Silica-supported HClO4 and KHSO₄ as reusable green catalysts for sulfonation of aromatic compounds under solvent-free conditions

A green protocol is described for sulfonation of aromatic compounds that has been accomplished using sodium bisulfite (NaHSO₃) in the presence of reusable green heterogeneous SiO₂/HClO₄ and SiO₂/KHSO₄ (Silica- supported HClO₄ and KHSO₄ catalysts) under conventional and solvent- free microwave irradiation. The reactions afforded very good yields of products within 3 to 5 hour under conventional conditions. However, the reaction times in microwave-assisted protocol are drastically reduced to 3 to 5 minutes (from 3 to 5 hour under conventional conditions) followed by increasing product yields. Moreover, the developed silica-supported catalysts could be recycled for at least three to four times.