Oxygen affinity of transition metal cations: A coherent descriptor elucidating catalytic oxygenate transformations

This work reports a method of developing oxygen affinity (OA) as a complementary descriptor to acidity for metal cations. The OA values are determined through the dissociation kinetics of the Metal-O complexes using in-situ far infrared spectroscopy. The measured OA scales the interaction of metal cations with various oxygenate groups to help elucidate the catalytic performance of metal cations in oxygenate transformations. • Oxygen affinity is developed as a descriptor of metal cation–oxygenate strength. • Kinetic model based on far IR absorbance is developed to determine oxygen affinity. • Metal cation catalyzed oxygenate transformations are elucidated by oxygen affinity. This work reports a complementary descriptor to acidity for metal cations - oxygen affinity, to scale the interaction strength of metal cations with oxygenate groups. A method for measuring oxygen affinity is developed in alkyl methylimidazolium chloride solvents using in-situ far infrared (FIR) spectroscopy. The relative oxygen affinity values of metal cations in metal chlorides with model compounds including cyclopentanol (for alcohols), cyclohexanone (for ketones), ethylene glycol diethyl ether (for ethers), and H 2 O are measured from the dissociation kinetics of the metal-oxygenate complexes. The concentrations of metal-chloride and metal-oxygenate complexes are determined during evaporative removal of oxygenate molecules according to the FIR absorbance of metal-Cl bonds. The oxygen affinity values are shown to reasonably account for the catalytic performances of metal cations in furfural-acetone condensation and glucose isomerization reactions. The oxygen affinity descriptor could be potentially exploited as an intrinsic property of metal cation broadly for metal chloride or metal oxide.