New method for the determination of adsorption space volume for sorbents with an arbitrary porous structure from excess gas adsorption isotherm measurements

A new method for the determination of adsorption space volume has been proposed, which is applicable to adsorbents with an arbitrary porous structure, including nonporous adsorbents with open surfaces. The method is based on the use of an experimental excess adsorption isotherm measured over a wide range of pressures in the equilibrium gaseous phase (as a rule up to 100–150 MPa) and the absolute adsorption isotherm equation with unspecified parameters in the most general form, given by statistical physics. The method has been tested for a number of adsorption systems, and it has been found that the result was always unambiguous, correct, and stable in the sense of input data.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2381–2385, December, 1995.     

Carbon dioxide adsorption on the microporous ACC carbon adsorbent

Adsorption isotherms of carbon dioxide on the microporous ACC carbon adsorbent and the adsorption deformation of the adsorbent were measured. The heats of adsorption at temperatures raising from 243 to 393 K and pressures from 1 to 5⋅10⁶ Pa were measured. In the low-temperature region (243 K), an increase in the amount adsorbed is accompanied by adsorbent contraction, and at high micropore fillings (a > 10 mmol g⁻¹) the ACC carbon adsorbent expands. At high temperatures, adsorbent expansion is observed in the whole region of micropore filling. At 243 K in the low filling region (a < 1 mmol g⁻¹), the heat of adsorption decreases smoothly from 27 to 24 kJ mol⁻¹. The heat of adsorption remains virtually unchanged in the interval 2 mmol g⁻¹ < a < 11 mmol g⁻¹ and then decreases to 8 kJ mol⁻¹ at a = 12 mmol g⁻¹. Taking into account the nonideal character of the gas phase and adsorbent deformation the heats of adsorption are strongly temperature-dependent in a region of high pressures. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1331–1335, June, 2005.     

Experimental evidence of the reversibility of the first stage of protein adsorption at a hydrophobic quartz surface near the isoelectric point

The reversibility of the first stage of adsorption of zwitterionic cytochrome c on a hydrophobic quartz surface was investigated using time‐resolved slab optical waveguide (SOWG) absorption spectroscopy. Using a novel prism‐free broadband coupling approach, absorbance data were collected successfully at a 50 ms time interval during the first few seconds after solution–surface contact. Near the isoelectric point where the cytochrome c molecules possess a net charge of zero and hence cannot be influenced by an electric field, the speed at which adsorption proceeded was found to be dependent on cytochrome c concentration as well as on surface hydrophobicity. It was also observed that the degree of protein adsorption increased as the surface hydrophobicity was increased. Within 6 s the adsorption process appeared to be reversible, as revealed by extremely low chi‐squared values when the absorbance data were fitted into the reversible Langmuir‐type kinetic model. The standard Gibbs free energy of adsorption was also calculated from the absorbance data. Copyright © 2003 John Wiley & Sons, Ltd.     

Adsorption behavior of end-functionalized polymers modified by iminium ion

In order to clarify the end-functionalization effect of polymers modified by iminium ion, a model compound with a modified chain end was prepared by directly reactingn-butyllithium withN-methyl-2-pyrrolidone and then quenching by the addition of methanol. It is shown that the model compound includes the iminium ion group and that the adsorption bands in the IR-spectra of the chemically modified polymer were similar to those of model compoundThe adsorption behavior of end-functionalized polystyrene(PS-X) and diblock copolymer of PS and polybutadiene(PB) which were modified by iminium ion(PS-PB-X), were examined over a wide range of molecular weights of these polymers. It is evident that the amount of adsorption increased significantly by the end-functionalization and this trend was especially strong in the low molecular weight polymers (M _w<-10⁵). The preference for the end-functionalized polymer was also examined by competitive and sequential adsorption experiments between different molecular weight species. It is shown that a prefered adsorption of PS-X with low molecular mass is pronouced over the adsorption of PS or PS-X with high molecular weight and in the combination of high and low molecular weight species of PS-X, a special enhancement of total adsorption can be observed. This enhancement is based on a combination of different adsorption layers formed by these two molecular weight species.     

Studies on the Physical Adsorption Equilibria of Gases on Porous Solids over a Wide Temperature Range Spanning the Critical Region—Adsorption on Microporous Activated Carbon

Adsorption equilibria of nitrogen and methane on microporous ( < 2 nm) activated carbon were measured for a wide temperature range (103‐298 K) spanning the critical region. Information relating to Henry constants, the isosteric heat of adsorption, and the amount of limiting adsorption were evaluated. All isotherms show type‐I features for both sub‐ and supercritical temperatures. A new isotherm equation and a consideration for the importance of the effect of the adsorbed phase volume allow this kind of isotherms to be modeled satisfactorily. The model parameter of the saturated amount of absolute adsorption (n⁰_t) equals the limiting adsorption amount (n^itm), leaving the physical meaning of the latter clarified, and the exponent parameter (q) proves to be an appropriate index of surface heterogeneity.     

Adsorption of Nitrogen on Silica Gel Over a Large Range of Temperatures

To study the mechanism of physical adsorption of supercritical gases, the adsorption equilibria of N₂ on silica gel for 103–298 K using 20 K increments and pressures up to 10 MPa were measured. A transition of the adsorption mechanism was proven on crossing the critical temperature, but the transition way observed is different from that observed with activated carbon. This causes a difference in the locations of the linear section of the n- _g isotherm at the near-critical temperature. Although the isotherm type is different on silica gel and on activated carbon in the sub-critical region, all isotherms in the supercritical region can be well modeled by a single model. It leads to the argument that the adsorption mechanism of supercritical gases is identical no matter what kind of adsorbent is used.     

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.     

Nonideality of binary mixtures in the adsorbent micropores

Analysis of a binary equilibrium in a distinct micropore showed that the noouniformity of the adsorption field results in a negative deviation of the adsorption solution from ideal behavior. This deviation is due to a decrease in the entropy and internal energy caused by rearrangement of adsorbed molecules. Different interactions between identical and different molecules in distinct micropores also result in a negative deviation from ideal behavior. A criterion was formulated that allows one to elucidate how the deviation from ideality depends on the nonuniformity of the adsorption space and the character of the interaction between molecules in a micropore. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No.6, pp. 1011–1017, June, 2000.     

Thermodynamic quantities of adsorption described by Freundlich isotherm

Thermodynamic analysis of the Freundlich adsorption isotherm was performed. Equations describing the thermodynamic characteristics of adsorption as a function of the adsorbed quantity are presented, and relationships between the excess differential and average molar thermodynamic adsorption quantities were obtained. It was shown that the thermodynamic approach does not contradict the molecular statistical theory and provides more general equations. A model of adsorption described by the Freundlich isotherm is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 175–177, January, 2000.     

Thermodynamic quantities of adsorption described by Freundlich isotherm

Thermodynamic analysis of the Freundlich adsorption isotherm was performed. Equations describing the thermodynamic characteristics of adsorption as a function of the adsorbed quantity are presented, and relationships between the excess differential and average molar thermodynamic adsorption quantities were obtained. It was shown that the thermodynamic approach does not contradict the molecular statistical theory and provides more general equations. A model of adsorption described by the Freundlich isotherm is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 175–177, January, 2000.     

Adsorption phenomena at high pressures and temperatures 5. Heats of excess and total adsorption of krypton on zeolite NaA

Thermodynamic principles for the calculation of differential heats of excess and absolute adsorption were considered. A set of isosteres of excess adsorption of krypton on zeolite NaA are presented, from which the coverage and temperature dependences of the heats of excess adsorption are calculated and analyzed. The reasons for infinitely high values of the excess heats at finite values of adsorption are discussed. The problems of recalculation of the excess adsorption to absolute adsorption are considered.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1946–1950, August, 1996.     

Liquid-phase hydrogenation of methyl oleate on a Ni/α-Al2O3 catalyst: A study based on kinetic models describing extreme and intermediate adsorption regimes

The kinetics of the hydrogenation of methyl oleate on a Ni/α-Al₂O₃ catalyst was studied in the absence of mass-transport limitation, at 398 ≤ T ≤ 443 K and 3.7 ≤ P_H2 ≤ 6.5 bar. The kinetic modeling was performed on the basis of elementary step mechanisms involving different regimes of competition between hydrogen and methyl oleate. Admitting a distinction between occupied-sites and covered-sites by the large molecule of methyl oleate, a rigorous proposal was made to link the seemingly separate kinetic models corresponding to the extreme modes of competitive and non-competitive adsorption, without having to draw the common distinction between two types of surface sites. General rate equations were formulated without expressing opinion a priori on whether the adsorption regime is competitive or non-competitive. Then, typical LHHW rate equations for both extreme adsorption regimes were straightforwardly derived as special cases. Statistical results demonstrated the inadequacy of the models approaching non-competitive adsorption to describe the experimental data but results did not allow a definite discrimination between rival models with competitive and semi-competitive adsorption. A mechanistic model featuring dissociative adsorption of hydrogen, molecule of methyl oleate interacting with a single atom of Ni, and second insertion of hydrogen as RDS, proved to be the best candidate to describe the experimental data satisfactorily with physically reasonable parameters. As a distinctive feature, the model considering semi-competitive adsorption gave additional indication that the adsorbed molecule of methyl oleate could cover up to seven surface sites. From this finding, the semi-competitive model seems to be more realistic than the competitive one.     

Dynamics of the adsorption of freons

The elution adsorption dynamics of some adsorption systems (freon-active carbon) has been studied at two temperatures. The peculiarities of the adsorption dynamics, which are determined by the properties of the adsorption system, have been established. A possibility to calculating elution curves based on the equilibrium adsorption layer model is shown. Preliminary results of the influence of the parameters of the adsorption system on the coefficient of the effective kinetic model are obtained.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1373–1376, June, 1996.     

Adsorption deformation in the microporous carbon adsorbent-benzene system and porous structure of adsorbents

The relative adsorption deformation of several microporous carbon adsorbents was studied as a function of the benzene adsorption at relative pressure ranging from 1·10⁻¹ to 1.0 and at 293 K. A correlation between the maximum compression of the sample and the characteristic energy of benzene adsorption of the Dubinin-Radushkevich equation was obtained. Using data on the adsorption deformation, it is possible to identify the region with a specific pore size that cannot be evaluated with the help of the Dubinin-Radushkevich equation. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No.6, pp. 1018–1022, June, 2000.     

不同锌铝比水滑石的合成及其吸附脱除水中邻苯二甲酸性能

采用共沉淀法合成一系列具有不同锌铝比的水滑石,并利用X射线衍射(XRD)、扫描电镜(SEM)、热重(TG)、氮气吸脱附及电感耦合等离子体质谱(ICP-MS)等表征手段对其结构与组成进行了测试。将上述水滑石材料用于吸附脱除水中邻苯二甲酸污染物,考察了不同锌铝比水滑石吸附邻苯二甲酸性能的差异。结果表明,在较低锌铝比时,随着水滑石锌铝比的增加,其对邻苯二甲酸的吸附量逐渐增大;当锌铝比较大时(6),随着锌铝比的增加,水滑石的吸附量基本保持不变。进一步选取锌铝比为6的水滑石,分别对其吸附邻苯二甲酸的动力学和热力学进行了研究,发现其吸附等温线和吸附动力学数据分别符合Freundlich等温吸附模型和准二级动力学模型,且循环吸附性能较好。     

不同锌铝比水滑石的合成及吸附脱除水中邻苯二甲酸性能的研究

采用共沉淀法合成一系列具有不同锌铝比的水滑石,并利用X射线衍射（XRD）、扫描电镜（SEM）、热重（TG）、氮气吸脱附及电感耦合等离子体质谱（ICP-MS）等表征手段对其结构与组成进行了测试。将上述水滑石材料用于吸附脱除水中邻苯二甲酸污染物,考察了不同锌铝比水滑石吸附邻苯二甲酸性能的差异。结果表明,在较低锌铝比时,随着水滑石锌铝比的增加,其对邻苯二甲酸的吸附量逐渐增大;当锌铝比较大时（>6）,随着锌铝比的增加,水滑石的吸附量基本保持不变。进一步选取锌铝比为6的水滑石,分别对其吸附邻苯二甲酸的动力学和热力学进行了研究,发现其吸附等温线和吸附动力学数据分别符合Freundlich等温吸附模型和准二级动力学模型,且循环吸附性能较好。     

Dynamic surface tension behavior of aqueous solutions ofN-dodecyl-N,N dimethyl aminobetaine chlorohydrate

The adsorption behavior ofN-dodecyl-N,N dimethyl aminobetaine chlorohydrate (DDAB·HCl) at the air/aqueous interface was studied for solutions in pure water and phosphate buffer (pH=7.4). The equilibrium surface tension versus concentration curves were used to estimate the equilibrium adsorption parameters and CMCs. The buffer solution has a lower CMC and shows higher surface activity below the CMC than the pure water solution. Data and calculations of the dynamic tension behavior at constant-area conditions showed that the adsorption processes of DDAB·HCl solutions are about 10 to 300 times slower than those predicted by a diffusion-controlled model. A mixed kinetics adsorption model with a modified Langmuir-Hinshelwood kinetic equation, which considers an activation energy barrier for adsorption, was applied to find the kinetic adsorption parameters. The dynamic tension behavior at pulsating-area conditions with large amplitude was also examined for frequencies up to 90 cycles/min. The tension amplitude responses depended strongly on the concentration and frequency. Comparisons of diffusion-controlled model predictions and pulsating area tension data confirmed the need to use a mixed kinetics model. The latter model can improve the fit over the diffusion-controlled model, but it does not quantitatively match the observed tensions.     

Adsorption of helium on zeolite NaA

Isotherms of helium adsorption on zeolite NaA were measured by the volumetric method under static conditions at 23–50 K and pressures from 1 Pa to 65 kPa. The Henry constants and the initial heat of helium adsorption were calculated; the isosteric heat was calculated, and its dependence on the adsorption was determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1291–1292, May, 1996.     

Ethanol-Water Separation in the PSA Process

An investigation was carried out on ethanol-water separation employing a PSA adsorption cycle with zeolite 3A as the adsorbent. The cycle was operated under the following operating variables: feed flow (2, 4, 6 and 8 L/h), adsorption temperature (200°C) and adsorption pressures (2, 4 and 6 bar). All experimental runs were performed under vacuum of 0.2 bar in the desorption step. The effect of these variables on the enrichment and recovery percentage of the product, on the productivity and on the total cycle time was studied, using the adsorption pressure as a parameter. The results showed that these variables significantly affect the responses of interest. We also studied the influence of such variables as: adsorption pressures, desorption pressures, flow rates and adsorption temperatures (T), on the enrichment, recovery, productivity and on the total cycle time. The data were obtained from operational cycles in a Kahle's system. The experiments were organized by a three level factorial design. The results obtained were compared with from the fitted empirical equations as well as with the corresponding surface responses and all variables showed to be influential. Last, by an optimal combination of the variables was obtained by means of the ridge analysis method and the optimized multi-response method.     

Comparison of molecular simulation of adsorption with experiment

Experimental measurements of adsorption yield the surface excess. The Gibbs surface excess is the actual or absolute amount of gas contained in the pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. Molecular simulation of adsorption yields the absolute amount adsorbed. Comparison of simulated adsorption isotherms and heats of adsorption with experiment requires a conversion from absolute to excess variables. Molecular simulations of adsorption of methane in slit pores at room temperature show large differences between absolute and excess adsorption. The difference between absolute and excess adsorption may be ignored when the pore volume of the adsorbent is negligible compared to the adsorption second virial coefficient (VB _1s ).