ZrO₂/g-C₃N₄ hybrid nanocomposite: an efficient and eco-friendly recyclable catalyst for the trimethylsilyl protection of hydroxyl groups and synthesis of a-aminophosphonates

Recently considerable attention has been devoted to heterogeneous catalysts. Generally, heterogeneous catalysts offer several advantages such as mild reaction conditions, high throughput and ease of work-up procedures. Among the heterogeneous catalysts investigated, polymeric mesoporous graphitic carbon nitrides (g-C₃N₄) has attracted much attention recently due to strong van der Waals interactions between the layers, g-C₃N₄ is chemically stable against acid, base and organic solvents and also thermogravimetric analysis (TGA) reveals that g-C₃N₄ is thermally stable even in air up to 600 ^oC, which can be attributed to its aromatic C-N heterocycles. More importantly, g-C₃N₄ is only composed of two earth-abundant elements: carbon and nitrogen. This not only suggests that it can be easily prepared at low cost, but also its properties can be tuned by simple strategies without significant alteration of the overall composition. The last approach is considered as the most efficient way to design of high-performance heterogeneous catalysts utilizing the g-C₃N₄ as catalyst support. An interesting phenomenon is that the modification is mainly focused on metal oxides. Zirconia (ZrO₂) is a physically rigid material with chemical inertness. It has high resistance against attacks by acids, alkalis, oxidants and reductants. In this study, ZrO₂/g-C₃N₄ hybrid nanocomposite has been shown to be an excellent catalyst for the conversion of alcohols and phenols into their corresponding trimethylsilyl ethers with hexamethyldisilazane (HMDS) under solvent-free condition and for the synthesis of a-aminophosphonates. In addition, the ZrO₂/g-C₃N₄ can be easily recycled after separation from the reaction mixture without considerable loss in catalytic activity.