Well‐Dispersed Trifluoromethanesulfonic Acid‐Treated Metal Oxide Nanoparticles Immobilized on Nitrogen‐Doped Carbon as Catalysts for Friedel–Crafts Acylation

Abstract Although strong acid‐treated metal oxides are useful heterogeneous superacid catalysts for various organic transformations, they usually have a limited density of acidic sites due to their low surface areas. Herein, heterogeneous trifluoromethanesulfonic acid immobilized nitrogen‐doped carbon‐incarcerated titanium nanoparticle (NP) catalysts have been developed that are composed of well‐dispersed, small Ti NPs (ca 7 nm) that are otherwise difficult to achieve using acid‐treated metal oxides. The catalysts showed high activity for Friedel–Crafts acylation with low titanium loading (2 mol%, <1 mg of metal for 1 mmol of substrate). A range of microscopic, spectroscopic and physicochemical studies revealed that the nitrogen‐doped carbon immobilized the trifluoromethanesulfonic acid and that the addition of metals further changed the nature of the acidic species and enhanced catalytic activity.