The formations of defective MgCl2 surfaces, and subsequent adsorption of Ti species and electron donor, as well as propylene polymerization over the Ziegler-Natta catalyst have been investigated using density functional theory (DFT) method. Twelve possible support models of regular and defective MgCl2 (110) and (100) surfaces were built. The individual adsorptions of titanium chlorides as mononuclear or dinuclear, and ethyl benzoate (EB) as electron donor, on these models were evaluated. The analysis of energies presented the cases of EB adsorption were generally more stable than titanium chlorides on both surfaces. Thus, EB as internal electron donor mainly prevented TiCl4 from coordinating on the MgCl2 surfaces where mostly non-stereospecific active sites could be formed. Exceptionally, A5 the site model with terminal Cl-vacancy on the MgCl2 support, presented stronger adsorption of TiCl4 than that of EB on (110) surface. Since the TiCl4 and ethyl benzoate (EB) would compete to adsorb on the support surface, it seems reasonable to assume that TiCl4 might predominately occupy this site, which can act as the most plausible active site for propylene polymerization. The first insertion of propylene monomer into the A5 active site model showed that it exhibited good regioselectivity but poor stereospecificity in the absence of electron donor. 相似文献
Vaporization of MgCl2 and other metal halides results in monomeric gas-phase species. Cocondensation of these species with organic diluents such as heptane yields highly activated solids which are precursors to MgCl2 supported “high-mileage” catalysts for olefin polymerization. These catalysts, prepared by treatment with TiCl4 followed by standard activation with aluminum alkyls display high activity for ethylene and propylene polymerization. MgCl2 can also be evaporated into neat TiCl4 to give a related catalyst. The concentration of MgCl2 in the diluent affects catalyst properties as does the nature of the diluent. TiCl3, 3TiCl3 · AlCl3, VCl3 and other metal halides are subject to similar activation. 相似文献
This paper is a comparative study of the performance of TiCl4 catalysts supported on recrystallized MgCl2 through different techniques for the polymerization of ethylene, propylene and ethylene-propylene copolymers. MgCl2 was dissolved in 1-hexanol and recrystallized through solvent evaporation, quick cooling and precipitation with SiCl4. The effect of the recrystallization conditions during the catalyst preparation on the chemical composition of catalysts was discussed with the help of IR spectroscopy. The variations of dealcoholation levels due to the different recrystallization techniques highly influenced the catalytic activity. The catalyst obtained through SiCl4 recrystallization was not only the most active, but it also showed the highest isotacticity indexes for propylene polymerization. 相似文献
The kinetics of propylene polymerization by superactive CH-catalyst prepared from toluene solution of MgCl2 · EH/PA/TiCl4–TEA/PES was investigated. The results are compared with CW-catalyst prepared from crystalline MgCl2/EB/PC/TEA/TiCl4–TEA/MPT (abbreviations given in the text). The former is four times more active than the latter and produces more isotactic polypropylene. The CH-catalyst has 25% of the Ti as isospecific sites as compared to 6.7% for the CW-catalysts. These sites have the same rate constant of propagation so that the higher polymerization activity of the CH-catalyst is attributable simply to a greater number of active sites. Differences in the kinetics of deactivation and of chain transfer for the two catalysts are described. 相似文献
Hydrogen effects for propylene polymerization were investigated with ultra low TiCl3 loading MgCl2-supported catalysts in which the electric states of Ti species can be almost uniform. Hydrogen did not affect the catalyst activity, while the efficiency of hydrogen as a chain transfer agent was found to depend on the Ti content of the catalyst and the stereospecificity of the polymerization sites: Hydrogen was effective for isospecific sites independent of Ti contents, but inert for aspecific sites only at the extremely low Ti content. These results were explained within the island model, where isospecific sites may be located in the islands with other Ti species in their surroundings acting, as a steric hindrance for isospecific polymerization and as hydrogen dissociation sites after deactivation. Most of the aspecific sites should be isolated only at the extremely low Ti content. These isolated sites have no other Ti species in their surroundings, i.e. no hydrogen dissociation sites, and are inert to hydrogen. 相似文献
In propylene polymerization with MgCl2‐supported Ziegler‐Natta catalysts, it is known that the reduction of TiCl4 with alkylaluminum generates Ti3+ active species, and at the same time, leads to the growth of TiClx aggregates. In this study, the aggregation states of the Ti species were controlled by altering the Ti content in a TiCl3/MgCl2 model catalyst prepared from a TiCl3 · 3C5H5N complex. It is discovered that all the Ti species become isolated mononuclear with a highly aspecific feature below 0.1 wt.‐% of the Ti content, and that the isolated aspecific Ti species are more efficiently converted into highly isospecific ones by the addition of donors than active sites in aggregated Ti species.
The MCM‐41 and SiO2 supported TiCl4 and TiCl4/MgCl2 catalysts with different molar ratios of Mg/Ti were synthesized and used for ethylene polymerization under atmospheric pressure. The nanochannels of MCM‐41 serve as nanoscale polymerization reactor and the polyethylene nanofibers were extruded during the reaction. The nanofibers were observed in SEM micrographs of resulting polyethylene. The effect of MgCl2 on catalytic activity and thermal properties of resulting polyethylene is investigated too. In the presence of MgCl2, the catalytic activity increased and more crystalline polyethylene with higher melting points were formed. However, no fibers could be observed in the polyethylene prepared by SiO2 supported catalysts. 相似文献
The article discusses recent results of kinetic analysis of propylene and ethylene polymerization reactions with several types
of Ti-based catalysts. All these catalysts, after activation with organoaluminum cocatalysts, contain from two to four types
of highly isospecific centers (which produce the bulk of the crystalline fraction of polypropylene) as well as several centers
of reduced isospecificity. The following subjects are discussed: the distribution of active centers with respect to isospecificity,
the effect of hydrogen on polymerization rates of propylene and ethylene, and similarities and differences between active
centers in propylene and ethylene polymerization reactions over the same catalysts. Ti-based catalysts contain two families
of active centers. The centers of the first family are capable of polymerizing and copolymerizing all α-olefins and ethylene.
The centers of the second family efficiently polymerize only ethylene. Differences in the kinetic effects of hydrogen and
α-olefins on polymerization reactions of ethylene and propylene can be rationalized using a single assumption that active
centers with alkyl groups containing methyl groups in the β-position with respect to the Ti atom, Ti-CH(CH3)R, are unusually unreactive in olefin insertion reactions. In the case of ethylene polymerization reactions, such an alkyl
group is the ethyl group (in the Ti-C2H5 moiety) and, in the case of propylene polymerization reactions, it is predominantly the isopropyl group in the Ti-CH(CH3)2 moiety.
Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 11, pp. 1911–1934.
The text was submitted by the authors in English. 相似文献
The surface composition of TiCl3-based Ziegler—Natta catalysts prepared by various methods was analyzed by ESCA to correlate the total amount of surface titanium with the catalyst activity in propylene polymerization. The ESCA peak ratio (Ti 2P3/2/Cl 2P) of the catalysts was measured to estimate the surface composition. The titanium index defined as the product of the (Ti/Cl peak ratio and surface area) was closely correlated with the catalyst activity in polymerization. This indicates that surface titanium concentration and surface area determine the catalyst activity. It was also found that removal of surface aluminum and chlorine at the catalyst preparation stage results in concentration of titanium at the surface and an increase in surface area. 相似文献
Data on the number of active centers (Cp) and propagation rate constants (Kp) have been obtained by means of polymerization quenching with 14CO of propylene and ethylene polymerization with supported titanium-magnesium catalysts (TMC) with different composition. In the case of propylene polymerization the Cp and Kp values have been measured separately for isospecific, aspecific and low stereospecific centers. Effects of MgCl2 support, internal and external donors are discussed on the basis of data obtained. Data on the strong effect of diffusion limitation at ethylene polymerization with number of TMC have been obtained and a set of methods have been used to exclude this effect. Data on Cp and Kp values at ethylene polymerization with low stereospecific and highly stereospecific catalysts are presented. 相似文献