Unraveling the Role of Single-Atom Catalysts Immobilized onto Metal–Organic Frameworks in Selective Acetylene Hydrogenation: An Implementation of the Active Site Isolation Strategy

The active site isolation strategy has long been used to perform selective hydrogenation of acetylene. Out of all of the approaches available for isolating active sites, the use of single-atom catalysts (SACs) provides an attractive route to enhancing selectivity. Herein, SACs in the form of metal alkoxides (where the metal varies from Sc to Zn) have been immobilized onto the organic linkers of the UiO-67 metal–organic framework (MOF), and density functional theory (DFT) calculations have been used to thoroughly examine their hydrogenation ability. Given that there are four pathways detrimental to C2H4 selectivity, namely, (a) C2H4 hydrogenation pathway, (b) C2H2 overhydrogenation pathway, (c) CHCH3 hydrogenation pathway, and (d) C2 oligomerization to 1,3-butadiene, UiO-67(Co) has been screened out to be the most efficient catalyst. Comparison with other studies shows that the hydrogenation ability of the UiO-67(Co) MOF is superior to that of many Pd-based catalysts and some non-Pd-based catalysts. Although isolation of active sites has been shown to effectively inhibit the production of green oils, our findings are contrary to the previously reported phenomena. Our study shows that the use of SACs does not guarantee inhibition of green oil formation─two C2 molecules can adsorb at the same active site and can undergo C–C coupling. Selectivity of the reaction is hampered when C2 oligomerization to 1,3-butadiene becomes more competent than the C2H4 desorption pathway. In the realm of SACs, the choice of modulating the ethylene adsorption mode/energy is not available, and hence, we have reported a new strategy to enhance C2H4 selectivity. Since the availability of H atoms is crucial for the hydrogenation process, the key to attaining selectivity is to reduce the availability of H atoms to an extent that ethylene hydrogenation can be prevented without hampering acetylene hydrogenation to ethylene. This criterion is accurately met by UiO-67(Co).