Immobilized copper-layered nickel ferrite on acid-activated montmorillonite, [(NiFe2O4@Cu)(H+-Mont)], as a superior magnetic nanocatalyst for the green synthesis of xanthene derivatives

Abstract

In this study, the immobilization of copper-layered nickel ferrite on the surface and in the cavities of acid-activated montmorillonite (H⁺-Mont) was investigated. In this context, magnetic nanoparticles (MNPs) of NiFe₂O₄ as the prime magnetic cores were prepared. Next, through the reduction of Cu²⁺ ions with sodium borohydride, the nanoparticles of Cu⁰ were immobilized on the nanocore-surface of NiFe₂O₄, and the constituent NiFe₂O₄@Cu MNPs were obtained. Moreover, through the activation of montmorillonite K10 (Mont K10) with HCl (4 M) under controlled conditions, the H⁺-Mont constituent was prepared. The nanostructured NiFe₂O₄@Cu was then intercalated within the interlayers and on the external surface of the H⁺-Mont constituent to afford the novel magnetic nanocomposite (NiFe₂O₄@Cu)(H⁺-Mont). The prepared clay nanocomposite was characterized using FTIR spectroscopy, SEM, EDX, XRD, VSM and BET analyses. The obtained results showed that through acid-activation, the stacked-sheet structure of Mont K10 was exfoliated to tiny segments, leading to a significant increase in the surface area and total pore volume of the H⁺-Mont constituent as compared to those of montmorillonite alone. SEM analysis also exhibited that the dispersion of NiFe₂O₄@Cu MNPs in the interlayers and on the external surface of acid-activated montmorillonite was carried out successfully, and the nanoparticle sizes were distributed in the range of 15-25 nm. The BET surface analysis also indicated that through the immobilization of NiFe₂O₄@Cu MNPs, the surface area and total pore volume of the H⁺-Mont system were decreased. The catalytic activity of (NiFe₂O₄@Cu)(H⁺-Mont) was further studied towards the synthesis of substituted 13-aryl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H) tetraones 3(a-k) and 3,3,6,6-tetramethyl-9-aryl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H) diones 5(a-k)via the pseudo-one-pot three-component cyclocondensation of 2-hydroxy-1,4-naphthoquinone (Lawsone)/dimedone and aromatic aldehydes in a mixture of H₂O-EtOH (1 : 1 mL) as a green solvent at 80-90 °C. The (NiFe₂O₄@Cu)(H⁺-Mont) MNPs can be easily separated from the reaction mixture by an external magnetic field and reused for seven consecutive cycles without significant loss of catalytic activity.