Tamas Mallat

Oxidation of alcohols with molecular oxygen on platinum metal catalysts in aqueous solutions

The platinum-catalyzed oxidation of alcohols with molecular oxygen in aqueous medium was discovered one and a half centuries ago [ 1,2]. The method has been improved with the general development of heterogeneous catalysis. At the beginning of this century it was found [ 31 that not only platinum, but dispersed palladium was also a good catalyst for the reaction. Platinum on various supports was tested and compared by Heyns [ 41 when he extended the method to the synthesis of L-ascorbic acid. The next milestone was the promotion of platinum metals with heavy metals such as Bi or Pb to overcome the deactivation problems and change the direction (selectivity) of the reaction [ 5-71. The present review will be focused on the application of biand multi-metallic catalysts in the aqueous phase oxidation of alcohols with molecular oxygen. The reactions discussed are the transformation of primary alcohols to aldehydes or carboxylic acids, and secondary alcohols to ketones. Special attention will be given to the various reasons for the loss of catalytic activity and the role of promoters in the suppression of deactivation. A detailed discussion of the earlier results of alcohol oxidation on platinum metal catalysts, especially in the field of carbohydrate chemistry, can be found in previous reviews [ 818 1. The mild reaction conditions and high selectivity in the oxidation of polyols made the method very attractive for carbohydrate chemistry. As an example, the catalytic route for the oxidation of D-glucose to D-gluconic acid based on the application of promoted Pd catalysts in aqueous alkaline medium can compete economically with the generally applied biochemical oxidation [ 19,201. Various chelating agents, detergent builders or food additives may be prepared by this way from relatively cheap starting materials of natural origin.

Shape-selective enantioselective hydrogenation on Pt nanoparticles.

The structure sensitivity of enantioselective hydrogenations on chirally modified metals was investigated using Pt nanoparticles of different shapes. All three samples had an average particle size of 10 nm, but the fraction of dominantly cubic, cubooctahedral, and octahedral particles varied with decreasing {100} and increasing {111} faces in the same order. In the absence of chiral modifier the hydrogenation of ethyl pyruvate was independent of the shape of the Pt nanoparticles; variation of the specific reaction rates did not exceed the experimental error on all self-prepared catalysts and on a commercial Pt/Al(2)O(3) used as reference. Addition of cinchonidine or quinine induced a significant rate enhancement by a factor of 4-15, and the rate was always higher with quinine. Also, 72-92% ees were achieved, and the reaction was shape selective: both the rate and the ee increased with increasing Pt{111}/Pt{100} ratio. A similar correlation in the hydrogenation of ketopantolactone confirmed that decarbonylation or aldol-type side reactions of ethyl pyruvate were not the reason for structure sensitivity. A combined catalytic and theoretical study revealed that the probable origin of the particle shape dependency of enantioselective hydrogenation is the adsorption behavior of the cinchona alkaloid. DFT studies of cinchonidine interacting with Pt(100) and Pt(111) terraces indicated a remarkably stronger interaction on the former crystallographic face by ca. 155 kJ/mol. The higher adsorption strength on Pt(100) was corroborated experimentally by the faster hydrogenation of the homoaromatic ring of the alkaloid, which fragment interacts the strongest with Pt during its adsorption. Thus, an ideal catalyst for the hydrogenation of activated ketones contains dominantly Pt{111} terraces, which crystallographic face is more active and affords higher enantioselectivity, combined with the higher stability of the modifier.

Asymmetric Hydrogenation of Activated Ketones on Platinum: Relevant and Spectator Species

The heterogeneous enantioselective hydrogenation of activated ketones over chirally modified platinum is reviewed with emphasis on identifying the role of the various species observed in this catalytic system. The past years have witnessed a continuous broadening of the scope of this catalytic system including new reactants and modifiers affording over 97% ee. New reaction pathways have been uncovered and the kinetic and mechanistic studies have been faced with a number of complicating factors caused by spectator species and interactions in solution and on the Pt surface. The previously proposed mechanistic models are critically assessed in the light of these new findings.

EPOXIDATION OF FUNCTIONALIZED OLEFINS OVER SOLID CATALYSTS

Heterogeneous catalytic epoxidation of functionalized olefins in the liquid phase has been reviewed, focusing on catalyst performance and its interrelation with the crucial parameters of the catalytic systems. Efficient catalysts include supported and mixed oxides, framework-substituted (“redox”) molecular sieves, layered-type materials, heterogenized homogeneous catalysts, and some others. Among the various substrates, allylic and homoallylic alcohols, and unsaturated carbonyl compounds have received most attention so far. The great variety of available catalysts enables selective epoxidation of most substituted olefins. The mechanistic understanding of heterogeneous catalytic epoxidation is still underdeveloped, rendering catalyst design rather empirical. A considerable potential for future development lies in the area of “heterogenization” of successful homogeneous catalysts especially for asymmetric epoxidation. Crucial requirements in the development of heterogeneous catalytic epoxidation catalysts are, besides good catalytic performance and cheap oxidant, recyclability and resistance to leaching of the active component. Some of the examples shown in the literature do not fulfill the latter requirement.

Enantioselective hydrogenation of 2-methyl-2-pentenoic acid over cinchonidine-modified Pd / alumina

A chiral alkanoic acid was prepared with up to 52% excess of the (S) enantiomer by hydrogenating anα,β-unsaturated carboxylic acid with a cinchonidine-Pd/Al2O3 catalyst system. Favourable conditions are: high surface hydrogen concentration (⩾ 60 bar hydrogen pressure, low catalyst concentration and apolar solvents), near ambient temperature and a cinchonidine/reactantmolar ratio of at least 0.4 mol%. It is proposed that high hydrogen solubility and the presence of 2-methyl-2-pentenoic acid reactant as dimers are advantageous for enantiodifferentiation.

Selectivity enhancement in heterogeneous catalysis induced by reaction modifiers

Abstract The application range of solid catalysts can be greatly extended by reaction or process modifiers , that is by simple addition of an inorganic or organic compound to the reaction mixture. The modifier, used in catalytic amounts, ideally interacts strongly with the active sites in a fashion which induces favorable changes in the outcome of the reaction. Evolution of the actual modified metal catalyst during reaction and the importance of in situ characterization in understanding these processes are illustrated using the examples of promotion by metal ions and nitrogen-containing bases. The major part of the review describes the advantages and limitations of employing N-base modifiers for tuning the performance of solid catalysts. Reactions discussed include chemo-, stereo-, enantio- and diastereoselective hydrogenations over metal catalysts, aerobic oxidation of alcohols with Pt and Pd, and epoxidation of allylic alcohols with titania–silica mixed oxides and alkyl hydroperoxides.

Allylic oxofunctionalization of cyclic olefins with homogeneous and heterogeneous catalysts

Heterogeneous catalysis and heterogenization of homogeneous catalysts are catch phrases of modern chemistry. Here we attempt to trace the development from homogeneous to heterogeneous catalysis in the allylic oxofunctionalization of cyclic olefins. The utility of this oxidative conversion is highlighted succinctly in the allylic oxidation of isophorone to ketoisophorone, which is a useful intermediate in the synthesis of carotenoids and fragrances. Competitive allylic oxidation and epoxidation are discussed briefly for the structurally simpler cyclohexene. Allylic oxidation of bicyclic α-pinene to verbenone is used as an example to illustrate that upon progressing to more complex cyclic olefins, competitive isomerization and structural rearrangements often result in very poor selectivity to the desired α,β-unsaturated ketone.

Enantioselective hydrogenation of α,β-unsaturated acids. Substrate-modifier interaction over cinchonidine modified PdAl2O3

Abstract X-Ray diffraction, IR measurements and catalytic hydrogenation of various substituted acrylic acids in apolar solvents, as well as molecular modelling provide new insight into the nature of the cinchonidine-substrate interaction and a general rule to predict the major enantiomer.

Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids

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.

Catalyst potential: a key for controlling alcohol oxidation in multiphase reactors

Abstract The possibilities and advantages of measuring the catalyst potential during alcohol oxidation with molecular oxygen and supported platinum metal catalysts is discussed. The steady-state catalyst potential is a sensitive indicator of the balance between oxidation and reduction processes taking place on the active sites. In this short review we focus on the application of the potential measurements for adjusting the rate of oxygen supply to the rate of alcohol oxidation and avoiding catalyst deactivation. Other uses are the control of selectivity in the consecutive reaction sequence alcohol → aldehyde → acid and the elucidation of the nature of catalyst deactivation. The partial oxidation of cinnamyl alcohol in a multi-phase reactor has been chosen as an example.