Enhanced The Thermal Stability Of Metal Organic Framework MIL-101Fe In Water Solution And The Study Of Solid Phase Crystallization

The metal-organic frameworks?MOFs?formed by the coordination of metal ion clusters and organic ligands have potential applications in gas adsorption storage and separation,catalysis,ion exchange and drug delivery,and have attracted wide attention in recent years.However,most MOFs materials are severely constrained by their poor hydrothermal stability.Therefore,how to design and synthesize MOFs with good stability has attracted much attention.At the same time,poor stability of MOFs is prone to breakage and recombination of coordination bonds under certain conditions.Structural transformation is caused by solvent removal and exchange of guest molecules and temperature stimulation.The performance optimization of this crystal phase transition to MOFs materials has also attracted the interest of researchers.Among many MOFs materials,Fe-based MOF have good performances in catalysis and adsorption separation.Among them,MIL-101Fe is widely favored by researchers due to its unique structure and large specific surface area.But the water stability of the material is relatively low,and it will be destroyed when it meets water at room temperature,thus seriously affects the application of the material.It is found that the stability of MIL-101Fe in water can be improved by adding AlCl₃·6H₂O,and the structure of MIL-101Fe can neither be decomposed nor destroyed;CoCl₂·6H₂O and NiCl₂·6H₂O can not enhance the stability of MIL-101Fe in water,so it is inferred that the presence of Al³⁺is the main reason for the stability of MIL-101Fe in water.Further studies have shown that MIL-101Fe remains stable in water at temperatures not higher than 80? when adding the Al³⁺.This provides a guiding direction for the design and synthesis of highly stable MOFs,which can be expanded for applications such as adsorption separation or catalysis of gases.In addition,the stability study results show that MIL-101Fe decomposes at low temperatures in aqueous solution.While at high temperatures,crystal phase transition occurs,which is inferred to be Al-MIL-53Fe by XRD and EDS.The second work of this thesis is based on the conclusion above,and the solid phase transformation of MIL-101Fe is realized by steam assisted method.By using water?H₂O?,methanol?MeOH?,ethanol?EtOH?,N,N-dimethylformamide?DMF?,N,N-dimethylacetamide?DMA?and acetone vapor for MIL-101Fe.Structural transformation studies have shown that MIL-101Fe in acetone vapor is converted to MIL-53Fe,which decomposes into ligand protonated ligand?H₂BDC?in water vapor,and becomes amorphous phase in ethanol vapor.While in DMF,DMA and methanol vapor,the MIL-101Fe crystallized is an unknown phase.Furthermore,the conditions for the conversion of MIL-101Fe to MIL-53Fe in acetone vapor at different temperatures and different reaction times were also investigated:200? for 24 h,220? for 24 h,and 220? for 3 h.The results show that the solvent orientation of acetone vapor can transform the low-density phase of MIL-101Fe into a high-density phase of MIL-53Fe through a combination of the breaking and recombination of the coordination bond in a short time.This vapor phase assists the transformation between crystals as a new reaction mechanism,which can save solvent,reduce reaction time,increase mass transfer rate,facilitate the exploration of crystal structure properties and applications,and provide new crystal synthesis and conversion ideas.