Impact of Metal and Heteroatom Identities in the Hydrogenolysis of C–X Bonds (X = C, N, O, S, and Cl)

Hydrogenolysis of complex heteroatom-containing organic molecules plays a large role in upgrading fossil- and biomass-based fuel and chemical feedstocks, such as hydrodeoxygenation and desulfurization. Here, we present a fundamental study contrasting the cleavage of C–X bonds in ethane, methylamine, methanol, methanethiol, and chloromethane on group 8–11 transition metals (Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) using density functional theory (DFT). Previous kinetic and DFT studies have shown that hydrogenolysis of unsubstituted C–C bonds in alkanes occur via unsaturated intermediates (e.g., *CHCH* for ethane) after a series of quasi-equilibrated dehydrogenation steps that weaken the C–C bond by creating C–metal bonds. However, the effects of the substituent group in CH3XHn on the required degree of unsaturation to cleave the C–X have not been systematically studied and are critical to understanding heteroatom removal. DFT-predicted free energy barriers indicate that the carbon atom in C–X generally cleaves after the removal of 2 H atoms (to form CH*) on group 8–10 metals regardless of the identity of the metal or the heteroatom. Group 11 metals (coinage metals: Cu, Ag, and Au) generally cleave the C–X bond in the most H-saturated intermediates with barriers close to thermal activation of C–X in gaseous CH3XHn molecules. The N-leaving group in C–N cleavage depends on the metal identity as it can leave fully dehydrogenated (as N*) on group 8 metals and partially or fully hydrogenated (as NH* or NH2*) on group 9–11 metals. Although O and S are both group 16 elements, C–S bonds always cleave to form S* (losing one H), while C–O bonds generally cleave to form OH* (without preceding H removal). Cl does not have H atoms to be removed before C–Cl cleavage in CH3Cl, and thus the C atom sacrifices an additional H atom to weaken the C–Cl bond on group 8 metals. This study of heteroatom removal from simple organic molecules is the first step to providing fundamental insights into H2-based upgrading of more complex organic molecules.