(109g) Kinetic Analysis of Competitive Hydrodeoxygenation of Lignin-Derived Molecules over Bulk MoO3

Ambient pressure hydrodeoxygenation (HDO) over MoO₃-based catalysts has exhibited advantageous performance in selectively cleaving C-O bonds in lignin-derived molecules without saturating the aromatic ring. While several studies have contributed to elucidating the HDO kinetic and mechanistic behavior of these catalysts using single-model compound systems, lignin deconstruction generates a plethora of molecules with different functional groups, so these studies are insufficient to provide knowledge of the possible interaction and competition between these molecules. Hence, studying a more representative, multi-model compound system is essential for gaining more insight into this complex system's kinetics. To address this, simultaneous gas-phase acetone and anisole HDO at 603 K and 1 bar H₂ partial pressure was studied over bulk MoO₃. Propene, propane, and benzene were the HDO products formed, showing a similar product distribution to the single-compound system. Selectivity to propene was considerably higher than benzene, even at three times higher anisole partial pressure. A negative anisole HDO (-0.97) rate order with varying acetone partial pressure suggested a strong inhibition effect on anisole HDO by acetone. Conversely, with increasing anisole partial pressure, a rate order of -0.07 was observed for acetone HDO, implying a weak impact of anisole co-feed on acetone HDO. These results imply competitive adsorption of the reactants on the active sites. 2,6-di-tert-butylpyridine co-feed studies indicated the second H-addition to the adsorbed intermediate as the rate-determining step. Kinetic modeling showed that the acetone adsorption constant is more than three times higher than the anisole adsorption constant. Additionally, we posit that electron density around the oxygen atom of the oxygenate dictates the relative adsorption based on a lower acetone-to-phenol adsorption constant ratio (2.8). Density functional theory (DFT) calculations of the adsorption energy on the active sites followed the same order of acetone > phenetole > anisole as the experiments suggested.