Ketonization reactions provide a feasible approach to remove oxygen and increase carbon chain length for conversion of biomass derived carboxylic acids. However, the reaction suffers from fast catalyst deactivation and low ketone selectivity. In this work, Lewis acidic heteroatom Ti-, Zr-, Ce-, and Sn-Beta zeolites were prepared using a twostep post-synthesis method and applied in vapor phase ketonization of propionic acid at 350 °C. Among these zeolites, Zr-Beta shows both the highest activity and selectivity. Characterizations indicate that Zr prefers the vacant tetrahedral site when the Zr content is < 7%, corresponding to the maximal fraction of vacant sites produced from dealumination of the parent H-Beta with a Si/Al ratio of 19. Extraframework Zr may also form at a Zr content ≥ 7%. The tetrahedrally coordinated framework Zr species (mainly in the structure of open sites) show Lewis acidic characteristic, and their density can be linearly correlated with the ketonization activity, indicating that these sites are active sites for ketonization. In contrast to rapid deactivation of H-Beta, the 7% Zr-Beta is stable for ketonization over 60 h, maintaining a conversion of ∼50% and a 3-pentanone selectivity > 96% at a space time of 2 h. The amount of coke deposition on Zr-Beta is about 1/3 of that on H-Beta, and the structure of Zr-Beta is preserved after 60 h of reaction. The results of this work indicate that Zr-Beta zeolite is a promising catalyst for ketonization with good stability and high selectivity toward ketone.

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CeO2–UiO octahedron catalysts derived from cerium-based metal–organic frameworks Ce–UiO-66 were synthesized for vapor-phase ketonization of propionic acid. Characterizations showed the CeO2–UiO octahedron is assembled from nanosized CeO2 crystallites with mesopores. XPS and Raman results indicated that more oxygen vacancies are formed in CeO2–UiO-450 catalyst than in CeO2–P prepared by a precipitation method. Intrinsic ketonization rates on CeO2–UiO-450 are improved relative to CeO2–P. At 350 °C, the turnover frequency based on acid–base pair on CeO2–UiO-450 (7.45 s–1) is 1.38 times higher than that on CeO2–P (5.41 s–1). Consistently, the activation energy is lowered from 130.5 kJ/mol for CeO2–P to 109.0 kJ/mol for CeO2–UiO-450. Infrared spectroscopy results showed that monodentate carboxylate is the active adsorption configuration, and its consumption is much faster on CeO2–UiO-450 than on CeO2–P. A linear correlation between the concentration of oxygen vacancy and intrinsic ketonization rate is found, indicating that the oxygen vacancy promotes ketonization on CeO2.

CeO2 rods, octahedrons, and cubes exposing well-defined (110), (111), and (100) surfaces, respectively, were synthesized and investigated for the catalytic ketonization of propionic acid. The intrinsic ketonization rates at 350 °C on the rods, octahedrons, and cubes are 54.3, 40.4, and 25.1 mmol·m–2·h–1, respectively, indicating that the (110) facet is the most active surface for ketonization. The reaction was tracked by both in situ infrared and mass spectroscopies under transient conditions, and the results showed that monodentate propionate, a minority surface species, is responsible for the formation of 3-pentanone. In contrast, bidentate propionate, a dominant species on all three surfaces, appears to a spectator for ketonization. Moreover, the ketonization activity can be correlated with relative concentration of monodentate propionate. A density functional theory study showed that the relative concentration of monodentate propionate (or the adsorption energy difference between monodentate and bidentate configurations) at high coverages is strongly dependent on the surface geometry. The stability of monodentate propionate on the (110) surface exposing both the O and Ce sites in the outermost layer with the well-separated Ce sites exhibits little dependence on the propionate coverage. In contrast, strong steric hindrance due to the top layer O atom and the closely packed Ce atoms in (111) destabilizes monodentate propionate significantly at high coverages. This study demonstrates that the surface geometrical structure of CeO2 can determine the abundance of the active monodentate propionate, which, in turn, will determine the catalytic activity of CeO2 for ketonization.