(143c) Tuning Shape and Position of Step-Shaped Isotherms of Flexible MOFs through Ligand Functionalization for Improved CO2/CH4 Separation

Natural gas is a key green energy solution towards alleviating climate change. One of the key challenges to using natural gas is its energy-intensive purification process. Adsorption-based technologies offer an energy-efficient way to extract methane as an energy source from other components in natural gas like carbon dioxide. Metal-organic frameworks (MOFs) are a highly tunable class of adsorbents known for their modular assembly of metal nodes and organic linkers. More than 500,000 possible MOF structures have been predicted in literature so far, each with different chemical modifications affecting their adsorption behavior. Among these materials, third generation porous framework materials with structural flexibility - also known as flexible MOFs - have gained considerable attention. Flexible MOFs have been shown to exhibit a gate-opening behavior in the presence of an external stimuli, such as guest gas molecules. This gate-opening behavior is characterized by the MOF transitioning between a closed, non-microporous structure and an open, porous phase through linker rotation. As a result of this behavior, flexible MOFs have step-shaped adsorption isotherms and can adsorb and release gas molecules over a narrow pressure range, allowing for efficient pressure swing adsorption and regeneration applications. Ligand functionalization has a dual effect on flexible MOF materials: it provides adsorption sites for a particular component and it changes the shape and position of the step-shaped adsorption isotherm. Here, different amounts of nitrogen sites were introduced into the structure of the flexible MOF ZIF-7 to provide high-energy adsorption sites for carbon dioxide, while tuning the shape and position of the stepped isotherms. This modification results in a functionalized surface that is more selective towards carbon dioxide than methane, and impacts the isotherm shapes for the two components differently. Binary adsorption of methane and carbon dioxide was studied using breakthrough experiments to investigate the persistence of the preferred adsorption of CO₂ under real mixture conditions. Experimentally calculated CO₂/CH₄ selectivity was compared to the selectivity predicted using the Osmotic Framework Adsorbed Solution Theory (OFAST) to investigate the applicability of OFAST for tuned ZIF-7 systems.