Preparation Of Multi-shelled Hollow Mesoporous Organosilica Nanospheres Supported Chiral Organic Catalysts And Its Application In Asymmetric Catalysis

Hollow nanomaterials have diverse potential applications in many fields such as catalysis,battery,chemical sensor,biomedicine and energy storage and conversion.In the past few decades,scientists have studied a range of different hollow materials,in-cluding polymers,metal oxides and inorganic-organic hybrid materials.Among these materials,multi-shelled hollow organosilica nanospheres（MHMOSs）have attracted widespread attention owing to their unique properties such as high specific surface area,low density,high load capacity and high chemical stability.These special struc-tures of MHMOSs are especially suitable for the design of catalysts with high-efficiency.However,chiral organocatalyst is difficult to be introduced into the structure of MHMOSs owing to the serious impact of organocatalyst on the well-controlled mor-phology of MHMOSs.In this paper,the MHMOSs,functionalized by chiral organoca-talyst,is prepared in a simple and cost-effective way and successfully applied in heterogeneous asymmetric organocatalysis,which has important research value and application prospects.In this paper,a simple and effective method for the preparation of MHMOSs was successfully developed.The surface morphology,acid loading capacity,chemical composition and pore structure of MHMOSs are systematically studied by nitrogen adsorption-desorption,SEM,TEM,IR,elemental analysis and acid-base titration.After the–SH group is oxidized to sulfonic acid and cinchona base-derived primary amine catalyst（QDNH₂）is immobilized via acid-base reaction,QDNH₂ and SO₃H-functionalized multi-shelled hollow organosilica nanospheres is obtained and applied in asymmetric Michael addition.The paper mainly includes the following contents:In the first chapter,the preparation of hollow nanomaterials and multi-shelled hollow nanospheres is reviewed.The advantages and applications of multi-shelled hollow nanospheres are summarized,and the significance of multi-shelled hollow nanospheres for heterogeneous catalysis is expounded.In the second chapter,a simple and effective soft template method under hydro-thermal condition is used to prepare various hollow mesoporous organosilica nano-spheres,including single-shelled（SS-SiO₂-SH）,double-shelled（DS-SiO₂-SH）and triple-shelled（TS-SiO₂-SH）,by adjusting molar ratios of TEOS,BTSE and MPTMS and of silicon sources in aqueous ethanol solution.Then the–SH group is oxidized to sulfonic acid by hydrogen peroxide,hollow nanosphere solid acids（SS-SiO₂-SO₃H,DS-SiO₂-SO₃H and TS-SiO₂-SO₃H）are obtained.After primary amine QDNH₂ is adsorbed into nanosphere via acid-base reaction,the multi-shelled hollow mesoporous organosilica nanospheres SS-SiO₂-SO₃H/QDNH₂,DS-SiO₂-SO₃H/QDNH₂ and TS-SiO₂-SO₃H/QDNH₂ are prepared.In the preparation process,it is found that the molar ratio of silicon precursor and solute in solution have significant effects on the morphology and structure of MHMOSs.When a single precursor MPTMS（2.0 mmol）is used,the inner layer of MHMOSs is too dense to form a multi-shelled hollow structure.Then,silicon source TEOS and ethyl bridged silicon source BTSE are add-ed to improve the spherical morphology and multi-shelled structure.The optimum molar ratio of mixed TEOS,MPTMS and BTSE is 0.5:0.7:0.3.In addition,under un-changed total volume of solvents,the volume of water is increased from 70 mL to 90 mL,the nanosphere size is reduced from about 200 nm to 70 nm.Under the optimal conditions,the as-fabricated SS-SiO₂-SH(530.3 m² g^-1,0.79 cm³ g^-1,3.8 nm),DS-SiO₂-SH(512.4 m² g^-1,0.81 cm³ g^-1,3.9 nm)and TS-SiO₂-SH(382.5 m² g^-1,0.38 cm3/g,3.8 nm)possess well-shaped spherical morphology and large specific surface areas,pore volumes and pore sizes.The loading capacities of thiol are 1.59 mmol g^-1,2.23 mmol g^-1 and 2.34 mmol g^-1,respectively.After the oxidation of–SH to-SO₃H The acid capacities with 0.35 mmol H⁺g^-1,1.50 mmol H⁺g^-1 and 0.80 mmol H⁺g^-1 are produced.According to the nitrogen contents determined by elemental analysis,the loading capacities of QDNH₂ in SS-SiO₂-SO₃/QDNH₂,DS-SiO₂-SO₃H/QDNH₂ and TS-SiO₂-SO₃H/QDNH₂ were 0.40 mmol g^-1,0.51 mmol g^-1and 0.47 mmol g^-1,respectively.The third chapter proposes to apply the above three heterogeneous catalysts to the asymmetric Michael addition reaction of 3-methyl-2-cyclohexen-1-one with trans-β-nitrostyrene.The experimental results indicate that DS-SiO₂-SO₃H/QDNH₂ with double-shelled structure has the fastest reaction rate with the yield（68%）and enantio-selectivity（83%ee）at 48 h.DS-SiO₂-SO₃H/QDNH₂ also exhibits good to excellent catalytic properties with 62-84%yields and enantioselectivities（83-96%）for other trans-β-nitrostyrenes bearing both the electron-withdrawing and electron-donating groups.After the end of the catalytic reaction,DS-SiO₂-SO₃H/QDNH₂ was recover-ed by centrifugal separation and directly applied in the following reactions.It is found that the 5^th-for DS-SiO₂-SO₃H/QDNH₂ still maintain good catalytic performances.