Propylene polymerization using MgCl2/ethylbenzoate/TiCl4 catalyst: Determination of titanium oxidation states

The chemical interaction of the catalyst MgCl₂/ethylbenzoate/TiCl₄ with the cocatalysts triethylaluminum and trisobutylaluminum was investigated to establish a relationship between the titanium oxidation states and the catalytic activity, polymer isotacticity, and polymer molecular weight in propylene polymerizations. This interaction was studied using different Al : Ti molar ratios by measuring the changes of the titanium oxidation states at different polymerization times. Both hydrogen and alkyl aluminum caused a reduction of Ti⁴⁺ species to lower oxidation states species Ti³⁺ and Ti²⁺. However, the Ti⁴⁺ species reduction appeared to be incomplete. It was found that the Ti⁴⁺ species undergoes a severe reduction as the Al : Ti molar ratio increases from 50 to 230 as overreduction takes place. This change of the Ti³⁺ species percentage with time was found to correlate with the rate–time profiles of propylene polymerization. From this observation, it would be fair to conclude that the trivalent titanium species is more likely to be the active titanium species for propylene polymerization than the aforementioned catalyst system. On the other hand, hydrogen addition was found to cause an increase in Ti³⁺ species. The increases in both hydrogen amount and/or Al : Ti molar ratio were found to cause a decrease in both molecular weight and polypropylene isotactic index. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 56–62, 2004     

The Role of CaO in the Ziegler–Natta Catalyst for Propylene Polymerization

A certain amount of CaO enhanced the activity of Ziegler–Natta catalyst during propylene polymerization, but decreased isotacticity. The results indicated that CaO accelerated the initial polymerization rates and improved catalyst against deactivation without effects on melting point, crystallinity and morphology of polypropylene. The electron spin resonance (ESR) results revealed that CaO facilitated the reduction of Ti⁴⁺ species.     

Observation of bimodal polyethylene derived from TiO2-supported zirconocene/MAO catalyst during polymerization of ethylene and ethylene/1-hexene

The bimodal polyethylene obtained from TiO₂-supported zirconocene/MAO catalyst was observed during polymerization of ethylene and ethylene/1-hexene. By means of XPS, it revealed that TiO₂ consisted of Ti³⁺ (BE = 462.6 eV) and Ti⁴⁺ (BE = 464.9 eV). The dual catalytic sites were attributed to the presence of Ti³⁺ and Ti⁴⁺ in TiO₂.     

Conductive PProDOT-Me2–capped Li4Ti5O12 microspheres with an optimized Ti3+/Ti4+ ratio for enhanced and rapid lithium-ion storage

Li₄Ti₅O₁₂ (LTO) has attracted much attention for its use as an anode material within lithium-ion batteries (LIBs), owing to its excellent safety and outstanding cyclability. Unfortunately, the poor electronic conductivity of LTO greatly limits the rate capability of the LIBs and, therefore, its practical applications. The conductivity of LTO can be enhanced significantly through thermal treatment under hypoxic conditions to form oxygen vacancies. The defective LTO with oxygen vacancies has a much higher electric conductivity, owing to partial reduction of the Ti ions (from Ti⁴⁺ to Ti³⁺). Too many oxygen vacancies generated in the LTO will, however, also cause severe lattice distortion, inhibiting ionic transfer. In this study, we optimized the Ti³⁺/Ti⁴⁺ ratio of LTO by annealing it under various conditions. We also modified the surface of LTO with a thin layer of poly (dimethyl 3,4-propylenedioxythiophene) (PProDOT-Me₂) through in situ chemical polymerization. Studies of the surface morphology and chemical composition confirmed that PProDOT-Me₂ had been successfully capped on the LTO surface. Combining the effects of the optimal Ti³⁺/Ti⁴⁺ ratio and the surface modification with PProDOT-Me₂ resulted in the charge and ionic transport properties of LTO both improving significantly. Accordingly, the nanocomposites displayed greatly enhanced rate capabilities, relative to those of the unmodified material.     

Propylene oxide polymerization using the diphenylzinc-acetone system in benzene at 60°C

Summary The diphenylzinc-acetone system was used as catalyst for propylene oxide polymerization in benzene solution at 60°C. This system as well as the diphenylzinc-water system is greatly influenced by the molar ratio of acetone to diphenylzinc and the maximum catalyst activity was found for a ratio of unity. GPC results strongly suggest the presence of more than one active species for the system.¹³CNMR analysis indicates that the resulting poly(propylene oxide) has a head-to-tail arrangement. This system was not an effective catalyst for the styrene oxide polymerization.     

On nonaqueous electrochemical behavior of titanium and Ti compounds

In this study, we examined the electrochemical behavior of titanium and Ti⁴⁺ compounds in THF solutions. Tetra butyl ammonium chloride (TBACl) was used as supporting electrolyte in order to increase the ionic conductivity of the solutions. Electrodeposition of pure titanium could not be obtained. A variety of analytical techniques have been used in conjunction with electrochemical methods in order to analyze the reduction process of Ti⁴⁺. These included Raman and UV-vis spectroscopy, ICP and CHNS elemental analyses. Ti⁴⁺ is being reduced to Ti³⁺ in TiCl₄/THF/TBACl solutions.In addition we show that metallic titanium can be electrochemically dissolved from an organo-metallic electrolyte solution comprising EtAlCl₂ and LiCl in THF. The product is Ti⁴⁺. While LiCl is insoluble in THF it reacts with EtAlCl₂ to form ionic species. Hence, these solutions possessed reasonable ionic conductivity.We could not obtain electroactive Ti⁴⁺ with TiBr₄ or TiI₄ as starting materials, in similar solutions.     

Uncalcined TS-2 immobilized Au nanoparticles as a bifunctional catalyst to boost direct propylene epoxidation with H2 and O2

Developing stable yet efficient Au–Ti bifunctional catalysts is important but challenging for direct propylene epoxidation with H₂ and O₂. This work describes a novel strategy of employing uncalcined titanium silicalite-2 (TS-2-B) to immobilize Au nanoparticles as a bifunctional catalyst for the reaction. Under no promoter effects, the Au/TS-2-B catalyst compared to the referenced Au/TS-1-B catalyst delivers outstanding catalytic performance, that is, exceptionally high stability over 100 hr, propylene oxide (PO) formation rate of 118 g_PO·hr⁻¹·kg_cat⁻¹, PO selectivity of 90% and hydrogen efficiency of 35%. The plausible relationship of catalyst structure and performance is established by using multiple techniques, such as UV–vis, high-angle annular dark-field scanning transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. A unique synergy of Au–Ti⁴⁺–Ti³⁺ triple sites is proposed for our developed Au/TS-2-B catalyst with the higher stable PO formation rate and hydrogen efficiency. The insights reported here could shed new light on the rational design of highly stable and efficient Au–Ti bifunctional catalysts for the reaction.     

Polymerization of some oxiranes using the diphenylzinc-butanone system in benzene at 60°C

The diphenylzinc-butanone system was used as polymerization catalyst for some oxiranes in benzene solution at 60°C. This system is greatly influenced by the molar ratio of butanone to diphenylzinc, and the maximum catalytic activity for propylene oxide and ethylene oxide was found for a ratio of unity. GPC results strongly suggest the presence of more than one active species for the system. ¹³C NMR analysis indicates that the resulting poly(propylene oxide) has a head-to-tail arrangement. For the polymerization of propylene oxide with butanone/diphenylzinc = 1, after an initial induction period, the reaction was first-order with respect to monomer with k = 2·51 × 10^?5 s^?1. Ethylene oxide polymerizations using butanone/diphenylzinc = 1 and 5 were also first-order with respect to monomer after an initial induction period with k = 7·80 × 10^?6 s^?1 and k = 5·71 × 10^?6 s^?1, respectively. The diphenylzinc-butanone system was not an effective catalyst for styrene oxide polymerization.     

Microstructure,optical and magnetic properties of TiO2/CuO nanocomposites synthesized by a two-step method

TiO₂/CuO composites in different ratios were prepared via a two-step method. X-ray diffraction and transmission electron microscopy results indicated that part of Cu²⁺ substituted Ti⁴⁺ in the TiO₂ lattice in the composite, leading to Cu²⁺-substituted sites in the TiO₂ lattice as well as Cu²⁺ species located in the CuO lattice. Scanning electron microscopy revealed a morphology change in the sample from a three-dimensional structure to a two-dimensional structure while forming an interface between TiO₂ and CuO. X-ray photoelectron spectroscopy and Raman spectra showed that there are oxygen vacancies (V_O) and Ti³⁺ in the lattice. UV–vis absorption spectra exhibited a widening of the absorption range and a decrease in the bandgap with increasing amount of CuO in the TiO₂/CuO composites. Additionally, the composites exhibited room-temperature ferromagnetism (RTFM), as can be explained by the indirect double-exchange model, which is related to V_O and the exchange interaction between the 3d orbitals of Ti³⁺ and Ti⁴⁺ at the interface.     

Electrochemical studies of titanium in fluoride-chloride molten salts

The interaction between titanium and Ti⁴⁺ ions (K₂TiF₆), the electroreduction reaction of Ti⁴⁺ ions and the anodic reaction of Ti in KCl–NaCl–KF melts with K₂TiF₆ at 973 K were studied by means of electrochemical and physical measurements. It was found that the fluoride ions played a very important role in these reactionsIn KCl–NaCl-3 wt % K₂TiF₆ molten salts with less than 3 wt % KF, the interaction reaction was considered to proceed as Ti⁴⁺+Ti=2Ti²⁺. If the bath contained more than 10 wt% KF, the reaction 3Ti⁴⁺+Ti=4Ti³⁺ occurred.The electrochemical reduction of Ti⁴⁺ (K₂TiF₆) ions in the molten salts with less fluoride ions was observed to proceed according to three reaction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+e=Ti²⁺, Ti²⁺+2e=Ti. In the case of the fluoride ion concentration being higher, two reduction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+3e=Ti were suggested.     

Epoxide polymerization

Summary Poly(propylene oxide), PPO, obtained using diphenylzinc, Ph₂Zn, and using the diphenylzinc-water system at various molar ratios H₂O/Ph₂Zn, were studied through ¹H NMR and IR spectroscopy establishing that the polymerization initiates by coordition of propylene oxide (PO) to a terminal zinc atom of the active species followed by consecutive insertions of PO units at this zinc atom. The PPO obtained showed Ph-and HO-, HO-and HO-end-groups depending on the molar ratio H₂O/Ph²Zn used, and also HO-and PhO-end-groups when oxygen was present in the polymerization system.     

Oxidative Dehydrogenation of Propane with CO2 Over Cr/H[B]MFI Catalysts

Abstract  

Cr/silicalite-1 and Cr/H[B]MFI catalysts were prepared by the impregnation method, and Cr/H[B]MFI were further treated by steaming. The catalysts were employed for the oxidative dehydrogenation of propane to propylene with CO₂ as the oxidant. Cr/H[B]MFI showed significantly higher catalytic activity than Cr/silicalite-1, and steamed Cr/H[B]MFI was superior in the reaction stability to Cr/H[B]MFI. The nature of the supported chromium species have been characterized by a number of physicochemical techniques, such as Raman, UV–vis and NMR. It is concluded that the steaming led to the auto-reduction of some Cr⁶⁺ to Cr³⁺, and resultant Cr³⁺ species might be located near the boron center in the borosilicate framework to counterbalance the negative charge of the framework. The transformation of Cr⁶⁺ species to Cr³⁺ species, facilitated by the steaming process and the presence of boron in the catalyst, is responsible for the enhanced stability of oxidative dehydrogenation of propane to propylene with carbon dioxide as the oxidant.     

Transition metal-silicate analogs of zeolites

The synthesis and characterization of three transition metal (Fe³⁺, Ti⁴⁺ and V⁴⁺)-silicate molecular sieves are discussed. The key factors for a successful incorporation of these metal ions in the growing silicate network during gel preparation/hydrothermal synthesis (e.g. avoidance of insoluble/sparingly soluble metal hydroxides/oxyhydroxides (Fe³⁺ and Ti⁴⁺) and alkali metal ions (Ti⁴⁺and V^4+/5+)) as well as the effects of post synthesis drying, calcination etc. on the stability of these metal ions in the framework are evaluated. Various spectroscopic and other techniques which are required to provide general as well as specific structural and textural information about the fate of these transition metal ions are discussed.     

Electrochemical studies of titanium ions (Ti4+) in equimolar KClNaCl molten salts with 1 wt% K2TiF6

The electrochemical reduction of Ti⁴⁺ ions in equimolar KClNaCl melts containing less K₂TiF₆ (1wt%) on a Pt electrode, was studied at 973 K by means of linear sweep voltammetry, stationary current density—potential curve and ac impedance methods. It was found that the reduction of Ti⁴⁺ ions in the molten salts proceeded in the following four consequent steps: Ti⁴⁺ + e = Ti³⁺; Ti³⁺ + e = Ti²⁺; Ti²⁺ + 2e = Ti(Pt alloy); Ti²⁺ + 2e = Ti(pure). And according to ac impedance measurements, the electroreduction reactions were reversible or quasireversible.     

Control of molecular weight distribution for polypropylene obtained by commercial Ziegler–Natta catalyst: effect of temperature

Polymerization of propylene was carried out by using MgCl₂-supported TiCl₄ catalyst in conjunction with triethylaluminium (TEA) as cocatalyst. The effect of polymerization temperature on polymerization of propylene was investigated. The catalyst activity was influenced by the polymerization temperature significantly and the maximum activity of the catalyst was obtained at 40 °C. With increasing the polymerization temperature, the molecular weight of polypropylene (PP) drastically decreased, while the polydispersity index (PDI) increased. The effect of the two-stepwise polymerization procedure on the molecular weight and molecular weight distribution of PP was studied and the broad PDI of PP was obtained. It was also found that the PDI of PP could be controlled for propylene polymerization through regulation of polymerization temperature. Among the whole experimental cases, the M _w of PP was controlled from 14.5 × 10⁴ to 75.2 × 10⁴ g/mol and the PDI could be controlled from 4.7 to 10.2.     

Effect of surface sites of TiO2 support on the formation of cobalt-support compound in Co/TiO2 catalysts

This study has addressed the effect of surface sites (as Ti⁴⁺ and Ti³⁺) of TiO₂ support on the formation of the cobalt-support compound on Co/TiO₂ catalyst. A 20% cobalt was prepared on each TiO₂ support having the different Ti⁴⁺/Ti³⁺ ratios covering on the surface (1.2, 1.3, 1.4, and 1.6). It can be concluded that the non-reducible compound as a Co-SCF preferred to form on the Co/TiO₂ catalyst when the most proportional site of surface TiO₂ became Ti⁴⁺. This is because the migration of cobalt cluster, which plays the significant role for creating Co-SCF, can be promoted by the increasing of Ti⁴⁺ sites resulting in the decrease of cobalt dispersion. The hyperfine patterns of ESR spectra demonstrated that the Co⁰(H_xTiO_y) was the simple chemical form of Co-SCF formed after standard reduction. Moreover, the structure of Co-SCF changed to be a higher non-uniform structure when the number of Ti⁴⁺ site on TiO₂ surface increased. The possible mechanism to form this compound has also been discussed.     

Kinetics of ring opening of propylene sulphide: 1. Alkali metal and cryptated alkali metal carbazyl salts

The reactivities of free ions and ion pairs have been determined for the ring opening of propylene sulphide with carbazyl salts, in THF at ?30°C. A large increase of the ion pair reactivity is observed on increasing the size of the counterion (Na⁺ < Cs⁺ < Na⁺ + |222|). Moreover, cryptated carbazyl ion pairs are three times more reactive than free ions in the same manner as thiolate species for the propagation of propylene sulphide in similar conditions.     

Zr4+/Ti4+ Codiffusion Characteristics in Lithium Niobate

Zr⁴⁺/Ti⁴⁺‐codoped LiNbO₃ plates were prepared by local codiffusion of stacked ZrO₂ and Ti metal films coated onto Z‐cut congruent LiNbO₃ substrates in wet O₂ at 1060°C. The metal and oxide films have different thicknesses and coating sequences. After diffusion, the Zr⁴⁺ doping effect on the refractive index of LiNbO₃ and the Li₂O out‐diffusion issue were studied by the prism coupling technique. The codiffusion characteristics of Zr⁴⁺ and Ti⁴⁺ were studied by secondary ion mass spectrometry. The results show that the Zr⁴⁺ doping has little contribution to the refractive index of the crystal. Li₂O out‐diffusion is not measurable. In the Zr⁴⁺‐only diffusion case, the diffusivity of Zr⁴⁺ is four times smaller than that of Ti⁴⁺. In the Zr⁴⁺/Ti⁴⁺ codiffusion case, the Ti⁴⁺ codiffusion assists the Zr⁴⁺ diffusion. The Zr⁴⁺ diffusivity increases linearly by two more times with the increase in initial Ti film thickness from 0 to 200 nm. On the other hand, the Zr⁴⁺ affects the Ti⁴⁺ diffusion little. Neither the ZrO₂ film thickness nor the coating sequence of Ti metal and ZrO₂ oxide films influences the diffusivity of the two ions. All the codiffusion characteristics are explained. A Zr⁴⁺/Ti⁴⁺ codiffusion model is suggested that consists of two independent diffusion equations with a Zr⁴⁺ diffusivity dependent of Ti⁴⁺ concentration and a constant Ti⁴⁺ diffusivity. In addition, the existence of a waveguide in the Zr⁴⁺/Ti⁴⁺‐codoped layer is verified experimentally, and the optical‐damage‐resistant feature of the waveguide is verified by two‐beam hologram recording experimental results.     

Homo- and copolymerization of ethylene and propylene with a soluble chromium catalyst

Polymerization of ethylene and propylene was carried out with a soluble $\text{Cr(C}{}_{17}\text{H}{}_{35}\text{COO}\text{)}{}_3\text{AlEt}{}_2\text{Cl}$ catalytic system in toluene. The system was found to be active only for ehtylene polymerization. From e.s.r. measurements of the catalytic system combined with some qualitative experiments, the active species was determined to be Cr²⁺. The reason why it should be incapable of polymerization of propylene was also discussed. The catalytic system showed some activity for ethylene-propylene copolymerization to give a random copolymer with a narrow molecular weight distribution.     

Studies on VOCl3/MgCl2/NaY/Al2Et3Cl3 complex support catalysts for the copolymerization of ethylene and propylene

Using VO²⁺ as a spin probe, a new method to obtain microenvironmental information on supports was developed which can be used in the choice of supports for coordination catalysts. Utilizing the above method, NaY was chosen as second support component. A complex support catalyst VOCl₃/MgCl₂/NaY/Al₂Et₃Cl₃ was prepared and used in ethylene–propylene copolymerization. Higher polymerization activity was obtained with this catalytic system. Alternating the ratio of two kinds of supports, the composition and sequence structure of copolymers could be controlled, which showed that NaY participated in the active species, affected the insertion of monomer, and changed the composition and sequence structure of copolymers. © 1994 John Wiley & Sons, Inc.