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.     

Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content

The present paper examines the adsorption of water by microporous carbons containing various amounts of surface oxygen and a smaller proportion of basic centres. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), confirms that the overall type IV isotherm is the sum of a type I isotherm associated with the specific interactions, and a type V isotherm reflecting the non-specific interactions. The principle of temperature invariance is followed by these isotherms, which indicates that modelling leads to the Dubinin-Astakhov equation.The present approach allows the prediction of water adsorption near room temperature, on the basis of the PSD and the density of oxygen present on the surface area of the micropores. It is assumed, to a first and good approximation, that the pores are slit-shaped and the oxygen distribution is random.     

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.     

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.     

Water adsorption by activated carbons in relation to their microporous structure

The present paper examines the adsorption of water by microporous carbons in the absence of specific interactions. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), shows that the type V isotherms follow the Dubinin-Astakhov (DA) equation and fulfill the requirement for temperature invariance. Furthermore, the parameters of the DA equation can be related in a simple way to structural properties of the model carbons. For a number of well-characterized carbons, the type V isotherms generated by combining model isotherms with the corresponding PSDs are in good agreement with the limiting isotherms at 293 and 310 K derived on the basis of a recent development of Dubinin’s theory. This approach will provide the basis for further studies including specific interactions.     

Comparison of molecular sieve properties in microporous chars from low-rank bituminous coal activated by steam and carbon dioxide

A Polish high volatile bituminous coal was subjected to air oxidation, carbonization and gaseous activation. The activation with steam and carbon dioxide was performed to low levels of burn-off: 5-25%. Sorption measurements of CO₂, as well as of organic vapours with increasing molecular sizes (CH₂Cl₂, C₆H₆, C₆H₁₂, CCl₄) were applied to evaluate the porous structure of the activated chars. Steam and carbon dioxide develop the microporous system according to the same mechanism—opening (burn-off 5-10%) and then widening of the narrow micropores. For char from the oxidized coal mainly a widening of the narrow micropores takes place. Comparing both activating agents, it was stated that for steam greater micropore volumes were obtained. This was confirmed by other authors for chars from brown coal and coking coal, but was in disagreement with the results for olive stones and carbon fibres. This would indicate the importance of the carbon precursor in the formation of the porous structure of carbon materials by different activating agents. In the region of studied burn-offs, among the micropore sizes useful for separation of gases and vapours with small molecules, micropore volumes with widths close to 0.4-0.5 nm are dominating. At very low burn-offs (5-10%), steam activation renders greater micropore volumes within these sizes, than does activation with carbon dioxide. But with increasing burn-off (15-25%), this phenomenon becomes reversed. This effect is still more accentuated for the preoxidized coal.     

Preparation and characterization of porous carbons from PAN-based preoxidized cloth by KOH activation

Porous carbons (PCs) were prepared from PAN-based preoxidized cloth with potassium hydroxide (KOH) as active reagent by the chemical activation method. The PCs have been systematically studied by the adsorptions of nitrogen, benzene and iodine. It has been found that the process parameters such as weight ratio of KOH to the starting material, activation temperature and activation time are crucial for preparing high quality PCs. A series of PCs with high BET surface area and well-developed porous structure in which micropores are dominant were obtained with less KOH and shorter activation time in comparison to the traditional methods. The optimum conditions for preparing PCs with high BET surface area from PAN-based preoxidized cloth were given, and the relationships between pore structure and adsorption property of PCs were explored.     

Preparation and pore control of highly mesoporous carbon from defluorinated PTFE

Porous carbon having more than 2000 m²/g of BET specific surface area was synthesized by defluorination of polytetrafluoroethylene (PTFE) at 473 K using sodium metal. The porous carbon as-prepared had a large amount of narrow mesopores 2-3 nm in pore width, together with micropores. Control of the pore structure was attempted by simple heat-treatment of the carbon in nitrogen, and change of the porous structures was characterized by nitrogen adsorption techniques. As a result, it was found that the ratio between micro- and mesopores was easily varied. Electric double layer capacitance was measured as one of the applications for the mesoporous carbon with specific porosity, and the effect of pore control on capacitance was investigated.     

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.     

Preparing activated carbon from various nutshells by chemical activation with K2CO3

We have prepared activated carbons by chemical activation with K₂CO₃ from five kinds of nutshells: almond shell (AM), coconut shell (CN), oil palm shell (OP), pistachio shell (PT) and walnut shell (WN). When prepared at 1073 K, the activated carbons from all the nutshells had the maximum specific surface areas. According to the maximum values of specific surface areas, the activated carbons prepared were classified into two groups: Group-L and Group-S; the former group included activated carbons with high specific surface area and the latter included those with lower specific surface area, respectively. It was found that K₂CO₃ effectively worked as an activation reagent, but differently in the temperature ranges below 800 and above 900 K. Due to impregnation, cellulose and hemi-cellulose were modified by K₂CO₃ and accordingly the weight loss behaviors of the nutshells were changed in the temperature range below 800 K. In the temperature range above 900 K, carbon in the chars was removed as CO gas by the reduction of K₂CO₃ to increase the specific surface area and the pore volume. It was deduced that the difference between the specific surface areas of Group-L and those of Group-S correspond to the difference between weight loss behaviors in the temperature range above 900 K.     

Preparation of colloidal microporous carbon spheres from furfuryl alcohol

Colloidal microporous carbon (MC) spheres with an average size of 260 nm-1.5 μm were prepared by carbonization of “nonstick” poly(furfuryl alcohol) (PFA) spheres, which were synthesized by a two-step polymerization of furfuryl alcohol (FA) involving slow polymerization (1st step) and sphere formation (2nd step). The high dispersibility of the MC spheres was simultaneously realized by removing surface functional groups of the PFA spheres with the evaporation-induced concentrated sulfuric acid (2nd step). By varying the temperature of slow polymerization, the spheres with different sizes could be simply synthesized. The studies also showed that surfactant and sulfuric acid treatment played an important role in the synthesis of colloidal PFA (MC) spheres.     

Controlling carbon microporosity: the structure of carbons obtained from different phenolic resin precursors

Carbons were prepared from resins synthesised using the phenolic precursors phenol, para methylphenol, para ethylphenol, para n-propylphenol, para isopropylphenol and 3,5-dimethylphenol. Loss of phenolic OH from these materials was followed using solid-state nuclear magnetic resonance. The surface areas of the carbons were determined using N₂ and CO₂ adsorption. No significant differences in the loss of phenolic OH were observed. Under the same carbonisation conditions, the para alkyl phenols gave carbons with wide micropores, while the phenol and 3,5-dimethylphenol gave carbons with narrow micropores. Grinding the cured resins prior to carbonisation was found to significantly increase the surface area of the carbons obtained, with the microporous surface area increasing rapidly with a fall in particle size, without a significant increase in burn-off. Higher carbonisation temperatures widened the micropore size distribution, as shown by fitting the CO₂ adsorption isotherm with the Dubinin-Astakhov equation. The ability to change the carbon micropore structure obtained from a simple, well defined precursor, has many potential applications in carbon molecular sieves, catalyst supports and the investigation of adsorption processes.     

Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution

The objective of this research was to develop activated carbon selection criteria that assure the effective removal of trace organic contaminants from aqueous solution and to base the selection criteria on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, a matrix of activated carbon fibers (ACFs) with three activation levels and four surface chemistry levels was prepared and characterized. In addition, three granular activated carbons (GACs) were studied. Two common drinking water contaminants, relatively polar methyl tertiary-butyl ether (MTBE) and relatively nonpolar trichloroethene (TCE), served as adsorbate probes. TCE adsorbed primarily in micropores in the 7-10 Å width range while MTBE adsorbed primarily in micropores in the 8-11 Å width range. These results suggest that effective adsorbents should exhibit a large volume of micropores with widths that are about 1.3 to 1.8 times larger than the kinetic diameter of the target adsorbate. Hydrophobic adsorbents more effectively removed both TCE and MTBE from aqueous solution than hydrophilic adsorbents, a result that was explained by enhanced water adsorption on hydrophilic surfaces. To assure sufficient adsorbent hydrophobicity, the oxygen and nitrogen contents of an activated carbon should therefore sum to no more than about 2 to 3 mmol/g.     

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.