Controlled alkali etching of MOFs with secondary building units for low-concentration CO2 capture

Low-concentration CO2 capture is particularly challenging because it requires highly selective adsorbents that can effectively capture CO2 from gas mixtures containing other components such as nitrogen and water vapor. In this study, we have successfully developed a series of controlled alkali-etched MOF-808-X (where X ranges from 0.04 to 0.10), the FT-IR and XPS characterizations revealed the presence of hydroxyl groups (–OH) on the zirconium clusters. Low-concentration CO2 capture experiments demonstrated improved CO2 capture performance of the MOF-808-X series compared to the pristine MOF-808 under dry conditions (400 ppm CO2). Among them, MOF-808-0.07 with abundant Zr–OH sites showed the highest CO2 capture capacity of 0.21 mmol g−1 under dry conditions, which is 70 times higher than that of pristine MOF-808. Additionally, MOF-808-0.07 exhibited fast adsorption kinetics, stable CO2 capture under humid air conditions (with a relative humidity of 30%), and stable regeneration even after 50 cycles of adsorption and desorption. In situ DRIFTS and 13C CP-MAS ssNMR characterizations revealed that the enhanced low-concentration CO2 capture is attributed to the formation of a stable six-membered ring structure through the interaction of intramolecular hydrogen bonds between neighboring Zr–OH sites via a chemisorption mechanism.

In this study, we synthesized a series of controlled alkalietched MOF-808-X (X: 0.04-0.10) materials with enhanced lowconcentration CO 2 capture capacity under simulated air conditions compared to the pristine MOF-808. Among these materials, MOF-808-0.07 exhibited a CO 2 capture capacity of 0.21 mmol g −1 under simulated air conditions, which is 70 times higher than that of the pristine MOF-808. Additionally, MOF-808-0.07 displayed excellent stability over 50 cycles of adsorption and desorption. In situ DRIFTS and 13 C CP-MAS ssNMR analysis revealed that the increased low-concentration CO 2 capture capacity is attributed to the formation of a stable six-membered ring structure through the interaction of intramolecular hydrogen bonds between neighbouring Zr-OH sites in the micro-mesoporous environment of MOF-808-X.

Results and discussion
MOF-808 was synthesized according to the reported method. 40,41 And MOF-808-X (X: 0.04-0.10) series were prepared by various degrees of alkali etching of MOF-808 (Scheme 1). In Fig. 1a, the FT-IR analysis of these samples reveals that the infrared absorption peaks at 1630 and 1400-1600 cm −1 , which correspond to the stretching vibration peak of C]O and the benzene ring, respectively, display varying degrees of weakening. This suggests that the benzene ring in MOF-808 has undergone degradation to different extents. In Fig. 1b, the powder X-ray diffraction (PXRD) patterns of the as-synthesized MOF-808 are shown, which match well with the simulated PXRD pattern obtained from single crystal analysis. 40 However, the PXRD peaks of the MOF-808-X (X: 0.04-0.10) series gradually weaken with increasing etching degree, until all XRD diffraction peaks disappear. Scanning electron microscopy (SEM) images of the as-synthesized MOF-808 exhibit octahedral morphology ( X-ray photoelectron spectroscopy (XPS) analyses were conducted to investigate the electronic structure of MOF-808 and MOF-808-X (X: 0.04-0.07) series (Fig. 1c). In Fig. 1d, the C 1s high-resolution spectrum of MOF-808 and MOF-808-X series displays two distinct binding energy peaks at 284.8 and 288.5 eV, corresponding to the binding energy peaks of C-C and C-O. The O 1s high-resolution spectrum of MOF-808 in Fig. 1e shows a binding energy peak of C-O-Zr bond at 532.5 eV. However, a new binding energy peak appeared in the O 1s HR-XPS spectrum at 530.5 eV, which gradually increased with the increase of the alkali etching degree of MOF-808, and the new binding energy peak was attributed to the Zr-OH generated by alkali etching. Moreover, it is obvious from Fig. 1f that the binding energy peak of Zr 3d is shied towards a lower binding energy in MOF-808-X series compared to the pristine MOF-808. These results suggest that electron-donating groups exist on the Zr site. N 2 adsorption and desorption isotherms were employed to further characterize the pore structure and BET surface area of MOF-808 and MOF-808-X series. The isotherms of these materials exhibit a typical type I adsorption pattern (as shown in Fig. S2 †), indicating the presence of micro-mesoporous structure. The BET specic surface area of MOF-808 was found to be 1614 m 2 g −1 , whereas for MOF-808-X (X: 0.04-0.07) series, the BET specic surface area gradually decreases with the increase in the degree of etching and is found to be 300, 229, 225, 221, and 144 m 2 g −1 , respectively. This suggests that the BET surface area changes as the degree of etching increases due to the gradual collapse of the MOF-808 framework.
Due to the presence of numerous Zr-OH sites in the MOF-808-X series, we were prompted to investigate the CO 2 adsorption characteristics of these materials. As shown in Fig  CO 2 adsorption isotherms of the MOF-808-X series demonstrate improved CO 2 adsorption at low pressures compared to the pristine MOF-808. Particularly, the MOF-808-X series materials with Zr-OH sites exhibit a strong affinity for CO 2 at low concentrations, as evidenced by the steepness of the CO 2 adsorption isotherms and the attainment of a plateau at very low pressures. Further analysis of the CO 2 adsorption behaviour (Fig. 2b) within a low-pressure range of 400 ppm reveals that MOF-808-0.07 exhibits a high CO 2 uptake of 0.28 mmol g −1 , which is comparable to the values obtained for MOF-808-0.04 (0.01 mmol g −1 ), MOF-808-0.05 (0.08 mmol g −1 ), MOF-808-0.06 (0.16 mmol g −1 ), and the pristine MOF-808 (0.008 mmol g −1 ). This highlights the signicantly enhanced CO 2 uptake and the interactions between CO 2 and Zr-OH sites in the MOF-808-X series materials compared to the pristine MOF-808. Additionally, the MOF-808-X series exhibits excellent thermal stability up to 200°C (Fig. S3 †).
The dynamic CO 2 capture performance of MOF-808 and MOF-808-X series were assessed in a xed-bed reactor packed with a column of simulated ambient air (400 ppm CO 2 and argon as balance gas) under ow conditions (5 mL min −1 ) at 298 K. The detailed experimental procedure is provided in the ESI. † Fig. 2c depicts the short-term CO 2 breakthrough process of pristine MOF-808 in simulated dry air conditions (0 RH%), resulting in low CO 2 capture capacities of 0.003 mmol g −1 . In contrast, MOF-808-X (X: 0.04-0.10) series exhibited long-term dynamic CO 2 breakthrough processes with enhanced CO 2 capture capacity compared to the pristine MOF-808. The dynamic CO 2 capture capacity of MOF-808-X (X: 0.04-0.10) series under simulated dry air conditions were 0.06, 0.09, 0.13, 0.21, and 0.205 mmol g −1 , respectively. Notably, the MOF-808-0.07 demonstrated the highest CO 2 capture capacity, which is a 70-fold increase in CO 2 uptake capacity compared to the pristine MOF-808. Although the MOF-808-X series exhibited lower CO 2 capture capacity than the 13X zeolite (0.39 mmol g −1 ) under simulated dry air conditions, they demonstrated faster adsorption kinetics than 13X zeolite, as illustrated by the sharper breakthrough prole for MOF-808-0.07 compared to 13X (Fig. 2e). Additionally, Fig. 2d indicates that MOF-808-0.07 exhibited almost the same CO 2 breakthrough curves under simulated dry and humid air conditions (0 and 30 RH%). In contrast, the 13X zeolite in Fig. 2f exhibited signicantly reduced CO 2 capture capacity under humid air conditions (30 RH%), indicating that MOF-808 has higher moisture resistance. Moreover, Fig. 2g demonstrates that MOF-808-0.07 exhibited stable CO 2 capture performance with minimal losses aer 50 cycles (Fig. S4 †). In addition, the MOF-808-0.07 aer CO 2 capture was evaluated by FT-IR, PXRD, SEM, XPS and N 2 adsorption and desorption isotherms, all results show the structure integrity for MOF-808-0.07 in CO 2 capture processing ( Fig. S5-S9 †). The above results indicate that the MOF-808-0.07 has superior CO 2 adsorption-desorption stability.
In order to illustrate the low-concentration CO 2 adsorption process of MOFs containing Zr-SBUs at low concentrations, we synthesized a series of MOFs with different M-SBUs, including MIL-101-Fe with Fe 3 -SBU cluster, MIL-101-Cr with Cr 3 -SBU cluster, and MIL-125-Ti with Ti 4 -SBU cluster. Through controlled etching, as conrmed by PXRD analysis (Fig. S10 †), we obtained MOFs with varying degrees of etching. The dynamic CO 2 capture results revealed that all MOFs with varying degrees of etching exhibited a CO 2 capture process, but their capture capacities were not comparable to that of MOF-  808-X with controlled etching with Zr 6 -SBU cluster (Fig. S11 †). This suggests that MOFs with higher coordination numbers exhibit superior CO 2 capture abilities.
To investigate the desorption kinetics of MOF-808-0.07 in dry air conditions, we employed temperature programmed desorption (TPD) to evaluate its desorption energy. The activation energies of desorption for MOF-808-0.07 were calculated using the method proposed by Cvetanovic and Amenomiya, by measuring the TPD-CO 2 signal at different heating rates, as presented in Fig. 3a and b. 42 Our results demonstrate that MOF-808-0.07 exhibits a higher desorption energy (56.51 kJ mol −1 ) than 13X zeolite (48.14 kJ mol −1 (ref. 42)) under simulated dry air conditions (Table S1 †), indicating that CO 2 adsorption by MOF-808-0.07 occurs through chemical adsorption.
In order to verify the adsorbed species in CO 2 capture for MOF-808-0.07, the in situ diffuse reectance infrared Fourier transform spectroscopy (in situ DRIFTS) of MOF-808-0.07 with adsorbing CO 2 in simulated dry air (MOF-808-0.07-CO 2 ) was carried out. Fig. 4a shows two distinct infrared absorption peaks at 1685 and 3000-3600 cm −1 in the in situ DRIFTS spectra of MOF-808-0.07-CO 2 aer heat treatment (140°C), corresponding to the stretching vibration peak of C]O (-OCO 2 H), and -OH (M-OH with broad peak and hydrogenbonding), respectively. The results display that the CO 2 adsorption within the MOF-808-0.07 framework is in the form of bicarbonate species and hydrogen bonding interactions under dry conditions. Furthermore, the heat-treated MOF-808-0.07 is subjected to in situ CO 2 adsorption again in dry conditions, the in situ DRIFTS spectra in Fig. 4b show obvious infrared absorption peaks in 1685 and 3000-3500 cm −1 corresponding to the stretching vibration peak of C]O (-OCO 2 H) and hydrogen-bonding. Aer heat treatment again, the infrared absorption peak of C]O and hydrogen-bonding gradually disappeared again (Fig. 4c), demonstration of the breaking of hydrogen bonding and the successful complete desorption of CO 2 . Further elucidating the adsorptiondesorption stability, the second in situ CO 2 adsorption also showed that the C]O and hydrogen-bonding infrared absorption peak gradually strengthens with various adsorption time (Fig. 4d). As a comparison, the control experiments of pristine MOF-808-CO 2 shows no obvious infrared absorption peak for CO 2 desorption at 140°C. And the heat-treated MOF-808 is subjected to in situ CO 2 adsorption again in dry conditions along with various time, the in situ DRIFTS spectra show no obvious change in infrared absorption peaks (Fig. S12 †). The results show that parent MOF-808 does not have low concentration CO 2 adsorption capacity. Based on the above in situ DRIFTS results, showing that the alkali etched MOF-808-0.07 has enhanced low concentration CO 2 capture capacity compared to parent MOF-808 under dry air conditions due to the presence of Zr-OH adsorption sites.
To elucidate the formation of -OCO 2 H species under dry conditions, solid-state cross-polarization magic-angle spinning (CP-MAS) 13 C NMR experiments were conducted on variant MOF-808-0.07 to investigate the change in chemical species before and aer capturing 13 CO 2 (isotopic gas).   results with the in situ DRIFTS data, it can be inferred that these shis are attributed to -OCO 2 H groups and -OCO 2 H groups involved in intramolecular hydrogen bonding, respectively.
Based on the above in situ DRIFTS and 13 C CP-MAS ssNMR characterizations, we proposed a possible mechanistic of lowconcentration CO 2 capture process in MOF-808 series. (1) When the two Zr-OH sites within the MOF-808-X framework are distanced apart, each Zr-OH site can adsorb one CO 2 molecule, forming Zr-O 2 COH species (Fig. 6a). (2) When the neighbouring Zr-OH sites within the MOF-808-X framework are in close proximity. As shown in Fig. 6b, rst, a Zr-OH site adsorbs a CO 2 molecule to form a Zr-O 2 COH species, and the Zr-O 2 COH species forms intramolecular hydrogen bonding with the neighbouring Zr-OH site. Subsequently, the neighbouring Zr-OH re-adsorbs a CO 2 molecule with it to form two opposing Zr-O 2 COH species, which interact to form a stable six-membered ring structure through the interaction of intramolecular hydrogen bonding to complete an adsorption process.

Conclusions
In conclusion, we have demonstrated that controlled alkali etching of MOF-808 leads to the formation of MOF-808-X series, which exhibit signicantly enhanced low-concentration CO 2 capture compared to the pristine MOF-808 under dry air conditions. Among the MOF-808-X series, MOF-808-0.07 displays the highest CO 2 capture capacity of 0.21 mmol g −1 in simulated dry air conditions, which is 70 times higher than the pristine MOF-808. The desorption kinetics of the MOF-808-0.07 also show higher desorption energy compared to the commonly used 13X zeolite. Our control experiments suggest that MOFs with high coordination numbers show higher CO 2 capture performance under dry air conditions. Furthermore, in situ DRIFTS and 13 C CP-MAS ssNMR results indicate that the enhanced low-concentration CO 2 capture is due to the formation of a stable six-membered ring structure through intramolecular hydrogen bonds between Zr-OH sites of neighbouring micro-mesoporous environments of MOF-808-X. Overall, these ndings suggest the potential of MOF-808-X series as promising materials for low-concentration CO 2 capture.

Data availability
The authors declare that all data supporting the ndings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest
There are no conicts to declare.