Production of jet-fuel precursors from volatile fatty acids using metal oxide-supported zeolitic catalysts

Abstract

This study investigates the vapor-phase ketonization of hexanoic acid (HA) to produce 6-undecanone (6-UN), a key intermediate for sustainable aviation fuels (SAF), using bifunctional catalysts composed of metal oxides supported on acidic zeolites. A screening of metal oxides (TiO₂, ZrO₂, CeO₂) supported on H-ZSM-5 identified TiO₂ as the most active phase. Although HA conversion decreased over time on stream (TOS), 6-UN selectivity increased up to 90%, due to the deactivation of Brønsted acid sites (BAS) that minimizes side reactions like cracking and aromatization. In contrast, Lewis acid sites (LAS), essential for ketonization, remained more stable. The effect of supporting TiO₂ on different zeolites (H-ZSM-5, H-β, and USY) was also evaluated. TiO₂/H-β showed the highest 6-UN yield due to its high LAS concentration and large mesoporous surface area (271 m² g⁻¹), highlighting the importance of acidity and textural properties. TiO₂/H-β also demonstrated strong stability when tested with crude bio-HA derived from grape pomace fermentation. Additionally, Na-exchanged H-ZSM-5 confirmed that removing BAS improved ketonization performance, though not as effectively as TiO₂/H-β, reinforcing the latter's superior catalytic behavior.