Synthesis Of Porous Aromatic Frameworks With Lewis Acid Active Centers And Their Functional Properties

Porous aromatic frameworks?PAFs?,constructed from rigid aromatic building units with total covalent bonds,are a new kind of porous materials.PAFs have many advantages such as low skeleton density,high specific surface area,large porosity,diverse structural features and high thermal and chemical stability.Thus they were widely explored to be applied in many research fields such as gas sorption and separation,heterogeneous catalysis,capture of harmful compounds and sensing.In this thesis,two novel PAFs?PAF-165 and PAF-166?were successfully synthesized and characterized in detail to confirm their structural features.Moreover,their applications in gas adsorption,CO₂/N₂ separation and heterogeneous catalysis were investigated.Firstly,triphenylboron with electron-deficient structure of boron center was selected as building units to construct novel PAFs containing Lewis acid active sites.By the copolymerization of triphenylboron monomer with 1,4-bis?bromomethyl?benzene and 1,3,5-tris?bromomethyl?benzene via Friedel-Crafts alkylation reaction,PAF-165 and PAF-166 were successfully synthesized.These two PAFs present great thermal stability and are stable up to 350 ^oC.N₂ sorption measurements suggest that both PAFs possess high surface areas.According to Brunauer-Emmett-Teller?BET?model,the surface areas of PAF-165 and PAF-166 are 867 and 754 m²/g,respectively.Since a large number of Lewis acid active sites?boron centers with electron-deficient structure?exist in the framework of resulted PAFs,they can effectively accept the electrons of gas molecules to enhance the interactions between the framework and guest,thus bring great improvement of their gas adsorption ability.Therefore,because of the presence of lone pairs of electrons of CO₂ molecule,CO₂ sorption of PAF-165 and PAF-166 was performed,and CO₂/N₂ separation performances of these two PAFs were simulated and calculated according to the ideal adsorbed solution theory?IAST?.Unsurprisingly,the results show that both PAFs present high sorption ability for CO₂as well as high CO₂/N₂ selectivity.At 273 K and 1 bar,the equimolar mixture selectivity of PAF-165 and PAF-166 are 106.2 and 196.5,respectively.Furthermore,C₂H₄ and C₂H₆ gas adsorption of PAFs were measured,and PAF-165 exhibited higher adsorption capacity,which might be reason of higher surface area of PAF-165 compared with PAF-166.Taking full advantage of Lewis acid active sites in the framework of PAFs and their high selective adsorption ability over CO₂,PAFs could act as efficient catalyst via forming frustrated Lewis pairs with basic substrate to activate of CO₂ molecules.Therefore,the cyclization of o-phenylenediamine catalyzed by PAF-165 and PAF-166 we conducted,as o-phenylenediamine is a basic substrate.Through a series of experiments,it was proved that the synthesized PAFs had excellent catalytic ability on the cyclization of o-phenylenediamine,and the conversion rate was up to 99% under proper reaction conditions.Meanwhile,PAF-165 and PAF-166 possessed good recyclability and can maintain the catalytic performance after five cycles.Recycled PAFs catalysts were characterized by a series of characterizations to confirm their chemical structures,surface morphologies and thermal stability.And the results showed that these two PAFs possessed good stability without any change of their structures,indicating PAFs were great heterogeneous catalysts.In summary,two novel PAFs,PAF-165 and PAF-166,containing Lewis acid active centers,were successfully synthesized,and these two PAFs had high surface areas and great thermochemical stability.Benefiting from the active sites in the framework,they presented good gas sorption abilities for CO₂,C₂H₄ and C₂H₆,and also had a high selective sorption effect over CO₂/N₂.Furthermore,by forming frustrated Lewis pairs with the reaction substrate to activate CO₂ molecules,PAF-165 and PAF-166 exhibited excellent activities in the cyclization of o-phenylenediamine,which paved the path for the application of frustrated Lewis pairs in heterogeneous catalysis.