ReOx as a Brønsted acidic modifier in glycerol hydrodeoxygenation: computational insight into the balance between acid and metal catalysis

A computational study for the competitive conversion of glycerol to 1,2-propanediol and 1,3-propanediol is presented, considering a two-step sequence of dehydration followed by hydrogenation. The elementary steps for dehydration, i.e., breaking of C–H followed by C–OH or vice versa, were studied computationally both on the Rh metal surface and the acid-modified ReOH–Rh surface in order to understand the role of the acid promoter. While the acid modifier can catalyze the C–OH cleavage, the activation energy for the C–H cleavage was found to be considerably smaller on both pure and acid-doped Rh(111) surfaces, and breaking the secondary C–H bond is kinetically favored over breaking the terminal C–H bond. This is in complete agreement with experimental protocols favoring the formation of 1,2-propanediol. Another potential feedstock, glycidol, was studied for the epoxide ring opening to yield 1,2-propanediol and 1,3-propanediol, and the reaction was found to be metal-catalyzed even in the presence of acid.