Studies on basicity of alkali metal cation exchanged β and X zeolites by pyrrole-IR spectroscopy

The strength of basic sites has been measured by pyrrole-IR on alkali metal cation exchanged β and X zeolites, as well as NaOH loaded Naβ. The influence of cation type and the structure of zeolites on their basicity has been studied. The acidic and basic properties of the samples were investigated by NH₃-TPD and isopropanol reaction. It was shown that the strength of basic sites on samples could be characterized by the shift of v_nh band in the pyrrole-IR spectra. The framework oxygen charges were calculated from the Sanderson electronegativity. The changes in basic properties with various alkali metal cation are consistent with the changes of local oxygen charges of the zeolite framework.     

The interactions of sorbates with gallosilicates and alkali-metal exchanged gallosilicates

Structures, energetics and vibrational frequencies of the interaction of adsorbates with H-aluminosilicates (H-AlZ), H-gallosilicates (H-GaZ), alkali-metal exchanged aluminosilicates (X-AlZ) and alkali-metal exchanged gallosilicates (X-GaZ), where X being Li, Na, or K, have been carried out at B3LYP and HF levels of theory with 6-31G(d) as the basis set. The charge compensating alkali-metal ions can affect the catalytically active site (Si–O–T where T=Al or Ga) by weakening the Si–O, Al–O, and Ga–O bonds as compared to their anionic frameworks. Comparing the net stabilization energies, ΔE_NSE, of the naked alkali-metal/H₂O adducts with those of the alkali-metal exchanged zeolite/H₂O systems, the latter amounts only to about 50% of the former, which is partly due to the destabilizing role of the negative zeolitic oxygen frameworks surrounding the cations. The interaction of sorbates with the alkali-metal exchanged gallosilicates can be employed to probe the field strength inside the catalytic frameworks as indicated by the plot of the binding energy, ΔE, versus 1/R_X–Ow², with R_(X–Ow) being the distance between the cationic nucleus and the oxygen atom of the adsorbate. The IR spectra of H₂O adsorbed on Na-AlZ are calculated to be 3584, 3651, and 1686 cm⁻¹. The obtained results are in excellent agreement with the very recent experimental IR spectra of water adsorbed on Na-ZSM-5 of Zecchina et al. (J. Phys. Chem., 100 (1996) 16 484). Other important features, i.e. the correlation between Δν_OH and, ΔE, R_(X–Ow), and 1/R_X–Ow², cationic size, demonstrate that the interactions of sorbates with alkali-metal exchanged gallosilicates are well approximated by electrostatic contribution.     

Study of physical and thermochemical properties of modified zeolites

The treatment of zeolite with a solution of NaOH at different concentrations creates modified forms of natural zeolites.These modified zeolites exhibit an increased uptake of caesium and a satisfactory uptake of cobalt. The structural changes in the modified zeolites were studied by XRD analysis, which reveals the mineral compositions of the studied materials. The thermodynamic properties of the modified zeolites were studied by means of DTA and thermogravimetry, and the chemical composition too was determined.     

碱金属离子X型沸石的固相离子交换及其催化性能的研究

本文考察了水合13X沸石与碱金属盐类间的固相离子交换反应,并对交换样品的比表面、孔容和异丙醇分解反应活性进行了测定。结果表明,这种类型的固相离子交换在室温下即可发生。体积较大的Cs^+比K^+难于交换,高温焙烧有利于有提高Cs^+的交换度。固相交换样品的比表面、孔容变化和异丙醇分解反应活性都与液相交换样品相同,只是交换度的函数,与交换方法无关。与K^+交换样品相比,Cs^+交换样品中Cs^+离子更多地占据超笼,导致更高的超笼局部碱性和脱氢反应活性。     

Thermal properties of Y-type zeolites

The thermal and structural properties of two parent NaY zeolites and of those modified by ion exchange (ReNaY, HNaY, FeNaY) were investigated by simultaneous thermal analysis (TG-DTA-DTG) and by X-ray diffraction spectroscopy. Both the intracrystalline water and the zeolite framework were in our attention. The impurities (Fe) located in the lattice as well as the ions which entered by ion exchange (Re, H, Fe) influence the properties of the zeolites. The values of the activation energy of the dehydration process prove that the water molecules are more strongly bonded in all modified samples than in the parent ones. As compared to the NaY zeolites, an increased thermal stability, of about 100°C was revealed for ReNaY or of about 180°C for HNaY, and a decreased stability, of about 50°C, for FeNaY samples. The temperature at which the lattice break-down beginsT _amf, estimated by following the X-ray diffraction patterns for samples heated in air at temperatures from 300 to 1100°C, is the temperature which may be related to the structural characteristics of the zeolites, i.e., to the lattice constant of the uncalcined materials. The XRD studies reveal the heterogeneity of the crystallites constituting the zeolite material from both the point of view of the lattice constant values and the thermal stability. As the temperatureT _amf, generally, does not coincide with the temperature of the first exothermic peak,T ₁, of the DTA curve, we suggest the temperatureT _amf to be taken as an unambiguous measure of the thermal stability.     

Chemisorption and physisorption of CO2 on cation exchanged zeolitesA,X and mor

Calorimetric measurements of the heat of adsorption of CO₂ on zeolites with variable content of mono- and divalent cations lead to common conclusions. High initial heats (up to 120 kJ·mol^–1 for NaA), generally associated with a slow and activated rate of adsorption, are found for high contents of Na⁺, Li⁺ or Ca²⁺. They are attributed to a limited number of chemisorption sites (0.3 per cage in NaA).Physisorption results in lower heats (25–50 kJ·mol^–1). The lowest values are obtained with partially or totally decationized zeolites. Transition metal cations induce frequently weaker interactions than IA or IIA. Finally the stronger the energy of adsorption is, the larger the adsorbed amount is.

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Zusammenfassung Kalorimetrische Messungen der Adsorptionswärme von CO₂ an Zeolithen mit einem unterschiedlichen Gehalt an ein- und zweiwertigen Kationen führen zu allgemeinen Schlußfolgerungen. Für einen hohen Gehalt an Na⁺, Li⁺ oder Ca²⁺ werden hohe Initiierungswärmen (bis zu 120 kJ·mol^–1 für NaA) gefunden, die immer in Verbindung mit langsamen und aktivierten Adsorptionsgeschwindigkeiten auftreten. Dies wird einer begrenzten Anzahl an Stellen für die Chemisorption zugeschrieben (0.3 pro -Cage in NaA). Physisorption verursacht niedrigere Wärmen (25–50 kJ·mol^–1). Die niedrigsten Werte erhält man mit teilweise oder total entkationisierten Zeolithen. Kationen von Übergangs-metallen verursachen häufig schwächere Wechselwirkungen als IA-oder IIA-Kationen. Je höher die Adsorptionsenergie, um so größer ist die adsorbierte Menge.

     

Vapor phase alkylation of aniline using ethanol over alkali and alkaline-earth exchanged zeolites-Y

Ethylation of aniline has been investigated over various alkali and alkaline earth metal exchanged zeolites-Y. The basic zeolites-Y were found to have higher activity than acidic zeolites-Y for N-alkylation.     

Strontium and cesium sorption of some Anatolian zeolites

Zeolite samples of Tertiary age obtained from different areas of Anatolia (Turkey) are classified as Clinoptilolite, Analcime or Heulandite and we have investigated their sorption capacity for Cs⁺ and Sr²⁺ ions from aqueous solutions. Quantitative analysis of the zeolite samples untreated and treated with chloride salts of Cs⁺ or Sr²⁺ in aqueous solutions, for Na, Mg, Al, Si, P, K, Ca, Ti, Mn and Fe were performed using EDXRF. Chemical analysis indicated that the Clinoptilolite type zeolite from the deposits of Cankiri-Corum Basin of Anatolia is the best sorber for Sr and Cs ions.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August, 1995.     

Thermal,structural and electrical properties

The chloro compound of 3-hydroxy-2-quinoxalinecarboxylic acid with nickel(II) has been prepared in ethanolic solution from which a solid compound was isolated. Spectral and magnetic measurements show that the nickel ions are in an octahedral environment. Thermogravimetry, differential thermal analysis and electrical conductivity data are reported for 3-hydroxy-2-quinoxalinecarboxylic acid and its nickel complex. The conductivity measurements indicate that electron/hole traps are emptied during heating of the complex but repopulation occurs in about 24 h at room temperature.     

Physicochemical and mechanical properties of a novel fiber extracted from the stem of common reed plant

Natural fiber usage rather than the synthetic fibers is attracted by researchers due to their special features such as biodegradable, inexpensive, easy availability, low density, and good thermal properties. This present work deliberates the characterization and testing of untreated and treated fibers extracted from the common reed plant stem. From the characterization, it reveals that the treated fibers had higher crystallinity index value with 75.41% and cellulose content having 64.56%. The thermal stability and mechanical properties of fiber was improved by alkali treatment. The surface roughness of the fibers due to the elimination of the noncellulosic substance on alkali treatment is evidenced by SEM.     

Catalytic isomerization of dihalobenzenes C6H3X2R in the presence of high-silica zeolites

Catalytic isomerization of dichloro- and difluorobenzenes was carried out under conditions of a gas-phase reaction occurring in the presence of metallosilicates with the pentasil-type zeolite structure, in which several silicon atoms are isomorphically replaced by Al, Ga, or Fe atoms. A correlation between the acid properties of these zeolites and their ability to induce dihalobenzene isomerization was found.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1920–1922, November, 1994.     

Thermal transformations in systems based on zeolites Y,X, and A containing zinc and sodium nitrates

Thermal transformations in systems formed by interaction of Zn and Na nitrates with Y, X, and A zeolites were studied by TG—DTA technique. Temperature regions of existence of adsorbed water, water of crystallization, and decomposition of NO₃ ^– anion were determined. These intervals depend on the composition, structure, method of preparation, and pre-treatment conditions of zeolite systems. The extent of NO₃ ^– decomposition depends not only on the zinc and sodium content but also on the presence of ammonia involved in NO₃ ^– reduction. The zeolite matrix strongly stabilizes the occluded NO₃ ^– anions. A portion of zinc oxide formed by zinc nitrate decomposition is probably localized inside the zeolite cavities as the [Zn—O—(ZnO) _n —Zn]²⁺ particles. The latter compensate charges of the isolated [AlO₄]^– tetrahedra.     

Formation of disperse ferromagnetic nanoparticles in zeolites in the course of thermal activation in oxygen

Samples of synthetic high-silica zeolites after high-temperature activation in oxygen are studied by ferromagnetic resonance. Observation of the fine structure of ferromagnetic resonance (FMR FS) indicates the formation of magnetic iron oxide nanoparticles in the course of thermal treatment of zeolites in oxygen. Features of the formation of a dispersed ferromagnetic phase in zeolites, geometric and magnetic characteristics of constituent nanoparticles are discussed.     

Precursor memory effect determining structural properties of sulfated zirconia

Properties of two catalysts tailored in the laboratory conditions by sulfation of commercial Zr-hydroxide and Zr-hydroxide of nitrate origin were compared with those of commercial SO₄-Zr-hydroxide. Equally thermally treated samples in the temperature interval 500–700°C, and having the same amount of sulfur, show different properties indicating memory effect of their solid parent materials. The catalyst obtained by sulfation of commercial Zr-hydroxide differs in amount of residual sulfates upon calcination from other two catalysts. The instability of sulfates in the previous case might be connected to the lowest surface area values of catalyst and the highest fraction of monoclinic phase observed at all applied calcination temperatures.     

Characterization of the acidic properties of zeolites by means of temperature-programmed desorption (TPD) of ammonia

Desorption energy distributions were calculated for temperature-programmed desorption (TPD) of ammonia from H zeolites of different type by means of regularization. This method does not require any limiting assumptions about the distribution function. It could be shown that the desorption energy distributions obtained are nearly independent of the experimental conditions and therefore they should represent a suitable measure for the distribution of the strength of acidic sites. The calculated desorption energy distributions for the ammonia desorption from the isolated bridging SiOHAl groups of H zeolites of different type significantly differ from each other in shape. The increase of the desorption energy of the main range of the distribution functions correlates well with the increase of the average acid strength of the SiOHAl groups with decreasing Al content of the zeolites.     

Physicochemical properties of mixed surfacant systems: sodium dodecyl benzene sulfonate with triton X 100

The physicochemical properties of a mixed surfactant system were studied under various conditions. The surfactants were anionic sodium dodecyl benzene sulfonate and nonionic Triton X 100. Variation of specific conductivity with concentration was used to determine the critical micelle concentration of anionic as well as the mixed surfactants. Iodine solubilization method was used to determine the CMC of the nonionic surfactant. The interaction parameter between the surfactant molecules were calculated. The wetting, foaming and detergent properties of mixed surfactant systems were studied. The variation of contact angle of the solution with teflon surface as a function of surfactant concentration was found to be a reasonably good method to determine the critical micelle concentration. Viscosity and cloud points were also determined. All these quantities are discussed. Received: 14 January 1998 Accepted: 11 June 1998     

Synthesis of isoamyl acetate usin NaX and NaY zeolites as catalysts

A liquid phase esterification reaction between glacial acetic acid and isoamyl alcohol has been studied using NaX and NaY zeolites as catalysts. The influence of calcination temperature, the amount of catalysts, reaction temperature, time of esterification and the molar ratio of the reactants has been investigated. Water insoluble products have been isolated from the reaction mixture and analyzed for the ester. Both NaX and NaY are found to be active as catalysts in the reaction. However, their catalytic activity varies with the reaction conditions and their calcination temperature. The reaction has been found to be 100% selective to ester formation. Catalytic activity of the zeolites has been correlated with their surface acidity.