About reactions occurring during chemical activation with hydroxides

The chemical activation of anthracites with hydroxides has been shown to be of interest for the production of activated carbons with a highly microporous structure. In a previous paper, attention was placed on the reactions occurring during the chemical activation of an anthracite by NaOH and KOH. In the present work, the process of chemical activation by hydroxides has been extended to different coal precursors to confirm that such a chemical activation process starts through a solid-solid reaction and continues as a solid-liquid reaction. In such a solid reaction, the reactivity of the solid (precursor) should be a key parameter. The importance of the carbon reactivity on its reaction with hydroxides has been confirmed: the lowest rank coal reacts much easily and has a much lower temperature for the beginning of reaction than the highest rank coal.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

Synthetic carbons activated with phosphoric acid: I. Surface chemistry and ion binding properties

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer at various temperatures in the 400-1000 °C range. The resulting carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration with calculation of proton affinity spectra, and copper adsorption from solution. The results indicate that the synthetic carbons obtained possess acidic character and show cation-exchange properties similar to those of oxidized carbons. However, the acidic compounds arising from treatment with phosphoric acid are tightly bound to the carbon lattice and are chemically and thermally more stable than those introduced by oxidative treatments. The largest amount of cation-exchange surface groups is introduced after activation at 800 °C. Infrared investigations showed that phosphorus compounds may be polyphosphates bound to the carbon lattice. Proton affinity distribution curves calculated from potentiometric titration experiments showed four types of surface groups on synthetic phosphoric acid activated carbons. Among them phosphorus-containing groups are the most important for the adsorption of heavy metal ions (copper) from acid solutions. Thus, carbons activated with phosphoric acid may be regarded as prospective cation-exchangers for the removal of heavy metals from water solutions.     

Dynamic adsorption on activated carbons of SO2 traces in air: I. Adsorption capacities

Sulphur dioxide is an atmospheric pollutant which, among numerous others, has to be eliminated by habitacle filters. Breakthrough curves of low concentration SO₂ streams through beds of activated carbons have been obtained. Two carbons were studied, an activated PAN fiber (CF) and a granulated activated carbon (CN) under SO₂ concentrations lower than 100 ppm. Carbon CN used ‘as received’ is able to trap SO₂ in air at concentrations as low as 2.5 ppm. At this concentration, the adsorption of SO₂ is essentially irreversible. The fraction of reversibly adsorbed SO₂ rapidly increases when SO₂ content in air increases from 2.5 to 100 ppm. As expected, the amounts of SO₂ adsorbed per gram of carbon are much smaller than in the case of high SO₂ contents in air (>1000 ppm). The presence of water in carbon micropores enhances both reversible and irreversible adsorption of SO₂. The reversibly adsorbed part is physisorbed while the irreversibly adsorbed part results in oxidation of SO₂ at the carbon surface. This oxidation was evidenced by TPD from carbon samples after adsorption. The mechanism of SO₂ adsorption is discussed in relation to the mechanisms proposed in literature for high SO₂ contents (>1000 ppm).     

Preparation of fibrous porous materials by chemical activation: 1. ZnCl2 activation of polymer-coated fibers

Fibrous porous materials (FPMs) have been prepared by coating a glass fiber with a solution of polymer and ZnCl₂, followed by stabilization in air and heat treatment in N₂. The ZnCl₂ was then removed by washing with D.I. water and HCl. Four kinds of polymers, a phenolic resin, polyacrylonitrile, poly(vinyl alcohol) and cellulose, were used to prepare solutions with ZnCl₂. The results showed that ZnCl₂ acts as a dehydration agent to promote the thermal cross-linking of polymer at a much lower temperature, leading to FPMs having much higher char yields and very high surface areas. The porosity was created in part by dissolution of the ZnCl₂ left in the charred coating. The activation temperature and ZnCl₂ concentration play an important role in porosity development. In the early stage of heating, the specific surface area, micropore and mesopore volumes increased with increasing temperature. As the activation temperature increases above 450°C, ZnCl₂ begins to volatilize out of the coating, and further charring and aromatization of the coating results in a dimensional contraction leading to a decrease in the micropore and mesopore volumes. It was observed that the specific surface area, as well as micropore and mesopore volumes, increased with increasing ZnCl₂ concentration. Pore size analysis showed that the FPMs activated with ZnCl₂ were mainly microporous. For FPMs activated with concentrated ZnCl₂ (66 wt.%), there is a remarkable and large mesopore size distribution in addition to the typical micropore size distribution. In addition, such FPMs have very high specific surface area, more than 1600 for PAN-based and 2500 m²/g of coating for cellulose-based FPMs.     

Adsorption of dyes on activated carbons: influence of surface chemical groups

The surface chemistry of a commercial activated carbon has been selectively modified, without changing significantly its textural properties, by means of chemical treatments, using HNO₃, H₂O₂, NH₃, and thermal treatments under a flow of H₂ or N₂. The resultant samples were characterized in terms of their surface chemistry and textural properties, and subsequently tested in the removal of different classes of dyes. It was shown that the surface chemistry of the activated carbon plays a key role in dye adsorption performance. The basic sample obtained by thermal treatment under H₂ flow at 700 °C is the best material for the adsorption of most of the dyes tested. For anionic dyes (reactive, direct and acid) a close relationship between the surface basicity of the adsorbents and dye adsorption was shown, the interaction between the oxygen-free Lewis basic sites and the free electrons of the dye molecule being the main adsorption mechanism. For cationic dyes (basic) the acid oxygen-containing surface groups show a positive effect but thermally treated samples still present good performances, showing the existence of two parallel adsorption mechanisms involving electrostatic and dispersive interactions. The conclusions obtained for each dye individually were confirmed in the colour removal from a real textile process effluent.     

Estimation of activated carbon adsorption efficiency for organic vapours: I. A strategy for selecting test compounds

Strategies for developing quantitative structure-affinity relationships (QSAfR) for the prediction of break-through performance of 31 chlorinated hydrocarbons on activated carbon have been studied. Two different approaches for the selection of a limited set of compounds for modelling were evaluated through the predictive power of the resulting QSAfR models. When the model was based on a training-set selected without a rational strategy, the developed QSAfR model showed poor predictive performance. Accordingly, such models have a limited capability to produce information concerning the important adsorbate related parameters influencing adsorption. By using a strategy where multivariate data analytical techniques are used in conjunction with statistical experimental design to select a balanced set of compounds for break-through performance evaluation, it was possible to develop QSAfR models with high predictive capability.     

SO2 removal from flue gas by activated semi-cokes: 1. The preparation of catalysts and determination of operating conditions

The objective of this work was to use waste semi-coke as the raw material to prepare catalysts of industrial-scale size for SO₂ removal from flue gas and to find the optimal preparation methods. Results showed that lignite semi-coke was a suitable raw material, and that the catalyst, prepared by pre-activating in an autoclave, oxidizing with HNO₃, loading with CuSO₄ and finally calcining at 700 °C, exhibited the best desulfurizing property with a sulfur retention of about 9.6% SO₂/100 gC at a reaction temperature of 90 °C. Also, the effects of H₂O content in the flue gas, reaction temperature and space velocity on the desulfurizing property were investigated to determine optimum operating conditions. An H₂O content of 7% was appropriate for catalysts in this work. In the temperature range 80-120 °C, the catalyst showed good performance for SO₂ removal and was gradually deactivated at temperatures above 120 °C. Space velocity exhibited an optimal value of 830 h⁻¹. The kinetic behavior varied with space velocity and the desulfurizing property was controlled by diffusion at space velocities below 830 h⁻¹, and controlled by adsorption or catalytic reaction at space velocities above 830 h⁻¹.     

Catalytic filamentous carbon: Structural and textural properties

Catalytic filamentous carbon (CFC) synthesized by the decomposition of methane over iron subgroup metal catalysts (Ni, Co, Fe or their alloys) is a new family of mesoporous carbon materials possessing the unique structural and textural properties. Microstructural properties of CFC (arrangement of the graphite planes in filaments) are shown to depend on the nature of catalyst for methane decomposition. These properties widely vary for different catalysts: the angle between graphite planes and the filament axis can be 0° (Fe-Co-Al₂O₃), 15° (Co-Al₂O₃), 45° (Ni-Al₂O₃), 90° (Ni-Cu-Al₂O₃). The textural properties of CFC depend both on the catalyst nature and the conditions of methane decomposition (T, °C). The micropore volume in CFC is very low, 0.001-0.022 cm³ g⁻¹ at the total pore volume of 0.26-0.59 cm³ g⁻¹. Nevertheless, the BET surface area may reach 318 m² g⁻¹. Results of the TEM (HRTEM), XRD, Raman spectroscopic, SEM and adsorption studies of the structural and textural properties of CFC are discussed.     

Simultaneous adsorption of MIB and NOM onto activated carbon: II. Competitive effects

The adsorption of an odour compound common in drinking water, 2-methylisoborneol (MIB), was studied on six activated carbons in the presence of six well-characterised natural organic matter (NOM) solutions. It was found that, although the carbons and the NOM solutions had a wide range of characteristics, the major competitive mechanism was the same in all cases. The low-molecular-weight NOM compounds were the most competitive, participating in direct competition with MIB for adsorption sites. Equivalent background compound calculations indicated a relatively low concentration of directly competing compounds in the NOM. Some evidence of pore blockage and/or restriction was also seen, with microporous carbons being the most affected by low-molecular-weight NOM and mesoporous carbons impacted by the higher-molecular-weight compounds.     

NaOH activation of anthracites: effect of temperature on pore textures and methane storage ability

The main purpose of this work was to prepare various active carbons from the same precursor at various activation temperatures, and investigate both porosity development and corresponding methane storage capacities. An anthracite was thus chemically activated with sodium hydroxide under nitrogen flow at temperatures ranging from 600 to 830 °C, with a constant mass ratio: hydroxide/anthracite = 3. The pore textures of the corresponding activated carbons were investigated using the adsorption isotherms of four probe molecules characterised by their increasing molecular diameters, namely CO₂, N₂, C₆H₆ and CCl₄. The changes occurring in each kind of pores were discussed and put in relation with the activation temperature. The specific volumes of different micro- and mesopore families were measured and discussed. Depending on the temperature range, two different activation mechanisms were evidenced. Methane storage isotherms at 20 °C and up to 3.5 MPa were measured for the investigated materials. Linear correlations between various textural parameters and methane storage capacities were given. Additionally, a number of results previously reported in the literature were confirmed by the present work.     

Formation of oxygen complexes by ozonation of carbonaceous materials prepared from cherry stones: I. Thermal effects

In this study, the feasible use of ozone to form oxygen complexes in chars prepared from cherry stones (CS) is investigated. CS were charred at 450, 600 or 900°C for 2 h in nitrogen. Char samples were ozonated over the 25-250°C temperature range for 1 h. Elemental chemical analysis was effected for a few selected samples. The oxygen complexes were successfully analyzed by Fourier infrared spectroscopy (FT-IR) and by titration methods. Thermal decomposition of ozone in the gas stream was also studied and the mechanism of the ozonation process dealt with. The ozonation treatment of CS chars was found to yield products with a relatively high concentration of a number of oxygen complexes. These include phenolic hydroxyl, quinonic, carboxylic acid, and ether structures. The content of lactonic structures was very low in the ozonated samples. The type and quantity of oxygen complexes depended on the ozonation and charring temperatures. The formation of oxygen complexes was favored when the charring of CS was effected at 450°C and when the ozonation of the char prepared at 600°C was performed at 100°C. The ozone content in the gas stream was very sensitive to the temperature increase in the reactor. Several reaction routes have been proposed for the transformation of ether, aromatic, and olefinic structures present in CS chars into oxygen complexes.     

Overcoming calcium catalysis during the thermal reactivation of granular activated carbon: Part II. Variation of steam-curing reactivation parameters

In order to overcome the deleterious effects of calcium catalysis during thermal reactivation, the authors have developed a methodology for first steam-curing spent GAC at 548-748 K, and then ramping the furnace temperature to 1023-1223 K while exposing the GAC to flowing N₂. In this article, the authors evaluated the influence of an array of parameters on the steam-curing plus ramped-temperature protocol that included curing time, curing temperature, ramped-to temperature and steam flow rate. Pore size distribution (PSD) measurements employed the density functional theory (DFT), and these revealed that the steam-curing time had the greatest influence on pore size distribution: increasing the steam-curing time from 15 to 60 min increased the <500 Å cumulative pore volume by ca. 10% and the 5.4 to 32 Å pore volume by ca. 12%. Several of the other process parameters exhibited only a slight effect on PSD. Furthermore, a spent GAC that first experienced the steam-curing and ramped-temperature protocol and then experienced acid washing had identical micropore volume as a spent GAC that first experienced acid washing and then experienced conventional reactivation. This confirmed that the steam-curing protocol overcame calcium catalysis and its destruction of microporosity.     

Synthesizing activated carbons from resins by chemical activation with K2CO3

We prepared activated carbons from phenol-formaldehyde (PF) and urea-formaldehyde (UF) resins by chemical activation with K₂CO₃ with impregnation during the synthesis of the resins. The influence of carbonization temperature (773-1173 K) on the pore structure (specific surface area and pore volume) and the temperature range at which K₂CO₃ worked effectively as an activation reagent, were investigated. The specific surface area and micropore volume of PF-AC and UF-AC increased with an increase of carbonization temperature in the range of 773-1173 K. We prepared activated carbon with well-developed micropores from PF, and activated carbon with high specific surface area (>3000 m²/g) and large meso-pore volume from UF. We deduced the activation mechanism with thermogravimetry and X-ray diffraction. In preparing activated carbon from PF, K₂CO₃ was reduced by carbon in the PF char. The carbon was removed as CO gas resulting in increased specific surface area and pore volume above 1000 K. In preparing AC from UF, above 900 K the carbon in UF char was consumed during the K₂CO₃ reduction step.     

The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon

The physicochemical properties, surface chemical structure and some catalytic properties of a series of carbons prepared by nitric acid oxidation of an activated carbon and subsequent heat treatment under vacuum and mild temperature conditions (423-573 K) were studied. The porous structure characteristics of the partially evacuated samples were estimated from low-temperature nitrogen adsorption data. The thermal analysis and the quantitative determination of surface functional groups by selective neutralization of bases and pH-metric titration were carried out. The dehydration of 2-methylpropan-2-ol was used as a test reaction. While gradual annealing in vacuum alters the surface only slightly, it does differentiate strongly the number and the acidic strength of the surface groups. Progressive heating under mild conditions removes mainly those surface groups that are located in macropores or on the outer surface of the carbon. According to TPD results, the decomposed surface groups are single carboxylic groups, as expected. The decomposition of single, strong carboxylic groups is accompanied by rearrangements of other carboxylic groups with the simultaneous formation of additional cyclic structures like anhydrides, lactones or lactols. Catalytic tests support our previous findings that oxidized carbons have a high dehydration activity. This activity is controlled not only by the number and the strength of acidic groups, but also by their accessibility. There exists an optimum concentration of surface acidic groups above which the activity decreases due to steric restrictions.     

Oxidative degradation of carbon blacks with nitric acid: II. Formation of water-soluble polynuclear aromatic compounds

Furnace black and acetylene black were oxidized with concentrated nitric acid at 100 °C for prolonged periods. The oxidized carbon black was dissolved/dispersed into alkaline solution and was size-fractionated into six fractions by ultrafiltration. The yields of the fractions revealed that oxidized furnace black contains oxygenated polynuclear aromatic compounds with a variety of molecular sizes, but oxidized acetylene black consists of only a great quantity of the largest size fraction, probably carbon black particles, and a scarce amount of the smallest size fraction. With oxidized furnace black, elemental compositions of all fractions except the largest molecular-size fraction were independent of the period of oxidation, suggesting that each fraction possesses a similar molecular structure. Noncarbon constituents such as oxygen and hydrogen increased with decreasing molecular size. The mean molecular weights of fractions were estimated to be in a range from ca. 400 to 1200 and more on the basis of elemental and functional group analyses. ¹³C-NMR and IR analysis showed that the molecules of fractions comprise phenolic, carboxylic, nitro, perhaps quinonic carbonyl groups, and aromatic carbons, but no aliphatic carbons. Ultraviolet and visible spectra of fractions denoted that absorption at higher wavelengths increased with increasing the molecular weights, indicating extension in the conjugated aromatic ring system. On the basis of the experimental results molecular structure models for the fractions were proposed.     

Adsorption of Hg(II) ions from aqueous chloride solutions using powdered activated carbons

Activated carbons were developed from Casurina equisetifolia leaves, by chemically treating with sulfuric acid (1:1) or zinc chloride (25%), at low (425 °C) and high (825 °C) temperatures. The resulting powdered activated carbons were applied for removing mercuric ions from aqueous solution at different agitation times and mercuric ion concentrations. The equilibrium data fitted well the Langmuir adsorption isotherm. The Langmuir adsorption capacities were 12.3 and 20.3 mg g⁻¹ for low temperature carbons and 43.9 and 38.5 mg g⁻¹ for high temperature carbons impregnated with H₂SO₄ and ZnCl₂, respectively. Studies of the effects of carbon dosage, NaCl concentrations and solution pH values were carried out for the more effective, high temperature carbons. Increasing NaCl concentration resulted in a significant decrease in the adsorption efficiency. Adsorption was high from solutions with low and neutral pH values and lower for solutions with alkaline pH values for the high temperature carbons.