L. B. Belykh

Formation and the nature of activity of Ziegler systems based on palladium β-diketonate complexes in hydrogenation catalysis

The catalytic properties and nature of Ziegler-type Pd(Acac)2 and Pd(Acac)2PPh3 based catalysts are studied in the hydrogenation of unsaturated compounds. The causes of an extremum appearing in the dependence of the specific activity of the catalyst in styrene and phenylacetylene hydrogenation on the proportions of the starting components are considered. The increase in the specific activity of the Pd(Acac)2 + AlEt3 catalytic system in hydrogenation as a function of the Al/Pd ratio arises from an increase in the degree of dispersion of the microheterogeneous system, an increase in the fraction of reduced palladium, and changes in the nature of the ligand shell. The inhibiting effect is caused by triethylaluminum adsorption on palladium nanoparticles. Palladium nanoparticle models are suggested.

Effect of the nature of the acido ligand in the precursor on the properties of nanosized palladium-based hydrogenation catalysts modified with elemental phosphorus

The effect of the nature of the acido ligand in the precursor and the modifying action of elemental phosphorus on palladium catalysts for hydrogenation are reported. The large turnover frequency (TOF) and turnover number (TON) values observed for styrene hydrogenation on the Pd blacks prepared in situ by PdCl2 reduction with hydrogen in DMF are due to the formation of fine-particle catalyst with a base particle size of 6–10 nm. This is explained by the high PdCl2 reduction rate and by the formation of a palladium cluster stabilizer—dimethylammonium chloride—in the reaction system via the catalytic hydrolysis of the solvent (DMF). The modifying action of elemental phosphorus on the properties of the palladium catalysts depends on the nature of the acido ligand in the precursor. In the case of oxygen-containing precursors at small P/Pd ratios, elemental phosphorus exerts a promoting effect, raising the TON and TOF values by a factor of about 9. In the case of palladium dichloride as the precursor, white phosphorus exerts an inhibiting effect. At the same time, it enhances the stability of the catalyst, raising the TON value at P/Pd = 0.3. The causes of these distinctions are considered.

Synthesis, Properties, and Activity of Nanosized Palladium Catalysts Modified with Elemental Phosphorus for Hydrogenation

The applicability of elemental phosphorus as a modifier of palladium catalysts for hydrogenation was demonstrated, and the conditions for the synthesis of nanoparticles that are highly efficient in hydrogenation catalysis were optimized. The modifying effect of elemental phosphorus depends on the P/Pd ratio; it is associated with changes in the catalyst dispersity and the nature of the formed nanoparticles containing various palladium phosphides (PdP2, Pd5P2, and Pd6P) and Pd(0) clusters. The main stages of the formation of palladium catalysts for hydrogenation were determined, and a model of an active catalyst, in which the Pd6P phosphide is the core of a nanoparticle and Pd(0) clusters form a shell, was proposed.

Nature of the modifying action of white phosphorus on the properties of nanosized hydrogenation catalysts based on bis(dibenzylideneacetone)palladium(0)

The catalytic properties and nature of the nanoparticles forming in the system based on Pd(dba)2 and white phosphorus are reported. A schematic mechanism is suggested for the formation of nanosized palladium-based hydrogenation catalysts. The mechanism includes the formation of palladium nanoclusters via the interaction of Pd(dba)2 with the solvent (N,N-dimethylformamide) and substrate and the formation of palladium phosphide nanoparticles. The inhibiting effect exerted by elemental phosphorus on the catalytic process is due to the conversion of part of the Pd(0) into palladium phosphides, which are inactive in hydrogenation under mild conditions, and the formation of mainly segregated palladium nanoclusters and palladium phosphide nanoparticles. By investigating the interaction between Pd(dba)2 and white phosphorus in benzene, it has been established that the formation of palladium phosphides under mild conditions consists of the following consecutive steps: Pd(0) → PdP2 → Pd5P2 → Pd3P. It is explained why white phosphorus can produce diametrically opposite effects of on the catalytic properties of nanosized palladium-based hydrogenation catalysts, depending on the nature of the palladium precursor.

The State of Palladium in the Nanosized Hydrogenation Catalysts Modified with Elemental Phosphorus

The size, nature, and surface state of nanoparticles formed by reduction of Pd(acac)2 with hydrogen in the presence of P4 have been elucidated by means of X-ray photoelectron spectroscopy, X-ray powder diffraction analysis, and transmission electron microscopy. The nanoparticles (average diameter of 5.6 nm) consist of Pd6P and palladium nanoclusters (at initial ratio P/Pd = 0.3). Dimethylammonium dihydro- and hydrophosphates are found in the surface layer of the catalyst nanoparticles. The nanoparticles are stabilized by ammonium salts formed via dimethylformamide hydrolysis.

Hydrogenation catalysts based on polynuclear palladium complexes with organophosphorus ligands

A new procedure is proposed for the preparation of hydrogenation catalysts. This procedure includes the synthesis of cyclic tetranuclear palladium complexes with bridging diphenylphosphide ligands followed by a reaction with Pd(CH3COO)2 in the presence of hydrogen to form nanosized particles. In the test catalysts, the ensembles of palladium atoms (or palladium hydrides) immobilized on supramolecular structures formed by the association of phosphinidene and phosphide complexes of palladium are responsible for the catalytic activity.

Functions of organoaluminum and proton donor compounds in the formation and functioning of nanosized Ziegler-type nickel-containing hydrogenation catalysts

The turnover frequency (TOF) and turnover number (TON) in styrene hydrogenation have been determined for catalytic systems based on Ni(acac)2 · nH2O (n = 0, 0.5, and 3.0), Ni(COD)2 (COD = cycloocta-1,5-diene), and AlEt3 at different Al/Ni molar ratios. The amount of water of crystallization determines the TON and TOF of the nickel catalysts and their dependence on the Al/Ni ratio. The TON and TOF of the nickel catalysts are markedly increased by the introduction of an optimum amount of a proton donor. The model of the ligand shell stabilizing the nickel nanoparticles has been refined. The refined model provides an explanation for the activating action of proton donor compounds in the formation of the hydrogenation catalysts. The functions of the organoaluminum compounds are not limited to Ni(II) reduction to Ni(0); they are also involved in the stabilization of the nickel-containing nanoparticles.

Formation, nature of activity, and hydrogenation catalysis by nickel bis(acetylacetonate)-lithium tetrahydroaluminate systems

A new approach to synthesis of nickel catalysts under the action of lithium tetrahydroaluminate was proposed which allows preparation of high-performance nanosized catalytic systems with well-reproducible properties. The major stages of formation and the nature of catalytically active species and inhibitors formed in the Ni(acac)2-LiAlH4 system were determined. The catalytic properties of the nickel nanoclusters were studied in relation to the nature and concentration of the proton-containing compounds. Factors responsible for the promoting action exhibited by these compounds were analyzed.

Palladium hydrogenation catalysts modified with aluminum- and phosphorus-containing compounds and with alcohols : Effect of modifiers

The catalytic properties of nanosize catalysts derived from Pd(acac)2 and triethylaluminum in hydrogenation of organic substrates were studied, and the optimal conditions for the catalyst preparation were found. The maximum observed on the plot of the catalyst specific activity vs. Al/Pd ratio was explained by experiments.

Nanosized hydrogenation catalyst based on palladium bisacetylacetonate and phosphine: Formation, the origin of activity, and properties

The nature and catalytic properties of a hydrogenation catalyst based on Pd(acac)2 and PH3 are considered. As demonstrated by a variety of physicochemical methods (IR and UV spectroscopy, 31P and 1H NMR, electron microscopy, and X-ray powder diffraction), nanoparticles consisting of various palladium phosphides (Pd6P, Pd4.8P, and Pd5P2) and Pd(0) clusters form under the action of dihydrogen during catalyst preparation. The promoting effect of phosphine at low PH3: Pd(acac)2 ratios is mainly due to the ability of phosphine to increase the extent of dispersion of the catalyst.