Particle distribution in a microporous material: experiments with zeolite A.

The theory on particle distribution and exchange equilibria in a microporous material is applied to experimental ion-exchange data involving zeolite Na-A and zeolite K-A, with silver ions as the exchanging species. The presented method enables direct evaluation of the measured data and consideration of nonequivalent particle sites. The isotherms of the K+ versus Ag+ exchange in zeolite K-A rise much more steeply, at low exchange degrees, than those of the Na+ versus Ag+ exchange in zeolite Na-A. This result implies a different course of the ion-exchange reactions. Spectroscopic measurements on dehydrated, partly silver-exchanged zeolites Na-A and K-A do indeed show that in zeolite Na-A, the Na+ ions occupying four-ring positions are exchanged faster for Ag+ than the Na+ ions occupying eight- and six-ring positions, while in zeolite K-A the exchange does not start with the four-ring ion but with six-ring ions, followed by the four-ring ion. These findings are consistent with the results obtained from evaluation of the ion-exchange data. The resulting thermodynamic quantities significantly differ from published reference values, which we suggest should be revised.     

Experimental and Modeling Study of Ion Exchange Between Aqueous Solutions and the Zeolite Mineral Clinoptilolite

Ion-exchange experiments were conducted at 25°C between the zeolite mineral clinoptilolite and aqueous solutions of Na⁺/Sr²⁺ (0.005, 0.05, and 0.5 N), K⁺/Sr²⁺ (0.05N), and K⁺/Ca²⁺ (0.05 N). The isotherm data were used to derive equilibrium constants and Gibbs energies for the ion-exchange reactions and Margules parameters for the zeolite solid solution. The Margules model, in combination with the Pitzer equations for activity coefficients of aqueous ions, was used to predict isotherms for ion exchange involving clinoptilolite and aqueous solutions of Na⁺/Sr²⁺, K⁺/Sr²⁺, and K⁺/Ca²⁺ over wide ranges of solution composition and concentration. The ion-exchange isotherms are strongly dependent on the total solution concentration. For Na⁺/Sr²⁺ ion exchange, isotherm values at 0.005 and 0.5 N predicted using thermodynamic parameters derived from the 0.05 N data showed excellent agreement with measured values. The model was also applied to calculations of aqueous composition based on the chemistry of coexisting zeolite phases. The results show that the aqueous composition can be predicted well from the composition of the zeolite, at least for systems that involved binary (two-cation) exchange. Because the thermodynamic model can be easily extended to ternary and more complicated mixtures, it may be useful for modeling ion-exchange equilibria in multicomponent systems.     

Oxidation of cyclohexane catalyzed by metal-ion-exchanged zeolites

The ion-exchange rates and capacities of the zeolite NaY for the Cu(II), Co(II), and Pb(II) metal ions were investigated. Ion-exchange equilibria were achieved in approximately 72 h for all the metal ions. The maximum ion exchange of metal ions into the zeolite was found to be 120 mg Pb(II), 110 mg Cu(II), and 100 mg Co(II) per gram of zeolite NaY. It is observed that the exchange capacity of a zeolite varies with the exchanged metal ion and the amount of metal ions exchanged into zeolite decreases in the sequence Pb(II) > Cu(II) > Co(II). Application of the metal-ion-exchanged zeolites in oxidation of cyclohexane in liquid phase with visible light was examined and it is observed that the order of reactivity of the zeolites for the conversion of cyclohexane to cyclohexanone and cyclohexanol is CuY > CoY > PbY. It is found that conversion increases by increase of the empty active sites of a zeolite and the formation of cyclohexanol is favored initially, but the cyclohexanol is subsequently converted to cyclohexanone.     

CXN天然沸石的研究1: 离子交换与结构性质表征

采用离子交换和焙烧等方法,对我国CXN天然沸石(STI型)进行改性。应用化学分析,粉末XRD,TG/DTA,^2^7AlMASNMR,^2^9SiMASNMR,低温N~2吸附等方法表征相关的结构、离子交换等性质。CXN沸石的晶胞组成Na~0~.~2Mg~0~.~1Ca~8~.~4[Al~1~7~.~2Si~5~4~.~8O~1~4~4].65H~2O,属富钙型。经离子交换脱Ca^2^+后的沸石在焙烧过程中伴随有骨架脱铝,骨架的热稳定性已由原样的500℃以下提高到700℃以上。交换改性后的沸石,呈现反映该沸石微孔特性的I型氮吸附等温线。     

Iranian natural clinoptilolite and its synthetic zeolite P for removal of cerium and thorium from nuclear wastewaters

The ion-exchange behaviors of an Iranian natural clinoptilolite and its modified forms as well as a relevant synthetic zeolite P were investigated toward cerium and thorium from nuclear wastewaters. Column experiments were performed on different exchangers in various conditions and the effect of parameters such as particle size, pH, temperature, and time were considered. The distribution coefficient, cation exchange capacity and some thermodynamic parameters were calculated. Ion-exchange isotherms and break-through curves were plotted. As a result, the selectivity of synthetic zeolite P from Iranian natural clinoptilolite toward cerium and thorium was compared with that of natural and cationic forms of clinoptilolite     

Equilibrium cation exchange on natural heulandite

Corrected selectivity coefficients are determined from the zeolite counter-ion composition and thermodynamic ion-exchange constants by studying ion-exchange equilibria in the system natural heulandite-binary aqueous solution for NaCl, KCl, CsCl, NH₄Cl, Pb(NO₃)₂, LiCl·H₂O, and Ca(NO₃)₂·4H₂O. The sorbent composition after ion exchange from multicomponent aqueous solutions of differently charged electrolytes can be calculated in advance using data for the equilibria of binary exchange.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1474–1477, July, 1990.     

Zeolite an efficient catalyst for the Biginelli condensation reaction

A zeolite catalyzed, single step and environmentally friendly process for synthesis of classical Biginelli reaction was investigated. For this reaction Transition metal/Y zeolites were prepared by microwave solid-state and aqueous solution ion-exchange methods. The yield of reactions was increased in order of CuY > CoY > NiY > MnY ≈ FeY > VY > CrY > ZnY for the solid-state zeolite ion-exchange and CuY > CoY > NiY > MnY > CrY > VY > ZnY > FeY for the aqueous solution ion-exchange. The solid-state ion-exchange zeolite by microwave irradiation showed higher activity compared to the aqueous solution exchange. The yield of the product in the present of CuY zeolite was in order of 22–50%.     

Adsorption of cesium from spent nuclear fuel basin water

Summary Batch equilibrium and kinetic measurements were performed for Cs⁺ exchange in silicotitanate zeolite (Ionsiv^® TIE-96) at 30 and 60 °C. The Langmuir isotherm equation provided a good fit of the equilibrium data. The heats of exchange reaction between Cs⁺ in the aqueous solution and Na⁺ in the zeolite structure were derived from the equilibrium data. The results indicate that the exchange mechanism is different from that of physical adsorption on heterogeneous materials. The apparent diffusion coefficients and activation energy were derived from the kinetic data and the values obtained for inter-diffusion of Cs⁺ and Na⁺ cations in the zeolite structure were 1.33 ^. 10^-12 and 1.04 ^. 10^-12 cm^{2 .} s^-1 at 30 and 60 °C, respectively. The activation energy for Cs⁺ was 1.7 kcal/mol, suggesting that the Cs⁺ cation can access easily all the sites in the zeolite framework. Thus, the exchange of Cs⁺ with Na⁺ in the zeolite was not hindered by ion-sieve effects.     

Distribution of adsorbed molecules along zeolitecavities

Conclusions From the experimental isotherm of adsorption on a zeolite, on the assumption of a quasiindependence of the molecules found in different cavities, it is possible to determine the distribution of the adsorbed molecules along the zeolite cavities. If the adsorption isotherm satisfies the Henry equation, then this distribution coincides with the Poisson distribution, while if it satisfies the Langmuir equation then the distribution coincides with the Bernoulli distribution.Translated from. Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 452–454, February, 1973.     

Ion-exchange foam chromatography : Part II. Rapid separations on heterogeneous cation-exchange foams

The possibility of using polyurethane-Varion KS heterogeneous cation-exchange foam for rapid separations in aqueous and alcoholic solutions was investigated. The exchange equilibria on the ion-exchange foam were attained rapidly and the break-through capacity of columns packed with the foam material was quite acceptable. Relatively high flow-rates could be applied without any appreciable loss in column performance. With foam columns, microgram and milligram amounts of copper could be retained and eluted quantitatively with hydroxyammonium chloride solution. The overall capacity of the examined ion-exchange foams was about 40% of the original capacity of beads. Cation-exchange distribution coefficients of cadmium(II), zinc(II), iron(III) and calcium-(II) were determined for foam material and conventional bead exchanger; the selectivity of both was about the same.     

Adsorption equilibrium modeling and solution chemistry dependence of fluoride removal from water by trivalent-cation-exchanged zeolite F-9

Fluoride in drinking water above permissible levels is responsible for human dental and skeletal fluorosis. In this study, therefore, the large internal surface area of zeolite was utilized to create active sites for fluoride sorption by exchanging Na+-bound zeolite with Al3+ or La3+ ions. Fluoride removal from water using Al3+- and La3+-exchanged zeolite F-9 particles was subsequently investigated to evaluate the fluoride sorption characteristics of the sorbents. Equilibrium isotherms such as the two-site Langmuir (L), Freundlich (F), Langmuir-Freundlich (LF), Redlich-Peterson (RP), Toth (T), and Dubinin-Radushkevitch (DR) were successfully used to model the experimental data. Modeling results showed that the isotherm parameters weakly depended on the solution temperature. From the DR isotherm parameters, it was considered that the uptake of fluoride by Al3+-exchanged zeolite proceeded by an ion-exchange mechanism (E = 11.32-12.13 kJ/mol), while fluoride-La3+-exchanged zeolite interaction proceeded by physical adsorption (E = 7.41-7.72 kJ/mol). Factors from the solution chemistry that affected fluoride removal from water were the solution pH and bicarbonate content. The latter factor buffered the system pH at higher values and thus diminished the affinity of the active sites for fluoride. Natural groundwater samples from two Kenyan tube wells were tested and results are discussed in relation to solution chemistry. In overall, Al3+-exchanged zeolite was found to be superior to La3+-exchanged zeolite in fluoride uptake within the tested concentration range.     

Characterization and Activity of Cr, Cu and Ga Modified ZSM—5 for Direct Conversion of Methane to Liquid Hydrocarbons

Direct conversion of methane using a metal-loaded ZSM-5 zeolite prepared via acidic ion exchange was investigated to elucidate the roles of metal and acidity in the formation of liquid hydrocarbons. ZSM-5 (SiO2/A12O3=30) was loaded with different metals (Cr, Cu and Ga) according to the acidic ion-exchange method to produce metal-loaded ZSM-5 zeolite catalysts. XRD, NMR, FT-IR and N2 adsorption analyses indicated that Cr and Ga species managed to occupy the alllmlnum positions in the ZSM-5 framework. In addition, Cr species were deposited in the pores of the structure. However, Cu oxides were deposited on the surface and in the mesopores of the ZSM-5 zeolite. An acidity study using TPD-NH3, FT-IR, and IR-pyridine analyses revealed that the total number of acid sites and the strengths of the BrSusted and Lewis acid sites were significantly different after the acidic ion exchange treatment.Cu loaded HZSM-5 is a potential catalyst for direct conversion of methane to liquid hydrocarbons. The successful production of gasoline via the direct conversion of methane depends on the amount of aluminum in the zeolite framework and the strength of the BrSnsted acid sites.     

The synthesis of mesoporous aluminosilicate using microcline for adsorption of mercury(II)

An economical mesoporous aluminosilicate was synthesized with microcline as starting material and the precursor 13X zeolite as seed for crystal structure on mesoporous walls. In this method, a mixture of microcline and Na2CO3 with a molar ratio of 1:1.05 was first calcined at 1093 K for 2.5 h. The calcined materials were mixed with 35 ml C16TMABr aqueous solution (containing 8.2 g C16TMABr) and the precursors of 13X zeolite, resulting in mesoporous aluminosilicate after crystallization of the solution at 378 K for 48 h and calcination of the powder at 823 K for 5 h. The as-synthesized sample has a uniform pore diameter distribution centered at 3.7 nm. The as-synthesized sample had BET surface area of 725 m2/g and BJH mean pore diameter of 3.7 nm. The FT-IR results revealed that the building units of 13X zeolite were inserted into the pore walls of the as-synthesized sample. The adsorption ratio of mercury(II) onto the as-synthesized adsorbent was about 95%. The adsorption process was found to be spontaneous and can be explained by particle diffusion and chemical ion-exchange mechanisms. The equilibrium concentration of mercury(II) using the as-synthesized sample as the adsorbent was under 1 microg/L, making the concentration of mercury meet the limit for drinking water in China as recommended by the World Health Organization.     

NH_4~ 改型钾丝光沸石及其离子交换性

用铵盐改型钾丝光沸石，通过Ｘ－射线衍射、红外分析、分配系数、离子交换等温线及交换容量等的测定，就其结构及交换性能进行了研究；并与铵改型钠丝光沸石和天然斜发沸石等进行比较，表明ＮＨ_4~ 改型后的钾丝光沸石对Ｋ~ 具有较高的交换容量和较好的Ｋ~ ／Ｎａ~ 分离效果。     

NH4^＋改型钾丝光沸石及其离子交换性

用铵盐改型钾丝光沸石,通过Ｘ－射线衍射,红外分析,分配系数,离子交换等温线及交换容量等的测定,就其结构及交换性能进行了研究;并与铵改型钠丝光沸石和天然斜发沸石等进行比较,表明ＮＨ４＾＋改型后的钾丝光沸石对Ｋ＾＋具有较高的交换容量和较好的Ｋ＾＋／Ｎａ＾＋分离效果。     

The Rectangular Isotherm Model for Adsorption Kinetics

The transient uptake response of an adsorbent particle, subjected to a step change in surface concentration, is considered. It is shown that, when the isotherm is highly favorable, the theoretical curves derived for a Langmuirian system reduce asymptotically to the much simpler form for a rectangular isotherm. The simple rectangular model provides a useful approximation even when the form of the actual isotherm is quite far from the rectangular limit.     

Studies on ion exchange equilibria and kinetics

The kinetics and equilibria of cation exchange HR+Na⁺=NaR+H⁺ with 001×7 strong acidic resin have been studied. The isotherm for this reaction has been determined, and the rational equilibrium constant, Ka, calculated. It is found experimentally that at low concentration, the reverse exchange RNa/H⁺ is faster than the forward exchange; on the contrary, at high concentration, the forward exchange is faster than the reverse exchange. The integral particle interdiffusion coefficients have been calculated and the film interdiffusion coefficients estimated.     

Conversion of methanol on a high-silica zeolite with divalent cations

The influence of divalent cations introduced into a high-silica zeolite by an ion exchange on its acidity and catalytic properties has been studied. The nature of the divalent cations has an influence on the distribution of the acid sites according to their strength, as well as on the activity and selectivity of action of the zeolite in the conversion of methanol. The primary conversion products of methanol, viz., dimethyl ether and ethylene, form on the acid sites of moderate strength. The hydrogen-redistribution reactions take place predominantly on the strong acid sites.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 114–118, January–February, 1986.     

Dispersion and Orientation of Zeolite ZSM‐5 Crystallites within a Fluid Catalytic Cracking Catalyst Particle

Confocal fluorescence microscopy was employed to selectively visualize the dispersion and orientation of zeolite ZSM‐5 domains inside a single industrially applied fluid catalytic cracking (FCC) catalyst particle. Large ZSM‐5 crystals served as a model system together with the acid‐catalyzed fluorostyrene oligomerization reaction to study the interaction of plane‐polarized light with these anisotropic zeolite crystals. The distinction between zeolite and binder material, such as alumina, silica, and clay, within an individual FCC particle was achieved by utilizing the anisotropic nature of emitted fluorescence light arising from the entrapped fluorostyrene‐derived carbocations inside the zeolite channels. This characterization approach provides a non‐invasive way for post‐synthesis characterization of an individual FCC catalyst particle in which the size, distribution, orientation, and amount of zeolite ZSM‐5 aggregates can be determined. It was found that the amount of detected fluorescence light originating from the stained ZSM‐5 aggregates corresponds to about 15 wt %. Furthermore, a statistical analysis of the emitted fluorescence light indicated that a large number of the ZSM‐5 domains appeared in small sizes of about 0.015–0.25 μm², representing single zeolite crystallites or small aggregates thereof. This observation illustrated a fairly high degree of zeolite dispersion within the FCC binder material. However, the highest amount of crystalline material was aggregated into larger domains (ca. 1–5 μm²) with more or less similarly oriented zeolite crystallites. It is clear that this visualization approach may serve as a post‐synthesis quality control on the dispersion of zeolite ZSM‐5 crystallites within FCC particles.