Adjusting the Acid Strength of Hybrid Solid Acids in Confined Nanospace

The acid strength is one of the most important issues in solid acid. Herein, we report an efficient strategy for controlling the acid strength of sulfonated polystyrene (PS-SO₃H) within mesosilica hollow nanospheres. The sulfonation of PS@SiO₂ core–shell nanospheres with surfactant un-removed, partially removed and totally removed results in the formation of PS-SO₃H@SiO₂ with core–shell, yolk–shell and hollow nanostructures, respectively. The results of ³¹P MAS NMR using triethylphosphine oxide as a base probe molecule show that the acid strength of solid acids depends on their nanostructures. The yolk–shell and hollow nanostructures respectively afford the strongest and the weakest acid strength, suggesting the different aggregation degree of PS-SO₃H confined in nanospace during the sulfonation process. All the solid catalysts afford much higher activity than Amberlyst resin in esterification reaction. The yolk–shell nanostructured solid acid with the strongest acid strength shows the highest activity, suggesting the positive effect of the acid strength on the catalytic activity.