J.A. Pajares

Preparation of activated carbon fibres from acrylic textile fibres

Abstract Acrylic textile fibres have been used to prepare activated carbon fibres (ACF). Characterisation by means of elemental analysis, XRD, SEM and low temperature nitrogen adsorption show that the properties of the acrylic ACF compare favourably with those of non-textile PAN, Kevlar and Nomex ACF. A particularly interesting, and never previously reported, feature was observed with fibres activated at 900°C. It was found, with one fibre in particular, that over a very limited range of burn-off between 40 and 50% the micropore volume tripled, the mean pore width suddenly increased, the mean stack height, Lc, suddenly decreased and the reactivity decreased by more than a half. The observed changes suggest a change in the mechanism of activation from one involving principally gasification of amorphous or more reactive carbon at low burn-off to one involving principally attack of individual crystallites and their reorganisation at higher burn-off.

Textural characterization of coals using fractal analysis

Abstract The aim of this study is to show how fractal analysis can be effectively used to characterize the texture of porous solids. The materials under study were series of coals oxidized in air at various temperatures for different time intervals. Data from mercury porosimetry determinations of samples was analyzed using fractal models. The methods employed were those proposed by Neimark, Friesen and Mikula and that developed by Zhang and Li. Some methods are able to supply a fractal profile or “fractal fingerprint” of materials, i.e. ranges of pore sizes with different fractal dimensions are detected. These fractal profiles are very sensitive to the oxidation treatment. The average fractal dimension can also be used as a valid parameter to monitor the textural evolution of the coals as the treatment progresses, as this behaves in a similar way to other textural parameters. The use of fractal analysis in conjunction with the results of classical characterization methods leads to a better understanding of textural modifications in the processing of materials.

Synthesis and characterization of pillared montmorillonite catalysts

Abstract Pillared montmorillonite was synthesized by the direct cation exchange method, varying the hydroxyl/aluminium and aluminium/clay ratios in the preparation in order to find the optimum conditions, within the limits of this study, for obtaining stable products with a microporous structure and high specific surface area, capable of being used as cracking catalysts. The minimum aluminium/clay ratio required was 7.5 meq Al/g clay, independent of the hydroxyl/aluminium molar ratio. The specific surface area of the original sample (61 m2/g) was increased by pillaring up to 366 m2/g. The structure of the aluminium oxide pillared montmorillonite proved to be stable up to 500°C.

Effect of gasification on the porous characteristics of activated carbons from a semianthracite

Abstract Activated carbons with degrees of burn-off between 20% and 80% have been prepared from a semianthracite char by gasification in steam at 1123 K. The adsorption isotherms of nitrogen at 77 K and CO2 at 273 K have been determined. The pore size distribution has been calculated from t plots, and helium and mercury densities. The ungasified char has a narrow pore size distribution and mainly consists of very fine microporosity. The pores have constrictions that do not allow adsorption of nitrogen, but the microporosity can be measured by the adsorption of CO2 at 273 K. Smic and CO2 (D-R) surface areas of the ungasified char are about the same. The gasification of the char broadens the micropore distribution. The small microporosity calculated from t plots increases with burn-off between 20%–35%. Gasification to higher degrees of burn-off causes burning of the walls between the adjacent pores and eliminates pore constrictions. This results in widening the micropores, creating meso and macroporosity and decreasing microporosity. When the degree of gasification is 80%, the small microporosity disappears almost completely, although the total porosity has been increased almost three times. While the nitrogen surface area increases with burn-off, the CO2 surface area decreases at higher degrees of burn-off.

Effect of coal preoxidation on the development of microporosity in activated carbons

Abstract Activated carbons have been prepared from a semianthracite preoxidised in air to different degrees. The activation has been carried out in steam at 850 °C to 50 ± 1% burnoff. The adsorption isotherms of nitrogen at 77 K and CO2 at 273 K have been determined. The adsorption isotherms indicate that the activated carbons obtained from non-oxidised coal has a poor porosity. The porosity increases with increase in the degree of coal preoxidation. The BET surface area increases from 160 m2g−1 in the case of activated carbon obtained from nonoxidised coal char to 847 m2g−1 for the activated carbon from oxidised coal char, t-plots obtained show that the degree of oxidation enhances both small and medium microporosity, the increase in small microporosity being much larger. The activated carbons obtained from preoxidised coal chars are largely microporous in character.

Characterization of activated carbons by the BET equation: an alternative approach

The adsorption isotherms of nitrogen at 77 K have been determined on two series of activated carbons and two samples of activated carbon fibres. The carbons were highly microporous and contained varying amounts of different forms of microporosity. Characterization of the carbons was undertaken using an alternative linear form of the BET equation and the results of the BET parameters obtained have been compared with those obtained from the usual classical linear form of the equation. The surface areas calculated from the two linear forms were similar while the C values were quite different. The C values obtained from the alternative linear form appeared to be more reasonable, being consistent with and related to the microporous character of these carbons. The alternative linear form gave BET plots which were more sensitive to deviations from linearity allowing the relative pressure range for the application of the BET equation to be determined without any ambiguity and more precisely.

Structural and textural evolution of Al- and Mg-rich palygorskites, I. Under acid treatment

Abstract The structural and textural evolution of two Spanish palygorskites, one magnesium-rich and another aluminium-rich, activated with hydrochloric acid of different concentrations, has been studied. The maximum activation is obtained in both cases with a 2 N concentration. The activation is much higher in the magnesium-rich palygorskite, which undergoes a greater dissolution of its octahedral sheet as well as a more extensive alteration of its structure and a greater increased creation of microporosity.

Structural and textural evolution under thermal treatment of natural and acid-activated Al-rich and Mg-rich palygorskites

The structural and textural evolution under thermal treatment of two Spanish palygorskites, one magnesium-rich and the other aluminium-rich, and also of the same palygorskites previously activated with HCl, has been studied. Using IR and X-ray techniques, the structural modification that the samples undergo on heating, is shown. By means of N2 adsorption-desorption isotherms and Hg porosimetry the changes in the texture were followed until 1000°C. The texture of both natural palygorskites is maintained until 600°C, despite the structural changes which they undergo. On heating at 1000°C a great decrease of surface area is observed. In samples previously activated with HCl, a greater evolution of the texture is evident. On heating above 300°C there is a sharp decrease of the surface area, owing to the disappearance of the microporosity created by acid-leaching. This decrease of the surface area is greater in the magnesium-rich sample.

Preparation of active carbons from coal: Part III: Activation of char

Active carbons with a burn-off of 52% have been prepared from four coals of different rank and origin after preoxidation to different degrees at 543 and 473 K, and further carbonization at 1123 K. The activation has been carried out with CO2 at 1123 K at two flow rates viz. 7 cm3 min−1 and 500 cm3 min−1. Active carbons have also been prepared from a preoxidized coal by activation to different degrees of burn-off between 10 and 80%. The effect of the degree of oxidation, the flow rate of the activating gas and the extent of burn-off on the porous structure development of active carbons has been examined. The active carbons prepared from unoxidized coal have poor textural characteristics (porosity, N2 and CO2 surface area). Nevertheless, the textural characteristics are enhanced as the degree of preoxidation of the coal is increased. The low flow rate of CO2 (activating gas) produces active carbons with a better microporous character. The degree of activation (the extent of burn-off) of the carbon determines the porous structure of the active carbon. At low degrees of burn-off (less than 50%) the product is largely microporous. At higher degrees of burn-off between 35–65% the product has a mixed porous structure and contains all types of pores. Active carbons with a given textural character can be obtained by controlling the degree of oxidation of coal and the degree of activation of the carbonized material.

Preparation of active carbons from coal Part I. Oxidation of coal

Abstract The oxidation of coals of different rank, origin and particle size has been studied at temperatures between 423 and 543 K and for time intervals between 6 h and 42 days. The chemical composition of the oxidised coal depends upon the coal rank, its particle size and the degree of oxidation as determined by the temperature and the time of oxidation. At higher degrees of oxidation, whether at higher temperatures for shorter time intervals or at lower temperatures for longer time intervals, the oxidised coal tends to approach similar chemical compositions. The weight of coal also changes on oxidation, the increase or decrease in weight depending upon the rank and the oxidation conditions. The rate and extent of oxidation decrease with increase in particle size because the larger particles slow down the diffusion of oxygen into the coal particles. The oxidation can eliminate completely the plastic properties of bituminous coal which inhibit the formation of anisotropic structures and enhance the development of a primary pore structure. The helium density increases with the degree of oxidation but the mercury density initially increases and then decreases. A minimum in the mercury density is obtained when the carbon content of the oxidised coal is around 85–90%. The oxidation significantly enhances the porosity and the surface area, the extent of increase depending upon the nature of the coal and the degree of oxidation. The changes in chemical composition, porosity and surface area with the degree of oxidation indicate that the oxidation of coal involves two different mechanisms, one operating at lower temperatures and the other at higher temperatures.