Fabrication Of Functionalized Silicon Nanomaterials For Efficient CO₂ Adsorption And Catalytic Conversion

As a major greenhouse gas,the continuous increase of CO₂ in the atmosphere has caused a series of environmental problems.On the other hand,CO₂ is also an abundant,inexpensive and nontoxic C1 building block,it is expected to become an important resource to replace oil and natural gas as future“carbon source”,the efficient and clean utilization of CO₂ resources has shown important scientific and economic values.Among the various products,the synthesis of cyclic carbonates via 100%atom-economical coupling of CO₂ and epoxides shows great prospect toward industrialization.However,in the catalytic CO₂ cycloaddition reaction,the complicated separation steps and low stability of catalysts as well as harsh reaction condition limit their large-scale production.Therefore,the development of efficient and stable heterogeneous catalysts by introducing new ideas and methods is the key to achieve chemical fixation and conversion of CO₂ under mild conditions.Based on the multi-active sites co-catalytic strategy,several novel silicon-based nanomaterial catalysts were designed and facilely constructed in the present work.The main research contents are as follows:(1)Novel cyclotriphosphazene-based periodic mesoporous organosilicas(PMO-CPF)with controlled pore structure were successfully developed by simple hydrothermal self-assembly method,and the materials were characterized by FT-IR,solid-state ¹³C and ²⁹Si NMR,XRD,N₂ adsorption-desorption and HR-TEM techniques.The developed catalysts of the CO₂adsorption capacity and catalytic conversion into various cyclic carbonates were thoroughly studied.The influence of different catalyst components and several key reaction parameters on catalytic activity were systematically investigated.The prepared PMO-CPF exhibit the structural characteristics of high specific surface area,multiple Lewis bases and ortho dual hydrogen bond donor(HBD)groups.Under the optimized conditions(90°C,2.0 MPa,6.0 h),the optimum PMO-CPF-20 combining with tetrabutylammonium iodide(TBAI)showed 98%propylene carbonate(PC)yield with above 99%selectivity.The PMO-CPF were proved to be versatile catalysts.In addition,the unusually stable catalyst could be reused five times in a row without a significant decrease of the catalytic efficiency or structural deterioration.(2)On the basis of the previous work,several triazine and hydrazo sites co-modified periodic mesoporous organosilicas(THPMO-x)were prepared via a simple hydrothermal self-assembly method in order to further improve the adsorption capacity of the material for CO₂,and the obtained catalytic materials were characterized by a variety of characterization techniques.The catalyst with excellent adsorption capacity and efficient conversion for CO₂was selected by regulating the content of silicone precursor in the structure.Excitingly,the CO₂ adsorption capacity of THPMO-25 was1183?mol/g.Meanwhile,this catalyst combining with TBAI exhibited excellent reactivity for their catalytic activity in the cycloaddition of epoxide and CO₂under mild,metal-and solvent-free conditions.Moreover,THPMO-25 showed well-ordered and stable mesoporous channel,which endowed the materials with a superior recyclability.Combining the FT-IR spectra of chemical interaction between CO₂and THPMO-25catalyst,a plausible synergistic catalytic mechanism of Lewis base and HBD groups was proposed.Wherein,the triazine units as Lewis bases played the role of activating CO₂,and the hydrazine units as HBD groups activating epoxide.(3)To replace co-catalyst in the reation,chitosan-silica nanomaterials modified by Br-based ionic liquids(CS-Si O₂@Im Br)was developed as catalysts of the cycloaddition reaction of CO₂ with various epoxides.Under the optimized reaction conditions(110 ^oC,2.0 MPa,7 h),excellent PC yield and selectivity were achieved,and the catalyst had universal applicability to other epoxides.In addition,the influence of the amount of the supported ionic liquids on the catalytic activity was studied.The prepared CS-Si O₂@Im Br exhibited multiple functionalities of HBD groups,nucleophilicity(Br^-)and Lewis bases property,and the synergetic effects promoted the reaction smoothly.The single-component catalyst was also easy to prepare and could be recycled by simple filtration,overcomed the drawbacks of difficult separation and water sensitivity for homogeneous ionic liquids.