(402b) Templating Robust Metal-Organic Frameworks for Carbon Capture

Carbon capture is an emerging group of technologies needed to mitigate climate change. Direct air capture (DAC) removes carbon dioxide (CO₂) by filtering the atmosphere. DAC has numerous advantages over other methods of carbon capture. Notably, DAC is the only technology that can achieve negative emissions. However, isolation of CO₂ is challenging due to its low atmospheric concentration and the presence of competing gasses such as water vapor. Efficient separation requires the development of robust new materials with enhanced selectivity for CO₂, especially in the presence of water vapor.

One rapidly growing class of solid sorbents are metal-organic frameworks (MOFs). MOFs are porous coordination polymers composed of metals connected by organic linkers. The most interesting aspect of MOFs is the extensive control over pore properties attained by varying the structural components or synthetic conditions. This tunability has allowed the strategic tailoring of MOFs for many diverse applications. The industrial relevance of MOFs was proven by our group’s previous material CALF-20, the first MOF used commercially for carbon capture. The majority of MOFs use carboxylate linkers assembled by the principles of reticular chemistry. Carboxylate linkers are favoured by MOF chemists due to the easily characterized ordered structures they form. Phosphonate linkers tend to form more stable MOFs. Yet, phosphonate MOFs are uncommon because their structures are difficult to control and characterize. This work demonstrates a novel method for tunable formation of highly stable phosphonate MOFs that are otherwise inaccessible. This method provides a fresh perspective by exploring the synthesis of unconventional MOFs in a guest-up templating approach.

This work has focused on our patented invention of hydrogen-bonded metal-organic frameworks (H-MOFs) as metastable intermediates to control phosphonate MOF properties. Kinetically inert hexaaquachromium(III) forms a framework with phosphonic acid linkers via charge-assisted hydrogen bonds. The H-MOF hydrogen bonds are weaker and thus more reversible than coordination bonds. Consequently, ordered networks form in a controlled manner. The structure of the resulting H-MOF is flexible and easily changed by exposure to guest molecules. Crystallographic experiments demonstrated guests with different shapes or chemical properties each cause the H-MOF to flex in a unique way. The flexibility of the H-MOF allows the guest molecule to dictate the properties of the pore. This is due to the guest contorting the host H-MOF to maximize host-guest interaction. That is, the guest shapes the framework to create a pore perfect for itself. The H-MOF can then be dehydrated irreversibly into a MOF. It is hypothesized the resulting MOF would retain a pore perfectly optimized for interactions with that particular guest. The pore structure is rigid and maintained after guest removal. This optimized pore would be selective for that guest upon re-exposure.

Presented herein is a proof-of-concept study to demonstrate the tunable formation of highly stable phosphonate MOFs with enhanced CO₂ selectivity for direct air carbon capture. Synthesized H-MOF powders were placed into a customized pressure vessel. The vessel was heated under supercritical CO₂, resulting in dehydration of the H-MOF (H-CALF-55) to a MOF (CALF-55). Interestingly, the resulting MOFs had substantially improved gas adsorption capacity relative to MOFs produced from dehydration without a template. Most notably, a threefold improvement in CO₂ adsorption capacity at low pressures was observed. Furthermore, the shape of the isotherm was tunable by varying pressure and dehydration rate. This allows optimization of the material for specific separation processes. CO₂ loaded into the pore of a H-MOF, is proposed to shape the pore to optimize interaction with CO₂. This conformation would be locked in by dehydration and result in a CO₂ selective MOF. Conventional MOF synthesis techniques would struggle to mimic this effect. Selectivity will be confirmed by competitive dynamic column breakthrough experiments. The high stability of CALF-55 was proven by thermal gravimetric analysis and gas adsorption. Negligible loss of CO₂ adsorption capacity was observed after 1 week in 6M hydrochloric acid and over 75% CO₂ capacity was retained after 1 month in boiling water.

Following these promising results, the ability of CO₂ to template H-MOFs was enhanced by increasing host-guest interaction during dehydration. An amine appended linker was synthesized and used to form an H-MOF. The addition of amine functional groups to the H-MOF linkers likely promotes carbamate formation with the CO₂ template. The stronger host-guest interaction would subsequently increase the ability of CO₂ to alter the H-MOF structure toward a perfect pore for itself. This effect was evaluated by gas adsorption experiments similarly to CALF-55. The CO₂ templated amine appended systems exhibited improvements in adsorption capacity relative to template-free synthesis. The effect of the CO₂ templating was greater than that observed for CALF-55. These results suggest an enhancement of the templating effect. Ongoing infrared spectroscopy experiments probe the formation of carbamates in these materials.

This templating method was also applied to the challenge of water-CO₂ competition in MOFs for DAC. It is hypothesized that binary mixtures of templates could be used to create distinct binding sites for CO₂ and water. The separation of water and CO₂ within the pore would reduce the competition between the gasses and improve the MOF performance in the presence of humidity. Binary templating was studied by adding controlled amounts of additives to the pressure vessel during dehydration. Ongoing experiments include assessing the heat of adsorption of each MOF. Competitive dynamic column breakthrough experiments will measure the separation performance of the materials in the presence of humidity. This proof-of-concept study will lay the foundation for a new method to produce selective MOFs and is expected to be broadly applicable to multiple industrially relevant gas separations.