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

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@Al-x-mSiO(2)) 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@mSiO(2) catalyst (Pd@Al-3-mSiO(2)) exhibited higher catalytic performance (98% conversion rate and 99% selectivity towards diesel-range alkanes) as compared with the Pd@mSiO(2) and conventional Pd/gamma-Al2O3 catalysts (35% and 70% conversion rates, respectively) at 260 ? and 3.0 MPa H-2. 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@Al-3-mSiO(2) catalyst, thereby reducing the loss of carbon atoms. Detailed characterization (Al-27 NMR, NH3-TPD, and in situ Py-FTIR) suggests that the incorporation of aluminum atoms brings not only Lewis acid sites, but also Bronsted acid sites via the formation of Si-OH-Al bonds. The synergy between the metallic Pd, Lewis-and Bronsted-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 C-C 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@Al-3-mSiO(2) catalyst showed good stability with a slight loss (conversion decreased from 98% to 90%) over five cycles.

Automated summary

A stable and efficient catalyst for selective hydrodeoxygenation (HDO) of fatty acid esters has been developed, which can produce diesel-range alkanes under mild conditions. The catalyst consists of a metallic palladium core and an outer mesoporous silica shell doped with aluminum atoms. It exhibits high catalytic performance and selectivity towards diesel-range alkanes in various experiments using model compound methyl palmitate and vegetable oils such as soybean and palm oils as raw materials.