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

Abstract

Isolated hafnium (Hf) sites were prepared on Silicalite-1 and SiO 2 and investigated for acetone conversion to isobutene. Characterization by IR, 1 H MAS NMR, and UV−vis spectroscopy suggests that Hf atoms are bonded to the support via three O atoms and have one hydroxyl group, i.e, (SiO) 3 Hf−OH. In the case of Hf/Silicalite-1, Hf−OH groups hydrogen bond with adjacent Si−OH to form (SiO) 3 Hf−OH•••HO−Si complexes. The turnover frequency for isobutene formation from acetone is 4.5 times faster over Hf/Silicalite-1 than Hf/SiO 2 . Lewis acidic Hf sites promote the aldol condensation of acetone to produce mesityl oxide (MO), which is the precursor to isobutene. For Hf/SiO 2 , both Hf sites and Si−OH groups are responsible for the decomposition of MO to isobutene and acetic acid, whereas for Hf/Silicalite-1, the (SiO) 3 Hf−OH•••HO−Si complex is the active site. Measured reaction kinetics show that the rate of isobutene formation over Hf/SiO 2 and Hf/Silicalite-1 is nearly second order in acetone partial pressure, suggesting that the rate-limiting step involves formation of the C−C bond between two acetone molecules. The rate expression for isobutene formation predicts a second order dependence in acetone partial pressure at low partial pressures and a decrease in order with increasing acetone partial pressure, in good agreement with experimental observation. The apparent activation energy for isobutene formation from acetone over Hf/SiO 2 is 116.3 kJ/mol, while that for Hf/Silicalite-1 is 79.5 kJ/mol. The lower activation energy for Hf/Silicalite-1 is attributed to enhanced adsorption of acetone and formation of a C−C bond favored by the H-bonding interaction between Hf−OH and an adjacent Si−OH group.

The synergistic effect of Hf-O-Ru bonds and oxygen vacancies in Ru/HfO2 for enhanced hydrogen evolution

“…Evidently, the V O -Ru/HfO 2 -OP catalyst prepared with a molar ratio of Ru to Hf of 1:1, an annealing temperature of 750 °C, the 50 mg PVP, and the ratio of O to P of 4: 50 showed the best electrocatalytic activity for HER (Supplementary Figs. [16][17][18][19].…”

“…However, it had been applied in thermal catalysis. As a Lewis acid site, isolated Hf facilitates acetone conversion to isobutene 19 . Pd/ HfO 2 has been reported to be highly active for methane combustion 20 .…”

Facile and fast synthesis of highly active Lewis acid MWW zeolite from pure silica ITQ-1

“…Specifically, the appearance of the 3510 cm −1 band suggests the formation of hydroxyl nests in the deboronated B-MWW zeolite. 36,42,43 After the hydrothermal regrowth, the substantial decrease in the intensity of the bands at 3510 cm −1 indicates that the Si-OH nests have healed significantly (Fig. 5b).…”

“…5b). 35,42 As mentioned, traditionally, the regrowth way for preparing Sn-MWW from deB-MWW requires high SDA (SDA/Si > 1.0) or a long cycle (>10 d). 22,23 In our new method, the SDA consumption is much lower and the growth is much faster.…”

Catalytically active Rh species stabilized by zirconium and hafnium on zeolites

“…UV-visible (UV-vis) spectroscopy indicates that Hf/S-1 exhibits absorption bands at 200 nm and 220–260 nm, which are ascribed to the isolated Hf( iv ) and HfO x clusters, respectively. 39–41 The Rh/S-1 sample shows absorption bands at 260 nm and 320 nm, ascribed to the Rh–O charge transfer and the 1 A 1g → 1 T 1g transition, respectively. 42 Thus, the absorption bands of RhHf/S-1 at 200 nm and 260 nm can be the superposition of two different bands (Fig.…”