Influence of exchangeable cations on the character of dehydration of phillipsite

Various ion-exchange forms of phillipsite (Phi) were prepared. Anomalous dependences of the extent of rehydration and heats of immersionQ _i on the temperature of thermovacuum treatment were revealed for the initial Na, K- and K-forms made by ion exchange. The curves pass through a minimum between 100 and 180°C, indicating structural changes accompanied by considerable contraction of the zeolite framework. No structural changes were observed when the ammonium form of phillipsite was evacuated at elevated temperatures; the curveQ _i =F(T) passes through a maximum at 250°C. It was calculated that NH₄-Phi is thermally much less stable than Na, K-Phi or K-Phi.     

The influence of exchangeable cations on the carbonization of biomass

Cations of calcium and potassium added to wood through ion exchange have unique effects on the thermal decomposition of the wood. Addition of calcium ions increases the decomposition temperature of the wood and affects the char yield only slightly, while addition of potassium ions reduces the decomposition temperature and significantly increases the char yield. These effects are distinct from the effects of salts of the same elements added to the same or similar materials through absorption of a solution of the salt. For instance, potassium carbonate absorbed on cellulose significantly increases its decomposition temperature, although it has the opposite effect when added to wood through ion exchange. The primary sites for ion exchange are believed to be glucuronic acids in the hemicellulose fraction, and it is therefore likely that the effects of exchanged cations are due primarily to their influence on the decomposition of this component. When naturally occuring inorganic species are removed from wood by acid washing, the char yield is reduced and the cellular structure of the wood can be lost during carbonization, particularly at high heating rates.     

Influence of the exchangeable cations on SO<Subscript>2</Subscript> adsorption capacities of clinoptilolite-rich natural zeolite

The sulfur dioxide adsorption on clinoptilolite-rich tuff from Bigadiç region of Western of Turkey and its modified forms (Na⁺, K⁺, Ca²⁺ and Mg²⁺) have been studied at 273 K and 293 K up to 100 kPa. The structural properties of clinoptilolites were studied by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR). The quantitative XRD analysis demonstrated that the Natural-B sample is mainly constituted by clinoptilolite (80–85%) with minor contents of quartz (7–8%), feldspar (5–6%) and mica-illit (4–5%). It was found out that the adsorption capacity and the affinity of SO₂ with clinoptilolite samples depended mainly on the type of exchanged cations and decreased as Na-B > K-B > Mg-B > Natural-B > Ca-B for both temperature. These results show that clinoptilolite-rich zeolites are considered potentially good adsorbents for SO₂ removal.     

Influence of the exchangeable cations on stability and rheological properties of montmorillonite suspensions

The effects of NaCl and CaCl₂ on the colloid stability and rheological properties of Na- and Ca-montmorillonite dispersions were studied. The distribution of cations between the surface and the bulk phase was determined. For both of monocationic montmorillonite, the critical coagulation concentration were 250 mmol NaCl/dm³ and 2 mmol CaCl₂/dm³. The changes in the Bingham yield stresses of Na- and Ca-montmorillonite dispersions as functions of the NaCl and CaCl₂ concentration could be explained in terms of the surface excess amount and equilibrium concentration of the cations, the second electroviscous effect and the formation of a gel structure.     

Carbon dioxide adsorption by synthetic mordenite isotherms and differential heat of adsorption

Carbon dioxide adsorption isotherms by synthetic mordenite are determined over the ranges 10^?5 atm<P<50 atm and ?77°C<T<+160°C, differential heats of adsorption calorimetrically measured over 10^?5atm<P<l atm and ?77°C< T< + 120°C.Differential heat curves show two adsorption ranges with different energetic characteristics. At the transition a very marked maximum appears.The integral molar entropy and specific heat of adsorbed phase are calculated.Two methods for adsorbed phase density determination are proposed:(i) The first, straight from maximal adsorbed amount at 50 atm. where adsorption isotherms level out.(ii) The second, based on adsorption potential theory with an elementary graphical determination.When the adsorbed phase density is so determined, Dubinin's equation can be successfully applied.     

Effects of exchanged surfactant cations on the pore structure and adsorption characteristics of montmorillonite

Ca-montmorillonite (Ca-Mont) was exchanged with two quaternary amines, tetramethylammonium (TMA) chloride and hexadecyltrimethylammonium (HDTMA) bromide, to study the surfactant ion exchange effect on the pore structure, surface characteristics, and adsorption properties of montmorillonite. The revolution of both the surface area and pore structure of montmorillonite was characterized based on classical and fractal analyses of the nitrogen isotherms as well as the XRD patterns. The change of surface characteristics was identified from FTIR patterns and zeta-potential plots. The adsorption isotherms of acid dye, Amido Naphthol Red G (AR1), were then measured to identify the effects of the ion-exchange process on the adsorption properties of montmorillonite. It was found that the exchange processes might induce an increase or decrease in the surface area, pore size, pore volume, and surface fractal dimension D of montmorillonite, depending on the size, the molecular arrangement, and the degree of hydration of the exchanged ion in the clay. On the other hand, it was also found that the hydrophobic bonding by conglomeration of large C(16) alkyl groups associated with HDTMA could cause positive charge development on the surface of montmorillonite, which was not observed for TMA-modified montmorillonite (TMM). The effects of the alteration of the surface characteristics of montmorillonites on their adsorption selectivity for acid dye were discussed.     

Recationation and adsorption parameter adjustment for natural mordenite

Partially dealuminated forms of natural mordenite after alkali treatment have been used to show that the adsorption rates are increased and the adsorption energies are decreased for various substances as the Si/Al ratio increases. For that reason, there is a maximum in the performance of the modified mordenite specimens as the adsorption energy decreases in the purification of air from CO₂ and in the separation of the major components of air (O₂+N₂). L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 03039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 2, pp. 126–131, March–April, 2000.     

Role of exchangeable cations on geometrical and energetic surface heterogeneity of kaolinites

Textural and energetic proprieties of kaolinite were studied by low-pressure argon adsorption at 77 K. The heterogeneity of four kaolinites (two low-defect and two high-defect samples) modified on their surface by cation exchange with Li+, Na+, or K+ was studied by DIS analysis of the derivative argon adsorption isotherms. The comparison between the derivative adsorption isotherms shows that the nature of the surface cation influences the adsorption phenomena on edge and basal faces. In the case of basal faces, two adsorption domains are observed: for the first one, argon adsorption is slightly sensitive to the nature of the surface cation; for the second one, argon adsorption energy depends on the nature of surface cation suggesting their presence on theoretically uncharged basal faces. This study also shows that the shape of elementary particles, as derived from basal and edge surface areas, changes with the nature of cation. This anomalous result is due to the decrease of edge surface area with increasing the size of the cation. This surface cation dependence can be accounted for the area occupied by the edge surface cations in the first argon monolayer.     

Effect of the exchangeable cations on the spectral properties of methylene blue in clay dispersions

Adsorption of a cationic dye, methylene blue (MB), on the surface of montmorillonite leads to the molecular aggregation of dye cations, reflected by significant changes of dye optical properties. Montmorillonite samples, saturated with various inorganic cations (mono-, bi-, and trivalent, including those of transition metals), were used. Influence of the exchangeable cations on the MB aggregation was tested. Various properties of cations were considered (charge, diameter, acidity, hydration energies). Both direct and potential indirect effects of the cations were taken into account, such as salting-out effect, influence of the ions on solvent polarity, influence on swelling, colloid properties of montmorillonite dispersions, cation hydration properties, hydrolysis, and interaction of the cations with the clay surface. The spectra of MB in dispersions of montmorillonite saturated with NH4+, K+, Rb+, and Cs+ were significantly different from those of other reaction systems. Direct association between large monovalent cations and basal oxygen atoms of silicate probably leads to a partial fixation of the cations, which affects the ion exchange reaction and dye aggregation. Thus, the presence of large monovalent cations leads to the formation of fewer ordered H-aggregates in favor of monomers and aggregates of lower size. In these cases, dye species absorbing light of low energies also appeared in significant amounts and were assigned to J-aggregates, characterized by a head-to-tail intermolecular association.     

Location of Mg cations in mordenite zeolite studied by IR spectroscopy and density functional theory simulations with a CO adsorption probe

The location of Mg cations in the channel of mordenite zeolite was studied using a combination of DFT simulations and IR spectroscopy of adsorbed CO. The calculated adsorption energies and frequencies of CO on Mg cations are in good agreement with the results from the IR spectra of adsorbed CO. It is found that the Mg cations can occupy the sites A, C, D and E in mordenite and the distribution of the Mg cations in these sites follows the priority order, site C > site A > site D and site E.     

The effect of layer charge and exchangeable cations on sorption of biphenyl on montmorillonites

Montmorillonies separated from the bentonites SAz-1 (Cheto, AZ, USA), and Cressfield (New South Wales, Australia) were used as starting materials. Reduced charge montmorillonites (RCMs) were prepared from these chemically different and Li-saturated montmorillonites via heating at temperatures in the range of 120–300°C. The residual exchangeable Li⁺ cations were then replaced with tetramethylammonium (TMA⁺) or hexadecyltrimethylammonium (HDTMA⁺) cations and the ability of the modified montmorillonites to adsorb biphenyl was investigated. Lower adsorption was observed for Li-montmorillonites than for the organoclays. The extent of adsorption was dependent on both the layer charge of montmorillonite and the size of alkylammonium cations. HDTMA-forms prepared from unheated Li-montmorillonites adsorbed biphenyl better than the organoclays prepared from RCMs. In contrast, the TMA-samples prepared from the Li-montmorillonites that were not heated showed low uptake of biphenyl probably due to high content of TMA⁺ cations. Reduction of the layer charge, resulting in lower content of TMA⁺ cations, increased sorption efficiency of both TMA-montmorillonites. The best adsorbents of biphenyl were HDTMA-SAz-1 prepared from the unheated Li-SAz-1 and TMA-Cressfield prepared from the Li-form heated at 180°C. These samples removed about 80% of biphenyl from its aqueous solutions      

Dilatometric study of the adsorption of heavy-metal cations on goethite

The specific volumes of adsorption of Cd, Co, Cu, Ni, Pb, and Zn on goethite determined by means of the dilatometric method are 21, 32, 32, 31, 31, and 42 cm3/mol, respectively, and are independent of pH. The effect of NaCl (up to 0.5 mol dm(-3)) on the specific volume of adsorption is rather insignificant. The specific volume of precipitation of corresponding hydroxides (determined experimentally and calculated) is about 60 cm3/mol. Apparently, the adsorbed heavy-metal cations lose half of their hydration water. The adsorption constant decreases as the pressure increases, and the effect becomes significant at pressures of > 10(7) Pa, i.e., more than 1 km of water column.     

Effects on the aromatic character of DNA/RNA nucleobases due to its adsorption onto graphene

In the last years, experimental/theoretical studies have shown that graphene has a strong affinity toward nucleobases, serving as a promising nanomaterial for self‐assembly, sensing, and/or sequencing of DNA/RNA constituents. Then, a complete picture of the properties of the nucleobase–graphene systems is required for its use in technological applications. This work describes a detailed quantum chemical analysis of the aromaticity of adsorbed nucleobases on graphene, comparing between aromaticity indexes based on magnetic, geometry, electron density, and electron delocalization properties of graphene–nucleobase systems. Contrary to the stated by magnetic‐based aromaticity criteria (such as nucleus‐independent chemical shifts), it is proved that the aromatic character of nucleobases is not increased/decreased upon binding on graphene. Therefore, magnetic aromaticity criteria are not recommended to analyze aromaticity in related systems, unless a fragmented scheme be adopted. Finally, these results are expected to expand the knowledge about the understanding of biomolecules‐graphene interactions.     

Modeling of the diffusion-kinetics-controlled adsorption of cations on a sorbent surface

A mathematical model for describing the sorption of two kinds of cations from dilute immobile liquid solutions on a flat sorbent surface was proposed. A criterion of the transition from the kinetics-controlled sorption mode, in which the selectivity of the sorbent is governed by the ratio between the rate constants of the reactions of the different cations from the solution and adsorption sites on the sorbent surface, to the diffusion-controlled mode, in which the selectivity is determined by the difference between the diffusion coefficient for the migration of the cations to the sorbent surface, was proposed. Formulas were obtained for calculating the surface concentration of adsorbed cations as a function of the time. The calculation results are in a reasonable agreement with the experimental data.     

Study of the adsorption properties of a natural Hungarian mordenite and its ion-exchanged derivatives

The texture and adsorption properties of a natural, Hungarian mordenite were investigated by electron microscopy and BET adsorption measurements. The properties of the ion-exchanged adsorbents were characterized by CO₂ and SO₂ isotherms. The measurements were evaluated on the basis of the pore-filling theory of Dubinin. We have found that at 20°C the H⁺-form of mordenite interacts preferably with CO₂, while on the Na⁺- and Ca²⁺-derivatives stronger adsorption of SO₂ takes place. This fact accounts for their applicability as selective adsorbents.

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, . CO₂ SO₂. . , 20°C H⁺- CO₂, Na⁺- Ca²⁺- SO₂ . .

     

氨在丝光沸石位置(VI)上的吸附

The adsorption of NH3 on (AO)3SiO*HAl(OA)2OSi(OA)3 cluster from the structure of site (VI) of mordenite Zeolite has been calculated by an adsorption model of hydrogen-bond form, quantum chemical method, CNDO/2 scheme. Variations of adsorption distance between NH3 and O* in the cluster were directly related to initial adsorption heats of NH3 on O*H group, jump frequency of the proton between NH3 and O* and frequency shift of IR band with O*H group. Adsorption distance of NH3 on O*H group has been calculated, the range was about 2.5 to 2.75A, and the most favourable adsorption distance with strong acid strength was 2.5A. It was related to the maximum initial adsorption heat which was 33.80 kcal/mol, the lowest activation energy of proton mobility between NH3 ane O*, and the largest frequency shift of IR bands with O*H group. This result qualitatively corresponded to experimental observations. It showed that the various representations of acidic strength are equivalent to each other which could be explained on basis of adsorptin model of hydrogen bond form.