SYNTHESIS OF MCM-41 BASED METAL COMPLEXES FOR CATALYTIC STUDIES

Abstract

Silica based mesoporous materials attracted researcher’s interest in various fields such as adsorption, catalysis, drug delivery and sensor technology, since their discovery in 1992. Mesoporous materials have pore size (2-50 nm) between micro and macro porous range. On the basis of mesostructure they can be differentiated ordered and disordered with broad/narrow pore-size distribution. M41S is the most discussed family of silica based ordered mesoporous materials reported by Mobil group. MCM-41 (hexagonal), MCM-48 (cubic), MCM-50 (lamellar) are the three members of this family having silica pore wall, narrow pore size distribution with different mesophases.MCM-41, the extensively studied member of M41S family due to its unique features such as controllable pore size (1.5- 20 nm) with wall thickness 1.5-2 nm, extremely high surface area, regular hexagonal 1D channel and high thermal stability. The tailorable properties of MCM-41 has opened a new area of research especially in the field of adsorption and catalysis. The most interesting facts about the MCM-41 are- its ordered structure with honeycomb arrangement of mesopores and presence of silanol (Si-OH) groups at the inner surface. MCM-41 has very high surface area due to mesopores in the framework, which makes it perfect to use in the field of catalysis. The synthesis of silica based mesoporous material follows the soft template strategy, which involves two common precursors i.e. Surfactant (templating agent) and silica source. The surfactant aggregates to form micelles in basic medium to provide a template followed by the condensation of silica precursor around this template. Mesoporous silica obtained after the removal of template in the final step. Quaternary ammonium ions with long alkyl chains (ex- cetyltrimethylammonium bromide) used as a template or structure directing agent and frequently used silica source are organic silica alkoxide such as tetraethylorthosilicate (TEOS). By changing the length of alkyl group of template, pore size of mesoporous silica can be changed. Physical and chemical properties of mesoporous silica can be altered by incorporating inorganic and organic moieties to the framework through free silanol group of pore surface. MCM-41 attracted attention in the field of catalysis as a support due to its high surface area and free Si-OH groups, which act as binding sites for the anchoring of homogeneous complexes on the surface. Grafting or post synthetic modification is the most commonly used method for the covalent attachment of metal complexes on the surface of MCM-41 to synthesize heterogeneous catalysts for various reactions. After immobilization the catalytic efficiency of metal complexes improved significantly due to the increase in the dispersion of active sites on large surface area of MCM-41.The strong interactions between the metal and the ligand generate significant stability for this kind of catalyst, which allows them to be reused in several cycles without losing their efficiency. Work presented in this thesis based on the development of mesoporous MCM-41 based heterogeneous catalysts for various organic transformations. For the sake of convenience, the presented work 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. In the second chapter, the benzimidazole derivatives were synthesized through the coupling of O-phenylenediamine and substituted benzaldehydes by using a heterogeneous catalyst MCM@CP@PYO@Cu, which was synthesized by anchoring the Cu-Schiff base complex on the surface of chloro-functionalized MCM-41 via grafting method. FT-IR, TGA, low angle Powder XRD, SEM, TEM, MP-AES and N2 adsorption-desorption analysis were used in the characterization of MCM@CP@PYO@Cu. 98% of product yield was obtained with this catalyst. The synthesized heterogeneous catalyst can be reused up to 7 runs with insignificant change in its catalytic properties. Reusability, high surface area, low metal loading, short reaction time and good to excellent product yield are the main advantages of the present catalyst. In the third chapter, a Palladium (II) Schiff base complex was implanted on the surface of MCM-41 to fabricate a simple and reusable catalyst using post-grafting methodology. The surface of MCM-41 initially modified with 3-chloropropyltriethoxysilane (CPTES), followed by the Schiff base ligand (PAL) and on reacting with PdCl2 results in the formation of MCM@CP@PAL@Pd complex. Characterization of the designed heterogeneous catalyst by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Microwave Plasma Atomic Emission Spectroscopy (MP-AES) and Brunauer–Emmett–Teller (BET) techniques indicated successful grafting of Palladium (II) complex inside the hexagonal channels of MCM-41. The MCM@CP@PAL@Pd catalyst was used for a good range of heck reactions using different aryl halides with styrene or alkyl acrylate esters. The catalyst has low metal content, provides magnificent yield (95%) in a short time period (20 minutes), displayed excellent structural and chemical stability and can be reused seven times without remarkable change in its catalytic efficiency.In the fourth chapter, a practical approach was used to fabricate a reusable and robust heterogeneous catalyst MCM@CP@PDC@Co via covalent anchoring of metal complex on MCM-41 nanostructure. Various appropriate techniques such as FTIR, P-XRD, TGA, BET, SEM, TEM, MPAES and XPS were used to examine its structural characteristics. The catalytic efficiency of the prepared catalyst was studied for the one pot synthesis of polyhydroquionoline derivatives. All the reactions were carried out at 80 ℃ under solvent free media. Operational simplicity, simple work-up, inexpensive and non-toxic catalyst, excellent yield (98%) are the key features of this protocol. Solvent free conditions and reusability of catalyst make this protocol more attractive and environment favorable. In the fifth chapter, we have reported synthesis, characterization and catalytic activity of an efficient and reusable silica-based catalyst MCM@CP@PAL@Cu. The catalyst was prepared by the anchoring of Cu(II)-Schiff base complex on mesoporous MCM-41 grafted with -chloro group on the surface. The obtained catalyst was characterized with various analytical methods such as FT-IR, Low angle P-XRD, TGA, N2 adsorption-desorption, SEM, TEM and MPAES. The catalyst composition and chemical state of Cu metal identified using X-ray spectroscopy. The catalyst MCM@CP@PAL@Cu employed for the conversion of various alkenes to their epoxides using tert-butyl-hydroperoxide (tert-BuOOH) oxidant and acetonitrile as a solvent. 99% yield of the epoxide obtained with model reaction and catalyst can be reused minimum six times. The materials and reagents, synthetic procedure, experimental details and supporting information have been provided in the sixth chapter of the thesis.