Evaluating the Risk of C–C Bond Formation during Selective Hydrogenation of Acetylene on Palladium

Palladium-based catalysts are known to promote the selective hydrogenation of acetylene to ethylene. Unfortunately, coupling reactions between the numerous surface intermediates generated in this process occur alongside. These side reactions are undesired, generating the so-called “green oil”, i.e., C4+ hydrocarbons that poison the active sites of the catalyst. The current work assesses the energetic and kinetic aspects of C4 side products formation from the standpoint of computational chemistry. Our results demonstrate that the C–C coupling of common surface species, in particular acetylene, vinylidene, and vinyl, is competitive with selective hydrogenation. These C–C couplings are particularly easy for intermediates where the C–Pd bond can largely remain intact during the coupling. Furthermore, the thus formed oligomers tend to be hydrogenated more easily, consuming hydrogen normally spent on acetylene hydrogenation. The analysis of site requirement suggests that isolated Pd2 ensembles are sufficient fo...