(407b) Ni-Based Intermetallic Catalysts for Chemoselective Hydrogenation: Translating Bulk Behavior to the Nanoscale Is Not so Easy!

The catalytic semi-hydrogenation of acetylene to produce ethylene is a common method for the removal of trace acetylene (~1%) in ethylene feed streams destined for ethylene polymerization.  Acetylene impurities in ethylene can cause deactivation of the polymerization catalyst if not removed from ethylene.  An effective catalyst for this reaction converts all of the acetylene to ethylene without further conversion of ethylene to ethane such that there is a net increase in the amount of ethylene.  Well-dispersed Pd supported on metal oxides exhibits high activity for acetylene removal, but limited selectivity and long-term stability.  Pd-Ag alloys, and more recently, intermetallic Pd-Ga compounds, demonstrate high selectivity towards ethylene and long-term stability.  Improved selectivity is a result of isolation of active Pd hydrogenation sites which reduces over-hydrogenation to form ethane, produce oligomerization products, and form coke on the catalyst surface.  A recent report demonstrated that intermetallic Ni-Zn may be a suitable replacement for Pd-based catalysts based on DFT calculations and experimental validation.  Replacing Pd-based catalysts with base metal Ni-based catalysts would be highly beneficial in terms of cost and environmental impact.  We report on the catalytic selectivity of unsupported bulk intermetallic Ni-Zn and to a lesser extent Ni-Ga catalysts for acetylene semi-hydrogenation.  Bulk intermetallic Ni-Zn and Ni-Ga catalysts contain little structural and compositional variance, a property that is not easily attainable with supported catalysts.  We demonstrate the addition of Ni-Zn improves selectivity to ethylene due to a reduction in acetylene oligomerization products rather than ethane over-hydrogenation as originally proposed.  Co-impregnation using nickel and zinc nitrates on a silica or alumina support results in the formation of ZnO which remains after a high temperature hydrogen treatment.  Additionally, there is no evidence supporting the formation of intermetallic Ni-Zn using a co-impregnation technique.  Direct deposition of Ni onto ZnO leads to the formation of an intermetallic Ni-Zn phase, but this catalyst shows low activity and no improved ethylene selectivity relative to a supported Ni/SiO₂ catalysts.  The inherent difference between the bulk and nanoscale Ni-Zn catalysts is due to the inability to drive enough Zn into Ni to form the high Zn-content Ni-Zn intermetallic required for high chemoselectivity and the reactive nature of the ZnO support.