Influence of pore structure and pretreatments of activated carbons and water effects on breakthrough dynamics of tert-butylbenzene

Several activated carbons differently pretreated (de-ashed, oxidized, reduced, wetted, frozen, and dried) were investigated using static (equilibrium adsorption of nitrogen and benzene) and dynamic (tert-butylbenzene (TBB)) adsorption methods. Treatments of carbons at relatively mild conditions leading to not great changes in their textural characteristics affect the TBB breakthrough concentration because of changes in the chemistry of the surfaces (oxidized or reduced) and the presence of water in airstream or pre-adsorbed on carbon beds. Oxygen-containing functionalities at a carbon surface change condition of competitive adsorption of nonpolar TBB and polar water molecules. Calculations of distribution functions of adsorption potential (A), energy (E), Gibbs free energy (Delta G) of adsorption of benzene and TBB and effective adsorption (first-order) rate pseudoconstant beta e over different ranges of relative exit TBB concentration c(t)/c(0) (from approximately 10(-5) to 0.01-0.4) reveal nonlinear effects caused by the size of carbon granules, the pore size distributions, the presence of water, oxidation or reduction of the surfaces and other treatments resulting in distribution functions f(y) with nonzero intensity in relatively broad ranges of the A, E, Delta G, and beta e values. There are many factors affecting the breakthrough parameters; therefore, simple linear relationships between these parameters and the structural characteristics (S(BET), S(DS), Vp, and V(DS)) are not observed.     

Cyclodextrins adsorbed onto activated carbons: preparation, characterization, and effect on the dispersibility of the particles in water

We have investigated the adsorption equilibrium of selected cyclodextrins onto activated carbons. A number of parameters were examined including the type of carbon material, the size of macrocyclic cavity, and the chemical nature of the oligosaccharide (e.g., neutral, anionic, or cationic cyclodextrin). Adsorption isotherm studies revealed that the maximum amount of cyclodextrin immobilized on the carbon surface is obtained for the native β-CD, while the adsorption capacity of the ionic cyclodextrins derivatives strongly depends on the net surface charge of the activated carbon. The affinity of cyclodextrins for activated carbons was further utilized to prepare modified activated carbons containing controlled amounts of cyclodextrins through an adsorption process. The resulting materials were characterized by N(2) adsorption-desorption volumetric measurements, FTIR and Raman spectroscopy, while the quantitative determination of the oligosaccharide content on activated carbons was performed by gravimetric measurements. On the basis of the Turbiscan results, it was found that the chemical structure of cyclodextrins, which are incorporated in the carbon framework, had significant influence on the dispersibility and stabilization of the solid particles in water. Agglomeration and precipitation of the carbon particles were markedly suppressed with substituted cyclodextrins whose hydroxyl groups were partially substituted by methyl or alkylammonium groups.     

Adsorption of water vapor on modified activated carbons

Summary The adsorption isotherms of water vapor on modified activated carbons are measured in order to study the role of various surface groups in the primary adsorption of water molecules on these adsorbents. These adsorption isotherms are analysed by means of the Dubinin-Serpinsky and Jovanovic equations, which take into account the special features of water vapor adsorption on microporous activated carbons. Numerical analysis of the measured adsorption isotherms by means of the above mentioned equations showed their limited applicability for interpreting adsorption mechanism of water molecules on activated carbons.

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Adsorption von Wasserdampf auf modifizierter Aktivkohle

Zusammenfassung Die Adsorptionsisothermen von Wasserdampf auf modifizierter Aktivkohle wurden gemessen, um die Rolle verschiedener Oberflächentypen auf die Primäradsorption von Wassermolekülen auf diesen Adsorbenzien zu untersuchen. Die Adsorptionsisothermen wurden mittels der Dubinin-Serpinsky- und Jovanovic-Gleichungen analysiert, welche die speziellen Eigenheiten von Wasser auf mikroporöser Aktivkohle berücksichtigen. Die numerische Analyse der gemessenen Adsorptionsisothermen mittles der genannten Gleichungen zeigte ihre limitierte Anwendbarkeit zur Interpretation von Adsorptionsmechanismen von Wassermolekülen auf modifizierter Aktivkohle.

     

Influence of surface heterogeneity on hydrogen adsorption on activated carbons

This study presents an experimental and theoretical analysis of the effect of surface heterogeneity on the capacity of 20 commercial activated carbons to adsorb hydrogen at 77 and 258 K and for maximum pressures of 20 bar. Some of the samples have been subjected to surface modification by impregnation or by surface oxidation prior to the hydrogen adsorption measurements. All the activated carbons have been analyzed by N₂ adsorption at 77 K using the thermodynamic isotherm presented in a previous study. The hydrogen adsorption capacity of the activated carbons has been well correlated to the micropore volume and the characteristic m₂ parameter of the thermodynamic isotherm accounting for the energy heterogeneity of the material. On the basis of the model presented here, we discuss how surface heterogeneity, in addition to the adsorption strength, might affect the ability of activated carbons and related materials to adsorb hydrogen.     

Influence of surface chemistry on the SAXS response of polymer-based activated carbons

Small-angle X-ray scattering (SAXS) measurements using contrast variation are reported for activated carbons prepared from poly(ethyleneterephthalate). The carbon surfaces are functionalized to different degrees by cold and hot nitric acid treatment. The latter treatment reduces the surface area by 75%, but the pore size distribution in the micropore range is hardly affected. Seven liquids, n-hexane, i-octane, i-propanol, cyclohexane, toluene, alpha-pinene, and nitrobenzene, in addition to water vapor, were used as contrast modifiers. Although the values of the specific surface area S(X) deduced from these measurements are relatively insensitive to the solvent, the detailed SAXS spectra reveal interactions occurring on different spatial scales that depend on the surface chemistry of the carbon and on the physicochemical properties of the solvent. In the most heavily oxidized sample, the amphiphilic molecule i-propanol stabilizes the surface structure, whereas nonpolar molecules make the rough surface smoother. In the untreated and the lightly functionalized carbons, water vapor at 50% relative humidity condenses only partially in the micropores at room temperature, whereas in the heavily treated sample condensation in the micropores is practically complete.     

Influence of surface chemistry on the adsorption of oxygenated hydrocarbons on activated carbons

The objective of this work was to study the adsorption of different oxygenated hydrocarbons (methanol, ethanol, 1 and 2-butanol, methyl acetate) on activated carbons from organic mixtures with cyclohexane. Three activated carbons prepared by thermal and chemical treatments of a commercial carbon were employed for this purpose. Their textural properties were found to be similar, whereas their surface chemistries were modified, as shown by temperature-programmed desorption coupled to mass spectrometry (TPD-MS) and X-ray photoelectron spectroscopy (XPS). The adsorption isotherms were obtained by depletion method, and the analysis of adsorbed species was evaluated by TPD-MS to obtain new insight into the interactions between the different hydrocarbons and the carbon surface. Ethanol leads to a high-energy interaction between its hydroxyl function and the oxygenated surface groups and also to a lower energy interaction between the aliphatic part of the molecule and the carbon material. The desorption activation energy for this hydrophilic interaction is high (50 to 105 kJ/mol), and it is related to the nature of the carbon surface groups. The relative importance of these two interactions depend on the size of the alcohol/methanol is similar to ethanol, whereas butanols lead to more dispersive interactions. Methyl-acetate cannot undergo this kind of strong interaction and behaves like cyclohexane, having desorption activation energies ranging between 25 and 45 kJ/mol no matter the molecule and the carbon surface chemistry.     

Porous structure of activated carbons and tert-butylbenzene breakthrough dynamics

The structural and adsorptive characteristics of six activated carbons were studied by means of nitrogen and benzene adsorption and water desorption. Tert-butylbenzene (TBB) breakthrough dynamics was analyzed by using several integral equations solved with a regularization/singular-value decomposition procedure. TBB interaction with texturally different activated carbons with the presence of preadsorbed or adsorbed water under dynamic conditions was illustrated by the breakthrough plots handled with several models. The influence of the type of activated carbons, their pore size distributions, water vapor, and TBB flow rate on the breakthrough times (tb) and the dynamic capacity of the carbon beds has been explored with better results for a carbon sample possessing a maximal contribution of mesopores at half-width x>1.5 nm among the carbons studied (which also appears on benzene adsorption) and a major contribution of microporocity as VDS/Vp approximately 0.88 and SK/SBET approximately 0.15. Another adsorbent, which is characterized by a similar total porosity but a larger micropore volume, a smaller contribution of mesopores (SK/SBET approximately 0.08), greater total and miroporous specific surface areas, and greater intensity of the pore size distribution at x<1.5 nm, shows the second result in dynamic TBB retention.     

Influence of KMnO4 oxidation on the electrochemical performance of pitch-based activated carbons

In this work, activated carbons (ACs) are obtained from petroleum pitch by the combination of a chemical treatment with different potassium permanganate (KMnO₄) amounts, i.e., 0, 0.5, 1.0, and 2.0 g, and a chemical activation with KOH at a constant KOH/pitch ratio of 3/1. The effects of the chemical activating agent on the surface morphology and porosity are evaluated with scanning electron microscopy and N₂ adsorption isotherms at 77 K, respectively. The specific surface area of the pitch-based ACs is increased with increasing the amount of KMnO₄ pre-treatment and showed the highest value of 2,334 m² g^?1 at 2 g of KMnO₄ amount. The electrochemical performance of AC electrodes is examined by cyclic voltammetry and galvanostatic charge/discharge characteristics in 6 M KOH electrolyte. Among the prepared ACs, 2.0 K-ACs possesses a specific capacitance as high as 237 F g^?1 and showed excellent electrochemical performance due to its suitable porous structure and low interface resistance.     

Adsorption of vapor mixtures of water and organic substance on activated carbons

The isotherms of water adsorption in the presence of an organic substance vapor with a specified concentration are calculated from experimental data on the joint frontal dynamics of adsorption of water vapor and several organic substances (benzene, hexane, cyclohexane, tetrachloromethane, and perfluorotripropylamine) on two samples of activated carbons. The influence of the organic substances on the equilibrium water adsorption decreases with an increase in the molecule size.     

Characteristics of interfacial water affected by proteins adsorbed on activated carbon

The influence of proteins (bovine serum albumin, BSA, and mouse gamma-globulin, IgG) physically adsorbed or covalently attached via coupling with N-cyclohexyl-N'-(2-morpholinoethyl) carbodiimide methyl-p-toluenesulfonate, CMC, to the surface of activated carbon SCN (spherical carbon with nitrogen) on the mobility of interfacial water molecules was studied by means of 1H NMR spectroscopy with freezing-out of bulk water at 180 < T < 273 K. Relaxation processes in the interfacial non-freezing water were investigated measuring transverse time t2 of proton relaxation dependence on the presence of proteins and CMC. The distribution function of activation free energy of relaxation (with a maximum at 20-22 kJ/mol) was calculated for the protein-water-carbon systems using a regularization procedure and the relationships between t2 and the amounts of the interfacial water unfrozen at T < 250 K assuming the Arrhenius-type dependence for t2(-1) on temperature. The state of unfrozen water in pores of SCN shows that the low temperature relaxation processes occur in narrow pores with half-width X < 1.5 nm.     

Improving methane storage on wet activated carbons at various amounts of water

Different mesoporous activated carbons were prepared by both chemical and physical activation processes and were examined for methane uptake in the presence of water.Methane isotherms were obtained at ...     

Energy characteristics of adsorbed water in active carbons according to the NMR relaxation data

Nuclear magnetic relaxation measurements were used to determine activation energy E _act of the motion of water molecules adsorbed in active carbons. The E _act value was found to depend on the filling of active carbon pores due to changes in the state of water molecules under adsorption. It was established that the E _act = f(p/p _s) plots, where p/p _s is the relative pressure of water vapor observed for microporous active carbons (FAS-1, 2, N-15, SKT-6A), are similar in form to the corresponding plots of changes in water adsorption heats. In particular, we concluded that the plateau in the E _act = f(p/p _s) dependences, as in the case of adsorption heats, reflects the volumetric filling of active carbon micropores with water. We show that a linear function describes the increase in E _act values for water upon the complete filling of micropores with an increase in the volume of adsorbed water clusters per one primary adsorption center (W ₀/a _m ). We establish that, for water in the FAS-3 sample, the deviation of E _act values from this linear function was due to the contribution from the vapor phase in the mesopores (x ₀ = 0.7−1.2 nm) that make up a considerable part of the active carbon’s porous system.     

Preparation of activated ordered mesoporous carbons with a channel structure

Activated ordered mesoporous carbons with a channel structure (AOMCs-CS) were successfully prepared by imposing CO(2) activation on ordered mesopore carbon C-FDU-15. It is found that the continuous carbon framework of the precursor C-FDU-15 plays an important role in keeping the order structure of the resulting AOMCs-CS. The mild activation (e.g., 31 wt % burnoff) does not impair the order degree. After that, the order degree gradually decreases with further increasing burnoff. However, the basic hexagonal mesostructure of C-FDU-15 can still be found in the AOMCs-CS when the burnoff is up to 73 wt %, although many carbon walls are punched and thus many larger mesopores and marcropores are generated. With increasing burnoff, the surface area and volume of micropores increase first and then decrease, and the surface area and volume of mesopores continuously increase. The highest measured Brunaruer-Emmett-Teller (BET) surface area, micropore volume, and total pore volume of the AOMCs-CS reach 2004 m(2)/g, 0.50 cm(3)/g, and 1.22 cm(3)/g, respectively.     

Prediction of the micropore structure parameters and adsorption properties of activated carbons

A regularity govering variations of volume and linear size of micropores in carbon adsorbents during their vapor-gas activation was found. A parameter was proposed that characterizes the degree of development of the micropore system in activating carbons and an initial carbonized material. The parameter is defined as the number (or surface area) of micropores in the volume unit of the micropore zones. This parameter allows one to rationalize the choice of carbonized materials for the preparation of activated carbons with specified adsorption properties and to establish the range of activation beyond which the structure of the micropores loses stability. Furthermore, the parameter serves to predict how activation affects micropore structure parameters and adsorption properties of carbons. This in turn indicates the optimal degrees of microporosity of carbons needed to attain required adsorption properties.     

Chemisorption of o-xylene on activated carbons

It was determined volumetrically that at temperature of gas-phase oxidative catalysis (473–573 K) o-xylene is chemisorbed on SKN and oxidized AG-3 carbons. The chemisorption is weak and reversible and is not accompanied by dissociation of o-xylene molecules, The active chemisorption centers are surface heteroatoms of nitrogen (in SKN carbon) or oxygen (in oxidized AG-3 carbon), promoting polarization of the C-H bond of the methyl group of o-xylene.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 3, pp. 370–374, May–June, 1986.     

全硅FER沸石上有机吸附物脱附行为的研究

运用热分析技术,研究吸附在体相和表面结构完美的单晶状疏水全硅FER沸石孔道中的有机化合物的脱附行为,测定亲和性指数AT值和负载量。所研究的吸附质为直链烷烃、直链烷基醇、直链烷基胺等,结果显示醇有较低的AT值,而直链烷烃有较高的AT值,胺类有最高的AT值。证明全硅FER沸石骨架对烷基、胺基呈现出强的"亲和性",而对羟基呈现出“憎性”。同时还发现吸附质的链长对脱附性质、AT值也有较大影响。     

Influence of electrochemically activated water on lignin

It has been shown that under the action of electrochemically activated water a change in the functional composition of the macromolecule of hydrolysis lignin and of lignin sulfonate takes place and, in addition, the latter is desulfurated.M. V. Frunze Simferopol' State University. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 425–427, 1987.     

Reactive adsorption of penicillin on activated carbons

A series of activated carbons with varied surface chemistry, obtained by wet oxidation and thermal treatment, was used for the removal of penicillin from low concentration aqueous solution. It was found that the carbon surface chemistry favors the degradation of the antibiotic, giving rise to various intermediates detected both in solution and in the adsorbed phase (deposited with the pore structure of the activated carbons). The confinement of penicillin molecules entrapped in the nanopores of activated carbons of acidic nature accelerates their degradation compared to that one in the bulk solution, which can be linked the strong local pH fall inside the pores. Degradation also takes place in activated carbons of basic pH, although the nature and partition of the intermediates formed differ from those in the acidic carbons. In both cases most of the breakdown products do not present therapeutic activity.