Unraveling the mechanism of propanoic acid hydrodeoxygenation on palladium using deuterium kinetic isotope effects

• A combined experimental and theoretical study of HDO of propanoic acid over Pd/C.
• Deuterium kinetic isotope effect showed αCH bond cleavage as rate-controlling.
• Experimental data consistent with DFT/microkinetic modeling predictions.
• Decarbonylation (DCN) was the main reaction pathway at conditions studied.

A combined experimental and computational kinetic isotope effect (KIE) study was performed for the catalytic hydrodeoxygenation (HDO) of deuterium-labeled propanoic acid (PAc-2, 2-D2) over Pd catalyst. For the experimental study, the kinetics were measured in a plug flow reactor over a 5 wt% Pd/C catalyst at 200 °C and 1 atm under differential conversion using a reactor feed consisting of 1.2% PAc and 5% or 20% H2, with balance helium. Different experimental KIE values for the high (kH/kD = 1.13 ± 0.04) and low (kH/kD = 1.62 ± 0.05) partial pressures of hydrogen were observed. Density functional theory calculations were performed to obtain the reaction parameters of the elementary steps involved in the HDO of PAc on Pd (1 1 1), and a microkinetic model was developed to estimate the KIE for the low hydrogen partial pressure case from first principles. The computed result (kH/kD = 1.49) is in good agreement with the experiment. In addition, the product distribution favored C2H6 and CO, suggesting decarbonylation (DCN) is the main reaction pathway. This provides strong evidence for the proposed mechanism for the formation of C2H6 on both Pd (1 1 1) and supported Pd nanoparticles presenting primarily that exposed crystal face.

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