Identification of Non-Precious Metal Alloy Catalysts for Selective Hydrogenation of Acetylene

The removal of trace acetylene from ethylene is performed industrially by palladium hydrogenation catalysts (often modified with silver) that avoid the hydrogenation of ethylene to ethane. In an effort to identify catalysts based on less expensive and more available metals, density functional calculations were performed that identified relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces. This analysis not only verified the facility of known catalysts but identified nickel-zinc alloys as alternatives. Experimental studies demonstrated that these alloys dispersed on an oxide support were selective for acetylene hydrogenation at low pressures.

Fig. 1 The calculations on the Pd surfaces show that the selectivity is determined by the differences in the adsorption strengths of ethylene and its barriers towards further hydrogenation. Modification of the Pd surface with Ag results in a weaker adsorption of acetylene and ethylene while the hydrogenation barriers are essentially unaffected.

Fig. 2. The acetylene and ethylene adsorption energies are to a good approximation described by the methyl adsorption energy (A) of the corresponding alloys. A slope of four (two) is observed for acetylene (ethylene) corresponding to four (two) bonds that are formed to the surface during adsorption. There is a certain window where high selectivity and activity is expected, and where known hydrogenation catalysts are found. Screening of ~70 bimetallic alloys (B) led to the identification of non-precious catalysts (NiZn). These catalysts have been synthesized and tested for their selectivity in the hydrogenation of acetylene in an ethylene stream.

Fig. 3. Testing of NiZn: We are showing the production of ethane as a function of acetylene conversion. The experiments verified the applied concept and the high potential of NiZn catalysts for the selective hydrogenation of acetylene. While being as selective as PdAg, their material costs are significantly lower.