IR investigation of polymerization centres of the phillips catalyst

The Nature of Surface CrOx Sites on SiO2 in Different Environments

“…The EPR spectrum of (:SiO) 4 Cr ?2 2 taken before exposure to ethylene showed a weak signal for Cr ?3 , suggesting that the minor amount of Cr ?3 is most probably responsible for the polymerization activity [30]. Similar high initial polyethylene polymerization activity was also found for the mononuclear (:SiO) 3 Cr ?3 model compound consistent with the role of Cr ?3 sites for ethylene polymerization [28]. Furthermore, the same initial ethylene polymerization rate was also obtained with a traditional CO-activated supported CrO x /SiO 2 catalyst consistent with the role of surface Cr ?3 sites for ethylene polymerization [32].…”

“…Additionally, as seen in the corresponding EXAFS, these surface sites are not Cr ?2 dimers because there is no signal for Cr-Cr in the second coordination shell [15]. The fraction of surface Cr sites interacting with ethylene has previously been proposed to vary from 10 to 55 % depending on the probe method [27][28][29], but these claims were based on indirect measurements and are not supported by this new direct XAS measurement indicating that all the dispersed Cr sites are involved [15]. The difference in the states of the surface CrO x sites on silica upon CO reduction and C 2 H 4 reduction were further demonstrated by adsorbing ethylene on a catalyst that had been pre-reduced with CO. Adsorption of ethylene on the Cr finger print in the pre-edge region.…”

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides

“…Instead, IR spectroscopy has shown to be a very powerful technique due to its ability to discriminate between different surface Cr 2+ species. [69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85] This is illustrated in Figure 9 for CO reduced Cr/SiO 2 catalysts. 77 Three families of anchored Cr 2+ ions have been singled out (labeled as A, B, and C), after chemisorption of CO.…”

“…In addition, the interaction between supported Cr 2+ and ethylene can be followed by IR spectroscopy. Detailed IR studies were carried out by the group of Zecchina [70][71][72][75][76]85,[133][134] and by Rebenstorf and coworkers. [79][80][81][82][83] Finally, oxygen causes reoxidation of reduced Cr to Cr 6+ with a bright chemiluminescence.…”

Selective Catalysis and Nanoscience: An Inseparable Pair

“…[134,136] IR spectroscopy of probe molecules has been preferentially used to discriminate among the different Cr II sites on the silica surface. [136,[166][167][168][169][170][171] The conclusion arising from all of these works is that on the CO pre-reduced A C H T U N G T R E N N U N G Phillips catalyst a distribution of Cr II sites is present, all active in the ethylene polymerization, but characterized by a different polymerization rate. The whole process of Cr VI anchoring, surface activation and successive CO reduction are represented in Figure 9b.…”