Abstract
While metal-organic frameworks (MOFs) are promising gas adsorbents due to their high surface area, microporosity, and abundance of CO₂ adsorption sites, their application as kinetically efficient CO₂ adsorbents is hindered by elevated diffusion resistance, partly resulting from the lack of mesoporosity. Fundamentally, this diffusion resistance arises from two factors: (1) tortuous pore network within the vast microporous structures and (2) collapse of micropore channels and interparticle mesopores due to the elevated local capillary stresses during thermal evacuation processes. Therefore, approaches that enhance the pore accessibility of MOFs are essential to improving their CO₂ uptake performance. Additionally, the role of coordinated water in CO₂ adsorption process within MOFs featuring coordinatively unsaturated metal sites (CUS) cannot be ignored, as water can act as both a competitor and a facilitator in CO₂ adsorption. Understanding when and how water plays a beneficial role in CO₂ capture is crucial for designing effective MOFs adsorbents. Therefore, this thesis focuses on addressing two fundamental challenges, enhancing pore accessibility and elucidating the role of coordinated water in CO₂ adsorption. Mg-MOF-74, a benchmark MOFs known for its high CO₂ uptake capacity at 0.1 bar, is selected as the model system in this thesis due to its abundant CUS sites and its representative low-coordinated framework structure.
The major contributions of this thesis are highlighted as follows:
In this thesis, defect engineering is effectively applied to low-coordinated MOFs. Missing cluster defects are introduced into low-coordinated Mg-MOF-74 using graphene oxide (GO) as a solid modulator, which helps to stabilize the otherwise collapse-prone defect-rich structures. The presence of defects leads to enhanced surface area and pore volume, providing improved pore accessibility for CO₂ molecules. As a result, a 19.3% enhancement in capture capacity at 0.1 bar and room temperature is achieved.
In an alternative approach to improve pore accessibility, supercritical CO₂-assisted drying is employed for low-coordinated MOFs to enhance pore accessibility. Applied to Mg-MOF-74, this technique achieves a 21.65% improvement in CO₂ adsorption quantity (0.1 bar, 25 ℃) compared to thermally evacuated samples. This improvement is attributed to the preservation of interparticle mesopores and increased accessibility to internal active sites.
To understand the water stability of Mg-MOF-74, this thesis systematically examines its stability in terms of morphology, crystallinity and porosity after water vapor and liquid water exposure at different conditions. Additionally, the effect of coordinated water connected with water chains in different states is isolated and studied during the CO₂ capture process. The study reveals that with optimal amounts of coordinated water connected with water chains in proper states, hydrogen-bonded networks are formed, serving as secondary adsorption sites that interact with and trap CO₂ molecules, thereby achieving an 12.3% increase in CO₂ capture capacity at 0.1 bar and room temperature. Advanced in situ techniques, including X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), provided deep insights into the water desorption process and the interaction between CO₂ molecules and water-containing frameworks, offering guidance for the design of effective CO₂ adsorbents.
Collectively, these findings not only advance the potential of MOFs for CO₂ capture in climate mitigation but also provide fundamental insights into their structure-function relationships, thereby expanding the applicability and utility of MOFs across various fields.