Mechanochemistry of metal-organic frameworks under compression

Mechanochemistry;Metal-organic frameworks;Mechanical energy;Shock wave

Miao, Yurun

关键词: Mechanochemistry;    Metal-organic frameworks;    Mechanical energy;    Shock wave;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/99321/MIAO-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Metal-organic framework (MOF) materials have drawn intense interest in the chemical community in the past decade. They exhibit a variety of advantages over conventional porous materials and show great promise for a rapidly expanding collection of applications, such as gas storage, separation, catalysis, sensing, and drug delivery. The investigation on the mechanochemistry of MOFs, however, has been very limited in spite of its relevance to the practical application of MOFs. Also, recent reports have demonstrated the possibility to use MOFs for mechanical energy absorption via an internal volume collapse mechanism. In this dissertation, the mechanical and mechanochemical behaviors of MOFs under compression, especially their energy absorption during plastic deformation, are explored.The structural impact of static, bulk compression has been examined on a prototypical MOF, UiO-66. Upon bulk compression at 1.9 GPa, the effective number for Zr-O bonds between Zr(IV) ions and carboxylate groups in UiO-66 decreased from 4.0 to 1.9, as determined by X-ray absorption spectra (XAS), and the internal free volume was synchronously collapsed. Consistent with the XAS data, IR spectra confirmed conversion of syn-syn bridging carboxylates to monodentate ligation, thus establishing mechanochemical reactions induced by external compression of MOFs and indicating great endothermicity during this process.The mechanical properties and energy absorption of single crystals of UiO-66, along with three isostructural UiO-type MOFs, have been measured under uniaxial compression. In-situ nanocompression experiments were used to measure the mechanical behavior of individual MOF nanocrystals within a TEM. The plasticity and endothermicity during deformation of MOFs shows a surprising potential for absorption and dissipation of mechanical shock. At compressive stress below 2 GPa, relatively small amounts of energy (<0.3 kJ/g) are absorbed by the compression of these MOFs. As the stress was increased, however, the energy absorption was significantly enhanced. Above 2 GPa, the energy absorption typically reaches 3-4 kJ/g, comparable to the energy release in the explosion of TNT. The response of another prototypical MOF, ZIF-8, under compression from dynamic shockwave generated by a table-top laser-driven flyer plate system, has been studied. The shockwave profile after propagation in ZIF-8 layer is significantly stretched in time compared to the direct impact at bare glass substrate, with significantly reduced peak intensity due to the formation of a two-wave feature in the shock profile. In post-mortem analysis on impacted ZIF-8, it has been discovered that the MOF crystals experience significant deformation with chemical decomposition near the impact surface, suggesting the energy absorption mechanism in MOFs via sacrificial structural collapse and bond breakage. Besides the mechanochemistry of MOFs, the controlled synthesis of three-dimensional (3D) superstructure of ZIF-8 has also been investigated. ZIF-8 is found to undergo self-templated crystallization from colloids of its amorphous isomorph. The wet-chemistry conversion process consisting of hydrothermal and solvothermal steps can be incorporated into a convenient aerosol process using an ultrasonic spray pyrolysis (USP) system to afford hierarchically structured ZIF-8 particles.