(426b) Effect of Sulfuric Acid and Solvent Environment in the Hydrogenolysis of Glycerol on ReOx-Ir Catalyst: A Combined Experimental and Computational Study

Supported multicomponent catalysts comprising noble metals like Re or Pt in contact with partially reduced metal oxides like ReO_x or WO_x are the most effective catalytic systems for selective hydrogenolysis of aqueous glycerol to 1,3-propanediol. Ambiguities regarding the active sites on these catalysts and the hydrogenolysis mechanisms/pathways, and inadequate understanding of the role of the aqueous acidic environment in which the reaction is typically carried out, have been impediments in making firm strides towards development of processes for this transformation. Combining batch hydrogenolysis experiments with density functional theory (DFT) calculations and detailed characterization of the ReO_x-Ir catalyst, mechanistic insights into hydrogenolysis of glycerol were obtained. The role and contribution of the aqueous acidic reaction medium was investigated using NMR relaxometry studies and were complemented with DFT calculations and molecular dynamics simulations. NMR relaxometry analysis of catalysts soaked in aqueous glycerol showed that competitive interaction of glycerol with the catalyst, relative to the solvent water, increased with glycerol concentration in solution. This explained the observed enhancement in the conversion of glycerol with glycerol concentration. NMR relaxometry analysis further showed that the presence of sulfuric acid increased the concentration of glycerol within the pores of the catalyst, relative to purely aqueous glycerol. DFT calculations suggested that the acidic environment enhanced the propensity for dissociative adsorption of glycerol on the catalyst. DFT calculations also suggested that the homogeneous acid catalyzed dehydration of glycerol was not the prominent reaction mechanism for glycerol hydrogenolysis. Hence, the promotional effect of sulfuric acid during hydrogenolysis was attributed to the two indirect contributions described earlier. DFT calculations showed the existence of kinetically preferred pathways for the formation 1,3-propanediol (1,3-PDO), compared to the formation of 1,2-propanediol (1,2-PDO). This explained the formation of 1,3-PDO as the dominant product of hydrogenolysis. However, NMR relaxometry analysis showed that 1,3-PDO preferentially interacted with the catalyst in its mixture with glycerol, compared to 1,2-PDO in similar mixtures. This suggested that 1-propanol (1-PO), another prominent product of hydrogenolysis, formed by successive hydrogenolysis of 1,3-PDO, despite its lower reactivity than 1,2-PDO. This investigation highlights the significance of understanding the competitive interactions of the reactant and solvent with the catalyst as well as those of the reactant/ intermediates/products with the catalyst during liquid-phase reactions.

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