Oxidation with air of cyclohexanone to carboxylic diacids on carbon catalysts

Carbons derived from phenolic resins were used as catalysts for the aqueous phase oxidation of cyclohexanone at 140°C under air pressure. The reaction yielded mainly adipic, glutaric and succinic acids. The samples were modified by heat treatments in CO₂, air or N₂ at different temperatures and characterized to determine their surface area and porosity and to evaluate the functional groups on the surface. Treatments under CO₂ or air increased the oxidation activity. The selectivity to adipic acid was maximum (33%) after activation in air which greatly increased the surface concentration of oxygen-containing functional groups. However, it was not possible to attribute the selectivity to specific acidic, neutral or basic groups present at the surface of carbons. This revised version was published online in June 2006 with corrections to the Cover Date.     

The cytotoxicity of highly porous medical carbon adsorbents

This study investigated the cytotoxic potential of novel activated carbon adsorbents (MAST Carbon International Ltd.) developed for medical applications such as extracorporeal therapies. Carbon adsorbents were assessed for their in vitro cytotoxicity against a V79 cell line using a material extraction method in combination with a colony formation assay. Results were compared to those from a commercially available cellulose-coated carbon adsorbent, developed for direct haemoperfusion. Initial findings demonstrated an inhibition of colony formation and an apparent cytotoxic effect. However, it was found that this inhibition occurred as a result of protein and ion adsorption by carbon materials possessing large surface area and highly developed porous structure. Consequently, these essential nutrients were unavailable to the cells during colony formation. Modifications to the cytotoxicity assessment methods were required in order to take into account nutrient loss. Subsequently it was determined that the carbon materials do not show a cytotoxic response towards the V79 cell line under the modified conditions employed. The suggested approach may be useful in the assessment of other biomaterials such as carbon nanotubes and other nanoparticles which possess large surface area. The preliminary data support the ongoing investigation of these adsorbents as candidates for use in extracorporeal therapies.     

Comparative study of nanopores in activated carbons by HRTEM and adsorption methods

Detailed analysis of nanopores (IUPAC micropores at pore half-width x < 1 nm) of carbonised porous phenolformaldehyde resin microbeads used as a precursor of activated carbon (AC) and CO₂ activated carbon (at 50% burn-off) has been performed on the basis of high-resolution transmission electron microscopy (HRTEM) image analysis and nitrogen adsorption data analysed using several density functional theory (DFT) methods. The results of quenched solid DFT (QSDFT) and nonlocal (NLDFT) are in agreement with the pore size distributions of nanopores based on the HRTEM image analysis. Development of porosity with progressive activation degree in a series of ACs leads to enhancement of the deviation of the pore shape from the used pore models. The TG/DTA data and Raman spectra show nonlinear but weak changes in the AC characteristics with increasing burn-off degree.     

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.     

Effect of mesoporosity on specific capacitance of carbons

The study compares the structural and electrochemical properties of 12 porous carbons based on phenolic resins, using both aqueous (H₂SO₄) and aprotic ((C₂H₅)₄NBF₄ in acetonitrile) electrolytes. It appears that they fit into the general pattern observed for other carbons. The present carbons have micropore volumes varying between 0.29 and 0.66 cm³ g⁻¹ and average pore widths L_o between 0.62 and 1.23 nm. Five samples are exclusively microporous, whereas seven also display a relatively important mesoporosity. This allows a direct comparison between pairs of carbons with similar micropore systems, with and without mesopores, in order to assess the role of mesoporosity in the electrochemical properties. It appears that mesopores have only a limited influence on the decrease in capacitance at high current density as opposed to earlier assumptions.