Hybrid zeolitic-mesostructured materials as supports of metallocene polymerization catalysts

摘要

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/AI 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 C 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, ~(27)A1-MAS-NMR, ICP-atomic emission spectroscopy and UV-vis spectroscopy.Catalysts supported over hybrid ZSM-5/AI-MCM-41 (Si/AI = 30-60) exhibited the best catalytic activity followed by those supported on Al-MCM-41 (Si/AI = 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.