Bicarbonate anion coordination assisted CO₂ capture by using urea–morpholine hybrid receptors in water

Abstract

The development of energy-efficient sorbents for aqueous CO₂ capture remains a significant challenge. This work presents a new design strategy by integrating a tertiary amine (morpholine) with a urea motif into a single molecular receptor. This structure enables autonomous, base-free CO₂ capture in water, where the urea groups provide complementary hydrogen-bonding sites for (bi)carbonate anions, while the morpholine moiety acts as an internal proton acceptor. The resulting receptors demonstrate a rapid uptake of CO₂ from a simulated flue gas (10% CO₂/N₂), achieving a capacity of up to 1.22 mmol g⁻¹. Spectroscopic studies (NMR, MS) and structural analysis of a model complex confirm that the capture proceeds via hydrogen-bond-stabilized bicarbonate formation. Crucially, the captured CO₂ can be completely released under remarkably mild conditions, either by heating at ca. 40 °C or by simple N₂ purging at ambient temperature. The receptors exhibit excellent recyclability over multiple capture–release cycles without capacity loss. This study highlights the potential of fine-tuning supramolecular interactions—particularly hydrogen bonding combined with a built-in base—to create low-energy, water-compatible CO₂ capture systems.

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Introduction

Effective capture of carbon dioxide (CO₂) is crucial for mitigating climate change, driving the search for materials that combine high adsorption capacity with low regeneration energy.^1–4 Conventional chemisorbents, such as inorganic bases, organic amines, and their polymeric or framework derivatives, typically rely on forming strong chemical bonds (e.g., carbonates or carbamates).^5–17 While effective for capture, this strong binding often necessitates substantial energy input for CO₂ release. The challenge is further amplified in the presence of water, a ubiquitous component in flue gas and direct air capture streams. In aqueous environments, CO₂ hydration readily forms carbonic acid (H₂CO₃), and the interaction between organic amines and CO₂ tends to yield (bi)carbonate salts, instead of carbamate.¹⁸ Consequently, achieving both high capture efficiency and energy-favorable desorption in water-rich media remains a significant scientific and technological challenge.

Hydrogen bonding has emerged as a key secondary interaction to regulate the properties of CO₂ capture and release. This is exemplified by strategic modifications in organic amine systems. Introducing intramolecular hydrogen-bond donors or acceptors, such as pyridine or amide groups adjacent to the reactive amines,^19–22 can stabilize CO₂ adducts (e.g., carbamate or carbamic acid) through internal hydrogen bonding.^23–26 Such stabilization not only modulates binding strength but can also lower the regeneration energy by shifting the acid/zwitterion equilibrium toward more readily released neutral species, as demonstrated in aminopyridine solvents.²⁷ In these systems, the intramolecular hydrogen-bonding network is a critical descriptor for achieving low viscosity and reduced regeneration temperatures (as low as 60–80 °C).^28,29 Similarly, in diamide–diamine macrocycles, the carbamate product is encapsulated and stabilized by a multiple hydrogen bonding network.³⁰ In aqueous media, the design principle shifts toward employing strong, charge-assisted hydrogen bonds to capture these anions.^31–33 A seminal advance in this direction is the work by Custelcean et al. on bis-iminoguanidinium compounds.³⁴ The rigid, planar guanidine units protonate readily to form guanidinium cations that engage in complementary, strong hydrogen bonds with carbonate or bicarbonate anions, driving the spontaneous crystallization of insoluble frameworks from water and thereby removing CO₂ from the air. Notably, regeneration of these sorbents, achieved by mild heating of the solid carbonate crystals to release CO₂, can occur at temperatures of 80–120 °C, underscoring how engineered hydrogen-bonding networks can enable energy–efficient capture–release cycles.^34–37 Collectively, these studies highlight hydrogen bonding as a versatile and powerful design element for tailoring CO₂ capture materials.

Like guanidium units, urea motifs can also bind oxyanions like carbonate and bicarbonate via anion coordination (i.e., hydrogen bonding), and therefore we sought to extend this supramolecular strategy with urea-based absorbents. While conventional oligourea receptors require exogenous strong bases (e.g., tetraalkylammonium hydroxides or fluorides) to activate CO₂ fixation,^38–44 we hypothesize that integrating a tertiary amine moiety directly into an oligourea scaffold may enable autonomous, base–free capture in water. In this design, the urea groups provide precise hydrogen–bonding sites to stabilize in situ formed HCO₃⁻/CO₃²⁻ ions, while the tertiary amine serves as an intramolecular proton acceptor,^45–47 offering additional electrostatic stabilization and facilitating the proton–transfer processes essential for both capture and subsequent release (Fig. 2a).

Guided by this rationale, we designed and synthesized a series of small molecules that combine morpholine (a cyclic tertiary amine, Fig. 1) with urea motifs. These morpholine–urea compounds were evaluated for CO₂ capture performance in water without any external organic base. Remarkably, they demonstrated rapid and efficient adsorption of CO₂ directly from water, achieving a capacity of up to 0.75 mol mol⁻¹ (1.22 mmol g⁻¹) for receptor 2. More importantly, the captured CO₂ could be rapidly released under heating at ca. 40 °C, highlighting the regulation effect of anion coordination.

Fig. 1 Schematic illustration of the design principles of morpholine–urea receptors and their chemical structures.

Results and discussion

The designed receptors 1 and 2 were prepared in two steps from the starting material 4-(2-aminoethyl) morpholine (Scheme 1), with the overall yields of 58% and 65%, respectively. Both receptors can be easily synthesized on the gram scale, and their structures are characterized and confirmed by NMR and MS spectroscopy (Fig. S8–S16). These two receptors display good water solubility (up to 3 M), and the control receptor 1^NO2 is not water soluble, which is directly synthesized from hydrazine hydrate and 4-nitrophenyl isocyanate (Fig. S1–S7).

Scheme 1 Synthetic procedure of 1 and 2. Conditions: (a) 4-nitrophenyl chloroformate, CH₂Cl₂, 0 °C, 80%; (b) dry CH₃CN, Et₃N, 50 °C, 72% for 1, 81% for 2.

The CO₂ capture performance of the two receptors was first evaluated in aqueous solution. A simulated flue gas (10% CO₂ in N₂) was bubbled at room temperature through 15 mL of a 20 mM aqueous solution of each receptor, while the uptake process was monitored in real time using a setup equipped with a high–precision CO₂ sensor (Fig. 2b). The initial pH of the receptor 1 solution was 8.96, consistent with the alkalinity provided by its morpholine group (pK_a ≈ 7.41 for N-methylmorpholine).^48,49 Upon CO₂ bubbling, the pH decreased to 6.77 for receptor 1 and to 6.99 for receptor 2, confirming the existence of the HCO₃⁻ anion and H₂CO₃. As recorded by the CO₂ sensor, the amount of captured CO₂ increased gradually and reached a plateau within 30 minutes (Fig. S26–S28). The determined CO₂ capacity reached 0.75 mol of CO₂ per mole of receptor for 2 (0.75 mol per mol of receptor, equivalent to 1.22 mmol g⁻¹, Fig. 2c), which is higher than that of receptor 1 (0.58 mol mol⁻¹). This difference can be attributed to the higher number of urea and morpholine groups present in receptor 2, which provide additional binding sites and basicity. The observed CO₂ uptake trends are consistent with the pH changes of the solutions. Notably, the absorption performance of both receptors is comparable to that of monoethanolamine (MEA), a benchmark industrial absorbent, which typically captures 0.4–0.5 mol of CO₂ per amine.^50–52 Nevertheless, these receptors are not suited for direct air capture applications, as their binding affinity is insufficient for the ultra-low CO₂ concentrations characteristic of ambient air (Fig. S30).

Fig. 2 (a) Mechanistic scheme of the formation of HCO₃⁻ and its binding with receptors in water. (b) A schematic diagram of the CO₂ capture experiment (gas input: 10% CO₂, 90% N₂; receptor solution: [receptor] = 20 mM, H₂O, room temperature). (c) The pH changes and the amount of CO₂ captured by using receptors 1 and 2.

To elucidate the CO₂ capture mechanism, NMR and mass spectrometry (MS) experiments were performed. Following the completion of CO₂ uptake, as indicated by pH changes after ca. 30 min of CO₂ bubbling through a receptor-containing aqueous solution (20 mM), the resulting solutions were subjected to spectroscopic characterization.

As recorded in the ¹H NMR spectra, the urea NH signals (Ha and Hb) exhibited characteristic changes upon CO₂ capture. For receptor 1, the original singlet at 6.78 ppm split into two doublets at 7.7 ppm and 6.8 ppm (Fig. 3a), whereas for receptor 2, the signal shifted from 6.0 ppm to 6.2 ppm (Fig. 3b). These changes correspond to the hydrogen bonding between the urea groups and the HCO₃⁻ anion produced upon CO₂ absorption. Additionally, protons adjacent to the morpholinyl nitrogen (H2 and H3) in both receptors showed downfield shifts, consistent with the protonation of the tertiary amine and potential hydrogen bonding with the HCO₃⁻ anion. The ¹³C NMR spectra further support the existence of the HCO₃⁻ anion. After CO₂ exposure, new peaks appeared at 160 ppm for 1 and 161 ppm for 2, which are assigned to the HCO₃⁻ anion and consistent with reported data from the literature. Moreover, the carbonyl carbon atoms of the urea moieties exhibited clearly downfield shifts, with the most pronounced change observed for 1 (Δδ ≈ 4 ppm). These shifts reflect the electronic perturbation induced by HCO₃⁻ anion binding. In addition, the post-capture aqueous solutions of 1 and 2 were analysed by ESI-QTOF mass spectrometry. Distinct peaks were observed at 406.20 and 675.43, assigned to the adducts of [1 + HCO₃]⁻ and [2 + HCO₃]⁻, respectively (Fig. 3c), providing direct evidence for the formation of bicarbonate complexes. The MS results combined with NMR data to support the absorption of CO₂ as the HCO₃⁻ anion binding complexes with receptors.

Fig. 3 ¹H and ¹³C NMR spectra of receptors (a) 1 and (b) 2 before and after CO₂ capture (10% DMSO-d₆/90% H₂O, 298 K). (c) ESI-QTOF spectra of receptors after CO₂ capture, indicating the formation of HCO₃⁻ binding complexes.

To obtain structural insights into the CO₂ capture process, crystallization trials were conducted. Due to the high-water solubility of receptors 1 and 2, single crystals of their HCO₃⁻ complexes could not be obtained. Instead, a control receptor 1^NO2, which bears terminal nitrophenyl groups and retains the bis(urea) hydrogen–bonding motif but exhibits lower water solubility, was employed. Slow vapor diffusion from acetonitrile/diethyl ether yielded single crystals of its HCO₃⁻ and CO₃²⁻ complexes (as tetramethylammonium salts) suitable for X–ray diffraction analysis (Fig. 4).

Fig. 4 Single crystal X-ray diffraction structures of (a) HCO₃⁻ and (b) CO₃²⁻ binding complexes of receptor 1^NO2. Solvent molecules and tetramethylammonium cations (TMA⁺) are omitted for clarity. Their anion binding structures with urea units are shown in the middle boxes. The HCO₃⁻ anion dimerizes through two complementary hydrogen bonds and interacts with urea units through three hydrogen bonds. The CO₃²⁻ anion is disordered and stabilized by eight hydrogen bonds with four urea units.

As shown in Fig. 4a, the HCO₃⁻ complex crystallizes in the space group C2/c, while the CO₃²⁻ complex crystallizes in the C2/m space group. Both structures display one–dimensional arrangements (along the c-axis or b-axis) stabilized by hydrogen bonds between the anions and the bis(urea) receptors. In the HCO₃⁻ complex, a 2 : 2 stoichiometry between the anion and 1^NO2 is observed. Every two HCO₃⁻ anions dimerize through two O–H⋯O hydrogen bonds (d = 2.597 Å), a recurring motif also reported in oligourea and guanidinium–based CO₂ capture systems. Such anti-electrostatic hydrogen bonding is also widely seen for hydroxy anion dimers in the solid state.^39,53–55 Each bicarbonate anion is further stabilized by three N–H⋯O hydrogen bonds from urea groups. Every two receptors adopt a parallel arrangement with an essentially linear conformation (Fig. S6).

In contrast, the CO₃²⁻ complex features a V–shaped receptor conformation. Each carbonate anion is engaged in eight N–H⋯O hydrogen bonds involving four different receptor molecules (Fig. 4b). The average N–H⋯O distance in the CO₃²⁻ complex (2.82 ± 0.01 Å) is notably shorter than that in the HCO₃⁻ analogue (2.89 ± 0.10 Å), reflecting stronger binding to carbonate. Along the b-axis, adjacent CO₃²⁻ ions are linked by urea groups from different receptors, forming an infinite one–dimensional hydrogen–bonded chain that extends into a rod–like framework structure (Fig. S7).

In contrast to receptors 1 and 2, the control receptor 1^NO2 captures CO₂ only in the presence of an external base, such as fluoride or tetramethylammonium hydroxide (TMAOH), in acetonitrile or DMSO. This requirement is attributed to the absence of the built–in morpholine group, a feature that enables autonomous proton capture in 1 and 2, and aligns with the behavior of other oligourea receptors reported earlier. Interestingly, once CO₂ is captured, the resulting HCO₃⁻ complex of 1^NO2 precipitates rapidly from acetonitrile solution within minutes, forming a hydrogen–bond–supported framework as seen in the solid state (Fig. 4). Such fast precipitation, commonly observed with guanidinium–based receptors, is seldom reported for urea–based systems. Furthermore, in the presence of TMA⁺ as the countercation, the absorbed HCO₃⁻ anion cannot be thermally released as CO₂ gas (Fig. S17–S25). Reversible CO₂ release occurs only when free protons (H⁺) are available in solution,⁵⁶ underscoring the crucial role of proton transfer in the regeneration step.

In the presence of TMA⁺ as the countercation, the absorbed HCO₃⁻ anion cannot be thermally released as CO₂. This is because thermal regeneration of the absorbent necessitates the complete protonation of absorbed HCO₃⁻ to form labile carbonic acid (H₂CO₃), which then dissociates to release CO₂. Nevertheless, during the heating of TMAHCO₃ complexes, no labile protons are available for combination with HCO₃⁻, thus precluding CO₂ release and absorbent regeneration. In contrast, the receptor utilized in our work allows the absorbed CO₂ to be converted into H⁺ and HCO₃⁻ during the absorption process. Specifically, the generated H⁺ resides on the protonated morpholine moieties, whereas HCO₃⁻ is connected to the urea groups via weak hydrogen bonds, constructing a fragile hydrogen-bonding network. This structural characteristic facilitates proton transfer from the protonated morpholine groups to HCO₃⁻ upon heating, which in turn promotes the formation of labile H₂CO₃ (Fig. 5a).

Fig. 5 (a) A schematic representation of the mechanism of CO₂ release. (b) Variable temperature ¹H NMR spectra of CO₂-binding receptors (10 mM, 10% DMSO-d₆/90% H₂O, 400 MHz). (c) Reversible CO₂ capture and release (upon heating at 40 °C).

For receptors 1 and 2, the captured CO₂ (in the form of HCO₃⁻) can be readily released under mild heating, as initially demonstrated by variable–temperature ¹H NMR experiments. A solution of receptor 1 after CO₂ bubbling (10 mM, 10% DMSO-d₆/90% H₂O) was examined while increasing the temperature from 298 K to 308 K (35 °C). Upon heating, the urea NH signal (Ha) gradually disappeared, while the adjacent proton Hb showed a clear upfield shift (Fig. 5b). At 318 K (45 °C), the spectrum closely resembled that of the free receptor, indicating complete release of the HCO₃⁻ anion. A similar trend was observed for the receptor 2. The recovery of the ¹H NMR signals corresponding to the free receptors confirms that hydrogen–bonding interactions with bicarbonate are effectively disrupted upon heating, resulting in the release of CO₂ gas. Such low–temperature CO₂ release is rarely reported, suggesting that hydrogen–bonding interaction can serve as a tunable tool for regulating both capture and release processes.

To assess the recyclability of the receptors, reversible CO₂ capture and thermal release were monitored in real time using the CO₂ sensor setup (Fig. 2b). An aqueous solution of receptor 1 or 2 (20 mM, 15 mL) was continuously purged with 10% CO₂ in N₂. After uptake reached completion (∼30 min), the solution was heated to 40 °C using a stir plate. The CO₂ sensor recorded a drop in outlet CO₂ concentration during the capture phase, followed by a return to the baseline level upon heating, which is consistent with the variable–temperature NMR results. This capture–release cycle was repeated multiple times without any loss in capacity (Fig. 5c), confirming excellent reversibility and stability of the receptors.

Furthermore, we found that CO₂ could also be liberated simply by purging the solution with pure N₂ gas (Fig. S31), a behavior similarly observed in other anion–based CO₂ capture systems where CO₂ is weakly bound. This result further underscores the low–energy character of CO₂ release in these urea–morpholine receptors. The integration of hydrogen–bonding motifs with a tertiary amine moiety enables effective capture while maintaining weak binding strength, thereby minimizing the energy penalty associated with regeneration, a feature attributed to the labile nature of the hydrogen–bonding network that stabilizes the bicarbonate anion.

Conclusions

In summary, we have designed and synthesized new urea–morpholine receptors capable of efficient and reversible CO₂ capture in water without adding external bases. The synergistic function of the urea hydrogen-bond donors and the tertiary amine proton acceptor facilitates direct CO₂ capture, forming stable bicarbonate complexes as verified by NMR, MS, and structural analysis using a crystalline model system. A key achievement is the exceptionally mild release of CO₂, achievable either by gentle heating (∼40 °C) or inert gas purging, which underscores the low-energy penalty associated with disrupting the labile hydrogen-bonding network. The receptors maintain full capture capacity over multiple cycles, demonstrating robust recyclability. Compared to the control receptor 1^NO2, which requires added base and exhibits irreversible binding, the integrated morpholine design is essential for autonomous proton management and energy-efficient regeneration. This work establishes hydrogen-bonding donor/acceptor integration as a powerful supramolecular strategy for developing new, energy-lean carbon capture materials operable under practical, moisture-rich conditions.

Author contributions

Data curation, investigation and methodology: Z. Sun, J. Wang, Q. Nie, N. Li, and Z. Liu. Project administration, supervision and funding acquisition: W. Zhao, X.-J. Yang, and B. Wu. Conceptualization and writing (original draft, reviewing & editing): W. Zhao and Z. Sun.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data supporting this article have been included as part of the supplementary information (SI). Supplementary information: details of syntheses, single crystal structures and NMR results. See DOI: https://doi.org/10.1039/d5dt03056k.

CCDC 2516702 (carbonate complex), 2516703 (bicarbonate complex), 2517602 (receptor 1^NO2) and 2517603 (receptor 1) contain the supplementary crystallographic data for this paper.^57a–d

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22101024) and the Beijing Municipal Natural Science Foundation (2222025). We thank Prof. Zheng Meng for helpful discussions and the setup of the CO₂ absorption equipment. We are also thankful for the support from the Analysis & Testing Center of Beijing Institute of Technology for data collection.

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