Elettra | SAXS, SISSI-Bio Reversible low-pressure CO₂ capture and release using flexible metal-organic framework films The urgent need for green Carbon Capture and Storage (CCS) technologies is clear, as fossil fuels still account for over 85% of global energy production, with CO2 emissions contributing up to 60% of global warming. Metal–Organic Frameworks (MOFs) hold significant promise in this context due to their high CO2 uptake capacities and their potential to overcome the limitations of current CCS methods, such as corrosion and high energy costs in the case of amine scrubbing, or energy-intensive adsorbent regeneration as for zeolites or molecular sieves that typically result in additional CO2 emissions. Thanks to their chemical tunability, MOFs have been extensively studied for CO2 capture, especially in gas mixtures, direct air capture, or under high-pressure conditions. However, relatively few studies have focused on CO2 adsorption and release from MOFs under moderate or low pressures, despite the importance of minimal energy input for costeffective and efficient CCS. To address this gap, MOFs with high structural stability and easy regeneration are essential, which can be realized when fabricating MOFs as film structures. Moreover, experimental insights are key to understanding and optimizing these materials for low pressure CO2 capture. Hence in this study, three complementary operando techniques are applied to investigate reversible CO₂ uptake and release by MOF films. These methods are generally applicable to MOFs of various topologies, independent of film fabrication protocols or substrate type. The herein introduced techniques include Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS), Infrared (IR) spectromicroscopy, and Quartz Crystal Microbalance with Dissipation monitoring (QCM-D; Fig. 1 a–c). The combination of GIWAXS and IR ensures to successfully unravel chemical features along with structural changes within the films, while QCM measurements allow to quantify the amount of adsorbed CO2. Especially for oriented MOF assemblies, as used within this study, features related to the uptake of guest molecules are often subtle and to resolve such, synchrotron radiation is essential. Hence key results were obtained by measurements conducted at the Austrian SAXS Beamline (GIWAXS) and the SISSI beamline at Elettra (IRspectromicroscopy). Herein, the interaction of CO2 with functionalized MOF film structures was examined along with the reversibility of CO2 ad-/desorption under moderate gas-flow conditions. For this purpose, hetero-epitaxially grown Zn-based MOF films of the type Zn₂L₂DABCO on Cu₂BDC₂/Cu(OH)₂ were investigated by systematically varying the bulkiness and electron-donating character of the bridging ligands L₂ (Fig. 1 d). Ligand selection was guided by literature reports: amino groups enhance the CO2 binding, while methyl groups improve stability under moisture and alkyl ethers increase flexibility. Functionalization with methoxy and methyl groups yielded more flexible MOF structures and significantly enhanced CO₂ uptake compared to non-functionalized Zn₂BDC₂DABCO films (Fig. 1 e). At ambient temperature and low pressure, Zn₂Me-BDC₂DABCO and Zn₂MeO-BDC₂DABCO showed CO₂ capacities of 0.17 mmol/g and 0.40 mmol/g, respectively, thus outperforming some MOFs under higher pressure conditions. This characteristic was further utilized to investigate the adsorption of CO2 molecules within the Zn2L2DABCO pores, examining how external stimuli – temperature and light – affect pore size transitions (large-pore (LP) to narrowpore phase (NP)) and, consequently, the structural and chemical environment experienced by the adsorbed CO2 molecules. At reduced temperature, methoxy- and methyl-functionalized films displayed distinct vibrational shifts, indicating stronger CO₂-framework interactions. QCM-D confirmed a 30% increase in uptake over unmodified Zn₂L₂DABCO. Moreover, vibrational modes associated with adsorbed CO₂ were notably stronger in the functionalized films, thus suggesting enhanced interaction with the framework. Among all functional groups studied, methoxy showed the strongest CO₂ affinity, which was entirely absent in the non-functionalized Zn₂BDC₂DABCO system. Additionally, at lower temperatures, increased framework flexibility influenced the amount of adsorbed CO₂, highlighting the dynamic behavior of the Zn₂L₂DABCO structure. To investigate the photo-responsiveness of the flexible Zn-MOF films under CO2 load, the photo-active azobenzene molecule was introduced into the pores, which was exploited to initiate repeatedly photo-triggered CO2 uptake and release (Fig. 1 f). Results indicated that interactions between azobenzene and CO₂ molecules supported an adaptive photo-response of the system (Fig. 1 g). Our findings demonstrate that light induces a significant structural response in these photo-responsive Zn-MOF films when CO2 molecules are present, allowing the reversible uptake and release of CO2 upon irradiation with 365 and 450 nm. Overall, the results highlight the promise of stimuli-responsive systems for ondemand CO₂ capture and release at low pressures, with structural reversibility under near-ambient conditions. This demonstrates the potential of tailored MOF film architecture for next-generation, energy-efficient CCS technologies. Moreover, the integrated methodologies presented in this study offer a broadly applicable platform for probing structural and molecular changes under operando conditions in stimuli-responsive films, with relevance extending to more complex systems such as mixed-matrix membranes and device-integrated assemblies. Elettra Highlights 2024–2025 26
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