Untersuchungen an oxidischen Katalysatoren. XXXVII. Zum Einfluß der Herstellungsbedingungen auf die Eigenschaften röntgenamorpher Al2O3?SiO2-Festkörper

Studies on Oxide Catalysts. XXXVII. On the Influence of Preparation Conditions on the Properties of Silica-Alumina Silica-alumina samples of different composition were prepared by coprecipitation from aqueous solutions at pH = 6 and pH = 9, respectively. Structures and properties of the solids were characterized (i.r., DTA, X-ray, molybdato method, surface acidity, catalytic activity in the cracking reaction of cumene). Solid composition and pH value of the precipitation medium equally exhibit influence on the structure of the silica-alumina (kind of incorporation of the aluminium) and thereby on the acidic and catalytic properties of the solids. Silica-alumina of equal composition but precipitated at different pH values may widely differ in their properties including activity-composition dependency.     

27Al-MAS-NMR- und IR-spektroskopische Untersuchungen zur Aluminium-Koordination in röntgenamorphen SiO2 ?Al2O3-Festkörpern

²⁷Al MAS N.M.R. and I.R. Investigations on the Aluminium Coordination in Amorphous Silica-Alumina Gels Amorphous silica-alumina of different Si/Al ratios have been investigated by ²⁷Al magic-angle spinning n.m.r. (MAS n.m.r.) and i.r. spectroscopy. Aluminosilicate framework structures were found for the Na⁺- and NH₄⁺-exchanged forms of the investigated gels. The thermal deammoniation of the NH₄⁺-exchanged gels, producing the H⁺ forms, causes an irreversible damage of the aluminosilicate framework. This process is accompanied by a change of the coordination state of a part of the aluminium, converting AlO₄ into AlO₆ units. The progress of the solid-state reaction upon further thermal treatment of the H⁺ forms is reflected by a considerable broadening of the n.m.r. spectra and a decrease of the intensity. The possibility that well-known results, including those of nonframework aluminium, obtained for comparable zeolitic systems may also be valid for amorphous solids, is discussed.     

Differential Scanning Calorimetry Evaluates Relative Proportions and Strength of Acid-Sites in Catalytic Materials and Adsorbents Alumina and Amorphous Silica-Alumina

Abstract

Differential scanning calorimetry (DSC) is employed to evaluate the relative proportions and strength of acid-sites in gammaalumina and amorphous silica-alumina through presorption of tri-ethylamine (TEA) followed by its thermal desorption in the DSC equipment. From the data obtained, total acid-sites in silica-alumina is found to be 2.3 times as large as those present in gamma-alumina, and 5–5% of total acidity in alumina is of Bronsted type whereas in silica-alumina this type comprises 28.9%' The Bronsted acidity strength in alumina and silica-alumina is identical although greatly different in quantity. Lewis acidity in silica-alumina is significantly stronger than that in alumina.     

Hydrocarbon production from synthesis gas over ZnO?Cr2O3+SA physical mixtures and Pd/SA impregnated catalysts

The performances of ZnO–Cr₂O₃+silica-alumina physically mixed and Pd impregnated on silica-alumina catalysts in the transformation of synthesis gas to hydrocarbons are compared in the present work. ZnO–Cr₂O₃ or Pd and silicaalumina are used as methanol synthesis and the hydrocarbon formation catalysts, respectively. The highest CO conversion corresponds to the highest relative amount of methanol synthesis active sites. The highest proximity between both types of active sites in the Pd imprenated on silica-alumina produces higher hydrocarbon selectivity and higher C₁ fraction than when using the physically mixed ZnO–Cr₂O₃+silica-alumina catalysts.     

General analysis of the measurements of the lateral crystal lattice moduli of semicrystalline polymers by x-ray diffraction and the application to nylon 6

A mathematical representation based on a linear elastic theory is proposed by which one may investigate the dependences of molecular orientation and crystallinity on the crystal lattice moduli and linear thermal expansion coefficients in the direction perpendicular to the chain axis as commonly measured by x-ray diffraction. In the theoretical calculation, a previously introduced model was employed in which oriented crystalline phase is surrounded by oriented amorphous phase and the strains of the two phases at the boundary are identical. The mathematical analysis indicated that the lateral crystal lattice moduli and linear thermal coefficients as measured by x-ray diffraction may be different from the intrinsic crystal moduli and linear thermal coefficients of a crystal unit cell, depending on the structure of the polymer solid. The numerical calculation was applied to nylon 6. As a result, it may be confirmed that the lateral crystal lattice moduli measured by x-ray diffraction are sensitive to the morphology of the bulk speciments and close to the intrinsic crystal moduli if the morphology of the test specimen can be represented by a parallel model with respect to the original stretching longitudinal direction.     

Ethylene polymerization over supported MAO/(nBuCp)2ZrCl2 catalysts: Influence of support properties

The catalytic system methylaluminoxane (MAO) and bis(n-butylcyclopentadienyl)zirconium dichloride ((nBuCp)₂ZrCl₂) was immobilized on commercial silica, silica-alumina and aluminophosphate calcined at different temperatures. The properties of the supports were determined by using N₂ adsorption-desorption isotherms at 77 K, FT-IR spectroscopy and SEM. After aluminium and zirconium impregnation, the catalysts were analyzed by ICP-AES, FT-IR and UV-vis spectroscopy. Ethylene polymerizations were carried out in a Schlenk tube at 70 °C and 1.2 bar of ethylene pressure. The polyethylene obtained was characterized by GPC, DSC and SEM.Catalysts supported on silica-alumina exhibited higher polymerization activity than those supported on silica and aluminophosphate. Besides, the activity of MAO/(nBuCp)₂ZrCl₂ catalytic system supported on silica-alumina and aluminophosphate decreased strongly with support calcination temperature, while remained almost constant when silica was employed as support. All these experimental features suggest a role of the support acid properties and hydroxyl group population in the generation of active polymerization species.     

Vibronic effects in Cu(II)-doped Ba2Zn(HCO2)6 x 4 H2O

The temperature-dependent electron paramagnetic resonance (EPR) spectrum of approximately 1% Cu(II) ions doped into Ba 2Zn(HCO2)6 x 4 H2O was analyzed at the Q-band frequencies over the temperature range 100-350 K to obtain structural information about the local environment. It can be concluded that the host crystal imparts a large orthorhombic strain which mainly corresponds to a tetragonal compression imposed onto the Cu(II)O6 species. This results in a copper center which adopts an orthorhombically distorted elongated geometry with the elongated axis perpendicular to the direction of the tetragonal compression due to the host crystal. There are two possible axes of elongation, and these represent two conformers separated by approximately 320 cm(-1). The thermal population of the higher energy level averages the g values, giving the observed temperature-dependent EPR spectra. The averaging process is between vibronic levels that are localized at two different minima of a single ground-state potential energy surface. These vibronic levels correspond to vibrational levels having different electronic properties. The determination of the host lattice strain parameters from the Cu(II) EPR spectra means that the guest ion is used as a probe of the environment of the Zn(II) site. The structural data derived from the lattice strain parameters are correlated with those from the Ba 2Zn(HCO2)6 x 4 H2O crystal structure.     

Thermal study of the Ce<Subscript>0.9</Subscript>Tb<Subscript>0.1</Subscript>O<Subscript>2</Subscript> pigment prepared by different synthesis

The inorganic ceramic compounds based on the CeO₂ belong into the group of high-temperature pigments. The pigments have been prepared by the classical dry process (i.e. solid-state reaction) in the temperature range from 1,300 to 1,600 °C and by the coprecipitation at the three different temperatures: 400, 600 and 1,100 °C. The principal of these pigments makes the host lattice of the CeO₂, which is doped by terbium ions. This incorporation of the doped ions leads to obtaining of the interesting dark orange colour after application into ceramic glaze. The aim of our research was to improve and optimize the synthesis conditions of these pigments. The samples were submitted to thermal analysis (TG–DTA) for determination of the temperature interval of the pigment formation and the thermal stability of pigments. The compounds were also measured from the point of view of their colouring, structure and particle size distribution.     

Emission of M2X+ cluster ions in thermal ionization mass spectrometry in the presence of graphite.

The emission of M2X+ cluster ions in thermal ionization mass spectrometry when graphite is loaded on the heating filaments was studied. The emission model of non-reductive thermal ionization of graphite was preliminarily discussed and factors influencing the thermal emission of M2X+ ions were investigated. The results show that the intensities of M2X+ cluster ions are related to ionic radius and crystal lattice energy, and possibly also to the solvation energies of ions. The intensities of M2Cl+ (M stands for K, Rb, and Cs) cluster ions, the M2Cl+/M+ ratios, and the 37Cl/35Cl ratios determined from M2Cl+ ion measurement usually increase with measurement time. The variation of the 37Cl/35Cl ratios determined from Cs2Cl+ ion measurement is lower than those based on K2Cl+ and Rb2Cl+ ion measurement, indicating the lowest isotopic fractionation.     

Sol–gel synthesis of Mn2O3/Al2O3/SiO2 hybrid nanocomposite and application for removal of organic dye

Journal of Sol-Gel Science and Technology - Nanocomposite of Mn2O3/Al2O3/SiO2 was prepared through an in situ sol–gel process, in which Mn2O3 nanocrystals were dispersed in the silica-alumina...     

Ab initio crystal structure predictions for flexible hydrogen‐bonded molecules. Part III. Effect of lattice vibrations

In crystal structure predictions possible structures are usually ranked according to static energy. Here, this criterion has been replaced by the free energy at any temperature. The effects of harmonic lattice vibrations were found by standard lattice‐dynamical calculations, including a rough estimate of the effects of thermal expansion. The procedure was tested on glycol and glycerol, for which accurate static energies had been obtained previously (Part II of this series). It was found that entropy and zero‐point energy give the largest contribution to free energy differences between hypothetical crystal structures, adding up to about 3 kJ/mol for the structures with lowest energy. The temperature‐dependent contribution to the energy and the effects of thermal expansion showed less variation among the structures. The overall accuracy in relative energies was estimated to be a few kJ/mol. The experimental crystal structure for glycol corresponded to the global free energy minimum, whereas for glycerol it ranked second at 1 kJ/mol. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 816–826, 2001     

Ethylene polymerization studies with supported cyclopentadienyl,arene, and allyl chromium catalysts

Highly active catalysts for low pressure ethylene polymerization are formed when chromocene, bis (benzene)- or bis (cumene)-chromium or tris- or bis (allyl)-chromium compounds are deposited on high surface area silica-alumina or silica supports. Each catalyst type shows its own unique behavior in preparation, polymerization, activity, isomerization, and response to hydrogen as a chain transfer agent. The arene chromium compounds require an acidic support (silicaalumina) or thermal aging with silica to form a highly active catalyst. At 90°C polymerization temperature arene chromium catalysts produced high molecular weight polyethylene and showed, in contrast to supported chromocene catalysts, a much lower response to hydrogen as a chain transfer agent. An increase in polymerization temperature caused a significant decrease in polymer molecular weight. Addition of cyclopentadiene to supported bis (cumene)-chromium catalyst led to a new catalyst which showed a chain transfer response to hydrogen typical of a supported chromocene catalyst. Polymerization activity with tris- or bis (allyl)-chromium appears to depend on the divalent chromium content in the catalyst. Changes in the silica dehydration temperature of supported allyl chromium catalyst have a significant effect on the resulting polymer molecular weight. High molecular weight polymers were formed with catalysts that were prepared using silica dehydration temperatures below about 400°C. Dimers, trimers, and oligomers of ethylene were usually formed with catalysts that were prepared on silica dehydrated much above 400°C. The order of activity of the different types of catalysts was chromocene/silica > chromocene/silica-alumina > bis (arene)-chromium/silica-alumina ? allyl chromium/silica.     

Einfluß von Gitterstörungen des Graphits auf die Bildung von Graphithydrogensulfat

Influence of Lattice Defects of Graphite on the Formation of Graphite Hydrogensulfate Various natural and synthetic graphites were oxidized with chromic acid in sulfuric acid to the 1st stage of graphite hydrogensulfate with the intent of determining the inhibition of intercalation by lattice defects. More oxidant than required by stoichiometry is consumed because slow total oxidation to CO₂ is superimposed even at room temperature. The activation energy for total oxidation is reduced by lattice defects. With some graphites, separation of the intercalation reaction from total oxidation was accomplished using calorimetry. The reaction enthalpy in the oxidation of well-crystallized graphites to the graphite salt is ?3.05 kJ/mol C.     

Metal site-mediated, thermally induced structural changes in Cr6+-silicalite-2 (MEL) molecular sieves

Cr(6+) ions were incorporated into the lattice sites of phase-pure silicalite-2 made using 3,5-dimethylpiperidinium as a structure-directing agent. The materials exhibited a remarkably well-resolved vibronic emission consisting of a high frequency progression of 987 cm(-1), which was assigned to the fundamental symmetric stretching mode of the (Si-O-)(2)Cr(═O)(2) group dominated by the terminal Cr═O stretch. A low frequency progression at 214 cm(-1), which was assigned to a symmetric O-Cr-O bending mode, was built on each band of the 987 cm(-1) progression. Studies of the vibronic structure of the emission spectrum as a function of temperature and Cr ion concentration reveal an abrupt change in the Franck-Condon factor of the emission at 20 K for samples with very low Cr concentrations (0.03 mol %). The change in the Franck-Condon factor is attributed to a temperature-induced structural change in the coordination sphere of the metal ion. This structural change was found to be accompanied by a concomitant structural change in the lattice structure of the silicalite-2. This structural change, as studied by temperature-dependent X-ray diffraction, did not involve a crystallographic phase change but an abrupt decrease in the unit cell volume, caused specifically by a decrease in the c-axis. This structural change was not observed in pure silicalite-2, indicating that it is not intrinsic to the silicalite lattice. Moreover, no similar structural change was observed at higher Cr loading (1 mol %). This suggests that the presence of the Cr ions and the changes in the coordination geometry they undergo at low temperature induced the observed contraction in the silicalite-2 lattice, in effect acting as a thermal switch that decreases the unit cell volume.     

Hydrothermal synthesis,characterization, and thermal properties of alumino silicate azide sodalite,Na<Subscript>8</Subscript>[AlSiO<Subscript>4</Subscript>]<Subscript>6</Subscript>(N<Subscript>3</Subscript>)<Subscript>2</Subscript>

First time we report the synthesis, structural characterization and thermal behavior of an unusual N₃ ^? containing alumino-silicate sodalite mineral. Azide sodalite, Na₈[AlSiO₄]₆(N₃)₂ has been synthesized under hydrothermal conditions at 433 K in steel lined Teflon autoclave. The structural and microstructural properties of azide sodalite mineral was characterized by various methods including FT-IR, XRD, SEM, TGA, and MAS NMR. Crystal structure have been refined by Rietveld method in \(P\bar 43n\) space group, indicating that the N₃ ^? sodalite has cubic in lattice. High temperature study was carried out to see the effect of thermal expansion on cell dimension (a ^o) of azide sodalite. Thermal behavior of sodalite was also assessed by thermogravimetric method.     

Zeit- und temperaturaufgelöste Röntgenpulverdiffraktometrie: Die Reaktion von (NH4)2SnF6 mit Ammoniak

Time and Temperature Resolved in situ X-Ray Powder Diffractometry. The Reaction of (NH₄)₂SnF₆ with Ammonia The thermal decomposition of (NH₄)₂SnF₆ under an atmosphere of ammonia is reported. The complicated reaction paths were illucidated by time and temperature resolved in situ x-ray powder diffractometry. It is shown that this technique is a powerful tool to observe structural changes during reaction. It offers also a valuable access to thermodynamic and kinetic data for solid state and gas phase reactions. (NH₄)₂SnF₆ decomposes under ammonia below room temperature to NH₄F and amorphous SnF₄ · x NH₃. At a temperature of 80°C an intermediate product, (NH₄)₄SnF₈, is formed, which decomposes at 140°C into (NH₄)₂SnF₆ and NH₄F. At 250°C (NH₄)[Sn(NH₃)F₅] and Sn(NH₃)₂F₄ are formed. The latter crystallises C-centered monoclinic with lattice constants a = 844.1(5) pm, b = 630.5(3) pm, c = 520.2(3) pm and b? = 114.02(7)°. At 330°C a further decomposition yields SnF₂(NH₂)₂ with a C-centered monoclinic cell and lattice constants a = 1 069(7), b = 325.3(2), c = 504.8(3) pm and b? = 105.83(7)°. Finally above 500°C tin metal is formed.     

Molar heat capacity and thermodynamic properties of crystalline Ho(Asp)Cl<Subscript>2</Subscript>·6H<Subscript>2</Subscript>O

The molar heat capacity, C _p,m, of a complex of holmium chloride coordinated with L-aspartic acid, Ho(Asp)Cl₂·6H₂O, was measured from 80 to 397 K with an automated adiabatic calorimeter. The thermodynamic functions H _T-H _298.15 and S _T-S _298.15 were derived from 80 to 395 K with temperature interval of 5 K. The thermal stability of the complex was investigated by differential scanning calorimeter (DSC) and thermogravimetric (TG) technique, and the mechanism of thermal decomposing of the complex was determined based on the structure and the thermal analysis experiment.     

Investigation of the acidity of Ni/Al2O3 and Ni/SiO2?Al2O3 catalysts

Acidity of Ni-modified alumina and silica-alumina catalysts was determined using n-butylamine titration and pyridine adsorption methods. Strong influence of Ni²⁺ ions on the Brönsted acidity of silicaalumina was observed. Improved Brönsted acidity of such system was confirmed by the results of the test reaction.     

Über Oxidsulfide mit Åkermanitstruktur CaLaGa3S6O,SrLaGa3S6O,La2ZnGa2S6O und Sr2ZnGe2S6O

On Oxidesulfides of Åkermanitetype-Structure CaLaGa₃S₆O, SrLaGa₃S₆O, La₂ZnGa₂S₆O, and Sr₂ZnGe₂S₆O The oxide sulfides were prepared for the first time (space group and lattice constants see “Inhaltsübersicht”). The atomic positions were refined from single crystal X-ray data for CaLaGa₃S₆O and La₂ZnGa₂S₆O. Problems concerning the metal distribution on the crystallographic positions are discussed by comparing interatomic distances. The ratio c/a of the new compounds is compared with that of isotypic compounds from literature.     

Synthesis, characterization and thermal expansion studies on europium titanate (Eu2TiO5)

The lattice thermal expansion characteristics of europium titanate (Eu₂TiO₅) have been studied by measuring the lattice parameter by high temperature X-ray diffraction technique (HT-XRD) in the temperature range 298–1573 K. Percentage linear thermal expansion and mean linear thermal expansion coefficients were computed from the lattice parameter data. The percentage linear thermal expansion in the temperature range 298–1573 K along a, b and c axes are 1.05, 1.15 and 0.95 respectively.