Joao Henrique Zimnoch dos Santos

The effect of silica dehydroxylation temperature on the activity of SiO2-supported zirconocene catalysts

Abstract A series of heterogeneous catalyst systems was prepared by the immobilization of bis(n-butylcyclopentadienyl) zirconium dichloride, (nBuCp)2ZrCl2, on silica supports activated at different temperatures. Silica dehydroxylation was evaluated in terms of retained metal loading and activity in ethylene homopolymerization. Characterization of the catalyst systems was accomplished by Rutherford back-scattering spectrometry and infrared spectroscopy. The highest metal loading (0.48 wt.% Zr/SiO2) was achieved with silica treated under vacuum at room temperature (298 K), but the catalyst showed only a minor polymerization activity which may be attributed to a large number of inactive Zr-support bidentate species formed at a high surface density of OH in silica. IR spectroscopic data show that, regardless of support activation temperature, a significant number of isolated OH groups remains after zirconocene fixation. The presence of bulky ligands in the catalyst molecule seems to prevent the remaining OH groups from reacting with additional metallocene complexes, keeping the metal loading around 0.35 wt.% Zr/SiO2 for silica activated between 373 and 723 K. High polymerization activity observed for the system based on 373 K-activated silica suggests a role for these OH groups in the generation of active alkylated species when methylaluminoxane and trimethylaluminum (contained in MAO itself) are added at the beginning of the polymerization reaction. IR analysis shows that TMA, which is a less sterically demanding compound than MAO, can effectively access the remaining OH groups consuming them thoroughly. Practically all the prepared systems presented activity in ethylene polymerization with MAO as cocatalyst, the highest activity (5.1×105 PE g mol−1 Zr h−1) having been obtained with silica dehydroxylated at 723 K.

Organosilicon-modified silicas as support for zirconocene catalyst

The metallocene (nBuCp)2ZrCl2 was grafted on partially dehydroxilated commercial silica (Grace 948) whose surface had been chemically modified by wet impregnation of an organosilane (Ph3SiCl, Me3SiCl, or Me2SiHCl) aiming at supported catalyst systems with well spaced α-olefin polymerization active centers. Final Zr loadings were determined by Rutherford Backscattering Spectrometry (RBS), and modifications at the silica surface after each preparation step were monitored by in situ FT-IR spectroscopy. These catalyst systems produced polyethylenes with narrow molecular weight distribution when methylaluminoxane was used as cocatalyst, with twice the activity of (nBuCp)2ZrCl2 supported on bare SiO2. Effects of support structure and chemical modification on catalyst performance are presented and discussed.

Linear low-density polyethylene synthesis promoted by homogeneous and supported catalysts

Linear low-density polyethylenes (LLDPEs) were obtained through the copolymerization of ethylene with 1-hexene using (nBuCp) 2 ZrCl 2 co-catalyzed by methylaluminoxane (MAO). For comparative reasons, the same metallocene was supported on silica (0.85 wt%Zr/SiO 2 ) by grafting. The copolymerizations were performed in toluene, at 1.6 bar of ethylene, 60°C and in an Al/Zr molar ratio of 2500. The 1-hexene concentration varied from 0 to 0.50 mol L -1 . The resulting copolymers were characterized by GPC, DSC and 13 C NMR. The catalytic activities remained close to 2 × 10 7 and 7 × 10 6 (g pol) mol Zr -1 bar -1 h -1 for the homogeneous and supported systems, respectively. For the homogeneous system, the catalytic activity and the comonomer incorporation increased with the 1-hexene concentration up to 0.30 mol l -1 . Higher comonomer concentrations led to a decrease in catalytic activity in the case of the homogeneous system. The highest comonomer incorporation (6.3mol%) was achieved with 0.50 mol l -1 of 1-hexene in the reaction medium. The supported system afforded lower comonomer incorporations (maximum 4.0 mol%). The effect of the 1-hexene incorporation can also be evaluated through the polymer properties, namely, crystallinity, melting temperature, molecular weight and polydispersity.

Effects of Al/Zr ratio on ethylene-propylene copolymerization with supported-zirconocene catalysts

Abstract Effects of heterogenization parameters on compositional and catalytic properties of Et(Ind) 2 ZrCl 2 supported on silica modified with MAO were evaluated using data on metal loading, catalyst activity in ethylene–propylene copolymerization, and polymer properties. The supported catalysts were prepared according to a 2 3 factorial design for multivariate analysis. The parameters studied were: MAO concentration in impregnation, Et(Ind) 2 ZrCl 2 concentration in grafting, and immobilization temperature for both metallocene and organoaluminum. The grafting solution was monitored by UV–VIS spectroscopy, while the resulting catalyst systems were characterized by inductively-coupled plasma-optical emission spectroscopy (ICP-OES), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS). At high statistical significance, both MAO and Et(Ind) 2 ZrCl 2 concentrations during preparation affected the determination of the final Al/Zr ratio on silica. The catalyst systems were tested in ethylene–propylene copolymerizations using external MAO as cocatalyst. Relationships were observed between (i) catalyst activity and the binding energy of Zr 3d 5/2 electrons and (ii) Al/Zr ratio on silica and the average ethylene incorporation. The polymer samples presented narrow molecular weight distribution, but according to differential scanning calorimetry (DSC), and cross-fractionation chromatography (CFC) data, catalyst systems with given Al/Zr ratios might yield different crystallites, suggesting a plural distribution of chemical composition.

Tailored Silica-Antibiotic Nanoparticles: Overcoming Bacterial Resistance with Low Cytotoxicity

New and more aggressive antibiotic resistant bacteria arise at an alarming rate and represent an ever-growing challenge to global health care systems. Consequently, the development of new antimicrobial agents is required to overcome the inefficiency of conventional antibiotics and bypass treatment limitations related to these pathologies. In this study, we present a synthesis protocol, which was able to entrap tetracycline antibiotic into silica nanospheres. Bactericidal efficacy of these structures was tested against bacteria that were susceptible and resistant to antibiotics. For nonresistant bacteria, our composite had bactericidal efficiency comparable to that of free-tetracycline. On the other hand, the synthesized composites were able to avoid bacterial growth of resistant bacteria while free-tetracycline has shown no significant bactericidal effect. Finally, we have investigated the cytotoxicity of these nanoparticles against mammalian cells to check any possible poisoning effect. It was found that these nanospheres are not apoptosis-inducers and only a reduction on the cell replication rate was seen when compared to the control without nanoparticles.

Optimization of a silica supported bis(butylcyclopentadienyl)-zirconium dichloride catalyst for ethylene polymerization

A series of heterogeneous α-olefin polymerization catalysts were prepared by the immobilization of bis(butylcyclopentadienyl)zirconium dichloride, (nBuCp) 2 ZrCl 2 , on a commercial silica support (Grace 948) using different procedures. The preparation parameters, namely, silica activation temperature, grafting temperature, grafting time, and solvent, were evaluated in terms of metal content on silica and ethylene homopolymerization activity. Metal contents were determined by Rutherford back-scattering spectrometry (RBS). In the temperature activation range between 373 and 723 K, silica surface saturation in Zr was found to be around 0.34 wt.-% Zr/SiO 2 . However, polymer polydispersity is shown to decrease with increasing support activation temperature. A better control in the generation of the active surface species was achieved with thermal pretreatment temperatures close to 723 K. The grafting reaction was seen to be immediate. Longer grafting times or higher temperatures bore deactivated species. Practically all the systems were active in ethylene polymerization in the presence of MAO, but the highest yield was obtained after grafting at 353 K for 1 h in toluene solution, employing silica pretreated at 723 K.

Effects of ethylene polymerization conditions on the activity of SiO2-supported zirconocene and on polymer properties

The effects of polymerization conditions were evaluated on the production of polyethylene by silica-supported (n-BuCp)2ZrCl2 grafted under optimized conditions and cocatalyzed by methylaluminoxane (MAO). The Al : Zr molar ratio, reaction temperature, monomer pressure, and the age and concentration of the catalyst were systematically varied. Most reactions were performed in toluene. Hexane, with the addition of triisobutilaluminum (TIBA) to MAO, was also tested as a polymerization solvent for both homogeneous and heterogeneous catalyst systems. Polymerization reactions in hexane showed their highest activities with MAO : TIBA ratios of 3 : 1 and 1 : 1 for the homogeneous and supported systems, respectively. Catalyst activity increased continuously as Al : Zr molar ratios increased from 0 to 2000, and remained constant up to 5000. The highest activity was observed at 333 K. High monomer pressures (≈ 4 atm) appeared to stabilize active species during polymerization, producing polyethylenes with high molecular weight (≈ 3 × 105 g mol−1). Catalyst concentration had no significant effect on polymerization activity or polymer properties. Catalyst aging under inert atmosphere was evaluated over 6 months; a pronounced reduction in catalyst activity [from 20 to 13 × 105 g PE (mol Zr h)−1] was observed only after the first two days following preparation.

Stabilization and solidification of Pb in cement matrices.

Pb was incorporated to a series of cement matrices, which were submitted to different testes of solidified/stabilized product. The leaching behaviors of aqueous solution were monitored by graphite furnace atomic absorption spectroscopy (GF-AAS). The mechanical strengths were evaluated by unconfined compressive strength (UCS) at 7 and 28 ages. Data are discussed in terms of metal mobility along the cement block monitored by X-ray fluorescence (XRF) spectrometry. Complementary techniques, namely, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), small angle X-ray scattering (SAXS) and X-ray diffraction spectroscopy (XRD) were employed in the characterization of the modified matrices. The Pb incorporated matrices have shown that a long cure time is more suitable for avoiding metal leaching. At pH 8 lower Pb leaching took place both for both short and long cure time. For a longer cure period there is a decreasing in the compressive strength. TGA and DRIFTS analyses show that the resistance fall observed in the UCS tests in the sample with Pb are not caused by hydration excess. XRF analyses show that there is a lower Ca concentration in the matrix in which Pb was added.

Ethylene and 1-butene copolymerization catalyzed by a Ziegler–Natta/Metallocene hybrid catalyst through a 23 factorial experimental design

In this work, a 2 3 factorial experimental design for the evaluation of ethylene– 1-butene copolymerization was employed. The following reaction parameters were used as independent variables: catalyst type, Al/Ti molar ratio and 1-butene concentration. The copolymerization was carried out using hybrid Ziegler – Natta/Metallocene catalysts with different titanium molar ratios. The catalyst activity and polymer characteristics were statistically analyzed through response surface methods. It was found that the catalyst type has a significant effect on activity, melt flow index and 1-butene content. Copolymers presented crystallinity values ranging from 46 to 58% and melt temperature in the 128– 131 8C range. Copolymer comonomer content varied from 2 to 6% in weight. q 2002 Published by Elsevier Science Ltd.

Selective silica‐based sorbent materials synthesized by molecular imprinting for adsorption of pharmaceuticals in aqueous matrices

The presence of pharmaceuticals in aqueous environmental matrices often requires efficient and selective preconcentration procedures. Thus, silicas (SILs) were synthesized by a molecular imprinting technique using an acid-catalyzed sol-gel process and the following drugs as templates: fluoxetine, gentamicin, lidocaine, morphine, nifedipine, paracetamol, and tetracycline. The materials were subjected to sorbent extraction assisted by ultrasonic treatment to remove the drugs and the consequent formation of molecular imprinted cavities. The surface area of the resulting materials ranged from 290 to 960 m(2)/g. Adsorption tests were performed with the molecular imprinting phases. In terms of the potential selectivity, the SILs were subjected to the adsorption of drugs from samples such as potable and surface water. The adsorption capacity remained in the range between 55 and 65% for both matrices, while for the nonimprinted SIL it remained between 15 and 20%.