串联扰动型金属有机框架玻璃的热诱导晶化及结构转变

Thermal Induced Crystallization and Structure Transformation of Metal–Organic Framework Glass Fabricated by Sequential Perturbation Method

晶化行为极大影响玻璃材料的加工赋形及力学、光、电等性能,但晶化过程中微观结构演变机制远不明晰;对于全新类别金属有机框架(MOF)玻璃,相关晶化及高温动态配位化学系统研究更为匮乏。动态高孔穴四重穿插钻石网型MOF–[Co(L1)_(2)]_(4n),在水分子配位及热去除多层级协同扰动下可不经高温熔融直接玻璃化。作为首个串联扰动法获得的传统MOF成功玻璃化实例,热重和差热分析发现去水扰动不定形态MOF在T_(g)=560 K处发生玻璃化转变,过冷液体于558~586K间稳定,随后在T_(c)=597 K处发生明显晶化,并进一步在T_(d)=695 K以上发生结构分解;原位变温(PXRD)跟踪并证实了MOF去水无定形→玻璃化→过冷液化→再晶化的相态演变过程;X射线全散射原子对分布函数(PDF)揭示了玻璃态及再晶化态样品的短–中–长程结构特征。基于CCDC数据库结构检索以及铜基同构MOF的合成,结合MOF玻璃晶化相高分辨PXRD谱的Rietveld拟合结构验证,确认了致密六重穿插钻石网MOF–[Co(L_(1))_(2)]_(6n)的生成。MOF过冷液体不同温度淬冷所制备玻璃保有气体可接触孔性并呈现温度依赖变化,揭示了相均一MOF过冷液体内部液态结构差异性。这一研究构建了MOF“晶态–扰动态–过冷液态–玻璃态–晶化态”多相结构演变有趣例证,揭示了MOF玻璃及其过冷液体独特的高温配位自识别和有序化结构自组装行为。

Introduction Metal-organic frameworks are a class of organic-inorganic hybrid and crystalline materials based on coordinatively bonded porous network.But the structure and function have been seldom investigated.The effective processing of MOF devices have always been hindered by the intrinsic feature of crystalline powders.The utilization of thermal rheology of glass material is expected to realize the effective preparation of grain-boundary-free and isotropic MOF glass membrane,which is one of the key goals of porous chemistry in the future.Different types of MOF glasses have been prepared by covering melt-quenching,mechanical or pressure treatment,bottom-up assembly in solution,ionic liquid-assisted melting and sequential perturbation.Nevertheless,the crystallization behavior of liquid and supercooled liquid greatly affects the processing and mechanical,optical,and electrical properties of glass materials,but the mechanism of microstructure evolution during the crystallization process is far from clear.It’s even less clear on systematic study of the crystallization and high-temperature dynamic coordination chemistry for new MOF glass.Methods A highly porous and flexible MOF of[Co(L1)_(2)]_(4n) with 4-fold interpenetrated dia-net frameworks and another reference compound of Cu(L1)_(2)]_(6n) with 6-fold interpenetrated dia-net and dense frameworks,were synthesized by solvothermal reactions.The vapor treatment and subsequent thermal dehydration generated the h-CoL1 and dh-CoL1,respectively.The TG and DSC measurements were used to confirm the phase evolution from amorphous,through glass transition,to supercooled liquid,until either quenched glass or high-temperature re-crystallized phase.The in-suit and temperature dependent PXRD further confirmed the structure transformation process.X-ray total scattering atom pair distribution function spectra uncovered the short-middle-long range structure characters of both the glassy and re-crystallized MOFs.Infrared and Raman spectroscopy were used to analyze the coordination geometry changes among different states of MOF.The porosity of glassy g-CoL1 and recrystallized rec-CoL1 were investigated by using CO_(2) adsorption–desorption curves at 195 K.Results and discussion The[Co(L1)_(2)]_(4n) can turn into glassy state directly without high-temperature melting,facilitated by the multi-level synergistic perturbations of water coordination and subsequent thermal removal.As the first successful example of glass generated from traditional MOF based on sequential perturbation.TG and DSC revealed a glass transition at T_(g)=560 K of the dehydrated and amorphous MOF, the subsequent formation of stable super-cooled liquid in the range of 558–586 K, thecrystallization of the super-cooled liquid at T_(c) = 597 K, and the final decomposition above T_(d) = 695 K. Crystalline sample c-CoL1,glassy g-CoL1 obtained by quenching super-cooled liquid at different temperatures (563, 573, 583 K), and recrystallized rec-CoL1 at593 K, at 195 K were tested CO_(2) for isothermal adsorption. Based on structure match in the CCDC database and the synthesis ofiso-reticular Cu-MOF, coupled with the Rietveld fitting analysis of PXRD spectrum, the structure of the recrystallized phase wasverified to be a dense, 6-fold interpenetrated, and dia-net framework of [Co(L1)_(2)]_(6n). Glass prepared by quenching MOF super-cooledliquids at different temperatures retained gas-accessible porosity and showed temperature-dependent variations, revealing the inherentstructure difference in phase-homogeneous MOF super-cooled liquids.Conclusions The highly flexible, porous, and 4-fold interpenetrated dia-net frameworks of [Co(L1)_(2)]_(4n) can be perturbated by watermolecule coordination and thermal dehydration to achieve structural disorder and direct glass transition without the need of traditionalmelting. The obtained MOF super-cooled liquid further transforms to a dense and six-fold interpenetrated dia-net MOF, through thecoordinative self-assembly during high-temperature crystallization process. Gas accessible porosity of the quenched MOF glasswas verified by gas sorption. Based on the relevance between the quenching temperature of super-cooled liquid and thecorresponding porosity of the MOF glasses, the internal liquid structure difference of the homogeneous MOF super-cooled liquidis revealed. This study presents a vivid example of the multiphase structure evolution of MOF, covering different steps ofcrystalline, perturbated, super-cooled liquid, glass, re-crystallized states. This work thus reveals the unique high-temperaturecoordinative self-recognition and well-ordered self-assembly of MOF super-cooled liquid. Such new era of high-temperaturedynamic chemistry also offers great opportunity for material processing and function loading of MOF based on the super-cooledliquid, glass and re-crystallized states of MOF.