CO2 capture from water using a copper/chromium-adenine supramolecularly assembled porous metal-organic material

CO 2 capture from water using a copper/chromium-adenine supramolecularly assembled porous metal-organic material Ekain Maiza-Razkin a , Garikoitz Beobide a,b , Oscar Castillo a,b,* , Antonio Luque a,b , Rub´ en P´ erez-Aguirre a,** , Sonia P´ erez-Ya˜ nez a,b a Departamento de Química Org´ anica e Inorg´ anica, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, Leioa E48940, Spain b BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, Leioa E-48940, Spain ARTICLE INFO Keywords: Porous metal-organic materials CO 2 capture Adsorption from water Adsorption quantification techniques ABSTRACT A supramolecular metal-organic porous material [CrCu 6 ( μ -adeninato-кN3:кN9) 6 ( μ -OH) 6 ( μ -OH 2 ) 6 ](SO 4 ) 1.5 (Cu 6 Cr) has been tested for CO 2 capture from water. The properties of this compound, insolubility in water, a flexible supramolecular structure, and protonable positions on the adeninato ligands, make it a potential candidate for this task. The experimental determination of CO 2 capture from water was performed using two techniques: magnetic sustentation, for first time, and gravimetric measurements. Both techniques verified the CO 2 capture providing complementary information. The magnetic sustentation technique measures the mass being incorporated into the porous material (which depends on the chemical form in which is being captured: physisorption, HCO 3 - or carbamate), whereas the gravimetric measurement quantifies the total mass of CO 2 being captured in the aqueous suspension of the Cu 6 Cr particles regardless the chemical form in which this capture takes place. After 1 hour of CO 2 bubbling (300 mL⋅min −1 ) into the aqueous suspension of Cu 6 Cr particles, capture mass values of 22.3 % and 17.1 % are measured by magnetic sustentation and gravimetric techniques, respectively. This difference is because the CO 2 is captured as H 2 CO 3 that reacts with the adeninato ligands to form adenine/HCO 3 - pairs. These values normalize to 5.9 HCO 3 - and 6.4 CO 2 molecules per Cu 6 Cr entity, which are close to the theoretical value of 6, because of the six adeninato ligands per heptameric Cu 6 Cr entity. CO 2 adsorption isotherms, adsorption/desorption kinetics and cycling stability are also reported. Kinetic studies provide a ΔH ads =-19.4 kJ/mol, which is a significantly lower than other CO 2 adsorbents. However, the cycling stability needs to be improved. 1. Introduction The increase of CO 2 in the atmosphere seems to be the one of the main driving force behind the temperature rise that our planet is currently experiencing [1]. Therefore, development of materials and technologies for the capture [2], storage [3] and/or valorisation [4] of CO 2 is of paramount importance. As CO 2 production is mainly concentrated on places such as power plants, industrial facilities and transport, focusing on this stage for CO 2 capture [5] is more desirable than direct air capture (DAC) technology [6], as the cost of capture increases as the concentration of CO 2 in the flue gas decreases [7]. The capture of CO 2 can be carried out both before (pre-) [8] and after (post-) combustion [9], and even in other types of processes such as so-called oxy-- combustion [10]. In general, post-combustion CO 2 capture is the least disruptive to existing industrial processes as the CO 2 capture component can be adapted to the existing infrastructure. In addition, CO 2 capture can be achieved by a variety of techniques, most commonly using absorption/adsorption techniques [11] or membrane technology [12]. The adsorption phenomenon offers advantages such as low energy cost, high efficiency, ease of operation and availability of multiple adsorbent materials [13–16]. Therefore, the adsorption capacity, kinetics and enthalpy of adsorbents are widely studied to analyse and evaluate their performance [17]. Most of the reported research focuses on the adsorption phenomenon from the gas phase with many examples of * Corresponding author at: Departamento de Química Org´ anica e Inorg´ anica, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, Leioa E-48940, Spain. ** Corresponding author. E-mail addresses: [email protected] (O. Castillo), [email protected] (R. P´ erez-Aguirre). Contents lists available at ScienceDirect Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou https://doi.org/10.1016/j.jcou.2025.103144 Received 20 December 2024; Received in revised form 30 May 2025; Accepted 30 May 2025 Journal of CO2 Utilization 98 (2025) 103144 Available online 5 June 2025 2212-9820/© 2025 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/bync-nd/4.0/ ). adsorbent materials such as MOFs, zeolites, silicates, activated carbon, organic and inorganic polymers, and biochar [12,18–23]. However, there are no significant reports of CO 2 adsorption occurring from an aqueous medium [24,25]. In general, CO 2 molecules can be trapped in a porous solid by physisorption or chemisorption. Both are possible for adsorption from the gas phase, but from the liquid phase, especially for aqueous solutions, chemisorption is more likely because CO 2 physisorption competes with H 2 O physisorption. In fact, chemisorption typically exploits the acidic nature of CO 2 by reacting with the basic sites present in the porous material [26]. Another desirable property of the CO 2 capture material, especially when looking for the valorisation of the captured CO 2 , is a low energy penalty for the CO 2 release. Unfortunately, the enthalpy for chemisorption is usually higher than for physisorption (in the order of 80–240 vs. 20–40 kJ/mol) which hinders the release of the captured CO 2 [27]. This is indeed a well-known drawback of the amine-based solutions used for commercial DAC [28]. In this work, we have focused on a previously reported [CrCu 6 ( μ - adeninato-кN3:кN9) 6 ( μ -OH) 6 ( μ -OH 2 ) 6 ](SO 4 ) 1.5 compound (Cu 6 Cr, Fig. 1) [29]. This compound has some interesting features that led us to believe that it could be useful for CO 2 capture: (i) porosity built up from discrete heptameric entities that are supramolecularly assembled (SMOF), (ii) adaptable nature of its supramolecular structure, (iii) protonable positions on the adeninato ligands that do not alter its heptanuclear molecular structure. In addition, the μ -adeninato-кN3:кN9 coordination of the adeninato ligand causes its acidity constant to shift from pKa =9.8 for the uncoordinated adeninate anion to pK a ≈7.2–7.4 for the coordinated adeninato ligand [30] which is above the pK a1 value of CO 2 (6.35) but close enough for the resulting μ -adenine-кN3:кN9 / HCO 3 - adduct to be relatively unstable, facilitating the CO 2 release by the reverse reaction in aqueous medium. Another feature of this work is the development of methods that allow the quantification of the adsorption of chemical species from an aqueous medium. This is important because the gravimetric and volumetric techniques developed for gas-solid adsorption measurement are not directly extrapolable to liquid-solid conditions. In this sense, we have previously reported a technique called “magnetic sustentation”, which allows the quantification of the adsorbed mass percentage in paramagnetic porous compounds (mainly MOFs and SMOFs) independent of the adsorbed molecule [29,31–35]. This technique is combined herein with gravimetric measurements that determine the amount of CO 2 being incorporated into the aqueous media in which the adsorbent particles are suspended. Both measurements provide complementary information, because the gravimetric measurement provides the amount of CO 2 being trapped in the water/porous material mixture, whereas the adsorbed mass measured by magnetic sustentation depends on the chemical form in which CO 2 is captured. Therefore, it is possible to verify and quantify not only the CO 2 capture but also to determine the chemical form in which it is incorporated (bicarbonate or carbamate) [36]. This work will demonstrate how new sorption measurement techniques, when combined with novel SMOF porous materials, can shift the paradigm of CO₂ capture from a gas-based approach to a liquid-based one. 2. Experimental 2.1. Synthetic procedures 2.1.1. Synthesis of [Cu 6 Cr( μ -adeninato-κN3:κN9) 6 ( μ 3 -OH) 6 ( μ -OH 2 ) 6 ] (SO 4 ) 1.5 (Cu 6 Cr) The synthesis was performed using a modification of the previously reported one [29], in which chromium(III) sulfate monohydrate is replaced by chromium(III) potassium sulfate dodecahydrate. All the chemicals were of reagent grade and were used as commercially obtained. Adenine (0.8 mmol, 0.108 g) was dissolved in a 20 mL water/- methanol mixture (1:1 vol ratio), and was heated under continuous stirring for 20 min. Then, a 20 mL aqueous solution of Cu(SO 4 )⋅5 H 2 O (0.8 mmol, 0.200 g) and CrK(SO 4 ) 2 ⋅12 H 2 O (0.2 mmol, 0.100 g) was added. Immediately, a green suspension was formed (pH =3.0). This suspension was dissolved through acidification with sulfuric acid until a light green solution was obtained at pH 1.5. Subsequently, the pH of the solution was shifted to 9.2 by adding triethylamine, and a green suspension was obtained, which was left in a crystallizer sealed with sealing film (Parafilm M), slightly holed to allow the slow solvent evaporation. After 3 days, the suspension recrystallized as green needle-shaped crystals that were filtered, washed 3 times with water and dried under ambient conditions. The samples were stored in a water saturated atmosphere because of its reversible crystal structure transitionfrom the hydrated crystalline form to the amorphous anhydrous. The purity of the samples was verified by Powder X-ray Disfraction (PXRD) (Figure S3.1). Yield: 50–60 % (based on Cr). Fourier Transform Infrared Spectroscopy (FTIR) (KBr pellets, cm −1 ): 3388vs, 3200sh, 1642vs, 1603vs, 1548 s, 1463 m, 1402 m, 1304 m, 1277 m, 1195 m, 1152 m, 1108 m, 1033w, Fig. 1. The structural units of Cu 6 Cr showing the π - π stacking interactions that create the 3D porous architecture. E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 2 and 935w. 2.2. CO 2 capture and quantification procedures For the determination of the CO 2 capture from water using the magnetic sustentation technique (Figure S2.1), 50 mg of compound Cu 6 Cr were added in a glass flask with 50 mL of H 2 O. The flask was sealed with a septum and left to stir with gentle agitation. Two needles were immediately inserted: one for the addition of the CO 2 source and the other one as a pressure outlet. The CO 2 was bubbled through the solution at a flow rate of 300 mL/min at 293 K for the required time. Later the aqueous suspension was transferred to the magnetic sustentation measuring device to determine the mass uptake, in terms of percentage, by comparison with the value of the pristine porous material [29]. Further details on the quantification of mass adsorption by magnetic sustentation can be found elsewhere [31–35]. A slightly different set-up was used to gravimetrically determine the amount of CO 2 captured (Figure S2.2). A Schlenk flask was used to incorporate a CO 2 reservoir by placing a balloon in the flask’s sidearm that contains a stopcock valve. The flask was sealed with a septum and a long and a short needle were inserted. During the first 10 minutes CO 2 was bubbled (300 mL/min flow) through the long needle into the aqueous suspension containing the porous materials (50 mg of Cu 6 Cr and 50 mL of H 2 O) and the pressure was released through the short needle. After this time, the sidearm stopcock valve was opened and the short needle (acting as the outlet of the system) was removed allowing the balloon to fill. The long inlet needle was then also removed. The system was kept gently stirring for the remainder of the experiment to ensure that the CO 2 reservoir was not completely emptied during this time. After the end of the specified adsorption time, the stopcock valve sidearm was turned down and the Schlenk flask was placed in a polylactic acid (PLA) holder equipped with a wide base to enable accurate weighting of the system. A short period (5 min) during which the system was exposed to CO 2 bubbling under open-circuit conditions has been introduced to minimize water loss by evaporation, which could affect the accuracy of the CO 2 measurement using the gravimetric technique. It is essential to make a blank using only 50 mL of water to determine its capacity to hold CO 2 and to calculate by difference the excess of stored CO 2 attributable to the porous material. This procedure gives the mass increase occurring in the whole system. For the determination of CO 2 adsorption isotherms, the experiments were performed under different CO 2 /N 2 ratios (0 %, 25 %, 50 %, 75 % and 100 %) established by two controlled flows of pure CO 2 and N 2 (Figure S2.3), with a total flow in the gas inlet of 300 mL/min. The CO 2 bubbling was prolonged for 2 h to ensure the equilibration of the adsorption process. The adsorbed mass in the porous solid was determined by the magnetic sustentation technique. Finally, experiments were carried out on the capture of CO 2 from a water-saturated CO 2 stream on dry particles of the porous material, using the same basic setup was employed but bubbling the pure CO 2 stream in liquid water reservoir prior to its contact with the particles of the porous material (Figure S2.4). At specified times, the particles were placed in a test tube containing water and placed on the magnetic sustentation device for their measurement. The results reported in this work are based on three separate experiments, and are given with their associated standard deviation (ESI). 3. Characterization 3.1. General characterization techniques The IR spectra were recorded on a FT/IR-8X 8400S Jasco spectrometer in the 4000–600 cm −1 spectral region with an attenuated total reflectance (ATR) adapter. Magnetic sustentation experiments were performed using a Newport Pagnell England Electromagnet Type C sourced with a Hewlett Packard 6655 A System DC Power Supply to provide a variable magnetic field under which the particles of the compound dispersed in water are attached to the lower end of the electromagnet pole until the magnetic field decreases to the point where gravity prevails and the particles to fall down [31]. The purity of the samples was assessed by powder X-ray diffraction, thermogravimetry and Fourier-transform infrared spectroscopy (FTIR). Powder X-ray diffraction (PXRD) patterns were collected on a Philips X’PERT powder diffractometer using Cu K α radiation (λ =1.5418 Å) over the 5 <2θ <70◦range with a step size of 0.02◦and an acquisition time of 2.5 s per step at 25 ◦C. pH changes were monitored using a Crison GLP 21 pH-meter. 4. Results and discussion 4.1. CO 2 capture CO 2 and N 2 gas adsorption experiments (Figure S3.1) were performed on degassed Cu 6 Cr samples showing negligible values in both cases (CO 2 : 0.12 mmol/g at 298 K; 0.5 % in weight. N 2 : 0.11 mmol/g at 77 K; 0.3 % in weight). This behaviour is explained by the flexible nature of its supramolecular porous structure, which collapses with the release of the solvent molecules present in the pores of the material leading to an amorphous compact material with almost no free space remaining [29]. The resulting compact material reverts to the original porous supramolecular structure when exposed to a moisture-saturated atmosphere, but the interaction with pure CO 2 in the gas phase (physisorption) is not strong enough as to drive a reversal to the porous structure. On the contrary, the compound is very stable when immersed in water over a wide range of pH (2–10) [29]. Therefore, CO 2 capture by bubbling it in an aqueous medium containing particles of this material will be tested. This approach requires characterization techniques that differ from those used for gas phase adsorption. Two techniques will be employed for this purpose: (i) magnetic sustentation technique, which allows the determination of the captured mass within paramagnetic porous materials (MOFs and SMOFs mainly), and (ii) a gravimetric technique which monitors the mass increase of the system involving the aqueous medium and the suspended porous material particles. The respective experimental set-ups are shown in Figures S2.1 and S2.2. The gravimetric measurement was performed, at 20 ◦C, by weighing the mass increase of the reactor and subtracting the amount of CO 2 captured by the volume of water (50 mL; see details in the experimental section). The mass of CO 2 captured by this volume of water is 33 ±2 mg. The same experiment carried out with the particles of the compound added should indicate a greater amount of CO 2 captured in the system if the material is active. The experiment was carried out with 500 mg of Cu 6 Cr to minimize the contribution of the 50 mL of water. The result shows that the Cu 6 Cr captured a mass excess of CO 2 of 85.7 ±4 mg, corresponding to 17.1 % of the mass of Cu 6 Cr adsorbent used. The normalized results of the CO 2 /H 2 CO 3 captured by Cu 6 Cr formula correspond to 6.4 mol CO2 /mol Cu6Cr (Table 1), which is close to the Table 1 Magnetic sustentation and gravimetric measurement provided CO 2 /H 2 CO 3 capture values for the Cu 6 Cr aqueous suspension (500 mg in 50 mL water) and water alone (50 mL). Experiments performed at 20ºC and 1 h equlibration. Gravimetric measurement Magnetic sustentation Δ mass (mg) Δ mass excess (mg) CO 2 (%) CO 2 / Cu 6 Cr H 2 CO 3 (%) H 2 CO 3 / Cu 6 Cr Cu 6 Cr 500 mg/ 50 mL 118.7 ±4.0 85.7 ±4.3 17.1 ±0.9 6.4 ±0.3 22.3 ±2.6 5.9 ±0.7 Water 50 mL 33 ±1.7 - ~0 -   E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 3 expected number of basic positions placed in the heptameric unit, the six adeninato ligands (Fig. 2a). Additionally, Cu 6 Cr sample undergoes a color change from dark green to a lighter green during the adsorption process (Fig. 2b), reverting to its initial color over time after the CO 2 bubbling has ceased. In contrast, the magnetic sustentation technique measures the mass percentage captured directly by the porous material, unlike the gravimetric technique, which measures the total CO 2 stored in both the water and the porous material. Consequently, the results of the two techniques are complementary but not necessarily equal. The captured mass percentage measured by this technique may differ from the result obtained from the gravimetric approach because the mass incorporated into the porous material depends on the chemical form in which CO 2 is captured. Physisorption of CO 2 and carbamate formation should imply equal values but the capture of CO 2 by the reaction of the concomitant H 2 CO 3 with the basic positions of the adeninato ligands to generate an HCO 3 - / adenine adduct should imply a greater mass capture when measured by the sustentation magnetic technique than by the gravimetric method. The results obtained at 20 ◦C (Table 1) are consistent with the capture of CO 2 in the form of coordinated adenine/HCO 3 - pairs. The capture mass value measured by the magnetic sustentation technique (220 mg/g) is higher than the values obtained by the gravimetric measurement (ca. 172 mg/g). However, when these data are normalized per heptameric entity, the values obtained by both techniques for captured CO 2 /H 2 CO 3 are relatively close (6.4(3) vs 5.9(7)). These values are, in fact,close to the theoretical uptake of 6 CO 2 molecules in the form of HCO 3 - /adenine pairs (Fig. 2). The combination of the two techniques therefore makes it possible to ensure that CO 2 is captured, but also to discern the chemical form in which it is captured. Magnetic sustentation measurements provide a straightforward method to obtain adsorption kinetic curves. Accordingly, timedependent adsorption isotherms (20 ◦C) were measured for CO 2 in an aqueous suspension of Cu 6 Cr particles, wet CO 2 flow in Cu 6 Cr particles and dry CO 2 flow in Cu 6 Cr particles (Fig. 3a). In the first case, the CO 2 capture values are much higher, obtaining a mass increase of 22.3 %. Under these conditions, the adsorption reaches the steady state after 1 hour. In comparison, when Cu 6 Cr particles are exposed to humid saturated and dry CO 2 fluxes, the values drop to 12.9 % and 0 %, respectively, after 180 minutes. In addition, the time required to reach saturation increases greatly when the Cu 6 Cr particles are exposed to a humid saturated CO 2 flux than when the particles are immersed in water. These values need to be put in context with other CO 2 capture materials [37,38]. Among these materials, amines stand out with a capture value of 30 wt% for MEA (monoethanolamine), which is the conventional solution widely used at a commercial scale [39]. Alkali and alkali earth metal oxides provides also greater CO 2 capture values, for example Mg 90 Fe 10 -AMS 10 captures 65 wt% at 300 ◦C under a flux of pure CO 2 [40]. However, the results obtained for Cu 6 Cr are significantly higher than the values reported for MOFs among which Mg 2 (dobpdc) stands out with a maximum adsorption capacity of 12.4 % (2.82 mmol⋅g −1 ) at 40 ◦C under wet flue gas conditions consisting of 15 % CO 2 , 80 % N 2 and 5 % water [41]. The combination of MOFs with carbonaceous materials allow increasing the CO 2 capture (for example, CuBTC@GO captures 39 wt% at 273 K and 1 bar) [42]. On the other hand, the adsorption of Cu 6 Cr under different CO 2 concentrations (CO 2 /N 2 mixtures, Fig. 3b) shows that the adsorption achieved after 1 h of experiment decreases with the decreasing of CO 2 concentration from a 22.3 % of mass capture to 16.2 % at 1:1 CO 2 /N 2 flux. Fig. 4a summarises the CO 2 adsorption kinetic curves for aqueous suspensions of Cu 6 Cr particles carried out at different temperatures (20, 30 and 40 ◦C). These experimental data sets show relatively good fit to the Langmuir model with correlation coefficients of 0.993, 0.981 and 0.975, respectively (Fig. 4b). Note that the saturation implies the same amount of adsorbed mass which further supports chemisorption via formation of HCO 3 – /adenine ensembles, rather than CO 2 physisorption, for which the saturation value would decrease with the temperature. The apparent activation energy (E a ) of adsorption can be calculated using the Arrhenius equation (Eq. 1): lnkads = − Ea(ads) RT +C(1) where k ads is the adsorption rate constant, and C is a constant. As shown in Fig. 4c, E a(ads) is obtained from the slope of the ln k vs 1/T plot and gives a value of 21.8 kJ/mol. A comparison with other CO 2 adsorbents will be given later. Fig. 2. (a) Schematic representation of the CO 2 capture in the form of HCO 3 - /adenine pairs involving the [Cu 6 Cr( μ -adeninato-κN3:κN9) 6 ( μ 3 -OH) 6 ( μ -OH 2 ) 6 ] 3+ entities. (b) Colour change of Cu 6 Cr sample during CO 2 bubbling. The pictures were taken while the particles were in the aqueous medium. E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 4 4.2. CO 2 release As mentioned above, it is desirable for the CO 2 adsorbent to be able to release the captured CO 2 without a large energy penalty. Therefore, the CO 2 release behavior of this compound was also measured under two conditions: (i) while in the aqueous media and (ii) after removal of the compound particles from the aqueous media. The release kinetic study was carried out by keeping the particles in the aqueous media (test tube) exposed to the air under constant temperature conditions using a thermostatic water bath and performing successive magnetic sustentation measurements at different times which does not require the particles to be removed from the aqueous media. These desorption measurements were performed at 20, 30 and 40 ◦C (Fig. 5a). The desorption kinetic curves can again be fitted to a Langmuir model (Fig. 5b). Based on this temperature dependence, an E a(des) value can be calculated for this process (41.2 kJ/mol; Fig. 5c). The obtained E a(ads) and E a(des) values are similar to those found for amines loaded in different adsorbents: 12.6 and 54.2 kJ/mol for hydroxyl ethylenediamine (AEEA) supported in HZSM-5 zeolite [43], 7.4 and 41–47 kJ/mol for tetraethylenepentamine (TEPA) supported on carbon nanotubes [44], and 19.6 and 51.1 kJ/mol for TEPA supported in TiO(OH) 2 [45]. Furthermore, the reaction heat in the adsorption process can be calculated using Eq. 2: ΔHads =Ea(ads)−Ea(des)(2) In Cu 6 Cr compound, E a(ads) and E a(des) values are 21.8 and 41.2 kJ/ mol, respectively, giving an adsorption heat (ΔH ads ) of −19.4 kJ/mol, corresponding to an exothermic process. The value obtained is lower than those corresponding to the optimized systems described above, in which different amines are incorporated in porous matrices to create heterogeneous adsorption devices, which show ΔH ads values ranging from −31.5 to −41.6 kJ/mol. The adsorption enthalpies for nonencapsulated amines are even higher: around (-80)-(-100) kJ/mol for primary and secondary amines, and (-50)-(-65) kJ/mol for ternary amines in which carbamate formation is forbidden, chemisorption relying on the formation of ammonium hydrogencarbonate pairs [46]. Therefore, Cu 6 Cr offers a significant energetic improvement over these materials. In addition to measuring the CO 2 remaining in the porous material, the pH value of the aqueous medium was also monitored. The initial pH of the CO 2 -saturated aqueous medium is approximately 3.5 and rises to 6.0 very fast after the CO 2 bubbling stops. However, the suspension containing 500 mg of Cu 6 Cr in 50 mL of water gives an initial pH value of 5 although CO 2 bubbling is maintained for 1 h, which indicates the presence of HCO 3 - in the media. In addition, when the CO 2 bubbling is stopped, the increase in pH is much slower, reaching a value of 5.6 (Figure S9.1). The desorption kinetics were analyzed by the magnetic sustentation technique after the crystals were removed from the aqueous media by fast filtering the particles and keeping them at ambient conditions (20 ◦C). After this time, the CO 2 trapped by the particles is measured using the magnetic sustentation technique and following to the procedure described in the experimental section. The minimum time for this procedure is 10 min and provides an almost complete release of the captured CO 2 . Consecutive FTIR-ATR measurements (Fig. 6) were also carried out on a recovered sample of Cu 6 Cr which had been subjected to 1 h of CO 2 bubbling in water (300 mL/min). The first measurement shows the υ as (C – – O) and υ s (C – – O) of the HCO 3 - anion at 1575 and 1325 cm −1 , respectively, which partially overlap with the signals of the pristine SMOF (see ESI). The second measurement, taken 5 min after the first, shows a significant decrease in both signals and after 20 min it can be said that both signals are no longer visible. These facts seem to indicate a faster release kinetic when the CO 2 molecules are transferred directly from the pore system of the material to the surrounding air than when this release takes place in the aqueous media. 4.3. Stability upon CO 2 capture/release cycling Up to four successive cycles of CO 2 capture and desorption steps were performed, with magnetic sustentation measurements taken after each step (Fig. 7). Each desorption step involved keeping the aqueous suspension in an open atmosphere at 40 ◦C for 2 h. The zero mass gain obtained indicate that the desorption was complete under these conditions. The adsorption step was allowed to equilibrate for 1 h (or 30 min). In both cases, the adsorption capacity is reduced with each new cycle, but the decline of the adsorbent is more acute for 1 h adsorption time than when it was kept for 30 min. After 4 cycles of 1 h bubbling Cu 6 Cr can hardly be considered as a CO 2 adsorbent material, while reducing Fig. 3. (a) CO 2 adsorption kinetic curves by Cu 6 Cr under different conditions: bubbling CO 2 in an aqueous suspension of Cu 6 Cr particles (green), exposure of Cu 6 Cr particles to humid saturated CO 2 flow (blue), and exposure of Cu 6 Cr particles to dry CO 2 flux (blue). (b) Bubbling CO 2 adsorption isotherm curve by Cu 6 Cr as a function of CO 2 percentage (CO 2 diluted with N 2 ; 2 h equilibration time). Temperature: 20 ◦C; total flux: 300 mL/min. E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 5 the bubbling time to 30 min, the adsorption capacity is slightly better retained, although after 4 cycles it was almost negligible. Bearing in mind that the pH of the aqueous media reaches a relatively acidic value during CO 2 bubbling, the greater decay observed with longer bubbling times has been attributed to a chemical instability of Cu 6 Cr under these conditions. In fact, as stated in the seminal work on this compound, the observed chemical stability is due to the kinetic inertness of the Cr(III) metal center and not due to thermodynamic stability. Therefore, prolonged exposure to these acidic conditions will result in a progressive collapse of its crystal structure. A final attempt to ensure the cyclability of the material during the CO 2 adsorption/desorption cycles was made using a 0.1 M NaHCO 3 solution instead of pure water. Firstly, the stability of Cu 6 Cr was checked by immersing a sample of this compound in the solution for 18 h at ambient temperature. No significant solubilization was observed, and the PXRD pattern showed not significant changes with respect to the pristine compound (ESI). The results after four cycles show a clear improvement in the stability of the sample, which still retains a 50 % of its initial adsorption capacity at the last fourth cycle, Fig. 4. (a) CO 2 adsorption kinetic curves of compound Cu 6 Cr at different temperatures. (b) Numerical fitting to the Langmuir model (see table for the fitting parameters). (c) Arrhenius plot. Fig. 5. (a) CO 2 desorption kinetic curves of compound Cu 6 Cr at different temperatures. (b) Numerical fitting to the Langmuir model (see table for the fitting parameters). (c) Arrhenius plot. E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 6 whereas the use of water implies that the compound loses its CO 2 adsorption capacity and amorphizes. 5. Conclusions To the best of our knowledge, this is the first time that a porous supramolecular metal-organic material (SMOF) has been used to capture of CO 2 from water[41]. This material composed of heptanuclear [CrCu 6 ( μ -OH) 6 ( μ -adeninato-кN3:кN9) 6 ( μ -OH 2 ) 6 ] 3+ entities counterbalanced by sulfate anions is not able to capture CO 2 in the absence of water, but in aqueous media it is very efficient at storing almost up to 6 CO 2 molecules (in the form of adenine/HCO 3 - pairs; the theoretical maximum) per heptanuclear entity. The pK a value of the coordinated adeninato ligand (7.2–7.4) and the pK a1 value of H 2 CO 3 (6.4) allow the acid-base reaction to take place, but at the same time they are close enough to avoid the resulting adenine/HCO 3 - pair being too stable as to require high energy for the CO 2 release. The obtained heat of adsorption (ΔH ads ) of −19.4 kJ/mol and the desorption mediated by ambient conditions support this hypothesis. We have used a novel technique to quantify CO 2 capture: magnetic sustentation in combination with a conventional gravimetric measurement. Whereas the gravimetric approach tells the mass amount of gaseous CO 2 being captured, the magnetic sustentation technique measures the mass being incorporated into the porous material particles. The results of the latter technique depend on the chemical form (physisorbed, HCO 3 - , carbamate) in which the CO 2 is being captured. Comparing the values obtained by the two techniques helps to identify the chemical form in which CO 2 is being captured. Magnetic sustentation measurements also made it possible to obtain the adsorption and desorption kinetic curves at different temperatures. This capability is crucial to determine the corresponding activation energies of adsorption and desorption through the Arrhenius equation. Overall, the manuscript provides both an innovative type of material and techniques that may be valuable in the search for more efficient CO 2 Fig. 6. (a) FTIR spectra of pristine Cu 6 Cr (red) and Cu 6 Cr subjected to CO 2 bubbling for 1 h (300 mL/min) during the CO 2 desorption outside the aqueous medium. Magnifications of the υ s (C – – O) (b) and υ as (C – – O) (c) signals of HCO 3 - at 1325 cm −1 and 1575 cm −1 , respectively. E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 7 capture processes that can cope with the presence of water. In the near future, we are interested in developing Cu 6 Cr-like materials that incorporate additional basic positions in the counterion, which could boost CO₂ capture to higher levels. CRediT authorship contribution statement Sonia P´ erez-Ya˜ nez: Writing – review & editing, Project administration, Methodology. Ekain Maiza-Razkin: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis. Oscar Castillo: Writing – review & editing, Writing – original draft, Supervision, Funding acquisition, Conceptualization. Rub´ en P´ erezAguirre: Writing – review & editing, Writing – original draft, Formal analysis, Conceptualization. Antonio Luque: Writing – review & editing, Validation, Formal analysis. Garikoitz Beobide: Writing – review & editing, Methodology, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been funded by Eusko Jaurlaritza/Gobierno Vasco (IT1722–22) and by the Spanish Ministry of Science and Innovation (PID2022–138968NB-C22 project funded by MCIN/AEI /10.13039/ 501100011033/ and by FEDER A way to make Europe and TED2021–129810B-C22 funded by MCIN/AEI/10.13039/ 501100011033 and Next Generation EU/PRTR). Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, ESF) is also acknowledged. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jcou.2025.103144. Data Availability Data will be made available on request. References [1] C.C. Emenekewe, R.U. Onyeneke, C.U. Nwajiuba, I.Q. Anugwa, O.U. Emenekewe, Determinants of consumption-based and production-based carbon emissions, Environ. Dev. Sustain (2023), https://doi.org/10.1007/s10668-023-04311-9. [2] C. Zhang, Y. Ji, C. Li, Y. Zhang, S. Sun, Y. Xu, L. Jiang, C. Wu, The application of biochar for CO 2 capture: influence of biochar preparation and CO 2 capture reactors, Ind. Eng. Chem. Res. 62 (2023) 17168–17181, https://doi.org/10.1021/ acs.iecr.3c00445. [3] H. Shaikh, S. Shah, S. Karim, M. Younas, S. Ali, S. Farrukh, M. Shah, S. Shah, Carbon dioxide (CO 2 ) gas storage and utilization. Green Energy and Technology, Springer, Cham, 2023, pp. 208–243, https://doi.org/10.1007/978-3-031-21444-8_ 8. [4] S. Sunkara, N. Pankhedkar, R. Gudi, Valorization of refinery flue gas through trireforming and direct hydrogenation routes, J. Chem. Eng. 102 (2024) 2136–2150, https://doi.org/10.1002/cjce.25187. [5] Zanatta, Materials for direct air capture and integrated CO 2 conversion: advancement, challenges, and prospects, M. ACS Mat. AU 3 (2023) 576–583. 〈https://pubs.acs.org/doi/10.1021/acsmaterialsau.3c00061〉. [6] E. Woods, V. Berrio, Y. Qiu, P. Berlin, N. Clauser, W. Sagues, Biomass composting with gaseous carbon dioxide capture, RSC Sustain. 2 (2024) 621–625, https://doi. org/10.1039/D3SU00411B. [7] D. Leeson, N. Mac Dowell, N. Shah, C. Petit, P.S. Fennell, A Techno-economic analysis and systematic review of carbon capture and storage (CCS) applied to the iron and steel, cement, oil refining and pulp and paper industries, as well as other high purity sources, Int. J. Greenh. Gas. Control 61 (2017) 71–84, https://doi.org/ 10.1016/j.ijggc.2017.03.020. [8] C.A. Scholes, K.H. Smith, S.E. Kentish, G.W. Stevens, CO 2 capture from precombustion processes—Strategies for membrane gas separation, Int. J. Greenh. Gas. Control 4 (2010) 739–755, https://doi.org/10.1016/j.ijggc.2010.04.001. [9] A. Mukherjee, J.A. Okolie, A. Abdelrasoul, C. Niu, A.K. Dalai, Review of postcombustion carbon dioxide capture technologies using activated carbon, J. Environ. Sci. 83 (2019) 46–63, https://doi.org/10.1016/j.jes.2019.03.014. [10] L.F. de Mello, R. Gobbo, G.T. Moure, I. Miracca, Oxy-combustion technology development for fluid catalytic crackers (FCC) – large pilot scale demonstration, Energy Procedia 37 (2013) 7815–7824, https://doi.org/10.1016/j. egypro.2013.06.562. [11] X. An, T. Li, J. Chen, D. Fu, 3D-hierarchical porous functionalized carbon aerogel from renewable cellulose: an innovative solid-amine adsorbent with high CO 2 adsorption performance, Energy 274 (2023) 127392, https://doi.org/10.1016/j. energy.2023.127392. [12] U. Khan, C.C. Ogbaga, O.-A.O. Abiodun, A.A. Adeleke, P.P. Ikubanni, P.U. Okoye, J.A. Okolie, Assessing absorption-based CO 2 capture: Research progress and techno-economic assessment overview, Carbon Capture Sci. Technol. 8 (2023) 100125, https://doi.org/10.1016/j.ccst.2023.100125. [13] R. Chakraborty, A. Asthana, A.K. Singh, B. Jain, A.B.H. Susan, Adsorption of heavy metal ions by various low-cost adsorbents: a review, Int. J. Environ. Anal. Chem. 102 (2020) 342–379, https://doi.org/10.1080/03067319.2020.1722811. [14] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Mouscou, R.A. Pierotti, J. Rouquerol, T. Sie-mieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984), Pure Appl. Chem. 57 (nº4) (2009) 603–619, https://doi.org/10.1351/ pac198557040603. [15] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982, https://doi.org/10.1002/bbpc.19820861019. [16] F. Rouquerol, J. Rouquerol, K. Sing, Adsorptionby, in: Powders and Porous Solids, 1, Academic Press, London, 1999, p. 26, https://doi.org/10.1016/B978-0-12598920-6.X5000-3. [17] H. Qiu, L. Lv, C. Pan, Q. Zhang, Critical review in adsorption kinetic models, J. Zhejiang Univ. Sci. A. 10 (2009) 716–772, https://doi.org/10.1631/jzus. A0820524. [18] M. Zhao, Y. Yang, X.-S. Gu, MOF based CO 2 capture: Adsorption and membrane separation, Inorg. Chem. Commun. 152 (2023) 110722, https://doi.org/10.1016/j. inoche.2023.110722. [19] C. Goel, S. Mohan, P. Dinesha, CO 2 capture by adsorption on biomass-derived activated char: a review, Sci. Total Environ. 798 (2021) 149296, https://doi.org/ 10.1016/j.scitotenv.2021.149296. [20] E.E. Ünveren, B.O. Monkul, S. Sarıoglan, N. Karademir, E. Alper, Solid amine sorbents for CO2 capture by chemical adsorption: a review, Petroleum 3 (2017) 37–50. [21] X. Fan, B.H. Tan, Z. Li, X.J. Loh, Control of PLA stereoisomers-based polyurethane elastomers as highly efficient shape memory materials, ACS Sustain. Chem. Eng. 5 (2017) 1217–1227, https://doi.org/10.1021/acssuschemeng.6b02652. [22] Z. Li, P.L. Chee, C. Owh, R. Lakshminarayanan, X.J. Loh, Safe and efficient membrane permeabilizing polymers based on PLLA for antibacterial applications, RSC Adv. 6 (2016) 28947–28955, https://doi.org/10.1039/C6RA04531F. [23] X.J. Loh, Poly(DMAEMA-co-PPGMA): dual-responsive “reversible” micelles, J. Appl. Polym. Sci. 127 (2013) 992–1000, https://doi.org/10.1002/app.37530. [24] C. Huang, L. Tian, J. Wang, L. Jiang, K. Zhang, Water-CO 2 wettability on sandstone surface with asphaltene adsorption: molecular dynamics simulation, Fuel 360 (2024) 130558, https://doi.org/10.1016/j.fuel.2023.130558. [25] J. Wang, S. Wang, Q. Xin, Y. Li, Perspectives on water-facilitated CO 2 capture materials, J. Mater. Chem. A 5 (2017) 6794, https://doi.org/10.1039/ C7TA01297G. Fig. 7. CO 2 capture/release cycling process under three different adsorption conditions. E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 8 [26] D. Li, C. Li, Efficient and reversible capture of CO 2 in CO 2 -binding organic liquids formed by 1,1,3,3-tetramethylguanidine and glycerol derivatives, ACS Sust. Chem. Engi 11 (2023) 5335–5344, https://doi.org/10.1021/acssuschemeng.2c05109. [27] P.C. Hiemenz, R. Rajagopalan, Principles of colloid and surface chemistry (Ed), Marcel Dekker Inc. 3 (1997) 405–456, https://doi.org/10.1021/ed064pA328.3. [28] L. Jiang, W. Liu, R.Q. Wang, A. Gonzalez-Diaz, M.F. Rojas-Michaga, S. Michailos, M. Pourkashanian, X.J. Zhang, C. Font-Palma, Sorption direct air capture with CO 2 utilization, Prog. Energy Combust. Sci. 95 (2023) 101069, https://doi.org/ 10.1016/j.pecs.2022.101069. [29] R. P´ erez-Aguirre, B. Artetxe, G. Beobide, O. Castillo, I. de Pedro, A. Luque, S. P´ erezY´ a˜ nez, S. Wutke, Ferromagnetic supramolecular metal-organic frameworks for active capture and magnetic sensing of emerging drug pollutants, Cell Rep. Phys. Sci. 2 (2021) 100421, https://doi.org/10.1016/j.xcrp.2021.100421. [30] G. Beobide, O. Castillo, J. Cepeda, A. Luque, S. P´ erez-Y´ a˜ nez, P.- Rom´ an, J. ThomasGipson, Metal–carboxylato–nucleobase systems: from supramolecular assemblies to 3D porous materials, Coord. Chem. Rev. 257 (2013) 2716–2736, https://doi. org/10.1016/j.ccr.2013.03.011. [31] N. Barroso, J. Andreo, G. Beobide, O. Castillo, A. Luque, S. Perez-Yanez, S. Wuttke, Magnetic sustentation as an adsorption characterization technique for paramagnetic metal-organic frameworks, Comm. Chem. 6 (2023) 1–90, https:// doi.org/10.1038/s42004-022-00799-w. [32] J. Pascual-Colino, R. P´ erez-Aguirre, G. Beobide, O. Castillo, I. de Pedro, A. Luque, S. Mena-Guti´ errez, S. P´ erez-Y´ a˜ nez, An in solution adsorption characterization technique based on the response to an external magnetic field of porous paramagnetic materials: application on supramolecular metal–adenine frameworks containing heterometallic heptameric clusters, Inorg. Chem. Front 10 (2023) 2250–2261, https://doi.org/10.1039/D2QI01994A. [33] N. Barroso, S. Dutta, J. Andreo, G. Beobide, O. Castillo, A. Luque, S. P´ erez-Y´ a˜ nez, S. Wuttke, Guest-induced breathing mediated selective alcohol recovery from water by MIL-88A(Fe) (Sep), J. Mat. Chem. A. 11 (2023) 21300–21311, https:// doi.org/10.1039/D3TA04110G. [34] S. Mena-Guti´ errez, J. Pascual-Colino, G. Beobide, O. Castillo, A. Castellanos-Rubio, A. Luque, E. Maiza-Razkin, J. Mentxaka, S. P´ erez-Y´ a˜ nez, Isoreticular chemistry and applications of supramolecularly assembled copper-adenine porous materials, Inorg. Chem. 62 (2023) 18496–18509, https://doi.org/10.1021/acs. inorgchem.3c02708. [35] S. Mena-Guit´ errez, E. Maiza-Razkin, J. Pascual-Colino, M.J. Araúzo-Bravo, G. Beobide, O. Castillo, A. Castellanos-Rubio, D. Gerovska, A. Luque, S. P´ erezY´ a˜ nez, Drug-delivery and biological activity in colorectal cancer of a supramolecular porous material assembled from heptameric chromium-copperadenine entities, J. Mater. Chem. B 12 (2024) 11156–11164, https://doi.org/ 10.1039/d4tb01521e. [36] W. Rudolph, D. Fischer, G. Irmer, Vibrational spectroscopic studies and density functional theory calculations of speciation in the CO 2 —water system, Appl. Spectr. 60 (2006) 130–144, https://doi.org/10.1366/000370206776023421. [37] X. Yun, D. Soo, J.J. Cheng Lee, W. Wu, L. Tao, C. Wang, Q. Zhu, J. Bu, Advancements in CO 2 capture by absorption and adsorption: a comprehensive review, J. CO 2 Util. 81 (2024) 102727, https://doi.org/10.1016/j. jcou.2024.102727. [38] J. Wang, L. Li, G. Qu, M. Cheng, Y. Ren, P. Lu, T. Zhang, Y. Wang, F. Wu, Advanced materials for CO 2 capture and storage: a comprehensive review of current progress and future prospects, J. Environ. Chem. Eng. 13 (2025) 115881, https://doi.org/ 10.1016/j.jece.2025.115881. [39] K. Li, W. Leigh, P. Feron, H. Yu, M. Tade, Systematic study of aqueous monoethanolamine (MEA)-based CO 2 capture process: techno-economic assessment of the MEA process and its improvements, Appl. Energy 165 (2016) 648–659, https://doi.org/10.1016/j.apenergy.2015.12.109. [40] F. Gong, Z. Huang, X. Wei, S. Liu, J. Lu, J. Ding, W. Wang, MgO promoted by Fe 2 O 3 and nitrate molten salt for fast and enhanced CO 2 capture: experimental and DFT investigation, Sep. Puri. Tech. 307 (2023) 122766, https://doi.org/10.1016/j. seppur.2022.122766. [41] J.H. Choe, H. Kim, M. Kang, H. Yun, S.Y. Kim, S.M. Lee, C.S. Hong, Functionalization of diamine-appended MOF-based adsorbents by ring opening of epoxide: long-term stability and CO 2 recyclability under humid conditions, J. Am. Chem. Soc. 144 (2022) 10309–10319, https://doi.org/10.1021/jacs.2c01488. [42] S. Shang, Z. Tao, C. Yang, A. Hanif, L. Li, D.C.W. Tsang, Q. Gu, J. Shang, Facile synthesis of CuBTC and its graphene oxide composites as efficient adsorbents for CO 2 capture, Chem. Eng. J. 393 (2020) 124666, https://doi.org/10.1016/j. cej.2020.124666. [43] H. Cheng, H. Song, S. Toan, B. Wang, K.A.M. Gasem, M. Fan, F. Cheng, Experimental investigation of CO 2 adsorption and desorption on multi-type amines loaded HZSM-5 zeolites, Chem. Eng. J. 406 (2021) 126882, https://doi.org/ 10.1016/j.cej.2020.126882. [44] A. Kachko, L.V. Ham, A. Bardow, T.J.H. Vlugt, E.L.V. Goetheer, Comparison of Raman, NIR, and ATR FTIR spectroscopy as analytical tools for in-line monitoring of CO 2 concentration in an amine gas treating process, Int. J. Greenh. Gas. Con. 47 (2016) 17–24, https://doi.org/10.1016/j.ijggc.2016.01.020. [45] M. Irani, K.A.M. Gasem, B. Dutcher, M. Fan, CO 2 capture using nanoporous TiO (OH) 2 /tetraethylenepentamine, Fuel 183 (2016) 601–608, https://doi.org/ 10.1016/j.fuel.2016.06.129. [46] Nabil E.L. Hadria, Dang Viet Quanga, Mohammad R.M. Abu-Zahra, Study of novel solvent for CO 2 post-combustion capture, Energy Procedia 75 (2015) 2268–2286, https://doi.org/10.1016/j.egypro.2015.07.414. E. Maiza-Razkin et al. Journal of CO2 Utilization 98 (2025) 103144 9