Ziegler Systems Research Articles

Competitive hydrogenation of alkynes and olefins: Application for the analysis of size sensitivity

In this study, the researchers applied the competitive hydrogenation of two similar terminal alkynes (or olefins) to develop a new kinetic approach to evaluate size effects in catalytic processes and differentiate active sites. This method is based on the analysis of differential selectivity, using phase trajectories of reactions. An analysis of the phase trajectories shows that the hydrogenation of terminal alkynes occurs on the same type of active sites of palladium catalysts (faces). The negative size effect in alkyne hydrogenation, under the action of palladium catalysts with a particle size of 6.6–24 nm, is associated only with the geometric factor. Contrary to alkynes, hydrogenation of olefins takes place on at least two types of active sites. The reasons for the dependence of the apparent turnover frequency of alkyne hydrogenation on the particle size for Ziegler systems are considered. Moreover, the hypotheses about the catalytically active sites of Pd-P nanoparticles in the hydrogenation of alkynes are discriminated.

Directed design of hydrogenation Ziegler systems

We reported the concept of a directed design of multicomponent catalytic systems of the hydrogenation of unsaturated hydrocarbons.

Kinematical structural stability

This paper gives an overview of our results obtained from 2009 until 2014 about paradoxical stability properties of non conservative systems which lead to the concept of Kinematical Structural Stability (Ki.s.s.). Due to Fischer-Courant results, this ki.s.s. is universal for conservative systems whereas new interesting situations may arise for non conservative ones. A remarkable algebraic property of the symmetric part of linear operators may generalize this result for divergence stability but leading only to a conditional ki.s.s. By duality, the concept of geometric degree of nonconservativity is highlighting. Paradigmatic examples of Ziegler systems illustrate the general results and their effectiveness.

Formation and properties of Ziegler systems based on nickel bis(dimethylglyoximate) in catalysis of hydrogenation reactions

The effect of the nature of the acidoligand in the nickel precursor on the activity of Ziegler systems in hydrogenation of alkenes was characterized qualitatively. The key steps of the reaction of nickel bis (dimethylglyoximate) with triethylaluminum determined by the methods of IR, NMR, and ESR spectroscopy include the reaction between AlEt3 and the OH group of the dimethylglyoximate ligand, formation of the product of C-alkylation as a result of addition of AlEt3 to the C=N bond, and the reduction of Ni(II) to Ni(0) complexes. The absence of nickel clusters and a low concentration of coordinatively unsaturated Ni(0) complexes is the most probable reason of low catalytic activity of the studied system in hydrogenation catalysis.

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.

The particle-forming process of SiO2-supported metallocene catalysts

Homogeneous metallocene catalysts for the polyolefin production show compared with conventional Ziegler systems remarkable advantages, like the possibilities to regulate the microstructure leading to optimized polymer properties. However, for industrial application it is necessary to immobilize the metallocenes on a heterogeneous support. The parameters which now influence polymerization kinetics, polymer growth, polymer morphology, the decisive particle fragmentation are demonstrated and summarized in a physical and mathematical model.

Nonlinear Damping as a Stabilizating Factor and Resolving Ziegler's Paradox

Nonlinear model obtained from notorious Ziegler model by adding nonlinear terms is considered. Ziegler's paradox is explained from nonlinear point of view. It is known that our world is nonlinear. In order to describe it, it is necessary to use nonlinear models. But linear models continue playing an independent and auxiliary (linearization) role as they are more simply. However there are situations general enough when paradoxical effects in linear models disappear after adding nonlinear terms. For example one of such situations is the Ziegler paradox. There is a jump of value of the critical force (at which the system loses stability) in a linear not conservative system in which damping is taken into account. In this paper it is shown that if one changes the Ziegler's system a little with adding nonlinear damping that the jump disappears. In the nonlinear system depending on values of parameters we have either a "soft" type of loss of stability with appearance a stable limit cycle or a "sharp" type of loss of stability. The most interesting and important fact is that the difference between "soft" and "sharp" losses of stability is equal the magnitude of the jump of the critical force in the linear Ziegler system.

Adjusting the Features of Active Metallocene Ziegler Systems fot Their Potential Use as Carbon–Carbon Coupling Catalysts in Organic Synthesis

A variety of bent metallocene dichloride/methylalumoxane catalysts, derived from the zirconocene complexes 10, 12-18, have been employed in intramolecular olefin-coupling reactions yielding monomeric or dimeric products. This was achieved by using optimized reaction conditions employing low substrate concentrations (1.0-1.8 M) and rather long reaction times. Under these particular conditions, 1,5-hexadiene was cyclodimerized to give 1-methylene-3-(cyclopentylmethyl)cyclopentane (11). 1,6-Heptadiene was cleanly converted to methylenecyclohexane, and cis-1,2-divinylcyclopentane (19) and cis-1,2-divinylcylohenxane (22) were cyclized to yield 2-methylenebicyclo[3.3.0]octane (20) and 7-methylenebicyclo[4.3.0]nonane (23), respectively. In many cases, the cyclization products were accompanied by isomers originating from double-bond shift reactions that often occur rapidly at these catalyst systems under the specific reaction conditions chosen to keep the products in the monomeric regime.

Stability loss delay in a Ziegler system

Stability loss delay in a Ziegler system is considered, on the assumption that the follower force is slowly increasing. An “input-output” function is constructed, enabling the time at which stability loss occurs to be predicted, given the initial magnitude of the force.

Ziegler systems as hydrogenation catalysts. Part 2. Dependence of the hydrogenation activity of CoBr 2/Li[AlH n (OR) 4- n] systems from the concentration of the reducing agent

The hydrogenation activity of Li[AlH n(OR) 4- n ]/CoBr 2 systems in THF depends on the number of H atoms n and on the R group of the reducing agent. The change of activity with increasing concentration of the alkoxyalanate can be described as a 1:1 equilibrium between the catalyst and the reducing agent, when n = 1 and R = t-butyl. In all other cases the range of activity is very small and can be described as the stepwise formation of a bis-adduct.

Ziegler systems as hydrogenation catalysts.: Part 1. CoBr 2/Li [AlH ( O- t-butyl) 3] 3 system as a catalyst for the hydrogenation of anthracene

The system Li[AlH ( O- t-butyl) 3]/CoBr 2 in THF is a very active catalyst for the hydrogenation of anthracene. The rising activity with increasing concentration of the reducing agent can be described in terms of the formation of a 1 : 1 adduct between the catalyst and the reducing agent. If the catalyst is kept in a hydrogen atmosphere prior to the addition of the substrate, its activity decreases rapidly. During the addition of the components of the catalyst, at most 0.5 mol H 2/Mol CoBr 2 are generated at once, followed by slow H 2 generation which can be described as two parallel first-order reactions. From the immediate H 2 generation it can be deduced that CoBr 2 is reduced to Co I and that the catalyst contains unchanged reducing agent.

Burkitt's lymphoma between African and American types in Turkish children: Clinical, viral (EBV), and molecular studies

Seventy-two Turkish children with Burkitt's lymphoma (BL) observed during a period of 22 years (1968-1990) have been analysed retrospectively. The diagnosis was established histologically according to WHO criteria. BL represented 50% of NHL in this series. The patients were staged according to Ziegler's system. The median age of patients was 5.5 years with a sex (M/F) ratio of 2.1/1. The most common primary site of tumor involvement was the abdomen (69.4%), which was followed by facial tumors, in particular the jaw and orbit (49.9%). There were 21 cases with jaw (29.1%) and 15 cases with orbital involvement (20.8%) at initial presentation. The majority of the patients (84.4%) were in advanced stages (C and D) at initial diagnosis. Facial tumors observed in Turkish children with BL were more similar to African Burkitt's lymphoma than American or European cases. High titers of antibodies against VCA and EA of EBV were also seen in our recent cases of BL. Two main treatment regimens, namely, single agent chemotherapy with cyclophosphamide (CYX) (1968-1974) and three drug (COM) combination chemotherapy, were used consecutively (1974-1988). COM has been shown to produce better results than single agent therapy. The clinical presentation, mean age, and high antibodies (IgG) to EBV and preliminary molecular studies revealed that BL appears to be in between African and non-African types in Turkish children. This will be further elucidated by direct examination of tumor cells for EBV and investigation of the molecular characteristics of Turkish tumors. Such studies are presently under way.

An IR study of the Ziegler system Co(acad)3/Mg(n-butyl)2 in THF

Applying IR spectroscopy for studying the Ziegler system Co(acac)3/Mg(Bu)2 (Bu=n-butyl) in tetrahydrofuran, it could be shown that the following reactions take place: reduction of cobalt (III), exchange of the acetylacetonate groups and their interaction with Mg(Bu)2.

ESR spectra of47Ti-enriched titanium catalysts for polymerization

For the first time ESR spectra of47Ti-enriched Ziegler system Ti-TIBA in toluene have been studied. For the ratio Ti/Al∼1, more accurate h. f. s. constants have been found. For Ti/Al∼1/15 no h. f. s. have been observed, which is ascribed to ordering due to the cooperative Jahn-Teller effect.

Kinetic characteristics of chain propagation and transfer to aluminium alkyl in the high temperature polymerization of ethylene on Ziegler-type titanium-magnesium catalysts

The concentrations of active propagation centres and rate constants for elementary acts of PE macrochain propagation and the limitations of its propagation by alumino-organic compounds in the polymerization of ethylene under conditions of the PE in a hydrocarbon solvent at 433 K on titanium-magnesium catalytic systems (based on a bulk catalyst and on a system with dissolved components), and also on a traditional Ziegler system based on titanium trichloride are determined by radiochemical methods. It is shown that titanium-magnesium catalytic systems have a several times higher content of active centres in their composition, and that the high temperature polymerization of PE under their action has larger values of the above constants than with the classical Ziegler system.

The initiation mechanism of isoprene polymerization on a heterogeneous ziegler system, as studied by a multiple pulse NMR method

High-frequency NMR spectroscopy together with multiple-pulse methods was used to study the transformations occurring during the polymerization of isoprene with a heterogeneous catalyti Ziegler complex at elevated temperatures. From the structure of terminal fragments in macromolecules, the occurrence of spontaneous chain termination processes in the system was indicated, with the formation of a metal-hydride bond capable of subsequent exchange reactions. The initiation mechanism could be explained by means of the model of Cosse, modified by a consideration of the π-allylic nature of the active centre of polymerization.

Interpretation of energy stability for Ti 2 7+ and Ti 2 6+ dimers

By using the interacting bonds method (IBM), it has been established that for the chloride Ziegler systems (TiCl4+AlR3) for polymerization the formation of Ti27+ and Ti26+ pairs is a thermodynamically favorable process.

Promotion by supports of the reactivity of propagating species of Ziegler supported catalytic systems for the polymerization and copolymerization of olefins

AbstractKinetics for the polymerization of ethylene and the copolymerization of ethylene and propylene were studied by using highly effective heterogeneous metal organic catalysts produced by coating different organic and inorganic supports with the components of Ziegler systems. The activity of a supported Ziegler catalyst is characterized by the physical parameters of the support structure and its chemical nature. The active role of magnesium‐containing supports was established for the formation and functioning of the propagating species on their surfaces. This role is expressed not only by an increase in the portion of transition metal included in the propagating species, compared with typical Ziegler catalysts, but also by an increase in the reactivity of the propagating species, change in the nature of the elementary processes for polymerization and copolymerization, including control of copolymerization constants, and modification of the molecular structure of the polymers and copolymers. It was shown that by choice of support it is possible to control the activity of the same catalytic system and characteristics of the structure and properties of the polymers it produces under identical polymerization conditions.

ChemInform Abstract: TERPENE PRODUCT DERIVATIVES FROM ISOPRENE OLIGOMERIZATION

AbstractDas Übersichtsreferat, vorgetragen auf dem Symposium über "World‐Wide Progress of the Petro, Organic, and Polymer Chemical Industries Division of Industrial and Engineering Chemistry, ACS/CSJ Chemical Congress, Honolulu, Hawaii, April 1979, beschreibt die Oligomerisierung von Isopren an modizifierten Ziegler‐Katalysatoren.

Kinetic characteristics of the propagation reaction in polymerization of ethylene using highly effective ziegler type catalytic systems containing magnesium

Concentrations of active centres and rate constants of macrochain propagation were determined by a radiochemical method in polymerization of ethylene on typical Ziegler systems: Al(C 2H 5) 3−TiCl 4(I), Al(C 2H 5) 3−CH 3OTiCl 3 (II) and system I modified by the addition of diphenylmagnesium and system II applied on an anhydrous magnesium chloride carrier. A relation was established between the proportion of transition metal used effectively and the chemical composition of the catalytic system, the concentration of titanium compound in the reaction zone and its content in catalyst samples used. It was shown that the application of these additives containing magnesium and a carrier increase the reactivity of active catalytic complexes—the rate constant of chain propagation increases from 2 to 20 times, reaching 2400±120 1./mole·sec at 70° in the system of Al(C 2H 5) 3−CH 3OTiCl 3/MgCl 2.