Phosphotungestic acid supported mesoporous MCM-41 coated NiFe₂O₄ magnetic nanoparticles as highly effective green nanocatalysts for coumarin and xanthene synthesis

Phosphotungestic acid (PWA) supported on mesoporous MCM-41 coated NiFe₂O₄ magnetic nanoparticles were prepared to be used as efficient and recyclable heterogeneous catalysts for the synthesis of coumarin and xanthene derivatives. The prepared catalysts can be easily recovered and reused for four times without significant decrease in the catalytic activity. The simple recovery of the catalyst is occurred by using an external magnet. The catalysts have notable advantages in addition to easily isolation process from the reaction medium such as easy preparation, simple procedures, high stability, well catalytic performance, low reaction time and green reaction conditions. These advantages make them very good heterogenous catalysts from an economic and environmental point of view. The catalysts were fully characterized by XRD, FT-IR, SEM, EDS, TEM, XPS, and VSM analyses. XPS confirmed the presence of inverse cubic spinel phase of NiFe₂O₄ and the contribution of Fe(III) − O and Ni(II) − O bonds in tetrahedral (T_h) and octahedral (O_h) sites. PWA was found to be well dispersed inside and outside the pores of MCM-41 and/or NF-MCM-41 that enhanced both the surface acidity and the catalytic activity. The catalyst 55 W/NF-MCM-41-II exhibited the highest acidity (E_i = 592.4 mV) and catalytic activity toward the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene (94.7%) and 7-hydroxy-4-methyl coumarin (82.1%). [Figure not available: see fulltext.].