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

• Cr(VI)/SiO2 reduced by ethylene and hexenes.
• Olefin oxidation products are retained on the surface of Cr/SiO2.
• Thermal decomposition products CO2 and H2 suggest formation of formate.
• Thermal decomposition products CO2 and CH4 suggest formation of acetate.
• Reduction of Cr(VI) by hexenes does not necessarily produce active sites.

The adsorption, reaction and desorption of ethylene or hexenes on Cr(VI)/SiO2 containing 1 or 3 wt.% chromium were monitored by thermogravimetry, differential scanning calorimetry and mass spectrometry. Reaction of Cr(VI)/SiO2 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)/SiO2 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 CO2 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.

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