A proposed mechanism for silica supported chromium HDPE catalyst activation

The decomposition of Cr-3(C2H3O2)(7)(OH)(2)/SiO2 in oxidizing, inert, and reducing environments was studied using isothermal kinetics, temperature-programmed reaction (TPRxn), and variable-temperature diffuse-reflectance infrared spectroscopy (VT-DRIFTS). Based upon these results a mechanism is proposed for the activation of silica-supported basic chromium acetate, The decompositions in N-2 and CO/N-2 mixtures appear very similar The activation energies are within experimental error, and both exhibit second-order rate dependencies with respect to the surface acetate species. TPRxn results show that the Cr compound influences support dehydroxylation, but VT-DRIFT spectra show no evidence of Cr bonding to the SiO2 surface, When CO is present, silica dehydroxylation appears to proceed via a water-gas shift type of reaction producing CO2 and H-2 rather than H2O. In oxygen, Cr compound decomposition occurs at temperatures 90 degrees C lower than in inert or reducing environments. The reaction orders are 1/2 for oxygen and 1 for the surface acetate species. The activation energy is comparable to that calculated for the other two media. VT-DRIFT spectra show that oxygen induces a greater degree of hydroxyl removal and formation of Cr-O/Cr=O bonds concurrent with and subsequent to recorded decomposition temperatures. In this way they support TPRxn findings and suggest Si-O-Cr bond formation. It appears that the rate-limiting step in the activation mechanism is removal of the acetate methyl group. In oxygen this involves dissociative activation of oxygen on the Cr center and subsequent combustion. In the other environments migration is necessary to allow hydrogen transfer between two acetate groups to form methane and leave behind a hydrocarbon fragment, This explains the different temperatures of decomposition in the various media and the second-order rate dependence upon acetate in N-2 and CO/N-2. The lack of apparent Cr-SiO2 bond formation in nonoxidizing environments allows surface migration of Cr compounds which makes this mechanism feasible. (C) 1996 Academic Press, Inc.