J.J.M. Órfão

Modification of the surface chemistry of activated carbons

Abstract A NORIT activated carbon was modified by different chemical and thermal treatments (including oxidation in the gas and liquid phases) in order to obtain materials with different surface properties. Several techniques were used to characterize these materials including nitrogen adsorption, chemical and thermal analyses, XPS, TPD and DRIFTS. The results obtained by TPD agree quantitatively with the elemental and proximate analyses of the oxidized materials, and qualitatively with the observations by DRIFTS. A simple deconvolution method is proposed to analyse the TPD spectra, allowing for the quantitative determination of the amount of each functional group on the surface. A multiple gaussian function has been shown to fit the data adequately, the parameters obtained for each fit matching very well the features observed in the experimentally determined TPD spectra. It is shown that gas phase oxidation of the carbon increases mainly the concentration of hydroxyl and carbonyl surface groups, while oxidations in the liquid phase increase especially the concentration of carboxylic acids.

Pyrolysis kinetics of lignocellulosic materials—three independent reactions model

Abstract The behaviour of biomass components (cellulose, xylan—representative of hemicelluloses—and lignin) was studied thermogravimetrically with linear temperature programming, under nitrogen and air. The results were compared and the pyrolysis kinetics of cellulose determined, assuming a first-order kinetic function. The thermal decomposition of xylan and lignin could not be modelled with acceptable errors by means of simple reactions. Thermograms were determined for pine and eucalyptus woods and pine bark, under inert (nitrogen) or oxidizing (air) atmosphere. The pyrolysis of these lignocellulosic materials was modelled with good approximation by three first-order independent reactions. One of these reactions is associated with the primary pyrolysis of cellulose, its parameters being previously determined and fixed in the model. The model parameters are the activation energies and pre-exponential factors for the pyrolysis of the remaining two pseudo-components and two additional parameters related to the biomass composition. A method to determine this composition was proposed. The results calculated in this way were compared with data from the literature and those determined thermogravimetrically in air, showing good agreement.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups

Activated carbons were used as catalysts for the oxidative dehydrogenation of ethylbenzene. The type and amount of functional groups on the surface of the carbon catalysts was modified by oxidative treatments in the gas or in the liquid phase, while heat treatments at different temperatures were used to selectively remove some of the functional groups. The performance of these catalysts was evaluated in terms of conversion, styrene yield and selectivity. The results show that the gas phase treatments lead to improved performance associated with an increase in the amount of carbonyl/quinone groups on the surface, which were identified as the active sites for the reaction. A good correlation between catalytic activity and the concentration of these surface groups was obtained.

Decolourisation of dye solutions by oxidation with H2O2 in the presence of modified activated carbons

The decolourisation of dye solutions by oxidation with H(2)O(2), using activated carbon as catalyst, is studied. For this purpose, three different samples, mainly differing in the respective surface chemistries, were prepared and characterized. Moreover, this work involved three pH levels, corresponding to acid, neutral and alkaline solutions, and six dyes belonging to several classes. The catalytic decolourisation tests were performed in a laboratorial batch reactor. Adsorption on activated carbon and non-catalytic peroxidation kinetic experiments were also carried out in the same reactor, in order to compare the efficiencies of the three processes. The non-catalytic reaction is usually inefficient and, typically, adsorption presents a low level of decolourisation. In these cases, the combination of activated carbon with hydrogen peroxide may significantly enhance the process, since the activated carbon catalyses the decomposition of H(2)O(2) into hydroxyl radicals, which are very reactive. Based on the experiments with the different activated carbon samples, which have similar physical properties, it is proved that the surface chemistry of the catalyst plays a key role, being the basic sample the most active. This is discussed considering the involvement of the free electrons on the graphene basal planes of activated carbon as active centres for the catalytic reaction. Additionally, it is shown that the decolourisation is enhanced at high pH values, and a possible explanation for this observation, based on the proposed mechanism, is given.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts: 3. Catalyst deactivation

Abstract Extended catalytic tests show that coke deposition is responsible for the deactivation of activated carbon catalysts in the oxidative dehydrogenation of ethylbenzene (ODE). Temperature-programmed desorption (TPD), DRIFTS, textural and elemental analyses of used catalysts have shown that not only the great majority of the micropores become blocked, but also that the amounts of oxygen and hydrogen in the catalyst composition increase with time on stream, leading to a material increasingly more reactive towards oxidation. It was observed that after a few days on stream, the rate of carbon gasification became larger than the rate of coke deposition, leading to a decrease in catalyst weight. Working under milder conditions (lower temperature and oxygen partial pressure) delayed this effect. The increase in the concentration of surface groups with time does not result in a proportional increase in the activity of the catalysts, because the majority of the groups created are not active for the ODE reaction.

Catalytic ozonation of sulphamethoxazole in the presence of carbon materials: catalytic performance and reaction pathways.

Two carbon materials (multi-walled carbon nanotubes, MWCNTs, and activated carbon) were investigated as ozonation catalysts for the mineralization of the antibiotic sulphamethoxazole (SMX). MWCNTs presented a higher catalytic performance than activated carbons, which was justified by their differences in surface chemistry and by the higher internal mass transfer resistances expected for activated carbons. 3-Amino-5-methylisoxazole and p-benzoquinone were detected as primary products of single and catalytic ozonation of SMX, whereas oxamic, oxalic, pyruvic and maleic acids were identified as refractory final oxidation products. The original sulphur of the SMX was almost completely converted to sulphate and part of the nitrogen was converted to NH4+ and NO3-. The presence of the radical scavenger tert-butanol during catalytic and single ozonation evidenced the participation of HO radicals in the oxidation mechanisms of SMX, especially in the mineralization of several intermediates. Microtox tests revealed that simultaneous use of ozone and MWCNTs originated lower acute toxicity. The time course of all detected compounds was studied and the transformation pathway for the complete mineralization of SMX by single and catalytic ozonation in the presence of the selected materials was elucidated.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts : 2. Kinetic modelling

A detailed kinetic study of the oxidative dehydrogenation of ethylbenzene in the presence of an activated carbon catalyst is presented. The partial pressures of ethylbenzene and oxygen, as well as temperature, were varied one at a time, covering a wide range of experimental conditions. The experimental results were found to be well described by a kinetic model in which the main reaction occurs by a redox mechanism involving quinone/hydroquinone groups on the surface of the carbon catalyst. The activation energies determined for the main reaction and for the formation of carbon oxides are comparable to those determined on coked oxide catalysts.

Production, characterization and application of activated carbon from brewer's spent grain lignin

Different types of activated carbon were prepared by chemical activation of brewers spent grain (BSG) lignin using H(3)PO(4) at various acid/lignin ratios (1, 2, or 3g/g) and carbonization temperatures (300, 450, or 600 degrees C), according to a 2(2) full-factorial design. The resulting materials were characterized with regard to their surface area, pore volume, and pore size distribution, and used for detoxification of BSG hemicellulosic hydrolysate (a mixture of sugars, phenolic compounds, metallic ions, among other compounds). BSG carbons presented BET surface areas between 33 and 692 m(2)/g, and micro- and mesopores with volumes between 0.058 and 0.453 cm(3)/g. The carbons showed high capacity for adsorption of metallic ions, mainly nickel, iron, chromium, and silicon. The concentration of phenolic compounds and color were also reduced by these sorbents. These results suggest that activated carbons with characteristics similar to those commercially found and high adsorption capacity can be produced from BSG lignin.

Cerium, manganese and cobalt oxides as catalysts for the ozonation of selected organic compounds

Several metal oxides, as well as metal oxides supported on activated carbon, were assessed as ozonation catalysts for the removal of selected organic compounds. Two transition metals (Mn, Co) and one rare earth element (Ce) were chosen for the preparation of the two series of catalysts. These materials were used in the ozonation of two aromatic compounds (aniline and sulfanilic acid) and one textile azo dye (CI Acid Blue 113). The results were compared with those obtained with non-catalytic ozonation. All the tested materials were found to be effective ozonation catalysts. Among the metal oxides, those containing mixtures of cerium and manganese or cerium and cobalt enabled the highest mineralisation degrees. After 120 min of reaction the TOC removal achieved with Ce-Mn-O was 63% for sulfanilic acid and 67% for aniline, while Ce-Co-O allowed TOC removals of 58 and 66%, respectively. With single ozonation, the mineralisation of sulfanilic acid and aniline solutions was 34% and 40% after identical reaction period. Regarding the metal oxides supported on activated carbon, cerium and manganese oxides were, in general, the most active for the degradation of the studied compounds.

Enhanced direct production of sorbitol by cellulose ball-milling

The catalytic conversion of lignocellulosic biomass to renewable and valuable chemicals has attracted global interest. Given the abundance of this renewable raw material and its reduced impact on the food chain, it is an attractive source for replacing fossil fuels and obtaining chemicals or fuels in the context of a sustainable economy. In this work, a catalyst (Ru/AC) was developed to perform, in a single step, hydrolysis and hydrogenation of cellulose to sorbitol. An activated carbon supported ruthenium catalyst was examined for the one-pot hydrolytic hydrogenation of cellulose and it has shown to be very active and selective for the conversion of cellulose into sorbitol. When microcrystalline cellulose was used, a conversion of 36% was reached after 5 hours of reaction, with a selectivity to sorbitol of 40%. On the other hand, ball-milled cellulose allowed attaining conversions close to 90%, with a selectivity to sorbitol of 50%. Moreover, if the catalyst was ball-milled together with cellulose, the selectivity to sorbitol could be further increased to almost 80%. The catalyst showed excellent stability after repeated use. In this work we combined hydrolysis and hydrogenation in one-pot (using heterogeneous catalysts instead of homogeneous), in the presence of a Ru/AC catalyst (without any support pre-treatment with acids) and pre-treated cellulose just by ball-milling (instead of using acids). For this reason, the results obtained in this work are one of the best values achieved when using supported metal catalysts to convert cellulose by an environmentally friendly process.