Efficient electrocatalytic acetylene semihydrogenation by electron-rich metal sites in N-heterocyclic carbene metal complexes

Electrocatalytic acetylene semihydrogenation is a promising alternative to thermocatalytic acetylene hydrogenation due to its environmental benignity and economic efficiency, but its performance is far below that of the thermocatalytic reaction because of strong competition from side reactions, including hydrogen evolution, overhydrogenation and carbon-carbon coupling reactions. We develop N-heterocyclic carbene-metal complexes, with electron-rich metal centers owing to the strongly sigma-donating N-heterocyclic carbene ligands, as electrocatalysts for selective acetylene semihydrogenation. Experimental and theoretical investigations reveal that the copper sites in N-heterocyclic carbene-copper facilitate the absorption of electrophilic acetylene and the desorption of nucleophilic ethylene, ultimately suppressing the side reactions during electrocatalytic acetylene semihydrogenation, and exhibit superior semihydrogenation performance, with faradaic efficiencies of >= 98 % under pure acetylene flow. Even in a crude ethylene feed containing 1 % acetylene (1 x 10(4) ppm), N-heterocyclic carbene-copper affords a specific selectivity of >99 % during a 100-h stability test, continuous ethylene production with only similar to 30 ppm acetylene, a large space velocity of up to 9.6 x 10(5) mL.g(cat)(-1).h(-1), and a turnover frequency of 2.1 x 10(-2) s(-1), dramatically outperforming currently reported thermocatalysts.

Automated summary

Electrocatalytic acetylene semihydrogenation using N-heterocyclic carbene-copper as a catalyst shows high selectivity and performance, suppressing side reactions and achieving faradaic efficiencies >= 98% in pure acetylene flow. This new catalyst outperforms currently reported thermocatalysts with specific selectivity >99%, continuous ethylene production with low acetylene content, high space velocity up to 9.6 x 10^5 mL.g(cat)^(-1).h^(-1), and a turnover frequency of 2.1 x 10^(-2) s^(-1) in a 100-hour stability test.