M. Molina-Sabio

Activated carbons from lignocellulosic materials by chemical and/or physical activation: an overview

Abstract Four series of activated carbons prepared from lignocellulosic materials (almond shells and olive and peach stones) by either physical activation—gasification (uncatalysed and iron catalysed) in CO2 or in a water-nitrogen mixture—of chars or direct chemical activation with ZnCl2 of the precursor were selected to show the comparative behavior of the activation procedures. Activation with CO2 opens and widens the microporosity of the char with even a shift to meso- and macroporosity, the ablation of the exterior of the particle being very important at high burn-off; the final activated carbon has a well developed micro- and macroporosity, with a relatively small contribution of mesoporosity. The iron catalysed CO2 gasification and gasification with water-nitrogen mixture produce carbons with a well developed macroporosity, although the latter has the advantage of maintaining a well developed micro- and mesoporosity. Direct chemical activation of the precursor with ZnCl2 produces, in only one step, a larger yield of activated carbon having microporosity as well developed as in the CO2 gasification of the char, with the advantage of producing a much larger mesopore volume.

The use of steam and CO2 as activating agents in the preparation of activated carbons

Four series of activated carbon have been prepared from carbonized olive stones. One of them, series D, was prepared using carbon dioxide as activating agent, and the other three, series AV, W, and H, with water vapor under different experimental conditions. Two of the series, D and H, were prepared in such a way that the gasification rate for both reactants was identical, in an attempt to reduce the effect of the relative differences in diffusion and accessibility of both gases to the interior of the particles. The changes in porosity of the original char during activation have been studied by adsorption of N2 at 77 K and CO2 at 273 K, as well as by mercury porosimetry. The results obtained show that carbon dioxide produces an opening, followed by widening, of narrow microporosity, whereas water vapor widens the microporosity from the early stages of the process, the resulting activated carbon exhibiting lower micropore volume. However, dilution of water vapor and high activation temperatures approach the development of total microporosity by steam to that of carbon dioxide, although there is a more important role of narrow microporosity widening in the former.

Preparation of activated carbon by chemical activation with ZnCl2

Abstract Several series of activated carbons have been prepared by chemical activation of peach stones with ZnCl 2 in order to show the effect of variables such as a precursor particle size, extent of impregnation, impregnation method, and carbonization temperature on surface area, porosity, and bulk density of the resulting activated carbons. The adsorption isotherms of n-butane at 273 K on all carbons prepared are of type I, with a defined plateau, the extent of which is a function of the preparation conditions. The main factor affecting the surface area and the micropore size distribution is the amount of Zn introduced in the precursor during impregnation. Partial gasification in CO 2 of the carbons produces a considerable developing of surface area and porosity, maximum for burn-offs around 60–70%. In this way, it is possible to prepare activated carbons with very high surface area (larger than 3000 m 2 /g) compatible with a granular form and reasonable bulk density.

Porosity in granular carbons activated with phosphoric acid

Abstract Three series of activated carbon have been prepared by heat treatment of peach stones impregnated with solutions of phosphoric acid, in order to analyze the effect of phosphoric acid on the yield, bulk density and porosity of the resultant activated carbons. The analysis of the adsorption isotherms of N2 at 77 K. CO2 at 273 K and n-C4H10, at 273 K shows that the amount of phosphorus introduced into the material. Xp. is the main factor conditioning the porosity and pore size distribution of the activated carbon. In general terms, the increase in Xp leads to an increase in the volumes of micro and mesopores of the carbon. Furthermore, there is a noticeable similarity between the volume of micropores and the volume occupied by the P2O5·.xH2O in the interior of the peach stones. This suggests that the microporosity is mainly caused by the phosphoric acid remaining in the impregnated material, which inhibits the contraction of the material during carbonization. On the other hand. it is suggested that the mesoporosity of the activated carbon, which is large only when the concentration of phosphoric acid is high, is mainly caused by the hydrolysis of the lignocellulosic material and subsequent partial extraction of some of its components during impregnation.

Effect of steam and carbon dioxide activation in the micropore size distribution of activated carbon

Abstract This work presents the different effects of steam and carbon dioxide activation of a char in both the development of microporosity and the micropore size distribution using immersion microcalorimetry of liquids with different molecular size (benzene, 2,2 dimethylbutane, iso-octane and α-pinene). The study has been carried out with three series of carbons, two of them prepared by steam activation and the third one by carbon dioxide activation, covering a wide range of burn-off (8–70%). The experimental results show that carbon dioxide activation mainly causes the creation of microporosity. However, steam activation widens the microporosity as from the early stages of the activation process, the resulting activated carbons exhibiting a lower micropore volume. The different porous structures produced by both activating agents is related to the oxygen surface groups in the carbon, as measured by temperature programmed desorption (TPD). Activation by carbon dioxide creates not only a larger number of groups evolving as CO but also these groups are thermally more stable than those produced by steam activation.

Textural and chemical characterization of microporous carbons

Several methods for the characterization of the porous texture (adsorption of vapors and gases, immersion calorimetry) and chemical nature—mainly oxygen surface groups (selective titration and temperature programmed desorption) of activated carbons are analyzed. The results for several series of activated carbons show that a simple and convenient evaluation of volume of micropores and heterogeneity of the micropore size distribution can be obtained by combining the results from adsorption of N2 at 77 K and CO2 at 273 K. On the other hand, for microporous carbons the micropore size distribution obtained by adsorption and immersion calorimetry are very coincident. It has been found for carbons oxidized with different chemicals that there is a correlation between selective titration and TPD. Finally, the relative importance of the porous texture and oxygen surface groups of activated carbon on the adsorption capacity and enthalpy of immersion for molecules with different polarity has been evaluated, and it has been shown that the specific interactions between the polar molecules and the surface groups play an important, although not exclusive, role in both adsorption and immersion. In the case of non-polar molecules, only the porous texture of the carbon conditions the amount adsorbed and the enthalpy of immersion.

The combined use of different approaches in the characterization of microporous carbons

Abstract Adsorption of N2 (77 K), CO2 (273 K), and several hydrocarbons (273 or 298 K) and n-nonane preadsorption for two series of activated carbons with more-or-less distorted type I isotherms have been analyzed by the Dubinin-Radushkevich (D-R), Langmuir, and αs methods. When the microporosity is narrow and uniform, all theoretical and experimental approaches lead to values of micropore volume that are in good agreement. For carbons with wide micropore size distributions the Langmuir, αs and D-R methods applied to N2 (77 K) (if for the latter a straight portion in the D-R plot can be drawn in the 0.05–0.3 range of relative pressure) yield the total micropore volume (as the conventional D-R plots for hydrocarbons) whereas n-nonane preadsorption and CO2 give the volume corresponding to narrow micropores, in this way allowing a more complete characterization of the microporosity.

ADSORPTION OF SUBSTITUTED PHENOLS ON ACTIVATED CARBON

The adsorption of phenol and the substituted phenols, 4-nitrophenol, 2,4-dinitrophenol, 4-chlorophenol, and 2,4-dichlorophenol in aqueous solution, has been determined at 298 K on a series of activated carbons, prepared from olive stones, having a wide range of burn-off (8–52%) and micropore size distributions. The adsorption process is controlled predominantly by the porosity of carbon when the microporosity is narrow in range. If the ragne of microporosity is wide then the adsorption process is affected by the chemical nature of the carbon and by the nature of the substituent group in the phenol. The adsorption isotherm of 2,4-dinitrophenol is a step function, which is interpreted as being due to the coexistence of neutral and anionic adsorbate species, and not due to a change in the orientation of the neutral adsorbate species.

Development of porosity in combined phosphoric acid-carbon dioxide activation

Abstract Five activated carbons with different porosity, prepared by chemical activation of peach stones with phosphoric acid, have been further activated in a carbon dioxide gas flow at 825 °C for different periods of time to cover a wide range of burn-off. The porosity of all activated carbons was determined by adsorption of N 2 (77 K), CO 2 (273 K) and n -C 4 H 10 (273 K). The main effect of gasification with carbon dioxide is similar to that described for other carbons, namely the creation and widening of existing pores, the predominance of one or another being a function of burn-off. However, since activation with phosphoric acid may produce very different pore size distributions, a common carbon dioxide activation process may produce very different effects, which range from the development of only micropores to the development of only mesopores, thus enhancing the differences among the initial chemically activated carbons.

Effect of steam activation on the porosity and chemical nature of activated carbons from Eucalyptus globulus and peach stones

The effect of steam activation of chars prepared from Eucalyptus globulus (EU) and peach stones (PS) on both the porosity development and the amount of oxygen surface groups is presented. The N2 (77 K) and CO2 (273 K) adsorption isotherms and the mercury intrusion measurements show that, apart from the differences in macroporosity caused by the different texture of the original precursors, the development of porosity upon activation is small for the EU char and important for the PS char. A somewhat parallel behaviour is found for the oxygen surface groups, as determined by infrared spectroscopy and temperature programmed decomposition (TPD): the development of microporosity for PS chars is accompanied by an increase in the number of oxygen surface groups stable at the activation temperature, the number being low for activated carbons from EU chars. This behaviour indicates the more important role of diffusion control of the water molecule to the interior of the particle when activating the EU char in respect to the PS char.