乙酸乙酯水溶液中水在3A分子筛上的吸附平衡与动力学

乙酸乙酯水溶液中水在3A分子筛上的吸附平衡与动力学;乙酸乙酯;水;吸附平衡与动力学;表面扩散系数;3A分子筛     

Adsorption of the enantiomers of 3-chloro-1-phenyl-propanol on silica-bonded chiral quinidine carbamate

The interactions of 3-chloro-1-phenyl-propanol with a quinidine carbamate-bonded chiral stationary phase under NPLC conditions were studied by measuring the adsorption isotherm data of its enantiomers by frontal analysis, modeling these data with a suitable isotherm model, and comparing the experimental overloaded elution band profiles with those calculated with this isotherm and the equilibrium dispersive model of liquid chromatography. The affinity energy distribution was calculated from the adsorption isotherm data. The results show that the surface of the adsorbent is heterogeneous and exhibits a bimodal adsorption energy distribution. This fact is interpreted in terms of the presence of two different types of adsorption sites on the stationary phase, nonselective and enantioselective sites. Albeit the bi-Langmuir isotherm model successfully accounts for the single-component data corresponding to both enantiomers, the competitive bi-Langmuir isotherm model does not allow an accurate prediction of the overloaded band profiles of the racemic mixture. Thermodynamic data are drawn for explanation. Some aspects of the retention mechanism are discussed in the light of the data obtained.     

Sildenafil citrate (Viagra) as a corrosion inhibitor for carbon steel in hydrochloric acid solutions

Abstract  

The inhibition effect of sildenafil citrate (Viagra) on the corrosion of carbon steel in 1 M HCl was studied by weight loss, polarization, electrochemical impedance spectroscopy, and UV–Vis spectrophotometry. The results showed that sildenafil citrate is a good corrosion inhibitor for carbon steel in acidic media and acts as mixed type (cathodic/anodic) inhibitor. The adsorption of the inhibitor was well described by the Langmuir adsorption isotherm. Some thermodynamic parameters such as adsorption heat, adsorption entropy, and adsorption free energy were calculated. Kinetic parameters such as the apparent activation energy and pre-exponential factor were calculated and discussed.     

Determination of the single component and competitive adsorption isotherms of the 1-indanol enantiomers by the inverse method

The inverse method of isotherm determination consists in calculating the numerical values of the coefficients of an isotherm model that give a set of chromatographic profiles in best possible agreement with the set of experimental profiles available. This method was applied to determine the adsorption isotherms of the 1-indanol enantiomers on a cellulose tribenzoate chiral stationary phase. Both single-component and competitive isotherms were determined by using no more than one or two overloaded band profiles. The isotherms determined from the overloaded band profiles agreed extremely well with the isotherms determined by frontal analysis. Several isotherm models were used and tested. The best-fit isotherm was selected by means of statistical evaluation of the results. The results show that the adsorption is best characterized with a model describing heterogeneous adsorption with bimodal adsorption energy distribution.     

Limits of the numerical estimation of the adsorption energy distribution from adsorption isotherm data using the expectation-maximization method

The limits of the use of the expectation-maximization (EM) method for the study of the heterogeneity of adsorbent surfaces were tested by calculating the adsorption energy distribution of systems having known degrees of heterogeneity. Connecting on-line two different columns allows the simulation of a heterogeneous system. The two columns used were endcapped, C(18)-bonded silica used as stationary phases and having different degrees of C(18) chain coverages (0.42 and 2.03 micromol/m(2)). The adsorption constants of phenol measured by frontal analysis (FA) are significantly different on these two columns. On each column, the adsorption behavior was best accounted for by a bi-Langmuir isotherm model, corresponding to a heterogeneous surface with a bimodal energy distribution. The difference between the adsorption energies on the weak adsorption sites of the two columns is 1.5 kJ/mol. The energy difference of their high energy sites is 2.2 kJ/mol. The EM method can readily distinguish between adsorption sites having energies that differ by more than 5 kJ/mol after more than 10 million iterations, but it cannot distinguish between adsorption sites for which this energy difference is less than 2 kJ/mol, even after 100 million iterations. For highly heterogeneous systems, (e.g., those with more than three different types of adsorption sites), the EM program does not converge necessarily towards the actual energy distribution function but toward a simpler one, having fewer adsorption sites that are almost equally spaced in the energy space. This failure of the EM program is related to the fact that, despite the excellent precision of the FA measurements (<1%), any series of adsorption data can be represented by several distinct AEDs. Thus, the degree of heterogeneity of RPLC adsorbents determined with the EM method might often be minimized, resulting in erroneous values of the isotherm parameters.     

On the applicability of Arrhenius plot methods to determine surface energetic heterogeneity of adsorbents and catalysts surfaces from experimental TPD spectra

Recovering adsorption energy distribution from experimental data belongs to most difficult problems of adsorption science. In the case when thermodesorption data are used as a source of information, that difficult problem is overcome by the common use of the Arrhenius plot methods. So, we decided to carry out an extensive model investigation to show, how reliable information concerning the surface energetic heterogeneity is obtained by using the Arrhenius plot methods. Like in our previous publications we have used the Statistical Rate Theory of Interfacial Transport to describe the adsorption/desorption kinetics. Our model investigations showed, that the Arrhenius plot methods, cannot provide reliable information about the surface energetic heterogeneity. Moreover, for strongly heterogeneous surfaces a linear relationship exists between the logarithm of the pre-exponential constant and the adsorption energy, for certain adsorption coverages. That kind of compensation effect has, so far, been ascribed to interactions between the adsorbed molecules. The failure of the popular Arrhenius plot method puts, as an urgent agenda, the development of reliable methods for recovering adsorption energy distribution from the thermodesorption data.     

Adsorption dynamics of gases and vapors on the nanoporous metal organic framework material Ni2(4,4'-bipyridine)3(NO3)4: guest modification of host sorption behavior

This study combines measurements of the thermodynamics and kinetics of guest sorption with powder X-ray diffraction measurements of the nanoporous metal organic framework adsorbent (host) at different adsorptive (guest) loadings. The adsorption characteristics of nitrogen, argon, carbon dioxide, nitrous oxide and ethanol and methanol vapors on Ni2(4,4'-bipyridine)3(NO3)4 were studied over a range of temperatures as a function of pressure. Isotherm steps were observed for both carbon dioxide and nitrous oxide adsorption at approximately 10-20% of the total pore volume and at approximately 70% of total pore volume for methanol adsorption. The adsorption kinetics obey a linear driving force (LDF) mass transfer model for adsorption at low surface coverage. At high surface coverage, both methanol and ethanol adsorption follow a combined barrier resistance/diffusion model. The rates of adsorption in the region of both the carbon dioxide and methanol isotherm steps were significantly slower than those observed either before or after the step. X-ray diffraction studies at various methanol loadings showed that the host structure disordered initially but underwent a structural change in the region of the isotherm step. These isotherm steps are ascribed to discrete structural changes in the host adsorbent that are induced by adsorption on different sites. Isotherm steps were not observed for ethanol adsorption, which followed a Langmuir isotherm. Previous X-ray crystallography studies have shown that all the sites are equivalent for ethanol adsorption on Ni2(4,4'-bipyridine)3(NO3)4, with the host structure undergoing a scissoring motion and the space group remaining unchanged during adsorption. The activation energies and preexponential factors for methanol and ethanol adsorption were calculated for each pressure increment at which the linear driving force model was obeyed. There was a good correlation between activation energy and ln(preexponential factor), indicating a compensation effect. The results are discussed in terms of reversible adsorbate/adsorbent (guest/host) structural changes and interactions and the adsorption mechanism. The paper contains the first evidence of specific interactions between guests and functional groups leading to structural change in flexible porous coordination polymer frameworks.     

超临界条件下甲烷在纳米活性炭表面的吸附机理

在273-373 K、0-10 MPa范围内测量了甲烷在纳米活性炭表面的吸附等温线和等量吸附热. 结果发现, 在实验涉及的温度范围内, 吸附平衡特性在低压下能够很好地遵循Dubinin-Astakhov (DA)微孔填充模型, 但是当压力超过特定范围时, 吸附等温线及等量吸附热测量数据都与DA模型计算结果发生了偏离, 吸附行为更接近单层定位吸附.文中参照Cerofolini对亚单层吸附提出的Freundlich-Dubinin-Radushkevich (FDR)混合模型, 对纳米活性炭在较高压力条件下的吸附使用通用Freundlich (GF)模型进行了修正, 从而提出了一种分段模型GFDA. 根据GFDA模型对甲烷在广泛的压力范围内在纳米活性炭表面的吸附机理进行了完整的解释, 并对纳米活性炭表面的能量非均匀性进行了分析.     

Mechanism of Methane's Adsorption on Nanometer Active Carbon at Supercritical State

Isosteric heats and adsorption isotherms of methane on nanometer active carbon were measured at supercritical temperature (273-373 K) and pressure from 0 to 10 MPa. The measured data agreed well with Dubinin-Astakhov (DA) model at lower pressure but failed when pressure exceeded a special range. General Freundlich (GF) equation was used to modify the DA equation at high pressure and thus formed a combined bisection model GFDA. The adsorption mechanism of methane on nanometer active carbon was raised according to GFDA model and the heterogeneous energy distribution of the adsorbent was analyzed.     

How to distinguish energetic surface heterogeneity from electrostatic interactions in the case of hydrogen ion adsorption from solution onto oxides

It was shown that the adsorption of uncharged particles onto energetically heterogeneous surfaces with Gaussian-like distribution of adsorption energy can be described by the same adsorption isotherm as the adsorption of hydrogen ions from solution onto homogeneous oxide surfaces with surface potentials given by a quasi-Nernst formula. It exemplifies that the separation of electrostatic from energetic factors in the case of ion adsorption onto heterogeneous oxide surfaces is very difficult. Nevertheless, application of calorimetric data for proton adsorption makes it possible to estimate both factors.     

Pressure-induced effects in the heterogeneous adsorption of insulin on chromatographic surfaces

The effect of increasing the average column pressure (ACP) on the heterogeneous adsorption of insulin variants on a C18-bonded silica was studied in isocratic reversed-phase HPLC. Adsorption isotherm data of lispro and porcine insulin obtained for values of the ACP ranging from 57 to 237 bar were fitted to the Langmuir-Freundlich and the Tóth equation. The resulting isotherm parameters, including the equilibrium adsorption constant and the heterogeneity index, were next used for the calculation of distribution functions characterizing the energy of interactions between the adsorbed insulin molecules and the stationary phase. It was observed that increasing the pressure by 180 bar causes a broadening of the distribution functions and a shift of the position of their maximum toward lower interaction energies. These findings suggest that, under high pressures, the insulin molecules interact with the stationary phase in a more diversified way than under low pressures. Additionally, the most probable value of the energy of the insulin-surface interactions becomes lower when the ACP increases. The pressure-induced changes in the interaction of insulin variants with the hydrophobic surface are attributed to a possible conformational flexibility of the molecular structure of this protein.     

Global statistical predictor model for characteristic adsorption energy of organic vapors-solid interaction: use in dynamic process simulation

Adsorption of Volatile Organic Compounds (VOCs) is one of the best remediation techniques for controlling industrial air pollution. In this paper, a quantitative predictor model for the characteristic adsorption energy (E) of the Dubinin-Radushkevich (DR) isotherm model has been established with R(2) value of 0.94. A predictor model for characteristic adsorption energy (E) has been established by using Multiple Linear Regression (MLR) analysis in a statistical package MINITAB. The experimental value of characteristic adsorption energy was computed by modeling the isotherm equilibrium data (which contain 120 isotherms involving five VOCs and eight activated carbons at 293, 313, 333, and 353 K) with the Gauss-Newton method in a statistical package R-STAT. The MLR model has been validated with the experimental equilibrium isotherm data points, and it will be implemented in the dynamic adsorption simulation model PROSIM. By implementing this model, it predicts an enormous range of 1200 isotherm equilibrium coefficients of DR model at different temperatures such as 293, 313, 333, and 353K (each isotherm has 10 equilibrium points by changing the concentration) just by a simple MLR characteristic energy model without any experiments.     

Influence of analysis conditions on low pressure adsorption measurements and its consequences in characterization of energetic and structural heterogeneity of microporous carbons

Nitrogen adsorption isotherms on nonporous and microporous carbons were thoroughly studied at low relative pressures. For nonporous carbons low pressure measurements seem to be unaffected by analysis conditions. However, these measurements on microporous solids may be affected by analysis conditions at relative pressures below 10^–4. It was shown that selection of proper equilibration time is crucial for correct measurements of equilibrium pressures during adsorption on microporous carbons. The isotherm shift induced by insufficient equilibration of the system may affect the surface heterogeneity and microporosity analysis. A comparison of the adsorption energy and pore volume distribution functions calculated from low pressure nitrogen adsorption isotherms measured at different equilibration times on a microporous carbon shows that this effect is smaller than it was expected.     

Examination of the surface heterogeneity of reversed-phase packing materials with solvent adsorption

The adsorption isotherms of acetonitrile, ethanol, 2-propanol, and THF were measured using frontal analysis on six columns packed with octadecyl RP stationary phase. The effect of the bonding density of the end-capped octadecyl bonded phase on the adsorption properties was measured. Adsorption isotherm data were collected from aqueous solutions of the four organic modifiers. The isotherm model for solvent adsorption was selected using two independent parameter estimation methods, the regression analysis and adsorption energy distribution. The fitted isotherm parameters were tested by modeling of overloaded elution bands with the aid of the equilibrium-dispersive model of chromatography. The surface heterogeneity estimations and the effect of the silanol groups on the adsorption of solvents were based on those data.     

Adsorption isotherms of CO2, CO,N2, CH4, Ar and H2 on activated carbon and zeolite LiX up to 1.0 MPa

The adsorption isotherms of CO₂, CO, N₂, CH₄, Ar, and H₂ on activated carbon and zeolite LiX were measured using a volumetric method. Equilibrium experiments were conducted at 293, 308, and 323 K and pressures up to 1.0 MPa. The adsorption isotherm and heat of adsorption were analyzed for two pressure regions of experimental data: pressures up to 0.1 MPa and up to 1.0 MPa. Each experimental isotherm was correlated by the Langmuir, Sips, Toth and temperature dependent Sips isotherm models, and the deviation of each model was evaluated. The Sips and Toth models showed smaller deviation from the experimental data of adsorbents than the Langmuir model. Isosteric heats of adsorption were calculated by the temperature dependent Sips model and are presented along with surface loading. From deviation analysis, it is recommended that the isotherm in the proper pressure range be used to appropriately design adsorptive processes.     

Surface heterogeneity and surface area from linear inverse gas chromatography. Application to glass fibers

A new method of measuring surface heterogeneity of non-porous adsorbents by inverse gas chromatography (IGC) is developed. In contrast with the methods described in the literature which are based on nonlinear chromatography (thermodynamics of adsorption) this method is based on linear chromatography (kinetics of adsorption). The mass balance equation of linear chromatography with axial diffusion term and the Langmuir kinetic equation on an open (non-porous) heterogeneous surface are solved by the method of Laplace transforms. The expressions for four semi-invariants of an elution profile are obtained. These are linear combinations of the moments of the distribution of sites in residence time of adsorbed molecule with coefficients determined from elution profile of non-adsorbing gas. Four semi-invariants of elution profiles of benzene and methane on a column packed with E-glass fiber were determined experimentally at temperatures around 100 degrees C. The mean, the standard deviation of the normal distribution in adsorption energy as well as the sticking coefficient and pre-exponential factor of the Frenkel equation are determined for benzene on E-glass. A method of the specific surface determination from linear IGC is proposed.     

Oleic acid adsorption-desorption isotherms on the surface of high-dispersity ferrites from a solution in carbon tetrachloride

The equilibrium adsorption method was used to comparatively study the adsorption-desorption isotherms of oleic acid on the surfaces of manganese and copper ferrites from a solution in carbon tetrachloride. The adsorption isotherms of the fatty acid were described in terms of the theory of volume filling of micropores. The theory was used to calculate the limiting adsorption values, characteristic energy, and porous space volumes. The isotherm of oleic acid adsorption on the surface of manganese ferrite from a solution in carbon tetrachloride was similar to the isotherms of fatty acid adsorption from solutions in heptane, whereas the isotherm of adsorption on the surface of copper ferrite was similar to the isotherms of fatty acid adsorption from hexane. The limiting adsorption from carbon tetrachloride was higher on the surface of manganese ferrite than on the surface of copper ferrite. The adsorption-desorption isotherms contained hysteresis loops.