Development of magnesium oxide–silver hybrid nanocatalysts for synergistic carbon dioxide activation to afford esters and heterocycles at ambient pressure†
Multi-metallic hybrid nanocatalysts consisting of a porous metal oxide host and metal satellite guests serve as a scaffold for multi-step transformations of divergent and energy-challenging substrates. Here we have developed a 3D porous MgO framework (Lewis basic host) with Ag0 nanoparticles (noble metal guest) for ambient pressure activation and insertion of CO2 into unsaturated alkyne substrates. The hybrid MgO@Ag-x (x = 2, 5, 7, 8 at% Ag) catalysts are synthesized by impregnating Ag+ ions in porous MgO cubes followed by reduction using NaBH4. Morphological (SEM, TEM, EDX mapping) and structural (PXRD, XPS) characterization reveal that the micron-sized hybrid cubes derive from self-assembly of ∼100 nm (edge length) MgO cubes decorated with ∼5 to 25 nm Ag0 NPs. Detailed XPS analysis illustrates Ag0 is present in two forms, <10 nm NPs and ∼25 nm aggregates. The MgO@Ag-7 catalyst is effective for inserting CO2 into aryl alkynes followed by SN2 coupling with allylic chlorides to afford a wide range of ester and lactone heterocycles in excellent yields (61–93%) and with low E-factor (2.8). The proposed mechanism suggests a CO2 capture and substrate assembly role for 3D porous MgO while Ag0 performs the key activation of alkyne and CO2 insertion steps. The catalyst is recyclable (5×) with no significant loss of product yield. Overall, these results demonstrate viable approaches to hybrid catalyst development for challenging conversions such as CO2 utilization in a green and sustainable manner.