Catalytic Applications of Surface and Structurally Modified Zirconia Nanoparticles for Synthesis of Biologically Important Molecules

Abstract

In this thesis, surface and structural modification of zirconia nanoparticles have been carried out to generate novel heterogeneous catalytic system with strong surface acid/base properties. The catalytic application of the modified zirconia nanocatalysts have been studied for environmentally benign synthesis of fine chemicals and biologically important molecules.

Transition metal doped, anion grafted zirconia nanomaterials namely SO42-/Fe2O3–ZrO2 and PO43-/SnO2-ZrO2 were prepared by a two-step method. The doped zirconia nanoparticles were initially synthesized by co-precipitation method using urea as a mild hydrolyzing agent. The anions were subsequently grafted onto the surface of the composite oxides. The resulting anion grafted composite oxides were characterized using XRD, FTIR, Raman, surface area measurement, UV-Vis, XPS, TPD, FESEM and HRTEM techniques. The Fe3+ ions substitute for the Zr4+ ions in the ZrO2 lattice to form a substitutional type solid solution in a limited composition range. The presence of well dispersed iron oxide species in the zirconia lattice was ascertained from the Raman and UV-Vis study. HRTEM analysis of the SO42-/Fe2O3–ZrO2 samples revealed the presence of well dispersed mixed oxide nanoparticles with sizes in the range of 10–30 nm. Structurally, the SnO2-ZrO2 system contains a mixed phase system consisting of Sn4+ substituted t-ZrO2 and Zr4+ substituted cassiterite SnO2 phase. Gradual morphological changes from flake like to spherical and finally to cuboid shape nanoparticles were observed with increase in SnO2 content in the SnO2-ZrO2 composite system. HRTEM study showed considerable mismatch along the grain boundary region for the two phases which can serve as potential active sites. XPS and FTIR studies of both class of composites confirmed the presence of grafted sulfate and phosphate functional groups. TPD study indicated an enhancement in number as well as the strength of the acidic sites as a result of transition metal doping in ZrO2 lattice followed by anion grafting. The catalytic activity of the SO42-/Fe2O3–ZrO2 material was evaluated for the synthesis of 1,8-dioxo-decahydroacridines by multicomponent condensation of dimedone, substituted aryl aldehydes and substituted anilines. Similarly, the catalytic activity of PO43-/SnO2-ZrO2 composites was evaluated for synthesis of biologically important 3-substituted indoles by multicomponent one pot condensation of indole, malononitrile and aryl aldehydes. Structurally diverse dioxodecahydroacridine and 3-substituted indole molecules were obtained with high yield and purity in short reaction time using the modified zirconia materials as catalyst.

Layered α-Zirconium phosphate (α-ZrP) material 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 surface area, porosity, interlayers spacing and acidic sites were noticed as a result of Zr-pillaring. The cesium exchanged phosphotungstic acid (CsxH3-xPW12O40) nanoparticles were dispersed in the structural micropores 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 upon pillaring and its retention in the composite was noticed from XRD. N2 sorption study revealed that the composite materials contain uniform micropores and display surface area in the range of 80-130 m2/g. The composite materials contain significantly higher amount of strong acidic sites compared to the ZP and ZZP materials. HRTEM and elemental mapping study indicated the presence of well dispersed CsxH3-xPW12O40 nanoparticles with size in the range of 8-15 nm. The nanocomposite materials were used as efficient heterogeneous catalyst for synthesis of spiroxyindoles by multicomponent one pot condensation of isatin, malononitrile, naphthol/1,3-diketones. Structurally diverse spiroxyindoles were synthesized in high yield and purity in short span of time using the CsxH3-xPW12O40-ZZP nanocomposites as catalyst under mild conditions.

A series of CaO–ZrO2 nanocomposite oxides containing 2–50 mol% of CaO were synthesized employing urea as a mild hydrolyzing agent. The nanocomposite oxides were characterized using XRD, Fourier analysis, TPD, Raman, XPS, TGA-DSC, FESEM, HRTEM, and BET surface area measurement techniques. Upto 20 mol% CaO, the Ca2+ ions substituted for Zr4+ ions in the zirconia lattice to form a substitutional solid solution. Beyond 20 mol% CaO, the presence of a mixed phase system consisting of the solid solution phase, CaO and nonstoichiometric Zr0.93O2 phase was observed. TPD study revealed a significant enhancement in the basicity of zirconia as a result of Ca2+ incorporation. XPS study confirmed presence of different lattice oxygen as potential basic sites. Raman spectral data indicated the presence of oxygen vacancy and distortion in oxygen sublattice due to Ca2+ incorporation to zirconia. The CaO–ZrO2 nanocomposite oxides were used as an efficient and recyclable heterogeneous base catalyst for synthesis of chromene analogues. Structurally diverse 2-amino-4H-chromenes and 2-amino-2-chromenes were synthesized in high yield and purity under mild condition using CaO-ZrO2 material as catalyst and multicomponent condensation approach.

Zirconia nanoparticles were synthesized by precipitation, urea hydrolysis, amorphous citrate and combustion synthesis methods. The zirconia surface was subsequently modified by grafting Ba2+ species. The Ba/ZrO2 materials were characterized using XRD, Fourier analysis, UV-vis-DRS, FESEM and HRTEM techniques. XRD study indicated selective stabilization of the tetragonal phase of zirconia in the presence of Ba2+ species. Fourier line profile analysis of the XRD peaks revealed that the average crystallite size of the zirconia nanoparticles is in the range of 5-15 nm. The surface area, basicity and barium content of the material depend strongly on the method of synthesis. The catalytic activity of the Ba/ZrO2 catalyst was evaluated for synthesis of β-nitro alcohols and 2-amino 2-chromenes. The β-nitro alcohols were synthesized by condensation of aryl aldehydes and nitromethane. The Ba/ZrO2 catalyst was found to be highly efficient for synthesis of both classes of compounds providing excellent yield and purity of the products.