Bifunctional Catalytic System Effective for Oxidative Dehydrogenation of 1-Butene and n-Butane Using Pd-Based Intermetallic Compounds

The catalytic performance of Pd-based intermetallic compounds supported on silica (PdmMn/SiO2: M = Bi, Fe, Ge, In, Sn, Zn) was investigated in oxidative dehydrogenation of 1-butene and n-butane. A remarkable increase in selectivity (and also yield) of dehydrogenation products (1,3-butadiene and/or 1-butene) was obtained when PdIn, PdBi, or Pd3Fe replaced monometallic Pd. Temperature-programmed reduction and X-ray diffraction (XRD) studies on the catalysts with various treatments (i.e., the fresh, spent, and calcined) suggested that redox of the second metal was involved in the catalysis. XRD, X-ray photoelectron spectra, and transmission electron microscopy-energy dispersive X-ray spectroscopy analyses using the spent PdIn/SiO2 catalyst revealed the formation of a core-shell structure consisting of a PdIn1-delta core and Pd-In2O3 composite shell formed by oxidative decomposition of PdIn during the reaction. A mechanistic study suggested that the presence of In atoms, adjacent to Pd atoms, effectively inhibits the undesired combustion by capturing O-2 as lattice oxygen. The incorporated lattice oxygen reacts with hydrogen atoms derived from the hydrocarbon by C-H activation over Pd sites, resulting in the formation of water and oxygen vacancy and/or the parent intermetallic phase. We thus reveal that a combination of C-H activation by Pd and the redox of the second metal provides a unique and effective dehydrogenation system.