(175g) Kinetic and Mechanism of Acetone Conversion to Isobutene over Hf/Silicalite-1 and Hf/SiO2

Hafnium (Hf)/Silicalite-1 nanosheets and Hf/SiO₂ catalysts were prepared and investigated for the reaction of acetone to isobutene. The dependence of activity on Hf loadings for the two catalysts differed. For Hf/SiO₂, the rate of isobutene formation increased linearly for Hf weight loadings below 2 wt%, and then increased further but at a diminishing rate. By contrast, no isobutene was formed on Hf/Silicalite-1 for a Hf loading < 2 wt%. However, for higher Hf loadings, the catalyst active increased more rapidly than that observed for Hf/SiO₂. This abrupt activity change is attributed to a change in the structure of the Hf species. The IR spectra of adsorbed pyridine show distinct differences, suggesting differences in the Lewis acid properties of Hf on Hf/SiO₂ and Hf/Silicalite-1. The dependence of the rate of isobutene formation over the two catalysts also differ. Hf/Silicalite-1 exhibits a second order dependence on acetone partial pressure, while the reaction order decreased to 1.6 for Hf/SiO₂. The apparent activation energies also differ for the two catalysts, the activation energy for Hf/Silicalite-1 being significantly lower than that for Hf/SiO₂. Further investigation of the mechanism of isobutene formation revealed that the first step is aldol condensation to form mesityl oxide and the second is the decomposition of this product to form isobutene and acetic acid, which is consumed within the reaction cycle. For Hf/Silicalite-1, isobutene formation is limited by the rate of aldol condensation on Hf sites, whereas for Hf/SiO₂, the decomposition of mesityl oxide limits the rate of isobutene formation.