MAO-modified Silica Research Articles

Titanium and Zirconium Permethylpentalene Complexes, Pn*MCpRX, as Ethylene Polymerization Catalysts

A family of group 4 permethylpentalene complexes, Pn*MCpRX (M = Ti, Zr; CpR = Cp, CpMe, CptBu, CpnBu, CpMe3, Ind; X = Cl, Me), has been synthesized and fully characterized by multinuclear NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction studies. These complexes were immobilized on an insoluble polymethylaluminoxane (sMAO), MAO-modified silica (ssMAO), and a MAO-modified layered double hydroxide (LDH-MAO). The effect of substitution around the Cp ligand was examined in relation to their performance (activity, Mw, PDI, polymer morphology) for ethylene polymerization measured both in solution and in slurry phase. Maximum solution-phase activities of 3585 kg/mol·h·bar were recorded at modest [Zr]:[Al] ratios of 1:250. These were compared to the activities recorded using the equivalent solid-supported complexes, and it was observed that sMAO was a superior support material with average increases in activity of 5.3 and 2.3 times relative to ssMAO and LDH-MAO, respectively. Most strikin...

Microporous and mesoporous supports and their effect on the performance of supported metallocene catalysts

A series of hybrid-supported catalysts was prepared by sequentially grafting Cp 2ZrCl 2 and ( nBuCp) 2ZrCl 2 (1:3 ratio) onto synthesized silica–zirconia xerogel, alumino-silicates, alumina, chrysotile and commercial MAO (methylaluminoxane)-modified silica. Supported catalysts were characterized by Rutherford backscattering spectrometry, atomic force microscopy, extended X-ray absorption fine structure spectroscopy, X-ray diffraction and nitrogen adsorption. The grafted metal content was between 0.21 and 1.00 wt.% Zr/SiO 2 or Zr/Al 2O 3. All of the systems were shown to be active in ethylene polymerization with methylaluminoxane as the cocatalyst. The catalyst activity and polymer molecular weight depended on the effects of the textural characteristics of the supports on the structure of the generated supported catalyst species. The highest activity in ethylene polymerization (ca. 6560 kgPE molZr −1 h −1) was reached with the supported catalyst using commercial MAO-modified silica and also presented the lowest surface roughness. Correlation between EXAFS data and polymer characteristics was extracted: the production of polyethylenes with higher molecular weight was associated with the reduction in the interatomic Zr–O distance of the supported catalysts, which was shown to be dependent on the nature of the support.

Synthesis of nanosized ZSM-2 zeolite with potential acid catalytic properties

Nanosized ZSM-2 zeolite crystals (∼100 nm) with suitable surface acid properties were prepared. ZSM-2 zeolite forms with improved acidity were obtained through thermal deammoniation and steam treatment. Detailed characterization of acidic ZSM-2 zeolite was carried out by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance infrared Fourier transform (DRIFT), nitrogen sorptometry, dynamic light scattering (DLS), thermogravimetry (TGA), potentiometric acidity measurements, and FTIR analysis of pyridine adsorption. Acid properties of ZSM-2 zeolite were significantly improved by conversion into its protonated (H-ZSM-2) and dealuminated (DEAL-ZSM-2) forms, in which bridged Si(OH)Al groups with Brönsted acidity (H-ZSM-2) and aluminium extraframework species with Lewis acid character (DEAL-ZSM-2) are generated. These acidic ZSM-2 zeolite forms also maintain adequate crystalline order and high surface area values, making this nanosized zeolite attractive for catalytic applications. In the context of catalyst supporting materials for the polymerization of olefins, the DEAL-ZSM-2 zeolite form showed Lewis acid properties similar to those of a traditional MAO-modified silica support. Thus, DEAL-ZSM-2 zeolite appears as a promising material to be evaluated as polymerization catalyst support when a reduction of the amount of high-cost MAO cocatalyst is desired.

Ethylene polymerization using tris(pyrazolyl)borate titanium(IV) catalyst supported in situ on MAO-modified silica

Abstract The immobilization “in situ” of soluble catalyst TpMs*TiCl3 (1) on MAO-modified silica (4.0 wt.% Al/SiO2) yields active supported catalyst for ethylene polymerization. This supported catalyst showed activities between 132 and 461 kg of PE/mol[Ti] h atm which varies according to the nature of cocatalyst (MAO > TMA > TiBA > IPRA). Upon activation with MAO, no significant impact on the activity has been observed at 30 and 60 °C; however at higher temperature (90 °C) the activity decreases substantially as result of a partial catalyst deactivation. Studies related to the influence of the [Al]/[Ti] molar ratio using TMA as cocatalyst have shown that this supported catalyst presented higher activities using Al concentrations as low as 25:1. The viscosity-average molecular weights ( M ¯ v ) of the PE's produced with 1/SMAO-4 are strongly influenced by the Al concentration varying from 0.41 to 13.6 × 105 g/mol. Resulting polyethylenes can replicate the support morphology. The scanning electron microscopy (SEM) studies revealed that the spherical morphology of the supported catalyst is replicated in the polyethylene particles.

Comparative study of propylene polymerization using Me2Si(RInd)2ZrCl2/SiO2–SMAO/AlR3 and Me2Si(RInd)2ZrCl2/MAO (R = Me, H)

A mechanistic analysis of propylene polymerization was performed, in which the catalyst system was Me2Si(R1Ind)2ZrCl2/SMAO/AlR32 (in situ supported catalyst onto MAO-modified silica) or Me2Si(R1Ind)2ZrCl2/MAO (homogeneous), where R1=H or CH3, cocatalyzed by AlR32=TEA (triethylaluminum), IPRA (isoprenylaluminum), or TIBA (triisobutylaluminum). The catalyst activity of the homogeneous system Me2Si(2-Me-Ind)2ZrCl2/MAO was almost 8 times higher than that observed for Me2Si(Ind)2ZrCl2/MAO (38 vs 4.6kg PP/g cath), while the polypropylene molar mass was 3 times higher (Mw: 93 vs 34kg/mol). Conversely, the in situ supported systems Me2Si(Ind)2ZrCl2/SMAO/AlR3 and Me2Si(2-Me-Ind)2ZrCl2/SMAO/AlR3 showed similar activities, ranging from 0.2 to 1.5kg PP/g cath. The molar mass of the resulting polymers prepared using the in situ procedure was dependent on the AlR3 nature and on the Al/Zr ratio. Generally, the heterogeneous catalysts produced PP with higher molecular weights than that obtained with homogeneous ones. The influence of the alkylaluminum, used as the cocatalyst, on the chain-transfer termination reaction to the alkyl compound was evident from the activity and the molecular weight of the produced polymers.

Supported modified zirconocene catalyst for ethylene polymerization

The reactivity of the silyl ether-substituted zirconocene [Zr{η 5-C 5H 4(CH 2) 3OSiMe 3}(η 5-C 5H 5)Cl 2] ( 1) towards dehydroxylated silica was evaluated and comparative studies with respect to the unmodified analogue [Zr(η 5-C 5H 5) 2Cl 2] ( 2) were performed. The different reactions were monitored by FT-IR spectroscopy. The resulting materials were activated with MAO and the catalytic systems were evaluated in ethylene polymerization. The catalysts were characterized by elemental analysis as well as by FT-IR and UV–vis spectroscopy. In addition to the classical reaction between Zr Cl bonds and acidic silanol surface groups, grafting of complex 1 occurs to some extent by reaction between the silyl ether group and reactive siloxane bridges on the silica surface. In contrast, complex 2, as one would expect, only reacts through the Zr Cl groups. Supporting zirconocene 1 on MAO-modified silica gives rise to a more efficient catalyst. The effects caused by these different grafting reactions, as well as the catalytic activities of the resulting materials, are presented and discussed.

Supported dichlorobis(3-hydroxi-2-methyl-4-pyrone)Ti(IV) catalysts: Evaluation on ethylene polymerization

Dichlorobis(3-hydroxi-2-methyl-4-pyrone)Ti(IV) complex was grafted on different inorganic supports, namely different kinds of SiO 2, MAO-modified silica, MCM-41, Al 2O 3, ZrO 2 and MgO. The resulting supported catalysts were shown to be active in ethylene polymerization using methylaluminoxane (MAO) as cocatalyst, most of them being even more active that the homogeneous complex. The highest catalyst activities were observed for the Ti complex supported on SiO 2 948 activated at 450 °C, MCM-41 and Al 2O 3.

Polypropylene Made with In-Situ Supported Me2Si(Ind)2ZrCl2 and Me2Si(2-Me-Ind)2ZrCl2 Catalysts: Properties Comparison

Propylene homopolymerizations were carried out using Me 2 Si(Ind) 2 ZrCl 2 and Me 2 Si(2-Me-Ind) 2 ZrCl 2 , MAO-modified silica, and common alkylaluminum cocatalysts. Supported catalysts were prepared by the in-situ immobilization technique. The effect of the type and concentration of alkylaluminium on propylene polymerization was evaluated using TEA (triethylaluminium) IPRA (isoprenylaluminium); and TIBA (triisobutylaluminum) as cocatalysts. The polymers were analyzed by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and scanning electronic microscopy (SEM). The effect of the type and concentration of alkylaluminum on the melting temperature and the molar mass of the polypropylene was the same for both catalysts. The polymers made with in-situ supported catalyst had lower melting points and, in almost all polymerization conditions, higher molar masses than those produced by homogeneous polymerization. Polypropylene samples made with Me 2 Si(2-Me-Ind) 2 ZrCl 2 had higher melting temperatures and molar masses than those made with Me 2 Si(Ind) 2 ZrCl 2 . SEM micrographs showed that the polymers obtained with in-situ supported systems had a well-defined morphology, confirming that the polymerization indeed took place onto the silica support.

Effect of MAO silica surface loading on ( nBuCp) 2ZrCl 2 anchoring, on catalyst activity and on polymer properties

Silica Grace 948 activated at 723 K was modified with MAO in aluminum contents comprised between 0.5 and 20.0 wt.% Al/SiO 2. Supported metallocene catalysts were prepared by grafting ( nBuCp) 2ZrCl 2 toluene solutions directly onto silica and onto MAO-modified silicas. The catalysts were characterized by a series of complementary techniques, namely, Rutherford backscattering spectrometry (RBS), Diffuse-reflectance Fourier transform spectroscopy (DRIFTS), ultraviolet-visible spectroscopy (UV-Vis) and electron probe microanalysis (EPMA). MAO-mediated systems presented higher Zr content comparing to directly silica supported system. For the MAO-modified silica systems, Zr content increases up to 2.0–4.0 wt.% Al/SiO 2 reaching 1.5 wt.% Zr/SiO 2. As the aluminum content increases, metal loading decreases achieving 0.6 wt.% Zr/SiO 2 for the supported catalyst having 20.0 wt.% Al/SiO 2. All the systems were shown to be active in ethylene polymerization having MAO as cocatalyst. Higher activity were achieved with MAO-mediated systems having 2.0–4.0 wt.% Al/SiO 2. The catalyst systems produced polyethylenes with high molecular weight and narrow molecular weight distribution. The preparative protocol was stepwisely monitored by DRIFTS. For lower MAO content (2.0 wt.% Al/SiO 2) residual isolated silanol groups were thoroughly consumed or perturbed after ( nBuCp) 2ZrCl 2 impregnation.

Polymerization of ethylene by the tris(pyrazolyl)borate titanium(IV) compound immobilized on MAO-modified silicas

The immobilization of soluble catalyst {TpMs∗}TiCl3 (TpMs∗HB(3-mesityl-pyrazolyl)2(5-mesityl-pyrazolyl)−) on silica and MAO-modified silicas containing 4.0, 8.0 and 23.0 wt.% Al/SiO2 yields active supported catalysts for ethylene polymerization. Among the supported catalysts studied by XRF spectroscopy, higher titanium content was obtained using MAO-modified silica containing 8.0 wt.% Al/SiO2 as support. For the ethylene polymerization reactions carried out in hexane at 60 °C using a combination of triisobutylaluminum (TiBA) and methylaluminoxane (MAO) (1:1), the activities varied between 24.4 and 113.5 kg of PE/mol [Ti] h. The highest activity is reached using MAO-modified silica containing 4.0 wt.% Al/SiO2 as support. The viscosity-average molecular weights (M̄v) of the PE’s produced with the supported catalysts varying from 1.44 to 9.94×105 g/mol with melting temperatures in the range of 125–140 °C. The use of other Lewis acid cocatalysts, including TiBA, diethylaluminum chloride (DEAC), and trimethylaluminum (TMA) resulted also in the formation of active catalysts for ethylene polymerization. However, the activities are lower than that one using a combination of TiBA and MAO. The viscosity-average molecular weights (M̄v) of PE’s are influenced by varying the cocatalysts as well as the Al/Ti molar ratio. The supported catalyst generated in situ under ethylene atmosphere is roughly four times more active than supported one containing 4.0 wt.% Al/SiO2.

Zirconium alkoxide complexes as catalysts for ethylene polymerization

The complexation of 2-hydroxy-1,4-naphtho-quinone and 3-hydroxy-2-methyl-4-pyrone with zirconium tetrachloride yielded complexes containing bidentate alkoxide ligands. In the presence of MAO or TIBA these complexes were found to be active for ethylene polymerization, producing polymers with high molecular weight. Both catalytic systems were also shown to be active when supported on silica or on MAO-modified silica.

Ethylene polymerization with catalyst systems based on supported metallocenes with varying steric hindrance

Abstract A series of metallocenes differing in the metal center (M=Zr, Ti, Hf), in the coordination sphere (RCp, with R=H, Me, iBu, nBu; Indenyl) and the bridge (Et, Me2Si) were studied as homogeneous systems and supported on silica and MAO-mediated (2.0 wt.% Al/SiO2) silica. The effect of these parameters were evaluated in the grafted metal content, in catalyst activity and in polymer properties. Metal content was determined by Rutherford backscattering spectrometry (RBS) and X-ray photoelectronic spectroscopy (XPS). Alkyl ligands seemed not to affect sensitively the final metal content, but indenyl complexes led to higher grafted metal content. Immobilization on MAO-mediated systems afforded higher metal contents. XPS measurements evidenced a reduction in Zr, Ti or Hf binding energy (BE) after immobilization on silica or on MAO-modified silica, in the latter two different species being observed after signal deconvolution. Monitoring of the surface reaction by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that at this aluminum content residual silanol groups still remains, bearing probably two kind of species: one supported on silanol groups and another on MAO counterpart. Inductive effects from substituent on ligands influence in the catalyst activity, but this electronic effect is reduced in comparison to the steric effect played by the silica surface. Resulting polymers showed higher molecular weight than those produced by the soluble systems.

Polypropylene obtained with in situ supported metallocene catalysts

Propylene homopolymerization was carried out with Me2Si(Ind)2ZrCl2 immobilized on commercial MAO-modified silica by the in situ supporting technique. Adsorption isotherm determination for this metallocene on silica indicated that the saturation level is reached at 2.0 wt.% Zr/SMAO. Catalyst systems were shown to be active in the absence of external MAO, being activated by common alkylaluminum cocatalysts, namely triethylaluminum (TEA), isoprenylaluminum (IPRA) and triisobutylaluminum (TIBA). The effect of the nature and concentration of cocatalyst on catalyst activity and on polypropylene (PP) properties was evaluated. Best catalyst activity was observed in low concentration of IPRA (1.4 kg PP/g cat h). The resulting polymers were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), 13 C nuclear magnetic resonance (NMR) spectroscopy, and scanning electronic microscopy (SEM). © 2003 Elsevier Science B.V. All rights reserved.

Hybrid supported zirconocene and niobocene catalysts on MAO-modified silicas

A series of supported metallocene catalysts were prepared by immobilizing ( nBuCp) 2ZrCl 2 and Cp 2NbCl 2 on bare and on MAO-modified silica. Both metallocenes were sequentially grafted on silica under different combinations. Metal loading (Zr, Nb and Al) was determined by X-ray fluorescence spectroscopy (XRF). Final metal content was shown to be dependent on passivation of silica with MAO, and on the grafting sequence. Higher metal contents (0.42 wt.% Nb/SiO 2 and 1 wt.% Zr/SiO 2) were achieved when the metallocene was grafted onto MAO-modified silica. Cp 2NbCl 2 derivatives were not active in polymerizing ethylene using MAO as co-catalyst (Al/Nb=2000). All the systems containing zirconocene derivatives showed high catalyst activity. Nevertheless, higher activities were achieved immobilizing ( nBuCp) 2ZrCl 2 on silica that was previously modified with MAO and Cp 2NbCl 2. Some immobilization protocols led to catalyst systems which were even more active than the homogenous counterpart. Effects of support structure and chemical modification on the catalyst performance are presented and discussed.

Polyethylenes produced with zirconocene immobilized on MAO-modified silicas

The effect of Al content on MAO-modified silicas was evaluated on catalyst activity, on polymer properties and on residual metal content in the resulting polyethylenes. MAO-modified silicas were prepared by impregnating MAO toluene solutions in concentration range between 0.5 and 20.0 wt% Al/SiO 2. Commercial MAO-modified silica (Witco) containing 24.4 wt% Al/SiO 2 was used for comparative reasons. The resulting modified-silicas were employed as supports for grafting ( nBuCp) 2ZrCl 2. Using external MAO as cocatalyst (Al/Zr=2000) no difference in catalyst activity was observed. Nevertheless, for Al/Zr=500, catalyst activities were shown to be higher for supported zirconocene systems containing 0.0–2.0 wt% Al/SiO 2 range. According to DSC analysis, one T m peak was detected for polymer obtained with catalyst prepared with 0.5 wt% Al/SiO 2 (135 °C), but two T m peaks were observed for polymers obtained with catalysts prepared with 10.0 wt% Al/SiO 2 (136 and 141 °C) and 20.0 wt% Al/SiO 2 (133 and 141 °C).