The Experts below are selected from a list of 273 Experts worldwide ranked by ideXlab platform
Tuula T. Pakkanen - One of the best experts on this subject based on the ideXlab platform.
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Modeling Coadsorption of Titanium Tetrachloride and Bidentate Electron Donors on Magnesium Dichloride Support Surfaces
Journal of Physical Chemistry C, 2014Co-Authors: Andrey S. Bazhenov, Peter Denifl, Timo Leinonen, Anneli Pakkanen, Mikko Linnolahti, Tuula T. PakkanenAbstract:Coadsorption of titanium tetrachloride and two representative bidentate electron donors on magnesium dichloride surfaces is systematically studied by means of periodic quantum chemical calculations. The two catalytically relevant surfaces in the Ziegler–Natta Catalysis, (104) and (110) surfaces of the MgCl2 support, are taken into account. Adsorption of TiCl4 leads to formation of three types of mononuclear species on the magnesium dichloride surfaces. However, TiCl4 alone cannot properly stabilize the support. Coadsorption of electron donors along with TiCl4, on the other hand, is shown to significantly improve the strength of TiCl4 adsorption on the magnesium dichloride surfaces. Our findings indicate the importance of electron donors as promoters of titanium tetrachloride adsorption. The model is readily extendable to evaluate other electron donors and binuclear titanium species.
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Theoretical investigations on Ziegler-Natta Catalysis: Coordination of the electron donors to titanium modified MgCl2 support
Journal of Molecular Catalysis A-chemical, 1997Co-Authors: Eini Puhakka, Tuula T. PakkanenAbstract:Abstract Theoretical ab initio methods have been used to study the formation of the support-catalyst complex and the coordination of electron donors to this complex. A Mg 4 Cl 8 TiCl 4 cluster was used to model the catalyst surface. Determination of the coordination geometry of the supported catalyst indicated that the TiCl 4 catalyst coordinates octahedrally to the surface of the support, leaving one coordination site of the titanium empty. This vacant coordination site is in a stereospecific position. The coordination of different electron donors (alcohols, ketones, esters and their model compounds) through their oxygen atom to the Mg 4 Cl 8 TiCl 4 cluster was also studied. On the basis of the interaction energies, released in the coordination of the donors, the alcohol donors bind more strongly to the support than the ketones and esters. The extra stability of the alcohols can be explained by hydrogen bonding. The coordination geometries of the electron donors studied can be estimated rather reliably even with small model compounds.
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Theoretical investigations on Ziegler-Natta Catalysis : Alkylation of the TiCl4 catalyst
Journal of Molecular Catalysis A-chemical, 1997Co-Authors: Eini Puhakka, Tuula T. PakkanenAbstract:Abstract Theoretical ab initio methods have been used to study the alkylation reaction of the MgCl 2 -supported TiCl 4 catalyst. Investigation of the reaction path indicated that the Al(CH 3 ) 3 co-catalyst inserts into the coordination sphere of the titanium atom of the catalyst so that a methyl group of Al(CH 3 ) 3 interacts with the titanium. The methyl group migrates from Al(CH 3 ) 3 to the vacant coordination site of the catalyst. When the catalyst loses one of its chlorine atoms to the co-catalyst during this reaction, the catalyst maintains its vacant site, but the position of the vacant site changes. The presence of the external electron donor coordinated to the co-catalyst makes the alkylation reaction energetically more favourable.
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Theoretical investigations on Ziegler-Natta Catalysis: models for the cocatalyst components
Journal of Organometallic Chemistry, 1996Co-Authors: Eini Puhakka, Tuula T. Pakkanen, Eero IiskolaAbstract:Theoretical ab-initio methods have been used to study the stability of the Ziegler-Natta cocatalyst complexes, which consist of aluminium trialkyl and silicon alkoxide compounds. The choice of the basis set indicated that the 6-31 G∗ basis set is the best for aluminium-, oxygen- and silicon-containing compounds. Examination of the interaction energies of the complexes showed that the aluminium compounds determine the stability of the cocatalyst complexes. The silicon alkoxides have a slight effect on the interaction energies, which are to some degree dependent on the size and number of the alkoxy groups of the silicon alkoxides.
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Theoretical investigations on Ziegler-Natta Catalysis: models for the interactions of the TiCl4 catalyst and the MgCl2 support
Surface Science, 1995Co-Authors: Eini Puhakka, Tuula T. PakkanenAbstract:Abstract Theoretical ab initio methods have been used to study the interaction between the TiCl 4 catalyst and the MgCl 2 support in the Ziegler-Natta Catalysis. Determination of the coordination geometry between the catalyst and the support was carried out by optimizing the structure of the TiCl 4 MgCl 2 complex. The optimized structure of the complex resembles the chain-like structure of β-TiCl 3 . The applicability of this model was tested by determining the most favourable place for a methyl group in the coordination sphere of the catalyst. Based on these investigations we found the catalyst complex to be octahedrally coordinated with one vacant coordination site. The effect of the support on the coordination geometry of the catalyst was also studied by enlarging the support model. The results indicate that the coordination geometry is only slightly dependent on the coordination of the magnesium atom.
Luigi Cavallo - One of the best experts on this subject based on the ideXlab platform.
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Accurate experimental and theoretical enthalpies of association of TiCl4 with typical Lewis bases used in heterogeneous Ziegler-Natta Catalysis.
Physical Chemistry Chemical Physics, 2017Co-Authors: R. Credendino, Dario Liguori, Giampiero Morini, Yury Minenkov, Fabrizio Piemontesi, Andrea Melchior, Marilena Tolazzi, Luigi CavalloAbstract:Adducts of TiCl4 with Lewis bases used as internal or external donors in heterogeneous Ziegler–Natta (ZN) Catalysis represent a fundamental interaction contributing to the final composition of MgCl2 supported ZN-catalysts. This study presents the accurate experimental evaluation, from titration calorimetry, of the formation enthalpy of TiCl4 adducts with 15 Lewis bases of industrial interest. In addition, we report the accurate energies of association of TiCl4 with the same Lewis bases from calculations at the DLPNO-CCSD(T) level of theory. These accurate experimental and theoretical association values are compared with selected methods based on density functional theory (DFT) in combination with popular continuum solvation models. Calculations suggest that the PBE-D3, and M06 functionals in combination with a triple-ζ plus polarization quality basis set provide the best performance when the basis set superposition error (BSSE) is not removed from the association energies. Cleaning the association energies with the BSSE with the counterpoise protocol suggests B3LYP-D3, TPSS-D3 and M06L as the best performing functionals. The introduction of solvent effects with the PCM and SMD continuum solvation models allows the DFT-based association enthalpies to be compared with the experimental values obtained from titration calorimetry. Both solvation models in combination with the PBE-D3, PBE0-D3, B3LYP-D3, TPSS-D3, M06L, and M06 functionals provide association enthalpies close to the experimental values with MUEs in the range of 10–15 kJ mol−1.
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Toward a Unified Model Explaining Heterogeneous Ziegler–Natta Catalysis
ACS Catalysis, 2015Co-Authors: Raffaele Credendino, Dario Liguori, Giampiero Morini, Luigi CavalloAbstract:We propose a model for MgCl2-supported Ziegler–Natta catalysts, which is capable to reconcile the discrepancies emerging in the last 20 years, in which attempts have been made to rationalize experimental data by molecular models. We show that step defects on the thermodynamically more stable (104) facet of MgCl2 can lead to sites for strong TiCl4 adsorption. The corresponding Ti-active site is stereoeselective, and its stereoselectivity can be enhanced by coordination of Al-alkyls or Lewis bases in the close proximity. The surface energy of the step defected (104) MgCl2 facet is clearly lower than that of the well accepted (110) facet.
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Molecular mechanics and mechanisms of regulation of the stereospecificity in Ziegler‐Natta Catalysis
Macromolecular Symposia, 2011Co-Authors: Gaetano Guerra, Luigi Cavallo, Paolo Corradini, Michele VacatelloAbstract:The model catalytic sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler- Natta polymerizations of olefins, are reviewed. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non-bonded interactions. This mechanism of enantioselectivity, which involves a chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems. The model sites for the homogeneous isospecific polymerization of propene are also able to account for the observed enantioselectivities in the regioirregular placements. Recent calculations relative to a peculiar catalytic model site, which should present a polymerization mechanism involving a regular back-skip of the chain, to the starting position after each monomer insertion, are shortly reviewed. The relevance of this polymerization mechanism to the comprehension of the stereospecificity of some homogeneous and heterogeneous catalytic systems is also briefly discussed.
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Moving up and down the Titanium Oxidation State in Ziegler−Natta Catalysis
Macromolecules, 2011Co-Authors: Naeimeh Bahri-laleh, Andrea Correa, Shahram Mehdipour-ataei, Hassan Arabi, Mehdi Nekoomanesh Haghighi, G. H. Zohuri, Luigi CavalloAbstract:DFT molecular modeling studies were undertaken to shed light on possible activation and deactivation mechanisms of Ziegler−Natta catalytic systems, as well as on the possible mechanisms for their reactivation by organohalides. We focused our efforts on Ti species attached to the (110) lateral cut of MgCl2. First, the possible activation of adsorbed TiCl4 leading to an adsorbed TiIII species bearing a Ti−alkyl bond and a coordination vacancy, which is a species able to undergo chain-growth, was considered. According to our calculations formation of the first active species can be easily rationalized by cleavage of a Ti−Cl bond of coordinated TiCl4 by AlEt3, followed by transalkylation promoted by another AlEt3 molecule. Second, we investigated the possible reduction of polymerization active TiIII species leading to polymerization inactive TiII species, and we found that a Ti−H bond, possibly formed after chain termination, is weaker than the Ti-Et (polymeryl) bond. Third, we investigated the mechanism of r...
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On the effects of methyl substituents on chelating ligands in models for homogeneous isospecific Ziegler-Natta Catalysis
Polymer, 2003Co-Authors: Luigi Cavallo, Gaetano Guerra, Paolo Corradini, Michele VacatelloAbstract:Abstract A model for homogeneous isospecific Ziegler-Natta polymerization, recently proposed by us, has been tested for its ability to predict the experimentally observed behaviour when the structure of the catalysts is changed by chemical substitution. In particular, conformational energy calculations have been performed on models of the catalytic site containing the ethylene-bis(1-indenyl) ligand substituted in various positions with methyl groups. In agreement with previous experiments, the analysis indicates that the presence of substituents at the 3,3′ positions strongly reduces the isospecific behaviour of this catalyst. On the contrary, calculations performed on model sites containing the non-chiral ethylene-bis(cyclopentadienyl) ligand suggest that the presence of methyl groups at 3,3′ positions should induce a high degree of isospecificity in this otherwise aspecific system. This theoretical prediction has been confirmed by recently announced experimental results.
Mika Härkönen - One of the best experts on this subject based on the ideXlab platform.
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Asymmetric Markovian analysis of the polymerization mechanism of propene by ziegler‐natta Catalysis and external alkoxysilane donors
Macromolecular Symposia, 2011Co-Authors: Riichirǒ Chûjǒ, Mika Härkönen, Jukka SeppäläAbstract:The formulae for conditional probabilities are derived with the term of NMR triad tacticities for the polymers whose polymerization process is described by an asymmetric Markovian process. The formulae obtained are applied to the polypropylene polymerized by Ziegler-Natta Catalysis with external alkoxysilane donors. For both, unfractionated samples and heptane-insoluble fractions, four evidences are clarified as follows: Don the transformation from symmetric to (fluctuating) asymmetric sites the transformation of one (hereafter, L) site is much more enhanced than that of the other (D) site, 2) monotonous increase of isotacticity with an increase of the concentration of the donor is due to the increase of pLL, 3) aromatic donors are more effective than aliphatic ones for the improvement of isotacticity, and 4) the improvement of isotacticity after an addition of alkoxysilane is not due to the contribution from both of L and D sites, but the competition between the larger positive contribution from the former and the smaller negative one from the latter.
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External Silane Donors in Ziegler–Natta Catalysis. A Three-Site Model Analysis of Effects on Catalyst Active Sites
Polymer Journal, 1995Co-Authors: Mika Härkönen, Jukka V Seppälä, Hannu SalminenAbstract:A three-site model of the active sites in a MgCl_2-supported Ziegler–Natta catalyst was used to simulate the effects of various external alkoxysilane donors on the catalyst active sites. The simulations correlated well with the experimental data, and it is evident that the polypropylene produced by this catalyst system originates mainly from three types of active sites. One site is a highly isospecific enantiomorphic site producing boiling heptane insoluble polymer. Two sites are responsible for the major part of the polymers of the heptane soluble firaction: a moderately isospecific enantiomorphic site (the fluctuating site) and a symmetric Bernoullian syndiospecific site. All the alkoxysilanes tested, including monoalkoxysilanes, exhibited a stabilizing role, making fluctuating sites more isospecific. In addition, the silanes having more than one alkoxy group were capable of deactivating active Ti-sites, and the selectivity of deactivation depends mainly on the size of the hydrocarbon part.
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External silane donors in Ziegler-Natta Catalysis: a two-site model simulation of the effects of various alkoxysilane compounds
Polymer, 1995Co-Authors: Mika Härkönen, Jukka Seppälä, Riichirô Chûjô, Y. KogureAbstract:Abstract The results of experiments with a supported Ziegler-Natta catalyst were interpreted by a two-site model of the active sites, in order to enhance the understanding of multiple types of catalyst active sites and in particular the role of the external donor. The applicability of the two-site model is discussed and the stochastic parameters of the model are correlated with both the experimental and molecular modelling data. The two-site model satisfactorily explains the major part of the active sites of this highly isospecific catalyst and is a suitable tool for analysing the effects of the donors on catalyst active sites. Nevertheless, it does not completely describe the behaviour of Mg-supported Ziegler-Natta Ti catalyst. There is probably more than one kind of active site producing the part of polypropylene that is soluble in boiling heptane. This study also supports the earlier suggestion that some of the changes in the population of active sites can be explained only through the formation of new sites, and more specifically through the existence of fluctuating sites that can be stabilized by external donors. The stability of these new sites depends on both the structural and electronic properties of the external donors.
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External silane donors in Ziegler-Natta Catalysis. Computerbased molecular model calculations on alkoxysilanes
Macromolecular Chemistry and Physics, 1992Co-Authors: Mika Härkönen, Lauri Kuutti, Jukka SeppäläAbstract:The electronic factors of external alkoxysilane donors used in modern Ziegler-Natta catalyst systems were estimated by quantum mechanical MNDO (modified neglect of diatomic overlap) calculations on model compounds. The calculated electron densities of oxygen atoms in alkoxy groups were plotted against the experimental activity and the isotacticity of propene polymerization. The steric hindrance and the volume of the compounds were investigated as structural factors important to the polymerization. The results indicate that the performance of the external alkoxysilane donor is affected by both electronic and structural factors. The average electron density around the electron-donating oxygens has some influence on the deactivating power of the donor, but when the hydrocarbon part is sterically large the sterical effects apparently dominate.
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External silane donors in Ziegler‐Natta Catalysis. An approach to the optimum structure of the donor
Macromolecular Chemistry and Physics, 1991Co-Authors: Mika Härkönen, Jukka SeppäläAbstract:Our earlier work about alkoxysilanes as external donors in high-activity Ziegler-Natta catalysts is broadened with studies on the performance of some other oxygen-containing silane compounds and with some studies on the effect of the structure of the hydrocarbon part of the alkoxysilane compounds: There are new informations about the optimum structure of an external silane-type donor. The alkoxy group proved to be the best electron-donating group, whereas the other oxygen-containing electron-donating groups tested worked less effectively. In some structures the oxygen atoms were sterically too hindered for effective deactivation. Some increase in the isotacticity was nevertheless observed even with the poor performance donors, which indicates that there could be a population of active centres that are very easy to deactivate. All the para-substituted alkoxy phenyl silane donors tested gave polypropene with high isotacticity and activity. Therefore, we conclude that the nature of the substituent does not markedly affect the performance of the donor — at least as long as the substituent has not a sterically free electron-donating group.
Yukihiro Kojima - One of the best experts on this subject based on the ideXlab platform.
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Theoretical studies on the role of bridging group of CGC type ligands for the Ziegler–Natta Catalysis
Journal of Organometallic Chemistry, 2009Co-Authors: Shogo Sakai, Yukihiro KojimaAbstract:The potential energy surfaces of the initial reactions of ethylene polymerization with the Ziegler–Natta Catalysis related to the constrained geometric catalysts (CGCs) were studied by the B3LYP density functional method. Three metals (Ti, Zr, and Hf) in the Ziegler–Natta Catalysis and eight bridging groups (BH, CH2, NH, O, AlH, SiH2, PH, and S) between cyclopentadienyl (Cp) and NH ligands were treated. The reaction occurs through two steps as that of Kaminsky type: the first step produces the complex without a barrier and the second is the insertion of ethylene into the metal–carbon bond through the transition state. The complex formation energy for each metal system correlates linearly to the electronegativity of the bridging atom for each row atom of the periodic table except for those of the BH-bridging systems. The energies of the reactions for the BH-bridging systems could be explained with the through-bond model as the reactions of ansa-metallocenes and the π back-donation of BN double bond.
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The Role of Bridging Group of Cyclopentadienyl Ligands for the Ziegler–Natta Catalysis: Theoretical Study
Topics in Catalysis, 2009Co-Authors: Shogo Sakai, Yukihiro KojimaAbstract:The potential energy surfaces of the initial reactions of ethylene insertion for the Ziegler–Natta Catalysis with bridging groups of Cp ligands were studied by ab initio MO and density functional methods. Three metals (Ti, Zr, and Hf) in the Zeigler-Natta Catalysis and eight bridging groups (BH, CH_2, NH, O, AlH, SiH_2, PH, and S) were treated. For the complex formation between ethylene and metallocenes, two type structures (vertical and horizon) were found. The vertical type structures are more stable in energy than the horizon types. The formation energy of the complex between ethylene and the metallocenes by incorporation of bridging atom or group is related to the geometrical hindrance and the bond interaction as shown in the case of boron bridging system.
Arno P. M. Kentgens - One of the best experts on this subject based on the ideXlab platform.
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Structural Characterization of Electron Donors in Ziegler–Natta Catalysts
The Journal of Physical Chemistry, 2020Co-Authors: E. S. Merijn Blaakmeer, Vincenzo Busico, Giuseppe Antinucci, Andrea Correa, Arno P. M. KentgensAbstract:Ziegler–Natta Catalysis is a very important industrial process for the production of polyolefins. However, the catalysts are not well-understood at the molecular level. Yet, atomic-scale structural information is of pivotal importance for rational catalyst development. We applied a solid-state NMR/density functional theory tandem approach to gain detailed insight into the interactions between the catalysts’ support, MgCl₂, and organic electron donors. Because of the heterogeneity of the samples, large line widths are observed in the carbon spectra. Despite this, good agreement between experimental and computational values was reached, and this shows that 1,3-diether based donors coordinate at (110) surface sites, while phthalates are less selective and coordinate at both (104) and (110) surface sites.
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Structural Characterization of Electron Donors in Ziegler-Natta Catalysts.
Journal of Physical Chemistry C, 2018Co-Authors: E. S. Merijn Blaakmeer, Vincenzo Busico, Giuseppe Antinucci, Andrea Correa, Arno P. M. KentgensAbstract:Ziegler–Natta Catalysis is a very important industrial process for the production of polyolefins. However, the catalysts are not well-understood at the molecular level. Yet, atomic-scale structural information is of pivotal importance for rational catalyst development. We applied a solid-state NMR/density functional theory tandem approach to gain detailed insight into the interactions between the catalysts’ support, MgCl2, and organic electron donors. Because of the heterogeneity of the samples, large line widths are observed in the carbon spectra. Despite this, good agreement between experimental and computational values was reached, and this shows that 1,3-diether based donors coordinate at (110) surface sites, while phthalates are less selective and coordinate at both (104) and (110) surface sites.
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Quadrupolar nutation NMR to discriminate central and satellite transitions: Spectral assignments for a Ziegler-Natta catalyst
Journal of Magnetic Resonance, 2017Co-Authors: E.s. Blaakmeer, Wouter M. J. Franssen, Arno P. M. KentgensAbstract:Abstract In this contribution we used solid state 35 Cl ( I = 3/2) quadrupolar NMR to study a MgCl 2 /2,2-dimethyl-1,3-dimethoxypropane (DMDOMe) adduct that serves as a model system for Ziegler-Natta Catalysis. Employing large Radio-Frequency (RF) field strengths we observe three spectral features with strongly varying line widths. The assignment of the spectra is complicated because of the large difference in quadrupolar interactions experienced by the different sites in the system. The satellite transitions (ST) of relatively well-defined bulk Cl sites are partially excited and may overlap with the central transition (CT) resonances of more distorted surface sites. We show that nutation NMR of the ST of I = 3/2 spins yields a unique pattern that makes a clear distinction between an extensively broadened central transition and the satellite transitions of a component with a smaller quadrupolar interaction. This allows us to unambiguously unravel the spectra of the MgCl 2 adduct showing that we observe CT and ST of the bulk phase of MgCl 2 -nanoparticles with a C Q of 4.6 MHz together with the CT of surface sites displaying an average C Q of ∼ 10 MHz.
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Solid-state NMR studies of Ziegler-Natta and metallocene catalysts.
Solid State Nuclear Magnetic Resonance, 2015Co-Authors: Koen C.h. Tijssen, E.s. Blaakmeer, Arno P. M. KentgensAbstract:Abstract Ziegler–Natta catalysts are the workhorses of polyolefin production. However, although they have been used and intensively studied for half a century, there is still no comprehensive picture of their mechanistic operation. New techniques are needed to gain more insight in these catalysts. Solid-state NMR has reached a high level of sophistication over the last few decades and holds great promise for providing a deeper insight in Ziegler–Natta Catalysis. This review outlines the possibilities for solid-state NMR to characterize the different components and interactions in Ziegler–Natta and metallocene catalysts. An overview is given of some of the expected mechanisms and the resulting polymer microstructure and other characteristics. In the second part of this review we present studies that have used solid-state NMR to investigate the composition of Ziegler–Natta and metallocene catalysts or the interactions between their components.
