Carbon dioxide capture and storage is essential to reducing greenhouse gas emissions and meeting net-zero emissions targets1,2.
A range of technologies are under development to meet the need for more energy efficient carbon capture. One promising strategy to improve on traditional aqueous amine technology is to use solid adsorbent materials for capture3,4,5. In particular, installation of reactive amine or hydroxide functional groups within a porous scaffold such as a metal-organic framework or a porous silica brings about selective reactivity with CO26,7,8,9,10, with the porous scaffold providing a large surface area for hosting the reactive groups while maintaining channels for CO2 transport. The increasingly complex adsorbent materials under consideration bring major challenges in the characterisation of new carbon capture chemistry, hindering the design of improved materials4.
Existing characterisation tools for understanding CO2 capture modes include single-crystal diffraction11,12,13, powder diffraction14, infra-red spectroscopy6,10,15, X-ray absorption spectroscopy16, and NMR spectroscopy8,17,18,19,20, each of which has strengths and limitations in terms of the materials that can be studied and the information that can be obtained. Solid-state NMR spectroscopy is a promising tool for investigating CO2 binding modes in adsorbents as there is no requirement for long-range ordering and detailed information about the local structure and dynamics of the CO2 can be obtained. However, different CO2 adsorption products often give rise to very similar signals in the NMR spectrum and assigning these signals to specific CO2 binding modes remains very challenging8,17,18,19,20.
The most common experiment with the NMR approach is to dose the candidate adsorbent with 13CO2 gas and perform 13C magic angle spinning (MAS) NMR experiments. These experiments are relatively straightforward to perform, but often lead to ambiguous identification of the adsorption products. For amine-functionalised materials, the 13C chemical shifts give poor differentiation between closely related ammonium carbamate, carbamic acid, and ammonium bicarbonate adsorption products, with the signals from these species showing very similar 13C chemical shifts17,18. A similar problem arises for bicarbonate and carbonate products in hydroxide-based materials20. The prediction of NMR parameters with density-functional theory (DFT) calculations21 can improve confidence in the structural assignments, and more advanced multi-nuclear NMR experiments can give improved differentiation between adsorption products17,19,22. However, there remains a pressing need for the exploration of new NMR methods for understanding CO2 capture chemistry23.
A representative emerging class of CO2 adsorbents are amine-functionalised metal-organic frameworks. The framework M2(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) (Fig. 1a) can straightforwardly be functionalised with amines to yield a family of (amine)–M2(dobpdc) adsorbents (Fig. 1b)14. These adsorbents have large capacities for selective and reversible CO2 uptake, and the adsorption thermodynamics can be tuned by varying the amine11,24,25,26,27, and the metal14,28,29. Importantly, these materials generally display steep adsorption isotherms making them promising for a range of energy efficient carbon capture applications24,25. Initial characterisation of CO2 adsorption modes in these materials has revealed a rich chemistry, with three CO2 adsorption products proposed to date: (i) ammonium carbamate chains (Fig. 1c), thought to be the dominant product in a range of variants11, (ii) carbamic acid pairs (Fig. 1d), identified in the Zn-based framework functionalised with the diamine dmpn (dmpn = 2,2-dimethyl-1,3-diaminopropane)17,24, and (iii) a mixed adsorption product (Fig. 1e) recently proposed for (dmpn)-Mg2(dobpdc)17. The adsorption thermodynamics of these three adsorption processes vary, motivating further characterisation to aid the design of metal-organic frameworks with the best CO2 capture performances.
a Structure of the metal organic framework M2(dobpdc). b Amine functionalisation yields (amine)-M2(dobpdc). Upon exposure to
CO2 various adsorption products can form. The most dominant adsorption products are: c ammonium carbamate chains, d carbamic acid pairs and e mixed ammonium carbamate–carbamic acid.
Here we leverage the crystalline and tuneable family of (diamine)–M2(dobpdc) adsorbents to perform a systematic computational exploration of solid-state NMR parameters for different CO2 adsorption products. We show that 17O solid-state NMR spectroscopy is a powerful probe of CO2 capture chemistry, providing unambiguous identification of carbamic acid formation and a detailed picture of the hydrogen-bonding environments.
Methods and Materials
All of the chemicals used in this project were purchased from commercial suppliers and were used without further purification. The ligand 4,4'-dihydroxy-[1,1'-biphenyl]-3,3'-dicarboxylic acid (H4dobpdc) was purchased from Hangzhou Trylead Chemical Technology. 17O-enriched CO2 gas was purchased from ICON/Berry & Associates, Inc, with ~20 at.% 17O.
Mg2(dobpdc) was synthesised according to a previously reported procedure11. Mg(NO3)2·6H2O (11.5 g, 45.0 mmol, 1.24 equiv), H4dobpdc (9.90 g, 36.0 mmol, 1.00 equiv), N,N-dimethylformamide (DMF) (90 mL), and methanol (110 mL) were mixed together in a 350 mL glass heavy wall pressure vessel (Chemglass, CG-1880-42). The reaction mixture was sonicated for 15 min until all of the solids had dissolved, and was then sparged with N2 for 1 h. The reaction vessel was sealed and heated at 120 °C with stirring for 21 h. This resulted in the precipitation of a white solid from solution. The solid was collected via vacuum filtration and quickly returned to the reaction vessel along with fresh DMF (250 mL). The reaction vessel was then heated to 60 °C for 3 h with stirring. Following this, the solid was again collected via vacuum filtration and returned to the reaction vessel with fresh DMF (250 mL) and again heated to 60 °C for 3 h with stirring. This washing process with DMF was repeated a total of three times, after which the solid was washed three more times in methanol (250 mL) at 60 °C to yield the desired product, Mg2(dobpdc). A small portion of the product (ca 0.1 g) was collected via filtration and activated for characterisation by powder diffraction (Supplementary Fig. 10) by heating to 60 °C in N2 for 15 h. The remaining Mg2(dobpdc) was stored in methanol.
Diamine-functionalised Mg2 (dobpdc) synthesis
Diamine-functionalised Mg2(dobpdc) materials were synthesised according to a procedure previously reported in literature11. Methanol-solvated Mg2(dobpdc) was filtered and washed with toluene (50 mL). The filtered MOF (ca 0.1–0.4 g) was then added to a toluene (4 mL) and diamine (1 mL) solution and left to soak for at least 12 h. The solid was then collected via vacuum filtration and washed with toluene (50 mL). e-2, dmpn, ee-2, i-2 and ii-2, functionalised Mg2(dobpdc) materials were activated by heating in an aluminium bead bath under N2 to 125 °C for 1 h, 150 °C for 1 h, 125 °C for 1 h, 130 °C for 1 h, and for 130 °C for 1.5 h, respectively. This activation step removes solvent as well as excess diamine, and ensures the samples are dry prior NMR sample preparation (see below). A portion (10–20 mg) was taken for powder X-ray diffraction analysis (Supplementary Fig. 10). To determine sample stoichiometries by 1H NMR (Supplementary Fig. 9, Supplementary Table 5), ~5 mg of the activated amine-functionalised MOFs was digested in a mixture of dimethyl sulfoxide (DMSO-d6) (1 mL) and two Pasteur pipette drops of deuterated hydrochloric acid (DCl) (35 wt% in D2O, ≥ 99 at.% D).
Preparation of amine-grafted SBA15
12 g of Pluronic P123 triblock copolymer, 90 g of distilled water, and 360 g of 2 M HCl aqueous solution were mixed in a Teflon-lined container. The mixture was stirred at 35 °C for about 2 h, until complete dissolution of P123. Then, 25.5 g of tetraethyl orthosilicate was added to this solution under vigorous stirring. Stirring was stopped after 5 min, and the mixture was kept under static conditions at 35 °C for 20 h, followed by 48 h at 100 °C in an autoclave. The solid product was collected by filtration, washed with distilled water, dried at ambient condition, and calcined at 550 °C in flowing air for 6 h.
Amine grafting was carried out as described elsewhere57. The SBA-15 support was dried at 120 °C for 4 h to remove residual moisture. Then, 1.0 g of the dried support was transferred to a multineck flask, to which 30 mL of toluene was added. The mixture was stirred at ambient temperature for 2 h, and 0.3 mL of water was added dropwise. The mixing continued for an additional 2.5 h. The temperature was then raised to 110 °C, followed by addition of 1 mL of propylamine silane or 3-[2-(2-aminoethylamino)ethylamino]propyl trimethoxysilane for propylamine-SBA15 (referred to as Pr-Si) and triamine-SBA15 (referred to as Tri-Si), respectively. The mixture was kept under reflux overnight. The grafted materials were filtered and washed with toluene followed by pentane, then dried at room temperature overnight, and archived in sealed vials. Activation before gas dosing for NMR studies was carried out by heating at 120 °C under flowing nitrogen for at least 1 h.
C17O2 dosing of amine functionalised adsorbents
The activated amine functionalised adsorbents were packed into 4 mm or 3.2 mm NMR rotors inside a nitrogen-filled glovebag, thereby excluding water as far as possible. Each sample was then evacuated for a minimum of 10 min in a home-built gas manifold, as described previously17. 17O-enriched CO2 gas (20 at.% 17O) was then used to dose the samples with gas at room temperature, before sealing the rotors inside the gas manifold with a mechanical plunger. (ee-2)-Mg2(dobpdc) was dosed for 0.5 h with a final gas pressure of 896 mbar. The (dmpn)-Mg2(dobpdc) sample for measurements at 20.0 T was dosed for 15 h with a final gas pressure of 1253 mbar, and the second independent (dmpn)-Mg2(dobpdc) sample for measurements at 23.5 T was dosed with 17O-enriched CO2 for 15 h with a final gas pressure of 1113 mbar. (e-2)-Mg2(dobpdc) was dosed for 1 h, and the final gas pressure was 448 mbar. (i-2)-Mg2(dobpdc) was dosed for 0.5 h, and the final gas pressure was 1116 mbar. The (ii-2)-Mg2(dobpdc) sample for measurements at 20.0 T was dosed for 0.75 h with a final gas pressure of 1015 mbar, and the second independent (ii-2)-Mg2(dobpdc) sample for measurements at 23.5 T was dosed with 17O-enriched CO2 for 0.5 h with a final gas pressure of 1039 mbar. For activated Mg2(dobpdc) (i.e., with no amines), activation was first carried out by heating in flowing nitrogen gas at 180 °C for 15 h. This sample was then packed in an NMR rotor (as above) and dosed with gas for 0.5 h, and the final gas pressure was 1075 mbar. For silica grafted amines, samples were dosed inside 3.2 mm NMR rotors for 0.5 h with final gas pressures for Pr-Si and Tri-Si of 1083 mBar and 993 mBar, respectively.
17O MAS and MQMAS experiments were performed using spectrometers equipped with a 20.0 T wide-bore and 23.5 T standard bore magnets, corresponding to a 1H Larmor frequencies, ν0, of 850 MHz and 1 GHz. For experiments at 20.0 T, experiments were performed with an MAS frequency, νR, of 14 kHz. 17O MAS spectra were acquired using a spin-echo pulse sequence with radiofrequency field strength, ν1, of ~50 kHz and a recycle delay of 0.05 s. NMR parameters were optimised experimentally and therefore the spin-echo experiments cannot be considered quantitative. MQMAS experiments were acquired using a z-filter pulse sequence58,59,60,61 with triple-quantum excitation/conversion pulses with ν1 ≈ 50 kHz and a central-transition selective π/2 pulse at ν1 ≈ 11 kHz. All MQMAS spectra are shown after shearing using the convention described in ref.58. For experiments at 23.5 T, a spectrometer equipped with a 3.2 mm HX double resonance probe was used with a MAS rate of 20 kHz. 17O MAS spectra were acquired using a spin-echo pulse sequence, with ν1 = 25 kHz, and with a recycle delay of 0.05 s. Chemical shifts are given in ppm, and are referenced relative to liquid H2O at 0 ppm.
The candidate structures were first geometry optimised using CASTEP21,62,63,64,65,66,67,68,69,70,71,72,73. This was done with (i) plane-wave basis set with an 80 Ry (1088 eV) cut-off energy, (ii) the on-the-fly generated ultrasoft pseudopotential (C17), (iii) a 1 × 1 × 3 k-point grid, (iv) the Perdew-Burke-Ernzerhol (PBE) functional with a G06 Van de Waals correction.
The NMR parameters were calculated using the same parameters and this gave values of δiso, anisotropy, asymmetry, CQ and ηQ which converged within 0.1 ppm, 0.25 ppm, 0.001, 0.0 MHz and 0.0, respectively, for the investigated oxygen and carbon nuclei at the selected k-point grid and cutoff energy.
For 13C and 17O NMR, the principal components of the chemical shielding tensor (σ11, σ22 and σ33 where σ33 ≥ σ22 ≥ σ11) were obtained directly from the CASTEP calculations, in terms of σxx, σyy and σzz where |σzz – σiso| ≥ | σxx – σiso| ≥ |σyy – σiso|. The principal components of the chemical shielding tensor were converted to chemical shift principal components using δ = –(σcalc – σref) where the reference values for 13C and 17O were 171.2 and 249.8 ppm, respectively. These values were obtained from CASTEP calculations on cocaine (13C)74 and the amino acids tyrosine and valine (17O)75, and correlation of the calculated values with the experimental values with a linear fit with a fixed gradient of –1.
The cluster models were created using Avogadro76 based off the models given in ref.23. Dangling silicon bonds at the surface edges were terminated by OH species50. All calculations were performed using the Gaussian 09 software77. Geometry optimisations and frequency calculations were performed on the model structures prior to the calculation of NMR parameters. Note: no imaginary values were observed in the frequency calculations, and as such the structures were determined to be at the true minima. All calculations were carried out at the CAM-B3LYP/pcS-2 level of theory.
In conclusion, this work shows that 17O NMR is an excellent probe of different CO2 adsorption products in amine functionalised adsorbents. In particular, 17O NMR can differentiate between ammonium carbamate chains and carbamic acids in a wide range of materials. Our measurements provide new support for ammonium carbamate chain formation in a series of (amine)–Mg2(dobpdc) variants, and also provide strong evidence for a recently proposed mixed ammonium carbamate–carbamic acid mechanism for the material (dmpn)–Mg2(dobpdc). We reveal carbamic acid formation in a previously poorly studied adsorbent, (ii-2)-Mg2(dobpdc), highlighting the prevalence of carbamic acid in frameworks with bulky amine groups. Finally, initial measurements on amine-grafted silica materials showcase the excellent versatility of the technique, and support the formation of ammonium carbamates in these materials, while also suggesting a new adsorption mode may be in operation. It is worth noting that care was taken to ensure no water was present during the preparation of samples. It is known that the presence of water would impact the CO2 adsorption mechanism and 17O NMR spectroscopy would be a powerful tool to explore additional mechanisms further. In the future, 17O NMR spectroscopy will be extended to a range of carbon capture technologies and will ultimately enable the design of improved materials that can help tackle the climate crisis.