Methanol adsorption by amorphous silica alumina in the critical temperature range

The methanol adsorption capacity of an amorphous silica-alumina was measured using an equilibrium technique. The experimental temperature range was of 140 to 260°C and the pure methanol pressure range was 0.1 to 1.2 MPa. A multilayer adsorption was found, also for temperatures above the critical temperature of the adsorbate. Based on the Jovanovic adsorption model, the mean residence times of the adsorbed molecules were calculated. Surprisingly, the heat of adsorption was found to be independent of the temperature in the multilayer adsorption range.Symbols Units a parameter defined by Equation (7) - a parameter defined by Equation (10) Pa - a(T) parameter in the Peng-Robinson equation Pa/(mol/m³)² - b parameter defined by Equation (8) - b parameter defined by Equation (11) Pa^–1 - b(T) parameter in the Peng-Robinson equation m³ mol^–1 - c constant in the BET equation - k Boltzmann constant, being 1.3806 · 10^–23 JK^–1 - K characteristic constant in the Peng-Robinson equation - m mass of one adsorbate molecule kg - p pressure Pa - q adsorption capacity g/g - Q heat of adsorption J mol^–1 - R gas constant, being 8.314 J mol^–1 K^–1 - T absolute temperature K - V molar volume m³ mol^–1 - x relative pressure (=p/p ₀) - active molecule area m² - residence time s - acentric factor     

Isothermal pressure dependence of the solubility of some triazines in supercritical CO2

 The solubility of two related series of each three triazine compounds has been determined in supercritical CO₂, at 40 °C and pressures between 80 and 220⋅10⁵ Pa. A high pressure small volume cell with adjustable optical path and built-in circulation pump has been constructed to be used with quartz windows in a Perkin Elmer UV-VIS spectral photometer. At 220 bar solubilities are between 0.3 and 25 g/L, dependent on the specific triazine structure. The isothermal pressure dependence of the solubility could be modelled using the pressure dependent dielectric constant of CO₂ as the only variable. Received: 30 October 1996/Revised: 3 April 1997/Accepted: 13 April 1997     

Adsorption of carbon dioxide on microporous carbon adsorbent PAU-10

Carbon dioxide adsorption on the microporous carbon adsorbent PAU-10 within the 177.8—423 K temperature and 0.1—5.13·10⁶ Pa pressure intervals was studied. The isosteres of absolute adsorption are well approximated by straight lines, which do not change their slope on going to temperatures higher than the critical temperature of CO₂. An increase in the differential molar isosteric heat of adsorption (q _st) at 0 < a < 1 mmol g^–1 is explained by the influence of the endothermic effect of adsorption expansion of the adsorbent. In the region of high pressures and nonideal gas phase, q _st is temperature-dependent.     

Thermodynamics of CO<Subscript>2</Subscript> adsorption on zeolite NaX in wide intervals of pressures and temperatures

The dependences of the differential molar isosteric heat of adsorption and entropy of adsorption of CO₂ on zeolite NaX were determined in wide temperature (196–423 K) and pressure (0.1 Pa to 5.4 MPa) intervals. In the initial region of adsorption (a < 1 mmol g^–1), the differential molar heat of adsorption increases from 40 to 43 kJ mol^–1 and then decreases to 33 kJ mol^–1. The heat of adsorption remains virtually unchanged at 3 mmol g^–1< a < 6.5 mmol g^–1 and decreases sharply at high fillings of zeolite micropores (a > 7 mmol g^–1). The heat of adsorption was found to be temperature-dependent. The region with the constant heats shrinks with the temperature increase, and the heats begin to decrease at lower fillings of micro pores. The dependences of the change in the differential entropy of the adsorption system on the amount adsorbed were calculated at different temperatures. The specific features of the behavior of the thermodynamic functions of this adsorption system in the initial and medium region of fillings kre associated with interactions of adsorbate molecules with Na⁺ cations and walls of large cavities. For high fillings, an increase in repulsion forces between adsorbed molecules results in a sharp expansion of the adsorbent and a decrease in the heat of adsorption.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1570–1573, August, 2004.     

Infrared Spectroscopic Study on Adsorption of Acetonitrile from Solution onto Several Oxide Surfaces at High Pressure

Adsorption of acetonitrile from toluene solution at a liquid-solid interface under pressures of up to 300 MPa was investigated by IR spectroscopy. The CN stretching vibration bands (v_CN) of adsorbed acetonitrile were observed at higher frequencies than those of the same species in the liquid phase. The shift on alumina-pillared montmorillonite (ALPM) was the largest (ca. 8 cm^-1) for the adsorbents studied. The v_CN intensities of adsorbed acetonitrile on ALPM and on alumina (Al₂O₃) considerably increased with increased pressure, indicating an increase acetonitrile adsorption due to compression. It was concluded that the total volume of the system was reduced by adsorption, and that the reduction was brought about not only by the formation of an adsorption bond but also by the change in the solvation of the adsorbate in the adsorbent pore.     

Multicomponent Equilibrium Adsorption on Heterogeneous Adsorbents

A generalized method for prediction of multicomponent adsorption is suggested based on representing that adsorbent volume as energetically inhomogeneous. The method depends on extending the Polanyi potential theory to mixture adsorption. The main feature of the method is that, at constant partial pressure and temperature the composition of an adsorbed phase is not uniform over its volume. Results of applying this theory to non-porous adsorbents have been considered. The prediction ability of the theory is confirmed for the strongly non-ideal system acetone–chloroform–graphitized carbon black. It was shown that the departure from ideal behavior of adsorbed phase is quite close to that for the liquid mixture. Another system considered was oxygen–nitrogen–anatase at 78 K. Although this mixture is ideal, it has been found that there is significant variation in composition over the adsorbed layer due to the difference in the interactions of the quadrupolar N₂ molecule and nonpolar O₂ molecule with the anatase surface.     

A new apparatus for measuring the vapour pressure of liquid mixtures excess Gibbs free energy of octamethylcyclotetrasiloxane + cyclohexane at 308.15 K

A new apparatus for measuring the vapour pressure of liquid mixtures is described. In conjunction with an automatic pressure controller, a capacitance manometer is used as a null device to isolate the liquid and vapour. The vapour pressure is measured with a precision mercury manometer. The continuous-dilution technique for sample introduction has been incorporated in the new apparatus, so that the composition range of a mixture can be covered in two runs. The accuracy of each measured quantity is: pressure, 3 Pa; temperature (IPTS-68), 0.002 K; volume, 0.002 cm³. G^E for cyclohexane + octamethylcyclotetrasiloxane (abbreviated throughout this paper as omcts) at 308.15 K has been determined: the minimum value of ?68 J mol^?1 occurs near x₂(omcts) = 0.5.     

Preparation and characterization of activated carbon from bagasse fly ash

Activated carbons from bagasse fly ash (BFA) were prepared by one step chemical activation using ZnCl₂ as activating agent, or combination method of chemical with CO₂ physical activation (physicochemical activation). The development of porosity was studied in correlation with the method of activation, activation temperature, and also the chemical weight ratio. A typical sample by the combination method at 600 °C and weight ratio of ZnCl₂:BFA = 2 exhibited micropore volume of 0.528 cc/g, mesopore volume of 0.106 cc/g and surface area of 1200 m²/g. For determining the adsorption capacity of the carbon samples in solutions, phenol and methylene blue equilibrium adsorption experiments were conducted. The properties and adsorption capacity of the synthesized activated carbons has been compared to commercial activated carbon (Norit^® SX Plus).     

Adsorption of perfluoropropane on the PAC microporous carbon adsorbent

The adsorption of perfluoropropane (N₃F₈) on the PAC microporous carbon adsorbent, which is close in properties to monoporous adsorbents, was studied at temperatures of 216, 243, 295, 318, and 343 K in the pressure interval from 1 to 1·10⁵ Pa. The adsorption isosters are well approximated by straight lines in the studied interval of temperatures and pressures. The dependence of the isosteric heats of adsorption on filling is described by a curve with a maximum in the region of high fillings. Such a curve is characteristic of adsorbents with a narrow pore size distribution.     

Adsorption of Gases, Vapors and Liquids by Microporous Adsorbents

Adsorption of Xe, Kr, Ar, N₂, O₂, H₂ CH₄, CO₂, He, and freons by PAU-10 and ACC microporous carbon adsorbents as well as by A and X zeolites was investigated over a wide range of pressures (0.1 Pa – 20 MPa) and temperatures (77, 120–600 K). The amount of gases, vapors and liquids adsorbed by microporous adsorbents increases steadily with increasing pressure and does not change dramatically if phase transitions occur in the adsorptive. Isosteres of adsorption constructed as a curve of ln P against f(1/T)_a retain a linear form over a wide range of pressures and temperatures. The slope of isosteres does not vary on going through the critical temperature of the gaseous phase. At high pressures, due to non-ideality of the gaseous phase and non-inert behavior of the adsorbent the differential molar heat of adsorption is dependent on temperature. At high fillings of micropores the differential molar isosteric heat capacities of adsorption systems show maxima that indicate the occurrence of structural rearrangements in the adsorbate.     

Removal of zinc metal ion (Zn) from its aqueous solution by kaolin clay mineral: A kinetic and equilibrium study

A laboratory batch study has been performed to study the effect of various physic-chemical factors such as initial metal ion concentration, solution pH, and amount of adsorbent, contact time and temperature on the adsorption characteristics of zinc (Zn²⁺) metal ions onto kaolin. It has been found that the amount of adsorption of zinc metal ion increases with initial metal ion concentration, contact time, solution pH but decreases with the amount of adsorbent and temperature of the system. Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn²⁺) on kaolin is a two steps process: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow decreasing intra-particle diffusion in the interior of the adsorbent which has also been confirmed by intra-particle diffusion model. The equilibrium time is found to be in the order of 60 min. Overall the kinetic studies showed that the zinc adsorption process followed pseudo-second-order kinetics among pseudo-first-order and intra-particle diffusion model. The different kinetic parameters including rate constant are determined at different initial metal ion concentration, solution pH, amount of adsorbent and temperature respectively. The equilibrium adsorption results are analyzed by both Langmuir and Freundlich models to determine the mechanistic parameters associated with the adsorption process. The value of separation factor, R_L from Langmuir equation also gives an indication of favorable adsorption. Finally thermodynamic parameters are determined at three different temperatures and it has been found that the adsorption process is exothermic due to negative ΔH° accompanied by decrease in entropy change and Gibbs free energy change (ΔG°).     

Electrochemical Investigation for Cu2+ Oscillatory Phenomena at the Liquid/Liquid Interface with a Specific Adsorption of Ion Pair Model

In this paper, a novel current oscillatory phenomenon for Cu²⁺ at the water/1,2‐dichloroethane interface is reported with cyclic voltammetry and potential‐step chronoamperometry. The small irregular current spikes were only observed near the site of the oxidation peak of CuCl₂^? and were mainly related to the Cu²⁺concentration in the aqueous phase. Our experimental results demonstrated that the current oscillation is caused by specific adsorption of ion pairs at the W/DCE interface between Cu²⁺ in the aqueous phase and TPB^? in the organic phase. Therefore, a specific adsorption of ion pair model has been formulated for the current oscillation at the liquid/liquid interface. The DFT calculation method was used to explain the mechanism of ion pair formation. The calculation results suggested that the TPB^?Cu²⁺TPB^? ion pair has the lowest‐energy state, thus providing qualitative support for the ion pair model. A probable mechanism for the observed current oscillation was also discussed in this paper. At the same time, a spectrophotometric experiment was performed to evidence a strong attractive interaction between Cu²⁺ and TPB^?.     

Adsorption of chlorobenzene and benzene on γ-Al2O3/atl>

Adsorption of chlorobenzene and benzene on -Al₂O₃ was investigated in the 413--572 K temperature region at an adsorbate partial pressure ranging from 2 to 1000 Pa. The adsorption isotherms were measured and the isosteric heats and the entropy characteristics of adsorption were determined. The experimentally found and theoretically calculated entropy changes upon adsorption were compared. The mobility of the molecules of both adsorbates in the adsorption layer was limited with respect to that predicted by the ideal two-dimensional gas model. The mechanism of adsorption of benzene and chlorobenzene is discussed.     

Rate constant and a detailed error analysis for C2H3 + H reaction

The production and reactions of vinyl radicals and hydrogen atoms from the photolysis of vinyl iodide (C₂H₃I) at 193 nm have been examined employing laser photolysis coupled to kinetic-absorption spectroscopic and gas chromatographic product analysis techniques. The time history of vinyl radicals in the presence of hydrogen atoms was monitored using the 1,3-butadiene (the vinyl radical combination product) absorption at 210 nm. By employing kinetic modeling procedures a rate constant of 1.8 × 10^?10 cm² molecule^?1 s^?1 for the reaction C₂H₃ + H has been determined at 298 K and 27 KPa (200 torr) pressure. A detailed error analysis for determination of the C₂H₃ + H reaction rate constant, the initial C₂H₃ and H concentrations are performed. A combined uncertainty of ±0.43 × 10^?10 cm² molecule^?1 s^?1 for the above measured rate constant has been evaluated by combining the contribution of the random errors and the systematic errors (biases) due to uncertainties of each known parameter used in the modeling. © 1995 John Wiley & Sons, Inc.     

Methods to Analyse the Texture of Alginate Aerogel Microspheres

Nitrogen adsorption at 77 K has been applied to the study of the texture of alginate aerogel microspheres obtained by CO₂ supercritical drying of alcogels. The limited volume shrinkage suggests that the aerogels preserve the texture of the hydrogels. Alginate aerogels presents a N₂ adsorption at small pressure higher than reference non-porous silica, to be attributed to the polarity of the surface or to a small microporous volume. The aggregated nanobead strings of the guluronic-rich gels accounts for a significant mesoporosity. The N₂ adsorption results correspond to electron microscopy observations for features smaller than 50 nm.     

Surface characterization and heats of adsorption of chromatographic alumina gel

The porous nature of chromatographic alumina gel has been investigated by adsorption/condensation processes and electron microscopy. Having 63% porosity, the gel is very porous. Total pore volume as determined by the fluid-displacement method is 0.497 cm³ g^–1. Its specific surface area, as determined by water vapor adsorption, is 225 m² g^–1. Micropore volume, as determined by utilizing Gurwitsch's rule, turns out to be 0.262 cm³ g^–1. The greater portion of the surface area and pore volume occurs in small and transitional pores, with average pore radii (hydraulic) less than 2.1 nm.Organic vapors, such as methyl ethyl ketone, acetone, methyl acetate, and methyl alcohol, were adsorbed on the gel between 0 and 36°C under vacuum, and the data were recorded on a Cahn-1000 electrobalance device. Isosteric heats of adsorption were calculated by applying the Clausius Clapeyron equation to the adsorption isosters at different surface coverages. Two types of adsorption processes, one with low activation energy and other with high activation energy can be distinguished. The increase in values ofq _st indicates that increasing temperature changes physical adsorption into chemisorption.     

Criteria of choice of absorption parameters in the grahame—parsons model: Cesium adsorption on bismuth

General equations for the two-dimensional pressure, the potential shift and for the change of the differential capacity due to the specific adsorption of ions at a constant electrode charge, are derived. On the basis of the equations derived, a new calculation method for the adsorption parameters in the Grahame—Parsons model is elaborated. With the example of the Cs⁺ adsorption data for the interface Bi/H₂O, the sensitivity of the various verification criteria of the adsorption parameters are compared and the advantages of the new calculation method are demonstrated.     

Adsorption characteristics of chloroform on modified zeolites from gaseous phase as well as its aqueous solution

Adsorption characteristics of chloroform from its aqueous solution on Na–Y and Li–Na–Y modified by SiCl₄ were measured and compared with those on Na–ZSM-5 and Na-Mordenite.No adsorption occurred on Na–Y with high hydrophilicity, while the siliceous faujasites became capable of adsorption and its amount increased with increase in the Si/A1 ratio. Adsorption isotherms are of Langmuir type, suggesting that adsorption proceeds by pore filling. The adsorption amounts expressed in volume on Na–Y with high hydrophobicity corresponded to their pore volumes.Adsorption characteristics of chloroform from gaseous phase on Na–Y with different Si/A1 ratio were also measured. The adsorption capability decreased with increasing Si/A1 ratio.Immersional heats of zeolites into water or chloroform were measured in order to evaluate the surface affinity to both solvents. Immersional heats into water were almost constant (about 500 mJ·m^–2) for zeolites with their Si/A1 ratio below 10. The heats decreased with an increase in the Si/A1 ratio above 10, then became almost constant (about 120 mJ·m^–2) over 30 in their ratio. Heats of immersion of Na–Y series into chloroform were almost constant irrespective of their Si/A1 ratio, but decreased slightly when the ratio exceeded 20.Adsorption characteristics of chloroform could be well related to immersional heats into both solvents.     

Tuning of Microenvironment in Covalent Organic Framework via Fluorination Strategy promotes Selective CO2 Capture

Herein, we have designed and synthesized two heteroatom (N, O) rich covalent organic frameworks (COF), PD-COF and TF-COF , respectively, to demonstrate their relative effect on CO₂ adsorption capacity and also CO₂/N₂ selectivity. Compared to the non-fluorinated PD-COF (BET surface area 805 m² g⁻¹, total pore volume 0.3647 ccg⁻¹), a decrease in BET surface area and also pore volume have been observed for fluorinated TF-COF due to the incorporation of fluorine to the porous framework (BET surface area 451 m² g⁻¹, total pore volume 0.2978 ccg⁻¹). This fact leads to an enormous decrease in the CO₂ adsorption capacity and CO₂/N₂ selectivity of TF-COF , though it shows stronger affinity towards CO₂ with a Qst of 37.76 KJ/mol. The more CO₂ adsorption capacity by PD-COF can be attributed to the large specific surface area with considerable amount of micropore volume compared to the TF-COF . Further, PD-COF exhibited CO₂/N₂ selectivity of 16.8, higher than that of TF-COF (CO₂/N₂ selectivity 13.4).     

Photo-responsive azo MOF exhibiting high selectivity for CO2 and xylene isomers

A combination of azo and acylamide ligands is used in the preparation of metal–organic frameworks. Light response research reveals that under UV–vis irradiation, the CO₂ adsorption of 1 declines as much as 21.4%. 1 exhibits excellent CO₂ adsorption selectivity over CH₄, O₂, CO, and N₂ gasses with IAST selectivity of 21–580 at 293 K. This MOF also has promising potential in separation of xylene isomers in the liquid phase with the adsorption of p-xylene of 265.15, o-xylene of 101.25 and m-xylene of 0 mg g^?1, respectively.