Rh/TiO2-Catalyzed Tandem Hydroformylation–Aldol Condensation of Propylene for the Synthesis of 2-Ethyl-2-hexenal: Experimental and DFT Study

To address the issues of complicated procedures and high costs associated with the conventional two-step synthesis of 2-ethyl-2-hexenal (2E2H), this paper develops a novel route for the one-step synthesis of this high-value intermediate via the tandem hydroformylation–aldol condensation of propylene. A series of supported Rh-based bifunctional catalysts were prepared using metal oxides as supports, and the catalytic performance of different supports in the tandem system was systematically investigated. Through TEM, SEM, XRD, and XPS characterizations, combined with DFT calculations, the polarization and activation mechanism of the C=O bond of n-butyraldehyde by TiO2 was elucidated. Under optimized conditions (90 °C, 4 MPa, 6 h), the as-prepared Rh/TiO2 catalyst achieved a propylene conversion of 49.2%, a total butyraldehyde selectivity of 75.8%, and 2-ethyl-2-hexenal selectivity of 24.2%. Furthermore, the catalyst exhibited good recycling stability and could be reused for five cycles without significant loss of activity. This work provides an effective strategy for the value-added upgrading of propylene resources and the design of tandem catalytic systems.