Synthesis Of Schiff Based Covalent Organic Frameworks And Their Adsorption And Separation Of Small Hydrocarbons

Covalent Organic Frameworks(COFs)is a kind of crystalline porous organic frameworks material formed by polymerization of organic building units,which have the characteristics of large specific surface area,high stability,diverse structure and strong design.Schiff base reaction is one of polymerization reactions for constructing COFs.At present,the synthesis of Schiff base COFs mainly include solvothermal synthesis,mechanical grinding,ionic liquid synthesis,microwave assisted synthesis,ultrasonic synthesis and room temperature solution synthesis.Solvothermal synthesis using acetic acid as catalyst is the most widely method.In this reaction,the structure and properties of the material are affected by acetic acid concentration and volume,reaction solvent type and corresponding proportion,activated solvent type,reaction time and reaction temperature.Considering synthetically the effect of reaction conditions,the structure and performance of materials could be optimized.High purity methane is widely used in aerospace,petrochemical engineering,and electronics.etc.Natural gas is the main source of methane,mainly composed of methane(the concentration may be different in each producing area.Generally,methane accounts for about 85%of natural gas),and other components including ethane and propane in different ratio.Therefore,obtaining high purity methane requires separation of ethane/methane and propane/methane.Ethylene(nearly 200 million tonnes worldwide)is a vital raw material in the petrochemical industry,for preparing polyethylene and other high-value products.Ethylene is produced by steam cracking or pyrolysis,and these processes might bring a small number of ethane and acetylene.Polymer grade ethylene(≥99.95%)is a critical factor in preparation of high performance polymers.Therefore,the efficient separation of ethane/ethylene and acetylene/ethylene is the key to prepare high purity ethylene.High purity methane and ethylene can be obtained by using cryogenic distillation.In recent years,pressure swing adsorption using porous materials as adsorbent has been widely concerned in gas separation.The core of this technology is the adsorbent,which also requires the development of different types of porous materials.Over the past two decades,researchs regarding to COFs has been increasing.This thesis focuses on the pore regulation of COFs and their adsorption and separation performance of small hydrocarbons.Organic building units are the basis for preparing COFs,which are easy to modify.Structural modification of COFs can be obtained by introducing special functional groups into organic building blocks.This will effectively adjust the pore surface,pore size,pore volume,and pore geometry of COFs.In the process of gas adsorption,the surface property of porous materials is very important,which determines the interaction between subject and guest.In this thesis,the synthesis of Schiff base type COFs containing trifluoromethyl(-CF3)and ethoxy groups,and the adsorption and separation performance of small hydrocarbons are studied.The details are as follows:1.Synthesis of a-CF3 modificatory TPCOF-CF3-s and its adsorption and separation properties of small hydrocarbons.A-CF3 modificatory TPCOF-CF3-s is prepared by using 2,4,6-triformyl-homoglucinol and 2,5-diamino-trifluorotoluene as monomers,and acetic acid as catalyst.During synthesis,more than twenty reaction conditions are screened by considering the amount and concentration of acetic acid,the type of reactive solvent,the type of activated solvent and the proportion of reactive solvent.TPCOF-CF3-8 prepared by n-butanol/o-dichlorobenzene(1:1,2 mL)as solvent and 6 M acetic acid as catalyst displays high crystallinity.TPCOF-CF3-8 is a typical microporous material.The BET(Brunauer-Emmett-Teller)surface area is 825 m2·g-1 and a primary pore size is about located at 1.5 nm.At 1 bar and 273 K,the CH4 uptake of TPCOF-CF3-8 is only 6.5 cm3·g-1.When the temperature increased to 298 K,the methane adsorption capacity decreased to 4.8 cm3·g-1.The CH4 uptake of TPCOF-CF3-8 is low,which is favorable in natural gas purification.In addition,at 1 bar and 273 K,the uptake of C2H6 and C3H8 for TPCOF-CF3-8 is 46 cm3·g-1 and 75 cm3·g-1,respectively.When the temperature rose to 298 K,the adsorption capacity of C2H6 is 30 cm3·g-1,and that of C3H8 is 63 cm3·g-1.The ideal gas adsorption solution theory can be used to predict the separation performance of binary gas components.At 273 K and 1 bar,the C3H8/CH4 and C2H6/CH4 selectivity of TPCOF-CF3-8 are 5016 and 38.8,respectively.At 298 K and 1 bar,the selectivity of C3H8/CH4 and C2H6/CH4 are 346.8 and 13.5.TPCOF-CF3-8 shows good potential in natural gas purification.2.Explore the preparation of TPCOF-CF3-s under other synthetic systems.Explored by three different synthetic systems,TPCOF-CF3-30～37 is prepared by using 2,4,6-triformylhomophenol and 2,5-diamino-trifluorotoluene as monomers.Powder X-ray diffractions(PXRD)show that TPCOF-CF3-35,TPCOF-CF3-30,and TPCOF-CF3-33 have obvious diffraction peaks(from high to low crystallinity)around 5 degree.The BET surface area of TPCOF-CF3-30,TPCOF-CF3-33,and TPCOF-CF3-35 is 106 m2·g-1,131 m2·g-1 and 559 m2·g-1,respectively.The pore sizes are mainly microporous(1.78 nm,1.72 nm and 1.55 nm,respectively),and pore volume is 0.11 cm3·g-1,0.26 cm3·g-1 and 0.35 cm3·g-1,respectively.In comparison,TPCOF-CF3-8 has better crystallization and larger surface area.The structure and properties of Schiff base COFs synthesized in different systems are quite different.3.Synthesis of COF-OCH2CH3-1 and COF-OCH2CH3-2 containing-OCH2CH3 and their adsorption and separation properties of C2 hydrocarbons.The planar triangular 1,3,5-tri(4-aminophenyl)benzene and 2,4,6-tri(4-aminophenyl)-1,3,5-triazine reacts with linear ethoxy-containing 5-diethoxy-p-phenyl diformaldehyde,respectively.Schiff base COFs(COF-OCH2CH3-1 and COF-OCH2CH3-2)containing-OCH2CH3 are synthesized under acetic acid catalysis.Nitrogen adsorption and desorption isotherms show that they are mesoporous.The BET surface area of COF-OCH2CH3-1 is 2319 m2·g-1,and that of COF-OCH2CH3-2 is 1709 m2·g-1.The primary pore size is about 3.09 nm and 2.89 nm,respectively.At 273 K and 1 bar,the uptake of C2H6 for COF-OCH2CH3-1 is 60.50 cm3·g-1 and the uptake of C2H4 is 52.08 cm3·g-1.At 298 K,the adsorption capacities of C2H6 and C2H4 for COF-OCH2CH3-1 is 35.53 cm3·g-1 and 30.75 cm3·g-1,respectively.At 1 bar and 273 K,the uptake of C2H6 and C2H4 for COF-OCH2CH3-2 is 43.60 cm3·g-1 and 36.86 cm3·g-1,respectively.The adsorption capacity of C2H6 and C2H4 for COF-OCH2CH3-2 is 26.30 cm3·g-1 and 22.45 cm3·g-1 at 1 bar and 298 K.Given the time constraints of the study,further investigations on the adsorption and separation of C2H6 and C2H4 with respect to COF-OCH2CH3-1 and COF-OCH2CH3-2 are on the way.