Fine-Tuning Of Nanopores Of Mesoporous Materials Supported Chiral Salen Mn(III) Catalysts

With the increasing demand for chiral products around the world, the efficient and green production method for chiral compounds has been widely concerned. Among the various methods to selectively produce single enantiomer, asymmetric catalysis is undoubtedly the most attractive one from the atom-economic point of view. Compared with the homogeneous asymmetric catalysis system, the heterogeneous one has the advantages of easy catalyst/product separation and simple catalyst recycling. And more and more interests have been focused on the studies of heterogenization of chiral complexes. The asymmetric epoxidation of unfunctionalized olefins is an important approach for synthesizing optically active epoxides, and thus is widely used in the synthesis of fine chemicals, such as pharmaceuticals, agrochemicals and perfumes. Chiral salen Mn(III) complexes have demonstrated activity and selectivity for the enantioselective epoxidation of unfunctionalized olefins under homogeneous condition. In recent years, the immobilization of chiral salen Mn(III) complexes on inorganic mesoporous materials has received much attention due to the advantages of heterogeneous catalysis systems.In most cases, the heterogenized chiral salen Mn(III) catalysts led to lower activity and enantioselectivity compared with the homogeneous counterparts for asymmetric epoxidation, and only a few immobilized catalysts exhibited comparable or even higher ee values than the homogeneous ones. The improvement of enantioselectivity was mainly attributed to the confinement effect of the nanopores. However, the effect of confinement effect on the catalytic performance of mesoporous material-supported chiral salen Mn(III) catalyst has seldom been systemically investigated for the asymmetric epoxidation. In this thesis, the effects of the pore structure and pore size of mesoporous supports and the distribution of active sites on activity and enantioselectivity of the heterogeneous catalysts were studied. And this study would be helpful for further understanding the confinement effect of the nanopores on the catalytic performance of heterogeneous chiral salen Mn(III) catalysts.A series of mesoporous MCM-41 and MCM-48 materials with different pore sizes were synthesized applying the alkylammonium salts with different alkyl chain lengths as templates. These materials were used as supports to immobilize three types of chiral salen Mn(III) complexes via organosilane modification. The results of XRD, FT-IR, DR UV-Vis, N2 sorption, elemental analysis and ICP-AES showed that the chiral salen Mn(III) complexes have been successfully immobilized inside the channels of mesoporous materials, and the heterogeneous catalysts maintain the characteristic mesoporous structures of corresponding parent supports. The as-synthesized heterogeneous chiral salen Mn(III) catalysts were highly active and enantioselective in the asymmetric epoxidation of unfunctionalized olefins. It is found that the catalytic performance of heterogeneous catalysts was closely correlated to the pore sizes of parent supports. Normally, the larger pore size of the supports was beneficial to obtain higher catalytic activity, and the compatible pore size with substrate would be responsible for the improved enantioselectivity in the olefin epoxidation.The influence of organosilane dosage on the catalytic performance of heterogeneous chiral salen Mn(III) catalysts was studied, and the optimum organosilane dosage for the preparation of heterogeneous catalysts was determined. The experiments revealed that the dosage of organosilane had a great effect on the catalytic performance of heterogeneous catalysts and the stability of active species. The higher content of organosilane would improve the stability of the catalysts and decrease the leaching of active sites, however, it could also increase the diffusional resistance of reactants to the active sites located on the inner surfaces of supports, and thus decrease the catalytic performance of heterogeneous catalysts.The co-condensation method was applied to immobilize the chiral salen Mn(III) complex so as to make the active species uniformly dispersed on the surface of supports. The heterogeneous catalysts (PMOs) were prepared through the co-condensation of TEOS and bridged organosilane precursor-containing salen Mn(III) complex. The as-synthesized catalysts showed low activity and enantioselectivity in the epoxidation of olefins due to the embedment of active species into the pore wall. In consideration of this, the organo-functionalized mesoporous materials were prepared by the co-condensation of TEOS and 3-aminopropyltriethoxysilane, and used to immobilize the chiral salen Mn(III) complex. The effects of organosilane dosage on the catalytic performance were studied, thus the optimum dosage of 3-aminopropyltriethoxysilane was presented. The prepared heterogeneous catalysts exhibited comparable activity and enantioselectivity to those of homogeneous counterparts for the epoxidation of unfunctionalized olefins, even at lower dosage of catalysts (0.6mol%), thus providing the greatly increased catalytic efficiency.In addition, the multi-step grafting method was also used to immobilize the chiral salen Mn(III) complexes on inorganic mesoporous materials. This method is simple and can be applied to immobilize unsymmetrical chiral salen Mn(III) complexes. The as-synthesized heterogeneous catalyst exhibited excellent enantioselectivity (>99.9% ee) than homogeneous catalyst (49.8% ee) for the epoxidation ofα-methylstyrene. Moreover, these catalysts obtained comparable activity and enantioselectivity to those of homogeneous counterparts for the epoxidation of indene and 1-phenylcyclohexene. The heterogeneous catalysts were stable and could be recycled three times without loss of enantioselectivity.