(308f) Selective Chemical Looping Combustion of Acetylene in Ethylene Mixtures By Bi2O3

We show that in the absence of molecular oxygen, lattice oxygen of bismuth oxide combusts acetylene in mixtures with ethylene, even in dilute concentrations, with selectivity exceeding 99% and can reduce acetylene concentrations to < 1 ppm. This high selectivity persists across 10 successive reduction/oxidation cycles in which ~20-30% of O atoms are removed during each reduction step and fully replenished during reoxidation in chemical looping operation. Structural characterization of the fresh and redox cycled bismuth oxide catalysts by X-ray absorption spectroscopy (XAS) indicates that despite significant changes in surface morphology observed in Scanning Electron Microscopy (SEM) micrographs, redox cycling does not affect the local coordination environment of Bi atoms. During stoichiometric reduction cycles, an inert dwell step is employed to redistribute lattice O atoms and relax any intraparticle O gradients during reduction due to slow diffusion of O atoms; the equivalence in surface acetylene combustion product evolution rates observed upon exposure of equivalent acetylene partial pressures to a partially reduced (~20% of O atoms removed) Bi₂O₃ surface before and after this dwell step indicates that lattice oxygen diffusion is facile and does not limit surface reaction rates. Measured C₂H₂ and C₂H₄ combustion rates therefore reflect surface reaction kinetics and indicate that the first-order rate constant for C₂H₂ combustion is ~400x greater than that of C₂H₄ at 663 K. Density Functional Theory (DFT) calculations suggest that the ~70 kJ mol^-1 difference in activation barriers for C-H activation of C₂H₂ and C₂H₄ is due to the difference in stability and hardness of the anions formed upon C-H scission. Selective acetylene combustion on Bi₂O₃ provides an alternative industrially relevant method for the purification of ethylene streams feeding ethylene polymerization reactors due to the conventional requirement to reduce acetylene concentration below 2 ppm to prevent poisoning of polymerization catalysts.