Beyond the use of modifiers in selective alkyne hydrogenation: silver and gold nanocatalysts in flow mode for sustainable alkene production.

We report on the excellent stereo and chemoselectivity of nanosized silver and gold catalysts in the three-phase hydrogenation of acetylenic compounds under flow chemistry conditions. The materials featuring metal nanoparticles in the range of 2-21 nm were prepared by spray deposition or incipient wetness impregnation of silver nitrate and sol immobilisation of gold chloride on different carriers (Al2O3, SiO2, TiO2, and carbon), followed by activation in various atmospheres. The samples were characterised by ICP-OES, N2 sorption, XPS, HAADF-STEM, and HRTEM, and evaluated in a continuous-flow flooded-bed micro-reactor. Both metals display optimal activities for particles below 5 nm, enabling stable operation at T = 373 K and P = 10 bar. While the performance of the silver catalysts is less influenced by the support, the gold nanoparticles exhibit significant activity only when deposited on TiO2, likely due to the strong metal-support interaction. Hydrogenations of functionalised alkynes reveal that silver and gold match, and in some cases exceed, the selectivity of benchmark palladium-based catalysts. Furthermore, in contrast to Pd, the Ag and Au samples require no modifiers, which brings fundamental and practical simplifications for their understanding and large scale manufacture. Therefore, these materials could be advantageously used for the continuous production of olefinic intermediates in the fine chemical and pharmaceutical industries.