Controllable Preparation Of Hierarchically Porous Metal-Organic Frameworks By A Template-Assisted Spray Drying Strategy And Their Applications

Metal organic frameworks(MOFs)are organic-inorganic hybrid materials with periodic network structure formed by self-assembly of multi tooth organic ligands and metal coordination centers(metal ions or metal clusters).Because of its large specific surface area,adjustable porosity and good stability,it is widely used in the fields of adsorption,sensing,gas storage and so on.However,the pore sizes of most MOFs reported in the literature are limited in the microporous range(pore size < 2 nm).Although the existence of micropores is helpful to improve the specific surface area,it hinders the mass transfer and diffusion of small molecules in the pores.In addition,it also hinders the application of MOFs materials in the fields of macromolecular reaction and separation.Up to now,the methods of synthesizing hierarchically porous MOFs materials mainly include extension ligand method,template method,posttreatment method and so on.However,the research on hierarchically porous MOFs materials mainly focuses on the preparation of MOFs with two kinds of pores.There are few reports on the new structure hierarchically porous MOFs containing more abundant pore structures such as micro-meso-and macropores to meet different application needs.Herein,a strategy for efficient construction of hierarchically porous MOFs possessing macro-meso-micropores using template-assisted spray drying followed by etching process is proposed.Taking ZIF-8 as an example,using polystyrene(PS)templates,the complete hierarchically porous ZIF-8(HP-ZIF-8)with all the three categories of pores can be easily fabricated.The effects of spray drying parameters,the sizes and the ratio of nano-ZIF-8 and PS templates on the morphology of the product are investigated.The application performances of HP-ZIF-8 in Knoevenagel reaction and adsorption of ofloxacin were further studied.The main research contents and results are as follows:1.HP-ZIF-8 were prepared by spray drying assisted with template method using nanosized ZIF-8 and PS templates as raw materials.The effects of spray drying parameters,nanosized ZIF-8 and PS particle size and proportion on the morphology and pore structure of HP-ZIF-8 were investigated.It is found that the mesopores of HP-ZIF-8 are formed by the close arrangement of nanosized ZIF-8,and the macropores are formed after the removal of PS templates;The mesoporous size can be changed by regulating the size of the nanosized ZIF-8,while the macroporous structure can be adjusted by changing the particle size and mass ratio of PS.The optimum preparation process conditions are as follows: inlet temperature=100 ℃,feed flow rate=22.5 m L/min,compressed gas velocity=536 L/h,precursor solid content=2 wt%,PS particle size=400 nm,and the mass ratio of nanosized ZIF-8/PS is 4:1.Under the optimum process conditions,the obtained HP-ZIIF-8 is spherical with an average particle size of2 μm.It has obvious three-stage pore structure,with microporous pore diameter of 1.2 nm,mesoporous pore diameter of 9 nm and macroporous pore diameter of 400 nm.This method was further extended to successfully prepare hierarchically porous HKUST-1 with good morphology and pore structure.2.The application performance of HP-ZIF-8 in Knoevenagel condensation was studied.The effects of catalyst type,dosage and reactant molar ratio on the conversion of benzaldehyde in Knoevenagel condensation were investigated.The results show that HP-ZIF-8 can accelerate the Knoevenagel reaction process and improve the conversion of benzaldehyde,which can achieve 100%conversion of benzaldehyde within 3 h,while the conversion of benzaldehyde is only 71% after 8 h for traditional microporous ZIF-8;With the amount of catalyst increasing to 30 mg,the complete conversion of benzaldehyde can be achieved in 2 h,while the conversion of benzaldehyde can be completed in 1 h when the molar ratio of malonitrile to benzaldehyde is 3:1.In addition,HP-ZIF-8 has good recycling performance,and the pore structure of the catalyst remains well.3.The application performance of HP-ZIF-8 adsorbing ofloxacin in water was further studied.The effects of adsorption temperature,ion concentration and adsorbent dosage on the adsorption effect of ofloxacin were explored.The results show that the adsorption capacity of ofloxacin increased with the increase of temperature;The adsorption capacity is the highest in KCl solution;When the addition amount of HP-ZIF-8 is 2 mg,the adsorption capacity can reach 674 mg/g;The adsorption process is fitted with the adsorption kinetic model,which accords with the pseudo second-order kinetic model,and chemical adsorption is dominant;The adsorption isotherm fitting of the adsorption experimental data is in line with the Freundlich isothermal adsorption model.