Sulfated iron oxide mesoporous silica [SO₄²⁻/Fe₂O₃–mSiO₂]: A highly efficient solid acid catalyst for the green production of pharmaceutically significant 7-hydroxy-4-methyl coumarin, 3,4-dihydropyrmidinone and hydroquinone diacetate

Due to the excellent physicochemical and fascinating surface properties of metal oxides, they are considered an important class of materials that have profound implications in the field of catalysis. Metal oxides have been extensively applied as heterogeneous solid acid catalysts for several important organic transformations, due to their excellent acid-base and redox properties. In this work, we prepared a series of sulfated iron oxide mesoporous silica (SO₄²⁻/Fe₂O₃-mSi₂O₃) catalysts with different loadings of SO₄²⁻. The catalysts were characterized by XRD, FTIR, TEM, SEM, EDX, and BET techniques. The surface acidity of the catalysts was measured by potentiometric titrations via the n-butylamine titration method and FTIR spectra of pyridine adsorption. The catalytic activity was tested towards the formation of three biologically active compounds; 7-hydroxy-4-methyl coumarin, hydroquinone diacetate, and 3,4-dihydropyrimidinone. The presence of SO₄²⁻ species was found to enhance the surface acid sites whether Brønsted or Lewis type, that in turn enhances the catalytic efficiency. 15 wt% SO₄²⁻ revealed the best results between the different contents of sulfate loaded on Fe₂O₃-mSi₂O₃ and the calcination temperature of 450 °C also exhibited the best results compared with 400 and 500 °C. The performance of the SO₄²⁻/Fe–Si catalysts revealed that they are efficient heterogeneous solid acid catalysts for these reactions when compared with other catalysts. Moreover, the catalysts are found to be reusable, thermally stable, eco-friendly, and they can be easily separated by filtration.