Direct fabrication of mesoporous carbons using in-situ polymerized silica gel networks as a template

Mesoporous carbons were synthesized by in-situ polymerized silica gel networks as a template. The co-condensation of carbon precursor (sucrose) and silica precursor (sodium silicate) followed by heat treatment generated a carbon/silica nanocomposite. After etching the silica template, mesoporous carbons were obtained. Under optimum synthesis conditions a mesoporous carbon with a high surface area of >800 m²/g and a narrow pore size distribution centered at 3 nm was produced. The three-dimensionally interconnected silica structures effectively functioned as the template for the porous carbon materials.     

The effect of silica template structure on the pore structure of mesoporous carbons

We have synthesized two kinds of mesoporous carbons using a spherical silica sol (SMC1 carbon) and an elongated silica sol (SMC3 carbon) as templates. Nitrogen isotherms and electrochemical experiments were performed to investigate the effect of the silica template structure on the pore structure of the resulting mesoporous carbons. When carbons produced using the same silica to resorcinol molar ratio were compared, both nitrogen isotherms and electrochemical studies revealed that the SMC3 carbons exhibit simpler pore connectivity than SMC1 carbons.     

Carbon coating of anatase-type TiO2 through their precipitation in PVA aqueous solution

Fine particles of photocatalytic anatase-type TiO₂ prepared through hydrolysis of titanium-tetraisopropoxide were coated by carbon through their precipitation in poly(vinyl alcohol) (PVA) aqueous solution, followed by heat treatment at high temperatures of 400-1000 °C in a flow of high purity Ar. Without carbon coating, the phase transformation from anatase to rutile started above 600 °C, but it was suppressed up to 800 °C with carbon coating. Suppression of the phase transformation depended on the amount of carbon coated, apparent suppression being observed with carbon content above 5 mass%. The amount of carbon coated on anatase was controlled by changing the concentration of PVA in the solution. In order to have a carbon content of about 5 mass%, a PVA solution with more than 2 mass% had to be used.     

Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y

Emerging applications such as gas storage require porous carbon materials with tailored structural and surface properties. Template synthesis approach to porous carbons offers opportunities for tailoring these properties. In this study, ammonium-form zeolite Y (NH₄Y) was used as a template and furfuryl alcohol (FA) was employed as a carbon precursor to prepare microporous carbons by simple impregnation method. The effects of synthesis conditions such as carbonization temperatures and heating rates on the pore structure of the microporous carbons were investigated. The thermal behaviors of FA-NH₄Y mixtures and zeolite/carbon composites were studied by thermogravimetric analysis (TGA). The physical, structural, and surface properties of the microporous carbons were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), elemental analysis, and physical adsorption of nitrogen. Microporous carbons with high surface areas, pore volumes and nitrogen-containing surface functional groups can be readily synthesized.     

Effect of oxygen functional groups on synthetic carbons on liquid phase oxidation of cyclohexanone

Synthetic carbons from phenolic resins were used as catalysts for the aqueous phase oxidation of cyclohexanone to C₄-C₆ dicarboxylic acids (adipic, glutaric and succinic acids) at 413 K under 50 bar total air pressure. The changes in microporous structure and surface chemistry, produced as a consequence of activation or heat treatment processes, were analyzed. Using CO₂ or air as activating agent increased significantly the surface area and the total pore volume responsible for the activity. The surface chemistry of the samples was also modified and was characterized by titration with bases of different strength and with HCl, by temperature programmed desorption, and by X-ray photoelectron spectroscopy. To determine the role of surface oxygen functionalities on the catalytic behavior of the carbons, heat treatments in nitrogen at different temperatures were used to selectively eliminate oxygenated groups. Thus, treatment at temperatures of 1173 K eliminating the carbonyl/quinone groups decreased the selectivity to adipic acid and dicarboxylic acids. Introducing quinone groups during the synthesis of the carbons also improved the selectivity to adipic acid, proving that the mechanism of oxidation involves the quinone type groups on the carbon surface.     

Microporous carbons prepared from cationic surfactant-resorcinol/formaldehyde composites

Microporous carbons have been synthesized by the carbonization of cationic surfactant-resorcinol/formaldehyde (RF) composites, which were themselves formed by electrostatic organic-organic interaction. The porous structure produced by the decomposition of the surfactant plays an important role for the gasification of the RF polymer at higher temperatures. The pore size of the carbon prepared from tetrapropylammonium bromide (TPAB)-RF, cetyltrimethylammonium bromide (C₁₆TAB)-RF and decyltrimethylammonium bromide (C₁₀TAB)-RF mixtures can be estimated as 0.53 nm from the Horvath-Kawazoe method using N₂ adsorption isotherms. Their pore size distributions were very narrow, showing that the microporous carbons derived from surfactant-RF mixture may have promise as adsorbents and membrane materials.     

Pore structure of activated carbons prepared by carbon dioxide and steam activation at different temperatures from extracted rockrose

The influence of the activation temperature on the pore structure of granular activated carbons prepared from rockrose (Cistus ladaniferus L.), extracted previously into petroleum ether, is comparatively studied. The preparation was carried out by pyrolysis of a char in nitrogen and its subsequent activation by carbon dioxide and steam (flow of water controlled to generate the same mol number per minute of water as well as carbon dioxide/nitrogen) at 700-950°C to 40% burn-off. The techniques applied to study the pore structure were: pycnometry (mercury, helium), adsorption (carbon dioxide, 298 K; nitrogen, 77 K), mercury porosimetry and scanning electron microscopy. The preparation by steam activation, especially at 700°C, yields activated carbons showing a total pore volume larger than those prepared by carbon dioxide activation. The pore structures present the greatest differences when the activations are carried out between 700 and 850°C and closer at higher temperatures. At high temperatures, the decrease of differences in pore development caused by carbon dioxide or steam is attributed to an external burn-off. The micropore structure of each activated carbon is mainly formed by wide micropores. At the lowest activation temperatures, especially at 700°C, steam develops the mesoporosity much more than carbon dioxide. At 950°C, a similar reduction of pore volume in the macropore range occurs.     

Carbon foams prepared from polyimide using urethane foam template

Polyimide and carbon foams were successfully prepared using polyurethane foams as a template. Impregnation of polyimide precursor, poly(amide acid), followed by imidization at 200 °C gave polyurethane/polyimide (PU/PI) composite foams, which resulted in PI foams by heating above 400 °C and then carbon foams above 800 °C. Foams carbonized at 1000 °C were graphitized by the heat treatment at 3000 °C, keeping foam characteristics. Two applications of these carbon foams, i.e., an adsorbent of ambient water vapor and a substrate of photocatalyst anatase TiO₂, were experimentally confirmed. For the former application, the present foam could be characterized by prompt adsorption of ambient water vapor. Some of carbon foams prepared were floating on water, even after loading photocatalyst anatase, which might be advantageous for photodecomposition of pollutants in water in respect to the UV rays efficiency.     

Carbon-coated anatase: the role of the carbon layer for photocatalytic performance

Carbon-coated anatase-type TiO₂ was prepared by the heat treatment of powder mixtures of photocatalyst TiO₂ (ST-01) with different carbon precursors, poly(vinyl alcohol), hydroxyl propyl cellulose and poly(ethylene terephthalate), at a temperature between 700 and 900 °C for 1 h in an Ar gas flow. Since the carbon layers formed on the TiO₂ particles were porous, the samples prepared showed a high adsorptivity, in addition to the photoactivity of TiO₂. The carbon coating was shown to suppress the phase transition from photoactive anatase to much less active rutile, but seemed to reduce the amount of UV radiation reaching the surface of the TiO₂ particles. A balance among different factors controlled by the carbon layer on the TiO₂ particles was required to get high photocatalytic performance, i.e., high rate constant for the photodecomposition of methylene blue. On the sample prepared at 850 °C with a carbon content of about 3.5 wt%, the highest rate constant in the present work was obtained, in which the transition from anatase to rutile was suppressed and carbon layer was thin enough to transmit UV rays.     

Synthesis methods for preparing microporous carbons with a structural regularity of zeolite Y

As reported in previous communications, novel porous carbons were synthesized by using zeolite Y as a template. The carbons possess a periodic ordering structure and high BET surface area with large micropore volume. In this work, the details of the synthesis methods for preparing the ordered microporous carbons were examined. It was found that the following two-step process, the filling of carbon into zeolite channels by impregnation of furfuryl alcohol and then chemical vapor deposition (CVD) of propylene, was indispensable for preparing carbon with highly periodic ordering. In addition, low-temperature CVD and the further heat treatment of zeolite/carbon composite after the CVD are key points for the appearance of both good long-range periodicity and very high BET surface area with almost no mesoporosity in the carbons.     

Liquid-phase adsorption of dyes and phenols using pinewood-based activated carbons

Physical properties of activated carbons prepared from pinewood at different activation times (0.5, 1.5, 2.7, and 4.0 h) in steam at 900 °C were studied. The adsorption equilibria and kinetics of three dyes and three phenols (phenol, 3-chlorophenol, and o-cresol) from aqueous solutions on such carbons were then examined at 30 °C. The adsorption isotherms of phenols could be well fitted by the Freundlich equation, and those of dyes were adequately described by the Langmuir-Freundlich equation. The effect of microporosity of the carbons on adsorption capacity was explored. Four simplified kinetic models including pseudo-first-order equation, pseudo-second-order equation, intraparticle diffusion model, and the Elovich equation were selected to follow the adsorption processes. The adsorption of all six adsorbates could be best described by the Elovich equation. The kinetic parameters of this best-fit model were calculated and discussed.     

Preparation of fibrous porous materials by chemical activation 2. H3PO4 activation of polymer coated fibers

Fibrous porous materials (FPMs) have been prepared by coating a glass fiber with an aqueous solution of poly(vinyl alcohol) (PVA) and H₃PO₄, followed by stabilization and heat treatment in air. The H₃PO₄ was then removed by washing with deionised water and NaOH. The results show that H₃PO₄ acts as a dehydration agent to promote pyrolytic and thermal crosslinking of PVA at a much lower temperature of 170 °C, leading to FPMs having much higher char yields and surface areas. The activation in air is of benefit to achieve higher surface areas as compared to using N₂. Utilizing a fiberglass mat to support coatings of PVA activated with H₃PO₄ results in much higher specific surface areas. The activation temperature, activation time and concentration of H₃PO₄ have strong effects on the surface area, pore size distribution and coating content of FPMs.     

Template synthesis of novel porous carbons using various types of zeolites

As previously reported, high surface area microporous carbons with long-range order can be synthesized by using zeolite Y as a template. In this work, an attempt is made to synthesize porous carbon using several other types of zeolites (zeolite β, ZSM-5, mordenite and zeolite L). Special attention is paid to whether the structural regularity of each zeolite can be transferred to the carbon structure as well as with the case of zeolite Y. The carbon filling method was then examined to see which gives the highest regularity to the carbon structure. It has been found that the optimum carbon filling method for zeolite Y is not an optimum one for the other zeolites and the degree of the regularity of long-range ordering in the carbons strongly depends on zeolite type. The order of the regularity in the resultant carbons is zeolite β>>zeolite L>mordenite>ZSM-5. The effect of zeolite type on the regularity is discussed in relation to the size and the shape of the zeolite channels.