Selective Chemical Looping Combustion of Terminal Alkynes in Mixtures with Alkenes

The selective combustion of terminal alkynes in mixtures with alkenes is demonstrated during anaerobic reduction half-cycles on bulk bismuth oxide (Bi₂O₃) as an approach to remove alkynes, which act as inhibitors in olefin polymerization. Bi₂O₃ combusts phenylacetylene in styrene, 3-methylphenylacetylene in 3-methylstyrene, propyne in propylene, 1-hexyne in 1-hexene, and 1-octyne in 1-octene, with alkyne combustion selectivities exceeding 96%. Near unity reaction orders for hydrocarbon consumption during reduction half-cycles are consistent with combustion pathways initiated by rate-determining initial C-H activation, which drive selective alkyne combustion through intrinsic differences in the first-order rate constants for alkyne and alkene combustion rather than preferential adsorption of alkynes on Bi₂O₃ surfaces. Computational assessments of initial C-H activation pathways for alkynes and alkenes on (010) α-Bi₂O₃ surfaces using density functional theory illustrate that heterolytic transition states which form proton-carbanion pairs on Bi-O sites kinetically favor the activation of alkynes rather than alkenes due to differences in C-H bond acidity, and the barrier for heterolytic C-H activation is dictated in part by the sum of the molecular deprotonation energy and the energy to bind an R^- carbanion to a Bi site in its transition-state geometry. These heterolytic reactivity channels during selective chemical looping combustion present novel routes for purifying olefin gas streams containing alkyne impurities.