Role of phosphorous in transition metal phosphides for selective hydrogenolysis of hindered C–O bonds

Abstract

C–O hydrogenolysis can upgrade biomass to higher value chemicals, but often requires the selective activation of sterically hindered C–O bonds. Previous work examining C–O hydrogenolysis of methyltetrahydrofuran (MTHF), a model biomass-derived molecule, has shown that Ni2P and Ni12P5 show higher selectivities toward activation of the hindered (3C–O) bond over the unhindered (2C–O) bond compared to pure Ni catalysts. These measured selectivity differences—favoring 3C–O activations for materials with higher P content—were consistent with calculated free energy barriers for the 2C–O and 3C–O activation pathways using density functional theory (DFT). However, the role of P in causing this shift in selectivity is still unknown. In this work we use DFT to study other transition metal phosphides (Co2P, Pd2P, Rh2P, Fe2P, and Ru2P) and contrast them to their pure metal counterparts to determine if the role of P in Ni2P materials is consistent across other transition metals. To do this, we constructed theoretical models of these other transition metal phosphides that were isostructural to the Ni2P(001) surface. In comparing the phosphide materials to their pure metal counterparts, we saw a nearly ubiquitous shift in selectivity towards hindered C–O activation. However, the magnitudes of these shifts were significantly varied, with only Ni2P and Pd2P predicted to show high selectivity toward 3C–O activation. Periodic trends and charge analysis suggest that the varied selectivity shifts (comparing metal-phosphide to pure metal) can be rationalized based on the electronegativity of the metal and the resultant charge-transfer between P and the nearby metal atoms, which typically results in metals with a positive partial charge showing greater 3C–O selectivity. These results help to deconvolute the electronic and geometric impacts of P incorporation into transition metal catalysts and identify new catalysts for selective C–O activation at hindered C-atoms.