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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 CeO2crystallites 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.

The conversion of C2–4carboxylic (acetic, propionic, and n-butyric) acids on HZSM-5 was studied at 350 °C. The reaction proceeds through ketonization (to acetone, 3-pentanone, and 4-heptanone, respectively), aldol condensation, and tertiary reactions to aromatics-rich hydrocarbons. The product distribution is influenced by the carbon chain length of carboxylic acids due to the shape selectivity of ZSM-5. The turnover frequency (TOF) of ketonization of carboxylic acids decreases while the TOF of aldol condensation of corresponding ketones increases with increasing carbon chain length. Langmuir–Hinshelwood mechanism analysis shows that the true activation enthalpy of both ketonization and aldol condensation increases with increasing carbon chain length, due to the increased steric hindrance for the formation of a C–C bond. However, the true Gibbs free activation energy increases for ketonization but decreases for aldol condensation with increasing chain length, due to a greater contribution of activation entropy of aldol condensation than that of ketonization.

Ketonization of biomass-derived propionic acid was investigated at 270–350 °C over CexZr1–xO2mixed oxides with varying Ce/Zr ratios. The acidity of CexZr1–xO2increases as Zr content is increased. The density of the strong base sites is reduced with increasing Zr content, while the density of the medium-strength base sites is maximized on Ce0.5Zr0.5O2. Diffuse reflectance infrared spectroscopy of propionic acid adsorption indicated that increasing Zr content in CexZr1–xO2favors monodentate adsorption over bidentate adsorption. The reaction rate varies with the composition of CexZr1–xO2. The intrinsic reaction rate can be correlated with the medium-strength acid/base ratio. The rate maximizes at a medium-strength acid/base ratio of ∼1.07 on Ce0.1Zr0.9O2. This value is close to 1, indicating that the balanced medium-strength acid–base sites play a crucial role in the ketonization. The balanced acid–base property influences propionic acid adsorption, enolate formation, and C–C coupling and therefore affects the ketonization rate.

Photocatalytic CO2reduction is a promising route to simultaneously address two critical issues related to the energy and environment. Herein, a direct Z-scheme photocatalytic system, Ti3+defective TiO2/CdS nanoflowers, was fabricated to enhance the production of C1–C3 products from CO2reduction. First, nanoflower-like TiO2(NF-TiO2) is prepared using a TiB2precursor to build in Ti3+defects. Then, the NF-TiO2/CdS heterojunctions are constructed viathe sulfurization of TiO2/Cd-MOF. Importantly, the optimal heterojunction photocatalyst NF-TiO2/CdS0.6exhibits a highly promoted CO2reduction activity not only for CO and CH4but also for C2–C3 hydrocarbons. The cumulative yields of CO and CH4are 1007.0 μmol/g and 83.5 μmol/g (2.5 h), respectively, and a yield of 55.3 μmol/g (2.5 h) of C2–C3 hydrocarbons is obtained. This enhanced yield of C1–C3 products is attributed to the synergistic effects of abundant Ti3+defects that increase adsorption and activation of CO2, the large surface area that provides sufficient active sites, and a direct Z-scheme heterojunction with a good intimate contact interface that effectively separates charge carriers and accumulates electrons with strong reducibility. Moreover, the photoluminescence analysis of hydroxyl radicals was conducted to provide evidence for the direct Z-scheme mechanism. This work provides a facile method for the construction of a TiO2-based direct Z-scheme photocatalytic system for improving photocatalytic performance toward CO2reduction.