Effect of Crystal Size on Acetone Conversion over SAPO-34 Crystals

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 and investigated for acetone conversion to isobutene. Characterization by IR, ¹H 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)₃Hf-OH. In the case of Hf/Silicalite-1, Hf-OH groups hydrogen bond with adjacent Si-OH to form (≡SiO)₃Hf-OH···HO-Si≡ complexes. The turnover frequency for isobutene formation from acetone is 4.5 times faster over Hf/Silicalite-1 than Hf/SiO₂. Lewis acidic Hf sites promote the aldol condensation of acetone to produce mesityl oxide (MO), which is the precursor to isobutene. For Hf/SiO₂, 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)₃Hf-OH···HO-Si≡ complex is the active site. Measured reaction kinetics show that the rate of isobutene formation over Hf/SiO₂ 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/SiO₂ 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.

Intensified Biobutanol Recovery by using Zeolites with Complementary Selectivity

A vapor-phase adsorptive recovery process is proposed as an alternative way to isolate biobutanol from acetone-butanol-ethanol (ABE) fermentation media, offering several advantages compared to liquid phase separation. The effect of water, which is still present in large quantities in the vapor phase, on the adsorption of the organics could be minimized by using hydrophobic zeolites. Shape-selective all-silica zeolites CHA and LTA were prepared and evaluated with single-component isotherms and breakthrough experiments. These zeolites show opposite selectivities; adsorption of ethanol is favorable on all-silica CHA, whereas the LTA topology has a clear preference for butanol. The molecular sieving properties of both zeolites allow easy elimination of acetone from the mixture. The molecular interaction mechanisms are studied by density functional theory (DFT) simulations. The effects of mixture composition, humidity and total pressure of the vapor stream on the selectivity and separation behavior are investigated. Desorption profiles are studied to maximize butanol purity and recovery. The combination of LTA with CHA-type zeolites (Si-CHA or SAPO-34) in sequential adsorption columns with alternating adsorption and desorption steps allows butanol to be recovered in unpreceded purity and yield. A butanol purity of 99.7 mol % could be obtained at nearly complete butanol recovery, demonstrating the effectiveness of this technique for biobutanol separation processes.