Hisayuki Nakatani

Mechanistic implications of the unprecedented transformations of ethene into propene and butene over Phillips CrOx/SiO2 catalyst during induction period

Abstract In this work, an industrial calcined Phillips CrOx/SiO2 catalyst engaged in the induction period through interaction with ethene at ambient conditions was characterized by temperature-programmed desorption (TPD) equipped with a quadrupole mass spectrometer (MS). It was unprecedented to find that the first hydrocarbon species had already been formed and was observed to be propene instead of the expected butene. In fact, butene is the second hydrocarbon species formed after propene during the induction period. This evidence strongly implied that the initiation in terms of Cr–carbon bond formation on the Phillips catalyst occurs through an ethene metathesis mechanism during the induction period. Three most possible routes regarding the ethene metathesis initiation and formation of the first hydrocarbon species propene during the induction period on the Phillips catalyst were proposed.

New aspects of the induction period of ethene polymerization using Phillips CrOx/SiO2 catalyst probed by XPS, TPD and EPMA

Abstract In this work, an industrial Phillips CrO x /SiO 2 catalyst engaged in the induction period through interaction with ethene at room temperature (RT) was studied by the combination of surface analytical methods: X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron probe microanalysis (EPMA). XPS method disclosed the reduction of surface chromate species (expressed as Cr(VI)O x ,surf ) into surface-stabilized Cr 3+ and Cr 2+ species (expressed as Cr(III)O x ,surf and Cr(II)O x ,surf , respectively) by monomer during the induction period and the simultaneous formation of two kinds of surface carbon species, namely, formaldehyde and unsaturated hydrocarbons. The hydrocarbon species were supposed to be composed of not only coordinated ethene, but also adsorbed longer olefin chains. TPD measurement confirmed the formation of formaldehyde. EPMA characterization revealed that the surface Cr aggregation occurred in a low extent during the induction period resulting in a few Cr islands in sizes of several microns on catalyst surface. It was demonstrated that reduction and alkylation of Cr species as well as initiation of ethene insertion had already occurred even in the induction period. A mechanism concerning the reactions occurred during the induction period on the Phillips catalyst was proposed. The Cr(II)O x ,surf species coordinated with one formaldehyde and one ethene is postulated to be the active sites precursor for the alkylation as well as initiation of the ethene insertion. The coordinatively-adsorbed formaldehyde molecules on the catalyst are supposed to be the very reason for the hindered ethene oligomerization rather than normal polymerization, as well as for the induction of the surface Cr aggregation.

Variation in the Isospecific Active Sites of Internal Donor-Free MgCl2-Supported Ziegler Catalysts: Effect of External Electron Donors

The stopped-flow polymerization of propylene was carried out using an internal donor-free MgCl 2 -supported Ziegler catalyst in the absence or presence of external electron donors. The variation in the isospecific active sites was investigated based on the isotacticity distribution of the poly(propylene) analyzed by the TREF method. Highly isospecific active sites derived from the highest isotactic fraction (elution temperature by TREF: >112 °C) exist in the electron donor-free catalyst system. The addition of external electron donors converted parts of the aspecific into isospecific active sites, but showed no effects on the highests isospecific active sites. The external electron donor sterically affects a coordination vacancy of each aspecific titanium species and, consequently, transfers if into an isospecific active site of high, but not highest isospecificity.

Catalyst residue effects on the heterogeneous oxidation of polypropylene

The chemiluminescence (CL) curves in oxygen from polypropylene with a residual Ziegler-Natta catalyst Titanium concentration ranging from 0.2 to 4.1 ppm were measured from 100 to 150 degrees Celsius and it was found that the intensity of CL increased and the time taken to reach the maximum CL intensity decreased with [Ti]. The temperature dependence of the time to maximum CL intensity showed an apparent activation energy, Ea, of 88.1±0.6 kJ/mol that was independent of [Ti] but the pre-exponential factor, A′, decreased with increasing [Ti] according to a power law: A′=4.7×10−8[Ti]−0.3. This has been interpreted as reflecting the power law dependence of [Ti]−0.33 for the distance between residual particles of catalyst as sites for the initiation of heterogeneous oxidation. This has been tested by stochastic modelling of the spreading of oxidation for a range of volume fractions, p0, of infectious sites. This modelling allows pseudo-CL profiles to be constructed over the temperature range 100–150 degrees Celsius. The modelling has been restricted to p0 values equivalent to higher initial [Ti] but the results show that Ea is independent of [Ti] and A′ shows a power-law dependence of [Ti]−0.29 consistent with the experimental data.

Formation, deactivation and transformation of stereospecific active sites on TiCl4/dibutylphthalate/Mg(OEt)2 catalyst induced by short time reaction with Al-alkyl cocatalyst

Abstract Formation, deactivation and transformation of stereospecific active sites on TiCl 4 /dibutylphthalate (DBP)/Mg(OEt) 2 Ziegler–Natta catalyst induced by short time reaction with triethylaluminum (TEA) cocatalyst (with TEA pretreatment time from 0 to 600xa0s) were investigated by stopped-flow propene polymerization combined with temperature rising elution fractionation (TREF) and GPC methods. It was demonstrated that both formation and deactivation of active sites with broad multiplicity in isospecificity on the catalyst are slow reactions with an induction period of ca. 0.2xa0s. It was most important to find that the formation of active sites with the highest isospecificity strongly depends on the interaction between the catalyst and cocatalyst (up to 60xa0s of pretreatment) even in the presence of internal donor. This newly observed phenomenon (according to our knowledge) suggested that the transformation of monometallic active sites (aspecific or less isospecific) into bimetallic active sites (highly isospecific) through reversible complexing with TEA cocatalyst (or its reaction product diethylaluminum chloride (DEAC)) in Ziegler–Natta catalysts cannot be overlooked even in the presence of internal electron donor. The existence of –OC 2 H 5 ligand in the catalyst most probably gave birth to a new group of active titanium species. The stability of active sites increases with increasing isospecificity in the early stage of pretreatment (up to 60xa0s of pretreatment). While all the active sites became relatively stable in the later stage of pretreatment (from 60 to 600xa0s of pretreatment). The extraction of internal donor DBP by TEA from the catalyst within the pretreatment procedure is found to initiate from 60xa0s of pretreatment resulting in slight transformation of isospecific active sites into aspecific sites.

Synthesis and thermal properties of poly(styrene-co-4-methylstyrene) produced with syndiospecific metallocene catalysts

Poly(styrene-co-4-methylstyrene) with a different content of comonomer units was prepared using the typical syndiospecific homogeneous catalysts, i.e. η-C5H5TiCl3 and η-C5(CH3)5TiCl3 combined with methylaluminoxane (MAO). The η-C5(CH3)5TiCl3 catalyst gave the polymers with higher syndiotacticity. The monomer reactivity ratios indicated that the copolymers obtained with η-C5H5TiCl3 and η-C5(CH3)5TiCl3 possessed random and slightly blocky structure, respectively. With an increase in the 4-methylstyrene comonomer units, the glass transition temperature of the copolymer increased linearly and the crystallization temperature increased followed by a decrease in the melting point.

Effect of hydrogenation on dynamic mechanical relaxation: 2. Syndiotactic polystyrene

Dynamic mechanical behavior of a series of partially hydrogenated polystyrenes with various degrees of hydrogenation has been investigated. The primary relaxation process ascribed to glass transition shifts to higher temperatures and broadens with increasing degree of hydrogenation. There exist two secondary dispersions in a temperature range from 110 to 250 K. The frequency dependence of these relaxation processes and the molecular dynamics simulation suggest that the secondary relaxations are associated with the chair-chair flipping and oscillatory rotation of the cyclohexyl group.

Preparation and characterization of hydrogenated syndiotactic polystyrene

Abstract Syndiotactic polystyrenes (sPS) with different molecular weights were hydrogenated over the Ni/SiO 2 and Pd-BaSO 4 catalysts. Although the Ni catalyst yielded a completely hydrogenated sPS with a lower molecular weight, the hydrogenation of sPS with a high molecular weight was incomplete. On the other hand, the Pd catalyst was capable of hydrogenating sPS with a high molecular weight. However, the hydrogenated sPS (HsPS) was found to contained a small quantity of isolated styrene units. From a detailed analysis of the HsPS by differential scanning calorimetry (d.s.c.) and X-ray, it was revealed that this HsPS has a potential as a crystalline material with an excellent heat-resistance.

High resolution transmission electron microscope observation of α-TiCl3

Abstract High resolution transmission electron microscopy (HRTEM) was used to observe the structure of α-TiCl3 in order to investigate the nature of active sites in Ziegler–Natta catalyst. The crystalline structures of the α-TiCl3 surface were directly observed in the HRTEM image and the electron diffraction pattern. The ordered arrangement of the crystal lattices in α-TiCl3 particles was clearly revealed. The rapid deterioration of the crystalline structure of the α-TiCl3 due to the exposure in ambient conditions was also observed on the atomic scale, reflecting its ultra-high sensitivity to the impurities in the air.