(237c) First Principles Insights into the Selective Conversion of Glycerol to 1,3-Propanediol: The Synergistic Effect of Metal and Metal Oxide

Glycerol, a by-product of the biodiesel production process, is a cheap and green feedstock to produce the high value 1,3-Propanediol (1,3 PDO), a monomer in the production of Polytrimethylene terephthalate (PTT) which has superior properties compared to the polyethylene and polybutylene counterparts. Mechanistic insights into the liquid phase hydrogenolysis of glycerol for the formation of 1,3 PDO on MO_x (metal oxide like ReO_x or WO_x) modified M1 (active metals like Ir/Ru/Pt) are critical to improve its yield, which is currently limited to around 40%. ReO_x modified Ir catalyst is reported to be the best among the investigated catalyst systems for this reaction and hence, unravelling the specific roles of the metal oxide and the active metal will also help in identifying cheaper and more efficient MO_x/M1 combinations for the reaction. Density functional theory (DFT) based calculations are performed to compute reaction pathways and activation barriers associated with the key steps in glycerol hydrogenolysis on clean Ir and ReO_x modified Ir catalysts as model catalyst systems. Our calculations suggest that the low conversion of glycerol, as observed on Ir alone, is due to the extremely high activation barrier associated with the OH removal of glycerol. The selectivity towards 1,2-PDO on Ir alone could be due to the lower activation barrier (lower by ~37 kJ mol^-1) for removal of the primary OH of glycerol than that of the secondary OH. On ReO_x/Ir catalyst, the partially reduced ReO_x clusters dispersed on metallic Ir easily abstracts the hydroxyl hydrogen of glycerol and facilitates its attachment to the cluster as an alkoxide. The preferential attachment of glycerol through its primary OH groups in the early stages of the reaction allows the direct hydro-deoxygenation of the un-attached secondary OH of glycerol on the Ir catalyst, converting it to 1,3-PDO. Progressive reduction of the ReO_x clusters in course of the hydrogenolysis reaction lowers the preference of attachment of glycerol through its primary hydroxyls and this explains the drop in 1,3-PDO selectivity with the increase in conversion of glycerol. Since the MO_x cluster is attached to the active metal M1 by direct metal-metal interactions, stronger interactions facilitate the formation of finely dispersed MO_x clusters which are crucial for the direct hydrogenolysis mechanism. Thus, metal-metal interaction strength is suggested to be a suitable guide for the choice of metal oxide to be dispersed on the active metal in the mixed catalyst system.