Calorimetric And Structural Evolution During Glass-forming Process For Tetrahedral Imidazolate Framework-4

In view of that the reports on the calorimetric and structural evolution of metal-organic frameworks are very limited,some work focusing on a tetrahedral imidazolate framework,TIF-4,which has potential to be melted and vetrified was demonstrated here.Combined with TG-DSC and many other techniques,we first systematically explored the thermal response and corresponding structural origin upon melting and glass-forming for TIF-4 crystal.Then,we obtained dimetal TIF-4 materials using metal cation exchange.The melting behavior of dimetal TIF-4 crystal and the glass transition behavior of melt-quenched glass were highlighted.Above all,the main results are listed as follows:?1?Besides the previously known desolvation-induced endotherm and melting event,an unknown endothermic response at the temperature range of 320-400°C also exists in the calorimetric behavior of TIF-4.Based on TA/MS and FT-IR study,it can be mainly ascribed to the evacuation of only a very small amount of solvent molecules accompanied by certain decomposition reactions.However,we really cannot exclude some oxidation exothermic contribution in which ammonia coming from primary amine are catalytically oxidized by Pt.No structural change occurs at the temperature range of 320-400°C except for physical expansion/contraction.Such physical expansion/contraction is associated with framework flexibility.?2?The crystalline structure of TIF-4 maintained stable in the selected exchange solvents?MeOH,acetone,CH2Cl2?for a certain period of time.The exchange efficiency of these three solvents increases in the order:MeOH<acetone<CH₂Cl₂.However,the solvent molecules retained in TIF-4 framework from synthesis cannot be completely removed via solvent exchange.The melting and glass transition behaviors of TIF-4 changed a lot after heat treatment in air.We found TIF-4 was totally converted into zinc oxide after heat treatment at 400°C for 12 h in air.Without changing the TIF-4 network,the adjustable extent of ligand Im ratio is higher than that of ligand mbIm.The melting point of as-prepared TIF-4 product increases firstly,and then decreases as the ligand Im ratio increases,but the glass transition temperature decreases.On the conctraty,the melting point of product decreases as ligand mbIm ratio increases,but the glass transition temperature increases.?3?Post-synthesis cation exchange with Mn²⁺or Co²⁺in TIF-4 crystal leads to a color change.The cation Mn²⁺or Co²⁺successfully replaced metal node Zn²⁺and become incorporated within the TIF-4 framework.The obtained dimetal structure is identical with parent TIF-4.Limited by the concentration of exchang solution,partial metal exchange was observed.Together with Zn and Mn/Co element,also O element are spread throughout the exchanged particles.It is likely that the framework was oxided,or accommdated guest molecules containing O element.?4?There are some cracks and defects on crystal surface after metal cation exchange.Those defects derectly influenced the X-ray diffraction quality of those dimetal TIF-4 crystals.Both the melting temperature and glass transition temperature of TIF-4 decrease as the incoming ion Mn²⁺or Co²⁺replace metal node Zn²⁺.In exchange solutions H₂O,EtOH and DMF,TIF-4 framework show higher affinity of Mn²⁺than Co²⁺.While TIF-4 framework show higher affinity of Co²⁺than Mn²⁺in solution MeOH.The exchanged content of Mn²⁺increases in the order:MeOH<H₂O<DMF<EtOH.And the exchanged content of Co²⁺increases in the order:H₂O<DMF<MeOH<EtOH.