(521cm) Trends in C–X Hydrogenolysis: Contrasting Functionalized Alkanes and Aromatics | AIChE

(521cm) Trends in C–X Hydrogenolysis: Contrasting Functionalized Alkanes and Aromatics

Authors 

Williams, D., University of Florida
Nguyen, V., University of Florida
Tavana, J., University of Maine
Al-Gharrawi, M., University of Maine
Schwartz, T., University of Maine
Hibbitts, D., University of Florida
The selective removal of heteroatoms plays a large role in biomass conversion (O, N removal), petroleum refining (S removal), wastewater treatment (Cl removal), and pharmaceutical syntheses (O, N, Cl removal). Kinetic and density functional theory (DFT) studies have shown that in hydrogenolysis of saturated CH3XHn molecules (X = C, N, O, S, Cl) on group 8–11 transition metals, the heteroatom and metal identities impact the extent of dehydrogenation prior to the C–X cleavage transition state (ACS Catal. (2020) p. 5086). Free energy barriers indicate that C–X activations on group 8–10 transition metals typically occur after removing two H atoms from the carbon atom regardless of heteroatom identity. The dehydrogenation of –NH2 prior to C–N activation varied significantly across metals, while the dehydrogenation of –OH was rare and dehydrogenation of –SH was universal. We have since extended previous study to assess the mechanisms that activate Ph–X in functionalized aromatic species, and consider the possibility of dehydrogenations (from the ring or X-group) prior to Ph–X cleavage. Experimental kinetic data (U. Maine) examining chlorobenzene hydrogenolysis and our theoretical data on Pd(111) conclude that C–Cl cleavage in chlorobenzene (C6H5Cl) occurs without any preceding dehydrogenation of C6H5Cl. Broadly, we saw that dehydrogenation of the ring can lead to lower-barrier transition states while identity of the “leaving group” remained consistent with our earlier work on CH3XHn­; for example, toluene prefers to cleave to form CH*, phenol to form OH*, benzenethiol to form S*. In general, understanding the mechanisms of heteroatom removal from simple organic molecules furthers fundamental insight into hydrogenolysis of more complex organic molecules.