Silica-supported Metallocene Catalyst Research Articles

Effects of Spatial Distributions of Active Sites in a Silica-Supported Metallocene Catalyst on Particle Fragmentation and Reaction in Gas-Phase Ethylene Polymerization

Effects of Spatial Distributions of Active Sites in a Silica-Supported Metallocene Catalyst on Particle Fragmentation and Reaction in Gas-Phase Ethylene Polymerization

Discovery of Polyolefin Granule Products That Can Be Deconstructed into Individual, Fully Separable Grains Provides New Insights into Grain Growth and Material Design

We have discovered a silica-supported metallocene catalyst that produces large, open, porous polyolefin granules that can be easily and quantitatively deconstructed into their constituent polyolefin grains (Gₚ) by lightly crushing any granule. Three such polymerization products have been examined as follows: two heterophasic, comprising isotactic polypropylene (iPP) and polyethylene (PE), and one monophasic, comprising iPP. Deconstruction of granules from each product was followed by quantitative examination by optical microscopy of hundreds of grains. These data have enabled the first statistically robust demonstrations of lognormal distributions of polymer grain size and sphericity. While the lognormal results are not surprising per se, they offer the first quantitative demonstration of such distributions, rather than the unimodal distribution used in physical models since the introduction of the multigrain model over 30 years ago. Three contributions to the lognormal size distribution of the polymer grains are identified: dominant among these is the distribution of simple spheroidal polymer grains. For these simplest of grains, we also give the special notation “primaries”. The two remaining more complex grain topologies are constructed from primaries. One of these topologies is created from two or more fused, or attached, polymer primaries. The third type of grain topology is constructed from three or more fused primaries where the primary aggregate contains at least one void within the grain. In all, the polymer grains range in average diameter from 40 to as large as 470 μm. The median Gₚ diameter measured for all grains in each of the three products ranges from 83 to 150 μm. Between 6 and 16% of polymer grains have optically detectable voids, and grains with voids have on average 40–100% larger diameters than both the aforementioned solid grains denoted primaries and fused primaries. A simple morphological model is proposed for the dispersion of size and shape observed. Based on the data and model, the smallest supported catalyst grain, from which the polymer primary is grown, is calculated independently for each polymer product. These are ∼4–7 nm in diameter, or within the range of basic SiO₂ building blocks reported or calculated for spray-dried silica. Polarized light microscopy examination of isolated spheroidal primaries shows that chains in the heterophasic products have a preferential orientation, with chains aligning more or less along the radius of each primary, whereas chains in the iPP product show no preferential orientation. The morphology, or texture, of iPP and high-density polyethylene (HDPE) in a polymer primary taken from one of the heterophasic granule products was examined by scanning transmission electron microscopy. This examination, together with another for a closely related heterophasic product made with the same catalyst, shows that these two normally immiscible polyolefins are dispersed or mixed via direct sequential polymerization on a ∼50-250 nm, and perhaps finer, scale. Thus, the results provide visual confirmation that, to accommodate the growth of the HDPE phase, the first-grown iPP phase fracture extends through the primary grains themselves as the grains expand with new polymer. We expect that the demonstration of nearly effortless quantitative production of small polyolefin spheres, potentially with controlled surface roughness, as well as the commingling of immiscible polyolefins achieved on this fine scale in the nascent granules, could spawn new creative opportunities for material design.

Early-stage particle fragmentation behavior of a commercial silica-supported metallocene catalyst

Olefin polymerization starts at the outer surface and at the macropores walls. Here fragmentation occurs by peeling off the catalyst in a layer-by-layer mode with, in ideal conditions, the simultaneous formation of bisection-type fractures across the catalyst inner domains.

Silica-supported metallocene catalyst poisoning: The effect of surface modification on the efficiency of the catalytic system

Cp2ZrCl2 was immobilized on silica by two methods: direct grafting (GS) and via entrapment within silica matrix, produced by the non-hydrolytic sol-gel method (NH). In the latter case, the surface was further modified by metal salts (Cr, Mo and W). The supported catalysts (three GS and four NH systems) were evaluated in ethylene polymerization, having methylaluminoxane (MAO) as the cocatalyst. Almost all of them resisted to at least 0.2 μg g−1 acetone (used as probe molecule for oxygenated poisons). NH systems bearing W or Mo (3 wt.%) on its structure were shown to be more productively efficient against poisoning. NH containing W in its structure tended to keep its catalyst activity even after the addition of the poison. Therefore, metal doping may be a suitable strategy to keep or increase the efficiency of entrapped zirconocene systems against oxygenated poisons. SAXS analyses show likewise that the smaller the primary spheres, the more protected will be the active site. Texture and microstructure of the catalytic systems and properties of the yielded polyethylenes reveal that the nature of the immobilization route affects the polymer properties (Mw and Tm).

Porous Organic Polymers-Supported Metallocene Catalysts for Ethylene/1-Hexene Copolymerization

Porous organic polymers (POPs) have received much attention in adsorption, separation, and catalysis. In this paper, porous organic polymers with different pore structure were used as metallocene catalyst supports, and ethylene/1-hexene copolymerizations were conducted using the POPs-supported metallocene catalyst. The pore structure of the prepared POPs and the supported metallocene catalyst were characterized by nitrogen sorption porosimetry and non-local density functional theory simulation, and the molecular chain structure of the produced ethylene/1-hexene copolymers were investigated through gel permeation chromatography (GPC), IR analysis, differential scanning calorimetry (DSC), and temperature rising elution fractionation (TREF). The results show that the loading amount of active sites varied with different pore structures of the POP supports, and the active species scattered in different pore sizes had a moderate impact on the molecular chain growth and the molecular weight distribution. The IR, DSC, and TREF analysis revealedthat different branching degree, double bond content, and chemical composition distributions were detected from the molecular chain structure of the ethylene/α-olefin copolymers from different POPs and silica-supported metallocene catalysts, despite their similar IR, DSC, and TREF curves due to the same active species. Scanning electron microscopy (SEM) showed that porous ethylene/α-olefin copolymers with varied surface morphology were obtained from the POPs-supported metallocene catalysts with different pore structure.

Modeling of molecular weight distribution of propylene slurry phase polymerization on supported metallocene catalysts

A mathematical model of the molecular weight distribution (MWD) based on a particle growth model and the kinetic scheme is developed to simulate the MWD of the slurry phase propylene polymerization on a silica-supported metallocene catalyst by means of the equations of moments. The model is used to predict molecular weight distribution, including the number-average molecular weight, the weight-average molecular weight, and the polydispersity index. The results show that the mass transfer has great influence on the polymerization reaction, and it can broaden the MWD especially; moreover, the MWD can be evaluated by simulation; the average molecular weight increases as pressure or temperature, and MWD shifts to long chain lengths as the effective diffusion coefficient increasing thought the influence is not remarkable; furthermore, the MWD's simulation results are calculated, which fit greatly with the experimental data.

Modeling and Experimental Evaluation of Single Particle Growth in Syndiotactic Polymerization of Styrene

A comprehensive mathematical model and experimental study of single particle growth for styrene polymerization over a silica-supported metallocene catalyst were investigated. The model was developed based on the modification of the well-known multigrain model (MGM) by introducing mesoparticle scale limitations. Thereafter, the model was employed to predict the effects of bulk phase temperature and catalyst properties (initial catalyst active site concentration and initial catalyst particle size) on the polymerization rate, degree of polymerization (DP), and the polydispersity index (PDI) of syndiotactic polystyrene (SPS). The simulation results showed a significant radial distribution of styrene concentration across polymer particle growth at different polymerization conditions. It was found that increasing the initial catalyst concentration and bulk phase temperature resulted in polymerization rate enhancement. In context, the polymerization rate decreased as the initial catalyst particle size increased from 20 to 50 μm. The results revealed that a uniform increase in DP of the polymer was obtained by increasing the initial catalyst concentration and the reaction temperature, while resulting in a decrease of the PDI value. Meanwhile, the DP and PDI values varied inversely under the influence of initial catalyst particle size within a period of time similar to the one needed in the catalyst decay. The simulated results in the study agree well with experimental data of SPS.

Morphological Interpretation of the Evolution of the Thermal Properties of Polyethylene during the Fragmentation of Silica Supported Metallocene Catalysts

Gas phase ethylene polymerizations on supported metallocene catalysts have been carried out in a stopped flow minireactor for times as short as 0.3 s. By recovering the polymer particles without altering their morphology it was possible to follow the evolution of the thermal properties of the polymer during the fragmentation of the particles. The melting temperature increased with reaction time (from 118 °C at 0.3 s to 131 °C at 180 s), and three distinct crystallization peaks can be observed during the studied time range, centered on 75 °C for very short reactions, 105 °C for longer reactions, and 115 °C (the usual crystallization temperature of HDPE) which becomes predominant only after 30 s of reaction. It is believed that the reason for this behavior is related to the confinement of the nascent polymer in the nanopores of the catalyst support. This confinement perturbs the chain crystallization by limiting the crystal growth. It is shown that measuring the thermal properties of the reactor powder with...

Combined single particle growth and population balance modeling in stereoregular polymerization of styrene

Modeling and experimental analysis for syndiospecific polymerization of styrene over silica-supported metallocene catalyst was carried out. A detail model was developed by coupling the single particle growth model (PGM) with particle population balance equation. The model was employed to predict the effects of intraparticle mass transfer limitations and the initial catalyst particle size on the rate of polymerization and the particle size distribution (PSD) of syndiotactic polystyrene (sPS). The single PGM, based on a modified polymeric multigrain model, was first utilized to calculate the single particle growth rate and polymerization rate under intraparticle mass transfer limitations and different initial catalyst particle sizes. Then, the model was solved simultaneously with particle population balance equation to estimate the PSD of sPS under the same limitations. The single PGM results showed a significant radial distribution of styrene concentration across polymer growth. It was further noticed that the diffusion resistance was most intense at the beginning of the polymerization reaction and the effects of polymerization rate were stronger. Moreover, it appeared that increasing the initial catalyst particle size led to lower rate of polymerization. The PSD simulation results revealed that the mass transfer limitation, as well as the initial catalyst particle size made a strong impact on the PSD of sPS. In addition, the simulation results obtained from this model showed good agreement results with experimental data of sPS.

Coupled single-particle growth and kinetics modeling for styrene polymerization over silica-supported metallocene catalyst

A comprehensive mathematical model for styrene stereoregular polymerization was carried out. This model was generated by coupling the single particle growth model (SPGM) with kinetics model, to predict the effect of intraparticle mass transfer resistance and initial catalyst size on the polymerization kinetics. SPGM was derived based on a modified multigrain model (MMGM) to calculate the spatial-time evolution of styrene concentration under intraparticle mass transfer limitations. Then, the SPGM was solved simultaneously with kinetics model to estimate the polymerization rate and molecular weight distribution (MWD) under the above mentioned limitations. The results show that a significant radial distribution of styrene concentration across polymer growing. Moreover, the diffusion resistance was most intense at the early step of the polymerization and the effects of the polymerization rate are more strongly. Additionally, it is appear that increasing the initial catalyst size leads to a decrease in the rate of polymerization. For MWD, the model simulation show that the diffusion resistance led to have an increase in the molecular weight within a period of time similar to the one needed in the catalyst decay. The validation of the model with experimental data given a agreement results and shows that the model is able to predict monomer profile, polymerization rate, and MWD of syndiotactic polystyrene.

Effects of amino groups and microstructure of organic mesoporous silica supported metallocene catalysts on ethylene polymerization

Mesoporous silica (MS), 3-aminopropyltriethoxysilane (APTES) modified mesoporous silica (AMS), bis(3-trimethoxysilylpropyl)amine modified mesoporous silica (BAMS) and APTES modified solid spherical silica (AS) were prepared and used to immobilize metallocene catalysts for ethylene polymerization. Gel permeation chromatography results showed that polyethylenes (PEs) catalyzed by AMS (or BAMS) supported metallocene catalysts at the molar ratios of Al/Zr = 100, 300 and 500 were of bimodal molecular weight distribution (BMWD); while PEs catalyzed by the above catalysts at the molar ratios of Al/Zr ≥ 800 were of monomodal molecular weight distribution (MMWD). However, MS (or AS) supported metallocene catalysts could only produce PEs with MMWD in spite of the molar ratio of Al/Zr. It was because that AMS (or BAMS) supported catalysts possessed two active sites for ethylene polymerization at low molar ratios of Al/Zr due to the combination effects of mesopore geometrical constraint and amino groups of the supports, which was confirmed by X-ray photoelectron spectroscopy. This brings forward a novel and easy method for the synthesis of polyolefin with BMWD.

Nascent morphology of syndiotactic polystyrene synthesized over silica-supported metallocene catalyst

The nascent morphology of semi-crystalline syndiotactic polystyrene (sPS) polymerized over silica-supported pentamethyl cyclopentadienyl titanium trimethoxide (Cp∗Ti(OCH3)3) catalyst in a liquid slurry polymerization has been investigated under various reaction conditions. The scanning electron microscopic analysis of nascent polymers reveals that sPS molecules grow as long nanofibrils of 30–50nm diameter and X-ray diffraction analysis shows the co-crystalline phases including both sPS and low molecular weight guest molecules of monomer and diluent. The energy dispersive X-ray spectroscopy also shows that the disintegration of silica primary particles occurs during the polymerization as evidenced by the uniform dispersion of silicon and aluminum in a polymer particle. The fibrous growth of the polymer inside a polymer particle leads to the shape replication of the original silica particles.

Rate and molecular weight distribution modeling of syndiospecific styrene polymerization over silica-supported metallocene catalyst

The kinetics of syndiospecific polymerization of styrene over silica-supported Cp∗Ti(OCH3)3/MAO catalyst has been investigated through experimentation and theoretical modeling. At low monomer concentrations, the polymerization rate increases almost linearly with monomer conversion, but the reaction rate becomes independent of monomer concentration at high bulk phase monomer concentrations. A kinetic model that incorporates the monomer partition effect between the solid and the liquid phases has been proposed. The model simulations show that the observed non-linear kinetics can be adequately modeled by the monomer partition model. The polymer molecular weight has also been found to increase with the monomer concentration and the polymer molecular weight distribution (MWD) is quite broad, suggesting that the catalytic behavior deviates from the single site catalytic polymerization model. The MWD broadening is modeled by a two-site kinetic model and a good agreement between the model and the experimental data has been obtained.

Preparation of Nanosheet Polyethylene with Heterogeneous Metallocene Catalyst and Formation Mechanism of Nanosheet Polyethylene

Silica with small and regular pore sizes was synthesized by hydrothermal crystallization and investigated with thermogravimetric analysis, X-ray diffraction, Fourier transform infrared spectroscopy, N2 adsorption-desorption, and scanning electron microscopy (SEM). The ethylene polymerization was carried out with the silica-supported metallocene catalyst, and the morphology of the resultant polyethylene was investigated with SEM. It was found that the silica could break up into sheet fragments instantaneously during polymerization and the polyethylene could replicate it to obtain nanosheet polyethylene. A possible formation mechanism of the nanosheet polyethylene is proposed.

Preparation, Characterization, and Activity of Silica Supported Metallocene Catalysts

Silica supported metallocene catalysts were prepared for ethylene polymerization by immobilizing bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2) onto both calcined silica, and silica treated with γ-aminopropyltrimethoxysilane or γ-mercaptopropyltrimethoxy-silane. Bis(cyclopentadienyl)zirconium chloride hydride (Cp2ZrClH) was also immobilized onto both calcined silica and vinyltrimethoxysilane treated silica. The catalysts were characterized by using direct current plasma atomic emission spectroscopy (DCP-AES), diffuse reflectance infrared spectroscopy (DRIFT), and X-ray photoelecton spectroscopy (XPS). It was found that both Cp2ZrCl2 and Cp2ZrClH were adsorbed on both calcined silica and silane treated silica, respectively. When γ-aminopropyltrimethoxysilane (APTMS) was used as the spacer molecule, some of the amine became protonated upon adsorption of Cp2ZrCl2. Initial catalytic tests showed that all of the complexes were catalytically active (methylaluminoxane as a cocatalyst), with the Cp2ZrClH supported complexes giving the highest activities, but still lower than the homogeneous catalyst, Cp2ZrCl2. The supported complexes produced polymer with higher molecular weight than the homogeneous catalyst. These observations, together with presented spectral evidence, support the view that the catalyst remained adsorbed in the presence of MAO.

Synthesis of Silica-Supported Metallocene Catalysts for Olefin Polymerization

Four supported metallocene catalysts were synthesized directly on silica [Si(CpZrCp), Si(CpZrInd), Si(IndZrCp), and Si(IndZrInd)]. The first ligand was chemically tethered to silica (internal ligand) while the second was free from silica (external ligand). The polymerization of ethylene and the copolymerization of ethylene with hexene were investigated. The activity of the catalysts and the molecular weights of the polymers were found to depend on the position of the ligands. In all cases, high-molecular-weight polyethylene with moderate polydispersity was obtained. Moreover, the copolymerization of ethylene with hexene provided a homogeneous polymer. Reaction of ZrCl4(OC4H8)2 with modified silica (SiCp).

Effect of Light on the Activity of Metallocene Catalysts in the Gas-Phase Polymerisation of Ethylene

It was found that halogen light can have a strong effect on the polymerisation activity of silica-supported metallocene catalysts. Three different metallocene catalysts have been tested in the gas-phase polymerisation of ethylene. The effect of activation depends not only on the type of transition metal, but also on the wavelength and intensity of light. The activation by light was found to be a reversible process.

SIO2-Supported Metallocene Catalysts for Propene Polymerization: Electron Microscopic Studies and Tomographical Reconstructions of Polymer Growth

Abstract Summary: The polypropylene growth process on silica supported metallocene catalysts was examined by electron microscopy and kinetic investigations. It was possible to identify different polymerization stages with different characteristic kinetic data in the slurry phase which led to the development of a refined polymer growth model [1-2]. Tomographical reconstruction of shortly polymerized catalyst particles obtained by electron micrographs of serial microtome sections turned out to be a useful tool to achieve a better understanding of the heterogeneous polymerization process and the influence of the support structure on polymer growth. 1. Introduction Single-site metallocene catalysts produce extremely uniform polymers with molecular weight distributions (MWD) of 2 or less and very narrow compositional distributions in comparison to conventional Ziegler-catalysts. For industrial gas phase or bulk polymerization in existing plants (drop-in-technology) it is necessary to fix the catalytically active components on a heterogeneous support.

Synthesis of Novel Hindered Amine Light Stabilizers (HALS) and Their Copolymerization with Ethylene or Propylene over Both Soluble and Supported Metallocene Catalyst Systems

Novel polymerizable hindered amine light stabilizers (HALS) such 1-(but-3-enyl)-2,2,6,6-tetramethylpiperidine (1), 1-(undec-10-enyl)-2,2,6,6-tetramethylpiperidine (2), 4-(but-3-enyl)-1,2,2,6,6-pentamethyl-3,4-dehydropiperidine (3), 2-(but-3-enyl)-2,6,6-trimethylpiperidine (4), 4-(but-3-enyl)-1,2,2,6,6-pentamethyl-4-piperidyl ether (5), 4-(undec-10-enylamide)-1,2,2,6,6-pentamethylpiperidine (6), 4-((N-n-butyl)-undec-10-enylamide)-1,2,2,6,6-pentamethylpiperidine (7), and bis(N-n-butyl-N-2,2,6,6-tetramethylpiperidine)-N-n-butyl-N-allyltriazine (8) were synthesized. All the aforementioned HALS monomers except for 5 and 8 were successfully copolymerized in fair to high yields with ethylene or propylene over eight different group 4 metallocene catalysts using methylalumoxane (MAO) as cocatalyst. Copolymerizations were also performed over a supported metallocene/SiO2/MAO/triisobutylaluminum(TIBA) catalyst system. The silica-supported metallocene catalyst system readily promoted copolymerization of the sterically...

Development of a refined poly(propylene) growth model for silica supported metallocene catalyst systems

The poly(propylene) growth process on silica supported metallocene catalysts has been examined by means of electron microscopy and kinetic investigations. It was possible to extend the polymerization process and to identify different polymerization phases with different characteristic kinetic data by lowering the reaction temperature in the slurry phase. EDX analysis of microtome sections of a polymerized model system led to the development of a refined polymer growth model. It was found that the addition of activating hydrogen reduces the initial diffusion limitations as well as a prepolymerization step with 1-octene in the bulk phase.