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 for selective hydrogenation and would suppress oligomerization. However, upon aging, the PdAg alloy is likely to undergo reverse segregation, and in this case, our computations suggest that the selectivity of the catalyst is lost, with enhanced C–C couplings interfering even more strongly. Hence, small Pd ensembles are crucial to avoid oligomerization side reactions of acetylene