Al-modified Pd@mSiO2 core-shell catalysts for the selective hydrodeoxygenation of fatty acid esters: Influence of catalyst structure and Al atoms incorporation

Abstract

The development of robust catalysts for selective hydrodeoxygenation (HDO) of fatty acid esters is key for production of the diesel-range alkanes under mild conditions. Here, we report a stable and efficient HDO catalyst (Pd@Alx-mSiO2) that composed of inner uniformly dispersed metallic palladium core and the outer mesoporous silica shell doped with aluminum atoms. Using methyl palmitate as a model compound, Al-modified Pd@mSiO2 catalyst (Pd@Al3-mSiO2) exhibited higher catalytic performance (98% conversion rate and 99% selectivity towards diesel-range alkanes) as compared with the Pd@mSiO2 and conventional Pd/γ-Al2O3 catalysts (35% and 70% conversion rates, respectively) at 260 °C and 3.0 MPa H2. Further, when using vegetable oils such as soybean and palm oils as raw materials, high yields of diesel-range alkanes (> 80 wt%) can be obtained under the mild conditions. The HDO reaction pathway was more dominant than the decarbonylation pathway when the reaction was catalyzed by Pd@Al3-mSiO2 catalyst, thereby reducing the loss of carbon atoms. Detailed characterization (27Al NMR, NH3-TPD, and in situ Py-FTIR) suggests that the incorporation of aluminum atoms brings not only Lewis acid sites, but also Brønsted acid sites via the formation of SiOHAl bonds. The synergy between the metallic Pd, Lewis- and Brønsted-acid sites is responsible for its high HDO activity under the mild conditions. Additionally, the core-shell structure enables fatty aldehyde intermediate preferentially adsorbed on the aluminum atoms in the outer silica shell and avoids direct contacting with metallic Pd, which inhibits the cleavage of CC bonds to some extent. On the other hand, due to the protective effect of outer silica shell that inhibits the leaching and agglomeration of metallic Pd, the synthesized Pd@Al3-mSiO2 catalyst showed good stability with a slight loss (conversion decreased from 98% to 90%) over five cycles.