Ni-Based Bimetallic Catalysts for Acetylene Semi-Hydrogenation

The removal of trace acetylene in ethylene streams destined for polyethylene production is a high-volume industrial catalytic-based process that must demonstrate high selective for alkyne hydrogenation over alkene hydrogenation.  The current industrial catalyst is a Pd-Ag alloy; Ag dilutes Pd ensembles that are unselective toward acetylene/ethylene hydrogenation.  The inclusion of Ag substantially reduces activity but increases selectivity to acetylene semi-hydrogenation.  Recent research efforts to replace Pd-Ag catalysts with base-metal catalysts demonstrated Ni-Zn alloy catalysts had comparable activity and selectivity [1].  Additionally, the computational aspects of this work demonstrated the activity and selectivity were highly sensitive to the composition of the Ni-Zn alloy catalyst.

We report on the synthesis and characterization of Ni-Zn catalysts that exhibit a higher selectivity than Pd-Ag for acetylene semi-hydrogenation in excess ethylene.  Ni nanoparticles (NPs) with an average diameter of 3, 6 and 10 nm synthesized by a high-temperature colloidal method were converted into Ni-Zn NPs by the injection of diethylzinc into hot solvent containing Ni NPs.  Wide-angle XRD demonstrates Ni NPs – regardless of their initial size – incorporated Zn in a 1:1 atomic ratio with respect to Ni.  The atomic composition of the Ni-Zn sample could not be altered by changing the amount of diethylzinc addition.  The Ni-Zn NPs were subsequently adsorbed from solution on the surface of mesoporous SBA-15 silica and further characterized.

X-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) were used to determine the oxidation state and local coordination environment of the metal atoms in Ni-Zn NPs.  XANES results demonstrate reduced Ni and Ni-Zn NPs form during colloidal syntheses with no required reductive pretreatment of the supported NPs.  This is an inherent advantage of the colloidal route compared to Ni-Zn catalysts prepared by traditional simultaneous or co-impregnation synthetic routes.  The latter catalysts must be reduced and the disparate thermal stability of Ni and Zn lead to catalysts with uncontrollable Ni:Zn ratios. 

Ni-Zn NPs are much more selective than Ni NPs for acetylene semi-hydrogenation which is in agreement with previous experimental and theoretical results [1].  DFT-based reaction energy calculations for acetylene and ethylene hydrogenation were compared over Ni and Ni-Zn surfaces to determine the origin of the experimentally-observed selectivity enhancement on the bimetallic catalyst.  A sequential hydrogenation mechanism from acetylene to ethane is not sufficient to capture the selectivity enhancement over Ni-Zn compared with Ni.  Additional in-situ characterization of the Ni and Ni-Zn catalysts was conducted to rectify the apparent contradictions between the DFT calculations and experimental observations. 

1.      F. Studt et al., Science 320 (2008) 1320-1322