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  • 1
    Electronic Resource
    Electronic Resource
    Titania–silica epoxidation catalysts modified by acetoxypropyl groups (2000)
    Springer
    Catalysis letters 64 (2000), S. 9-14 
    Details
    ISSN: 1572-879X
    Keywords: titania–silica hybrid aerogel ; organic modification ; acetoxypropyltrimethoxysilane ; epoxidation ; cyclohexene ; cyclohexenol ; tert‐butylhydroperoxide
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Mesoporous titania–silica aerogels with TiO2SiO2 = 1:9 weight ratio and varying amounts of covalently bound 3‐acetoxypropyl groups were prepared from 3‐acetoxypropyltrimethoxysilane, tetramethoxysilane and titanbisacetylacetonatdiisopropoxide. The textural properties of the hybrid aerogels were strongly influenced by the concentration of acetoxypropyl groups. The BET surface area and specific pore volume were significantly lowered upon introduction of the polar organic modification. Despite these less favorable textural properties, all modified aerogels were remarkably more active epoxidation catalysts, and in the epoxidation of cyclohexenol also more selective, than the unmodified titania–silica aerogel. For example, incorporation of only 2 mol% acetoxypropyl groups lowered the BET surface area from 813 to 339 m2 g-1, whereas the selectivity to 2,3‐epoxy‐cyclohexenol increased from 76 to 90%, and the reaction time (required to achieve 70% TBHP conversion) decreased from 46 to 5 min. The higher epoxide selectivity and the small enhancement in cis/trans ratio in the epoxide may indicate an electron pair donor interaction between the ester carbonyl group and the Ti active site.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019074617565
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  • 2
    Electronic Resource
    Electronic Resource
    Transient behavior of the enantioselective hydrogenation of a hydroxymethylpyrone (2000)
    Springer
    Catalysis letters 69 (2000), S. 129-132 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective ; hydrogenation ; cinchonidine ; cinchonine ; palladium ; pyrone
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Various 2‐pyrone derivatives are important intermediates in the synthesis of biologically active compounds. Pd chirally modified by cinchona alkaloids has a potential in the enantioselective hydrogenation of 4‐hydroxy‐6‐methyl‐2‐pyrone to the corresponding 5,6‐dihydropyrone. A study of various parameters (solvent, temperature, pressure, concentration) and catalyst systems (Pd/alumina and Pd/titania, modified by cinchonidine or cinchonine) revealed striking variations of the reaction rate and enantioselectivity with conversion. This transient behavior is interpreted by the effect of competitive adsorption and hydrogenation of the substrate and modifier.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019007014666
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  • 3
    Electronic Resource
    Electronic Resource
    On the imaging mechanism of monatomic steps in graphite (2000)
    Springer
    Applied physics 71 (2000), S. 441-447 
    Details
    ISSN: 1432-0630
    Keywords: PACS: 61.16.Ch; 68.35.Bs; 81.05.Tp
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract. The shape and the atomic arrangement of monolayer steps of graphite have been characterized by STM. The origin of the appearance of the imaged features along the steps is discussed, addressing for the first time both morphological and electronic considerations. Extended Hückel theoretical calculations of nanotubes are used to identify the contribution of the electronic structure to the STM image of monolayer steps. We show that mechanical tip–sample interactions dominate the imaging process of graphite, leading to step deformation during scanning and negative STM contrast of the atom positions in the hexagonal unit cell.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003390000570
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  • 4
    Electronic Resource
    Electronic Resource
    Monitoring of the Gas Phase Composition: A Prerequisite for Unravelling the Mechanism of Decomposition of Solids. Thermal decomposition of cobalt oxalate dihydrate (2000)
    Springer
    Journal of thermal analysis and calorimetry 60 (2000), S. 735-758 
    Details
    ISSN: 1572-8943
    Keywords: cobalt oxide reduction and oxidation ; decomposition of cobalt oxalate ; mechanism of thermal decomposition of solids ; pulse thermal analysis ; thermal analysis combined with mass spectrometry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The complexity of the processes occurring during cobalt oxalate dihydrate (COD) decomposition indicates that an interpretation of the mechanism based only on the TG curve is of little value. Mass change alone does not allow deeper insight into all of the potential primary and secondary reactions that could occur. The observed mass changes (TG) and thermal effects (DTA/DSC) are a superposition of several phenomena and thus do not necessarily reflect COD decomposition alone. Investigation of the mechanism of decomposition requires the application of different simultaneous techniques that allow the qualitative and quantitative determination of the composition of the gaseous products. Composition of the solid and gaseous products of COD decomposition and heats of dehydration and oxalate decomposition were determined for inert, oxidizing and hydrogen-containing atmospheres. Contrary to previous suggestions about the mechanism of cobalt oxalate decomposition, the solid product formed during decomposition in helium contains not only metallic Comet, but also a substantial amount of CoO (ca 13 mol%). In all atmospheres, the composition of the primary solid and gaseous products changes as a result of secondary gas-solid and gas-gas reactions, catalyzed by freshly formed Comet. The course of the following reactions has been investigated under steady-state and transient conditions characteristic for COD decomposition: water gas shift, Fischer-Tropsch, CO disproportionation, CoO reduction by CO and H2, Comet oxidation under rich and lean oxygen conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010135003141
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  • 5
    Electronic Resource
    Electronic Resource
    Hydrophobic titania–silica aerogels: epoxidation of cyclic compounds (2000)
    Springer
    Topics in catalysis 11-12 (2000), S. 369-378 
    Details
    ISSN: 1572-9028
    Keywords: titania–silica aerogel ; epoxidation ; organic modification ; methyltrimethoxysilane ; phenyltrimethoxysilane ; cyclohexene ; cyclohexenol ; tert-butylhydroperoxide
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Titania–silica mixed oxides with covalently bound methyl or phenyl groups were prepared from the corresponding alkyltrimethoxysilane and tetramethoxysilane using a sol–gel process and ensuing low-temperature supercritical extraction with CO2. Prehydrolysis of the organically modified silicon precursor was applied to compensate for their lower sol–gel reactivity compared to the precursors of the other constituents. Trihexylamine was used as a gelation catalyst. The dominantly mesoporous aerogels were characterized by thermal analysis, N2 adsorption, infrared spectroscopy, 29Si- and 13C-NMR spectroscopy, and transmission electron microscopy. Thermal analysis and 29Si MAS NMR measurements indicated covalent incorporation of the modifying group. 13C NMR spectra confirmed the integrity of the incorporated modifying groups. The materials were tested in the epoxidation of cyclohexene and cyclohexenol with tert-butylhydroperoxide. NH3 adsorption studied by DRIFT spectroscopy indicated no significant acidity of the aerogel surfaces, which was traced to the use of trihexylamine as gelation catalyst. Addition of zeolite 4 Å or NaHCO3 did not improve the performance of the unmodified and phenyl-modified aerogel, corroborating the suppressed acidity of the surface. Interestingly, the phenyl-modified aerogel was more selective in the epoxidation of cyclohexenol, when using polar solvents (ethanol or acetone), compared to the reaction in toluene.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1027202604286
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