Study Of Nitrogen-containing Organic Polymers For Catalytic CO₂ Cycloaddition Reactions

The rise of CO₂content in the atmosphere poses a serious threat to the ecological environment and human survival.On the other hand,CO₂is also a readily available industrial raw material.And CO₂capture and utilization are strategically important for achieving the emission peak and carbon neutrality.It is worth noting that the cycloaddition reaction of CO₂to epoxides is a promising chemical reaction,since it is line with the principle of green chemistry,and cyclic carbonates have a wide range of applications.However,the chemical inertness of CO₂hinders this cycloaddition reaction,it is meaningful to synthesize suitable catalysts to activate CO₂molecules.Among them,nitrogen-containing organic polymers are a class of important catalysts.They can interact with CO₂to activate them,and can adjust the structure and performance of the catalyats through functional mofification.However,metal-modified nitrogen-containing organic polymers suffer from problems such as expensive monomers and complex preparation processes,and ionic nitrogen-containing organic polymers have problems such as the use of cocatalysts and organic solvents.In this thesis,nitrogen-containing organic polymers with high catalytic activity were prepared by the functionlization of functional groups and introduction of hierarchical pores,which were applied to catalyze the CO₂cycloaddition reaction.Polyethyleneimine（PEI）containing abundant amines was selected to form a Lewis acid-base binary catalytic system（PEI/Zn X₂）together with Zn X₂（X=Cl,Br,I）as cocatalysts.Among them,the primary and secondary amines in PEI and Lewis acidic zinc ions could activate epoxides,the nitrogenous groups（primary,secondary and tertiary amines in PEI）as Lewis bases could active CO₂molecules,the cycloaddition reaction of CO₂to epoxides could be efficiently promoted by the synergetic effects of Lewis acid-base sites.The screening of cocatalysts showed that the synergistic catalytic effect of PEI and Zn I₂was the best,and they could efficiently convert CO₂with propylene oxide（PO）into propylene carbonate（PC）at the conditions of 100°C,1.0 MPa and 3.0 h,its yield and selectivity were 95%and96%,respectively.Due to the problem of easy deliquescence of Zn X₂in air,a series of coordination metal-modified pyridine-polyethyleneimine catalysts（M-Py-PEI）were further prepared to avoid the use of Zn X₂and to improve the catalytic efficiency under milder conditions.Among them,epoxides could be activated by coordinated metal ions,CO₂molecules could be activated by secondary and tertiary amines.The catalytic system of Co-Py-PEI/Bu₄NBr could efficiently convert CO₂with PO into PC at the conditions of 30°C,1.0 MPa and 24 h,its yield and selectivity were both 99%.Co-Py-PEI showed great catalytic activity after seven runs,In addition,Co-Py-PEI/Bu₄NBr exhibited high catalytic activity for epoxides with small molecular size.The use of cocatalysts was not conducive to the purification of the products and industrial applications,and the easy loss of transition metals in the catalytic process could lead to catalyst deactivation and negative environmental impacts.In this study,single-component nitrogen-containing organic polymers were further developed,and a series of pyridine-based ionic porous organic polymers Py-POP-X（X=Cl,Br）were obtained by a one-pot method.Py-POP-X（X=Cl,Br）possessed plentiful hierarchical pores and high surface area.The plentiful hierarchical pores were conductive to the enrichment of CO₂and the mass transfer of reactants/products,the pyridine groups were conductive to the activation of CO₂,and the halogen ions promoted the ring-opening of epoxides,the presence of catalytically active functional groups and porous structures facilitated the catalytic reaction.The Py-POP-Br had high CO₂adsorption performance(1.72 mmol g^-1,273 K and 0.1MPa),and it could efficiently catalyze the cycloaddition reaction of CO₂and PO without the use of metal/cocatalyst/solvent（120°C,2.0 MPa and 8.0 h）,the yield and selectivity of PC were 97%and 99%,repectively.After four runs,Py-POP-Br still exhibited great catalytic activity with yield and selectivity of 92%and 99%for PC.In order to further improve the catalytic activity of single-component nonmetallic catalyst,a boron-modified pyridine-based ionic porous organic polymer（B-POP-Cl）with both Lewis acidic and basic sites was successfully synthesized by introducing boron-containing groups with Lewis acidity.Among them,epoxides could be activated by the Lewis acidic boron-containing group,and the presence of the Lewis basic chloride ions was conducive to the ring-opening of epoxides.Moreover,B-POP-Cl had high CO₂adsorption capacity(1.46 mmol g^-1,273 K and0.1 MPa)due to its plentiful hirearchical pores and high surface area.Under the conditions of 120°C,2.0 MPa and 5.0 h without cocatalysts and solvents,B-POP-Cl could efficiently catalyze the cycloaddition reaction of CO₂to PO,and the yield and selectivity of PC were 94%and 99%,repectively.