Acidic ionic liquids immobilized on nano silica sulfuric acid for highly effective catalytic synthesis of 4H-pyrano[2,3-c] pyrazole and dihydropyrano[c]chromene derivatives

We report a simple, clean, and straightforward method for the non-covalent immobilization of protic acidic ionic liquids on nano silica sulfuric acid. Initially, two organic zwitterions were prepared by reacting 1,4-dimethylpiperazine with 1,4-butane sultone under varying molar ratios and reaction temperatures. The resulting 1,4-dimethylpiperazinum functionalized mono- or bis-butyl sulfonate organic salts were then anchored onto nano silica sulfuric acid using a non-covalent ion-pair immobilization approach. In the following, the prepared catalysts were fully characterized using various techniques, including FTIR, H-1 and C-13 NMR, SEM, EDX, TEM, TGA, and BET. Finally, the catalytic performance of the resulting organic-inorganic nanohybrids was then investigated for the synthesis of 4H-pyrano[2,3-c]pyrazole and 3,4-dihydro pyrano[c]chromene derivatives. These ion-pair immobilized protic ionic liquids offer a simplified and cost-effective immobilization process without the need for coupling agents or complicated chemical reactions to anchor onto the support and represent simple recovery and reusability accompanied with improved thermal stability compared to their non-supported forms, making them superior choices.