Design, Synthesis and Applications of Layered Double Hydroxides (LDHs), Mixed Metal Oxides (MMOs) and Nanocomposites as Efficient Sorbents and Multi-Functional Catalysts

The research work presented here is focused on designing new synthetic techniques for LDHs, MMOs and nanocomposites for better control over surface area, particle size, morphology and catalytic activity as compared to conventional techniques. Materials synthesized via these methods were thoroughly characterized and used in various model chemical reactions. This thesis is delineated into three parts. The first part describes the synthesis of ordered mesoporous NiAl mixed metal oxides (MMOs) from NiAl layered double hydroxides (LDHs) through a soft template method using pluronic-F127 as the structure directing agent. Ordered mesopores were obtained by the thermal decomposition of as-synthesized LDHs at different temperatures. The effects of the pluronic-F127 amount and the calcination temperature on the pore size distribution of the MMO were investigated. NiAl MMOs exhibited excellent catalytic activities in the Knoevenagel condensation of benzaldehyde with acidic methylene group containing malononitrile. Finally, the dependence of catalytic activity on the surface properties of NiAl MMOs was investigated. The pore diameter and the pore volume of NiAl MMOs were well correlated with catalytic performance of the catalysts. MMO obtained from the calcination of NiAl-F1273%LDH at 750˚C for 5 hours gave the highest conversion (> 99%) in the Knoevenagel condensation in 30 minutes. Optimum pore diameter for the model reaction described here was 7.7 nm, which gave rise to more than 99% conversion with 100% selectivity. Ethanol gave the best conversion at 60˚C. The regenerated catalyst showed 93.0%, and 89.0% of the initial catalytic activity after the first and the second regeneration cycles, respectively. In the second part, a sonochemical method was employed in the synthesisof nickel aluminum layered double hydroxides (NiAl-LDH) and the materials wereused as adsorbents for the removal of the reactive azo dye, Remazol Brilliant Violet (RBV-5r). The experimental data obtained for microstructure were compared and both the arrangement and orientation of the intercalated dye species were examined using Molecular dynamics (MD) simulations. The obtained materials were characterized by X-ray diffraction (XRD), nitrogen sorption (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Fourier transformation infrared spectroscopy (FTIR). The adsorption characteristics were studied in a batch process by optimizing different parameters such as calcination temperature, contact time, initial dye concentration, solution pH, and solution temperature. NiAl-LDH material synthesized by sonochemical (SC) methods and calcined at 250˚C (NiAl-C250SC) showed the best dye removal efficiency (100% removal in 6 minutes) with an adsorption capacity of 150 mg/g at 25˚C and at pH=6. The reusability of the dye loaded material was investigated by replicating the adsorption-desorption cycle. The results show that the material could be regenerated without significant loss of the adsorption capacity. The regenerated adsorbent showed 95.9%, and 95.7% of the initial adsorption capacity after the first and the second regeneration cycles, respectively. XRD and FTIR results for LDH before and after the dye adsorption showed that removal of the dye is due to intercalation of the organic dye molecule into the LDH structure where a net increase in the basal spacing from 7.48 Å to 8.71 Å is observed. Molecular dynamics (MD) simulations further suggest that the dye molecules arrange in the interlayer space as a monolayer with the main axis horizontal to the layer plane. The calculated d-spacing values were in good agreement with the experimental results. The third part demonstrates the synthesis of activated carbon templated Copper Aluminum mixed oxide (CuAl MO) catalysts for the direct imine formation by oxidative coupling of alcohols and amines under solvent free conditions. Among the catalysts, CuAl MO20%C (catalyst synthesized by adding 20% activated carbon) shows the best activity and selectivity for this reaction. Here, air is used as the oxidant which is considered as the most economical and green oxidant among different oxidizing agents. Pyridine adsorption results confirmed that the presence of higher number of Lewis acidic sites enhance the catalytic activity of the material. Various alcohol and amine substrates were smoothly converted into the corresponding imines in good to excellent yields. According to catalytic activity studies and TG-MS data, surface oxygen availability and facile reversibility of oxygen readsorption on the surface account for the superior activity and high durability of the CuAl MO20%C catalyst. The regenerated catalyst showed 92% conversion with 100% selectivity even after the 4th reuse. In the fourth part, magnetically recyclable Ni/Graphene nanocomposites were synthesized via an in situ reduction growth process for selective reduction of nitroarenes into corresponding azoxybenzene at room temperature and under atmospheric pressure. Here, hydrazine hydrate (N2H4.H2O) is used as the reducing agent which generates N2 and water as byproducts. The catalyst exhibits a 100% conversion and selectivity to the target product without the use of any external additives. Under the optimized conditions, a variety of structurally different nitroarenes were selectively transformed to their corresponding azoxy products in high conversions. Furthermore, a high stability and recyclability of the catalyst were also observed under the investigated conditions.