Examinando por Autor "Paredes, B."

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Ethylene polymerization over supported MAO/(nBuCp)2ZrCl2 catalysts:Influence of support properties.
(ELSEVIER, 2007) Van Grieken, R.; Carrero, A.; Suarez, I.; Paredes, B.
The catalytic system methylaluminoxane (MAO) and bis(n-butylcyclopentadienyl)zirconium dichloride ((nBuCp)2ZrCl2) was immobilized on commercial silica, silica-alumina and aluminophosphate calcined at different temperatures. The properties of the supports were determined by using N2 adsorption-desorption isotherms at 77 K, FT-IR spectroscopy and SEM. After aluminium and zirconium impregnation, the catalysts were analyzed by ICP-AES, FT-IR and UV-Vis spectroscopy. Ethylene polymerizations were carried out in a Schlenk tube at 70 ºC and 1.2 bar of ethylene pressure. The polyethylene obtained was characterized by GPC, DSC and SEM. Catalysts supported on silica-alumina exhibited higher polymerization activity than those supported on silica and aluminophosphate. Besides, the activity of MAO/(nBuCp)2ZrCl2 catalytic system supported on silica-alumina and aluminophosphate decreased strongly with support calcination temperature, while remained almost constant when silica was employed as support. All these experimental features suggest a role of the support acid properties and hydroxyl group population in the generation of active polymerization species.
Hybrid zeolitic-mesostructured materials as supports of metallocene polymerization catalysts
(ELSEVIER, 2012) Carrero, A.; Grieken, Rafael van; Paredes, B.
The potential application of hybrid ZSM-5/Al-MCM-41 zeolitic-mesostructured materials as supports of metallocene polymerization catalysts has been investigated and compared with the behaviour of standard mesoporous Al-MCM-41 and microporous ZSM-5 samples. Hybrid zeolitic-mesostructured solids were prepared from zeolite seeds obtained with different Si/Al molar ratios (15, 30 and 60), which were assembled around cetyltrimethylammonium bromide (CTAB) micelles to obtain hybrid materials having a combination of both zeolitic and mesostructured features. (nBuCp)2ZrCl2/MAO catalytic system was impregnated onto the above mentioned solid supports and tested in ethylene polymerization at 70 Cº and 5 bar of ethylene pressure. Supports and heterogeneous catalysts were characterized by X-ray powder diffraction, nitrogen adsorption-desorption isotherms at 77 K, transmission electron microscopy, 27Al-MAS-NMR, ICP-atomic emission spectroscopy and UV-vis spectroscopy. Catalysts supported over hybrid ZSM-5/Al-MCM-41 (Si/Al = 30-60) exhibited the best catalytic activity followed by those supported on Al-MCM-41 (Si/Al = 30-60). However, catalyst supported on ZSM-5 gave lower polymerization activity because of its microporous structure with narrower pores and lower textural properties than hybrid and mesoporous materials. Although higher acid site population shown by hybrid materials could contribute to the stabilization of the metallocene system on the support, in this case their better catalytic performance is mainly ascribed to the larger textural properties.