Preparation Of Magnetic Hierarchical Porous MOFs And Study Of Adsorption Performance

The improvement of modern industrialization and urbanization has brought about increasingly serious environmental pollution problems,and there is an urgent need to develop the technology of efficient environmental pollution treatment.Thanks to the emergence of various new materials（ferroferric oxide,ion exchange resins,molecular sieves,ultra-high cross-linked polymers,covalent organic frameworks,metal organic frameworks,etc.）,adsorption science and technology have been greatly developed.Among them,porous ferroferric oxide（Fe3O4）and its composite adsorbents with metal organic frameworks（MOFs）have attracted attention due to their high specific area and separability under external magnetic field.MOFs materials have been extensively studied due to their rich variety,high specific surface area,high porosity,and flexible and controllable properties.However,most MOFs have microporous characteristics,which are not conducive to mass transfer and limits their application in the field of pollutant adsorption and removal.Therefore,researchers have tried to improve the application performance of MOFs by pore control and the combination of MOFs and magnetic Fe3O4,and have achieved some results.However,so far,scientific issues such as how to efficiently control the pore structure of MOFs,how changes in the pore structure of MOFs affect the adsorption performance of pollutants,and how to achieve efficient combination of magnetic Fe3O4 and MOFs materials have not yet been resolved.In response to the above problems,we developed a new strategy for regulating the porous structure of MOFs by exploring the formation mechanism of hollow porous structure in Fe3O4,and realized the efficient combination of Fe3O4 and MOFs by analyzing the internal relationship between the preparation systems of them,and studied the effect of novel hierarchical porosity on the adsorption performance.The main research contents and conclusions are as follows:（1）Hollow magnetic microspheres（MHPMs）have been prepared using hydrothermal method.Besides,the carbonized products of MHPMs（MHPMs-C）have also been prepared via calcination in an inert gas atmosphere.The key role of structure-directing agent sodium polyacrylate（PAAS）in the formation of MHPMs structures was uncovered via characterizing and analyzing the differences between the two magnetic products.MHPMs and MHPMs-C were used as adsorbents for neutral red adsorption.The experimental results show that the embedded PAAS makes MHPMs a better adsorbent,which can quickly adsorb and remove neutral red molecules from water.The adsorption kinetics of MHPMs follow the pseudo-second-order kinetic model(the second-order kinetic constant is 0.775 g mg^-1 min^-1).Moreover,the fitting result of the Langmuir isotherm model shows that the adsorption of neutral red by MHPMs is monolayer chemical adsorption,and the maximum adsorption capacity is 61.5 mg g^-1.At last,the results of the adsorption mechanism research indicate that the active sites of MHPMs for the adsorption of neutral red is carboxyl groups,which also makes the MHPMs adsorb selectively cationic dye molecules containing only amino groups.（2）Inspired by the formation of the hollow mesoporous structure of MHPMs,Poly dimethyl diallyl ammonium chloride（PDDA）was selected as the structure-directing agent for synthesizing Hp ZIF-8-10（D）with hierarchical porosity via a one-step method in an aqueous system.And a series of target products were prepared by adjusting the amount of PDDA added.The PDDA in the system will cause defects in the structure of ZIF-8 product and reduce the specific surface area,but the negative effects can be balanced to a certain extent by increasing the proportion of ligands.The time-dependent experimental results of Hp ZIF-8-10（2）show that the formation mechanism of the gradient porous structure mainly includes the Oswald ripening process after the nucleus reunion and the crystal orientation growth induced by PDDA.Among the series of products,Hp ZIF-8-10（1.5）has the best adsorption performance on tetracycline hydrochloride（TH）and chloramphenicol（CP）,which is significantly better than ZIF-8 with microporous structure.According to Langmuir isotherm model,the maximum adsorption capacities of TH and CP by Hp ZIF-8-10（1.5）are 277.8 mg g^-1 and 29.1 mg g^-1,respectively.In summary,the introduction of hierarchical porous structures into the microporous frameworks helps to expose much more active sites of the adsorbent and improve the mass transfer efficiency,which finally significantly enhances the adsorption capacity of the target pollutants.（3）The 2-methylimidazole in the ZIF-8 preparation system can be used as both an alkali source and an organic ligand.A one-step approach in an aqueous system has been developed to synthesize magnetic organic-metal framework composite material M-ZIF-8-10.First,the organic ligand 2-methylimidazole can be used as the alkali source in the coprecipitation method.The obtained Fe3O4 nanoparticles exhibited superparamagnetism,and their saturation magnetization could reach 61.3 emu g^-1.Secondly,the consumption of ligands by Fe3O4nanoparticles in the mixed system did not significantly affect the formation and growth of ZIF-8 crystals.The resulting composite product M-ZIF-8-10 still exhibited superparamagnetism(saturation magnetization is 18.8 emu g^-1),and the specific surface area is up to 958 m² g^-1.The product M-ZIF-8-10 can quickly and efficiently adsorb Congo red（CR）in water and the process could be described by the pseudo-second-order kinetic model indicating that chemical adsorption occurred.In addition,the Langmuir isotherm model fitting results show that the adsorption of CR molecules is single layer adsorption,and the maximum adsorption capacity is up to 210.5 mg g^-1.By further introducing the structure-directing agent PDDA into the mixed system,a superparamagnetic product M-Hp ZIF-8 with hierarchical porous structures(saturation magnetization is 13.5 emu g^-1 and specific surface area is 582 m² g^-1)can be obtained,and the maximum adsorption capacity of CR was increased to 255.8 mg g^-1.In short,the coprecipitation system for preparing Fe3O4 and the ZIF-8 synthesis system can be linked with each other through basic organic ligands,and the efficient composite of magnetic Fe3O4 and MOFs was realized.（4）MIL-53（Al）-FA（3）product with three-dimensional hierarchical porous structures composed of two-dimensional nanoplate units has been synthesized by using strong alkaline sodium metaaluminate as Al source without any other additives.The ex-situ time-dependent experimental results show that the incompletely deprotonated ligand（HOOC-CH=CH-COO-）that still contains a carboxyl group will gradually participate in the coordination reaction and induce the formation of two-dimensional nanoplate as a capping agent.While the formation of three-dimensional hierarchical porous structure is attributed to the inducing effect HOOC-CH=CH-COO-and the bridging effect of fully deprotonated ligand（-OOC-CH=CH-COO-）.The surface of MIL-53（Al）-FA（3）has abundant carboxyl groups,which can be used to adsorb and anchor the precious metal ion Ag⁺,and obtain the supported catalyst through in-situ reduction.In addition,MIL-53（Al）-FA（3）with hierarchical porosity can effectively capture gaseous iodine:The adsorption capacity is 1124.3 mg g^-1 when the temperature is 70°C(gaseous iodine concentration is up to 138.21 mg L^-1).Thanks to two-dimensional structure of MIL-53（Al）-FA（3）and the small diffusion resistance in the pores,the adsorption capacity can still reach 794.4 mg g^-1 at room temperature(20～30°C,gaseous iodine concentration is as low as 6.70～13.65 mg L^-1).The experimental results show that a metal organic framework with carboxylate ligand can be prepared by introducing an appropriate alkali source and the ligands in the system can be used to control the morphology and structure of the product.（5）The deprotonated fumaric acid in the MIL-53（Al）-FA preparation system can also be used as both an alkali source for coprecipitation and an organic ligand for coordination reaction.Hence,the magnetic metal organic framework composite material M-MIL-FA with hierarchical porosity has been synthesized via a one-step method in the aqueous phase.First,the obtained Fe3O4 nanoparticles using deprotonated fumaric acid as an alkali source exhibited superparamagnetism,and their saturation magnetization could reach 52.7 emu g^-1.Secondly,the resulting hierarchical porous composite product M-MIL-FA with specific surface area of527 m² g^-1 also exhibited superparamagnetism（saturation magnetization is 6.5 emu g-1）.Compared with MIL-53（Al）-FA,the specific surface area of M-MIL-FA is significantly reduced.In addition to the presence of magnetic particles with low specific surface area,the consumption of ligands by Fe3O4 nanoparticles also has a negative effect on the growth of MIL-53（Al）-FA.The product M-MIL-FA can effectively capture gaseous iodine,and can be desorbed in ethanol showing good recyclability.In a word,the experimental results further confirm that the coprecipitation system of Fe3O4 and the synthesis system of MOFs can be connected to each other through an appropriate alkali source,which would effectively simplify the preparation process of magnetic metal-organic framework composite materials.