Preparation Of MOFs Grafted On SBA-15 Composite Catalysts And Their Application In Catalytic Conversion Of CO₂

Recently,the cycloaddition reactions of CO₂ with epoxides forming cyclic carbonates have been widely studied to fix CO₂.However,CO₂ is thermodynamically stable,various catalysts have been used to promote the reactions.However,the developed catalysts often require harsh reaction conditions,co-catalysts and solvents,but result in low activities.In this thesis,a series of MOFs/SBA-15 composite catalysts were synthesized by using layer-by-layer assembly method to coordinate the unfunctionalized or pre-functionalized ligands with metals in SBA-15 channels.Using the confinement and structure orientation of SBA-15 channels,a series of MOFs/SBA-15 composite catalysts were synthesized.And the catalytic performances of the composite catalysts for the cycloaddition of CO₂ to propylene oxide were investigated.The research contents and main results obtained were as follows:（1）ZIF-8 nanocrystals were loaded into SBA-15 mesoporous channels through layer-by-layer assembly method,and a series of hybrid materials with different contents of initial loaded Zn,assembly layer numbers and coordination states of ZIF-8were prepared.The successful loading of ZIF-8 was proved by various characterizations such as XRD,FT-IR,N₂ adsorption-desorption isotherms,SEM,SAXS.And the catalytic performances of these composite catalysts for the cycloaddition of CO₂ to propylene oxide were investigated in detail.It was found that when the mass ratio of Zn（NO₃）₂·6H₂O to SBA-15 was 0.5:1,and loading number of ZIF-8 layers was 4,more metal active sites in the hybrid catalyst could be exposed,thus the catalytic activity was the best,which was significantly better than that of pure ZIF-8.In addition,due to the effective protection of ZIF-8 by the mesoporous structure of SBA-15,the catalyst showed better recyclability than pure ZIF-8.（2）On the basis of the above research,in order to further improve the activity and stability of the catalysts,the ligand MOF-5-Br was pre-functionalized to introduce the basic active center Br-.Subsequently,MOF-5-NEt₃Br/SBA-15 hybrid catalysts with different loading content of MOF-5-NEt₃Br were synthesized through layer-by-layer assembly method.Meanwhile,the physicochemical properties were characterized by the means of XRD,FT-IR,TEM.The results showed that pure MOF-5-NEt₃Br did not possess effective pore structures,and the mesoporous structure of SBA-15 was maintained after the loading of MOF-5-NEt₃Br.Due to the introduction of basic active sites and the uniform dispersibility of MOF-5-NEt₃Br in the pores of SBA-15,the composite catalysts MOF-5-NEt₃Br/SBA-15 exhibited much higher catalytic activity.When the number of MOF-5-NEt₃Br layers in SBA-15 was 2,the MOF-5-NEt₃Br/SBA-15 presented a yield of 41.4%under the reaction conditions of 1MPa,120 ^oC,6 h,which was significantly higher than that of pure MOF-5-NEt₃Br（19.9%）.Especially when the reaction time was extended to 22 h,the yield reached up to 97%and after six consecutive cycles,a high yield of 81%was maintained.（3）On the basis of the above research,the central metal ion（Zn）was expanded into Cr,Co,Cu and Ni ions to further confirm the generality of this synthetic strategy.MOF-5-NEt₃Br（Cr）/SBA-15,MOF-5-NEt₃Br（Co）/SBA-15,MOF-5-NEt₃Br（Cu）/SBA-15 as well as MOF-5-NEt₃Br（Ni）/SBA-15 were synthesized respectively,and their performance in catalyzing the CO₂ cycloaddition was preliminarily investigated.The results showed that these catalysts could be used to catalyze the reaction and MOF-5-NEt₃Br（Co）/SBA-15 displayed the best catalytic performance with a catalytic yield of 82.8%under the reaction conditions of 1 MPa,120 ^oC,and 22 h.