Synthesis,characterization And CO₂ Capture Of Nitrogen-containing Metal-Organic Frameworks

Metal-Organic Frameworks?MOFs?are hybrid porous crystalline materials with high surface areas and diverse pore channels which make MOFs have great potentials in gas storage/separation.The feature of MOFs is their structures can be easily tuned by changing the metal ion,organic ligand or coordination topology.Nitrogenous MOFs include N-only and N-assist coordination species,which are more flexible and designable.In this work,we take the MOFs with nitrogenous ligand as research objects to explore the effects of synthesis method,functional linker,solvent post-treatment,linker extension and hybrid metal nodes on MOFs structure and properties.And study the relationships between MOFs structure and their CO₂adsorption/separation performance,as well as further conversion of CO₂.The main work includes:1.Based on mixed linker H₂bdc and dabco,we have synthesized three pillared-layer MOFs with kagome?kgm?lattice via room temperature synthesis method,namely MBD-rt?M=Co,Ni,Zn?.The isostructural MBD-rt possess two different pore channels:triangular and hexagonal.We proposed that rapid nucleation through room temperature synthesis is the key to obtain the kgm crystal form,which can provide new idea to produce the MOFs with special crystal form.Among three MBD-rt,ZnBD-rt shows the best CO₂ separation properties,and its IAST selectivities on CO₂/CH₄ and CO₂/N₂ are higher than those of the isomeric square lattice?sql?form of ZnBD-rt,indicating the crystal form of polymorphic MOFs has effects on gas separation performance.2.Four monohalogenated pillared-layer MOFs have been systematically synthesized by using mixed linker H₂bdc-X?X=F,Cl,Br and I?and dabco with zinc salt,namely ZnBD-X.ZnBD-X has the similar framework structures to the parents ZnBD_sql and their BET surface area and pore volume decreased along with the increasing of halogen atomic number.ZnBD-Br exhibits the highest CO₂ uptake 46.1cm³/g at 298 K and 1 bar among the four,even higher than non-halogenated ZnBD_sql,due to the combined effects of polarizability and surface area.Meanwhile,ZnBD-I have the highest IAST selectivity and working capacity of CO₂/CH₄?50/50?and CO₂/N₂?15/85?,indicating the effect of halogen's polarizability is more significant in CO₂/CH₄ and CO₂/N₂ separation.3.The effects of solvent post-treatment to the MOFs structure have been investigated by immersing kgm form MBD_kgm in different solvents.ZnBD_kgm and NiBD_kgm maintained their crystallinity in common organic solvent such as chloroform,acetone,acetonitrile and ethanol.Surprisedly,ZnBD_kgm and CoBD_kgm can be transferred into sql form through methanol immersion,while NiBD_kgm cannot.The proposed mechanism is related to the-OH group and molecular size of methanol.The SEM results showed that ZnBD_kgm transferred into sql form with the morphology changing from rod-like to tube-like,which provide new ideas for MOFs morphology control.Finally,we attempted to synthesize kgm form ZnBD-X through rapid nucleation but only ZnBD-Fkgm can be obtained,due to the steric hindrance of larger atom size of Cl,Br and I.And also ZnBD-Fkgm in kgm form can be turned into sql form in methanol as expected.4.To avoid the framework interpenetration caused by ligand extension,which lead to lower surface area,we chose?NiHbtc?formed by H₃btc with Ni salt as 2D layer instead of M₂?O₂CR?₄ paddle-wheel layer and succeeded to construct a new pillared-layer MOFs with pillar-ligand Azpy,namely Ni-btc-Azpy.Fortunately,the Ni-btc-Azpy is non-interpenetrated due to the narrow channel of?NiHbtc?layers.The activated Ni-btc-Azpy shows high BET surface area and pore volume of 2218 m²/g and 0.92 cm³/g,respectively.The henry law selectivity for CO₂/CH₄ and CO₂/N₂ are6.26 and 17.35,respectively.Meanwhile,the IAST selectivities at 298 K within 1 bar are 5.1 and 20 for CO₂/CH₄?50/50?and CO₂/N₂?15/85?,respectively.5.Bimetallic zeolite imidazolate framework has been synthesized in methanollic solution via room temperature synthesis method,namely Zn/Co-ZIF.The structure analysis showed that Zn/Co-ZIF has SOD topology,same as ZIF-8 and ZIF-67.The Zn²⁺and Co²⁺ions are highly dispersed in the framework as the random metal nodes in a similar ratio with feed ratio based on ICP-AES and EDX results.For physical adsorption properties,Zn/Co-ZIF shows slightly higher selective CO₂ adsorption over CH4 and N2.For chemical properties,Zn/Co-ZIF has relatively more Lewis basic sites than ZIF-8 or ZIF-67 and exhibits better catalytic conversion in reaction of converting CO₂ into cyclic carbonate with epichlorohydrin without solvent and co-catalyst and can be reused over three cycles without activity decreasing.Moreover,Zn/Co-ZIF has much higher water stability than ZIF-67 through moisture exposure and water immersion test.