Engineered assembly of water-dispersible nanocatalysts enables low-cost and green CO2 capture

Catalytic solvent regeneration has attracted broad interest owing to its potential to reduce energy consumption in CO2 separation, enabling industry to achieve emission reduction targets of the Paris Climate Accord. Despite recent advances, the development of engineered acidic nanocatalysts with unique characteristics remains a challenge. Herein, we establish a strategy to tailor the physicochemical properties of metal-organic frameworks (MOFs) for the synthesis of water-dispersible core-shell nanocatalysts with ease of use. We demonstrate that functionalized nanoclusters (Fe3O4-COOH) effectively induce missing-linker deficiencies and fabricate mesoporosity during the self-assembly of MOFs. Superacid sites are created by introducing chelating sulfates on the uncoordinated metal clusters, providing high proton donation capability. The obtained nanomaterials drastically reduce the energy consumption of CO2 capture by 44.7% using only 0.1 wt.% nanocatalyst, which is a ∽10-fold improvement in efficiency compared to heterogeneous catalysts. This research represents a new avenue for the next generation of advanced nanomaterials in catalytic solvent regeneration.

T he growing trend of fossil fuel consumption and anthropogenic CO 2 emission has gained global attention as an urgent environmental issue 1,2 . To overcome this challenge, the Paris Agreement was signed in 2015 by consensus to keep the Earth's temperature rise well below 2°C by mid-century 3,4 . Despite this, CO 2 emission has been relentlessly increasing and approaching 40 GtCO 2 /year due to persistent release from key industrial sectors 5 . A strategy to reduce CO 2 emissions while keeping existing industrial assets is carbon capture and storage (CCS). However, given the limited number of large-scale CO 2 capture plants and the noticeable reduction in fossil fuel prices, hundreds of CCS facilities will need to be constructed by 2030 for the successful achievement of the Paris Agreement targets 6 . Chemical solvent CO 2 absorption-desorption, as the most viable and widely accepted technique for carbon separation, is not inherently green and its high energy demand for solvent regeneration indirectly contributes to global CO 2 emissions. Additionally, the energy-intensive nature of separation strongly impacts the economics of the process, leading to a reluctance to invest in CCS projects. Hence, it is of high importance to deploy a diverse portfolio of energy-efficient and green CO 2 capture technologies with net-zero emissions.
The remarkable energy consumption of CO 2 separation is mainly attributed to the high solvent regeneration temperature (above 100°C) required to accelerate CO 2 desorption kinetics 7,8 . Recently, catalytic solvent regeneration has emerged as an interesting approach to effectively promote the CO 2 desorption rate and reduce the required regeneration energy 9,10 . One of the distinguishing features of catalytic regeneration is its potential to critically reduce the CO 2 desorption temperature below 100°C and accordingly pave the way for utilizing lower grade heat resources, such as solar hot water, as a green approach for solvent regeneration 7,11 . In this regard, a wide range of commercial and synthetic heterogeneous catalysts have been tested for catalyst-aided solvent regeneration, though their low efficiency and operational difficulties are still a significant barrier toward large-scale implementation 11 .
Metal-organic frameworks (MOFs) are an emerging class of porous materials with highly favorable catalytic properties owing to their large pore volume, abundant metal sites, and tailorable structure 12,13 . These features have made MOFs potential candidates for developing advanced nanocatalysts. Yaghi et al. recently reported the synthesis of an acidic MOF-SO 4 catalyst by treating a zirconium-based MOF (MOF-808) with aqueous sulfuric acid 14,15 . They showed the strong Brønsted acidity of MOF-808-SO 4 originates from the chelating sulfates attached to the unsaturated metal clusters which release protons into the reaction medium. However, the microporosity of MOFs is still a major concern for the fabrication of super-efficient nanomaterials, particularly in catalysis [16][17][18] . Additionally, there are other acidstable MOFs desired for the fabrication of MOF-SO 4 nanocatalysts which have not yet been explored. To this end, it is of great interest to exploit advanced MOF-SO 4 materials with targeted physicochemical features for low-temperature CO 2 desorption, thereby taking a step further toward establishing green CO 2 capture technologies and achieving the objectives of the Paris Climate Accord.
Herein, we present a new strategy for the fabrication of engineered water-dispersible nanocatalysts with hierarchical micromesoporous structures ( Fig. 1 and Supplementary Fig. 4). In this technique, we used acidic Fe 3 O 4 nanoclusters as a versatile substrate for the modulated self-assembly of different MOFs with broad structural diversity. The presence of carboxylates surrounding the core surface leverages the creation of active mesoporosity throughout the network of assembled MOFs. Specifically, the grown mesoporous shell facilitates the subsequent coordination of chelating sulfate moieties onto the metal clusters, thus allowing for a higher density of Brønsted acid sites. The accommodation of approachable acidic sites through the tailored hierarchical structure, as well as the nanofluidic aspect readily enables water-dispersible Fe 3 O 4 @MOF-SO 4 nanocatalyst to actively participate in CO 2 desorption reactions and drastically reduce energy consumption.

Results
Water-dispersible acidic Fe 3 O 4 substrate. We first examined the potential of acidic Fe 3 O 4 nanoclusters as favorable waterdispersible supports for the formation of advanced nanocatalysts. Ferric ammonium citrate, containing both iron (Fe 3+ ) and citric acid, was selected as a cheap and environmentally benign precursor for the fabrication of carboxylated Fe 3 O 4 nanoclusters (Fe 3 O 4 -COOH). One of the benefits of the citrate assembly approach is that the carboxylate groups on the surface of the nanocluster can serve as proton donor sites. Additionally, carboxylates can also provide stable hydrogen bonds to water molecules, allowing great dispersibility and facilitating the nanofluidic behavior of Fe 3 O 4 -COOH in aqueous solvents ( Supplementary Fig. 5). The combination of these two features enables the Fe 3 O 4 -COOH to be simply utilized as a waterdispersible acidic catalyst during the continuous CO 2 absorption-desorption operation without any modification to the process configuration.
The surface modification is achieved via two steps by the assembly and adhesion of citrate ions to the Fe 3 O 4 surface (Fig. 2a). Upon partial reduction of Fe 3+ to Fe 2+ in the nonaqueous solution and formation of single-crystal Fe 3 O 4 nanoparticles, citrate ions easily attach to the surface owing to the strong coordination affinity between carboxylate groups and Fe 3+ /Fe 2+ ions, resulting in aggregation of single nanoparticles to form large Fe 3 O 4 nanoclusters 19,20 . Helium ion microscopy and scanning electron microscopy (SEM) showed the spherical clusters ranging in size from ⁓100 to 300 nm (Fig. 2b, c and Supplementary Fig. 6). Specifically, transmission electron microscopy (TEM) indicated the aggregated nanoclusters are composed of small adhesive Fe 3 O 4 nanoparticles with about ⁓2-5 nm size ( Fig. 2d). High-angle annular dark-field (HAADF), energydispersive X-ray spectroscopy (EDX) mapping, and elemental line scanning confirmed the homogeneous morphology of Fe 3 O 4 nanoclusters with uniform Fe and O distribution throughout the structure ( Fig. 2e and Supplementary Fig. 7). In addition, T 2g and E g peaks were identified as the prominent vibrational modes in the Raman spectrum which confirm the successful formation of the Fe 3 O 4 structure (Fig. 2f).
Different characterization methods were utilized to confirm the successful assembly of carboxylic acid groups (COOH) on the surface of Fe 3 O 4 -COOH nanoclusters. Fourier transform infrared spectroscopy (FTIR) displayed two characteristic peaks at ⁓1340 and ⁓1610 cm −1 corresponding to the carbonyl group (C=O) and C-OH stretching of the carboxylic acid group (Fig. 2g) 21 . X-ray photoelectron spectroscopy (XPS) revealed that the majority of Fe 3 O 4 -COOH surface is covered by O (⁓58.9%) and C (⁓29.4%), while the share of Fe was ⁓9.1% (Fig. 2h, i and Supplementary  Fig. 8). In contrast, the elemental analysis showed the bulk is mainly composed of Fe (⁓87.0%), which is ⁓28-fold greater than that of C (⁓2.9%) in the bulk sample ( Supplementary Fig. 9   hydrophilicity of Fe 3 O 4 -COOH, its aqueous solution was mixed with silicon oil, toluene, and dichloromethane, and no Fe 3 O 4 -COOH was observed in the non-aqueous solvents, highlighting the catalytic potential of Fe 3 O 4 -COOH in phase induced areas, such as CO 2 capture using liquid-liquid phase change solvents (Fig. 2g) 22 . These results indicate that Fe 3 O 4 -COOH can be viably used as a water-dispersible substrate for further surface modification, having tunable physicochemical motifs that are of interest for catalytic solvent regeneration in CO 2 capture.
Tailoring the properties of Fe 3 O 4 @MOF. To appraise the modulated self-assembly of different MOFs on acidic Fe 3 O 4 -COOH substrate, seven MOFs, including ZIF-8, ZIF-67, MIL-100(Fe), MOF-Fe(II), HKUST-1, UiO-66, and UiO-66-NH 2 with various metal (zinc (Zn), zirconium (Zr), cobalt (Co), copper (Cu), ferrous (Fe 2+ ), and ferric (Fe 3+ )) and ligand (benzene-1,4-dicarboxylic acid (H 2 BDC), 2-aminoterephthalic acid (H 2 BDC-NH 2 ), 2,5-pyridinedicarboxylic acid (H 2 BDC-N), benzene-1,3,5-tricarboxylic acid (H 3 BTC) and 2-methylimidazole (2-Melm)) combinations were utilized (Fig. 3a). After growing MOFs, the zeta potential of Fe 3 O 4 -COOH substrate in all cases changed from −43.1 mV to greater values (ranging from −1.7 to −19.5 mV), indicating that the surface of the substrate is primarily covered by metal-ligand assembly ( Supplementary Fig. 10). This variation was due to the coordination of citrate ions and thereby diminution in the negative surface charge onto the nanocluster. In detail, the citrate groups of Fe 3 O 4 -COOH nanocluster can be easily dissociated and create a negatively charged surface, while hydroxyl groups of MOFs have a lower dissociation constant than those of carboxylates resulting in a decrease in the negative surface charge of Fe 3 O 4 @MOFs. The successful preparation of Fe 3 O 4 @MOF core-shell structure was further confirmed by TEM images, nitrogen adsorption-desorption analysis, FTIR, XRD, Raman spectroscopy, and XPS characterization ( uniformed spherical shapes, similar to that of pristine Fe 3 O 4 -COOH without MOF coating, to semispherical conjunct core-shell structure, like Fe 3 O 4 @MIL-100(Fe). This structural diversity is mainly imputed to the unique characteristic features of MOFs. As MOFs are created from different metal-ligand coordination, a series of frameworks with various physicochemical properties, crystal size, and surface roughness can be obtained . Furthermore, the high density of carboxylic acid sites on the rough surface of the substrate, as a potential nucleation site, can effectively manipulate the size and shape of the crystal, justifying the super small MOF crystals spotted on the exterior surface of core-shell structures compared to their pristine MOFs with large crystals. In addition to the surface morphology, we investigated the porosity of Fe 3 O 4 @MOFs, particularly relative to those of pristine MOFs. As observed in Fig. 3l, the total pore volume of core-shell materials increased to ⁓0.02-0.20 cm 3 /g, whereas a negligible porosity was detected for the acidic core itself. The difference in the porosity of Fe 3 O 4 @MOFs can likely be attributed to the different pore architectures of MOFs on the exterior shell side. Notably, a comparison of pore volume exhibited that Fe 3 O 4 @UiO-66 has the maximum pore volume of ⁓0.20 cm 3 /g among core-shell materials, highlighting the determining role of MOF type in the created porosity. Interestingly, we found that the modulated core-shell nanomaterials benefit from a hierarchical micro-mesoporous structure, while pristine MOFs are generally microporous materials . For example, the mesoporosity, i.e., mesopore divided by total pore volume, of the modulated UiO-66 shell (⁓85.4%) was much greater than that of pristine UiO-66 (⁓11.1%) synthesized by the conventional procedure. The created mesoporosity in the coating could be ascribed to the induced missing-linker defects during the modulated self-assembly of MOFs on the acidic core. Similar to the carboxylic acid linkers used for the synthesis of MOFs, Fe 3 O 4 -COOH can partially play the role of ligand, change the bonding energy of metal-ligand coordination and manipulate the MOF formation mechanism. Accordingly, the MOF layers cannot smoothly grow on the carboxylic acid-rich substrate which can create mesoporosity and engineer the pore structure by hindering the bridging linkers and changing the coordination environment during their modulated self-assembly.
To confirm the role of missing-linker defects in the formation of engineered core-shell materials, thermogravimetric analysis was used ( Fig. 3e and Supplementary Fig. 23). Because of the thermal decomposition of organic ligands from 100 to 350°C, the weight reduction ratio of core-shell structure (ΔW C-S ) to pristine MOF (ΔW P ) in this temperature range was considered as an indicator of missing-linker deficiency 23 . Although Zn-, Co-, and Fe-based MOFs showed a high linker deficiency, the predominance of metal coordination through the network could have a negative impact on the self-assembly process, resulting in the poor formation of MOF structure (Fig. 3m). This is compatible with their low coating layer weights. In contrast, both Cu-and Zr-based MOFs displayed a good linker deficiency (⁓56.2-91.5%), as well as high coating weight (⁓30.1-34.7 wt%) and homogenous coating , likely due to the optimum surface energy of Fe 3 O 4 -COOH@Cu and Fe 3 O 4 -COOH@Zr compared to their corresponding organic ligands [24][25][26] . These results demonstrate that the self-assembly of MOFs on Fe 3 O 4 -COOH can be used as a simple platform to prepare advanced mesopore-induced core-shell materials with tailored properties, specifically more defects, and unsaturated metal sites.
The formation of Fe 3 O 4 @MOF-SO 4 . Defect-engineered MOFs are promising platforms for developing advanced functional nanomaterials for various catalytic applications, including photoand electrocatalysis, as they can provide active metal sites with a strong affinity towards a broad range of functional moieties 12 . We, therefore, with respect to our interest in acidic nanocatalysts, treated the engineered Fe 3 O 4 @MOFs with diluted sulfuric acid to introduce sulfate moieties through their defected structures. The general procedure for Fe 3 O 4 @MOF sulfation in the present study is schematically demonstrated in Fig. 4a and Supplementary Fig. 28. The primary advantage of the reported sulfation method is that Fe 3 O 4 @MOF-SO 4 can be simply prepared by dispersing Fe 3 O 4 @-MOFs into the aqueous solution of sulfuric acid (0.05 M, pH ⁓1.3) at room conditions which makes it a viable approach for large-scale implementation. FTIR analysis of Fe 3 O 4 @MOF-SO 4 disclosed the presence of sulfur compounds in 800-1300 cm −1 region, including both S-O (⁓800-950 cm −1 ) and S=O (⁓1000-1300 cm −1 ) bonds, thus confirming that SO 4 −2 species were successfully coordinated with active metal sites (Fig. 4b, c and Supplementary Fig. 29) 27 . In addition, elemental line scanning profiles also revealed that there is a good distribution of sulfur across the treated core-shells, while no sulfur was detected before acid treatment (Fig. 4d, e). Since the sulfates can take different coordination positions on the surface of uncoordinated metal clusters, missing-linker deficiency in defect-engineered core-shells can positively manipulate the chelating mode of sulfate by providing additional space, resulting in the improved sulfation yield.
To explore the versatility of the aqueous sulfation method for Fe 3 O 4 @MOF, Fe 3 O 4 @HKUST-1, Fe 3 O 4 @UiO-66, and Fe 3 O 4 @UiO-66-NH 2 with the highest pore volumes were selected for posttreatment. TEM analysis illustrated the Fe 3 O 4 @MOF structures well preserved their core-shell structures at low pH values (i.e., ⁓1.3), with a shell thickness changing from ⁓10 nm in Fe 3 O 4 @HKUST-SO 4 (Fig. 4f, g) to ⁓20 nm in Fe 3 O 4 @UiO-66-SO 4 (Fig. 4i, j) and ⁓40 nm in Fe 3 O 4 @UiO-66-NH 2 -SO 4 (Fig. 4l, m). HAADF and EDX maps reconfirmed the homogeneous distribution of metallic and organic elements corresponding to their MOF structures (Fig. 4h, k, n). Nevertheless, it was found that the amount of sulfur elements varies among Fe 3 O 4 @MOF-SO 4 materials, and Fe 3 O 4 @HKUST-SO 4 possesses the least sulfur content when compared with those of Zr-based core-shell structures. This observation was further examined by high-resolution XPS analysis, showing the sulfur species on the exterior surface of Fe 3 O 4 @MOF-SO 4 materials ( Fig. 4o-q and Supplementary Fig. 30). Notably, XPS peaks did not appear at 164-174 eV (the typical range of binding energy for sulfur, S 2p), validating the low sulfur signals in its EDX. These results suggest that the chemical stability of Fe 3 O 4 @MOF materials is a key contributor to the aqueous sulfation process. For instance, HKUST-1 framework is composed of Cu 2+ ion pairs chelated by carboxylate bridges with paddle-wheel units; however, the high concentration of protons at low pH values can potentially accelerate the hydrolysis of Cu-O bonds, resulting in the partial disassembly of MOF crystals 28 . Similarly, Fe 3 O 4 @HKUST-1 could not entirely preserve its structural stability at harsh acidic conditions, as the shell side partially dissociated ( Supplementary  Fig. 31) and the average shell thickness remarkably diminished after the sulfation process. Furthermore, the unstable Cu bond could prohibit both bridging and chelating mode of sulfate coordination which verifies the poor efficiency of Catalytic CO 2 desorption performance. Catalytic solvent regeneration, a recently emerged technique, has garnered wide attention, because of its low-temperature operation and high energy efficiency. The acidic nanocatalysts can act as a good proton donor, supplying the excess amount of protons required for the carbamate breakdown reaction, and promote CO 2 desorption at temperatures less than 100°C (the boiling temperature of water at atmospheric pressure) 30 . To illustrate that the prepared water-dispersible nanomaterials can be used to accelerate CO 2 desorption reactions, we first examined the catalytic performance of acidic Fe 3 O 4 -COOH nanoclusters during the regeneration of CO 2 -rich monoethanolamine (MEA, 5 M) at 88°C (Fig. 5a). As shown in Fig. 5b, adding a small amount of Fe 3 O 4 -COOH (0.1 wt%) significantly increased the kinetics of CO 2 desorption, resulting in ⁓27.3% less energy consumption, when compared with that of blank MEA solution without using any catalyst. By increasing the amount of Fe 3 O 4 -COOH from 0.1 to 1 wt%, CO 2 was released more quickly from the solvent; however, the energy efficiency parameter (i.e., the absolute of relative heat duty reduction versus the amount of catalyst used) significantly dropped from 2.73 to 0.66, respectively. Thus, a concentration of 0.1 wt% was established to be the optimal value of water-dispersible nanocatalysts, almost 10 times less than previously reported values, owing to the abundant active acidic sites on the surface of Fe 3 O 4 -COOH, as well as the Brownian motion and nanofluidic behavior of magnetic nanoclusters in the solvent 31,32 . These results highlight the potential of Fe 3 O 4 -COOH as a versatile substrate for the synthesis of acidic water-dispersible nanocatalysts which can be easily added at low concentrations during the continuous operation of CO 2 absorption-desorption processes.
Besides the CO 2 desorption performance of Fe 3 O 4 -COOH, the catalytic behavior of defect-engineered Fe 3 O 4 @MOFs was explored. Figure 5c, d shows that all prepared core-shell materials improved the kinetics of CO 2 desorption, ranging from Fe 3 O 4 @ZIF-8 to Fe 3 O 4 @HKUST-1 with the least and the best performance, respectively. From these observations, it appears that a MOF coating had a negative impact on the catalytic efficiency of Fe 3 O 4 -COOH substrates. Indeed, Fe 3 O 4 @HKUST-1   (as the best of core-shell materials) depicted a relative heat duty of 74.1%, whereas 72.7% was recorded when solely using Fe 3 O 4 -COOH. This could be attributed to the replacement of Brønsted acid sites (i.e., carboxylates) with Lewis acid sites (uncoordinated metal clusters) with less proton donation capability 33 . Contrarily, enriching Brønsted acid sites through the hierarchical structure of core-shell structures (i.e., Fe 3 O 4 @MOF-SO 4 ) resulted in a distinct catalytic performance. The Fe 3 O 4 @UiO-66-SO 4 succeeded to desorb 80.9% more CO 2 compared with that of the blank solution at similar operating conditions. In addition, we found a decreasing trend in the differential desorbed CO 2 with the regeneration time (from 24 to 36 min), highlighting the substantial influence of catalyst on the kinetics of CO 2 desorption (Fig. 5e, f). To further investigate the catalytic performance of the Fe 3 O 4 @UiO-66-SO 4 , its corresponding relative heat duty was compared with those of Fe 3 O 4 -COOH and commercialized solid acid catalysts, including metal oxides (Al 2 O 3 , V 2 O 5 ) and zeolites (H-Beta and HZSM-5) (Fig. 5g and Supplementary Fig. 32). The Fe 3 O 4 @UiO-66-SO 4 exhibited the lowest required energy for the regeneration of CO 2 -rich MEA solution with a relative heat duty of 55.3%, mainly due to its large pore volume, predominant mesoporous structure, abundant Brønsted acid sites, and nanofluidic behavior ( Supplementary  Figs. 33 and 34). These findings also suggested that the performance of other prevalent heterogeneous nanocatalysts is not comparable with the water-dispersible materials at low catalyst concentrations (less than 0.1 wt%).
To specifically explore the performance of water-dispersible nanomaterials at low concentrations (0.01, 0.05, and 0.1 wt%), their cyclic CO 2 absorption-desorption capacity was measured (Fig. 5h,  i). As discussed, H-Beta zeolite failed to display any sensible promotion throughout the course of the CO 2 desorption operation. Unlike H-Beta, all Fe 3 O 4 -COOH, Fe 3 O 4 @UiO-66, and Fe 3 O 4 @UiO-66-SO 4 demonstrated comparable cyclic CO 2 capacity, even at an extremely low concentration of 0.01 wt%. For instance, adding Fe 3 O 4 @UiO-66-SO 4 nanocatalysts increased the cyclic capacity of CO 2 from 0.21 mol CO 2 /mol MEA in the blank solution to 0.30, 0.33, and 0.38 mol CO 2 /mol MEA with 0.01, 0.05, and 0.1 wt% concentrations of nanocatalyst, respectively, which are comparable with those of commercialized catalysts with ⁓10to ⁓100-fold higher concentrations (⁓1.0-1.1 wt%) 34 . Since acidic catalyst allows for enhanced cumulative CO 2 desorption during the solvent regeneration process, the solvent can absorb more CO 2 in the next absorption cycle, leading to the better performance of the solvent (in terms of equilibrium and kinetics) in the absorption column. The stability of Fe 3 O 4 @UiO-66-SO 4 was assessed via ten cycles of consecutive CO 2 absorption-desorption operation (Fig. 5j). As seen, the relative heat duty of MEA solution increased by only ⁓9% over the first four cycles (region 1) and remained stable throughout the last six cycles (region 2). In addition, no significant changes were observed in the XPS spectra, XRD patterns, and SEM images of Fe 3 O 4 @UiO-66-SO 4 , before and after the cyclic operation ( Supplementary Fig. 35), confirming the excellent recyclability of these water-dispersible nanocatalysts.
Besides all targeted features of Fe 3 O 4 @UiO-66-SO 4 for catalystaided solvent regeneration, its distinguished catalytic performance, particularly when compared with Fe 3 O 4 -COOH, could be assigned to its special proton donation mechanism. Throughout the solvent regeneration, CO 2 molecules are generally released by the carbamate breakdown reaction. However, the yield of this reaction is highly dependent on the number of active protons in the reaction medium supplied by the amine deprotonation reaction (Fig. 6) 35 . Owing to the endothermic nature of all reactions, a high operating temperature (⁓120-140°C) is required for spontaneous proton transfer and bond cleavage, resulting in high-quality steam use and subsequently high energy consumption 36 . In Fe 3 O 4 @UiO-66-SO 4 , the adsorbed water molecules on the surface of uncoordinated Zr clusters can participate in a hydrogen bond with a sulfate moiety chelated to another neighboring Zr center 14 . This specific arrangement of sulfate and water moieties results in the formation of superacid sites (H 0 ≤ −14.5; see Supplementary Table 1) with distinct proton donation ability, accelerating the carbamate breakdown reaction and allowing for enhanced CO 2 desorption at low regeneration temperatures (less than 100°C). We note that all water-dispersible nanocatalysts could successfully recover their released protons during the CO 2 absorption process, as compared to heterogeneous solid acid catalysts which need to be protonated via an acid washing processes. The results indicate that the unique privileges of water-dispersible nanomaterials (i.e., ease of use, low operating concentration, and high efficiency) can potentially make the implementation of catalytic solvent regeneration industrially affordable. In order to gain a greater understanding of the carbamate breakdown catalyzed by Fe 3 O 4 @UiO-66-SO 4 , density functional theory (DFT) calculations were performed. Starting from a defect active site with HSO 4 − and H 2 O coordinating the two Zr atoms, the addition of carbamate resulted in the displacement of the H 2 O by the carbamate carboxylate group (Fig. 7a). This configuration places the HSO 4 − hydroxyl oxygen within~3 Å of the carbamate nitrogen. While the exact order of proton transfer is unclear, the protonation of the carbamate nitrogen by either the HSO 4 − or the nearby H 2 O resulted in the breakage of the N-C bond, coupled with the formation of a C-O bond from the H 2 O, leading to stable products of MEA and HCO 3 − (Fig. 7b). Attempts to protonate the carbamate nitrogen and break the C-N bond in the absence of the water molecule were unsuccessful, suggesting that the presence of H 2 O and the formation of HCO 3 − as a product are essential in the catalytic reaction.

Discussion
In summary, we have developed a new and effective methodology to synthesize a series of water-dispersible nanocatalysts with engineered properties and nanofluidic behavior for lowtemperature CO 2 desorption. A promising feature of the current method is that the carboxylate-rich surface of Fe 3 O 4 -COOH nanoclusters allows for the modulated growth of MOFs on its structure. Importantly, we observed a transition from the microporous structure in pristine MOFs to a hierarchical micromesoporous network in Fe 3 O 4 @MOFs. Different metal ion and organic ligand combinations can be selected for the tailored selfassembly of MOFs on the magnetic core, resulting in a broad diversity of physicochemical properties. The obtained materials with structural defects and active metal centers can serve as a potential platform for the coordination of chelating sulfates with superacidity. Enhanced kinetics of CO 2 desorption was obtained for all Fe 3 O 4 -COOH, Fe 3 O 4 @MOF, and Fe 3 O 4 @MOF-SO 4 materials. As a proof of concept, Fe 3 O 4 @UiO-66-SO 4 exhibited the best catalytic performance (44.7% reduction in energy consumption) with excellent recyclability (⁓9% loss over five cycles) using a low catalyst concentration of only 0.1 wt%. The comprehensive characterizations revealed that the missing-linker defects and superacidic sites through the hierarchical network of Fe 3 O 4 @UiO-66-SO 4 are key contributors to its advanced proton donation capability and thus accelerated CO 2 desorption performance. We anticipate that this approach will open up a new avenue to the synthesis of water-dispersible nanocatalysts in the area of catalytic CO 2 absorption-desorption and take one further step toward the utilization of green energy (e.g., solar hot water) in the portfolio of energy-efficient CO 2 capture.

Methods
Synthesis of acidic Fe 3 O 4 -COOH nanoclusters. Typically, ferric ammonium citrate (3.25 g) and NaOAc (6 g) were dissolved in 100 mL EG under vigorous stirring at room temperature. The formed hazy solution was transferred to a stainless-steel autoclave (150 mL capacity) and heated at 200°C for 10 h. Then, the autoclave was gradually cooled to room temperature and the black solid precipitants were separated via the application of an external magnet. The obtained nanoclusters were repeatedly washed with acetone, ethanol, and water to remove unreacted or physically attached molecules. During each washing step, the nanoclusters were suspended in the solvent, ultrasonicated for 30 min, and separated by an external magnet. Finally, the resultant materials were dried at 80°C overnight under a severe vacuum and labeled as Fe 3 O 4 -COOH.

Synthesis of conventional Fe 3 O 4 nanoparticles.
To compare the structural properties of Fe 3 O 4 -COOH nanoclusters with a benchmark, conventional Fe 3 O 4 nanoparticles were synthesized according to the previously reported coprecipitation method with minor modifications 37 . Firstly, 298.5 mg of FeCl 2 ·4H 2 O and 810 mg of FeCl 3 ·6H 2 O were dissolved in 100 mL of water and sonicated for 15 min. The temperature was increased to 60°C, and while keeping the mixture under vigorous agitation, NH 4 OH solution was added as an oxidation agent to trigger the iron oxide precipitation. When the pH of the solvent reached 11, the temperature was increased to 80°C and held for 2 h. The black precipitates were separated by an external magnet and washed with a copious amount of water to remove extra NH 4 OH reactants and OH − ions. Eventually, the obtained products were dried in a vacuum oven at 80°C overnight, labeled as Fe 3 O 4 nanoparticles and stored for future use.  4 were magnetically separated, dispersed in 250 mL ultra-pure hot water (ca. 60°C), and sonicated for 15 min, followed by magnetic separation and supernatant removal. Finally, the products were dried using a vacuum oven at 150°C for 48 h and stored for future use.
Theoretical calculations. The initial structure of UiO-66 was taken from Cavka et al. 38 , and truncated and protonated based on the work of Sittiwong et al. 39 . This structure comprises four 1,4-benzene-dicarboxylate (BDC) linkers surrounding the defect active site on the Zr 6 O 4 (OH) 4 node created by removing one linker, and seven linkers truncated to formate, for computational efficiency. To balance the charge of the system, hydrogens were added to the terminal carboxylates of BDC, and hydrogen was removed from the bridging OH of the defect active site to compensate for the removal of the BDC linker ( Supplementary Fig. 3a). Following initial geometry optimization, a water molecule was added to the active site. The protonation of bridging oxygen and coordination of a hydroxyl group to a Zr atom was found to be more energetically favorable than an intact H 2 O by~0.26 eV (Supplementary Fig. 3b).
To model the effects of sulfation, an H 2 SO 4 molecule was added to the hydroxylated defect active site, which spontaneously reacted with the adjacent hydroxyl group coordinated to the Zr to form HSO 4coordinated to one Zr, and H 2 O coordinated to the other Zr, during geometry optimization ( Supplementary Fig. 3c). All DFT calculations were performed using Gaussian 16 Revision C.01 40 . The 6-31G(d,p) basis set was employed for all atoms except Zr, which was treated using the double-ζ of the Stuttgart-Dresden pseudopotential 41,42 from the Basis Set Exchange 43 . The M06-L DFT functional 7 was used for all calculations. For all geometry optimizations, the formate hydrogens and terminal carboxyl carbons were frozen to maintain the overall structure of UiO-66, while all other atoms were allowed to relax. This methodology has been validated previously for catalytic reactions on UiO-66 39 .