Understanding Selective Hydrodeoxygenation of 1,2- and 1,3-Propanediols on Cu/Mo<sub>2</sub>C via Multiscale Modeling
Kyung-Eun You, Salai Cheettu Ammal, Zhexi Lin, Andreas Heyden
Abstract
The hydrodeoxygenation (HDO) mechanism of 1,2- and 1,3-propanediols has been investigated over a Cu/Mo2C catalyst using density functional theory to understand the effect of vicinal hydroxyl groups on the HDO activity and product selectivity. To correlate the simulation results with experimental data, a microkinetic continuous stirred-tank reactor model was developed, and the activity of these diols was studied under both ultrahigh vacuum (UHV) and near ambient pressure reactor conditions. Our microkinetic model predicted a one order of magnitude higher turnover frequency for the HDO of 1,3-propanediol relative to 1,2-propanediol. Intramolecular H-bonding plays an important role in reducing the activation barrier for the rate-determining O–H bond scission step that occurs via a concerted H-transfer from the neighboring hydroxyl group. Under simulated UHV conditions, the major products are for both diols, the kinetically favored dehydrogenation products; however, a higher selectivity toward the thermodynamically favored HDO products was observed under higher pressure conditions and longer residence times typical for most chemical reactor studies. Our analysis revealed that the HDO of 1,2-propanediol follows a similar mechanistic pathway to glycerol due to the presence of adjacent hydroxyl groups in both molecules; in contrast, the HDO of 1,3-propanediol follows a different HDO mechanism.