Hydrodeoxygenation of propanoic acid over silica-supported palladium: effect of metal particle size

The effects of metal nanoparticle size on the hydrodeoxygenation (HDO) of propanoic acid (PAc) over Pd/SiO2 catalysts was investigated. Strong electrostatic adsorption (SEA) was used to prepare catalysts with Pd nanoparticles ranging between 1.9 to 12.4 nm. The particle sizes were determined by chemisorption (O-2-H-2 titration) and scanning transmission electron microscopy (STEM). The HDO was carried out in a continuous gas-phase reactor at 200 degrees C and 1 atm at differential conversion. The reaction followed decarbonylation and hydrogenation pathways to yield ethane (C2H6) and propionaldehyde (EtCHO), respectively. While the catalytic TOF remained constant between 3.0-12.4 nm, it decreased by a factor of 2-3 with decreasing particle size down to 1.9 nm. The reaction rate is therefore considered to be largely structure-insensitive over the range studied. The selectivity toward EtCHO increased as the particle size increased, indicating hydrogenation is favored on single crystal Pd(111) and Pd(100) planes versus corners and edges. For decarbonylation to produce C2H6, reaction rate orders with respect to PAc (similar to 0.5) and H-2 (similar to 0), and the apparent activation energy (similar to 12 kcal per mole), were found to be the same for both 2.0 and 12.4 nm particle sizes. In contrast, the reaction rate order with respect to PAc (similar to 1.0) and H-2 (similar to 0.3) was different for hydrogenation to produce EtCHO. These differences are explained by a change in the rate-determining step for the HDO of propanoic acid.