Multifunctional heterogeneous catalysts highly performing in the conversion of carbon dioxide: Mechanistic insights

A series of novel multi-functional heterogeneous catalysts was efficiently synthesized with the objective of creating a synergistic activation while unravelling the role of each single catalytic component. Spherical silica nanoparticles embedding Sn as single site (Sn-SiO₂) were prepared via a straightforward and time-saving procedure. The silica surface was selectively decorated with two different imidazolium-based moieties bearing primary or secondary alcohols in order to investigate if the presence of two co-catalysts (Lewis acid site and alcohol functionality) may induce a synergic activation improving the overall catalytic performances. The solids were fully characterized through ¹¹⁹Sn, ²⁹Si and ¹³C solid state NMR as well as via N₂-physisorption, transmission electron microscopy, ICP-OES and chemical combustion analysis. FT-IR of adsorbed CO₂ was employed to obtain some mechanistic insights on the interactions between CO₂ and the surface of the novel solids. All the synthesized catalysts were compared selecting the challenging CO₂ fixation on either cyclohexene oxide or styrene oxide to produce the corresponding cyclic carbonates. The best solids display excellent catalytic performances outcompeting with other totally heterogeneous catalysts previously reported in the literature. Finally, the true heterogeneous nature of the catalyst was proved in consecutive catalytic cycles. The characterization of the material after reuses allowed confirming the stability of the solid under the selected reaction conditions.