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.