Active Centers Of Catalyst Research Articles

Change in the structure of a cathodic catalyst based on turbostratic carbon modified by CoTMPP in the course of corrosion interaction

The change in the structure of a cathodic catalyst based on turbostratic carbon (carbon black) modified by products of pyrolysis of cobalt tetra(pmethoxyphenyl)porphyrin in the course of corrosion tests in 0.5 M H2 SO 4 at a temperature of 60°C is studied. It is shown that corrosion treatment can result, in the case of multicomponent systems, both in changes in the support surface and in a decrease in the amount of active catalyst centers on the whole.

Catalytic decomposition of polystyrene. The role of acid and basic active centers

The influence of the acid and basic properties of a catalyst on the selectivity of polystyrene transformation was studied. The properties of silicaalumina SiO2–Al2O3(45%) dotted with NaOH from 1 to 20wt% and γ-Al2O3 containing 1 to 8wt% NaOH or H2SO4 were examined using the test reactions of cumene (acid) and diacetone alcohol (base) transformation. These catalysts were used in polystyrene transformations. It was found that polystyrene decomposition involves thermal and catalytic transformation. In the first case, two main reaction pathways are involved: gradual depolymerization to the monomer (styrene) and pyrolysis leading to the formation of different volatile oligomers (dimers, trimers, tetramers.). The latter species react solely over the catalyst active centers. Linear dimers of styrene activated by Brønsted acid sites undergo decomposition to styrene and ethylbenzene as well as to toluene and α-methylstyrene. Consecutive transformation of the products leads to a simultaneous hydrogen (H+ and H−) production and coke formation. The latter ions are active in hydrogen transfer reactions which causes hydrogenation of styrene to ethylbenzene. In the presence of base catalysts selective styrene formation takes place and transformation leading to coke are slower hence ethylbenzene formation is suppressed. In the presence of acid catalysts linear dimers can also isomerize to cyclic derivatives which after dealkylation give benzene and methylindane. The later product in isomerization and hydrogen transfer reactions forms methylindene and naphthalene. The higher styrene oligomers reactions are catalyzed by both acid and basic sites leading mainly the monomer, i.e. styrene formation.

Synthesis and investigation of photocatalytic activity of lanthanum-titanium oxides promoted by metal-oxide systems based of copper and silver

We have investigated factors affecting the formation of the structure and properties of active centers of catalysts of hydrogen synthesis of organic compounds on the basis of hybrid lanthanum-titanium oxide systems based on copper and silver. It has been shown that the conditions for the preparation of synthesized materials substantially affect their physicochemical and catalytic properties. We have revealed main regularities for the formation of catalysts investigated under nonhydrothermal conditions. It has been determined that this process is affected by such factors as the concentration of organic compounds, temperature, the pH of the reactive medium, the amount of the catalyst and the prompting phase.

Separation and Polymerization of Active Species of the Neodymium Chloride Isopropanol Complex/Triisobutylaluminium as Catalyst

The reaction product between NdCl3·3iPrOH and Al(i-Bu)3 as catalysts was separated into a soluble and an insoluble part in hexane,and isoprene polymerization behaviors and characteristics of hexane-soluble and insoluble fractions were examined.The results indicated that the hexane-soluble fraction of the catalyst aging with the(Nd+Al) preformation had no activity at room temperature and at 70 ℃,and the catalytic precipitate obtained at 70 ℃ had no active too.The catalysts with the(Nd+IP+Al) preparation pattern showed activity of polymerization at both room temperature and 70 ℃.We speculated that there are two catalyst active centers of(isoprenyl)2NdCl and(isoprenyl)NdCl2 in the polymerization system.The catalytic prepared methods had no effect on the microstructures of polyisoprene(about 96%).The molecular mass of the polyisoprenes obtained by the suspension catalyst and precipitate catalyst was high(Mn100×104) and the molecular mass distribution was narrow(Mw/Mn2.0),while the polyisoprene obtained by the clear solution catalyst had lower molecular mass(Mn30×104) and a very wide molecular mass distribution(Mw/Mn5.0).

ChemInform Abstract: Dynamic Processes in Active Phase-Support Interactions

Abstract This paper constitutes an attempt to discern the main dynamic interactions between active phase and support in catalytic reactions. The focus is on the structural or textural changes and mobility phenomena that take place during the reaction, and which as a consequence, have a direct bearing on the real nature of the active catalyst centers, as well as on ageing and deactivation processes. We begin with an inventory of the various types of interactions, distinguishing between those which bring about changes in the active phase and those exhibiting, as a consequence, a modification of the catalytic reaction. Three groups of processes, the dynamic nature of which has important consequences, will be examined: (i) those dealing with the changes in texture (in particular, dispersion) of the active phase; (ii) those originating from reactions in the solid state and (iii) those linked to movements of mobile surface species (spillover) between the active phase and support. Catalysis-driven processes , as opposed to those simply developing along with catalysis, without interaction between both phenomena will be particularly discussed.

Transformations of tetradecane on La/Al2O3 + ZSM catalysts

The behavior of La/Al2O3 + ZSM catalysts has been studied in the process of the hydrogen-free conversion of tetradecane. The structure and state of active centers of catalysts have been investigated using a combination of physicochemical and instrumental methods (TPD of ammonia, IR spectroscopy, electron microscopy). It was found that the catalysts are characterized by the presence of acid (Bronsted and Lewis) sites and M0 and Mn+ metal centers. Lanthanum in different oxidation states can be included in the acid sites. The degree of tetradecane conversion and the product composition are determined by the SiO2/Al2O3 ratio of the zeolite and the conditions of the process. The mechanism of the cracking and transformations of tetradecane on La/Al2O3 + ZSM catalysts is discussed.

Investigation of Ethylene Polymerization Kinetics over Ziegler-Natta Catalysts: Employing Moment Equation Modeling to Study the Effect of Different Active Centers on Homopolymerization Kinetics

AbstractIn the present work, ethylene polymerization kinetics was modeled using moment equations. According to the results obtained, the molecular weight distribution of each active center follows a Schultz-Flory distribution. However, the molecular weight distribution of polymer produced is much broader than a Schultz- Flory distribution. Besides, the order of polymerization with regard to monomer concentration is higher than unity. Moreover, the catalyst active centers deteriorate in the presence of hydrogen and consequently catalyst yield drops; nevertheless, polymerization kinetics is not mainly affected by hydrogen. Hydrogen also reduces polymer molecular weight since it is a strong transfer agent in olefin polymerizations. Notwithstanding, it does not affect polydispersity index. Last of all, although increasing cocatalyst concentration does not influence the activity of active centers, it lessens the molecular weight as a transfer agent

Application of Monte Carlo simulation method to polymerization kinetics over Ziegler–Natta catalysts

AbstractIn the current work, the Monte Carlo simulation method was applied to ethylene polymerization over Ziegler–Natta catalysts. As expected, polymerization over each center of a Ziegler–Natta catalyst leads to a polymer having a Schultz–Flory molecular weight distribution. Notwithstanding, the total molecular weight distribution obtained by all catalyst centers together is at least twice as broad as that of each center. As another interesting finding, the introduction of hydrogen to the reaction deactivates the catalyst active centers and thereby reduces the catalyst activity. Nevertheless, it does not mainly affect the polymerization kinetics. In addition, the polymer molecular weight falls as hydrogen is added to the reaction since it acts as a strong transfer agent. The same effect is seen when cocatalyst concentration increases. Hydrogen also widens the polymer molecular weight distribution. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 45–56, 2009

Propylene polymerizations with a binary metallocene system—Chain shuttling caused by trimethylaluminium between active catalyst centers

AbstractPropylene was polymerized at varying trimethylaluminium (TMA) concentration with a homogeneous binary metallocene catalyst system activated by methylaluminoxane (MAO) in an attempt to better understand interactions between active catalyst sites and to clarify the role of the TMA as a chain shuttling agent. TMA‐free polymerization conditions were obtained by chemical treatment of MAO solution with 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT). A binary catalyst system consisting of catalyst precursors diphenylmethyl(cyclopentadienyl)(9‐fluorenyl)zirconium dichloride (1) producing high Mw syndiotactic polypropylene and rac‐dimethylsilylbis(4‐tert‐butyl‐2‐methyl‐cyclopentadienyl)zirconium dichloride (2) producing low Mw isotactic polypropylene was investigated. At the studied polymerization conditions, chain shuttling between the active catalysts caused by TMA was confirmed. The chain shuttling reactions caused changes in catalyst activity, molecular weights, melting behavior, and polymer microstructure. We propose that TMA is capable to transfer a growing polymer chain from catalyst 2 to catalyst 1, and a stereoblock copolymer is formed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1364–1376, 2007

Katalizatory i proces koordynacyjnej polimeryzacji etylenu - eksperyment i teoria

Experimental and theoretical studies of coordination polymerization of ethylene have been performed in the presence of precursors - the complexes of Ti, Zr, Hf, V halides with organic Lewis bases, magnesium support [MgCl2(THF)2] and organoaluminium compounds. It was found the correlation between a nature of halide in precursor and vanadium catalyst activity (Table 1, Figs. 1-3). The energetic profiles of insertion (Table 2) and termination [beta-elimination (Fig. 4) or hydrogen transfer (Table 3)] reactions have been computed. Calculated values of insertion barriers as well as of ethylene and active center bond energy correspond well with polymerizations results. Agostic interactions found in the active center, assist in stabilisation of transition metal carbon bond. In the titanium-magnesium catalysts, containing oxygen-donor ligands [tetrahydrofurfurol - Formula (I), its anion - Formula (II) or ethylene glycol dimethyl ether - Formula (III)], the possibility of transformation of O,O'-bidentate ligands into dangling monodentate ones was shown. This may play a crucial role in the catalyst activity. The structures of active centers in catalysts investigated have been proposed (Fig. 7) and has been demonstrated by theoretical way.

Dialkoxy-Substituted, C1-Symmetric Metallocenes: Synthesis and Catalytic Behavior in the Propylene Polymerization Reaction

The synthesis of a series of C1-symmetric metallocene complexes rac-[1-(5,6-dialkoxy-2-methyl- 1-η5-indenyl)-2-(9-η5-fluorenyl)ethane]zirconium dichlorides (alkyl: n-butyl, n-hexyl, n-octyl, n-decyl) is described. These complexes are versatile catalysts in the polymerization of propylene after in situ activation with triisobutylaluminum (TIBA) and Ph3C[B(C6F5)4] in toluene and heptane solution. All catalysts show higher solubility and improved polymerization properties in industrially used hydrocarbon solvents (e.g. heptane). However, the molecular weights and isotacticity values of the resulting polypropylene materials are decreased compared to the ethoxy-bridged analogue rac- [1-(5,6-ethylenedioxy-2-methyl-η5-indenyl)-2-(9-η5-fluorenyl)ethane]zirconium dichloride. A possible explanation is based on enhanced interaction of the active catalyst centers with Al(III) scavenger molecules even at low Al : Zr ratios, leading to reversible chain transfer.

Kinetics of the platinum catalyzed hydrazoic acid decomposition in acidic media

The kinetics and the mechanism of the hydrazoic acid decomposition over the 1% Pt/SiO 2 catalyst in the solutions of perchloric and nitric acids were studied at [HN 3] 0=5×10 −3 to 5×10 −2 M; [H +]=0.5–9 M (HClO 4 or HNO 3); T=44–75 °C and a solid-to-liquid ratio S/L=0.05–0.15 g/ml. It was shown that in 0.5–8 M HClO 4 and 0.5–5 M HNO 3 the hydrazoic acid catalytic decomposition proceeds as a heterogeneous disproportionation of HN 3 molecules with a formation of the ammonium ions and the gaseous nitrogen as reaction products. The reaction deceleration and the increase of the apparent activation energy with the increase of the solution acidity is explained by the competitive adsorption of the hydrogen ions on active catalyst centers. In the concentrated (>6 M) nitric acid, the decomposition of hydrazoic acid proceeds due to a catalytic oxidation of adsorbed HN 3 molecules by nitronium ions NO 2 +, existing in the nitric acid solutions in some equilibrium concentration. Mechanisms of the both process are postulated.

The platinum catalyzed hydrazine decomposition in non-nitrate acidic media

The stoichiometry, kinetics and the mechanism of the hydrazine decomposition over the 1% Pt/SiO 2 catalyst in the solutions of perchloric and sulphuric acids were studied at [N 2H 5 +] 0 = 2×10 −3 to 0.25 M; [H +] = ∼10 −4 (pH 4) to 3.4 M (HClO 4 or H 2SO 4); T=40–90 °C; and a solid:liquid ratio, S/L=0.02–0.2 g/ml. It was shown that the hydrazine catalytic decomposition proceeds with a formation of the ammonium ions and the gaseous nitrogen as reaction products in all studied range of the experimental conditions. It was found that the apparent reaction order with respect to [N 2H 5 +] depends on the hydrazine initial concentration and increases from about 0.2 to 1 when [N 2H 5 +] 0 decreases from 0.2 to 0.01 M. The reaction deceleration and the increase of the apparent activation energy with the increase of the solution acidity is explained by the competitive adsorption of the hydrogen ions on active catalyst centers. The mechanism of the hydrazine heterogeneous catalytic decomposition in acid non-nitrate solutions includes the adsorbed hydrazine dissociation as a limiting step of the reaction.

A facile and effective method for the distribution of Mo/HZSM-5 catalyst active centers

Post-steaming treatment of Mo/HZSM-5 catalysts results in more molybdenum species migrating into and residing in the HZSM-5 zeolite channels. This is confirmed by XRF and XPS measurements. 1H MAS NMR and 29Si MAS NMR also demonstrate that the number of free Bronsted acid sites decreases in the Mo/HZSM-5 catalysts that underwent post-steaming treatment, compared to untreated Mo/HZSM-5 catalysts. As a result, the deactivation rate constant (kd) on the Mo/HZSM-5 catalyst after post-steaming treatment for 0.5h is much smaller, and the catalyst therefore shows remarkable stability in the probe reaction of methane dehydro-aromatization. The results suggest that a more beneficial bi-functional balance between active Mo species for methane activation and acid sites for the following aromatization is developed over those Mo/HZSM-5 catalysts that have experienced post-steaming treatment for 0.5h, in comparison with the untreated Mo/HZSM-5 catalysts.

Catalytic Hydrogen Transfer over Magnesia, xv. Preliminary Studies of Active Centers of Catalysts

Reduction of 7-tridecanone and 1-phenyl-1-hexanone by 2-propanol at 573-673 K in the presence of MgO has been studied. Acidic as well as strong and weak basic sites of MgO surface have been blocked in the reaction of the oxide with triethylamine, phenol or benzoic acid (1000 μmol/g of catalyst), respectively. Retention of activity of MgO treated with triethylamine or phenol in ketones reduction by 2-propanol has been observed. Only residual activity of MgO poisoned by benzoic acid has been noted. The exclusive participation of moderate basic centers of MgO in catalytic reduction of ketones by 2-propanol has been ascertained.

Oxidation of Long Chain Primary Alcohols to Acids over the Quaternary Ammonium Peroxotungstophosphate Catalyst System

Factors influencing the catalytic properties for long chain primary alcohol oxidation to acid over quaternary ammonium peroxotungstophosphate catalyst were investigated. The catalysts were characterized by means of IR and NMR techniques. Based on the experimental results, the catalyst active centers for the title reaction were elucidated and optimal reaction conditions given.

Inversion of Methane on Transition-Metal Complexes: A Possible Mechanism for Inversion of Stereochemistry

The inversion of methane bound to first-row transition-metal ions from Sc+ to Cu+ is systematically investigated using the B3LYP method, a hybrid density-functional-theory method of Becke and Lee, Yang, and Parr. The computed transition states for the methane inversion on the M+(CH4) complexes have a Cs structure in which one pair of C−H bonds is about 1.2 Å in length and the other pair is about 1.1 Å. The barrier height for the methane inversion is significantly decreased from 109 kcal/mol for free methane to 43−48 kcal/mol on the late transition-metal complexes, Fe+(CH4), Co+(CH4), Ni+(CH4), and Cu+(CH4). Since each activation energy involves the binding energy of the complex (16 kcal/mol on the average), the actual barrier height should be lower by this quantity if measured from the dissociation limit. The inversion of methane can therefore occur at the transition-metal active center of catalysts or enzymes under ambient conditions through a thermally accessible transition state, and it would reasonably lead to inversion of stereochemistry at a carbon atom in catalytic reactions of hydrocarbons. We propose that a radical mechanism based on a planar carbon species may not be the sole source of the observed loss of stereochemistry in transition-metal-catalyzed hydrocarbon hydroxylations and other related reactions.

Dynamic processes in active phase-support interactions

This paper constitutes an attempt to discern the main dynamic interactions between active phase and support in catalytic reactions. The focus is on the structural or textural changes and mobility phenomena that take place during the reaction, and which as a consequence, have a direct bearing on the real nature of the active catalyst centers, as well as on ageing and deactivation processes. We begin with an inventory of the various types of interactions, distinguishing between those which bring about changes in the active phase and those exhibiting, as a consequence, a modification of the catalytic reaction. Three groups of processes, the dynamic nature of which has important consequences, will be examined: (i) those dealing with the changes in texture (in particular, dispersion) of the active phase; (ii) those originating from reactions in the solid state and (iii) those linked to movements of mobile surface species (spillover) between the active phase and support. Catalysis-driven processes, as opposed to those simply developing along with catalysis, without interaction between both phenomena will be particularly discussed.

On the nature of active centers in catalysts for propylene oxidation to acetone

Acid-base properties of binary tin-oxide catalysts for the oxidation of propylene to acetone have been studied. It has been revealed that active catalysts should have both Bronsted acid- and base centers.

The mobility of chemisorbed molecules and the surface regeneration of active centers in the dehydration of isopropanol on aluminum oxide and aluminosilicate

1. By a differential isotope method involving14C the authors have investigated the surface mobility of chemisorbed molecules of isopropanol during its dehydration in an adsorption layer on aluminum oxide and aluminosilicate. 2. The chemisorbed alcohol molecules possess marked surface mobility which plays a decisive part in the mechanism of surface regeneration of the active catalyst centers in the process of dehydration. 3. The cessation of the reaction long before the adsorbed alcohol is completely used up is explained by the hypothesis that there is local overpopulation of the active sectors by water formed by the reaction; this hinders further surface regeneration and repetition of the elementary events of dehydration.