Preparation, Luminesecent Properties Of Bipyridine-proline Grafted Bimodal Mesoporous Silica And Its Application In Asymmetric Catalysis

Heterogeneous asymmetric catalysis has many advantages, such as easy separation and purification for molecules, regenerated and recycled use for catalysts.Besides, the special electronic effect or space confinement effect between supports and species will improve the catalytic activity and stereoselectivity in some cases.Mesoporous silica nanoparticles are good supports in heterogeneous catalysis,because they have large specific surface area and pore volume, adjustable mesoporous pore diameter, high mechanical strength, good thermal stability, easy to modify internal and external surface. Particularly, bimodal mesopores silicas(BMMs) as the host materials are of bimodal mesoporous structures with ordered small mesopores of around 3 nm and the large mesopores of uniform intra-nanoparticle, which is more favorable for the diffusion of guest molecules, and unique double channel structures make the reactants and products molecules easy sorption and diffusion in the mesopores channels, so as to improve the efficiency of reaction. In this thesis, the BMMs as support, the pyridine proline derivatives were immobilized on it via hydrogen bonding and coordination bonding. A series of heterogeneous catalysts were prepared, in which the proline-grafted hybrid materials were used as catalysts for the asymmetric Aldol reaction. The research contents and main conclusion are as follows;1. Various capacities of(2S,2’S)-N,N’-([2,2’-bipyridine]-3,3’-diyl) bis(pyrrolidine-2-carboxamide)(Z1) were successfully grafted onto the bimodal mesopores surfaces of BMMs by post-synthesis method. The structural properties and textural parameters of resultant hybrid materials were characterized by XRD, SEM,N2 adsorption-desorption isotherms, UV-vis DRS, FT-IR, TGA, elemental analysis and fluorescence measurements. The luminescent behaviors of Z1-BMMs demonstrated that their fluorescence intensity and position in emission spectra strongly depended on the various mass ratio of Z1 with BMMs. These results illustrated that Z1 sites were highly dispersed in the mesopores of BMMs. Meanwhile,the proline-grafted hybrid materials were used as catalysts for the asymmetric aldol reaction, and their catalytic performances presented a good activities(yield up to 80%)and high stereoselectivities(ee up to 85%), compared with that of homogeneous catalysts.2. In order to improve the structural stability of Z1 in BMMs, different metal ions like Co2+, Ni2+, Zn2+ and Cu2+ were firstly grafted on BMMs, then Z1 was immobilized via the coordination bond between metal and pyridine group. The obtained catalysts were donated as Z1MBMMs(M=Co, Ni, Zn, Cu). Asymmetric aldol reaction as the model reaction, the catalytic activity of heterogeneous catalystswere better than their corresponding metal complexes catalysts. The XRD、SEM、TEM、FT-IR、UV-vis DRS results illustrated that the bimodal mesoporous structure benefit the uniform distribution of Z1 and absorption or diffusion of molecule.Meanwhile, we find that the immobilization of Z1 on BMMs Than that by coordination bonding was more stable than hydrogen bonding.3. The result of recycling reactions of above heterogeneous catalysts showed that the second reaction was similar to first reaction performance, but in third reaction, the catalytic activity was reduced. Combining with the analysis of characterization methods, some possible reasons for the decreasing of activity of heterogeneous catalyst were put forward, as follow:(1) XRD, SEM, TEM results illustrated that the stability of catalyst in the reaction process was affected, the channel structure were destroyed to some extent after series of recycling, especially at the third time;(2) TG,UV-Vis DRS profiles illustrated that some organic molecules which were by-produced in the asymmetric aldol reaction were deposited on the mesoporous surface, resulting in that the reactants could not easily diffuse into the active sites;(3)ICP data indicated that with the increasing of recycling times, Z1 and metal were strongly leached, which is the main reason for catalyst deactivation. In a word, the investigations for decreasing of catalytic activity need to be further done.