Cooperative carbon capture and steam regeneration with tetraamine-appended metal–organic frameworks

Abstract

Natural gas has become the dominant source of electricity in the United States, and technologies capable of efficiently removing CO₂ from natural gas-fired power plants emissions could reduce their emission intensity. However, given the low partial pressure of CO₂ in the flue stream, separation of CO₂ is particularly challenging. Taking inspiration from the crystal structures of diamine-appended metal–organic frameworks exhibiting two-step cooperative CO₂ adsorption, we report a family of robust tetraamine-functionalized frameworks that retain cooperativity, leading to the potential for exceptional efficiency in capturing CO₂ under the extreme conditions relevant to natural gas flue emissions. The ordered, multimetal coordination of the tetraamines impart the materials with extraordinary stability to adsorption-desorption cycling with simulated humid flue gas and enable regeneration using low-temperature steam in lieu of costly pressure or temperature swings.

One Sentence Summary:

Cooperative CO₂ adsorption under challenging conditions, together with steam regeneration, is achieved using tetraamine-functionalized metal–organic frameworks enabling novel CO₂ capture processes.

Carbon dioxide (CO₂) emissions from fossil fuel combustion and industrial processes account for as much as 65% of anthropogenic greenhouse gas emissions (1–3). Carbon capture and sequestration (CCS), wherein CO₂ is separated from the flue emissions of large point sources and permanently sequestered underground, is widely recognized as an essential component of strategies for meeting the ambitious climate targets established at the Paris Climate Conference (4). The development of capture technology has largely focused on coal flue emissions (5), but worldwide use of natural gas is projected to exceed that of coal by ~2032, necessitating the rapid development of CCS technology for natural gas emissions (6). The U.S. Department of Energy (DoE) has set an ambitious target of 90% capture of the CO₂ from natural gas flue streams (7), which is particularly challenging given that the CO₂ concentration in natural gas combined cycle (NGCC) emissions is typically only ~4%, compared to ~12 to 15% for coal emissions (7, 8). Additionally, NGCC emissions contain high concentrations of O₂ (12.4%) and water (8.4%), and thus effective technologies must be stable and maintain CO₂ capture performance in the presence of these species (7, 9).

Aqueous amine solutions, the most mature carbon capture technology to date (10), are susceptible to oxidative and thermal degradation and have low CO₂ cycling capacities (11). Porous solid adsorbents such as zeolites, silicas, and metal–organic frameworks are emerging as promising CO₂ capture materials because of their high surface areas, lower intrinsic regeneration energies, higher stabilities, and tunable surface chemistries (12, 13). Taking inspiration from amine-functionalized silicas (14), we and others have shown that amine-functionalized metal–organic frameworks can capture CO₂ in the presence of water (15, 16). A major consideration for adsorptive CO₂ capture from natural gas flue emissions is that the low partial pressure of CO₂ requires materials with high adsorption enthalpies. In turn, higher temperatures or lower pressures are needed to desorb captured CO₂, and these regeneration conditions are both costly and can substantially impact performance (12). The use of steam for CO₂ recovery has been proposed as a cost-effective strategy for amine-functionalized adsorbents (17–22), particularly because low-grade steam is inexpensive and readily available in most industrial processes. Unfortunately, engineering adsorbents with stability in the presence of steam is an ongoing issue (19, 23–25).

We recently reported the promising CO₂ capture properties of the diamine-appended framework mmen–Mg₂(dobpdc) (mmen = N,N′-dimethylethylenediamine, dobpdc⁴⁻ = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) (Fig. 1A) (26). In this material, CO₂ adsorption proceeds through a cooperative mechanism in which the gas inserts into the Mg–N bonds to form chains of oxygen–bound carbamate species charge-balanced by neighboring ammonium groups running along the pores (27). This mechanism gives rise to unusual step-shaped CO₂ adsorption profiles and improved material CO₂ cycling capacities that can be achieved with smaller pressure or temperature swings than are required for traditional adsorbents (27). The CO₂ capture properties can further be tuned by varying the appended diamine, leading to adsorbents with remarkable potential for CO₂ capture from coal flue gas emissions (Fig. 1B) (28–31) and even NGCC emissions (32).

Fig. 1. Diamine versus tetraamine coordination in M₂(dobpdc).

(A) Illustration of a hexagonal channel of Mg₂(dobpdc) viewed in the ab-plane, using single-crystal X-ray diffraction data for the isostructural framework Zn₂(dobpdc). (B) The diamine-functionalized material features coordination of one diamine to each Mg²⁺ site (28) whereas (C) tetraamines can coordinate to two Mg²⁺ sites. (D) Tetraamines explored in this work and their abbreviations. (E,F) Single-crystal x-ray diffraction structures (100 K) of isostructural Zn₂(dobpdc) functionalized with 3–3-3 and 3–4-3 tetraamines, respectively. The tetraamines span metal centers across the pore that are 10.4637(11) Å apart (3–3-3) and 16.8312(19) Å apart (3–4-3). Green, light blue, gray, red, blue, and white spheres represent Mg, Zn, C, O, N, and H, respectively.

Nonetheless, these materials are susceptible to diamine volatilization upon regeneration, which has limited their applicability in a practical capture processes (30). We now report that tetraamine-functionalized Mg₂(dobpdc) materials (Fig. 1C) exhibited high thermal stability and cooperatively captured CO₂ at concentrations as low as parts per million (ppm), enabled by a two-step ammonium carbamate chain-formation mechanism. The ordered, multimetal coordination mode achieved with the tetraamines dramatically increased the stability of these adsorbents and enables CO₂ desorption with inexpensive steam for the first time in amine-functionalized Mg₂(dobpdc).

Synthesis and structure of tetraamine-functionalized Mg₂(dobpdc)

Our pursuit of tetraamine-functionalized Mg₂(dobpdc) was motivated by structural analysis of diamine-appended analogs, which indicated that tetraamines with chain lengths on the order of two N-alkylethylenediamines could bridge nearest neighbor metals across the pore (Fig. 1, B and C) (28). Such multiple metal coordination could yield adsorbents exhibiting cooperative CO₂ capture and greatly enhanced thermal and hydrolytic amine stability. Soaking Mg₂(dobpdc) with various tetraamines (Fig. 1D) in toluene (see the Supplementary Methods) produced loadings of ~1 tetraamine per Mg²⁺ site, or twice the desired 1:2 tetraamine:metal ratio (Fig. 1C), as determined by ¹H nuclear magnetic resonance (NMR) spectra of acid-digested samples. The desired loading could be achieved through subsequent thermal activation (see the Supplementary Methods and tables S1 and S2 and figs. S3 and S4). Thermogravimetric analysis indicated that the tetraamine-grafted Mg₂(dobpdc) materials were resistant to further diamine loss up to ~250 to 290°C (figs. S3 and S4). Here, we refer to the tetraamines using shorthand based on the number of carbon atoms in the alkyl groups bridging the amine moieties, for example N,N′-bis(3-aminopropyl)-1,3-diaminopropane and N,N′-bis(3-aminopropyl)-1,4-diaminobutane are referred to as 3–3-3 and 3–4-3, respectively (Fig. 1D).

Single-crystal x-ray diffraction (XRD) data obtained for 3–3-3 and 3–4-3-appended variants of the isostructural framework Zn₂(dobpdc) revealed that the tetraamines coordinate in a highly ordered fashion (Fig. 1, E and F). Tetraamine 3–3-3 bound two metal centers separated by a distance of 10.4637(11) Å (Fig. 1E) whereas 3–4-3 bridges metal sites were separated by 16.8312(19) Å (Fig. 1F). Longer tetraamines could ostensibly also be accommodated in the framework and bridge metal atoms at even greater distances. In 3–3-3-functionalized Zn₂(dobpdc), there was extensive intramolecular hydrogen bonding, whereas 3–4-3–Zn₂(dobpdc) primarily exhibited intermolecular hydrogen bonding between adjacent tetraamines along the pore direction. These experimental structures confirmed the tetraamine coordination mode in M₂(dobpdc) and accounted for the extremely high thermal stability of these materials (see below).

Adsorption properties and optimization for natural gas combined cycle post-combustion capture

Remarkably, all of the Mg₂(dobpdc)(tetraamine) variants exhibited sharp step-shaped CO₂ adsorption profiles consistent with cooperative adsorption and ammonium carbamate chain formation (Fig. 2 and figs. S14 to S16) (27, 28, 30, 32). In each case, the CO₂ adsorption capacity at 30°C approached the theoretical capacity of two CO₂ molecules per tetraamine. Interestingly, with the exception of Mg₂(dobpdc)(3–3-3), the frameworks exhibited two-step adsorption profiles, with each step corresponding to half of the theoretical capacity. Analogous two-step adsorption profiles were previously observed in bulky diamine-appended variants of Mg₂(dobpdc) and was attributed to steric conflict between neighboring ammonium carbamate chains across the pore (30). In the case of tetraamine-appended Mg₂(dobpdc), we hypothesize that initial chemisorption of CO₂ occurred at one amine end, generating ammonium carbamates running along one vertex of the hexagonal pore. The formation of the second set of ammonium carbamate chains likely required reorientation of the unreacted, bound amines.

Fig. 2. CO₂ uptake in tetraamine-appended Mg₂(dobpdc).

(A) Adsorption isobars obtained through thermogravimetric analysis of Mg₂(dobpdc)(tetraamine) under pure CO₂ at atmospheric pressure. Dashed lines indicate the theoretical capacities for binding of one and two CO₂ molecules per tetraamine. (B) Adsorption (filled circles) and desorption (open circles) isotherms for CO₂ uptake in Mg₂(dobpdc)(3–4-3) at 90°, 100°, 110°, and 120°C. The CO₂ pressures in an untreated NGCC flue emission stream (40 mbar) and following 90% capture (4 mbar) are indicated by light gray and dark gray lines, respectively.

Consistent with this proposed mechanism, increasing tetraamine length coincides with a decrease in the material step separations up to 3–3-3, for which there is only a single adsorption step. Two-stepped adsorption returns in the case of slightly larger 3–4-3, which we attribute to this tetraamine likely bridging two metals at a longer distance than 3–3-3 and the other smaller tetraamines (Fig. 1E,F). Furthermore, when appended with triamines 3–3 (bis(3-aminopropyl)amine) and 3–4 (N-(3-aminopropyl)-1,4-diaminobutane), which can only form one set of ammonium carbamate chains, the Mg₂(dobpdc) frameworks exhibited CO₂ capacities that correspond to adsorption of one molecule of CO₂ for every two Mg²⁺ sites (figs. S12 and S13). For practical applications, a single-step adsorption profile, or a two-step adsorption with closely spaced adsorption steps, is desirable to maximize the operating range over which the full material adsorption capacity can be accessed (32).

The temperature of the first adsorption step for Mg₂(dobpdc)(3–3-3) and Mg₂(dobpdc)(3–4-3) is 163° and 150°C, respectively. The temperatures are among the highest reported for any amine-appended variant of Mg₂(dobpdc) (26, 28–30, 32). Traditional adsorbents with Langmuir-type adsorption profiles typically show optimal performance at the lowest possible adsorption temperature, where the adsorption capacity is maximized. The high adsorption step temperatures for the tetraamine-appended materials directly correlated with adsorption steps at low pressures sufficient to enable 90% CO₂ capture from a NGCC flue stream (Fig. 2B and figs. S14 to S16) (7). For example, stepped CO₂ adsorption isotherm for Mg₂(dobpdc)(3–2-3) at 75°C indicated that the material should be able to reduce the CO₂ content in a NGCC flue stream to 0.4% (fig. S16). Additionally, our preliminary results (figs. S28 and S39) indicate that these materials may also be suitable for direct capture of CO₂ from air. While the majority of tetraamine-appended materials exhibit stable CO₂ cycling capacities, Mg₂(dobpdc)(3–3-3) shows a gradual decrease in CO₂ cycling capacity over time (fig. S10), thus Mg(dobpdc)(3–4-3) was chosen for further study due to its practical step position and fundamentally interesting two-step adsorption behavior.

The adsorption data for Mg₂(dobpdc)(3–4-3) suggested that in a post-combustion capture process from NGCC flue emissions, this framework could achieve a 90% CO₂ capture rate, referring to the removal of CO₂ to a residual concentration of 10% that of the feed (reduction from 40 mbar to 4 mbar at atmospheric pressure). Single-component isotherms collected between 90° and 120°C reveal that step-shaped CO₂ capture at 4 mbar (corresponding to 90% capture) was retained up to 90°C under dry conditions (Fig. 2B). Using the Clausius-Clapeyron relationship, we calculated differential adsorption enthalpy (Δh_ads) and entropy (Δs_ads) values of 99 ± 3 kJ/mol and 223 ± 8 J/mol·K, respectively, at a loading of 1 mmol CO₂/g (figs. S19 and S20). These values followed a correlation previously observed between Δh_ads and Δs_ads values determined for diamine-appended Mg₂(dobpdc) (fig. S21) (28). The Δh_ads value for Mg₂(dobpdc)(3–4-3) is among the highest reported for amine-functionalized materials. Such a high enthalpy of adsorption should enable CO₂ capture at high temperatures and result in lower energy consumption in a temperature-swing adsorption process by minimizing the required temperature swing (33–37). Additionally, Mg₂(dobpdc)(3–4-3) exhibited minimal adsorption-desorption hysteresis at atmospheric pressure (fig. S11).

Cooperative adsorption mechanism

We used in situ infrared (IR) and solid-state NMR spectroscopies to characterize Mg₂(dobpdc)(3–4-3) before and after CO₂ adsorption to investigate the two-step cooperative adsorption mechanism. In situ diffuse reflectance IR spectroscopy (DRIFTS) characterization of the framework in the presence of CO₂ revealed a characteristic carbamate C–N stretch at 1339 cm⁻¹ and a C–O stretch at 1689 cm⁻¹ (see isotopic difference spectrum, Fig. 3A) (27–30). In addition, spectra collected simultaneously with a volumetric, equilibrium CO₂ isotherm at 120°C indicated that the ammonium carbamate species was responsible for cooperative adsorption. Distinct features corresponding to hydrogen bonding between neighboring ammonium carbamate chains were observed in the N–H regions of the difference spectrum (3300 to 3000 and 3500 to 3400 cm⁻¹) and distinct hydrogen bonding environments were characterized for each of the two adsorption steps (fig. S26).

Fig. 3. Spectroscopic investigation of CO₂ adsorption in Mg₂(dobpdc)(3–4-3).

(A) Raw in situ DRIFTS spectra of activated 3-4-3-Mg₂(dobpdc) (grey curve), 3-4-3-Mg₂(dobpdc) dosed with 400 mbar of ¹²CO₂ and ¹³CO₂ at 120°C (dark blue and yellow curves, respectively), and the isotopic difference spectrum (light blue curve). The ¹³CO₂ spectrum was used as a baseline, isolating vibrations caused by inserted CO₂. Vibrations corresponding to diagnostic carbamate bands are labeled. (B) Room-temperature ¹³C solid-state magic angle spinning NMR (16.4 T) of Mg₂(dobpdc)(3-4-3) dosed with 1038 mbar of ¹³CO₂. The resonance at 162.6 ppm was assigned as carbamate. (C) ¹H⟶¹³C heteronuclear correlation (contact time 100 μs) spectrum and correlation assignments. (D) Predicted structures of Mg₂(dobpdc)(3-4-3), Mg₂(dobpdc)(3-4-3)(CO₂), and Mg₂(dobpdc)(3-4-3)(CO₂)₂ obtained from structural relaxations with vdW-corrected DFT. Green, grey, red, blue, and white spheres represent Mg, C, O, N, and H atoms, respectively.

Corroborating evidence for ammonium carbamate formation was obtained from magic angle spinning solid-state NMR experiments. A single carbamate resonance was observed at 162.6 parts per million in the ¹³C NMR spectrum of Mg₂(dobpdc)(3–4-3) dosed with 1.04 bar of ¹³CO₂ (Fig. 3B), which coincides with resonances reported previously for ammonium carbamate formed upon ¹³CO₂ adsorption in diamine-appended Mg₂(dobpdc) (38). The presence of a single carbamate feature suggested that all of the chemisorbed CO₂ was in the same chemical environment after adsorption at 1 bar. We also observed five resonances for the tetraamine alkyl carbons, supporting a symmetric reaction between the tetraamine and CO₂ at full capacity (Fig. 3B). Two-dimensional heteronuclear correlation data further revealed that CO₂ reacted with the primary amine and that a secondary ammonium group formed nearby the carbamate (Fig. 3C) (38), and solid-state ¹⁵N NMR data confirmed the symmetric formation of ammonium and carbamate species (fig. S27). The NMR and IR results confirmed that our tetraamine-functionalized Mg₂(dobpdc) variants operated through the envisaged two-step cooperative adsorption mechanism.

The structures of Mg₂(dobpdc)(3–4-3) after adsorption of CO₂ to half and full capacity were further investigated using van der Waals (vdW)-corrected density functional theory (DFT) calculations (Fig. 3D). Lattice parameters obtained from high-resolution powder XRD diffraction experiments under vacuum and after exposure to 1 bar CO₂ were used as starting points for structural relaxations (fig. S29 and table S4). The computed binding energies (ΔE_ads) for the structures are within ±10 kJ/mol of the corresponding experimental Δh_ads values (table S7) (38). At full CO₂ capacity (i.e., two molecules of CO₂ per tetraamine), our calculations support two-step cooperative insertion of CO₂, and the calculated NMR chemical shifts for this structure are in good agreement with the experimental values (table S8) (38). The computed ΔE_ads values indicate that the half capacity structure was more stable than the full capacity structure, which further supports our proposed two-step cooperative adsorption mechanism. In addition, the tetraamines in our optimized structure of activated Mg₂(dobpdc)(3–4-3) bridge the same set of metals spanning ~16.7 Å in the ab-plane of the framework, consistent with the single-crystal structure of Zn₂(dobpdc)(3–4-3). Comparing the activated and full CO₂ capacity structures to synchrotron PXRD patterns by Rietveld analysis show reasonable fits (figs. S32 and S33 and table S9). Taken together, our results from XRD, NMR, IR, and DFT data indicate that tetraamine-functionalized frameworks indeed enhance cooperative CO₂ adsorption relative to diamine-appended materials.

Carbon capture from flue gas with steam desorption

Because flue gas streams are saturated with water, any candidate carbon capture material must be able to adsorb CO₂ under humid conditions. When exposed to CO₂ streams with ~2.6% H₂O (see Supplementary Methods) (32), Mg₂(dobpdc)(3–4-3) again exhibited step-shaped isobars, with steps shifted to higher temperatures relative to those under dry CO₂ (fig. S40) as has been reported previously for diamine-appended Mg₂(dobpdc) (29, 30, 32). Notably, this temperature shift indicated that water enhanced CO₂ binding in the material and should promote CO₂ capture from a flue gas stream (29, 30, 32). Furthermore, essentially no changes in the CO₂ adsorption profile and capacity were observed after the material was held under flowing air at 100°C for 12 h, indicating the stability of the framework to oxygen in the flue gas stream (fig. S43).

We also performed larger scale breakthrough experiments at 100°C under the same humid conditions to simulate a real fixed-bed adsorption process. Under these conditions, Mg₂(dobpdc)(3–4-3) exhibited a high CO₂ capture rate of 90% with a breakthrough capacity of 2.0±0.2 mmol/g (Fig. 4A and figs. S45 to S48). It also exhibited minimal tetraamine volatilization during the course of 1000 CO₂ adsorption-desorption cycles performed under humid conditions using a thermogravimetric analyzer, and the material remains crystalline with a stable cycling capacity of 13.6 g/100 g (Fig. 4B and fig. S50 and S51). Thus, Mg₂(dobpdc)(3–4-3) could cycle a large quantity of CO₂ while meeting the U.S. DoE target for capturing 90% of the CO₂ from natural gas flue emissions (7).

Fig. 4. CO₂ adsorption in Mg₂(dobpdc)(3-4-3) in the presence of water.

(A) CO₂ breakthrough profile for Mg₂(dobpdc)(3-4-3) under simulated humid (~2.6% H₂O) natural gas flue gas (4% CO₂ in N₂) at 100°C and atmospheric pressure. (B) Extended temperature-swing cycling of Mg₂(dobpdc)(3-4-3) carried out using a thermogravimetric analyzer at atmospheric pressure. Adsorption conditions: humid (~2.6% H₂O) 4% CO₂ in N₂ for 5 min at 100°C; desorption conditions: humid (~2.6% H₂O) CO₂ stream at 180°C for 1 min. (C) Infrared spectra showing 15 cycles of adsorption under humid CO₂ (bright green spectra) and desorption under simulated steam (light grey spectra, estimated 51 to 65% water content, balance N₂). Increasingly darker colored curves indicate progressive cycles. Spectra of the activated framework under flowing steam/N₂ were used as a baseline. (D) Plot of the integrated area for the carbamate band in C (~1290-1360 cm⁻¹) versus adsorption cycle number, illustrating stable formation of the carbamate species across 15 cycles.

The high breakthrough capacity and exceptional stability of Mg₂(dobpdc)(3–4-3) motivated us to explore the use of steam in place of heating at 180°C to desorb CO₂ from the material. Low-grade steam could offer advantages over both temperature swing and pressure swing adsorption processes for post-combustion CO₂ capture (17–22). The viability of using steam for CO₂ desorption was tested in a custom-built flow-through cell for in situ DRIFTS equipped to cycle between flowing humid CO₂ and simulated steam (estimated 51 to 65% water content, balance N₂, see table S10) at 120°C and ~1 atm (Fig. 4C,D). The IR spectra collected for Mg₂(dobpdc)(3–4-3) over the course of 15 cycles showed repeated growth and disappearance of both the ammonium (2800–1800 cm⁻¹) and C–N (1339 cm⁻¹) bands associated with carbamate chain formation, indicating the stability of the cooperative insertion mechanism to steam regeneration conditions. Postcycling isobaric analysis of the material revealed that step-shaped adsorption was retained to a high capacity (~16.6 g/100, see figs. S52 to S55 and table S11), and ¹H NMR analysis revealed that the tetraamine loading remained unchanged (table S12). In stark contrast, the representative diamine-appended material Mg₂(dobpdc)(e-2)₂ (e-2 = N-ethylethylenediamine) (28, 30) exhibited substantial amine volatilization after only a single steam desorption cycle (table S13 and fig. S56). The impressive performance of Mg₂(dobpdc)(3–4-3) underscores the stabilization afforded by the multimetal coordination modes.

Outlook

We have developed a class of tetraamine-functionalized metal–organic frameworks that exhibit cooperative CO₂ adsorption and greatly enhanced stability compared to previous diamine-functionalized materials as a result of multiple, ordered metal–amine interactions. Critically, the nature of amine coordination in these materials gives rise to two-step adsorption of CO₂ and large adsorption enthalpies suitable for CO₂ capture from simulated natural gas flue streams. The top-performing material, Mg₂(dobpdc)(3–4-3), achieved a large CO₂ adsorption capacity of 2.0 ± 0.2 mmol/g in the presence of water while meeting the DoE target for 90% CO₂ capture from natural gas flue emissions. Most strikingly, however, the enhanced stability of these tetraamine-functionalized frameworks enables regeneration using direct steam contact, a pathway that could afford significant energy savings compared to traditional processes. Overall, the molecular-level design strategy implemented here represents a step-change in the industrial viability of adsorption-based CO₂ capture from natural gas flue emissions, a separation of growing importance in the global energy landscape.