Direct synthesis of 2-ethylhexanal via n-butanal aldol condensation–selective hydrogenation reaction integration over a highly stable Pd/TiO2 catalyst†

2-Ethylhexanal (2EH) is an important intermediate for the production of 2-ethylhexanoic acid (2-ETA), and studying the direct synthesis of 2EH through heterogeneous catalysis of n-butanal aldol condensation–selective hydrogenation reaction integration is of great significance for the industrial production of 2-ETA. Herein, a series of Pd/TiO₂ catalysts were prepared and characterized by XRD, CO pulse adsorption, TEM, CO₂/NH₃-TPD and H₂-TPR. Their catalytic performance for direct synthesis of 2EH from n-butanal was investigated in a stainless steel autoclave first. The results showed that Pd/TiO₂ prepared by the excessive impregnation method, loaded with 0.5 wt% of Pd and reduced at 400 °C, showed excellent catalytic performance: 95.4% conversion of n-butanal and up to 99.9% selectivity of 2EH. The particle size of Pd, the base strength and the amount of acid/base of the catalyst are the main factors affecting its catalytic performance. The smaller the particle size of Pd, the greater the base strength and the larger the amount of acid/base of the catalyst, the better its catalytic performance. Then the integrated reaction mechanism was established based on the results of characterization and FT-IR of online sampling. Furthermore, the continuity of the reaction process was achieved on a fixed-bed reactor. Under the reaction conditions of a liquid hourly space velocity (LHSV) of 7.2 h⁻¹, a reaction temperature of 190 °C and a H₂ pressure of 3.2 MPa, the conversion of n-butanal and the selectivity of 2EH were 91.2% and 89.8%, respectively. During the continuous operation for 300 h under optimal conditions, the conversion of n-butanal decreased slightly first and then stabilized, while the selectivity of the target product 2EH remained basically unchanged.