Mechanism and Kinetics of Acetone Conversion to Isobutene over Isolated Hf Sites Grafted to Silicalite-1 and SiO₂

Isolated hafnium (Hf) sites were prepared on Silicalite-1 and SiO2 investigated for acetone conversion to isobutene. Characterization by IR, 1H MAS NMR, UV-vis spectroscopy suggests that Hf atoms are bonded the support via three O have one hydroxyl group, i.e, (?SiO)3Hf-OH. In case of Hf/Silicalite-1, Hf-OH groups hydrogen bond with adjacent Si-OH form (?SiO)3Hf-OH···HO-Si? complexes. The turnover frequency isobutene formation from is 4.5 times faster over Hf/Silicalite-1 than Hf/SiO2. Lewis acidic promote aldol condensation produce mesityl oxide (MO), which precursor For Hf/SiO2, both responsible decomposition MO acetic acid, whereas complex active site. Measured reaction kinetics show rate Hf/SiO2 nearly second order in partial pressure, suggesting rate-limiting step involves C-C between two molecules. expression predicts a dependence pressure at low pressures decrease increasing good agreement experimental observation. apparent activation energy 116.3 kJ/mol, while 79.5 kJ/mol. lower attributed enhanced adsorption favored H-bonding interaction an group.