Control of molecular weight distribution for polypropylene obtained by a commercial Ziegler–Natta catalyst: Effect of a cocatalyst and hydrogen

The polymerization of propylene was carried out with an MgCl₂‐supported TiCl₄ catalyst (with diisobutyl phthalate as an internal donor) in the absence and presence of hydrogen (H₂) as a chain‐transfer agent. Different structures of alkylaluminum were used as cocatalysts. The effects of the alkyl group size of the cocatalyst, H₂ feed, and feed time on the propylene polymerization behaviors were investigated. The catalyst activity significantly decreased with increasing alkyl group size in the cocatalyst. The molecular weight and polydispersity index (PDI) increased with increasing alkyl group size. With the introduction of H₂, the catalyst activity increased significantly, whereas the molecular weight and PDI of polypropylene (PP) decreased. Additionally, the effect of the polymerization time in the presence of H₂ on the propylene polymerization was studied. The molecular weight distribution curve was bimodal at short polymerization times in the presence of H₂, and we could control the molecular weight distribution of PP by changing the polymerization time in the presence of H₂. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Significant variation of molecular weight distribution (MWD) of polyethylene induced by different alkyl-Al co-catalysts using a novel surface functionalized SiO2-supported Ziegler-Natta catalyst

In this work, a novel surface functionalized SiO₂-supported Ziegler-Natta catalyst was developed for ethylene polymerization. A state-of-the-art control of some unique characteristic catalytic properties on the surface functionalized SiO₂-supported catalyst analogized from the previously reported poly[propylene-co-(7-methyl-1,6-octadiene)]-supported Ziegler-Natta catalyst had been successfully realized through a precise molecular level catalyst design on the silica gel surface functionalized with linear long alkene chains through the reaction between unsaturated alcohol 9-decene-1-ol and hydroxyl groups on the silica surface. The active sites of this SiO₂-supported catalyst prepared in this study showed very stable activity without obvious deactivation for more than 50 h and the molecular weight distribution of polyethylenes varied significantly from narrow and unimodal to broad and multimodal by solely changing the type of Al-alkyl cocatalyst from diethylaluminumchloride to triethylaluminum.     

Partial oxidation of methane to CO and H2 over nickel and/or cobalt containing ZrO2, ThO2, UO2, TiO2 and SiO2 catalysts

The influence of different metal oxide supports (i.e. ZrO₂, ThO₂, UO₂, TiO₂ and SiO₂) on the performance of Ni- and/or Co-containing catalysts [Ni and/or Co/MO₂ mole ratio (where M=Zr, Th, U, Ti or Si)=1.0] in the oxidative methane-to-syngas conversion at very low contact time (GHSV=5.2×10⁵ cm³ g⁻¹ h⁻¹ at STP) was investigated. The nickel-containing ZrO₂, ThO₂ and UO₂ catalysts (with or without pre-reduction by hydrogen at 500°C) showed good performance in the process; the order of their performance is NiO–ThO₂>NiO–UO₂>NiO–ZrO₂. The NiO–TiO₂ showed appreciable catalytic activity only after its reduction at 800°C. However, this catalyst and the NiO–SiO₂ catalyst showed poor performance in the process. These two catalysts are also deactivated very fast, mostly because of sintering of Ni and/or formation of catalytically inactive binary metal oxide phases by solid–solid reaction at the high catalyst calcination and/or catalytic reaction temperature. Although the Ni-containing ThO₂, UO₂ and ZrO₂ catalysts showed good performance, carbon deposition on them during the process is fast. However, because of the addition of cobalt to these catalysts (with Co/Ni=1.0), the rate of carbon deposition on them in the process is drastically reduced. This Co addition however resulted in a significant decrease in both the conversion and selectivity; the decrease in the selectivity was small.     

用负载型钛系催化剂制备的乙丙共聚物链结构的影响因素

以负载型钛系催化剂为主催化剂,采用常压淤浆聚合工艺进行乙烯与丙烯共聚合,考察聚合条件对共聚合行为和产物结构的影响。以三乙基铝/三异丁基铝(TiBA)混合物为助催化剂时,随着TiBA用量增加,共聚物中聚丙烯(PP)链段越来越长,且长PP链段数量越来越多,而长聚乙烯(PE)链段数量逐渐减少;空间位阻大的外给电子体虽然可以提高PP的规整度,但不利于形成长PP链段;适当降低聚合温度更有利于形成长PP链段;产物中嵌段共聚物的含量随TiBA用量的增加而提高。所以,改变助催化剂的组成、使用不同的外给电子体或调节聚合温度都可以调控乙烯-丙烯共聚物的链结构及其分布。     

Grafting of methyl methylacrylate onto isotactic polypropylene film using supercritical CO2 as a swelling agent

Grafting of methyl methacrylate (MMA) onto isotactic polypropylene (iPP) film was carried out by free radical polymerization using supercritical (SC) carbon dioxide (SC CO₂) as a solvent and swelling agent. The iPP film was first impregnated with the monomer MMA and initiator benzoyl peroxide (BPO) using SC CO₂ at 308.15 K. After releasing CO₂, the MMA molecules in the film were grafted onto the iPP at a higher reaction temperature. Using this method, the grafting level and the morphology can be controlled by the soaking time, pressure and concentrations of MMA and BPO in the fluid phase. The products were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide angle X-ray diffraction (WAXD).     

Effect of monomer feed and production rate on the control of molecular weight distribution of polyethylene in gas phase reactors

The control of the polymer molecular weight distribution via altering the monomer to hydrogen molar ratio in fluidized bed reactors is examined. The molar ratio is altered by manipulating the monomer and hydrogen feed rates using nonlinear model predictive controller. The simulation revealed promising results however a trade-off between utilizing both the monomer and hydrogen flows simultaneously and the hydrogen flow exclusively exists. Utilization of the monomer and hydrogen flows together favors the rapid transition to the required MWD but at the expense of higher purge and inconsistent production rate. Exclusive use of the hydrogen intake leads to steady production rate and less consumption of the purge. However, longer time is needed to achieve the desired MWD. The same phenomena are observed when discrete mechanism for product withdrawal is implemented.     

Molecular weight distribution broadening of polypropylene by periodic switching of hydrogen and catalyst additions

This study presents a feasibility study of the broadening of the polypropylene molecular weight distribution produced using a multisite Ziegler‐Natta catalyst in a continuous liquid‐pool polymerization reactor. The broadening is achieved by operating the reactor under periodic forcing of both hydrogen and catalyst feed flows. Model‐based dynamic optimization is used to determine the cycle period and peak width for these inputs. Through simulation it is shown that limited widening (～ 30%) of the molecular weight distribution can be achieved in case of a limited removal of hydrogen from the reactor. The results also show that the forced removal of hydrogen from the reactor could potentially double the polydispersity index of the produced polymer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Investigation on the polymer particle growth in ethylene polymerization with PS beads supported rac-Ph2Si(Ind)2ZrCl2 catalyst

The mechanism of polyethylene particle growth was investigated using poly(styrene-co-divinylbenzene) (PS beads) supported rac-Ph₂Si(Ind)₂ZrCl₂ catalyst. From the analysis of the resulting polyethylene particles by SEM (scanning electron microscopy) and EPMA (electron probe microanalysis), it was found that the active species are located on the surface layer of catalyst particles and that the catalytic species are uniformly distributed throughout the polymer particles, whereas the cores of PS beads, which lack a potential active species, were not disintegrated during polymerization. These results suggest that the PS beads supported catalyst also follows the fragmentation and replication process as frequently observed with the MgCl₂ supported Ziegler–Natta catalysts.     

Enhanced photocatalytic decomposition of 4-nonylphenol by surface-organografted TiO2: a combination of molecular selective adsorption and photocatalysis

n-Octyl-grafted TiO₂ (C8-TiO₂) was prepared as a model of a photocatalyst with high molecular adsorption selectivity, and its photocatalytic activity for the decomposition of dilute 4-nonylphenol (an endocrine disrupter, ca. 2 ppm) in water was investigated. The catalyst was highly active in the presence of concentrated phenol (1000 ppm), and the 4-nonylphenol was decomposed in 180 min, while pristine TiO₂ (P-25) under the same conditions showed much lower activity. The high C8-TiO₂ activity was ascribed to the molecular selective adsorption of the organic molecules on the alkyl-grafted hydrophobic surface. Infrared spectra showed that the grafted alkyl groups were gradually decomposed under photoirradiation. The model catalyst demonstrated that molecular selective adsorption is important for removal of low-concentration contaminants in the presence of other, more concentrated compounds.     

Control of molecular weight distribution in propylene polymerization with Ziegler–Natta/metallocene catalyst mixtures

Propylene polymerization was investigated with a sequential addition of Ziegler–Natta and metallocene catalysts. From the fact that the molecular weights of polypropylene (PP) produced with Ziegler–Natta and with metallocene catalysts differ, it was possible to control the molecular weight distribution (MWD) of PP with a sequential addition of methylaluminoxane and rac-ethylenebis(indenyl)zirconium dichloride followed by triethylaluminum and magnesium dichloride-supported titanium tetrachloride catalyst. The obtained PP exhibited a wide MWD curve with shoulder peak. The position and height of each peak was controlled with the variation of polymerization time for each catalyst as well as the amount of each catalyst. The MWD of PP prepared with sequential addition of catalysts was much wider than that of PP obtained from each catalyst. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2213–2222, 1998     

Control of molecular weight distribution and tensile strength in a free radical styrene polymerization process

A new method is presented for modeling and controlling polymer molecular weight distribution (MWD) and tensile strength in a batch suspension polymerization of styrene. The molecular weight distribution is modeled by computing the weight fraction of the polymer in different chain length intervals. Tensile strength is then related to the modeled molecular weight distribution using a correlation available in the literature and based on the concept of a threshold molecular weight. This method enables the design of operating conditions for a batch suspension polymerization reactor, which will theoretically yield amorphous polystyrene with a desired tensile strength. Two numerical examples are presented to illustrate the feasibility of the proposed method. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1017–1026, 1998     

Epoxidation of propylene by using [π-C5H5NC16H33]3[PW4O16] as catalyst and with hydrogen peroxide generated by 2-ethylanthrahydroquinone and molecular oxygen

The epoxidation of propylene catalyzed by a reaction-controlled phase transfer catalyst [π-C₅H₅NC₁₆H₃₃]₃[PW₄O₁₆] is investigated. The H₂O₂ is generated by the oxidation of 2-ethylanthrahydroquinone (EAHQ) with molecular oxygen in the organic solvent. Under mild conditions, the selectivity for propylene oxide, based on propylene, is 95%, and the yield, based on 2-ethylanthrahydroquinone, is 85%. During the epoxidation, the catalytic system is homogeneous. However, after the H₂O₂ is used up, the catalyst can be recovered as a precipitate and can be reused. After the epoxidation reaction, 2-ethylanthraquinone can be regenerated to 2-ethylanthrahydroquinone by catalytic hydrogenation, and no coproduct is produced.     

Enhancement of Pd-Pt/Al2O3 catalyst performance in naphthalene hydrogenation by mixing different molecular sieves in the support

The hydrogenation of naphthalene in hexane was carried out over the catalyst Pd-Pt supported on alumina mixed with different molecular sieves including ZSM-5, Beta, USY and SAPO-11. The catalyst was characterized by N₂ sorption, NH₃-TPD, TPR, and H₂-O₂ titration. The results indicated that the catalyst performance for naphthalene hydrogenation is related to its acid property, pore structure, Pd-Pt dispersion, and metal-support interaction, which may be well regulated by mixing proper molecular sieves in the alumina support. The catalyst Pd-Pt supported on a two-component support of Al₂O₃-SAPO-11 is provided with proper acidity, large pores and moderate metal dispersion; it exhibits excellent performance in the saturated hydrogenation of naphthalene to decalin (over it a complete conversion of naphthalene is observed and the selectivity to decalin exceeds 90% at 260-300 °C).     

A study of carbon nanotube formation by C2H2 decomposition on an iron based catalyst using a pulsed method

The synthesis of multiwalled carbon nanotubes has been studied using a pulsed catalytic method. Acetylene was used as carbon generation reactant and silica or alumina supported iron as catalysts. Different metal loadings and various reaction temperatures were evaluated. The carbon growth over the catalysts has been followed gravimetrically ‘in situ’. The reaction was stopped after different pulse numbers in an attempt to control both the diameters and the lengths of the carbon nanotubes, which were characterised by transmission electron microscopy (TEM). A consecutive step growing mechanism is used to describe our experiments.     

Determination of pore size distribution and adsorption of methane and CCl4 on activated carbon by molecular simulation

A combined method of grand canonical Monte Carlo (GCMC) simulation and statistics integral equation (SIE) for the determination of pore size distribution (PSD) is developed based on the experimental adsorption data of methane on activated carbon at ambient temperature, T=299 K. In the GCMC simulation, methane is modeled as a Lennord-Jones spherical molecule, and the activated carbon pore is described as slit-shaped with the PSD. The well-known Steele’s 10-4-3 potential is used to represent the interaction between the fluid molecule and the solid wall. Covering the range of pore sizes of the activated carbon, a series of adsorption isotherms of methane in several uniform pores were obtained from GCMC. In order to improve the agreement between the experimental data and simulation results, the PSD is calculated by means of an adaptable procedure of deconvolution of the SIE method. Based on the simulated results, we use the activated carbon with the PSD as the prototype of adsorbent to investigate adsorption. The adsorption isotherms of methane and CCl₄ at 299 K in the activated carbon with the PSD are obtained. The adsorption amount of CCl₄ reaches 20 mmol/g at ambient temperature and pressure. The results indicate that the combined method of GCMC and SIE proposed here is a powerful technique for calculating the PSD of activated carbons and predicting adsorption on activated carbons.     

Homo- and copolymerization of 5-ethylidene-2-norbornene with ethylene by [2-C5Me4-4,6-Bu2C6H2O]TiCl2/AlBu3/Ph3CB(C6F5)4 catalyst system and epoxidation of the resulting copolymer

5-Ethylidene-2-norbornene (ENB) polymerization and copolymerization with ethylene (E) catalyzed by constrained geometry catalyst 2-tetramethylcyclopentadienyl-4,6-di-tert-butylphenoxytitanium dichloride (1) were studied in the presence of AlⁱBu₃ and Ph₃CB(C₆F₅)₄ (TIBA/B). The catalyst system shows moderate activity on the homopolymerization of ENB, and higher activity on the copolymerization of E/ENB under atmospheric pressure of ethylene. ¹H NMR analysis indicates that ENB was regioselectively incorporated into the copolymer backbone through the endocyclic double bond, leaving the ethylidene double bond unreacted. The ethylidene group contained in the poly(E-co-ENB) was quantitatively converted to the epoxy group with m-chloroperbenzoic acid, producing functionalized poly(E-co-ENB)s. The parent and functionalized copolymers have been characterized using IR, NMR, DSC, and GPC techniques.     

Ionic conduction and dielectric properties of yttrium doped LiZr2(PO4)3 obtained by a Pechini-type polymerizable complex route

We report on the ion transport properties of Li_1+xZr_2-xY_x(PO₄)₃ (0.05?≤ x?≤?0.2) NASICON type nanocrystalline compounds prepared through a Pechini-type polymerizable complex method. Structural properties were characterized by means of powder X-ray diffraction, Raman spectroscopy and electron microscopy with selected area electron diffraction. Impedance spectroscopy was utilised to investigate the lithium ion transport properties. Y³⁺ doped LiZr₂(PO₄)₃ compounds showed stabilized rhombohedral structure with enhanced total ionic conductivity at 30?°C from 2.87?×?10^?7 S?cm^?1 to 0.65?×?10^?5 S?cm^?1 for x=0.05 to 0.20 respectively. The activation energies of Li_1+xZr_2-xY_x(PO₄)₃ show a decreasing trend from 0.45?eV to 0.35?eV with increasing x from 0.05 to 0.20. The total conductivity of these compounds is thermally activated, with activation energies and pre-exponential factors following the Meyer-Neldel rule. The tanδ peak position shifts to the high-frequency side with increasing yttrium content. Scaling in AC conductivity spectra shows that the electrical relaxation mechanisms are independent of temperature.     

Enhancement of photocurrents due to the oxidation of water and organic compounds at BiZn2VO6 particulate thin film electrodes by treatment with a TiCl4 solution

Photocurrents due to water oxidation at BiZn₂VO₆ (E_g 2.4 eV) particulate thin film electrodes were largely enhanced by pre-treatment with an aqueous TiCl₄ solution. Photocurrents for BiZn₂VO₆ electrodes with no TiCl₄ treatment were also enhanced by the addition of organic compounds such as methanol and trimethyl amine to the aqueous electrolyte. Interestingly, such enhanced photocurrents by organic compounds were further enhanced by the TiCl₄ pre-treatment. EDAX and SEM investigations showed the formation of a flock-like TiO₂ overlayer on BiZn₂VO₆ particles after the TiCl₄ treatment. The photocurrent enhancement by the TiCl₄ pre-treatment is thus mainly attributed to the necking effect of the flock-like TiO₂ overlayer, which facilitates the transport of photogenerated electrons within the BiZn₂VO₆ particulate thin film electrode.     

Uniform dispersion of Ni nano particles in a carbon based catalyst for increasing catalytic activity for CH4 and H2 production by hydrothermal gasification

A carbon based nickel (Ni) catalyst was prepared by ion exchange method in which a large amount of Ni is dispersed as almost uniform nano particles. The method ion exchanges the ion exchangeable sites in an ion exchange resin with Ni followed by carbonization at 500 °C. Two different catalysts were prepared by ion exchanging Ni from an aqueous solution containing same amount of Ni ion concentration but at different pH of the solution. The amount of Ni ion exchanged in both cases was about 15%. The metal load after carbonization was about 47% and 46% in the two catalysts, respectively. The XRD pattern and TEM images of the catalysts showed that Ni particles in NiWB500 (pH = 8.8) were bigger in size in comparison to the Ni particles in NiLG500 (pH = 9.4). The BET surface area was 178 and 183 m²/g, respectively. The catalytic hydrothermal gasification (CHTG) experiments at 350 °C, 20 MPa, and 50 h⁻¹ LHSV for 50 h with organic water containing 0.2% and 2% TOC concentration showed that conversion was almost 100% with NiLG500 (pH = 9.4) catalyst and 100% and 96% with NiWB500 (pH = 8.8) catalyst, respectively. The XRD and TEM patterns of the two catalysts after 50 h gasification run showed higher sintering in NiLG500 (pH = 9.4) in which Ni particles were smaller in size.