Promoted ceria catalysts for alkyne semi-hydrogenation

Abstract

CeO2 is a highly selective catalyst for the partial hydrogenation of alkynes. However, due to its limited H2 splitting ability, a high operating temperature is required for the reaction, hampering the practical exploitation of this abundant oxide. In this work, we demonstrate that gallium promotes the activity of CeO2 for the semi-hydrogenation of acetylene and methylacetylene, enabling a reduction of the operating temperature to 373 K, while maintaining an outstanding ethylene and propylene selectivity (80?97%), even in the presence of excess alkene in the feed. Oligomers comprised the main secondary product, while the selectivity to the corresponding alkane did not exceed 2%. The characterization of mixed Ce-Ga oxides reveals that the progressive incorporation of gallium into the ceria structure, forming a solid solution, boosts the oxygen storage capacity and the reducibility of the material. This is ascribed to the facilitated H2 activation on the Ga-promoted samples, as confirmed by in situ infrared spectroscopy and density functional theory simulations. The interplay between the advantage brought by the decreased barrier for H2 cleavage and the disadvantage due to the increased number of oxygen vacancies governs the reactivity of the CeGaOx catalysts in alkyne hydrogenation. The composition in the optimal catalyst, containing a molar Ce:Ga ratio of 95:5, was extrapolated to other trivalent cations. Indium incorporated in the ceria lattice favored the low-temperature H2 activation and led to an activity enhancement that surpassed that of gallium. However, aluminum did not form a solid solution with ceria and caused no effect. This work comprises the first application of promoted cerias in hydrogenation catalysis and may prompt further developments in olefin purification.