Fe-phosphomolybdic acid/mesoporous SiO₂: A Highly efficient catalyst for the synthesis of Medicinally important organic compounds

In this study, we synthesized Fe-exchanged phosphomolybdic acid catalysts supported by mesoporous SiO₂ (FePMA/mSiO₂) using an ion-exchange method. The FePMA catalysts were prepared with different ratios of iron and PMA protons (x = 0.17, 0.33, 0.50, 0.67, 0.83, and 1.0). Moreover, the 0.17 FePMA/mSiO₂ catalyst was subjected to calcination at different temperatures of 350, 400, and 450 °C. The as-prepared catalysts were characterized through Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction technique(XRD), Transmission electron microscope (TEM), Scanning electron microscope (SEM), and Brunauer-Emmett-Teller technique (BET). The surface acidity of these catalysts was assessed via nonaqueous-potentiometric titration by using n-butylamine. The FT-IR analysis was performed to determine acid sites (Lewis acid and Brønsted acid sites) by chemisorbed pyridine. With increasing iron content, there was a significant reduction in the Brønsted sites and an increase in the Lewis acid sites giving a corresponding decrease in the Brønsted-to-Lewis ratio. Catalytic performance was evaluated through the synthesis of 3,4-dihydropyrimidinone, 7-hydroxy-4-methylcoumarin and hydroquinone diacetate. The results indicated that excessive iron loading diminished catalytic activity and surface acidity due to the reduction in the number of protons from PMA. The highest catalytic performance was achieved using the 0.17 FePMA/mSiO₂ catalyst calcined at 350 ℃. Moreover, the as-prepared catalysts were easily eliminated from the mixture of reactions by filtration and demonstrated excellent thermal stability, reusability, and environmental friendliness.