Synthesis And Reversible Ammonia Adsorption Of Flexible Metal-Organic Frameworks

In recent years,air pollution caused by NH₃ emissions is getting worse,NH₃ is not only directly harmful to human health in the atmosphere,but also attracts people’s attention for its involvement in the formation of PM_2.5.Studies have shown that NH₃ is an important promoter for the formation of PM_2.5.As the most alkaline gas in the atmosphere,it reacts with acidic polluting gases（SO₂,NO_x,etc.）,which is beneficial to form fine particles of ammonium salts.The content of ammonium salt in PM_2.5 can be as high as 40%-60%,which is an important component of haze.In China,annual emission of NH₃ pollution into the atmosphere reaches up to 10.2 million tons,which is the highest in the world and brings serious environmental pollution.Industrial and agricultural emission of a large amount of NH₃,meanwhile,China needs to synthesize at least 60million tons of NH₃ to be applied in chemical industry,light industry,fertilizer,pharmaceutical industry,and synthetic fiber industry every year,of which 85%are used in nitrogen fertilizer production to ensure the national grain supply.Therefore,reduction and recycling of NH₃ is an important task that needs to be solved.In addition,NH₃,as a hydrogen-rich fuel,is an excellent hydrogen-energy carrier without carbon dioxide emissions,hence,it has great potential in the field of fuel cells.Due to the corrosivity and destructiveness of NH₃,its safety and stability problems need to be broken through in the storage and transportation of NH₃.Whether it is to reduce NH₃ pollution or to conduct stable and safe NH₃adsorption/storage research,adsorbents both play a key role.In particular,various new types of adsorbents have been emerged in recent years.Metal-organic frameworks（MOFs）are typical representatives for new adsorbents,they have many advantages such as structural diversity,adjustable pores,large specific surface area,and flexible structure transformation,making them have attractive application prospects in gases adsorption and separation,molecular catalysis,CH₄ storage,drug release,etc.The studies of MOFs adsorption on NH₃ have been reported in recent years,suggesting a great advantage in the structural diversity and adsorption amount.Flexible MOFs,as a special kind of material with flexible structural change,have unique advantages in NH₃ adsorption.According to the stimulating response of NH₃ on MOFs,flexible MOFs can stably adsorb a large number of NH₃ molecules while perform moderate structure conversion,and release ammonia molecules at a certain temperature to achieve structural reversibility.Compared with most rigid MOFs,this special property of flexible MOFs can effectively be applied to NH₃ adsorption by overcoming the strong destructive action of NH₃.This work mainly focuses on three series of flexible MOFs and compares them with some classic rigid MOFs.The synthesis and characterization of the materials were carried out,and the structural flexibility of different flexible materials for NH₃ adsorption was also explored.Finally,the performance of NH₃ adsorption and regeneration ability of the materials were investigated.The main research contents and conclusions are as follow:（1）M（INA）₂（H₂O）₄（M=Cu,Co,Ni,Cd,Zn）（INA series）materials were prepared by traditional hydrothermal synthesis and NH₃-assisted synthesis method.The structure and stability of samples were analyzed by XRD,SEM,TG-MS,FT-IR,TEM and N₂ adsorption/desorption techniques.The structural transformation properties of a series of M（INA）₂（H₂O）₄ and M-2（INA）（M=Cu,Co,Ni,Cd,Zn）materials were studied.And utilize the special structure transformation phenomenon,the use of flexible MOFs in NH₃ adsorption is proposed.The results show that obtained M-2（INA）（M=Cu,Co,Ni,Cd）materials have good pure NH₃ and H₂O/NH₃ adsorption capacities,their adsorption capacity of pure NH₃ is up to 12–13 mmol/g,and the adsorption amount in H₂O/NH₃ environment is 6.4,6.5,6.4 and 5.2 mmol/g,respectively;the adsorption capacities of Zn（INA）₂ in pure NH₃ and H₂O/NH₃ environment are both 6 mmol/g.All materials can be regenerated by activation in the temperature of 120–150?C and the adsorption performances have no loss after three cycles,which were reusable NH₃ adsorbent materials.（2）In order to improve the NH₃ adsorption capacity by ligands modulation,Cu（BDC）（DMF）,Zn₃（BDC）₃（H₂O）₃·4DMF and Cd（BDC）（DMF）（BDC series materials）were obtained by solvothermal synthesis.Samples were analyzed by XRD,TG,H₂O and N₂ adsorption/desorption techniques.Structural flexibility properties of the three materials were investigated in the adsorption and desorption of small molecules（DMF,H₂O and NH₃）.The results show that the M（BDC）（M=Cu,Zn,Cd）series materials have high NH₃ adsorption properties,among which Cu（BDC）has a repeatable adsorption capacity of 17 mmol/g,the adsorption amount of other two materials is 14 and 7.4 mmol/g,respectively.After adsorption of NH₃,the structures of M（BDC）materials were converted into M（BDC）（NH₃）₂（M=Cu,Zn,Cd）.The NH₃ bound in this conversion is relatively stable,and the removal of NH₃ can be achieved by vacuum activation at 250°C for 2 h,and the stable adsorption amount can be maintained in cycles test.Although the NH₃ adsorption capacities of BDC series materials are increased,there is a problem that activation and regeneration is difficult.（3）In order to simultaneously realize the high adsorption capacities and the easier regeneration conditions through ligands modulation,M（NA）₂（H₂O）₄ and M（NA）₂（M=Zn,Co,Cu,Cd）（NA series）materials were prepared by NH₃-assisted synthesis and solid solvent synthesis,respectively.The structure,stability and flexibility of samples were analyzed by XRD,SEM,EDS,TG,FT-IR,H₂O and CO₂ adsorption/desorption techniques.It is concluded that the NA series materials are also a kind of material with flexible structure.The NH₃adsorption performances of the materials were also tested.Among them,Zn（NA）₂ showed two-step adsorption,and the adsorption amount was low at low pressure.When the pressure is increased to 0.22 atmosphere,the layer structure will open and absorb a large amount of NH₃,finally reaching the adsorption amount of 10.2 mmol/g.The adsorption amounts of Co（NA）₂,Cu（NA）₂ and Cd（NA）₂ were 17.5,13.4 and 6 mmol/g,respectively.All four materials can be regenerated at 150?C and retain NH₃ adsorption capacity after cycles,simultaneously realize high adsorption capacity and easy regeneration.（4）Compared with flexible MOFs,several rigid MOFs are listed for NH₃adsorption studies.Cu-BTC and ZIF-8 were obtained by solid solvent synthesis,MIL and MOSs series materials were prepared by solvothermal synthesis.The results show that Cu-BTC has a high NH₃ adsorption capacity,but NH₃ will cause irreversible damage to its structure;the structure of ZIF-8 is very stable,but its NH₃ adsorption amount is very low.Therefore,if rigid MOFs can be used for NH₃ adsorption applications,it is necessary to ensure high stability of the structure and adsorption capacity of small polar molecules.It is confirmed that MIL series materials have strong structural stability in NH₃ adsorption,MIL-100and MIL-101 have high and repeatable adsorption capacities of 8 and 10 mmol/g.In addition,the structures of MOSs protected by antennas are also very stable and have strong ability to adsorb polar small molecules.MOS-1 has an NH₃adsorption capacity of 11.5 mmol/g,and the MOSs series materials maintain their performance stability after several cycles.However,compared with flexible MOFs,the adsorption capacity of rigid MOFs is commonly lower than12 mmol/g.The results show that MOFs with very stable structures are necessary for NH₃ adsorption applications,they have no advantages in adsorption capacity and binding stability of NH₃.