DEVELOPMENT OF SILICA BASED MESOPOROUS MATERIALS FOR CATALYSIS

Abstract

The silica based mesoporous materials discovered by Mobile Oil Company in 1992 have attracted great interest due to their wide application in a variety of areas such as adsorption, catalysis, sensor technology, chromatography and drug delivery. The pore size of these materials lies between 2 to 10 nm and can be controlled and modified by using synthetic strategies. These materials replaced zeolite molecular sieves, which were confined to the pore size of around 1.5 nm. Like microporous molecular sieves, these materials have large surface area and pore volume. Moreover, High concentration of surface hydroxyl groups makes the modification of these materials easier. MCM-41, MCM-48 and MCM-50 are among the well-known silica based mesoporous materials. MCM-41 is known for its hexagonal mesoporous structure. When it comes to MCM-48 and MCM-50, they are associated with cubic and laminar structure respectively The synthesis of silica based mesoporous material follows the soft template strategy in which aggregates of surfactant (neutral or charged) are used as a template over which silicates (esters of ortho silicic acid) are polymerized. At the end, the templet gets removed by means of either extraction or calcination. For the sake of convenience, the work represented in the thesis has been divided into the following six chapters: The first chapter of the thesis includes the brief introduction and literature survey regarding the silica based mesoporous materials functionalized with metal complexes of nitrogen and oxygen containing ligands. The modified silica based mesoporous materials show application in various field such as catalysis, sensing and gas adsorption. In the second chapter, a new and green heterogeneousMCM-41@PDCA-Co catalyst has been prepared via immobilization of cobalt (II) complex onto the surface of MCM-41. This was characterized by different techniques such as FT-IR, SEM, TEM, TGA, low angle Powder XRD, MP-AES and nitrogen sorption. The MCM-41@PDCA-Co has been applied in the synthesis of polyhydroquinoline (PHQs) and its derivatives in 98% yield which is four component reaction under solvent free condition at 100°C. The synthesized catalyst can be easily recovered from the reaction mixture by simple filtration method. This can be reused several times without any significant loss in stability and activity. In the third chapter, a new heterogeneous catalyst has been prepared through the immobilization of Ni(II) complex of N2,N6-bis(3-(triethoxysilyl)propyl)pyridine-2,6 dicarboxamide on MCM-41. The MCM-41 immobilized Ni complex has been characterized by different techniques such as FT-IR, low angle powder XRD, TGA, SEM, TEM, MP-AES and N2 sorption studies and used as a catalyst in the synthesis of bezimidazoles and quinazolinones through the coupling of substituted benzaldehyde with o-phenylenediamine and anthranilamide respectively. The prepared catalyst showed tolerance towards both electron-withdrawing and electron-donating functional group. High yield, ease in recovery by simple filtration and reusability up to five times without any considerable loss in catalytic efficiency are the main advantages of the catalyst. In the fourth chapter, a new hybrid catalyst has been prepared by the immobilization of copper (II) α-diimine complex on the surface of mesoporous silica, MCM-41. The symmetrical α-diimine ligand has been prepared from dimethyl glyoxal and p-amino benzoic acid (PABA). The anchored α-diimine ligand imposed a stable planar coordination geometry around the central copper (II) ions. The catalyst has been characterized by different analytical techniques such as FT-IR, N2 sorption, small angle X-ray diffraction, thermogravimetric analysis (TGA), MP-AES, SEM and TEM. The PXRD and TEM measurements showed the mesoporosity in the prepared heterogeneous catalyst. The catalyst has been tested in the conversion of alkene into epoxide using tert-butyl hydroperoxide (tert-BuOOH) as oxidant in acetonitrile medium. The catalytic results demonstrate that the catalyst is highly active and selective for the epoxidation of alkenes. The catalyst can be recycled and reused several times without significant loss of activity. In the fifth chapter, a MCM-41-supported palladium (II) α-diimine complex has been synthesized via anchoring of 3-chloropropyltriethoxysilane on mesoporous silica MCM 41, followed by grafting α-diimine ligand through the covalent approach to bind with Pd(II) ion. Different analytical technique such FT-IR, TGA, low angle powder XRD, SEM, TEM, ICP-MS and N2 sorption were used to characterize the complex. The complex serves as a highly active and recyclable catalyst for Suzuki-Miyaura coupling reaction affording a variety of biaryls with good yields. The materials and reagents, synthetic procedure, experimental details and supporting information have been provided in the sixth chapter of the thesis.