The cyclization of 3-hydroxy alkynes and the carboxylation of terminal alkynes both with CO2 are two attractive strategies to simultaneously reduce CO2 emission and produce value-added chemicals. Herein, the differential activation of alkynes over atomically precise Ag nanoclusters (NCs) supported on Metal-organic framework-derived highly-open mesoporous CeO2 (HM-CeO2) by reserving or removing their surface captopril ligands is reported. The ligand-capped Ag NCs possess electron-rich Ag atoms as efficient π-activation catalytic sites in cyclization reactions, while the naked Ag NCs possess partial positive-charged Ag atoms as perfect σ-activation catalytic sites in carboxylation reactions. Impressively, via coupling with HM-CeO2 featuring abundant basic sites and quick mass transfer, the ligand-capped Ag NCs afford 97.9% yield of 4,4-dimethyl-5-methylidene-1,3-dioxolan-2-one for the cyclization of 2-methyl-3-butyn-2-ol with CO2, which is 4.5 times that of the naked Ag NCs (21.7%), while the naked Ag NCs achieve 98.5% yield of n-butyl 2-alkynoate for the carboxylation of phenylacetylene with CO2, which is 15.6 times that of ligand-capped Ag NCs (6.3%). Density functional theory calculations reveal the ligand-capped Ag NCs can effectively activate alkynyl carbonate ions for the intramolecular ring closing in cyclization reaction, while the naked Ag NCs are highly affiliative in stabilizing terminal alkynyl anions for the insertion of CO2 in carboxylation reaction.
Keywords: Ag nanoclusters; CO2 fixation; CeO2; alkynes; surface ligands.
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