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

Abstract

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., C 4 + hydrocarbons that poison the active sites of the catalyst. The current work assesses the energetic and kinetic aspects of C 4 side products formation from the standpoint of computational chemistry. Our results demonstrate that the CC coupling of common surface species, in particular acetylene, vinylidene and vinyl, are competitive with selective hydrogenation. These CC 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 Pd 2 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 CC couplings interfering even more strongly. Hence, small Pd ensembles are crucial to avoid oligomerisation side reactions of acetylene.

Selective ensembles in supported palladium sulfide nanoparticles for alkyne semi-hydrogenation

“…The second potential side reaction investigated is oligomer formation, which was studied by modeling ethyne-ethyne coupling (Fig. 5c ) and alkyne-vinyl paths providing similar insights due to scaling relations 62 (Supplementary Fig. 19 , 21 ).…”

Competition of C‐C bond formation and C‐H bond formation For acetylene hydrogenation on transition metals: A density functional theory study

“…The selectivity for 1,3‐butadiene formation can be examined by the difference between reaction barrier for vinyl hydrogenation and coupling reaction of acetylene and vinyl, as shown in Figure b. The reaction barrier of coupling is in general higher than hydrogenation, inducing the hydrogenation of C 2 intermediates is faster than C‐C coupling . The selectivity for 1,3‐butadiene formation is found to follow the order of Cu(111) > Pd(111) > Ag(111) > Rh(111) > Pt(111) > Ir(111).…”

Engineering the Surface Architecture of Highly Dilute Alloys: An ab Initio Monte Carlo Approach

“…Indeed, it is well-known that the alloy structure and composition strongly affect the adsorption energy of surface intermediates [23][24][25][26] and the catalytic performance during a chemical process. 27 Accordingly, a number of experimental and theoretical studies investigating the structure of alloy systems under vacuum [28][29][30][31][32] versus reactive conditions 13,14,20,22,[33][34][35][36][37][38][39][40][41] (i.e. in the presence of adsorbates) have appeared in the literature.…”