Group 13-Doped Hollow Silica Nanospheres: Tailoring Acidity for Sustainable CO2 Conversion

Hollow silica nanospheres (NSs) are emerging as versatile supports for heterogeneous catalysis due to their high specific surface area and accessible internal cavities via the mesoporous shell. In this work, NS synthesis was optimized and group 13 metal cations (Al3+, Ga3+, In3+) were incorporated into the silica structure to tailor Lewis and Brønsted acidity. The influence of loading was investigated, selecting Al3+ as the target metal cation. Imidazolium functionalization yielded bifunctional catalysts. Materials were characterized by Transmission Electron Microscopy (TEM), Nitrogen Physisorption, Thermogravimetric Analysis (TGA), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), and Elemental Analysis (EA). Solid-state NMR spectroscopy was performed on 27Al, 71Ga, 13C, and 29Si nuclei, while the chemical environment of indium was investigated using X-ray Photoelectron Spectroscopy (XPS). 31P ssNMR with trimethylphosphine (TMP) as a probe molecule, and NH3-TPD analyses were performed to evaluate the acidity. All bifunctional solids were highly active catalysts for converting CO2 into cyclic carbonates via epoxide cycloaddition. The indium-doped bifunctional solid was the most efficient, showing stability and recyclability. Activity correlated with acid site nature and strength, offering insights into key design principles. The material remained active under different conditions and with different epoxides, showing competitive activity versus catalysts reported in the literature.