The selective hydrogenation of alkynes to alkenes is of substantial interest due to the importance of removing alkyne impurities from alkene-based monomer feedstocks used in the production of high-quality polymers. Current industrial catalysts are based on palladium (usually alloyed with silver) because of their combination of high activity and selectivity, but the use of noble metals results in high catalyst cost. It has been recently demonstrated that phosphides of Pd and Ni exhibit promising properties for the selective HYD of alkynes relative to their parent metals. In the current project, Ni-Pd bimetallic phosphides having low Pd contents and varying P/M molar ratios (M = Ni+Pd) are being investigated for selective hydrogenation of phenylacetylene. A series of low-loading NixPy/SiO2 catalysts (2.5 wt% NixPy) were prepared from precursors having a range of P/Ni molar ratios with X-ray diffraction used to identify supported Ni3P, Ni12P5 and Ni2P phases. Substitution of a small amount of Pd for Ni in Ni2P/SiO2 has been successful, resulting in a phase-pure Ni1.80Pd0.20P/SiO2 catalyst. Phenylacetylene (PA) hydrogenation studies were carried out in an atmospheric pressure flow reactor over the temperature range of 30-75 °C. The PA conversion decreases as the P/Ni molar ratio increases due to dilution of surface Ni ensembles by P atoms, which decreases the number of active Ni sites as indicated by CO chemisorption measurements. The selectivity for the desired styrene product increases with increasing P/Ni of the NixPy phase; despite low PA conversion, P-rich Ni2P/SiO2 exhibits 100% styrene selectivity. The introduction of Pd into Ni2P to form Ni1.80Pd0.20P substantially increases the PA conversion, while the styrene selectivity decreased only slightly. Studies are underway to determine the optimal Pd content of Ni2-xPdxP/SiO2 catalysts and to probe the surface chemical properties of the Ni-Pd phosphide catalysts.

Nickel-palladium phosphide catalysts for the selective hydrogenation of alkynes

ACS Spring 2022

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