Low-Energy CO2 Release from Metal-Organic Frameworks Triggered by External Stimuli.

Abstract

Groundbreaking research over the past 15 years has established metal-organic frameworks (MOFs) as adsorbents capable of unprecedented gas adsorption capacity. This has encouraged the contemplation of their use in applications such as increasing the storage capacity in natural gas fuel tanks, or the capture of carbon dioxide from coal-fired flue gas streams. However, while the gas adsorption capacity of MOFs is large, not all stored gas can be readily released to realize the efficient regeneration of MOF adsorbents. This leads to an increase in energy requirements, or working capacities significantly lower than the amount of gas adsorbed. This requirement for low energy means to efficiently release more stored gas has motivated the research in our group toward the triggered release of the stored gas from MOFs. Using CO2 as a typical gas adsorbate, we have developed three new methods of releasing stored gas with external stimuli that include light induction swing adsorption, magnetic induction swing adsorption, and their combination, denoted as LISA, MISA and MaLISA, respectively. LISA: Light, being naturally abundant, is particularly interesting for reducing the parasitic energy load on coal-fired power stations for regenerating the CO2 adsorbent. We showed that, by incorporating light-responsive organic linkers, exposure of light to a gas-loaded MOF promoted localized movement in the linkers, expelling around 80% of the adsorbed gas, just from the use of concentrated sunlight. Variation of the light-responsive components such as silver nanoparticles in MOFs allowed the response to be moved from UV to visible wavelengths, improving safety and light penetration depth. MISA: In order to expand this discovery to larger scales, more penetrating forms of radiation were sought. MOFs incorporated with magnetic nanoparticles (Magnetic Framework Composites, MFCs) were developed, and absorb the alternating magnetic fields exceptionally efficiently. The rapid heating of magnetic particles delivers local temperature increases to the otherwise thermally insulating MOF material, and in optimized conditions release all adsorbed gas in a matter of minutes. MaLISA: The triggered release methods of LISA and MISA may be combined in MFCs that also contain light-responsive groups. Both stimuli were employed and cooperative enhancement of gas releasing efficiency were found, minimizing the overall energy requirement even further. Initial calculations of the energy costs for these processes have shown them to have the potential to exceed any other reported method, following optimization. Encouragingly, the efficiency of the process was found to increase at larger scales, prompting further research in this area toward widespread deployment.