H.U. Blaser
Novartis
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Featured researches published by H.U. Blaser.
Journal of Molecular Catalysis | 1991
H.U. Blaser; H.P. Jalett; J. Wiehl
Abstract The effect of several organic acids and bases used as solvents and additives for the enantioselective hydrogenation of α-ketoesters with cinchona-modified platinum catalyst is described. Carboxylic acids have a positive influence on the enantioselectivity of the reaction, both as solvents and as additives. The best results are observed in acetic acid where the optical yields increase between 8 and 12% compared to toluene or ethanol as solvents. In addition, extremely low modifier concentrations are necessary. For the hydrogenation of ethyl pyruvate, using a special wide pore Pt/Al2O3 catalyst and 10,11-dihydro-O-methylcinchonidine as modifier, we obtained 95% enantioselectivity, the highest value ever reported for any chiral heterogeneous catalyst. Bases are not suitable as solvents but in some cases they have a beneficial effect as additives at low concentration.
Studies in Surface Science and Catalysis | 1988
H.U. Blaser; H.P. Jalett; D.M. Monti; J.F. Reber; J.T. Wehrli
Abstract A detailed investigation of supported platinum catalysts modified with cinchona alkaloids for the enantioselective hydrogenation of pyruvic acid esters has been carried out. It was confirmed that pretreatment of the catalysts has a strong effect on their performance, which is dependent both upon the catalyst type and the solvent used. Modified catalysts are up to 100 times more active than without cinchona alkaloids. Many commercially available catalysts are active with enantioselectivities of 20-90%. There is no simple correlation between performance and either the total or metal surface area. The most important process variables are pressure, temperature and modifier concentration. Any change in the combination Pt catalysts / cinchona alkaloid / α-ketoester leads to almost complete loss of enantioselectivity. Some mechanistic aspects of this interesting catalytic system are discussed.
Studies in Surface Science and Catalysis | 1991
H.U. Blaser; H.P. Jalett; D.M. Monti; Alfons Baiker; J.T. Wehrli
The effect of catalyst and modifier structure on selectivity and activity for the enantioselective hydrogenation of ethyl pyruvate with cinchona modified Pt/Al2O3 catalysts has been investigated. It has been found that the platinum dispersion and the method of catalyst preparation have a decisive influence on the catalytic performance. In order to get high optical yields, the platinum dispersion should be lower than 0.2 - 0.3. The texture of the support is less important but must be optimized with respect to pore size in order to get high enantioselectivity. Modifiers derived from cinchonidine lead to an excess of (R)-ethyl lactate while cinchonine gives preferentially the S enantiomer. Changes in the O-C9-C8-N part of the cinchona alkaloid have a decisive influence both on sign and size of the optical induction while the nature of the substituent in the quinuclidine part, R1, has only a minor effect. Partial hydrogenation of the quinoline rings results in a decreased enantioselectivity. Conclusions concerning the mode of action of the enantioselective catalyst are discussed.
Journal of Molecular Catalysis | 1989
J.T. Wehrli; Alfons Baiker; D.M. Monti; H.U. Blaser; H.P. Jalett
Abstract The influence of the reaction medium upon the enantioselective hydrogenation of α-ketoesters to the corresponding α-hydroxyesters has been studied, using a commercial platinum-on-alumina catalyst modified with cinchonidine. For the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate, the optical yield as well as the initial reaction rate were strongly dependent upon the type of solvent used. With few exceptions, the optical yield and the activity decreased linearly with increasing dielectric constant of the solvent. With suitable solvents (e.g. ethanol or toluene), both the hydrogenation activity and the selectivity were independent of substrate concentration over a wide range. This indicates a zero-order dependence of the reaction rate with respect to α-ketoester. The influence of conversion on the optical yield was studied for the hydrogenation of three different α-ketoesters. In all cases the optical yield was constant until completion of the reaction. With repeated use, the catalyst showed a moderate decrease in activity, whereas the high enantio selectivity was maintained.
Catalysis Letters | 1997
Peter Baumeister; H.U. Blaser; M. Studer
The accumulation of hydroxylamines during the catalytic hydrogenation of several aromatic nitro compounds could be reduced from >40% to <1% by the addition of catalytic amounts of vanadium promoters, resulting in a faster reaction and purer products.
Journal of Molecular Catalysis | 1989
J.T. Wehrli; Alfons Baiker; D.M. Monti; H.U. Blaser
Abstract The effect of platinum particle size on the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate in the liquid phase was studied using a series of Pt/γ-Al 2 O 3 catalysts modified with cinchonidine. The catalysts containing 5 wt.% platinum were prepared by the incipient wetness technique with H 2 PtCl 6 as precursor. Reduction at different temperatures, redispersion in air and sintering were employed to change the platinum dispersion from 0.27 to 0.6, corresponding to a particle size range of 1.7–4.0 nm. Within this particle size range, activity (expressed as initial turnover frequency) and enantioselectivity were higher for the larger particles. However, an increase in selectivity was not observed when the platinum particle size was increased by sintering. This indicates that the method applied for particle size control has an influence on the performance of the catalysts.
Studies in Surface Science and Catalysis | 1993
H.U. Blaser; Hans-Peter Jalett
Abstract The enantioselective hydrogenation of α-ketoacids using modified heterogeneous catalysts can be carried out with moderate to good (
Applied Catalysis | 1989
H.U. Blaser; H.P. Jalett; D.M. Monti; J.T. Wehrli
Abstract The effect of thermally pretreating Pt/Al 2 O 3 catalysts in hydrogen on enantioselectivity and activity in the liquid-phase hydrogenation of α-keto esters and the structure and oxidation state of the catalysts was studied. Thermal treatments in hydrogen at 673 K gave 15–20% higher optical yields of the α-hydroxy esters produced. In addition, the activity of the catalysts was up to three times higher for the activated catalysts. The processes taking place during the pretreatment of commercial and laboratory-made catalysts were characterized by temperature-programmed reduction (TPR), carbon monoxide and nitrogen adsorption and X-ray diffraction. The TPR profiles showed three well resolved peaks which were assigned to a platinum oxide shell on a platinum core (200–400 K), residual platinum salts (400–550 K) and carbon deposits that originate from the liquid-phase preparation processes (600–700 K). The amount and reducibility of the platinum oxide was determined by TPR and correlated with the activity of the catalysts in the sense that strongly oxidized catalysts show much lower activities. There is no simple correlation between the improvement in selectivity and the processes taking place during thermal activation, but there are indications that residual platinum salts have a negative influence on enantioselectivity.
Catalysis Letters | 1991
J. L. Margitfalvi; H.P. Jalett; E. Tàlas; Alfons Baiker; H.U. Blaser
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.
Studies in Surface Science and Catalysis | 1997
H.U. Blaser; D. Imhof; M. Studer
Summary The modifier concentration dependence of rate and enantioselectivity (e.e.) was investigated for the hydrogenation of ethyl pyruvate catalyzed by hydrocinchonidine (HCd) modified platinum catalysts. A model assuming the reversible adsorption of one (described by K m ) or two molecules of HCd (K m2 ) on unmodified active sites (Pt u ) was used. Good agreement between measured and calculated data was obtained for different types of Pt catalysts, several solvents and also for experiments in presence of additives. It was shown that high enantioselectivities were obtained only for catalytic systems with a high k m /k u ratio (rate constants of the modified and unmodified sites) and high intrinsic selectivity (s). No correlation exists between the observed maximum e.e. values and the absolute size of K m or k m . The results in presence of small amounts of quinoline and triethylamine indicate a competitive adsorption with HCd on modifiable sites, whereas thiourea seems to adsorb irreversibly.