Selective ketonization of propionic acid on Fe-MFI zeolites: crucial roles of acid strength and density†

The ketonization reaction offers a convenient approach to remove oxygen and increase the carbon chain length of carboxylic acids without consuming H₂. Conventional zeolites with a strong Brønsted acid site (BAS) are very active for ketonization; however, they suffer from low ketone selectivity and fast deactivation owing to the facile secondary and tertiary reactions occurring on the strong BAS. Herein, a series of Fe-MFI zeolites (Si/Fe = 80–180) with a weaker BAS, i.e., Fe–OH–Si, than Al-MFI were prepared, characterized and tested for ketonization of propionic acid at 350 °C and atmospheric pressure. Compared with Al-MFI-180, although Fe-MFI-180 with its weaker BAS strength moderately reduces the activity for propionic acid ketonization (turnover frequency (TOF) of 6.97 and 3.80 min⁻¹, respectively), it significantly reduces the activity of secondary (aldol condensation) and tertiary (aromatics formation) reactions (3-pentanone conversion TOF of 1.85 and 0.33 min⁻¹, respectively), resulting in 3-pentanone as the dominant primary product as well as improved stability. The Fe-MFI zeolites not only showed high TOF (2.00–3.80 min⁻¹) but also improved the selectivity for 3-pentanone and enhanced stability compared to Al-MFI with strong BAS. These results demonstrate a strategy for weakening the strength of BAS of zeolites to reduce the activity of the secondary and tertiary reactions and thereby improve the selectivity and stability of ketonization of carboxylic acids.