I.F. Silva

An experimental and theoretical study of the adsorption of aromatics possessing electron-withdrawing and electron-donating functional groups by chemically modified activated carbons☆

Abstract The adsorption of model aromatic compounds (aniline and nitrobenzene) on chemically tailored activated carbons has been systematically investigated. Adsorption experiments at controlled solution pH conditions confirmed that both electrostatic and dispersive adsorbate/adsorbent interactions can have a significant influence on the equilibrium uptakes of ionic and nonionic adsorbate species. For aniline (a weak electrolyte), maximum uptakes were found on oxidized carbon surfaces at solution pH near the adsorbates point of zero charge (pH PZC ). In contrast, nondissociating nitrobenzene uptakes were enhanced on heat-treated surfaces with graphene layers unperturbed by electron-withdrawing functional groups, particularly at solution pH ~ pH PZC . A theoretical model that can successfully account for the observed trends is hereby proposed as a much needed predictor of the experimental conditions and adsorbent surface chemical properties that will maximize the uptake of aromatic compounds by activated carbons.

Microcalorimetric study of acid sites on ammonia- and acid-pretreated activated carbon

Abstract This study was designed to determine the value of microcalorimetry as a probe of acid/base sites on carbon surfaces. Calorimetric studies of ammonia adsorption on acid and ammonia pretreated activated carbon (BDH) samples demonstrated that the technique does titrate acid sites (at equilibrium) according to their relative strengths. However, only in conjunction with other techniques, including Boehm titration, point of zero charge (PZC) and temperature programmed desorption (TPD) is it possible to determine the probable identity of the acid sites present on a given carbon. Collective consideration of the data from all techniques suggests that ammonia pretreatments create a surface whose chemistry is completely different from that of the original carbon. Not only are new acidic sites created, but a high concentration of basic sites are introduced as well. Thus, the final surface is clearly amphoteric, to an extent which is largely dictated by the pretreatment temperature.

Hydration of α-pinene over zeolites and activated carbons dispersed in polymeric membranes

Abstract The acid catalysed hydration of α-pinene yields a complex mixture of monoterpenes, alcohols and hydrocarbons. By controlling the reaction variables is possible to make it selective to the terpenic alcohols, namely α-terpineol. In this paper the results of the hydration reaction of α-pinene catalysed by polydimethylsiloxane (PDMS) membranes filled with zeolite USY, zeolite beta or a surface modified activated carbon, are reported. The reaction is carried out at 50°C, using aqueous acetone as solvent. The activity and selectivity towards α-terpineol of the composite membranes are compared with those of the free catalysts.

The effect of α-terpineol on the hydration of α-pinene over zeolites dispersed in polymeric membranes

Abstract The hydration of α-pinene over catalytic PDMS membranes loaded with a USY zeolite is studied. The concentration profiles of reagent and products exhibited a pronounced initial induction period followed by a rapid increase of the reaction rate, suggesting an autocatalytic behaviour. The effects of the main reaction product, α-terpineol, on the membrane transport properties are investigated. A simple diffusion-kinetic model which fits experimental concentration data quite well has been developed.

Vanadium as a catalyst for NO, N2O and CO2 reaction with activated carbon

The kinetics of the reaction of NO, N 2 O and CO 2 with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N 2 O and CO 2 was studied (450-900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N 2 O conversion the main products obtained were N 2 , N 2 O, CO and CO 2 but for CO 2 conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V 2 O 5 /V 6 O 13 .

Oxidation of pinane using transition metal acetylacetonate complexes immobilised on modified activated carbon

The performance of a new solid catalyst is studied. Copper and cobalt acetylacetonate complexes are chemically anchored to functionalised activated carbon. These catalysts are active and highly selective to 2-pinane hydroperoxide in the oxidation of pinane at room temperature and atmospheric pressure.

OXYDEHYDROGENATION OF CYCLOHEXANOL OVER CARBON CATALYSTS

Abstract The reaction of cyclohexanol over carbon catalysts with surfaces of different chemical nature was studied. The catalyst samples were prepared by treating a parent activated carbon with HNO3, H2O2, O2 and N2O. The catalytic tests were performed in a packed-bed reactor at 350 °C. Dehydration as well as dehydrogenation of the substrate occur under the reaction conditions. The main products obtained are cyclohexene, cyclohexanone, benzene and phenol. Dehydration occurs preferentially over the carbon treated with HNO3, while the best selectivity to cyclohexanone is achieved with the carbon treated with H2O2. The catalyst sample treated with N2O is highly selective to phenol. A mechanism involving nitro groups on the carbons surface is proposed.

Uncatalysed and MoO3-catalysed carbon-oxygen reaction: A kinetic study

Abstract A kinetic study was made of the reaction of activated carbon with oxygen, uncatalysed and catalysed by MoO 3 , using a microbalance to record the loss of weight as a function of time. Orders and activation energies were measured. MoO 3 was found to be a moderate catalyst for this reaction. The effect of loading on reactivity is linear for low loadings and shows saturation above 2.5 wt%. It is proposed that this type of reaction, involving three phases and two interfaces, can be explained by surface catalysis at the gas/catalyst interface with continuous diffusion of carbon atoms through the catalyst.

Uncatalysed and catalysed CO2 reaction using metal catalysts and binary vanadium mixtures supported on activated carbon

Abstract The kinetics of the reaction of CO 2 with activated carbon without catalyst and impregnated with precursor salts of Mg, Ba, Pb, Cu and Fe and their binary mixtures were investigated. Binary mixtures of those metals with vanadium oxide were also studied. The conversion of CO 2 was studied (300°C to 900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC–MS on line. CO was the main product together with some unreactant CO 2. The best synergetic effects were observed for samples doped with Ba+V, Mg+V, Fe+V, Cu+V and Ba+Fe oxides. Adsorption of CO 2 (20°C to 100°C) increased considerably in the presence of catalysts but binary oxide mixtures had no synergistic effects. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The active phases appear to be metal carbonates and/or metal oxide phases.

Kinetics, in situ X-ray diffraction and environmental scanning electron microscopy of activated charcoal gasification catalyzed by vanadium oxide, molybdenum oxide and their eutectic alloy

Mechanisms of carbon oxidation catalyzed by vanadium pentaoxide (V2O5) and molybdenum trioxide (MoO3) have been a subject of controversy. Two complementary in situ techniques, X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM), were used in this work to study the gasification of an activated charcoal catalyzed by the two metal oxides, their eutectic alloy and the binary mixture with the eutectic composition. Gasification experiments were carried out at relatively low temperatures (300–650 °C) in an XRD cell (1 atm) and ESEM (2.2 Torr) to monitor phase transformations and morphological changes of oxide catalysts, respectively. The experimental results showed that MoO3 and V2O5 particles in contact with active carbon surfaces are reduced to oxides with lower oxidation states, e.g. MoO2 and V6O13, respectively. The reduction of MoO3 to MoO2 on the carbon surface prevents the sublimation of MoO3 which takes place readily on a quartz surface under the same conditions. The formation of V6O13, on the other hand, causes more extensive spreading of the catalyst on carbon surfaces, since V6O13 has a lower melting point than V2O5. Based on the XRD and ESEM observations, it is clear that phase transformations of metal oxides during gasification depend on their interactions with carbon surfaces. The phase transformations of the metal oxides, play, in turn, a significant role in carbon gasification, as evident from the kinetic data obtained for the catalytic gasification of the activated charcoal sample. The synergy observed between the components of the eutectic mixture is discussed, comparing the XRD and ESEM observations.