CsxH3-xPW12O40 nanoparticles dispersed in the porous network of Zr-pillared α-zirconium phosphate as efficient heterogeneous catalyst for synthesis of spirooxindoles

Abstract

Layered α-zirconium phosphate (α-ZrP) was prepared by reflux method using ZrOCl2.8H2O and H3PO4 as precursor. The α-ZrP material was pillared with Zr-oxyhydroxy nanoclusters to prepare Zr-pillared α-zirconium phosphate (ZZP). An enhancement in interlayer spacing, surface area and porosity was noticed as a result of Zr-pillaring. The cesium exchanged phosphotungstic acid (CsxH3-xPW12O40) nanoparticles were dispersed in the porous matrix of ZZP material to prepare CsxH3-xPW12O40-ZZP nanocomposite systems. The nanocomposites were characterized using XRD, FTIR, UV–vis-DRS, TGA-DTA, XPS, N2 sorption, TPD, FESEM and HRTEM techniques. The expansion in layer structure of α-ZrP upon pillaring and its subsequent retention in the composite material was noticed from XRD. UV–vis and FTIR study indicated structural integrity of the CsxH3-xPW12O40 nanoclusters in the ZZP interlayer. The surface area of the composite materials was in the range of 80–130 m2 g−1. The composite materials contained significantly higher amount of medium and strong acidic sites compared to the ZP and ZZP materials. Microscopic study suggested the presence of hierarchical nanospheres with diameter between 150 and 200 nm. The presence of well dispersed CsxH3-xPW12O40 nanoparticles with size between 8 and 15 nm was confirmed from HRTEM study. The nanocomposite materials were used as efficient heterogeneous catalyst for synthesis of spirooxindoles by multicomponent one pot condensation of isatin, malononitrile, naphthol/1,3-diketones. Structurally diverse spirooxindole derivatives were synthesized in high yield and purity in short span of time using the CsxH3-xPW12O40-ZZP nanocomposites as catalyst under mild conditions.