Thermographic and mass spectrometric study on zeolite dehydration and deammoniation

Thermography and thermal desorption with mass spectrometric monitoring have been applied to dehydration, deammoniation, and dehydroxylation in cationic, NH₄, H, and dealuminated forms of synthetic zeolites of faujasite and mordenite types, as well as natural mordenite. The dehydration temperatures for the synthetic materials are only slightly dependent on the cation form. As the window size decreases and the Si/Al ratio in the framework increases, the deammoniation and dehydroxylation temperatures rise in the sequence Y < M < M_h.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, No. 4, pp. 504–508, July–August, 1989.     

The investigation of redox properties of palladium-zeolite catalysts

Palladium catalysts based on A, X, Y, chabazite, erionite, mordenite, TsVM, and TsVK zeolites are studied by the methods of temperature-programmed reduction and hydrogen desorption. Palladium is shown to be located in zeolite pores in all catalysts except Pd/mordenite for which 24 % of the palladium is located on the external surface of the zeolite. The dispersity of the palladium on the zeolites is determined by the amount of H₂ evolved in the decomposition of the -phase of PdH and by the value of the O/Pd ratio obtained by palladium oxidation in the 100 to 500 °C temperature range. According to the O/Pd ratio, the catalysts are divided into two groups: samples based on X, Y, chabazite, erionite, and mordenite (O/Pd=0.4 to 0.7), and catalysts containing A, TsVM, and TsVK zeolites (O/Pd=0.2 to 0.3). The catalysts of the first group are weakly active in the oxidative acetoxylation of propylene, while all of the catalysts of the second group except Pd/NaA are very active. The conclusion is drawn that the oxidation of palladium is not the limiting stage in the oxidative acetoxylation of propylene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 642–648, April, 1995.     

Features of the Distribution of Aluminum in Mordeniteclinoptilolite Catalysts for the Hydroisomerization of n-Hexane

The content of tetrahedral, extraskeletal, and ion-exchanged aluminum in zeolites consisting of natural and decationized forms of mordeniteclinoptilolite rocks after acidic activation were studied. The results were compared with acidity and catalytic activity properties of modified samples of rocks and synthetic mordenite in the hydroisomerization of n-hexane.     

Methanol conversion on high-silica zeolites with different alumosilicate modules and prepared in different ways

High-silica zeolites in the H-form that differ in the SiO₂/Al₂O₃ ratio and the method of synthesis, have similar acidity spectra but different distributions of the acidic centers with respect to strength. A symbatic change in the concentration of the strongly acidic centers and of the degree of conversion of methanol has been observed, as well as a change in selectivity for C₂–C₄ aliphatic and aromatic hydrocarbons. It has been established that the stages of dehydration of methanol and dimethyl ether, and of aromatization and alkylation occur on strongly acidic centers of high-silica zeolites, and polymerization reactions on weakly acidic centers.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 3, pp. 380–383, May–June, 1987.     

Formation and characterization of natural polysaccharide hollow nanocapsules via template layer-by-layer self-assembly

Two different natural zeolites having different phase compositions were obtained from different regions of Turkey and modified by ion-exchange (0.5M NH(4)NO(3)) and acid leaching using 1M HCl. The natural and modified samples were treated at low temperature (LT), high temperature (HT) and steam (ST) conditions and characterised by XRF, XRD, BET, FTIR, DR-UV-Vis, NH(3)-TPD and TGA. Ion-exchange with NH(4)(+) of natural zeolites results in the exchange of the Na(+) and Ca(2+) cations and the partial exchange of the Fe(3+) and Mg(2+) cations. However, steam and acidic treatments cause significant dealumination and decationisation, as well as loss of crystalline, sintering of phases and the formation of amorphous material. The presence of mordenite and quartz phases in the natural zeolites increases the stability towards acid treatment, whereas the structure of clinoptilolite-rich zeolites is mostly maintained after high temperature and steam treatments. The natural and modified zeolites treated at high temperature and in steam were found to be less stable compared with synthetic zeolites, resulting in a loss of crystallinity, a decrease in the surface area and pore volume, a decrease in the surface acidity as well as dealumination, and decationisation.     

On the location of Lewis acidic aluminum in zeolite mordenite and the role of framework-associated aluminum in mediating the switch between Brønsted and Lewis acidity

Lewis acidic aluminum in zeolites, particularly acidity that is inherent to the framework, is an indeterminate concept. A fraction of framework aluminum changes geometry to octahedral coordination in the proton form of zeolite mordenite. Such octahedrally coordinated aluminum is the precursor of a Lewis acid site and its formation is accompanied by a loss in Brønsted acidity. Herein, we show that such Lewis acid sites have a preferred location in the pore structure of mordenite. A greater proportion of these Lewis acid sites resides in the side-pockets than in the main channel. By reverting the octahedrally coordinated aluminum back to a tetrahedral geometry, the corresponding Brønsted acid sites are restored with a concomitant loss in the ability to form Lewis acid sites. Thereby, reversible octahedral–tetrahedral aluminum coordination provides a means to indirectly switch between Lewis and Brønsted acidity. This phenomenon is unique to Lewis acidity that is inherent to the framework, thereby distinguishing it from Lewis acidity originating from extra-framework species. Furthermore, the transformation of framework aluminum into octahedral coordination is decoupled from the generation of distorted tetrahedrally coordinated aluminum, where the latter gives rise to the IR band at 3660 cm⁻¹ in the OH stretching region.

Framework-associated aluminum is demonstrated to facilitate a reversible switch between Lewis and Brønsted acidity in zeolites with the Lewis acid sites preferentially populating the side-pockets in the case of mordenite.     

High-resolution 27Al NMR study of the states of aluminum atoms in modified natural mordenite

High-resolution ²⁷Al NMR has been applied to the states of aluminum in modified natural mordenite. Acid treatment of the natural mordenite and heat treatment of the ammonium form alter the coordination state of 6–8% of the aluminum atoms from tetrahedral (in the framework) to octahecral (at the inner surfaces of the zeolite cavities). In that state, the aluminum can exchange with Na⁺, NH₄ ⁺, and other cations. This change in coordination state should have a considerable effect on the catalytic parameters of zeolites.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, No. 2, pp. 241–244, March–April, 1987.     

State and properties of ion-exchanged cations in zeolites: 1. IR spectra and chemical activation of adsorbed molecular hydrogen

Adsorption isotherms and of adsorbed molecular hydrogen indicate that H₂ is weakly adsorbed by alkali-metal forms of faujasites, mordenite, and high-silica zeolite ZSM-5. The alkaline-earth forms of the same zeolites adsorb hydrogen somewhat more strongly; nevertheless, the hydrogen molecules adsorbed by the barium form of mordenite are in the hindered rotation state. Molecular hydrogen is most strongly adsorbed by the zinc and cadmium forms of the high-silica zeolite. In this case, molecular hydrogen is strongly polarized and undergoes heterolytic dissociative adsorption, yielding acidic hydroxyl groups and cation-bound hydride ions.     

Effect of the nature of the ion-exchange cation on the acidity of ZSM-5 zeolites

The acidic properties of the initial and copper-, cerium-, and cobalt-containing high-silica TsVK and TsVN zeolites of the ZSM-S type with SiO21A1203 ratios of 68 and 37, respectively, were investigated by IR spectroscopy and programmed thermal desorption of ammonia. It was shown that the introduction of copper, cerium, and cobalt cations leads to redistribution of the acid centers in the Ts VK and Ts VN zeolites according to their strength, while hardly changing their overall concentration. The strength of the Brnsted acid centers of CuTsVK, CeTsVN, and CoNTsVK zeolites correlates with their activity in the selective reduction of nitrogen oxides by hydrocarbons.     

Influence of the crystallinity of pentasil-type zeolites on their acidity

The acid properties of intermediate products in crystallization of pentasil-type zeolites have been investigated using programmed thermal desorption of ammonia and adsorption of pyridine. It was found that acidity increases with increasing crystallinity due to proton-donor centers. A correlation was established between this change in acidity and methanol TPSR data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 1, pp. 48–51, January–February, 1995.     

Oligomerization of ethylene of NiO-dealuminized mordenite catalyst

Conclusions When the amount of aluminum in a nickel-containing mordenite with variable degree of dealuminization is increased the total acidity of the zeolite increases, the concentration of the electron-acceptor centers, recorded by the EPR method, passes through a maximum, while the degree of ethylene conversion varies within narrow limits.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 223–225, January, 1978.     

Thermal desorption of cations from polysilicates and zeolites in an external electric field

The desorption of cations from crystalline polysilicates and polysilicic acids, A and Y zeolites, and natural mordenite in the NH₄, H, and dealuminized forms under the action of a strong electrostatic field at temperatures ranging from 20 to 600°C has been studied. The desorption of sodium and potassium cations was detected above a temperature of 150–200°C, and magnesium, calcium, and aluminum cations were not detected. Potassium cations are desorbed at lower temperatures than are sodium cations. Zeolite NaA retains cations more strongly than-does zeolite NaY. The observed spectra are consistent with the concentrations of the cations and the structural features of the samples studied.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 626–629, September–October, 1988.     

Some electroanalytical aspects of using pulse polarography for determination of Ge(IV)

Summary Presented data give some informations of analytical importance as a result of pulse polarographic investigations of Ge(IV) in KCl solutions within pH range 3–12 at Ge concentration from 1×10^–4 to 2.5×10^–6 M. It was shown that Ge(IV) can be polarographically active in acidic solution but its reduction interferes with hydrogen gas development. The addition ofp-quinone enables the determination of Ge(IV) without this interfering effect.Suggested explanation of the observed changes in polarographic curves dependent on pH and Ge concentration based on the existence of several polarographically active forms.

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Elektroanalytische Bemerkungen zur pulspolarographischen Bestimmung von Ge(IV)

Zusammenfassung Unsere Ergebnisse bieten einige Informationen über die pulspolarographische Bestimmung von Ge(IV) in KCl-Lösungen innerhalb pH 3–12 und bei Ge-Konzentrationen zwischen 10^–4 und 2,5×10^–6 M. Es wurde gezeigt, daß Ge(IV) in saurer Lösung polarographisch aktiv ist, seine Reduktion aber durch Wasserstoff-Entwicklung gestört wird. Der Zusatz vonp-Chinon beseitigt diese Störung. Die Änderung der polarographischen Kurven je nach Ge-Konzentration und pH beruht vermutlich auf der Existenz verschiedener polarographisch aktiver Formen.

     

Effect of zeolite matrix structure on the stabilization of silver clusters

Silver clusters characterized by an absorption band at 320–340 nm are found to be easily stabilized by zeolites of mordenite type. Size of the clusters is discussed on the basis of results on introducing silver clusters into the zeolites with various channel diameters and on electron microscopy and low-angle X-ray scattering (LAXS) data.     

Removal of gadolinium ions from solutions using granulated zeolites

The possiblity of the use of granulated zeolites for the removal of gadolinium ions from solutions has been investigated. Measurements of gadolinium ion concentrations in solutions and of the distribution of gadolinium ions in column filligns (granulated zeolites), after passing the solution through columns filled with various granulated zeolites (zeolite 4A, zeolite 13X and synthetic mordenite), have shown that gadolinium ions can be effectively removed from nearly neutral or acidic solutions using granulated mordenite. The influency of pH, column filling and the flow rate of the solution through the column on the efficiency of gadolinium ion removal and on the distribution of gadolinium ions in the columns has been investigated. On the basis of the results obtained, the conditions for the most efficient removal of gadolinium ions from solutions have been obtained.     

The interaction mechanisms between aqueous solutions of137Cs and134Ba radionuclides and local natural zeolites for an deactivation scenario

¹³⁷Cs and¹³⁴Ba were removed from synthetic aqueous solutions by means of natural zeolites of Slovakian origin. The equilibrium sorption behavior of Cs and Ba ions onto clinoptilolite and mordenite were studied under static as well as dynamic experimental conditions. Both Freundlich and Langmuir isotherms describe satisfactory by Cs and Ba adsorption on the zeolites studied. The elution of Cs and Ba ions from zeolite columns after the loading cycle was undertaken additionaly, in order to compare column operating runs of various exchanged zeolite forms.     

Adsorption of methanol on sodium faujasites and mordenites

Calorimetry and IR spectroscopy were used to study the adsorption of methanol on faujasites and mordenites with different Si/Al ratios. It was shown that the disturbance of the OH bonds in CH₃OH weakens and the heat of adsorption decreases as the cation content in the zeolites decreases. The protic acid sites in decationized mordenite form strong hydrogen bonds with the adsorbed molecules.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 476–479, March, 1993.     

Measurements of Heat of Zeolite Dehydration by Scanning Heating

A procedure for measurement of the heat of zeolite dehydration by scanning heating has been designed. Simultaneous data on heat flow (DSC) and mass loss (TG) are required for evaluation. The heating rate depends on the experimental conditions (point-spread function, sample mass, crucible design, and calorimetric reproducibility). Dehydration measurements have three advantages as compared with the sorption procedure: i) one can investigate samples with irreversible dehydration; ii) no approximation model is needed for calculation of the partial molar heat of dehydration; and iii) the procedure is not labor-consuming.The procedure was tested on the natural zeolites heulandite, chabazite and mordenite. The results are close to those measured by the sorption procedure. The partial molar heat of dehydration was found to depend on the water content. It increases from 50 to 87 J mol^–1 K^–1 for heulandite, from 53 to 81 J mol^–1 K^–1 for chabazite, and from 51 to 71 J mol^–1 K^–1 for mordenite.The approximation of the heat of sorption by linear regression was found to be wrong. Detection of a phase transitioN after this approximation has no meaning.This revised version was published online in November 2005 with corrections to the Cover Date.     

Catalytic properties of zeolites in hydrogenation of acetaldehyde and isobutyraldehyde

Conclusions Zeolites of various types (A, faujasite, chabazite, erionite, and mordenite with Na, Ca, and Nd cations) are highly active and selective catalysts for the hydrogenation of acetaldehyde and isobutyraldehyde in a 200–350°C interval and an H₂ pressure of 30–50 atm, while zeolites NaM and NaA possess the maximum hydrogenating activity.Translated from Izvestiya Akademiya Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1847–1850, August, 1978.