Utilisation of on-line acoustic irradiation as a means to counter-effect catalyst deactivation in heterogeneous catalysis

The effect of power ultrasound has been studied in the hydrogenation of 1-phenyl-1,2-propanedione (enantioselective hydrogenation) and linoleic acid (fat hardening). Furthermore the hydrogenation of d-xylose to xylitol and citral to citronellal and citronellol were studied under the influence of acoustic irradiation. We have investigated the effect of on-line acoustic irradiation of these heterogeneously catalysed liquid-gas-solid systems in which it was found to counter-effect catalyst deactivation, increase the reaction velocity and to enhance the selectivity of various heterogeneously catalysed reactions.     

Ultrasonic Irradiation in Enantioselective Hydrogenation of 1-Phenyl-1,2-Propanedione

In the presence of ultrasound both the initial hydrogenation rate of 1-phenyl-1,2- propanedione and the enantiomeric excess of the main product, i.e. (R)-1-hydroxy- 1-phenylpropanone, increased by 75 % and 52 % in toluene, respectively, as compared to the silent hydrogenation.     

Effect of modifier structure in the enantioselective hydrogenation of 1-phenyl-1,2-propanedione

Three different modifiers, (-)-cinchonidine, 10,11-dihydrocinchonidine and (+)-cinchonine were investigated in the enantioselective hydrogenation of an unsymmetrical dione, 1-phenyl-1,2-propanedione, over a commercial Pt/Al₂O₃ catalyst. The catalytic activity, as well as the regio- and enantioselectivity in the hydroxyketones and diols formed were compared.     

Effect of ultrasound in enantioselective hydrogenation of 1-phenyl-1,2-propanedione: comparison of catalyst activation, solvents and supports

The enantioselective hydrogenation of 1-phenyl-1,2-propanedione was carried out over Pt/Al2O3, Pt/SiO2, Pt/SF (silica fiber), Pt/C catalysts modified with cinchonidine under ultrasonic irradiation. The initial rate, regioselectivity and enantioselectivity were investigated for different catalyst pretreatments, solvents and ultrasonic powers. The ultrasound effects were very catalyst dependent. The sonication significantly enhanced enantioselectivity and activity of the Pt/SF (silica fiber) catalyst. For the other Pt supported catalysts the reaction rate, enantioselectivity and regioselectivity increased moderately. The choice of solvent influenced the impact of ultrasound effect, namely in mesitylene, which has the lowest vapor pressure, the highest ultrasound enhancement was observed. The effect of sonication on catalysts surface was studied by transmission electron microscopy and scanning electron microscopy (SEM). No significant change in the metal particle size distribution due to sonication was observed. However, in the case of the Pt/SF catalyst, acoustic irradiation induced morphological changes on the catalyst particle surface (SEM), which might be the cause for enhancement of the initial reaction rate and enantioselectivity.     

The Effect of Chemical Reducing Agents in the Synthesis of Sol-Gel Ru-Sn Catalysts: Selective Hydrogenation of Cinnamaldehyde

The influence of chemical reduction on the properties of bimetallic Ru-Sn/SiO₂ catalyst was studied. Prepared sol-gel catalysts were reduced by NaBH₄, KBH₄, H₂, and CH₂O. Physical and catalytic properties of reduced catalysts were compared to non-reduced ones in the liquid-phase hydrogenation of cinnamaldehyde. The influence of boron impregnation was investigated. The observed hydrogenation activity and selectivity towards cinnamylalcohol were effected by reduction pretreatment. The NaBH₄ reduced catalyst exhibited the highest activity and selectivity. Reduction pretreatment influenced the pore sizes, active metal surface as well as content of carbon impurities. Traces of the reduction agents were detected in the catalysts. Boron impregnation presumably improved the catalyst activity, but had a minor effect on the selectivity.     

A combined NMR, DFT, and X-ray investigation of some cinchona alkaloid O-ethers

Structures and conformational behavior of several cinchona alkaloid O-ethers in the solid state (X-ray), in solution (NMR and DFT), and in the gas phase (DFT) were investigated. In the crystal, O-phenylcinchonidine adopts the Open(3) conformation similar to cinchonidine, whereas the O-methyl ether derivatives of both cinchonidine and cinchonine are packed in the Closed(1) conformation. Dynamic equilibria in solutions of the alkaloids were revealed by combined experimental-theoretical spin simulation/iteration techniques for the first time. In the (1)H NMR spectra in CDCl3 and toluene-d8 at room temperature, Closed(1) conformation was observed for the O-silyl ethers as a separate set of signals. For O-methyl ether derivatives Closed(1) could be separated only at -30 degrees C in CDCl3 or toluene-d8 and for O-phenylcinchonidine at -70 degrees C in CDCl3/CD2Cl2. The ratio between the Closed(2) and Open(3) conformers was estimated by analyzing the vicinal coupling constant (3)J(H9,H8) at ambient and low temperatures. The observed conformational equilibria of O-(tert-butyldimethylsilyl)cinchonidine in CDCl 3 and toluene-d8 are in good agreement with the theoretically estimated equilibrium populations of the conformations according to Boltzmann statistics. The conformational equilibria of four cinchona alkaloid O-ether solutes in CDCl3 and toluene-d8 are discussed in the light of their relevance to the mechanism of 1-phenyl-1,2-propanedione (PPD) hydrogenation over cinchona alkaloid modified heterogeneous platinum catalysts. It was demonstrated that the conformation found to be abundant in the liquid phase has no direct correlation with the enantioselectivity of the PPD hydrogenation reaction.     

Mechanism of the chemo–bio catalyzed cascade synthesis of <Emphasis Type="Italic">R</Emphasis>-1-phenylethyl acetate over Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, lipase,and Ru-catalysts

One-pot synthesis of R-1-phenylethyl acetate was investigated starting from acetophenone hydrogenation performed over Pd/Al₂O₃ and PdZn/Al₂O₃ catalysts followed by acylation of the intermediate secondary alcohol, R-1-phenylethanol, over an immobilized lipase. Furthermore, the performance of a third type of catalyst, Ru supported on hydroxyapatite (HAP) was evaluated for racemization of S-1-phenylethanol in one pot together with the two other catalysts. The main objectives of this work were to separate the effects of different catalysts and to reveal the reaction mechanism. For this purpose not only acetophenone, but also (R,S)-1-phenylethanol, S-1-phenylethanol, R-1-phenylethyl acetate, and styrene were used as reactants in combination with Pd/Al₂O₃, lipase and Ru/HAP as catalysts. The results revealed that the main side product, ethylbenzene, was formed in two different ways, via dehydration of (R,S)-1-phenylethanol to styrene, followed by its rapid hydrogenation to ethylbenzene, and via debenzylation of the desired product, R-1-phenylethyl acetate to ethylbenzene. The true one-pot synthesis, however, was demonstrated over Shvo’s catalyst, but Ru/HAP was not sufficiently active in the racemization step. Ru/Al₂O₃ was a promising catalyst for racemization of S-1-phenylethanol and for dynamic kinetic resolution of (R,S)-1-phenylethanol, when using only small amounts of the acyl donor ethyl acetate. The challenge in racemization is that the activity of heterogeneous Ru catalysts was inhibited by esters.     

Towards a Molecular Understanding of Asymmetric Heterogeneous Catalysis: Hydrogenation of 1-Phenyl-1,2-Propanedione1

The present work comprises a detailed investigation of a complex reaction system, revealing features of reaction mechanisms that are general for asymmetric heterogeneous catalysis. Heterogeneous enantioselective hydrogenation of 1-phenyl-1,2-propanedione was studied over cinchonidine modified Pt catalysts producing (R)-1-hydroxy-1-phenylpropanone as the main product with an enantiomeric excess (ee) of 65% at maximum yield, which could be further increased above 90% due to kinetic resolution. The results of kinetic studies in batch and continuous reactors, catalyst screening and characterization results, as well as quantum chemical calculations, are summarized, and pertinent mechanistic aspects are discussed.     

Deactivation in liquid-phase hydrogenation

Importance of investigating catalyst deactivation in the synthesis of fine chemicals is illustrated by several examples in three-phase hydrogenation reactions, performed in batch and continuous reactors. This revised version was published online in June 2006 with corrections to the Cover Date.