Putting an ultrahigh concentration of amine groups into a metal–organic framework for CO2 capture at low pressures

Hydrazine can be grafted in CPO-27-Mg/MOF-74-Mg to provide an ultrahigh concentration of amine groups on the pore surface, giving an exceptionally high CO2 capture performance, especially at extremely low pressures.


Introduction
Applications of porous materials for capturing CO 2 from ue gas (ca. 150 mbar) and from air (ca. 0.4 mbar) have become an increasingly attractive area of research due to their importance in many applications related to the environment, energy and health. [1][2][3][4][5][6][7] Compared with post-combustion capture, direct air capture (DAC) is more challenging, since the CO 2 adsorption ability of most porous materials is low at trace CO 2 concentrations. 8 Among the various types of adsorbents, porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) with designable and modiable pore surfaces are very attractive for CO 2 capture.  To increase the CO 2 uptake at extremely low pressures, incorporating alkylamine groups on the pore surface is the most effective strategy so far, [46][47][48][49][50] because of the chemisorption of CO 2 associated with the formation of zwitterionic carbamate or carbamic acid. For instance, [Mg 2 (dobpdc)(eda) 1.6 ] (eda-Mg 2 (dobpdc), eda ¼ ethylenediamine, H 4 dobpdc ¼ 4,4 0 -dihydroxy-(1,1 0 -biphenyl)-3,3 0 -dicarboxylic acid) 48 holds the records for gravimetric and volumetric CO 2 adsorption capacities with 2.85 mmol g À1 and 2.72 mmol cm À3 , respectively, at 298 K and 0.4 mbar, this is because it has a high concentration of free amine groups at 3.86 mmol g À1 or 3.69 mmol cm À3 . Obviously, the CO 2 adsorption does not reach saturation at this condition, which is ascribed to the strong intermolecular hydrogen bonds between two adjacent amine groups that need to be opened by higher pressure CO 2 . 47,51 As a consequence, its CO 2 uptake at 0.4 mbar sharply drops to 0.120 mmol g À1 or 0.115 mmol cm À3 at a slightly higher temperature of 323 K. To overcome this problem, replacing the eda molecule with an even shorter diamine can be a good strategy, which not only reduces the intermolecular hydrogen bonding locks but also reduces the adsorbent weight.
Hydrazine (N 2 H 4 ) can be considered as the smallest/shortest diamine, and its basicity is only slightly weaker than that of alkylamines (Table S1 †). 52 It should be noted that the long/large length/size of eda and its derivatives is the main reason for choosing the large-pore [Mg 2 (dobpdc)] instead of the simpler small-pore [Mg 2 (dobdc)] (H 4 dobdc ¼ 2,5-dihydroxyl-1,4-benzenedicarboxylic acid) as a host framework. 49 Nevertheless, to the best of our knowledge, an N 2 H 4 -graed PCP has not been reported so far. Herein, we show that N 2 H 4 can be used to functionalize the prototypical small-pore PCP [Mg 2 (dobdc)] without the formation of the intermolecular amine-amine interaction, achieving an ultrahigh concentration of useful amine groups in both the gravimetric and volumetric points of view, as well as a series of new benchmark CO 2 capture performances, ranging from air to ue-gas conditions.

Results and discussion
The hypothetical structure of [Mg 2 (dobdc)(N 2 H 4 ) 2 ], i.e. [Mg 2 (dobdc)] fully functionalized by one N 2 H 4 molecule per Mg(II) ion, was simulated using molecular mechanics (MM) combined with periodic density functional theory (PDFT). During structural optimization, the host framework and N 2 H 4 molecules were considered as rigid and exible, respectively. The simulation results showed that the void ratio of [Mg 2 (dobdc)(N 2 H 4 ) 2 ] remains at 36.5%, and the smallest aperture diameter is ca. 4.8Å (Fig. 1a), which should be enough for the diffusion and accommodation of CO 2 molecules. Furthermore, the shortest possible intermolecular N/N separations between pairs of adjacent amine groups on the ab-plane and along the c-axis were simulated as 3.81 and 4.30Å, respectively ( Fig. 1b and c), which are much longer than the values for typical N-H/N hydrogen bonds. For comparison, we also simulated the structure of eda-Mg 2 (dobpdc), giving relatively short N/N separations of 3.06 and 3.07Å on the ab-plane and along the c-axis, respectively ( Fig. 1d and e), which can be assigned as a kind of weak hydrogen bonding. 53 Yellow microcrystalline [Mg 2 (dobdc)] (1) was suspended in anhydrous toluene under N 2 atmosphere at room temperature, and upon the addition of a toluene solution of anhydrous N 2 H 4 , the colour of the solid instantaneously changed to light yellow. The light yellow product was washed with toluene and then successively soaked in hexane and methanol to remove the excess N 2 H 4 . Elemental analysis gave a chemical formula of [Mg 2 (dobdc)(N 2 H 4 ) 1.8 ] (1a) for the light yellow product. Pawley renements of the powder X-ray diffraction (PXRD) patterns showed similar unit-cell parameters for 1 and 1a and conrmed the retention of framework integrity aer N 2 H 4 graing ( Fig. S1 †). PXRD and thermogravimetry-mass spectrometry analyses showed that 1a can be fully activated at 403 K ( Fig. S1 and S2 †), with negligible release of the N 2 H 4 molecules. N 2 sorption isotherms measured at 77 K gave apparent Langmuir surface areas of 1925 and 1012 m 2 g À1 and pore volumes of 0.67 and 0.30 cm 3 g À1 , respectively, for 1 and 1a (Fig. S3 †). Considering that their crystallographic pore volumes are 0.66 and 0.31 cm 3 g À1 , respectively, the N 2 sorption data further conrmed the good sample crystallinity and purity, as well as the chemical formula of 1a derived from elemental analysis. 1a possesses a high concentration of surface-appended N 2 H 4 molecules of 6.01 mmol g À1 or 7.08 mmol cm À3 .
At 298 K and 1 bar (Fig. 2a), 1a showed a CO 2 uptake of 5.51 mmol g À1 or 6.49 mmol cm À3 , corresponding to 1.65 CO 2 molecules per formula unit or 0.919 CO 2 molecule per N 2 H 4 molecule, which are lower than the values for 1 (8.04 mmol g À1 or 7.40 mmol cm À3 ) (Table S2 †), this is because the relatively large pore volume of 1 can accommodate additional physically adsorbed CO 2 . At 0.15 bar, the gravimetric uptake of 1a  (5.18 mmol g À1 ) is still lower than that of 1 (5.71 mmol g À1 ). However, the volumetric uptake of 1a (6.10 mmol cm À3 ) is 17% higher than that of 1 (5.25 mmol cm À3 ), illustrating the stronger CO 2 binding affinity of the amine groups. At a much lower pressure of 0.4 mbar, corresponding to the CO 2 concentration in air, 1a showed an exceptionally high CO 2 uptake of 3.89 mmol g À1 or 4.58 mmol cm À3 , which is not only a drastic improvement over 1 (0.09 mmol g À1 or 0.08 mmol cm À3 ), but also obviously higher than the previous records (Table S2 †). The CO 2 sorption kinetics of 1a were also evaluated using thermogravimetric analysis under dry air at 298 K. A CO 2 uptake of 3.66 mmol g À1 or 16.1 wt% (3.71 mmol g À1 or 16.3 wt% for the single-component CO 2 adsorption isotherm at 0.4 mbar and 298 K) was observed aer ca. 950 min by switching the atmosphere from pure N 2 at 403 K to dry air at 298 K. The equilibrium time is much longer than that at higher CO 2 partial pressures, being similar to known examples such as eda-Mg 2 (dobpdc) (1000 min, Fig. S4 †). 12,48 Interestingly, even at a higher temperature of 328 K, 1a still exhibits a relatively high CO 2 uptake of 1.04 mmol g À1 or 1.22 mmol cm À3 at 0.4 mbar, which is 2-3 times higher than the previous records (Table S3 †).
The coverage-dependent CO 2 adsorption enthalpy (Q st ) of 1a was calculated using the Clausius-Clapeyron equation and the Virial tting method using isotherms measured at 298, 313, and 328 K (Fig. 2b, S5 and S6 †). Among various isotherm models, only the Langmuir-Freundlich (LF) one gave a fair tting. Based on the Clausius-Clapeyron equation, the Langmuir-Freundlich isotherms gave a very high Q st of 118 kJ mol À1 at zero-coverage, while the original isotherms (without data tting) gave nearzero-coverage Q st of 90 kJ mol À1 . These Q st values are higher than the values of most reported materials (Table S2 †) and indicative of chemisorption. On the other hand, the Virial tting gave similar results as the Clausius-Clapeyron equation using original isotherms. In situ infrared (IR) spectra of 1a showed that the primary amine peaks at 3330 and 3287 cm À1 weakened under CO 2 atmosphere. The residue primary amine groups can be assigned to those anchoring on the Mg(II) sites. Meanwhile, two new absorption peaks appeared at 3397 and 1250 cm À1 . The former new peak may arise from the stretching band of the N-H bond and/or carboxylic O-H bond, while the latter can be assigned to the stretching band of the C-N bond of carbamic acid. The peak for the asymmetric deformation of the ammonium group at $2200 cm À1 was invisible, indicating the absence of proton transfer, which is consistent with the relatively long intermolecular separation of hydrazine molecules (Fig. S7 †). 54,55 Aer being heated under vacuum at 403 K, this peak disappeared, indicating the chemisorption of CO 2 is reversible. The formation of carbamic acid was further conrmed via 13 C NMR measurements with the observation of a broad resonance peak centered at d ¼ 162 ppm (Fig. 3). 22 The CO 2 adsorption and desorption behaviours of 1a under mixed-gas and kinetic conditions were analyzed via thermogravimetry (Fig. 4), in which the sample was blown repeatedly under conditions between a 15 : 85 CO 2 /N 2 (v/v) mixture at 313 K (the typical ue-gas environment) and a pure N 2 ow (a typical regeneration method for temperature-vacuum swing adsorption (TVSA) like process) at 403 K. It should be noted that the regeneration temperature is lower than for other alkylamine functionalized PCPs. By switching from pure N 2 at 403 K to the mixed gas and allowing the temperature to automatically decrease to 313 K aer 17 min, a weight increase of 17.0% was observed, corresponding to volumetric CO 2 uptake of 3.86 mmol g À1 or 4.55 mmol cm À3 , which is 78% of the single-component isotherm uptake at 313 K and 0.15 bar. The  CO 2 uptake further increased to 18.7% (4.25 mmol g À1 or 5.01 mmol cm À3 , i.e., 87% of the isotherm value) by maintaining the temperature at 313 K for an additional 23 min. Although the adsorption and desorption processes for 1a were not allowed to reach equilibrium, as its relatively small pores are not benecial for fast guest diffusion, the CO 2 adsorption/desorption working capacities obtained here are much higher than all the reported values for other materials under similar conditions and time scales (Table S4 †). Aer 5 such adsorption-desorption cycles, there was nearly no change in the adsorption capacity, illustrating the high thermal stability and good CO 2 adsorption-desorption reversibility of 1a, as indicated by the thermogravimetry-mass spectrometry analyses (Fig. S2 †).
A pure temperature swing adsorption (TSA) process for 1a was further carried out, and a working capacity of 3.52 mmol g À1 or 4.15 mmol cm À3 was obtained between the adsorption in a 15 : 85 CO 2 /N 2 (v/v) mixture at 313 K for 48 min and the desorption in a pure CO 2 ow at 413 K (optimized) for 54 min. It should be noted that, even though it was obtained in a relatively short time which did not allow the adsorption/desorption to reach equilibration, this working capacity is higher than previously reported values obtained in similar and longer adsorption/desorption times (Table S4 †). To evaluate the regeneration energy for CO 2 desorption, the heat capacity of 1a was quantied via differential scanning calorimetry. About À200 J g À1 was evolved as the material was cooled from 413 to 313 K (Fig. S8 †). With these data (see calculation method in the ESI †), approximately 3.02 MJ of energy would be required to regenerate 1 kg of CO 2 from 1a (Fig. 4b). 49,56 The regeneration energy for CO 2 desorption from 1a (3.02 MJ kg À1 ) is higher than those of mmeda-Mg 2 (dobpdc) (2.34 MJ kg À1 ) and state-of-theart amine-based solutions (2.6 MJ kg À1 ), but lower than that of monoethanolamine (3.5 MJ kg À1 ). 49 To simulate more practical CO 2 capture applications, we also measured the column breakthrough curves of 1a using a binary 10 : 90 CO 2 /N 2 (v/v) mixture at 313 K and 1 bar. As shown in Fig. 5a and S9a, † the breakthrough point (outlet concentration > detection limit) of CO 2 was observed at a mixture injection amount of 44.3 mmol g À1 or 52.2 mmol cm À3 , exceeding all the reported values even measured at a lower temperature of 298 K ( Table S5, † note that a lower temperature generally leads to a higher adsorption amount, at the same pressure), including 42.2 mmol g À1 or 38.8 mmol cm À3 for 1 and 25.5 mmol g À1 or 50.2 mmol cm À3 for the recently discovered microporous copper silicate SGU- 29. 7 Aer that, the outlet CO 2 concentration rose gradually to the inlet value at 51.6 mmol g À1 or 60.8 mmol cm À3 . The capability of 1a for trace CO 2 capture was investigated by column breakthrough tests using a 1 : 999 CO 2 /N 2 (i.e. 1000 ppm CO 2 ) mixture at 298 K (Fig. 5b). The CO 2 breakthrough and saturation points were observed at 4210 and 4400 mmol g À1 , or 4960 and 5180 mmol cm À3 , respectively.
The CO 2 uptake of the adsorbent 1a in the columns can be calculated (see ESI † for calculation method) as 4.89 and 4.22 mmol g À1 , or 5.76 and 4.97 mmol cm À3 , for the 10 : 90 and 1 : 999 CO 2 /N 2 mixtures, respectively, which are both ca. 98% of the values obtained from the single-component adsorption isotherms (4.97 mmol g À1 or 5.85 mmol cm À3 at 313 K and 0.1 bar, 4.30 mmol g À1 or 5.07 mmol cm À3 at 298 K and 1 mbar). Suffering from weak CO 2 /N 2 selectivity and/or low adsorption rate, many adsorbents show a much lower CO 2 uptake under mixed-gas conditions compared to single-component adsorption isotherms (Table S5 †). The exceptionally high adsorption efficiency of 1a implies that its CO 2 diffusion rate and CO 2 /N 2 selectivity are high enough under such mixed-gas breakthrough conditions.
We also tested the CO 2 capture performances and stabilities of the columns under high humidity. 9-13 Aer the breakthrough curves reached equilibrium under the above mentioned dry conditions (0% RH, RH ¼ relative humidity), the columns were activated under He ow at 403 K, cooled down to the measurement temperature of 313 K, saturated using humid He (82% RH), and then switched to humid CO 2 /N 2 mixtures (82% RH, no change to other parameters) to start a new breakthrough experiment. Remarkably, the breakthrough curve for the humid 10 : 90 CO 2 /N 2 mixture almost overlapped with that measured at the dry condition (Fig. 5a). For the humid 1 : 999 CO 2 /N 2 mixture, the CO 2 breakthrough point slightly (2.6% compared with that observed for the dry mixture) shortened to 4100 mmol g À1 or 4830 mmol cm À3 (Fig. 5b and S9b †). These observations indicated that water has little effect on the CO 2 adsorption capacity of 1a, even when the CO 2 concentration is extremely low. It should be noted that H 2 O can strongly compete with CO 2 for the adsorption sites in most porous materials, and only a handful of examples have demonstrated such a high waterproof CO 2 capture performance (Table S5 †).
As shown in Fig. 5, the same activation/breakthrough procedures were applied to the columns once again, and the breakthrough curves almost overlapped with each other, indicating the high stability of the adsorbent 1a and the column even under high humidity. This also indicates that 1a is particularly selective for CO 2 over water, and conrms that the graed N 2 H 4 molecules were not replaced by water molecules.

Conclusions
In summary, we demonstrated that the classical small-pore PCP [Mg 2 (dobdc)] can be modied by the shortest diamine N 2 H 4 to obtain a very powerful CO 2 adsorbent [Mg 2 (dobdc)(N 2 H 4 ) 1.8 ].
Thanks to the ultrahigh concentration of amine groups and the chemisorption of CO 2 associated with carbamic acid formation, this material exhibited exceptionally high CO 2 adsorption capacity at low pressures (from 0.4 mbar to 150 mbar) and a wide range of temperatures (from 298 to 328 K), well exceeding previous records. More importantly, these CO 2 adsorption performances can be maintained under mixed-gas kinetic conditions, even in the presence of high humidity.