of polyfurfuryl alcohol was studied up to 600 °C. Micropores appear in the carbon as early as 300 °C along with a significant amount of mesopores. As the pyrolysis temperature is increased, microporosity is retained, but the mesoporosity disappears. At 600 °C the material is microporous with a monodisperse pore size distribution centered at 4–5 A. Infrared, X-ray photoelectron, and nuclear magnetic resonance spectroscopies, in combination with thermogravimetric analysis provide evidence that between 300 and 400 °C, both polyaromatic domains decorated with hydrogen and oxygen (hetero) atoms and partially decomposed polymer chains coexist. The unreacted polymer and heteroatoms induce mesoporosity by buffering the micropores created by polyaromatic domains. Raising the pyrolysis above 400 °C releases the buffering material, thereby collapsing the mesopores.

A novel type of thin-film catalyst has been developed that combines high activity with selective transport across a micron-scale membrane film. Nanoporous carbon was used as a support for 12-tungstophosphoric acid with composite films synthesized by two distinct approaches: in-situ polymerization and adsorption from solution. Ion sputtering and XPS were used in combination to characterize the high dispersion of the catalyst within the film in the former case and a gradient in catalyst concentration in the latter. Both methods produce intact Keggin structures as demonstrated by FTIR spectroscopy. Gas permeation experiments indicate a preservation of separation capacity with O2/N2 permeability ratios as high as 6.5. The catalytic films were benchmarked using the decomposition of methyl tert-butyl ether at 55 °C and demonstrate remarkable increases in both conversion and simultaneous separation of decomposition products. Integrated conversions in a semi-batch reactor configuration far exceed the thermodynam...

A model for the structure of a non-graphitizing carbon is built by randomly incorporating between 0.5% and 4% non-hexagonal rings into an extended sp2 carbon sheet. The sheets are created using Cerius2 Modeling software, optimized using a DREIDING force-field, and turbostratically layered to create a three-dimensional structure. We show that the calculated pair distribution function (PDF) for models containing approximately 1% non-hexagonal rings is remarkable similar to the experimental PDF obtained from neutron scattering by a non-graphitizing carbon prepared from polyfurfuryl alcohol pyrolyzed at 1200 °C. No attempt was made to fit the model to the experimental data, and the model has no adjustable parameters except for the fraction of non-hexagonal rings.

Porous solids are of great technological importance due to their ability to interact with gases and liquids not only at the surface, but throughout their bulk. Although large pores can be produced and well controlled in a variety of materials, nanopores in the range of 2 nm and below (micropores, according to IUPAC classification) are usually achieved only in carbons or zeolites. To date, major efforts in the field of porous materials have been directed towards control of the size, shape and uniformity of the pores. Here we demonstrate that porosity of carbide-derived carbons (CDCs) can be tuned with subångström accuracy in a wide range by controlling the chlorination temperature. CDC produced from Ti3SiC2 has a narrower pore-size distribution than single-wall carbon nanotubes or activated carbons; its pore-size distribution is comparable to that of zeolites. CDCs are produced at temperatures from 200-1,200 degrees C as a powder, a coating, a membrane or parts with near-final shapes, with or without mesopores. They can find applications in molecular sieves, gas storage, catalysts, adsorbents, battery electrodes, supercapacitors, water/air filters and medical devices.

Poly(ethylene glycol), which has a negligible carbon yield upon pyrolysis, was used as a template to study the controlled formation of mesoporosity in pyrolytic carbons. A series of carbons was produced from mixtures of poly(ethylene glycol) and poly(furfuryl alcohol) with 25, 50 and 75% composition by weight and an Mn of 300 to18 500 g/mol of template. Polydisperse dextran adsorption reveals a maximum in uptake for 8000 g/mol and 50% templated carbons, while materials from 75% mixtures or those from less than 2000 g/mol template yielded negligible dextran uptake. These results correlated well with the intensity ratio of a broad peak between 7 and 11° 2θ in the X-ray diffraction spectrum and the 002 diffraction peak and also qualitatively with micrographs of the internal microstructure of the carbons. The results suggest a templating process dominated by both the molecular size of the template and the rate of expulsion of decomposed template material during the formation of the solid.

Using a transient analysis of permeation, i.e. the time lag method, as a function of temperature and pressure, it is shown that each of the parameters needed to fully evaluate adsorption and diffusion can be obtained in situ. These experiments were conducted on a supported tubular nanoporous carbon membrane, 5.1 cm 2 in area and prepared by ultrasonic deposition of poly(furfuryl alcohol) onto a porous stainless steel support. The permeation experiments were conducted at temperatures ranging from 25 to 225°C and over a range of pressures from 100 to 700 kPa. Under these conditions the fluxes ranged from 10 −7 to 10 −4 mol/m 2 /s with permeances ranging between 10 −12 and 10 −9 mol/m 2 /s/Pa. Heats of adsorption were found to be 2.5, 2.21, 3.05 and 2.52 kcal/mol for N 2 , O 2, Ar and CO 2 , respectively, and generally lower than those reported for granular nanoporous carbons. The apparent activation barriers to diffusion were also found to be low at 2.06, 5.87, 4.12, 5.89 and 2.19 kcal/mol for He, N 2 , O 2 , Ar and CO 2 . These results point to the presence of two parallel pathways for transport — the major one through the nanopores but a second through a few defect pores. Assumed to be on the order of 50 nm in diameter, these defects were calculated to represent a total area fraction of 3.43×10 −9 .

A novel method for producing carbon membranes for ultrafiltration applications is presented using a spray deposition and pyrolysis of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures on macroporous stainless steel supports. The poly(ethylene glycol) or PEG employed as a carbonization template creates a mesoporosity that leads to pores in the ultrafiltration range. Scanning electron microscopy (SEM) shows that the membranes consisted of 12- to 15-m thick carbon films. Gas permeation and water permeability data were used for the calculation of mean pore sizes, which were found to decrease with decreasing average molecular weight of the PEG template. Ultrafiltration of a polydisperse dextran solution was used to quantify the retention properties of the membranes. Molecular weight cutoffs determined from dextran retention data were shown to vary with template molecular weight: values of 2 × 10 4 ,3 .5 × 10 4 , and 6 × 10 4 gm ol −1 dextran were measured for respective templates of 2000, 3400, and 8000 g mol −1 PEG. For PEG molecular weights of 2000 or below, the templating effect was ill defined, membrane film cracking became more prominent, and membrane selectivity and reproducibility were adversely affected. © 2002 Published by Elsevier Science B.V.

A theoretical model developed predicts the thickness of a polymer film, coated on a rotating and translating cylindrical support. A model for the deposition process can be used to determine optimal operating conditions. It was validated experimentally and shows that the film-thickness morphology is quite sensitive to certain operating parameters. For example, particular sets of operating parameter values can produce highly nonuniform film thicknesses. Selected examples of the model calculations are discussed, as well as their implications.

A 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 O2N2=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.

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 2 O/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.

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.

Nanoporous membranes--porous membranes generally having a porosity below 1 nm--have attracted the attention of many researchers because of their potential for technological advances in gas separations and shape selective catalysis. Permeation experiments often constitute a significant contribution to the characterization of these membranes. As the dimensions of a pore approach that of the molecule, transport generally becomes extremely sensitive to the molecular dimensions of the probe gas and very high separation factors have been reported for ceramic, zeolite, and nanoporous carbon membranes of this type. The purpose of this article is to introduce a technique that can be used to dissect the relative contributions of the overall permeances through molecular sieving membranes.

The local atomic structure of nanoporous carbons produced by pyrolysis of poly(furfuryl alcohol) at various temperatures has been studied using neutron diA raction. Atomic pair distribution functions (PDFs) were obtained from the neutron data. Structure models have been ® tted to the PDFs to understand the ® ne features of atomic ordering. It has been found that carbons produced at 800 and 12008C are made of stacked graphene sheets, that are more or less curved. The 4008C-processed carbon has a heavily distorted nonplanar structure. For each sample we have determined the extent of the structural coherence, the average density within the structurally coherent regions and the proportion of carbon atoms, which are trigonally coordinated. We have also produced microscopic models for the local structure of the nanoporous carbons investigated.

Nanoporous carbons (NPCs) are useful in adsorptive separations and catalysis, owing to their ability to discriminate between molecules on the basis of size and shape. This property arises from their narrow pore size distribution, which is typically centred at a size corresponding to 0.5 nm. Despite this level of nanoregularity. there is no long-range order within these materials. Structural coherence dissipates to extinction at distances longer than 1–1.2 nm. For this reason, these nanoporous materials are complex solids and offer an intriguing problem in structural simulation and modelling. We show that modelling the spatial complexity of NPCs can be overcome by their chemical simplicity. Recognizing that the structures are comprised of trigonal sp2 carbon and imposing chemical and physical constraints on the possible outcomes of the simulation provide a means to surmounting the modelling problem presented by the intrinsic disorder. By this approach, models of the solid can be arrived at that ma...

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.

The use of a membrane reactor offers the possibility of shifting the extent of reaction beyond the normal equilibrium position by continuously depleting one of the products. One such case is the de...

The adsorption isotherms of 16 halocarbons have been obtained in the mid to high range of pressures and at room temperature on a zeolite-like nanoporous carbon molecular sieve Carbosieve G (CSG). All the isotherms display Type I structure in the range investigated. The agreement between the pore volume of the adsorbent calculated based upon the liquid densities of the one-carbon, C 1 , halocarbons at room temperature and that determined from the nitrogen adsorption isotherm obtained at 77 K is good. The capacity of CSG for these C 1 molecules ranges between 4.7 and 7.5 mmol/g. For the larger two-carbon species, C 2 , the adsorption is lower, in the range from 3.1 to 4.7 mmol/g. A plot of these equilibrium capacities versus the molecule size shows how the extent of adsorption varies with molecule size. The pores with sizes in the near vicinity of 5 A are found to predominate, a result which agrees well with pore size determinations made earlier by other means.

Two carbon adsorbents, Ambersorb-600 and Ambersorb-563 (A-600 and A-563), were compared for vapor-phase trichloroethylene (TCE) adsorption from humid air streams. These adsorbents retained capacity for TCE in humid environments and were regenerable in situ. Enhanced desorption, and hence, increased working capacities, were achieved with bimodal pore size distributions and hydrophobic surface chemistry. Vapor-phase TCE isotherms confirmed that both of these adsorbents have high capacities for TCE. Only a small difference between the micropore size distributions of A-563 and A-600 was determined by room-temperature methyl chloride adsorption and the modified Horvath-Kawazoe model. Besides differences in particle size and pore volume there was a measurable, but small change, in the fraction of the pores in the ultramicropore range (5 {angstrom} or smaller) of the A-600 adsorbent versus that of A-563. In packed-bed breakthrough curve experiments, A-600 displayed a sharper mass-transfer zone than A-563, but maintained essentially the same capacity for TCE in a humid environment. Both materials were amenable to in-situ regeneration, and the A-600 provided higher overall working capacity than that of A-563.

Carbogenic molecular sieve membranes supported on macroporous sintered stainless steel flat plates have been synthesized by pyrolysis of a (poly)furfuryl alcohol−acetone solution imbibed within the...

Porosimetry is a fundamental characterization technique used in development of new porous materials for catalysis, membrane separation, and adsorptive gas storage. Conventional methods like nitrogen and argon adsorption at cryogenic temperatures suffer from slow adsorption dynamics especially for microporous materials. In addition, CO2, the other common probe, is only useful for micropore characterization unless being compressed to exceedingly high pressures to cover all required adsorption pressures. Here, we investigated the effect of adsorption temperature, pressure, and type of probe molecule on the adsorption dynamics. Methyl chloride (MeCl) was used as the probe molecule, and measurements were conducted near room temperature under nonisothermal condition and subatmospheric pressure. A pressure control algorithm was proposed to accelerate adsorption dynamics by manipulating the chemical potential of the gas. Collected adsorption data are transformed into pore size distribution profiles using the Horvath-Kavazoe (HK), Saito-Foley (SF), and modified Kelvin methods revised for MeCl. Our study shows that the proposed algorithm significantly speeds up the rate of data collection without compromising the accuracy of the measurements. On average, the adsorption rates on carbonaceous and aluminosilicate samples were accelerated by at least a factor of 4-5.

A comprehensive study has been undertaken to establish the primary factors that control transport of oxygen and nitrogen in polymer-derived carbogenic molecular sieves (CMS). Characterization of the nanostructure of CMS derived from poly(furfuryl alcohol) (PFA) indicates that significant physical and chemical reorganization occurs as a function of synthesis temperature. Spectroscopic measurements show a drastic decrease in oxygen and hydrogen functionality with increasing pyrolysis temperature. Structural reorganization and elimination of these heteroatoms lead to a measurable increase in the unpaired electron density in these materials. High-resolution transmission electron microscopy and powder neutron diffraction are used to probe the corresponding changes in the physical structural features in the CMS. These indicate that as the pyrolysis temperature is increased, the structure of the CMS transforms from one that is disordered and therefore highly symmetric to one that is more ordered on a length scal...

The conversion of ethylbenzene to styrene by oxidative dehydrogenation is compared over several carbogenic molecular sieves. At 300 °C, Carbosieve G deactivated rapidly due to its nanoporous struct...

Carbon monoxide hydrogenation studies on silica- and alumina-supported Rh catalysts conducted in our laboratories as well as those reported in the literature suggest that the addition of early transition-metal oxides to the catalyst surface exerts a promotional influence on active Rh centers. This promotion leads to enhanced oxygenate selectivity. Explanations of the promotional influence of the early transition-metal oxides fall into three categories: (i) kinetic stabilization of small Rh aggregates, (ii) wetting and spreading of the transition-metal oxide, leading to decoration of the surface of Rh crystallites, and (iii) oxidative stabilization of Rh aggregates, altering Rh interactions with sub-carbonyls during reaction. Characterization of the RhMo γ- Al 2 O 3 system provides supporting evidence for both kinetic stabilization and oxidative stabilization of Rh aggregates and suggests that these processes contribute to the enhanced oxygenate selectivity of these materials. Hydrogen and carbon monoxide chemisorption results provide evidence for highly dispersed and oxidatively stabilized Rh aggregates. Even after extended on-stream testing (> 10 h), where metal aggregation is unavoidable, high-resolution transmission electron microscopy, associated energy dispersive spectroscopy, and the use of fast-Fourier transform post-processing to produce optical diffractograms highlight differences in the metal-aggregate morphology.

This study highlights the nature of the interactions between the transition metals in the Rh-Mo/γ-Al2O3 system and the underlying alumina surface. The investigation was carried out in the context of exploring the retrograde dehydration to dimethyl ether of primary alcohols produced at active metal sites during CO hydrogenation. Both native γ-Al2O3 and Rh-Mo/γ-Al2O3 samples were doped with KNO3 in an attempt to create an alkaline-metal oxide layer on the support surface which would screen alcohols from underlying Lewis acidity. The samples were characterized by microreactor-based methanol dehydration studies, analysis of infrared spectra of adsorbed pyridine, and ammonia saturation and temperature-programmed desorption. As expected, the dehydration activity of the samples was strongly correlated to their potassium loading and, hence, their Lewis acidity. More intriguing, however, are the insights gained in this analysis concerning the nature of the interactions between Mo, Rh, and the Lewis acid sites on the alumina surface. Our results are consistent with the principle, suggested in the Mo/γ-Al2O3 literature, that our Mo(CO)6 precursor interacts primarily with nonacidic hydroxyl groups on the dehydrated alumina surface. Furthermore, we provide strong evidence for the diminution of acidity brought on by the adsorption of Rh on Lewis acid sites of Mo/γ-Al2O3.

Carbogenic molecular sieves (CMS) are microporous, high-carbon solids. Because of their microporosity, they share with zeolites the ability to separate molecules on the basis of size and shape. For example, nitrogen is produced from air by pressure swing adsorption over packed beds of CMS. Other shape-selective separations also can be accomplished over CMS. Despite these apparent similarities CMS are quite different from zeolites. Instead of being crystalline, they are globally amorphous having no long-range order. Despite their disorder the microporosity can be controlled through choice of precursor, additives, and thermal history. Highly variable surface chemistries—acidic, basic, neutral, and free radical—also set CMS apart from zeolites. Here the properties of CMS are reviewed in detail. Synthesis properties and applications, both separative and catalytic, are covered. Finally, microstructural features of CMS are put in context with recognized features of other carbon materials which feature minimal surfaces with high local curvature.

Catalytic membrane reactors are of particular interest for reactions that are severely thermodynamically limited. A case study is developed comparing and contrasting cyclohexane dehydrogenation with isobutane dehydrogenation in a palladium membrane reactor. The former is an often chosen model dehydrogenation, while the latter is of high, current industrial interest. Based on the literature and modeling it is concluded that cyclohexane dehydrogenation accentuates the effect of the membrane on supra-equilibrium conversion. More favorable thermodynamics, stoichiometry and kinetics for cyclohexane versus isobutane dehydrogenation each contribute. To achieve similar levels of membrane-assisted conversion, the reactor volume required for isobutane dehydrogenation would need to be at least 750 times larger than that required for cyclohexane based upon published kinetics for commercial catalysts. Rate limitations at the catalyst rather than for hydrogen transport through the membrane tend to dominate. The hydrogen deficiency in the reaction zone resulting from the membrane transport also leads to enhanced rates of coke formation resulting in catalyst and membrane deactivation. This study points out that more effort needs to be expended on the development of new dehydrogenation catalysts with high activity per unit volume and high resistance to deactivation.

Experimental evidence is provided for adsorptive shape selectivity in the separation of the azeotrope between HF and 1,1,1,2-tetrafluoroethane (134a) over pyrolyzed poly(furfuryl alcohol)-derived carbogenic molecular sieve (PPFA-CMS). The separation can be accomplished over coconut charcoal or Carbosieve G on the basis of the differences in the extent of equilibrium adsorption of HF and 134a. On these adsorbents 134a is more strongly bound than HF, thus it elutes much more slowly from the bed. The heat of adsorption for 134a in the vicinity of 200 C on Carbosieve G is {approximately}8.8 kcal/mol. In contrast, when the same azeotropic mixture is separated over PPFA-CMS prepared at 500 C, 134a is not adsorbed. As a result 134a elutes from the bed first, followed by HF. The reversal is brought about by the narrower pore size and pore size distribution of the PPFA-CMS versus that for Carbosieve G. Thus the separation over PPFA-CMS is an example of adsorbate shape selectivity and represents a limiting case of kinetic separation.