Libor Červený

Competitive Catalytic Hydrogenation in the Liquid Phase on Solid Catalysts

Abstract Problems of heterogeneously catalyzed hydrogenation have received increased attention recently. Although the process ranks among those most widely used in the industrial practice, knowledge of it is still inadequate. One of the interesting problems, still also unexplored from the theoretical point of view, concerns competitive catalytic hydrogenations, i.e., reaction systems in which more than one compound react simultaneously. Basically, these systems can be divided into two classes: (1) various functional groups enter the reaction and (2) the reacting functional groups are the same. Competing groups capable of hydrogenation may be contained in one or several molecules.

Opportunities Offered by Chiral η6-Arene/N-Arylsulfonyl-diamine-RuII Catalysts in the Asymmetric Transfer Hydrogenation of Ketones and Imines

Methods for the asymmetric transfer hydrogenation (ATH) of ketones and imines are still being intensively studied and developed. Of foremost interest is the use of Noyori’s [RuCl(η6-arene)(N-TsDPEN)] complexes in the presence of a hydrogen donor (i-PrOH, formic acid). These complexes have found numerous practical applications and have been extensively modified. The resulting derivatives have been heterogenized, used in ATH in water or ionic liquids and even some attempts have been made to approach the properties of biocatalysts. Therefore, an appropriate modification of the catalyst that suits the specific requirements for the reaction conditions is very often readily available. The mechanism of the reaction has also been explored to a great extent. Model substrates, acetophenone (a ketone) and 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline (an imine), are both reduced by this Ru catalytic system with almost perfect selectivity. However, in each case the major product is a different enantiomer (S- for an alcohol, R- for an amine when the S,S-catalyst is used), which demanded an in-depth mechanistic investigation. Full-scale molecular modelling of this system enabled us to visualize the plausible 3D structures of the transition states, allowing the proposition of a viable explanation of previous experimental findings.

Shape-selective synthesis of 2-acetylnaphthalene via naphthalene acylation with acetic anhydride over large pore zeolites

Liquid phase acylation of naphthalene with acetic anhydride was investigated over large pore zeolites faujasite, mordenite and beta. It has been shown that mordenite and faujasite were easily deactivated in contrast to zeolite beta. The highest activity and long term stability of zeolite beta was achieved with Si/Al ratio about 37. At lower Si/Al ratios a rapid deactivation of zeolite beta was observed, while at Si/Al ratios higher than 60 the activity of this zeolite was very low. Zeolite beta was found to be an optimum catalyst for naphthalene acylation exhibiting a selectivity to 2-acetylnaphthalene over 80%. Deactivation of the zeolite is probably caused by the decomposition of the acylating agent followed by condensation reactions of its decomposition products and/or formation of higher aromatic compounds which are not easily desorbed from the zeolite channels. The excess of naphthalene in the reaction mixture and stepwise adding of acetic anhydride improved the naphthalene conversion and the stability of the zeolite used.

Competitive hydrogenation in alkene–alkyne–diene systems with palladium and platinum catalysts

Competitive hydrogenation of alkene, alkyne and diene substrates (C6–C8) over palladium and platinum catalysts were studied at 20°C and atmospheric pressure. Selectivities of these reactions were determined and the substrates relative adsorption coefficients calculated. It was found that hydrogenations of alkynic and dienic substrates were preferred in alkyne–alkene and diene–alkene systems, respectively. In these systems palladium catalyst selectivity was higher than selectivity of the platinum catalyst, due to higher relative adsorption coefficients of corresponding substrate couples on the palladium catalyst.

Preparation of tartaric acid modified Raney nickel catalysts: study of modification procedure

Chiral modification of Raney nickel using (2R,3R)-tartaric acid was studied. The prepared catalyst was used in enantioselective hydrogenation of methylacetoacetate (MAA) to methyl-(3R)-hydroxybutyrate. The influence of the most important modification parameters, such as pH, temperature, time, concentration of the modifier and the presence of a co-modifier, on the optical yield of MAA hydrogenation was systematically investigated. From the data obtained, a considerable influence of modifying conditions on the resulting enantioselectivity of the catalyst was evident. The optical yield increased with an increasing of the modifying temperature and time. Dependencies of the optical yield on the tartaric acid concentration and on the modifying pH passed through a maximum. Therefore, there exists an optimal value of modifying pH, at which a minimal catalyst amount is leached to the modifying solution. Furthermore, significant adsorption of tartaric acid and subsequent complex formation of nickel and tartaric acid occurs on the catalyst surface. It was found that the presence of sodium bromide in the modifying solution resulted in a lower degree of the nickel leaching, and a decrease of the residual aluminum content in the catalyst increased the optical yield of the reaction.

Structure effects in hydrogenation reactions on noble metal catalysts

Molecular structure effects in catalytic hydrogenations on a group of noble metals (Pt, Pd, Rh) were discussed. The method of competitive hydrogenation allowed the assessment of the relation between the molecular structure and the rate of a surface reaction in parallel with the stability of the surface complex. Experimental results, presented as selectivity, reactivity and adsorptivity parameters, were interpreted by means of molecular modelling. This communication reviews the research work that has been carried out in the recent past, with intention to contribute to the general theory of molecular structure effects in heterogeneous catalysis.

Characterization of chirally modified Raney nickel and compounds of tartaric acid and nickel

Abstract Samples of Raney nickel modified by (2R,3R)-(+)-tartaric acid and samples of prepared complexes of tartaric acid with nickel were characterized using atomic absorption spectroscopy (AAS), organic elemental analysis (OEA), thermogravimetric analysis (TGA), X-ray diffractometry (XRD), infra-red spectroscopy (FTIR), spectroscopy in ultraviolet and visible region (UV–VIS–NIR (DRS)), determination of total surface using BET method, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with X-ray microanalyzer (SEM–EDAX). It was found that the content of aluminium in the modified catalysts decreases during the modification due to aluminium leaching to the modifying solution. Simultaneously, increase of the carbon content, which represented an adsorbed amount of tartaric acid, was determined. Furthermore, during modification in the presence of NaBr, its partial adsorption was observed. The total surface area of modified catalysts was lower than in the case of Raney nickel and further decreased in the catalysts modified without NaBr. Using XPS, two kinds of nickel atoms were identified on the surface of modified catalysts, i.e. Ni0 and Ni2+, while the surface of Raney nickel contained only Ni0. It was found by XPS analysis that the ratio of Ni2+:Ni0 was approximately 1:1 on the catalyst surface.

Hydrogenation of 2-octyne and 1-heptene on platinum and palladium catalysts

Abstract The catalytic hydrogenation of 2-octyne and 1-heptene was investigated in methanol and cyclohexane solutions on platinum and palladium catalysts, with silica gel and active charcoal as the carrier. The effects of all these parameters on the hydrogenation of the individual substrates and of their equimolar mixture were studied. It was found that the competitive hydrogenation of these substrates is a suitable model for determination of the selectivity of hydrogenation of the triple bond, even if very selective catalysts are used. The effects of the catalyst, carrier and solvent on the hydrogenation process are discussed. The palladium catalysts were much more selective in the hydrogenation of the triple bond than the platinum ones; the double bond of 1-heptene was isomerized on the former, and these catalysts also were somewhat more active.

Competitive catalytic hydrogenation in systems of unsaturated hydrocarbons and nitrocompounds

Abstract Competitive catalytic hydrogenation was studied in binary systems of aromatic and aliphatic nitrocompounds (nitrobenzene, 2-nitrotoluene, 3-nitrotoluene, 4-nitrotoluene, 1-nitropropane) and in systems of nitrocompound–unsaturated hydrocarbon (cyclohexene, 1-methylcyclohexene and 1-tert-butylcyclohexene) on palladium, platinum and rhodium supported catalysts in liquid phase. Acquired kinetic parameters enabled discussion of structural effects in the course of heterogeneous catalytic hydrogenation. In the case of nitrocompounds, potential differences in values of selectivities of competitive hydrogenations were possible to unambiguously attribute to variance in adsorptivities of these substances. In the case of competitive hydrogenations in systems of unsaturated hydrocarbon–aromatic nitrocompound, depending on the amount of aromatic amine produced, selectivity increase was observed in favor of aromatic nitrocompound, which led to an entire suppression of unsaturated hydrocarbon hydrogenation. Ultimate counter-behavior was observed in the systems of unsaturated hydrocarbon–aliphatic nitrocompound.

The kinetics of methyl sorbate hydrogenation

Abstract Methyl trans -2-hexenoate was prepared with a two-step synthesis from trans,trans -2,4-hexadienoic acid ( sorbic acid ). The first step was the methanol esterification of sorbic acid catalyzed with sulfuric acid. The second step was a catalytic hydrogenation of the resulting methylester using metallic catalysts based on Pt, Pd, Rh and Ni carried on an activated carbon and γ -alumina. During the hydrogenation reactions, in all cases, monoolefinic methyl trans -2-hexenoate was produced as the main reaction intermediate. Varied isomers of methyl 3- and 4-hexenoates resulted from the reactions as the side intermediates. The resulting intermediates further hydrogenated to a common product, methyl hexanoate. Palladium was the most apposite catalyst, due to its high activity and significant selectivity to the production of methyl trans -2-hexenoate. At the end, the raw intermediate was rectificated under the atmospheric pressure.