Green pathway to the synthesis of some spirooxindole derivatives using NGO/PMA as a new and effective solid acid catalyst

Heteropoly acids as nontoxic, dual Lewis–Br ø nsted acids with high thermal stability can develop acid‐catalyzed reactions. Combining these catalysts with other materials is of great interest to enhance their efficiency and capability. Despite all the outstanding properties of heteropoly acids, their solubility in water and most organic solvents limits their application. So, in presented research, immobilization of phosphomolybdic acid (PMA) on prepared N‐doped graphene oxide (NGO/PMA) was done to synthesize a solid acid, green and effective catalyst for the multicomponent synthesis of three kinds of spirooxindole structures. The preparation of new NGO‐based solid acid catalyst was performed through a three‐step method and identified by FTIR, TEM, mapping elemental analysis, SEM, EDX, and XRD techniques. Reusability of the catalyst, short reaction times (1:20–2 h), green condition, and easy workup as well as high yield of products (90–95%) are some merits of this research.