A. Albiniak

The pore structure of activated chars of brown coal humic acids obtained at increased rate of carbonization

Brown coal humic acids, after pyrolysis at a heating rate of 600°C/min, have been activated in steam to different burn-offs. Carbonaceous adsorbents were obtained with well developed micro-, meso-, and macroporous structures, and nitrogen BET surface areas of about 1000–1500 m2/g. An increase in the heating rate during pyrolysis to 600°C/min (from the standard rate of 5°C/min) strongly enhances macropore development during subsequent activation, although micro- and mesopore volumes remain similar. The pore structure parameters of the activated chars based on the adsorption of nitrogen have been compared with those obtained from benzene and cyclohexane adsorption. The differences observed are less pronounced for benzene than for cyclohexane. The kind of nonporous carbonaceous standard employed, whether ungraphitized (Elftex 120) or graphitized (Spheron 6–2700), has little effect upon the results of calculations of the mesopore size distribution, but a marked effect on parameters derived from the α5 plot. For those chars that, in addition to their microporous system, were characterised by a significant contribution of mesoporosity, the values of Vm (from the BET plot) are visibly lower than the respective values of VB (from the position of point B). An attempt was made to use the existence of these differences in calculations of the surface area of the mesopores.

Cheap adsorbent. Part 1: Active cokes from lignites and improvement of their adsorptive properties by mild oxidation

In this article, cheap adsorbents were produced starting from two different lignites. About 500 kg of each coal was pyrolyzed in a rotary kiln at semi-pilot scale. Characterization of the obtained chars are made in terms of porosity development, surface functional groups and adsorptive properties determined for both 4-nitrophenol and lead. A post-oxidative treatment at low temperature is proposed in order to improve the adsorptive capacities of the initial chars by the introduction of oxygen containing functional groups, which are widely involved in chemisorption. The conditions of the post-treatment were chosen to be easily exported in an industrial process, for example during the cooling step. The importance of both lignite characteristics (ashes content, water content) and pyrolysis conditions is demonstrated. The positive effect of a simple post-oxidative treatment is shown by a strong increase in the adsorptive capacities, and correlated to the evolution of the porosities and surface oxygen functionalities. Considering unit area, the properties of our active cokes are similar to those of commercial active carbon, but their specific areas are less developed (about 400 m2 g−1).

Exfoliation and textural modification of anthracites

Abstract Two anthracites (La Mure, France and Hongay, Vietnam) have been exfoliated after intercalation in a perchloric and nitric acid solution followed by a thermal shock treatment. The anthracites are extensively exfoliated as shown by scanning electron microscopy, and this treatment is responsible for a significant development of mesoporosity and a great increase of specific surface area. Hongay anthracite was then steam activated, and specific surface areas of 1200 m2/g have been obtained with a well developed mesoporous texture. Moreover, after exfoliation, the anthracite reactivity with water vapour during steam activation is increased by a factor of 3. This shows that anthracites may be an important source of raw materials for the production of active carbons with a large pore size distribution.

Following changes in the porous texture of Nomex-derived activated carbon fibres with the molecular probe technique

A series of Nomex-derived activated carbon fibres (ACFs) activated to different burn-offs (BOs) together with two series derived from the previous ACFs through chemical vapour deposition (CVD) treatments were used as adsorbents. The adsorption of molecules of different size, namely, dichloromethane, benzene, cyclohexane and carbon tetrachloride was used to monitor the textural evolution of microporous carbon fibres owing to these treatments. The analysis of the results showed that carbon deposit has taken place mainly at the entrance of the pores rather than in the interior. The information obtained about the porous structure and the molecular sieve behaviour of the samples in the different series was also useful for establishing the possible uses of these materials: thus, the obtained ACFs could be used for adsorbing volatile organic compounds (VOCs) in the field of environmental control while CVD-treated ACFs could be used as adsorbents for gas separations. In particular, the combined use of CH2Cl2 and C6H6 could be used to determine whether an adsorbent is a good carbon molecular sieve (CMS) for CO2/CH4 separation.

Effects of FeCl3 (intercalated or not in graphite) on the pyrolysis of coal or coal tar pitch

Addition of iron chloride to coal tar pitch or coal before carbonization speeds up mesophase growth and increases the yield of an iron loaded microporous char. After steam activation a mesoporous activated carbon with specific adsorbents properties can be obtained. Iron oxide (magnetite) at the internal surface of the carbon plays the role of a basic centre for the catalytic oxidation of polluting gas. The present work deals with the effect on the pyrolysis and activation processes of the mode of introduction of FeCl3-either free or intercalated in graphite (GIC)—in coal tar pitch (CTP), in coking coal itself, or in a mixture of coal and CTP. Lewis acid effects on oil, methane, dihydrogen and carbon monoxide evolution could be correlated with those on mesophase growth, on char yield and on the properties of the steam activated chars. The study shows that free FeCl3 loading can be advantageously used to prepare cheap adsorbents.

Porosity development in steam activated chars from mixtures of coal tar pitch with graphite-FeCl3 intercalation compounds

The effect of addition of the iron graphite intercalation compound (GIC-FeCl3) to coal tar pitch, used as a carbon precursor, on the porosity development in the resulting steam activated chars, was studied. Different conditions of chars preparation have been considered. Benzene (298 K), nitrogen (77 K), carbon dioxide (298 K) and water (298 K) sorption measurements were taken as basis for the porosity evaluation. The changes in the development of pores of different categories, caused by added amounts of GIC-FeCl3 to coal tar pitch, temperature of carbonization and activation, and the degree of activation, are discussed.

Effect of MoCl5 addition on the carbonization and the subsequent steam activation of pitch and coking coals

Abstract The present work is devoted to the study of the role of MoCl 5 and its by-products on reactions involved in pore formation during the preparation of activated carbon for the manufacture of carbon-supported catalysts. The carbonaceous precursors were a coal tar pitch and three bituminous coals of increasing rank. Continuous detection of evolved tar and gas during carbonization was implemented and the data were correlated with the porosity development during carbonization and subsequent steam activation. It was found that MoCl 5 promotes the microporosity growth during carbonization by dehydrogenation reactions leading to cross-linking and condensation reactions. The pores left in the char, as well as the residual molybdenum compounds, favour the development of the porosity during steam activation.

Effect of iron enrichment with GIC or FeCl3 on the pore structure and reactivity of coking coal

The effect of a Lewis acid addition to a coking coal on the porosity and reactivity towards steam of the resulting iron enriched coal chars are studied. GIC (FeCl3 graphite intercalation compound) or free FeCl3 are used as iron containing additives. Coal iron enrichment was performed using either directly FeCl3 in vapour phase, or by mixing of coal and additives in decaline or by common grinding of coal and additives under argon. Iron enriched coals were carbonized at 750°C (heating rate = 5°C min) and activation made with pure steam at 800°C to a burn-off off of 50 wt%. The pore structures of coal chars before and after activation were evaluated on the basis of CO2 and C6H6 sorption at 25°C. A significant development of the microporosity is observed in the iron enriched char before activation and its steam reactivity is also increased. After activation, BET surface area values are increased in presence of iron, and porosity is mainly microporous.

Graphitization, intercalation, and exfoliation of cokes and anthracites: a comparative study

Abstract In this work, we will compare the ability to graphitize of two petroleum cokes, isotropic and anisotropic, and of two varieties of anthracites, in order to understand their subsequent ability to give exfoliated carbons. Both series of derived materials obtained by heat treatment (1500–2600 °C) of these four precursors are compared after the action of an excess of mixture containing water, perchloric acid, and nitric acid, which were used to obtain exfoliated graphite from very small particles. The intermediate products are shock heated to induce the thermal decomposition of the perchloric acid present either in the microtexture or in the graphitic structure. This ultimate treatment gave exfoliated materials, the specific surface of which strongly depends on the nature of the raw material, its ability to graphitize.

Pore structure and reactivity of chars obtained by pyrolysis of coking coals containing MoCl5

Three coking coals were pyrolysed in the presence of the Lewis acid MoCl 5 under an inert atmosphere at a final temperature of 750°C, the ultimate aim of this work being the elaboration of active carbon supported catalysts for use in hydrodenitrogenation reactions. The values of the parameters characterizing the porous structure of the chars before and after steam activation were determined using CO 2 and C 6 H 6 sorption measurements at 25°C. Char from the lowest rank coal had the most developed ultramicroporosity (pore width < 0.4 nm) and the presence of MoCl 5 in the precursor does not seem to exert much influence. In the case of the higher rank coals, a significant development of the ultramicroporosity was observed in the molybdenum enriched chars before activation and their reactivity toward steam was increased. After steam activation, the structure of the activated chars was mainly microporous, but a significant development of the mesoporosity was observed.