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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
    Adsorption mode of ethyl pyruvate on platinum: an in situ XANES study (2000)
    Springer
    Catalysis letters 66 (2000), S. 109-112 
    Details
    ISSN: 1572-879X
    Keywords: ethyl pyruvate ; in situ XANES ; enantioselective hydrogenation ; platinum
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The adsorption of ethyl pyruvate on Pt(111) has been studied by in situ XANES measurements in the presence and absence of hydrogen. Depending on the hydrogen and ethyl pyruvate pressure, the C and O K‐edge spectra exhibit distinctly different angular dependence. Without hydrogen ethyl pyruvate is oriented preferentially perpendicular to the surface, indicating bonding via the O lone pairs. In the presence of hydrogen the mean orientation is more tilted towards the surface. Likely, ethyl pyruvate also interacts with Pt via its π system under these conditions. The observed angle‐dependent shift of the energy of the π* and σ* resonances indicates the coexistence of differently adsorbed ethyl pyruvate species. The experimental findings demonstrate the importance of the in situ approach for unraveling the adsorption mode of ethyl pyruvate in the enantioselective hydrogenation over cinchona‐alkaloid‐modified Pt.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019047622959
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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    Supercritical Fluids; Opportunities in Heterogeneous Catalysis (2000)
    Springer
    CATTECH 4 (2000), S. 128-143 
    Details
    ISSN: 1572-8811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract The use of supercritical fluids (SCFs) in technology and research has been triggered by several unique features of these media. SCFs allow improvement of technical processes such as extraction, separation, particle formation and chemical reactions. However, it has been just in the last decades that SCFs have gained their importance in science and industrial applications, due to the relatively high pressures usually required when working with these media and the rigorous demands on the equipment used. Opportunities provided by the use of supercritical media in chemical reaction design have been the focus of several reviews(1-17). Here, we concentrate on a discussion of opportunities that SCFs provide in heterogeneous catalysis. First, we will consider the intrinsic properties of SCFs, then discuss the effect of these properties on chemical reactions, especially focusing on heterogeneously catalyzed reactions. Finally, we illustrate some of the opportunities of SCF applications in heterogeneously catalyzed reactions, using pertinent case studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1011943227355
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  • 8
    Electronic Resource
    Electronic Resource
    Supercritical Fluids; Opportunities in Heterogeneous Catalysis (2000)
    Springer
    CATTECH 4 (2000), S. 128-143 
    Details
    ISSN: 1572-8811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract The use of supercritical fluids (SCFs) in technology and research has been triggered by several unique features of these media. SCFs allow improvement of technical processes such as extraction, separation, particle formation and chemical reactions. However, it has been just in the last decades that SCFs have gained their importance in science and industrial applications, due to the relatively high pressures usually required when working with these media and the rigorous demands on the equipment used. Opportunities provided by the use of supercritical media in chemical reaction design have been the focus of several reviews(1-17). Here, we concentrate on a discussion of opportunities that SCFs provide in heterogeneous catalysis. First, we will consider the intrinsic properties of SCFs, then discuss the effect of these properties on chemical reactions, especially focusing on heterogeneously catalyzed reactions. Finally, we illustrate some of the opportunities of SCF applications in heterogeneously catalyzed reactions, using pertinent case studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1011915815968
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  • 9
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    Nuclear scaffold/matrix attached region modules linked to a transcription unit are sufficient for replication and maintenance of a mammalian episome (2004)
    National Academy of Sciences
    Details
    Publication Date: 2004-07-22
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 10
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    Varicella-zoster virus infection of human neural cells in vivo (2004)
    National Academy of Sciences
    Details
    Publication Date: 2004-07-09
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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