Exploring Pressure-Induced Structural Changes and CO2 Adsorption in Flexible Metal-Organic Frameworks
Metal-organic frameworks (MOFs) have attracted considerable attention over the past several decades as a class of novel hybrid porous materials. Their reversible CO2 adsorption capabilities have marked them as promising candidates for selective carbon capture. Flexible MOFs feature dynamic frameworks that can reversibly change structure in response to external stimuli, thereby exhibiting unique performance as CO2 adsorbents. The "gate-opening effect" of flexible MOFs can lead to a dramatic improvement in CO2 uptake capacity. Specifically, high pressure has the potential to induce structural changes in flexible MOFs, thereby altering interaction mechanisms of CO2 adsorption. This thesis focuses on the structural changes of flexible MOFs and their host–guest interactions with CO₂ under high pressure, investigated using in situ X-ray diffraction (XRD) and vibrational spectroscopy. The synergistic effect of high pressure and high temperature on CO2 adsorption in the MOFs was also investigated. Three types of flexible MOFs with different structures and properties were studied under high pressure. MIL-53(Al), a flexible MOF with a wine-rack topology, exhibited a remarkable negative linear compressibility (NLC) along the b axis under pressures. Under simultaneous high-pressure and high-temperature conditions, MIL-53(Al) demonstrated significantly enhanced CO2 adsorption. Detailed spectral analysis elucidated the chemisorptive nature of host-guest interactions between the framework and CO2. Zn2(BDC)2DABCO, featuring pillared square grid nets based on paddle-wheel motifs, exhibited pressure-induced phase transition attributed to the flexibility of the dinuclear Zn paddle-wheel unit and the “kneecap” structure in BDC linker. The CO2-loaded Zn2(BDC)2DABCO exhibited continuous unit cell contraction and two CO2 adsorption sites under high pressure. The CO2 uptake capacity was slightly enhanced under simultaneous high pressure and high temperature conditions. At last, the 2D layered MOF ELM-11 and its precursor pre-ELM-11 were investigated under high pressure. The NLC was observed for Pre-ELM-11 above 1.91 GPa, resulted from a two-stage compression process involving interlayer compression and layer sliding, while ELM-11 displayed a continuous compression dominated by interlayer contraction. The CO2-loaded ELM-11 exhibited increased compressibility and revealed the emergence of new CO2 adsorption sites under high pressure. Overall, these findings demonstrate the pressure-induced structural changes and underlying mechanisms of flexible MOFs, advancing our understanding of MOFs’ potential for carbon capture across a broad pressure-temperature range.
Metal-organic frameworks, CO2 adsorption, Structural change, High pressure, High temperature
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