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  • 1
    Electronic Resource
    Electronic Resource
    Role of the modifier in the enantioselective hydrogenation of ethyl pyruvate over Pt/Al2O3 catalyst (1990)
    Springer
    Catalysis letters 6 (1990), S. 281-288 
    Details
    ISSN: 1572-879X
    Keywords: Enantioselective hydrogenation ; ethyl pyruvate ; Pt/Al2O3 ; cinchona alkaloids ; modifier
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The time dependence of the conversion and enantioselectivity during the hydrogenation of ethyl pyruvate has been studied over an industrial Pt/Al2O3 catalyst. Various cinchona alkaloids were used as modifier and two different modes were applied for their introduction into the reaction system. The dependence of the enantioselectivity on conversion is strongly influenced by the mode of introduction and the structure of the modifier used. The conversion dependence of the enantioselectivity is attributed to the chemical transformations of the parent alkaloid observed under hydrogenation conditions. Experimental evidence is shown for the dynamic nature of the interaction between the modifier and the catalyst.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00763994
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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
    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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  • 4
    Electronic Resource
    Electronic Resource
    Enantioselective hydrogenation of ethyl pyruvate. Influence of oxidative treatment of cinchonidine-modified platinum catalyst and hemiketal formation in alcoholic solvents (1994)
    Springer
    Catalysis letters 29 (1994), S. 115-124 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective hydrogenation ; Pt/alumina ; cinchonidine ; ethyl pyruvate ; protonation ; hemiketal formation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The reasons for the increase in the rate and enantiomeric excess after oxidative (aerobic) treatment of Pt/alumina in ethanol have been investigated. It is demonstrated that this treatment results in the formation of acetic acid and consequently in the protonation of the quinuclidine n1 of cinchonidine. This favours the cinchonidine-pyruvate interaction and improves enantioselectivity. In addition, the reaction rate is enhanced due to acid catalysis of the carbonyl reduction. NMR and UV measurements indicate the rapid transformation of ethyl pyruvate to the corresponding hemiketal in primary alcohols as solvents. It is shown that the possible involvement of this hemiketal (and that formed between cinchonidine and ethyl pyruvate) as an intermediate in the pyruvate hydrogenation mechanism can be excluded.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00814258
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  • 5
    Electronic Resource
    Electronic Resource
    Chiral nitrogen compounds as new modifiers for the enantioselective hydrogenation of ethyl pyruvate (1995)
    Springer
    Catalysis letters 31 (1995), S. 143-151 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective hydrogenation ; chiral modifier ; ethyl pyruvate ; Pt/alumina
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The enantioselective hydrogenation of ethyl pyruvate to (R)- or (S)-ethyl lactate has been studied over alumina- and carbon-supported Pt-metal catalysts modified by various heterocyclic N-compounds and substituted amides. The reactions were carried out under mild conditions in acetic acid; other solvents had a detrimental effect on enantioselectivity. An enantiomeric excess (ee) of 67% and a rate acceleration by a factor of 6, compared to the unmodified catalyst, was observed with alumina-supported Pt modified by (R)-1-(1-naphthyl)ethylamine. In contrast, carbon-supported Pd, Ru and Rh were non-selective and only little active. The studies indicated that besides naphthyl or quinolyl groups, two separate phenyl groups or one phenyl group together with two amino groups can provide a suitable anchoring of the chiral modifier on the Pt surface. The nature of interaction between the modifiers and ethyl pyruvate is briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00808828
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  • 6
    Electronic Resource
    Electronic Resource
    Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids (1995)
    Springer
    Catalysis letters 34 (1995), S. 1-9 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective hydrogenation ; supercritical fluids ; ethyl pyruvate ; cinchonidine ; Pt/alumina ; CO-poisoning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate has been studied using gases under supercritical conditions as solvents. The catalyst was a 5 wt% Pt/alumina modified with cinchonidine. In supercritical ethane the reaction time could be reduced by a factor of 3.5 compared to toluene under similar conditions, without any loss in enantioselectivity. A further advantage of ethane is that the enantioselectivity remains high even at high catalyst/reactant ratio, which is interesting in view of a possible application of a continuous fixed-bed reactor for this reaction. A strong catalyst deactivation was observed in supercritical CO2, which is due to the reduction of CO2 on Pt as indicated by FTIR.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00808315
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  • 7
    Electronic Resource
    Electronic Resource
    Enantioselective hydrogenation of ethyl pyruvate on tin promoted Pt/Al2O3 catalysts (1991)
    Springer
    Catalysis letters 10 (1991), S. 325-333 
    Details
    ISSN: 1572-879X
    Keywords: Pt/Al2O3 ; promotion by tin ; modification by tin tetraalkyls ; surface organometallic complexes ; enantioselective hydrogenation ; ethyl pyruvate ; dihydrocinchonidine
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The enantioselective hydrogenation of ethyl pyruvate has been studied on a Pt/Al2O3-dihydrocinchonidine catalyst promoted with different amount of tin. The surface reaction between hydrogen adsorbed on Pt and tin tetraalkyls is used for the tin introduction. This reaction leads to the formation of surface organometallic complexes (I), with SnR(4-x) moieties anchored to the platinum surface. The enantioselectivity of the Pt/Al2O3-dihydrocinchonidine catalyst is found to change only slightly upon promotion with tin, while the rate of ethyl pyruvate hydrogenation depends strongly on the amount and the form of tin introduced. The hydrogenation activity is suppressed completely at relatively low tin coverage (Sn/Pts 〈 0.06). The highest hydrogenation rate is measured over catalysts containing complex (I) (Sn/Pts = 0.025) on the platinum surface. On Sn-Pt alloy type active sites, which are formed after decomposition of (I) in hydrogen, the rate of hydrogŋation is considerably lower than on the unpromoted reference Pt/Al2O3 catalyst.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00769167
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  • 8
    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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  • 9
    Electronic Resource
    Electronic Resource
    Titania-silica aerogels with superior catalytic performance in olefin epoxidation compared to large pore Ti-molecular sieves (1996)
    Springer
    Topics in catalysis 3 (1996), S. 421-436 
    Details
    ISSN: 1572-9028
    Keywords: titania-silica ; mixed oxide ; sol-gel ; aerogel ; epoxidation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The paper provides a short overview on the various Ti- and Si-containing epoxidation catalysts, with a focus on the development and characterisation of titania-silica mixed oxides prepared by the sol-gel-aerogel method. Titania-silica aerogels, dried by semicontinuous extraction of the solvent with supercritical CO2 at low temperature, possess high surface area, an amorphous mesoporous structure, and a rather even distribution of Ti in the silica matrix. The catalytic activities of mixed oxides, silica-supported titania and Ti-substituted molecular sieves (TS-1, Ti-Β and Ti-MCM-41) are compared in the epoxidation of 1-hexene, 1-octene, cyclohexene, cyclododecene and norbornene, based on literature data. The activities (i.e. the amount of converted olefin per unit time and catalyst weight) vary in a broad range of several orders of magnitude. It is shown that the low temperature aerogel containing 20 wt% TiO2 is superior to any other Ti- and Si-containing catalyst for the epoxidation of cyclic olefins. In the epoxidation of linear alkenes TS-1 has outstanding activity. Ti-substituted large and ultra-large pore molecular sieves require further development concerning their catalytic performance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02113865
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