(521cm) Trends in C–X Hydrogenolysis: Contrasting Functionalized Alkanes and Aromatics
AIChE Annual Meeting
2023
2023 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Poster Session: Catalysis and Reaction Engineering (CRE) Division
Wednesday, November 8, 2023 - 3:30pm to 5:00pm
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 8â11 transition metals, the heteroatom and metal identities impact the extent of dehydrogenation prior to the CâX cleavage transition state (ACS Catal. (2020) p. 5086). Free energy barriers indicate that CâX activations on group 8â10 transition metals typically occur after removing two H atoms from the carbon atom regardless of heteroatom identity. The dehydrogenation of âNH2 prior to CâN activation varied significantly across metals, while the dehydrogenation of âOH was rare and dehydrogenation of âSH was universal. We have since extended previous study to assess the mechanisms that activate PhâX in functionalized aromatic species, and consider the possibility of dehydrogenations (from the ring or X-group) prior to PhâX cleavage. Experimental kinetic data (U. Maine) examining chlorobenzene hydrogenolysis and our theoretical data on Pd(111) conclude that CâCl 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 âleaving groupâ remained consistent with our earlier work on CH3XHnÂ; 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.