Isosteric Enthalpy Research Articles

Adsorption of water vapor on MgCl2 × 6H2O salt surface

The adsorption of water vapor on dry solid films of MgCl2 × 6H2O salt was studied as a function of partial pressure of water over the temperature range 240–340 K using a flow reactor coupled to a modulated molecular beam mass spectrometer. The uptake of water was found to be completely reversible. The adsorption data are well described by a Freundlich isotherm with the heterogeneity parameter close to 0.5. The isosteric enthalpy of adsorption was found to be –(44.7 ± 1.2) kJ mol−1, independent of the salt surface coverage in the range (0.8–30) × 1015 molecule cm−2. An empirical equation is proposed for the amount of water adsorbed on MgCl2 × 6H2O as a function of relative humidity. Uptake of H2O on MgCl2 × 6H2O is observed to be much higher than on NaCl indicating that the amount of surface adsorbed water on mixed salts will be drastically dependent on the salt sample composition. The observed results support previous considerations that under atmospheric conditions sea salt particles are probably enveloped by a MgCl2 × 6H2O brine.

Adsorption of Chloroaromatic Models for Dioxins on Porous Carbons: The Influence of Adsorbate Structure and Surface Functional Groups on Surface Interactions and Adsorption Kinetics

Polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran species are classes of extremely toxic compounds generated in very low concentrations in postcombustion gases and these may be removed by adsorption on porous carbons. Their extreme toxicity and very low volatility prevent detailed study of their adsorption characteristics, and therefore, models for dioxins have been used in this study. Chlorobenzene, 2-chlorotoluene, 1,3-dichlorobenzene, and 2-chloroanisole were used as models to investigate factors influencing the adsorption characteristics of dioxins on porous carbons. Adsorption studies were carried out under conditions of very low concentration and temperatures up to 453 K, which simulate those found in dioxin abatement systems. Adsorption of 2-chloroanisole on three carbons with various micro/mesoporous structures showed that microporous structure was a critical adsorbent characteristic under these conditions. A microporous activated carbon was selected for detailed thermodynamic and kinetic studies of adsorption of chloroaromatic species in relation to adsorbate structure and adsorbent surface functional groups. Virial equation analysis of adsorption isotherms was used to determine the Henry’s Law constants and isosteric enthalpies of adsorption at zero surface coverage to compare adsorbate−adsorbent interactions. The van’t Hoff equation was used to determine the enthalpy of adsorption as a function of surface coverage. The role of surface functional groups on adsorption thermodynamics was investigated by oxidizing and reducing the carbon in nitric acid and hydrogen, respectively. The important factor influencing adsorption at very low concentrations is the adsorbate adsorbent interaction. Oxidation of the carbon adsorbent only has a small effect on the isosteric enthalpy of adsorption. The adsorption kinetics for each isotherm pressure increment were described by the stretched exponential equation. The activation energies and enthalpies of activation were calculated as a function of surface coverage for adsorption kinetics of chloroaromatic species. The planar molecules studied had lower activation energies and enthalpies of activation than isosteric enthalpy of adsorption indicating that a site-to-site surface hopping mechanism is the main factor in determining the adsorption kinetics. In comparison, 2-chloroanisole is nonplanar with a methoxy group giving rise to a larger minimum cross-section size and higher barrier to diffusion than isosteric enthalpy of adsorption at low surface coverage leading to the adsorption kinetics being mainly determined by diffusion through constrictions in the porous structure under these conditions. The isosteric enthalpies of adsorption initially increase with increasing surface coverage and this is attributed to π−π interactions of planar aromatic molecules confined in microporosity. The trends in the kinetic barriers and isosteric enthalpies of adsorption with surface coverage for 2-chlorotoluene are similar irrespective of adsorbent oxidation/reduction, indicating that surface functional groups only have a relatively small effect on adsorption characteristics.

Adsorption of atrazine from aqueous electrolyte solutions on humic acid and silica

The adsorption of, the still widely used, herbicide atrazine on model soil components, such as humic acid and humic acid–silica gel mixtures, was investigated in a series of batch experiments, under different experimental conditions (ionic strength, temperature, and pH). The investigation aimed at obtaining an estimate of the contribution of each of the soil components on the adsorption of atrazine from aqueous solutions. The kinetics of atrazine adsorption on humic acid showed two steps: a fast step, of a few hours duration, and a second slow step, which continued for weeks. The kinetics of adsorption data gave a satisfactory fit to the Elovich equation. Τhe adsorption of atrazine on the test substrates was found to be reversible in all cases. The atrazine uptake data on the test substrates were fitted best with the Freundlich adsorption isotherm. The ionic strength of the atrazine aqueous solutions did affect the amount of the atrazine adsorbed on the test substrates, suggesting that electrostatic forces between atrazine molecules and soil play a significant role in the adsorption process. An increase of temperature resulted in a decrease of atrazine adsorption on humic acid at low atrazine equilibrium concentrations. However, for higher levels of equilibrium concentrations (⩾3 mg/L) the amount of atrazine adsorbed onto the test substrate increased as temperature increased. The calculated isosteric enthalpies of adsorption ranged between slightly exothermic at low atrazine uptake and slightly endothermic at high atrazine uptake, all values being in the range of physisorption.

Characterisation of porous hydrogen storage materials: carbons, zeolites, MOFs and PIMs

Porous materials adsorb H2 through physisorption, a process which typically has a rather low enthalpy of adsorption (e.g. ca. 4 to 7 kJ mol(-1) for MOFs), thus requiring cryogenic temperatures for hydrogen storage. In this paper, we consider some of the issues associated with the accurate characterisation of the hydrogen adsorption properties of microporous materials. We present comparative gravimetric hydrogen sorption data over a range of temperatures for different microporous materials including an activated carbon, a zeolite, two MOFs and a microporous organic polymer. Hydrogen adsorption isotherms were used to calculate the enthalpy of adsorption as a function of hydrogen uptake, and to monitor the temperature dependence of the uptake of hydrogen. Under the conditions investigated, it was found that the Tóth equation provided better fits to the absolute isotherms compared to the Sips (Langmuir-Freundlich) equation at low pressures, whereas it appeared to overestimate the maximum saturation capacity. The isosteric enthalpy of adsorption was calculated by either: fitting the Sips and Tóth equations to the adsorption isotherms and then applying the Clausius-Clapeyron equation; or by using a multiparameter Virial-type adsorption isotherm equation. It was found that the calculated enthalpy of adsorption depended strongly upon the method employed and the temperature and pressure range used. It is shown that a usable capacity can be calculated from the variable temperature isotherms for all materials by defining a working pressure range (e.g. 2 to 15 bar) over which the material will be used.

Monomer and micellar adsorptions of CTAB onto the clay/water interface

In this study, the monomeric and micellar adsorption properties of clay samples in aqueous cetyltrimethylammonium bromide (CTAB) solutions were investigated as a function of ionic strength, CTAB equilibrium concentration, and temperature. It was found that the efficiency and effectiveness of CTAB adsorption decrease with increasing temperature, which is the exothermic nature of the adsorption. The sorption of CTAB onto clay increased at the pH range of 3–9 then the adsorbed amount did not change significantly in the range of pH 9–11. The isosteric adsorption enthalpy was found to be − 69.05 kJ/mol. Another observation was that the isotherm shape shifted from an L-shape to an S-shape with increasing temperature. The calculated entropy value (227.46 Jmol − 1 K − 1 ) was quite high and positive. The adsorption capacity increased with increasing ionic strength, as expected. The isotherm shapes for all electrolytes exhibited the change in the case of non-electrolyte and both the efficiency and effectiveness of adsorption decreased with increasing electrolyte concentration. The experimental data concerning adsorption of CTAB at 293 K were applied to the Langmuir model and a high fit was obtained, particularly for monomeric adsorption.

Predicting helium and neon adsorption and separation on carbon nanotubes by Monte Carlo simulation

The adsorption of helium and neon mixtures on single-walled carbon nanotubes (SWCNTs) was investigated at various temperatures (subcritical and supercritical) and pressures using canonical Monte Carlo (CMC) simulation. Adsorption isotherms were obtained at different temperatures (4, 40, 77 and 130 K) and pressures ranging from 1 to 16 MPa. Separation factors and isosteric enthalpies of adsorption were also calculated. Moreover, the adsorption isotherms were obtained at constant specific temperatures (4 and 40 K) and pressures (0.2 and 1.0 MPa) as a function of the amount adsorbed. All of the adsorption isotherms for an equimolar mixture of helium and neon have a Langmuir shape, indicating that no capillary condensation occurs. Both the helium and the neon adsorption isotherms exhibit similar behavior, and slightly more of the helium and neon mixture is adsorbed on the inner surfaces of the SWCNTs than on their outer surfaces. More neon is adsorbed than helium within the specified pressure range. The data obtained show that the isosteric enthalpies for the adsorption of neon are higher than those for helium under the same conditions, which means that adsorption of neon preferentially occurs by (15, 15) SWCNTs. Furthermore, the isosteric enthalpies of adsorption of both gases decrease with increasing temperature.

Adsorption and Desorption of H2 on Graphite by Molecular Dynamics Simulations

We report equilibrium properties and rate constants for adsorption and desorption of hydrogen on graphite as a function of temperature, from 70 to 390 K, using equilibrium molecular dynamic simulations. Below 170 K, we found that isotherms can be modeled with Langmuir isotherms, while at higher temperature, Henry’s law applied. The isosteric adsorption enthalpies and entropies were calculated for different loadings and showed a decrease as the loading increased, compatible with a nonideal adsorbate. These results are in good agreement with previous results from the literature. The observed adsorption and desorption rate constants were, however, not described by Langmuir kinetics. We propose a set of rate equations which follow from the law of mass action with activities as variables rather than concentrations. The unidirectional rates combine at equilibrium to give the Langmuir isotherm with the same result for the Langmuir constant. This kinetic behavior is interpreted as the consequence of a mobile adsorbed phase. The effect may be important for fuel cell performance or hydrogen storage in carbonaceous materials.

Lithium‐Doped Conjugated Microporous Polymers for Reversible Hydrogen Storage

Hi de hi-drogen: An extremely large hydrogen storage capacity is shown by Li-doped conjugated microporous polymers (CMP). These materials have an isosteric enthalpy of hydrogen adsorption up to 8.1 kJ mol−1, and an excellent hydrogen adsorption of 6.1 wt % was achieved at 77 K under an ambient pressure of 0.1 MPa (1 bar). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Adsorption Equilibrium of Water on a Composite Adsorbent Employing Lithium Chloride in Silica Gel

This paper presents a new method to develop a composite adsorbent hygroscopic salt inside silica gel which can eliminate corrosion effects. A salt solution would be formed when a conventional composite adsorbent adsorbed water and corroded the adsorbent bed quickly. The adsorption equilibrium of water on a composite adsorbent (lithium chloride confined to silica gel) was measured by the sorption isosteric method. The temperature T which ranged from (308 to 368) K was controlled by a thermostatic bath, and the absolute pressure was in the range (0 to 25 000) Pa using a membranometer. The results showed that though the isosteric enthalpy (the average isosteric enthalpy is 42.82 kJ·mol−1) is lower than the conventional composite adsorbent, the new composite adsorbent still had a larger water sorption capacity (x = 0.702 g·g−1) in comparison with that of pure silica gel. The Freundlich equation, the modified Freundlich equation, and the Dubinin−Astakhov (D-A) equation are used to fit the experimental data. In...

A study of the pore size distribution for activated carbon monoliths and their relationship with the storage of methane and hydrogen

By adsorption of different gases and simulation methods it was studied the characterization of microporous monoliths in activated carbons from coconut shells in relation to the storage capacity of gases that present energetic interest. Adsorption isotherms of nitrogen, methane, carbon dioxide and hydrogen at different temperatures were measured at sub-atmospheric pressures. Additional adsorption isotherms of methane were performed at room temperature and high pressures (up to 4.5 MPa). A Grand Canonical Monte Carlo simulation of adsorption on slit pores was carried out for these gases. The simulated data were adjusted to experimental data to optimize the models. Different parameters such as micropore volume, pore size distribution and differential isosteric enthalpy of adsorption, were studied and related to the hydrogen and methane storage capacity for these materials.

Heterogeneous adsorption and catalytic oxidation of benzene, toluene and xylene over spent and chemically regenerated platinum catalyst supported on activated carbon

The heterogeneous adsorption and catalytic oxidation of benzene, toluene and o-xylene (BTX) over the spent platinum catalyst supported on activated carbon (Pt/AC) as well as the chemically treated spent catalysts were studied to understand their catalytic and adsorption activities. Sulfuric aqueous acid solution (0.1N, H 2SO 4) was used to regenerate the spent Pt/AC catalyst. The physico-chemical properties of the catalysts in the spent and chemically treated states were analyzed by using nitrogen adsorption–desorption isotherm and elemental analysis (EDX). The gravimetric adsorption and the light-off curve analysis were employed to study the BTX adsorption and oxidation on the spent catalyst and its modified Pt/AC catalysts. The experimental results indicate that the spent Pt/AC catalyst treated with the H 2SO 4 aqueous solution has a higher toluene adsorption and conversion ability than that of the spent Pt/AC catalyst. A further studies of H 2SO 4 treated Pt/AC catalyst on their catalytic and heterogeneous adsorption behaviours for BTX revealed that the activity of the H 2SO 4 treated Pt/AC catalyst follows the sequence of benzene > toluene > o-xylene. The adsorption equilibrium isotherms of BTX on the H 2SO 4 treated Pt/AC were measured at different temperatures ranging from 120 to 180 °C. To correlate the equilibrium data and evaluate their adsorption affinity for BTX, the two sites localized Langmuir (L2m) isotherm model was employed. The heterogeneous surface feature of the H 2SO 4 treated Pt/AC was described in detail with the information obtained from the results of isosteric enthalpy of adsorption and adsorption energy distributions. Furthermore, the activity of H 2SO 4 treated Pt/AC about BTX was found to be directly related to the Henry's constant, isosteric enthalpy of adsorption and adsorption energy distribution functions.

Thermodynamics and kinetic studies of biosorption of a basic dye from aqueous solution using green algae Ulothrix sp.

This study addresses removal of a basic dye, methylene blue, from aqueous solutions by using dried Ulothrix sp. biomass as biosorbent. The effects of the initial dye concentration, contact time, temperature, solution equilibrium pH, biosorbent dosage, and mixing rate on biosorption of the dye have been investigated. It was found that 30 min is sufficient in order to reach adsorption equilibrium. The amount of methylene blue adsorbed onto Ulothrix sp. increased with increasing equilibrium pH and mixing rate, in contrary, it decreased with increasing temperature and sorbent dosage. The process followed the pseudo-second-order kinetic model. The isosteric enthalpy and entropy values were calculated as −11.8 kJ/mol and 37.5 J/(mol K), respectively. In addition, the results suggest that the physical interactions between sorbent particles and sorbate ions play an important role for the adsorption of methylene blue onto the biosorbent.

Heterogeneous ruthenium dye adsorption on nano-structured TiO 2 films for dye-sensitized solar cells

TiO 2 particles of single-phase anatase nanocrystallites were prepared by the hydrolysis of titanium-tetraisopropoxide under acidic condition and characterized by XRD, FE-SEM, and BET analysis. The adsorption isotherms of dye molecule on TiO 2 particles were obtained at three different temperatures (298.15, 313.15, 333.15 K) and the experimental data were correlated with Sips isotherm model. Also the isosteric enthalpies of dye adsorption were calculated by the Clausius–Clapeyron equation. The influence of heterogeneous adsorption of cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium dye (N719) on the energy conversion efficiency of solar cell was investigated on the basis of photocurrent–potential curves. The results showed that the conversion efficiency of dye-sensitized solar cell was highly dependent on the heterogeneous adsorption properties of N719 dye on TiO 2 films.

High selective purification of flavonoids from natural plants based on polymeric adsorbent with hydrogen-bonding interaction

The efficient purification method of high purity flavonoids from natural plants was reported. A series of polymeric adsorbents with novel structure were synthesized based on the copolymerization of methyl acrylate (MA) and ethylene glycol dimethacrylate (EDGMA). Functional groups, such as ester, amino or amide group, were introduced into the adsorbent matrix, respectively, to produce the hydrogen-bonding interaction and enhance the adsorption selectivity towards flavone compounds. The influences of matrix structure and functional groups of synthesized adsorbents on the adsorption selectivity were investigated. The resins were applied to purify flavonoids in natural plants. It was illuminated that the adsorbent No. 3B with 15% EGDMA content and amide groups performed optimal selectivity to flavone compounds in Scutellaria barbata D.Don, from which the purity of flavonoids in extracts was obtained more than 50%, obviously higher than that from commercial adsorbents. The result of adsorption thermodynamics experiment showed that the isosteric adsorption enthalpy of No. 3B was in the range of 25–30 kJ/mol, which testified that the adsorption mechanism was related to hydrogen-bonding interaction. The method showed its universality via good effects on the purification of total flavonoids from Ginkgo biloba L., Radix puerariae and Hypericum perforatum L.

Equilibrium Adsorption Measurements of Pure Nitrogen, Carbon Dioxide, and Methane on a Carbon Molecular Sieve at Cryogenic Temperatures and High Pressures

A detailed experimental study of the adsorption behavior at equilibrium of pure nitrogen, methane, and carbon dioxide gases on a commercial carbon molecular sieve (Shirasagi MSC 3K-161) is reported at temperatures between 115 K to 323 K and pressures up to 5 MPa. A volumetric-type apparatus was used to obtain over 200 excess (Gibbs) adsorption capacity data over this range of pressure and temperature with an estimated uncertainty of 4 %. The absolute adsorption isotherms were type I in the IUPAC classification with the adsorption capacity at constant pressure increasing significantly with decreasing temperature. For each gas, the adsorption data were regressed to a four parameter Toth equation to represent the temperature and pressure dependence of the data with a relative standard uncertainty of 4 %. The optimized parameters from the Toth equation included the isosteric enthalpies of adsorption that were 17 kJ·mol−1, 27 kJ·mol−1, and 18 kJ·mol−1 for N2, CO2, and CH4, respectively.

Pore size distribution and supercritical hydrogen adsorption in activated carbonfibers

Pore size distributions (PSD) and supercriticalH2 isotherms have been measured for two activated carbon fiber (ACF) samples. The surfacearea and the PSD both depend on the degree of activation to which the ACF has beenexposed. The low-surface-area ACF has a narrow PSD centered at 0.5 nm, while thehigh-surface-area ACF has a broad distribution of pore widths between 0.5 and 2 nm. TheH2 adsorption enthalpy in the zero-coverage limit depends on the relative abundanceof the smallest pores relative to the larger pores. Measurements of theH2 isosteric adsorption enthalpy indicate the presence of energy heterogeneity in both ACFsamples. Additional measurements on a microporous, coconut-derived activated carbon arepresented for reference.

Synthesis of the hydrophobic–hydrophilic macroporous poly divinylbenzene/poly(sodium acrylate) IPN resin and adsorption performance for berberine

AbstractThe macroporous polydivinylbenzene/poly(methyl acrylate) interpenetrating polymer network (PDVB/PMA IPN) was prepared by the sequential suspension polymerization method, and was modified to be hydrophobic–hydrophilic macroporous polydivinylbenzene/poly (sodium acrylate) IPN (PDVB/PNaA IPN) by converting the PMA to PNaA under the condition of base. The effects of different mass ratio of the two networks and different cross‐linking degree of the second network on the pore structure and adsorption capacity of PDVB/PNaA IPN resin were studied. The PDVB/PNaA IPN resin whose adsorption quantity is the biggest was chosen to study further. The pore structure, the weak acid exchange capacity, the water retention capacity, and the swelling ability of PDVB/PNaA IPN resin were measured. The study focused on the adsorption isotherms of berberine at different temperatures. Isosteric adsorption enthalpy, adsorption Gibbs free energies can be calculated according to thermodynamic functions. The results show that the saturated adsorption quantity of berberine is up to 109.4 mg ml−1 (wet resin) by the way of dynamic adsorption and desorption experiment. The resin could be reused by the mixture with 0.5% sodium chloride and 80% ethanol. On the one hand the hydrophobic PDVB in the PDVB/PNaA IPN resin has the ability of adsorption using π–π interaction, and on the other hand the hydrophilic PNaA in the PDVB/PNaA IPN resin has the ability of adsorption using ion exchange interaction. An important conclusion can be drawn that the PDVB/PNaA IPN resin has a promising application prospect in extracting and separating quaternary ammonium type alkaloids such as berberine. Copyright © 2009 John Wiley & Sons, Ltd.

Thermodynamic and NMR investigations on the adsorption mechanism of puerarin with oligo‐β‐cyclodextrin‐coupled polystyrene‐based matrix

AbstractBACKGROUND: Adsorption of puerarin on native resin polystyrene (PS) and oligo‐β‐cyclodextrin‐coupled matrix (PS‐CDP) was studied for interactions between the adsorbents and the adsorbates. The sorption mechanism on PS‐CDP was investigated using the isosteric heat approach and nuclear magnetic resonance (NMR) spectroscopy.RESULTS: The equilibrium adsorption data of puerarin on the two matrices PS and PS‐CDP (polystyrene‐based matrix before and after coupling by oligo‐β‐cyclodextrin) in the temperature range 288–318 K were well fitted to the Freundlich adsorption isotherm model. The energetic heterogeneity of the media was observed based on the result that the values of isosteric enthalpy were quantitatively correlated with the fractional loading of puerarin adsorption. The more heterogeneous surface of PS‐CDP compared with PS was attributed to the complexation between puerarin and β‐cyclodextrin (β‐CD). NMR studies validated the formation of an inclusion complex puerarin/β‐CD.CONCLUSION: Thermodynamic and NMR studies confirmed that multi‐interaction cooperatively governed the isolation of puerarin from aqueous solution on PS‐CDP matrix. Copyright © 2009 Society of Chemical Industry

Adsorption Performances for Vanillin from Aqueous Solution by the Hydrophobic-Hydrophilic Macroporous Polydivinylbenzene/Polyacrylethylenediamine IPN Resin

采用分步悬浮聚合法制备了具有疏水性能的聚二乙烯基苯（polydivinylbenzene，PDVB）为第一网，具有亲水性能的聚丙烯酰乙二胺（polyacrylethylenediamine，PAEM）为第二网的疏水／亲水大孔聚二乙烯基苯／聚丙烯酰乙二胺（PDVB／PAEM）互穿聚合物网络（interpenetrating polymer networks，IPN），研究这类疏水／亲水IPN组成的树脂对吸附质的吸附热力学和吸附动力学；测定了该树脂对香兰素在不同温度下的吸附等温线，吸附等温线符合Freundlich等温吸附方程，利用热力学函数关系计算出了吸附焓、自由能和熵变．实验表明，PDVB／PAEMIPN树脂中疏水性的PDVB网具有疏水作用吸附能力，亲水性的PAEM网具有氢键作用吸附能力．吸附动力学数据符合Lagergren二级速率方程，颗粒内扩散是吸附速率的主要控制步骤，可采用HSDM模型加以描述．

Adsorption and desorption of hydrogen on metal–organic framework materials for storage applications: comparison with other nanoporous materials

Hydrogen adsorption on porous materials is one of the possible methods proposed for hydrogen storage for transport applications. High pressure experimental studies of a wide range of porous materials have obtained maximum hydrogen excess capacities of 6-8 wt% at 77 K for metal-organic frameworks (MOFs) and porous carbon materials. Grand canonical Monte Carlo (GCMC) simulation studies indicate that higher hydrogen capacities are possible for covalent organic frameworks (COFs). Currently, the maximum isosteric enthalpies of adsorption of approximately 13 kJ mol(-1) at 77 K have been observed experimentally for metal-organic framework materials and this is higher than for COFs, where the maximum predicted from GCMC simulations is approximately 8 kJ mol(-1). Metal-organic framework materials have structural diversity and scope for modification of surface chemistry to enhance hydrogen surface interactions. The synthesis of MOFs with stronger H(2)-surface interactions to give similar hydrogen capacities at much higher temperatures than 77 K is required and eventually, materials that have these high capacities at ambient temperatures with rapid adsorption/desorption characteristics are necessary for applications as hydrogen storage materials for transport applications. The current methods envisaged for increasing adsorption at higher temperatures involve modification of the surface chemistry, in particular, the inclusion of open metal centres to increase hydrogen surface site interactions, and utilisation of the framework flexibility are discussed.