(689f) Synthesis, Characterization and Reactivity of Heteroatom Single Site Pairs for Selective Ethylene Conversion

Insoo Ro, Chithra Asokan, Phillip N. Christopher

Department of Chemical Engineering, University of California-Santa Barbara, CA

The recent shift in primary feedstocks for the chemical industry in the United States from naphtha derived from oil to light alkanes derived from natural gas has sparked interest in the development of heterogeneous catalytic processes for the selective conversion of ethylene into light olefins to overcome reduced supplies.[1] Uniquely, atomically dispersed Rh and Ni cations on certain oxide supports have been observed to drive ethylene conversion chemistry with selectivity to products, such as butenes and propylene, which are not produced with any reasonable selectivity on supported metallic Rh or Ni clusters. Previous reports proposed that co-localization of the isolated Rh or Ni cation and nearby acidic oxide sites enables the unique ethylene conversion chemistry. [2, 3] However, detailed insights into the how these sites act cooperatively and whether the reactive behavior can be tuned to enhance selectivity to targeted products and promote stability is lacking. The primary shortcomings of the previously explored catalytic systems are the lack of ability to control independently the acid site characteristics and the low stability of dispersed Rh and Ni cations that sinter to form metallic clusters under reductive conditions.

In this talk, we will discuss the synthesis of a class of heterogeneous catalysts consisting of co-localized isolated Rh and inorganic acidic active site pairs. The acidic sites such as ReO₄ and WO_x are atomically dispersed on Al₂O₃ support, and then Rh were selectively deposited on acidic sites on supports via the strong electrostatic adsorption, as we reported previously.[4] In situ UV-vis spectroscopy, in situ Raman, in situ FT-IR, and HRTEM-EDS characterizations was performed to analyze the structure of the WO_x and ReO_x acidic sites on the supports to characterize the active site pairs. We will show how active site structures and properties influence the reactivity in selective ethylene conversion chemistry to elucidate the individual and cooperative roles of the two active sites. We believe that fundamental understanding on active sites of ethylene dimerization will facilitate the design of active catalysts with high selectivity and good stability.

[1] Peplow, M., The Great Gas Gold Rush. Nature 2017, 550, 26-28.

[2] Iwamoto, M.; Kosugi, Y., Highly Selective Conversion of Ethene to Propene and Butenes on Nickel Ion-Loaded Mesoporous Silica Catalysts. The Journal of Physical Chemistry C 2007, 111, 13-15.

[3] Serna, P.; Gates, B. C., A Bifunctional Mechanism for Ethene Dimerization: Catalysis by Rhodium Complexes on Zeolite Hy in the Absence of Halides. Angewandte Chemie International Edition 2011, 50, 5528-5531

[4] DeRita, L.; Dai, S.; Lopez-Zapeda, K.; Pham, N.; Graham, G. W.; Pan, X.; Christopher, P., Catalyst architecture for stable single atom dispersion enables site-specific spectroscopic and reactivity measurements of CO adsorbed to Pt atom, oxidized Pt clusters and metallic Pt clusters on TiO2. Journal of the American Chemical Society, 2017, 139, 14150-14165.