Structural and electronic effects in metallocene catalysts studied by x‐ray techniques

Abstract

A series of metallocenes, namely, [Cp 2 ZrCl 2 ], [(MeCp) 2 ZrCl 2 ], [(nBuCp) 2 ZrCl 2 ], [(iBuCp) 2 ZrCl 2 ], [(tBuCp) 2 ZrCl 2 ], [Et(Ind) 2 ZrCl 2 ], [Et(IndH 4 ) 2 ZrCl 2 ], and [MeSi 2 (Ind) 2 ZrCl 2 )] were analyzed by extended x-ray absorption fine structure (EXAFS) and x-ray photoelectron spectroscopy (XPS). Complementary techniques, UV-vis spectroscopy and cyclic and differential pulse voltammetry, were employed to characterize the organometallic complexes. The catalysts were evaluated in ethylene polymerization, having methylaluminoxane (MAO) as the cocatalyst, and the resulting polymers were characterized by gel permeation chromatography. The structural and electronic effects caused by the coordination sphere around the metal center and their effects on the catalytic activity and polymer characteristics are discussed.

The Role of the Support in the Performance of Grafted Metallocene Catalysts

“…However, the textural properties of the support do not only affect catalyst activity by means of steric effects. Cp 2 ZrCl 2 and (nBuCp) 2 ZrCl 2 possess Zr–C interatomic distances of 2.34 and 2.35 Å, respectively 39. In the corresponding supported systems, this distance contracts to between 1.5 and 2.2 Å, which suggests that the pore diameter of the support may influence the structure of metallocene catalyst within the pores.…”

Catalisadores metalocênicos suportados para a produção de poliolefinas: revisão das estratégias de imobilização

“…Haag et al 67,68 mostraram que o metal de transição sofre uma redução de densidade eletrônica quando ligado ao oxigênio do suporte. Além disso, Silveira 90 e Silveira et al 69,70 mostraram em seus estudos que a distância Zr-C é reduzida quando o metaloceno é imobilizado, provavelmente, para compensar a redução da densidade eletrônica do Zr, como mostrado por Haag et al. 67,68 Certamente, e do ponto de vista eletrônico apenas, esse efeito de redução da densidade eletrônica do Zr pela ligação com o oxigênio do suporte, que agora faz parte da esfera de coordenação do metaloceno, leva a uma maior força de ligação entre o Zr e o metileno do polímero em crescimento, aumentando a barreira energética para inserção de olefinas coordenadas em relação ao caso homogêneo.…”

X-ray Spectrometry

“…That is, SQUID 2 acts as a low-noise preamplifier. The developed a mini-calorimeter offered an energy resolution of 40 eV for an 55 Fe source and 200 eV for an 241 Am source, respectively. As typically represented by roentgenograms, X-ray imaging using X-ray film has played an important role in the history of X-ray applications.…”

“…Among these, metallocenes of group 4, especially Zr, are active in catalytic olefin polymerization. Since the coordination sphere around the metal center is responsible for the structural and electronic effects, which in turn affect the activity and selectivity of the catalyst, Silveira et al (241) analyzed the structure of various Zr metallocenes using EXAFS and X-ray photoelectron spectroscopy. They found that electronic effects are important for the catalytic activity of unbridged metallocenes, but structural factors represented by Zr-C interatomic distance do not play a key role.…”