Temperature dependence of methanol adsorption on sodium and polyhydroxyaluminum montmorillonites

Changes that occur in the surface properties and porous structure of montmorillonite upon the substitution of its exchange cations by polyhydroxyaluminum cations are studied. Preliminary thermovacuum treatment noticeably affects the adsorption properties of polyhydroxyaluminum montmorillonite with respect to methanol. The dependences of the adsorption heat on the amount of adsorbed methanol are established based on a set of isosteres of CH₃OH adsorption on dehydrated sodium and modified montmorillonites measured in a wide temperature range.     

Modification of Potassium Chabazites Derived from Fly Ash by Dosing Extra Cations: Promoted CO2 Adsorption Capacities and Fine‐Tuned Frameworks

Potassium chabazite (K‐CHA), a typical microporous zeolite with excellent CO₂ separating properties, was synthesized with waste fly ash and modified via cation dosing treatments using cesium and zinc cations, respectively. The resulting CHAs were analyzed by XRF, XRD, FT‐IR, SEM, and N₂ physisorption, whose CO₂ adsorption properties were then tested on the reorganized TGA apparatus. It showed from XRF data that cesium and zinc cations were successfully imported in the original K‐CHA by cation dosing, but the CHA microstructures and morphologies of K‐CHA were perfectly retained as confirmed by XRD, FT‐IR, SEM and N₂ physisorption. Since there were still over 9 potassium cations per unit cell in cation dosed Cs‐CHA and Zn‐CHA, they both maintained the favored properties of K‐CHA as “molecular trapdoors”. In the following adsorption experiments, the CO₂ uptakes of Cs‐CHA and Zn‐CHA at 333 K and 1 bar, compared with K‐CHA, elevated from 1.70 mmol · g^–1 to 2.34 and 2.03 mmol · g^–1, and the import of zinc cation also presented a positive effect on the adsorption kinetics. Detailed comparisons suggested modifications with cesium and zinc cations fine‐tune the CHA complying with different mechanisms, and CHAs modified via cation perform more approvingly than fully ion‐exchanged ones, providing us important insights into CHA modifications and applications in practice.     

Synthesis and adsorption investigations of zeolites MCM-22 and MCM-49 modified by alkali metal cations

Ion exchange was made on MCM-22 and MCM-49 zeolites with different Si/Al molar ratios, with Li⁺, Na⁺, K⁺, and Cs⁺ ions and the study of the influence of alkali metal cations on CO₂ adsorption properties was performed. The degree of ion-exchange decreased for larger cations (Cs⁺) apparently due to steric hindrances. The exchange with different cations led to a decrease in the surface area and the micropore volume. Our study shows that the adsorption capacity of the tested zeolites depends significantly on the nature and the concentration of the charge-compensating cations. The highest CO₂ adsorption capacity was obtained on the MWW zeolites with the lowest Si/Al molar ratio and the Li⁺ or K⁺ cations.     

Adsorption of volatile organic compounds in porous metal–organic frameworks functionalized by polyoxometalates

The functionalization of porous metal–organic frameworks (Cu₃(BTC)₂) was achieved by incorporating Keggin-type polyoxometalates (POMs), and further optimized via alkali metal ion-exchange. In addition to thermal gravimetric analysis, IR, single-crystal X-ray diffraction, and powder X-ray diffraction, the adsorption properties were characterized by N₂ and volatile organic compounds (VOCs) adsorption measurements, including short-chain alcohols (C<4), cyclohexane, benzene, and toluene. The adsorption enthalpies estimated by the modified Clausius–Clapeyron equation provided insight into the impact of POMs and alkali metal cations on the adsorption of VOCs. The introduction of POMs not only improved the stability, but also brought the increase of adsorption capacity by strengthening the interaction with gas molecules. Furthermore, the exchanged alkali metal cations acted as active sites to interact with adsorbates and enhanced the adsorption of VOCs.     

The effect of the dehydration conditions of chitosan-based polymeric adsorbents on the adsorption of nickel cations

The structure and adsorption properties (with respect to water vapor and nickel cations) of chitosan samples dehydrated by either lyophilization after precipitation with NaOH or Na₂CO₃ or drying in air are investigated. IR spectroscopy and X-ray diffraction data testify that the lyophilization causes both changes in the conformation of chitosan macromolecules and a substantial rise in the crystallinity of the polymer to compare with the air dried one. Chitosan structure ordering resultant from the lyophilization noticeably increases its sorption ability with respect to both water vapor and nickel cations. For example, the sorption capacity of the lyophilized chitosan with respect to nickel cations is as high as 4.5 mmol/g and, thus, is threefold higher than that of air-dried chitosan. The comparative analysis of water adsorption isotherms in the region of low degrees of filling indicates that chitosan structure ordering resultant from the lyophilization changes the number and the energy of primary adsorption sites.     

Surface properties of catalytic aluminas modified by alkaline-earth metal cations: A microcalorimetric and IR-spectroscopic study

Lewis acidic properties of transition aluminas whose surfaces have been doped with alkaline-earth metal cations (Ca²⁺ and Ba²⁺) were studied by means of the room temperature adsorption of carbon monoxide. The vibrational features of CO adsorbed at the surface of doped aluminas were investigated by IR spectroscopy in comparison with pure parent aluminas, while the quantitative and energetic features were studied by adsorption microcalorimetry. Various CO adspecies were found to form at the surface of both pure and doped-alumina, owing to the structural heterogeneity of the Al₂O₃ surface and to the presence of alkaline-earth metal cations. The surface heterogeneity was revealed by different v_co stretching frequencies, namely v_co≈2230, 2218 and 2205 cm⁻¹ for coordinatively unsaturated tetrahedral Al³⁺ cations in different crystallographic configurations, and v_co≈2186 and 2172 cm⁻¹ for coordinatively unsaturated Ca²⁺ and Ba²⁺ cations, respectively. Heats of adsorption of ≈80, 70 and 55 kJ/mol were assigned to the formation of Al³⁺/CO complexes, ≈45 kJ/mol for Ca²⁺/CO and ≈30 kJ/mol for Ba²⁺/CO complexes. The latter value was estimated through a correlation curve existing between v_co stretching frequencies and adsorption enthalpies. This correlation, already proposed in the past for CO adsorbed on non-d/d⁰/d¹⁰ metal cations, has been revisited and confirmed here, by including Al₂O₃ data for which an apparent lack of correlation between the two parameters was first observed. With respect to pure alumina, the population of Lewis acidic sites was found to be significantly depressed by the presence of alkaline-earth cus metal cations. These acidic sites are intrinsically weaker than tetrahedral cus Al³⁺ cations, as witnessed by smaller upward shifts of the v_co stretching frequencies with respect to CO gas and lower heats of adsorption, in accordance with expectations from the charge/ionic radius ratios. Ca²⁺ cations were found to compete in adsorbing CO with Al³⁺ cations more efficiently than the larger Ba²⁺ cations. In the case of CaO/Al₂O₃ systems outgassed at 1023 K, a thin surface layer of calcium aluminate, not detected by XRD or HRTEM, was suggested to form.     

Effects of activation conditions on the structural and adsorption characteristics of pinecones derived activated carbons

This study was conducted to understand and optimize the activation process for the production of a low-cost activated carbon (AC) using a renewable and plentiful biomass waste, pinecones. This was achieved by tracking the changes in porous structure, surface chemistry and adsorption properties of the AC produced using different activating agents, activation temperatures, holding times and heating rates. Generally, produced ACs were predominantly microporous with small external surface area and were different in terms of H/C and O/C ratios. Study of Pb²⁺ cations adsorption on these samples proved the high affinity of the pinecones derived ACs to this cation. The best adsorption behaviour was recognized in sample prepared by impregnation with H₃PO₄ at weight ratio of 2, then heating at 400?°C for 2?h at 5?°C/min heating rate. This sample possessed the highest BET surface area (1335 m²/g). The adsorption process obeyed the pseudo-first-order and Freundlich model slightly better than the pseudo-second-order kinetics and Langmuir model. The high Langmuir maximum adsorption capacity of 418?mg/g supports the applicability of the produced AC for the removal of Pb²⁺ cations from wastewater.     

Toward a Better Understanding on the Adsorption Behavior of Aromatics in 12R Window Zeolites

Benzene adsorption behavior in a large family of 12R window zeolites (X, Y, EMT, Beta and LTL) has been examined by means of in-situ FTIR spectroscopy and correlated with the zeolite structure, the type and number of counter-ions, and the negative charge on framework oxygen atoms of zeolites. The effect of coadsorption of HCl, NH₃ and CH₃NH₂ on the benzene location has also been studied. The present work illustrates that besides the benzene adsorption on counter ions of zeolites, the 12R windows could also be the adsorption sites for benzene. Upon adsorption of coadsorbates such as HCl, NH₃ and CH₃NH₂, the migration of preadsorbed benzene molecules from one type of adsorption sites towards another, i.e. from 12R windows towards the cations for HCl and opposite direction for NH₃ and CH₃NH₂, has been evidenced. The lack of adsorption of benzene on 12R windows of NaBeta even upon coadsorption of a series of basic molecules reveals that benzene adsorption on 12R windows is most likely governed by a molecular recognition effect where benzene molecule and 12R window should have the adapted chemical and structural properties like in enzyme-substrate system and zeolites can be referred to as solid enzymes or zeo-enzymes. This paper indicates also that the adsorption properties of zeolites can be modified and accommodated by introduction of a co-adsorbate.     

Interaction of cadmium nitrate with the surface of functionalized organosilicas

The adsorption of Cd^II cations on the surface of amorphous macroporous silicas chemically modified by β-cyclodextrin and its functional derivatives was studied. The adsorption of Cd^II cations was shown to follow the equation of the Freundlich isotherm for the heterogeneous surface. Analysis of the adsorption kinetic curves showed that two parallel processes occurred on the surface of β-cyclodextrin-containing silicas. A substantial increase in the adsorption of Cd(NO₃)₂ is a result of the formation of uncharged supramolecular structures on the surface of silica adsorbents. The composition of these structures depends on the polarizability of functional substituents of β-cyclodextrins.     

Adsorption of kryptates of two-charged cations on a mercury electrode

The adsorption of complexes formed by cations of calcium, strontium, and barium with a macrobiocyclic ligand (Kryptofix® 222 of composition C₁₈H₃₆N₂O₆) is studied as a function of its concentration in 0.1 M solutions of corresponding chlorides by a method of measuring the differential capacitance on a stationary mercury drop. The adsorption parameters of the kryptates studied are determined by a method of regression analysis on the basis of the model of two parallel capacitors supplemented by the Frumkin adsorption isotherm. The differential-capacitance curves, calculated on the basis of these parameters, are compared with experimental data. The determined values of adsorption parameters for kryptates of single-and two-charged cations made it possible to confirm the earlier assumption about a deep mutual relation between the behavior of complexes under study and their structure.     

Physicochemical and catalytic characteristics of La-H-ZSM-5 zeolite in converting dimethyl ether to the mixtures of gasoline hydrocarbons: Effect of ion exchange conditions

The effect of the manner and conditions of introducing lanthanum cations into NH₄-ZSM-5 zeolite on the properties of catalysts for the conversion of dimethyl ether into the mixtures of gasoline hydrocarbons is studied. The physicochemical properties of synthesized catalysts are studied by means of temperature-programmed ammonia desorption, the adsorption of benzene, atomic absorption spectroscopy, differential scanning calorimetry, and thermogravimetry. It is shown that the degree to which lanthanum cations are replaced by ammonium cations both depends on the conditions of ion exchange in the zeolite and affects its acidity spectrum and the selectivity of the formation of paraffin hydrocarbons with isostructure. It is concluded that an increase in the amount of introduced lanthanum leads to an increase in the content of iso-paraffins from 69 to 76 wt % and a decrease in the content of aromatic hydrocarbons from 10.5 to 5.5 wt % and that of durene from 1.5 to 0.2 wt % in the products.     

Adsorption and diffusion of nitrogen, methane and carbon dioxide in aluminophosphate molecular sieve AlPO4-11

The knowledge about the adsorption and diffusion properties (specially about diffusion) of aluminophosphate molecular sieves is very scarce in the literature. These materials offer interesting properties as adsorbents as they have a polar framework and do not contain charge-balancing cations. In this work, the adsorption isotherms of nitrogen, methane and carbon dioxide over an AlPO₄-11 sample synthesized in our laboratories have been measured with a volumetric method at 25, 35, 50 and 65 °C over a pressure range up to 110 kPa. The adsorption capacities of each gas are determined by the strength of interaction with the pore surface (carbon dioxide > methane > nitrogen). The equilibrium selectivity to carbon dioxide is quite high with respect to other adsorbents without cations due to the polarity of the aluminophosphate framework. The adsorption Henry’s law constants and diffusion time constants of nitrogen, methane and carbon dioxide in the synthesized AlPO₄-11 material have been measured from pulse experiments. A pressure swing adsorption (PSA) process for recovering methane from a carbon dioxide/methane mixture (resembling biogas) has been designed using a dynamic model where the measured adsorption equilibrium and kinetic information has been incorporated. The simulation results show that the proposed process could be simpler than other PSA processes for biogas upgrading based on cation-containing molecular sieves such as 13X zeolite, as it can treat the biogas at atmospheric pressure, and it requires a lower pressure ratio, to produce high purity methane with high recovery.     

Composite Water Sorbents of the Salt in Silica Gel Pores Type: The Effect of the Interaction between the Salt and the Silica Gel Surface on the Chemical and Phase Compositions and Sorption Properties

The influence of the inorganic salt-silica gel surface interaction on the chemical and phase compositions and sorption properties of composites of the salt in silica gel pores type is studied. Two possible interaction mechanisms are considered: (1) the ion-exchange adsorption of metal cations on the silica gel surface from a solution of a salt (CaCl₂, CuSO₄, MgSO₄, Na₂SO₄, and LiBr) and (2) the solid-phase spreading of a salt (CaCl₂) over the silica gel surface. The adsorption of metal cations on the silica gel surface in the impregnation step affords ≡Si-OM ⁿ⁺¹ surface complexes in the composites. As a result, two salt phases are formed in silica gel pores at the composite drying stage, namely, an amorphous phase on the surface and a crystalline phase in the bulk. The sorption equilibrium between the CaCl₂/SiO₂ system and water vapor depends on the ratio of the crystalline phase to the amorphous phase in the composite.     

Adsorption of Toluene Vapors on Some Organomontmorillonites

The changes in the surface properties and porous structure of montmorillonite are investigated upon the substitution of tetramethylammonium or pyridinium cations for exchange Na⁺ cations. It is established that preliminary thermal treatment in vacuum substantially affects the adsorption properties of tetramethylammonium and pyridinium montmorillonites in relation to toluene. The dependences of the heats of adsorption and desorption on the amount of adsorbed substance are determined in a wide temperature range using the results of measuring the isosters of toluene adsorption and desorption by dehydrated organomontmorillonites.     

Adsorption of CO2 and N2 in Na–ZSM-5: effects of Na+ and Al content studied by Grand Canonical Monte Carlo simulations and experiments

Zeolite crystals with cations present, such as ZSM-5, are widely used for gas sequestration, separations, and catalysis. One possible application is as an adsorbent to separate CO₂ from N₂ in flue gas mixtures. Typically, the zeolite framework is of a SiO₂ composition, but tetravalent Si atoms can be replaced with trivalent Al atoms. This change in valence creates a charge deficit, requiring cations to maintain the charge balance. Experimental studies have demonstrated that cations enhance adsorption of polar molecules due to strong electrostatic interactions. While numerous adsorption studies have been performed for silicalite-1, the all-silica form of ZSM-5, fewer studies on ZSM-5 have been performed. Grand Canonical Monte Carlo simulations were used to study adsorption of CO₂ and N₂ in Na–ZSM-5 at T = 308 K, which is ZSM-5 with Na⁺ counter-ions present. The simulations suggest that a lower Si/Al ratio (or higher Na⁺ and Al content) substantially increases adsorption at low pressures. At high pressures, however, the effect of the Al substitutions is minor, because the Al^?/Na⁺ sites are saturated with guest molecules. Similarly, a lower Si/Al ratio also increases the isosteric heat of adsorption at low loading, but the isosteric heats approach the silicalite-1 reference values at higher loadings. Comparison of simulations and experimental measurements of the adsorption isotherms and isosteric heats points to the importance of carefully considering the role of charge on the Na⁺ cations, and suggest that the balancing cations in ZSM-5, here Na⁺, only have partial charges.     

Determining Sodium Kryptate Adsorption Parameters on a Mercury Electrode in 1 M Na2SO4

The adsorption of a complex of sodium cations with a macrocyclic ligand (Kryptofix^R 222, composition C₁₈H₃₆N₂O₆) as a function of its concentration in 1 M Na₂SO₄ is studied by measuring the differential capacitance on a stationary Hg drop. Adsorption parameters of sodium kryptate are found using a regression analysis method and various versions of a model of two parallel capacitors complemented with the Frumkin adsorption isotherm. The differential capacitance curves, calculated on the basis of these, are compared with experimental data. The difference in model versions that most adequately describe the adsorption data, established for systems in 0.1 and 1 M Na₂SO₄, is explained by the influence of the supporting electrolyte on the adsorption layer structure. Conclusions are made on the absence in the system under study of the salting-out from the bulk solution and on a change in the properties of an adsorption layer of sodium kryptate in the region of potentials of the anodic adsorption–desorption peak following expansion of the adsorption region.     

Ethylene and propylene adsorption and the formation of hydrocarbon complexes with NO2 upon interaction of adsorbed olefins with an NO + O2 mixture over HZSM-5 and CuZSM-5 zeolites studied by TPD and ESR

Adsorption of C₂H₄ and C₃H₆ on copper in oxidized samples of CuZSM-5 is found to increase with the copper concentration; simultaneously, olefin adsorption on the Br?nsted acid sites decreases. The Cu²⁺ cations in the square-planar coordination exhibit higher reactivity in olefin adsorption than copper cations in the square-pyramidal coordination. Thermal treatment of CuZSM-5 with hydrogen results in regeneration of the Br?nsted acid sites for olefin adsorption and the disappearance of Cu²⁺ cations, the active sites of adsorption, due to the reduction of Cu²⁺ to Cu⁺ and Cu⁰. Desorption peaks appear in the TPD spectra upon the interaction between the adsorbed hydrocarbons and NO₂. These peaks are not observed upon separate adsorption of the reactants, and they are likely due to decomposition of NO₂-hydrocarbon complexes over both the Br?nsted and copper-containing sites of the zeolite     

Acidity of Alginate Aerogels Studied by FTIR Spectroscopy of Probe Molecules

Supercritical drying of alginate gels is an efficient way to prepare aerogels with high surface area (>300 m² · g⁻¹). FTIR spectroscopy allows to monitor the adsorption of NH₃ from the gas phase onto the acid sites of the alginate. Free carboxylic groups are effective Brønsted sites, whereas the divalent cations used in the ionotropic gelation present the properties of Lewis sites. The ratio between Brønsted and Lewis sites provides infomation on the role of pH in alginate gelation and suggests that non-buffered gelation by transition-metal cations is a mixed ionotropic-acid process.     

Effects of Cetyltrimethylammonium Bromide and Heptane on the Surface Properties and CO2 Adsorption of Zeolite NaX

Zeolite X was synthesized by the gelation method for use as a CO₂ adsorbent. The zeolite was prepared from a sodium silicate solution and Al(OH )₃ as silicon and aluminum source, respectively. The effects of cetyltrimethylammonium bromide (CTAB ) and heptane addition on surface properties, morphology, and CO₂ adsorption capacities were investigated. The CO₂ adsorption capacity of the synthesized zeolite with the addition of CTAB and heptane increased as demonstrated by the higher CO₂ uptake (3.00 mmol/g), which was due to the higher surface area and pore volume. Interaction between CTAB micelles and sodium cations with aluminosilicate anions led to an increase in the nucleation rate and smaller crystalline sizes. The addition of heptane resulted in higher positively charged CTAB micelles and improved the affinity between the micelles and aluminosilicate anions.     

Equilibrium adsorption of <Emphasis Type="Italic">n</Emphasis>-hexane and carbon tetrachloride vapors on sodium and polyhydroxyaluminum montmorillonites

Changes in the sorption and energy characteristics of montmorillonite upon the substitution of polyhydroxyaluminum cations for exchange sodium ions are studied. Based on the temperature dependence of the equilibrium parameters of adsorption, it was found that the substitution of polyhydroxyaluminum cations for exchange ions results in an increase of micropore volume and the heat of adsorption. The curves of heats of adsorption for C₆H₁₄ and CCl₄ have extreme characters. The observed maxima are related to the interactions of adsorbate molecules with and between active sites due to their compaction during the filling of micropore volume.