Reduction of the Phillips catalyst by various olefins: Stoichiometry, thermochemistry, reaction products and polymerization activity

Abstract

The adsorption, reaction and desorption of ethylene or hexenes on Cr(VI)/SiO 2 containing 1 or 3 weight% chromium was monitored by thermogravimetry, differential scanning calorimetry and mass spectrometry. Reaction of Cr(VI)/SiO 2 with ethylene at a temperature of 200 °C always included some oligomerization, making it impossible to identify the stoichiometry of the initial redox reaction. Reaction of Cr(VI)/SiO 2 with 1-hexene or cyclohexene at 150 °C indicated an adsorption stoichiometry of one olefin per chromium. Heats of reaction were consistent with oxidation of the olefin to an aldehyde or ketone, or even to a carboxylic acid or ester. Oxidation products were retained on the surface. Thermal desorption of surface products resulted in CO 2 evolution at temperatures of 265-285 and 390-415 °C; with the co-products indicating decomposition of formate and acetate species, respectively. Reduction of Cr(VI) with ethylene led to sites active for ethylene polymerization after thermal desorption of the redox products, whereas reduction with hexenes did not generate active sites irrespective of thermal desorption of products.

Ligands Make the Difference! Molecular Insights into CrVI/SiO2 Phillips Catalyst during Ethylene Polymerization

“…In summary, the first relevant result emerging from our spectroscopic study is the identification, during the reduction of Cr VI /SiO 2 in ethylene, of 6-fold-coordinated Cr II species in interaction with a methylformate molecule. This novel concept is in very good agreement with recent findings by McDaniel and co-workers, who demonstrated that under commercial ethylene polymerization conditions oxygenates may remain attached to the chromium sites . The next questions are whether these hindered Cr II sites are involved in ethylene polymerization and whether methylformate remains attached to the Cr sites during ethylene polymerization, participating in the whole reaction.…”

“…Our new spectroscopic evidence reveals that the chromium site involved in ethylene polymerization is a divalent ion, 6-fold coordinated, and in interaction with an external nucleophilic ligand (mostly methylformate). This study provides the first spectroscopic identification, at the molecular level, of the oxygenated species adsorbed on the catalyst during the induction period inferred by indirect TG, DSC, and MS experiments . We also introduce the important concept that oxygenates remain in the Cr coordination sphere also during ethylene polymerization.…”

“…This study provides the first spectroscopic identification, at the molecular level, of the oxygenated species adsorbed on the catalyst during the induction period inferred by indirect TG, DSC, and MS experiments. 107 We also introduce the important concept that oxygenates remain in the Cr coordination sphere also during ethylene polymerization. Additionally, we demonstrate that ethylene polymerization occurs also if a fraction of the chromate sites is not reduced, implying that Cr VI sites might have a role in the first steps of the catalysis in cooperation with the active Cr II sites, as already proposed in the literature.…”

“…This novel concept is in very good agreement with recent findings by McDaniel and co-workers, who demonstrated that under commercial ethylene polymerization conditions oxygenates may remain attached to the chromium sites. 107 The next questions are whether these hindered Cr II sites are involved in ethylene polymerization and whether methylformate remains attached to the Cr sites during ethylene polymerization, participating in the whole reaction. DR UV−vis−NIR and FT-IR spectroscopies help in answering these questions.…”

Comparison of Support Effects on Phillips and Metallocene Catalysts

“…At about 400 • C, the chromium is oxidized into the hexavalent form, which spreads out and becomes anchored onto the support as chromate or dichromate surface esters [3,[5][6][7][8][9][10][11][12][13][14]. These species are then reduced by ethylene to a lower valence, expanding the potential coordination sphere, alkylating the Cr, and forming the coordinatively unsaturated active sites [3][4][5][15][16][17][18][19][20]. Consequently, and somewhat unusually for inorganic industrial catalysts, the active sites are individually attached to the support.…”