Fluorinated Metal–Organic Framework–Polymer Mixed Matrix Membrane with Tunable Hydrophobic Channel for Efficient Pervaporation of Butanol/Water

The permeability–selectivity trade‐off of membrane is a major challenge limiting the development of pervaporation (PV) technology. Rational design of high‐performance mixed matrix membranes (MMMs) has the potential to break off the trade‐off. Herein, a solvent‐assisted linker exchange‐based strategy is reported to introduce fluoroalkyl into metal–organic framework‐808 (MOF‐808). The pore size of fluorinated MOF‐808 can be adjusted with fluoroalkyl of different chain length (like trifluoromethyl and pentafluoropropyl). Then, the fluorinated MOF‐808/polyether block amide (PEBA) MMMs are prepared for the PV of n‐butanol/water. Compared with pristine PEBA membrane, 20 wt% 3F‐MOF‐808(P)/PEBA MMMs (3F = ‐CF3 group; P = postmodification method) exhibit 69% increase in permeation flux and 33% increase in separation factor in the PV of 2.5 wt% n‐butanol aqueous solutions at 70 °C. Based on Grand Canonical Monte Carlo and molecular dynamics simulations, fluorinated MOF‐808 shows better butanol affinity (pull effect) and stronger water repulsion ability (push effect). And the “push–pull effect” between fluorinated MOF‐808 with butanol/water is helpful to enhance the PV performance of MMMs. The application of “push–pull effect” provides a new strategy for the rational design of high‐performance MMMs, which is of great significance for the in‐depth research and application of PV technology.


Introduction
Biobutanol has characteristics of high energy density, low vapor pressure, and high economic efficiency, which has been extensively used as fuel additives, chemical additives, and plasticizers. [1]Recovering butanol from low concentration of butanol aqueous solution has high economic benefits. [2]raditional recovery processes include adsorption, [3] extraction, and distillation. [4]However, these methods have the drawbacks of high energy consumption, complex equipment, and high cost.Therefore, developing a separation strategy with low energy consumption, low cost, and high efficiency is of great significance in response to carbon peaking and carbon neutrality policies.Pervaporation (PV) has the advantages of mild operating conditions, low energy consumption, and no pollution, and has become an important means of industrial recovery of bio butanol. [5]Currently, polymer membranes are commonly used in PV process, such as polydimethylsiloxane, polyether block amide (PEBA), polyethylene. [6]Generally speaking, the "trade off" effect between membrane permeability and selectivity greatly limits the development and application of PV.Mixed matrix membranes (MMMs) combine the adsorption and diffusion capacity of polymers and the high selectivity of fillers, making it a good choice for efficient separation processes. [7]To overcome the trade-off effect in MMM, a reasonable design with good compatibility and defect free fillers is an effective strategy.
Metal-organic framework (MOF) is a type of porous material composed of self-assembled metal nodes and organic linkers, which has broad applications in adsorption, sensing, catalysis, separation, and other fields. [8]Compared with traditional inorganic fillers in MMM, MOF has the advantages of large specific surface area, high porosity, modifiability, and high compatibility with polymer.This allows MOFs to be incorporated into polymer membranes for gas separation and liquid purification processes. [9]However, the pore properties (such as size, adsorption sites, and functionalized modification nodes) and long-term stability of MOFs greatly determine their application. [10]Therefore, it is necessary to design MOFs with high stability and suitable pore size for specific system.Zirconium-based MOFs (Zr-MOFs), as a branch of MOFs family, have extensive membrane separation potential due to their high stability, large molecular transport channels, and flexibility. [11]UiO-66, [12] MOF-808, PCN-222 (Zr), and MOF-801 have been shown to significantly improve membranes separation performance. [13]The ordered channels and good dispersity of Zr-MOFs play a crucial role in separation process. [14]Liu et al. [15] prepared UiO-66 hollow fiber PV membrane for liquid purification.The fast molecular transport channel in UiO-66 is believed to be the reason for significant improvement in flux.Luo et al. [16] prepared a dense NU-906(Zr) membrane, which exhibited high permeation flux (1.41 kg m À2 h À1 ) and separation factor (2630) in the PV dehydration of n-butanol.This indicates that the pore size and high stability of Zr-MOFs are key to improving the performance of PV.In order to design and synthesize MOFs for MMM with high flux and selectivity, ligand exchange, functional group grafting, and metal anchoring methods are often used. [17]Common synthesis methods include the postmodification method and de novo synthesis method, with the former having milder reaction conditions than the latter. [18]For Zr-MOFs, the functionalized hydroxyl groups on Zr 6 cluster nodes provide a flexible postmodification platform.In order to obtain Zr-MOF-based membranes with high separation selectivity, various postmodification strategies have been developed, including atomic layer deposition, solvent-assisted ligand insertion, [19] and solvent-assisted linker exchange (SALE). [20]Luan et al. [21] introduced UiO-66-NH-(CH 2 ) 3 SO 3 H into the polybenzimidazole (PBI) matrix to prepare MMM for proton exchange.Their results indicate that the separation factor of the modified Zr-MOF/PBI membrane is increased by 4 times than that of pristine PBI membrane.Yang et al. [22] prepared functionalized UiO-66-CE C -based MMM for CO 2 /N 2 separation.The postmodified Zr-MOFs exhibit good dispersibility, compatibility, and stability in polymer membrane.Its selective adsorption performance is greatly improved.In addition, through postmodification strategies, hydrophobic/hydrophilic functional groups can be easily introduced into Zr-MOFs. [23]This may be a new method to adjust the structural properties and porthole effect of MMM based on Zr-MOFs.However, there is currently no relevant research to explore the PV process mechanism of Zr-MOF-based MMMs.
To better determine the characteristics of the crystal structure and stability of Zr-MOFs, X-ray diffraction (XRD) analysis was carried out (Figure 2a).The diffraction peaks of MOF-808, PM-MOF-808, and 3F-MOF-808(D) are consistent with the simulated diffraction peak.The results showed that after postmodification, the introduction of fluorinated groups did not destroy the crystal structure of MOF-808.To evaluate the stability of Zr-MOFs in PV process, PM-MOF-808 was immersed in 2.5 wt% nbutanol aqueous solutions for 2 months, and the XRD diffraction peak was still matched with the simulated diffraction peak, which demonstrated the excellent stability of PM-MOF-808.In addition, Fourier transform infrared spectroscopy (FT-IR) was used to analyze the groups' stretching vibration peaks in Zr-MOFs.As shown in Figure 2b, -COOH exhibits in-phase and out-phase stretching vibration peaks at 1560 and 1381 cm À1 .In addition, the C-H bending vibration peaks of Zr─O, O─H, and benzene rings in Zr 6 cluster are concentrated at 942, 756, 703, 654, and 573 cm À1 .New vibration peaks appeared at 1203 and 1156 cm À1 in 3F-MOF-808, corresponding to the -CF 3 group.It is worth noting that the new stretching vibration peaks appeared at 1328 and 1036 cm À1 in 5F-MOF-808 are different from the peaks of -CF 3 group, which are considered to be the characteristic peaks of C─F bonds in -CF 2 CF 3 group.Infrared spectroscopy (IR) characterization confirmed the successful synthesis of postmodified Zr-MOFs.
N 2 adsorption/desorption technology was used to determine the porosity of Zr-MOFs.As shown in Figure 2e, all Zr-MOFs show type I adsorption isotherm.The Brunauer-Emmett-Teller (BET) surface areas of MOF-808, 3F-MOF-808(P), 5F-MOF-808(P), and 3F-MOF-808(D) are 1450, 1225, 1078, and 651 m 2 g À1 , respectively.The pore volumes of them are 0.82, 0.75, 0.61, and 0.44 cm 3 g À1 , respectively.In addition, MOF-808 has large pores with 1.45 nm and small pores with 0.75 nm.After introducing fluorinated functional groups, the pores of 3F-MOF-808(P) decreased to 1.25 and 0.55 nm, while 5F-MOF-808(P) decreased to 1.21 and 0.53 nm.The disordered internal structure of 3F-MOF-808(D) leads to lower porosity and smaller pore size.The adsorption behavior of n-butanol and water in PM-MOF-808 was determined using gravity vacuum vapor sorption test (BSD-VVS, BSD Instrument, Beijing, China).The adsorption capacity of 3F-MOF-808(P) for the vapor of n-butanol and H 2 O at 30 °C could reach 413.95 and 560.74 mg g À1 , respectively, which was higher than that of 5F-MOF-808(P) (Figure S2a,b, Supporting Information).The reason was that the pore size of 3F-MOF-808(P) was higher than that of 5F-MOF-808(P).The curves of relative adsorption capacity of 3F-MOF-808(P) and 5F-MOF-808(P) for n-butanol and water over time are shown in Figure S2c,d, Supporting Information.For 3F-MOF-808(P), the relative adsorption capacity of butanol only reached 0.93 after 237 min, while the relative adsorption capacity of water could only reach 0.17 after 271 min.Adsorption of butanol and water on 5F-MOF-808 (P) exhibited the same characteristics.The results showed that PM-MOF-808 had good adsorption selectivity for butanol.Hydrophobically modified MOF-808 was suitable for the separation of n-butanol/water.X-ray photoelectron spectroscopy (XPS) analysis was performed on Zr-MOFs to further validate the successful introduction of fluorinated ligand.From the fine scan XPS spectrogram, it can be seen that PM-MOF-808 and 3F-MOF-808(D) exhibit clear F 1s peak (Figure 2f ).However, there is no F 1s characteristic peak on MOF-808 (Figure S3, Supporting Information).
Zr-MOFs/PEBA membranes were prepared through simple blending method.The thickness of all MMMs was controlled to approximately 30-35 μm.The surface morphology of the membrane was characterized by scanning electron microscope (SEM).Zr-MOFs had good dispersion in PEBA matrix even when its loading reached 20% (Figure 3a).As the loading rate increased from 0% to 20%, more Zr-MOFs in MMMs could be clearly observed in SEM images (Figure S4a-d, Supporting Information).Figure 3b showed a cross-sectional image of 20% 3F-MOF-808(P)/PEBA with a thickness of 32 μm.The adsorption-diffusion mechanism of the PV process is crucial for improving the selectivity of butanol/water separation.As shown in Figure 3c, compared to the pristine PEBA membrane, the swelling degree of 20% Zr-MOFs/PEBA in 2.5 wt% n-butanol aqueous solutions significantly increased.The swelling degrees of 3F-MOF-808(P)/PEBA and 5F-MOF-808(P) were 21.5% and 18.8%, respectively, indicating the preferential adsorption for n-butanol of PM-MOF-808/PEBA membrane.The crystal models of Zr-MOFs were constructed in Material Studio 2020 (MS).The free volume of n-butanol (Connolly radius = 2.53 Å) in the optimized structure of Zr-MOFs was calculated (Figure 3e-g).The free volume of 5F-MOF-808 was higher than that of 3F-MOF-808 and MOF-808, which indicated that the adsorption capacity of postmodification modified MOF-808 on n-butanol was increased.It was beneficial to improve the selectivity of butanol/water separation.In addition, the swelling degree of 3F-MOF-808(P)/PEBA increased as the amount of loading added from 0% to 20% (4.73% to 21.46%) (Figure S4e, Supporting Information).The increased loading quantity of Zr-MOFs is beneficial for the adsorption and diffusion of butanol within MMMs.The water contact angle tests were measured to determine the hydrophobicity of Zr-MOFs/PEBA (Figure 3d).When water droplets come into contact with the surface of the pristine PEBA membrane, the contact angle of water was 91.9°, which was attributed to the hydrophobicity of PEBA.When 20% MOF-808 particles were loaded into the PEBA matrix, the water contact angle increased to 99.3°, indicating that the loaded MOF-808 particles had an impact on the roughness and tension of the membrane surface.With the increase of 3F-MOF-808(P) in PEBA, the contact angle of MMMs further increased to 101.9°, which was attributed to the hydrophobic groups in 3F-MOF-808 (Figure S4f, Supporting Information).The strong electron withdrawing effect of F atom reduced the polarity and surface energy.This would enhance the hydrophobicity of MMMs.Moreover, 20% 5F-MOF-808(P)/PEBA had lower surface energy, which further increased the contact angle of the membrane.In addition, TGA curves showed that all Zr-MOFs/PEBA MMMs could maintain their structural stability at 240 °C, confirming their good thermal stability (Figure S5, Supporting Information).
The butanol/water separation performance of Zr-MOFs/ PEBA MMMs was evaluated by a PV setup (Figure S6, Supporting Information).2.5 wt% n-butanol aqueous solution was used as the feed solution in all PV processes.As shown in Figure 4a-b, the permeation flux of all of the 20% Zr-MOFs/PEBA membranes was gradually enhanced with temperature.The flux of MOF-808/PEBA MMMs increased by 86.3% than pristine PEBA membrane, but the separation factor almost unchanged.The increased permeability could be attributed to the highly porous structure of the loaded MOF-808 nanoparticles.The fast transport channel of MOF-808 was beneficial for reducing the mass transfer resistance within the PEBA membrane.The flux of 3F-MOF-808(P)/PEBA MMMs slightly decreased compared to MOF-808/PEBA MMMs, while the separation factor increased by 33%.The introduction of -CF 3 groups divide the pores of 3F-MOF-808(P) into hydrophobic channel with approximately 6 and 10 Å.The pore size of 5F-MOF-808(P) became smaller (about 6 Å).The "push-pull effect" of hydrophobic channels in PM-MOF-808 and the molecular sieving effects induced by them are beneficial for improving the separation selectivity of MMMs.Notably, the permeation flux of 20% 5F-MOF-808(P)/PEBA at 70 °C is only 1089 g m 2 h À1 .The reason might be that introduction of -CF 2 CF 3 groups blocked some pores of MOF-808, thus limiting the transport passage of the liquid.The Arrhenius equation was commonly used to describe the relationship between permeation flux and temperature.The apparent activation energies of butanol and water in different MMMs were calculated based on the fitting results (Figure 4c).It is clear that the apparent activation energy of butanol is greater than that of water in all MMMs, indicating that the n-butanol in MMMs is more sensitive to feed temperature.Among them, the n-butanol in 3F-MOF-808(P)/PEBA MMMs has the highest apparent activation energy (37.67 kJ mol À1 ) (Figure 4g).It is shown that 3F-MOF-808(P)/PEBA MMMs have better selectivity for butanol.Therefore, 3F-MOF-808(P)/PEBA MMMs were selected to study the effect of MOF loading amount on the permeation performance of MMMs.As shown in Figure 4d,e, with the increase of temperature, the PV performance of MMMs with different loading significantly increased.For example, in contrast to the flux and separation factor of the pristine PEBA membrane, the values of 20% 3F-MOF-808/PEBA MMMs were significantly higher (2556.34g m 2 h À1 and 28.5).The "push-pull effect" of fluorinated pores can effectively prevent the transport of water molecules, facilitate the adsorption of butanol molecules, and exhibit enhanced separation performance.The apparent activation energies of butanol and water in MMMs with different MOF loading amounts were calculated based on the fitting results of the Arrhenius equation (Figure 4f ).
As the 3F-MOF-808(P) loading increased, the activation energy of butanol increased from 30.57 to 36.67 kJ mol À1 and that of water increased from 16.55 to 24.76 kJ mol À1 .It is shown that the n-butanol in 3F-MOF-808 has a more sensitive response to temperature than water (Figure 4h).In addition, the increasing rate of water activation energy (49.6%) was greater than that of butanol (20%), demonstrating the slower diffusion of water molecules.Figure 4i shows that the flux of 20% 3F-MOF-808(P)/ PEBA MMMs was significantly enhanced by the increasing of n-butanol concentration, but the separation factor increased first and then decreased.The increasing flux in MMMs is attributed to the coupling effect caused by the hydrogen bonds between n-butanol and water molecules.The reason of the separation factor trend with butanol concentration may be that the increase of n-butanol concentration is beneficial for the adsorption on the surface of MMMs, but not good for the internal diffusion.When the n-butanol concentration was 2 wt%, the membrane showed the highest separation factor.The effect of the loading amounts of fluoro groups in PM-MOF-808/PEBA MMMs was also investigated.As shown in Figure S8, Supporting Information, IR characterization confirmed the successful synthesis of 3F-MOF-808(P)-X and 5F-MOF-808(P)-X.As the TFA content increases, the vibration peaks of -CF 3 groups in 3F-MOF-808(P) was gradually increasing (Figure S8a, Supporting Information).Similarly, as the PFPA content increases, the vibration peaks of -CF 2 CF 3 groups in 5F-MOF-808(P) was also strengthen (Figure S8b, Supporting Information).When the addition amount of TFA was greater than 6.0 mmol or the addition amount of PFPA was more than 17.5 mmol, the content of fluoro groups was not further increased significantly, indicating the completely introduction of fluoro groups.Subsequently, the stability of 3F-MOF-808(P)-X and 5F-MOF-808(P)-X was also investigated.XRD spectra indicated their excellent chemical stability (Figure S9, Supporting Information).With the aim of researching the n-butanol/water separation performances of PM-MOF-808(P)/ PEBA MMMs with different amount of fluoro groups, all MMMs were prepared with a 20 wt% PM-MOF-808-X loading.The PV temperature was set to 60 °C.Figure S10a, Supporting Information, shows the impact of TFA addition on the performance of 3F-MOF-808(P)/PEBA MMMs.When the amount of TFA increased from 1.5 to 7.5 mmol, the permeation flux of MMMs gradually decreased, while the separation factor increased.It could be considered that the increasing amounts of fluoro groups reduced the effective pore size of MOF-808, resulting in a decrease of free volume.However, the inhibitory effect of the -CF 3 groups to water molecules was beneficial for enhancing the selectivity for n-butanol molecular.Figure S10b, Supporting Information, indicates the impact of PFPA addition on the performance of 5F-MOF-808(P)/PEBA MMMs.When the addition amount of PFPA increased from 3.5 to 17.5 mmol, MMMs also showed reduced permeation flux and increased separation factor.The introduction of the -CF 2 CF 3 groups blocked the pores of MOF-808, thereby limiting the transport passage of liquid components.However, the enhancement of pore hydrophobicity was considered the main reason for the improvement of separation selectivity.
The crystallinity and internal defects of Zr-MOFs have been proven to be key factors affecting the PV performance of MMMs.As shown in Figure 2, XRD, IR, and XPS results confirmed the successful preparation of 3F-MOF-808(D).It can be observed from the SEM images that 3F-MOF-808(D) had no more classic octahedral crystal structure.The reaction environment of MOF-808 was changed by the introduction of trifluoroacetic acid in the de novo synthesis, resulting in decreased porosity and disordered channels (Figure 5a).In contrast, 3F-MOF-808(P) derived by the SALE method had better crystallinity, less defects, and more ordered pores than 3F-MOF-808(D).In order to investigate the influence of crystallinity and pore size of 3F-MOF-808 on the separation performance of MMMs, 3F-MOF-808(P)/PEBA and 3F-MOF-808 (D)/PEBA were used to test the PV of 2.5 wt % butanol/water.As shown in Figure 5b, the permeation flux of 20% 3F-MOF-808(P)/PEBA was 144% higher than that of 20% 3F-MOF-808(D)/PEBA (70 °C), while the separation factor of these two membranes was close (Figure 5c).This indicted that the ordered stacking pores in 3F-MOF-808 directly affected the permeation flux and the hydrophobic functional groups in the pores played a key role in molecular sieving.As shown in Figure 5d, the E a of butanol and water in 3F-MOF-808(P)/PEBA MMMs were 37.67 and 24.76 kJ mol À1 , respectively, which were higher than that of 3F-MOF-808(D)/PEBA MMMs (27.28 and 18.04 kJ mol À1 ).Therefore, MMMs with ordered pores exhibited better "push-pull effect" in PV process.
In order to evaluate the durability of the fluorinated MOF-808 MMMs, the 20% 3F-MOF-808(P)/PEBA MMMs was assigned for long-term PV process.During 60 h test at 70 °C, 20% 3F-MOF-808(P)/PEBA consistently maintained stable permeation flux and separation factor (Figure 5e).After the PV experiment, the SEM image of 20% 3F-MOF-808(P)/PEBA MMMs had no significant change compared with the one before separation test, which indicated that the introduction of 3F-MOF-808 could effectively enhance the thermal stability and mechanical strength of MMMs (Figure S11, Supporting Information).As shown in Figure 5f and Table S2, Supporting Information, the separation performance of 3F-MOF-808(P)/PEBA membrane in this work is better than that of the most literature-reported membranes.
The dissolution-diffusion mechanism is the dominant factor affecting the separation performance of PV.To deeply understand the adsorption behavior of n-butanol and water in Zr-MOFs, all crystal models were established using Zr 6 clusters linked by organic ligands.Subsequently, the adsorption sites of butanol/water in Zr-MOFs were calculated based on the Grand Canonical Monte Carlo method. [24]H 2 O molecule in MOF-808 forms an enhanced H•••O hydrogen bond (3 Å) with carboxyl oxygen atoms.However, there are weaker interaction forces between butanol and MOF-808, demonstrating the poor butanol affinity.In contrast, due to the presence of -CF 3 or -CF 2 CF 3 groups in PM-MOF-808, H 2 O cannot form hydrogen bonds with carboxyl oxygen atoms in PM-MOF-808, while hydroxyl groups in n-butanol molecules can form H•••O hydrogen bonds (%2.2 Å).This indicates that PM-MOF-808 has good affinity for butanol.The subject-object interaction force can be obtained through simulation calculations. [25]The interaction energy of MOF-808 for H 2 O and butanol is À13.4 and À37.51 kJ mol À1 , respectively (Figure 6a).The interaction energy of 3F-MOF-808 for n-butanol is stronger than that of MOF-808, while 5F-MOF-808 has the highest interaction energy for n-butanol (À44.3 kJ mol À1 ).The theoretical calculation results demonstrated that PM-MOF-808 had the better butanol affinity.As shown in Figure S12a-c, Supporting Information, the density functional theory simulation indicated that the separation factors of all Zr-MOFs for butanol/water separation were consistent with the experimental results.The total adsorption capacity of 5F-MOF-808 significantly decreased due to the blockage of the pores by the -CF 2 CF 3 functional group.To investigate the diffusion behavior of butanol and water in Zr-MOFs, all MS models were established based on the adsorption results (Figure S12d,e, Supporting Information).The molecular dynamics (MD) simulation was used to explore the self-diffusion coefficients of n-butanol/water in Zr-MOFs. [26]Based on the equation and the mean squared displacement results (Figure S12f,g, Supporting Information), the self-diffusion coefficient of butanol in PM-MOF-808 was significantly higher than that of MOF-808 (Table S1, Supporting Information).This indicates that the butanol molecules diffuse in PM-MOF-808 more quickly than in MOF-808.The selfdiffusion coefficient of water in PM-MOF-808 decreased compared with in MOF-808, indicating that the hydrophobic functional groups are not conducive to the diffusion of water molecules.Finally, MD simulation revealed that small molecules in 2.5 wt% butanol/water solution could rapidly diffuse in Zr-MOFs within 10 ps (Figure S13, Supporting Information).The simulation results provide guidance for the design and synthesis of high-performance Zr-MOF-based MMMs, and provide strong evidence for explaining the molecular diffusion behavior in Zr-MOFs.

Conclusion
In conclusion, we proposed a strategy for designing highperformance fluorinated MOF-808/PEBA MMMs for PV separation n-butanol aqueous solution based on the "push-pull effect".Specifically, the fluorine atoms and carboxyl atoms of the Zr 6 cluster in fluorinated MOF-808 are prone to form hydrogen bonding interaction with the hydroxyl hydrogen atoms in n-butanol, exhibiting a pull effect on n-butanol molecules and a push effect on water molecules.Characterization results indicated that PM-MOF-808 has better crystallinity and more ordered internal channels than that of 3F-MOF-808(D).Subsequently, the Zr-MOFs/PEBA MMMs were successfully prepared for PV separation of butanol/water mixture.Due to the ordered hydrophobic channels and better butanol affinity of 3F-MOF-808(P), the PV permeation flux and separation factor of 20% 3F-MOF-808(P)/PEBA MMMs increased by 69% and 33% than that of pristine PEBA membrane, respectively.Molecular simulations showed that PM-MOF-808 has enhanced hydrogen bonding interaction force with butanol molecules, while the adsorption capacity for water molecules was significantly weakened.The rational design of functionalized MOFs based on the "push-pull effect" is conducive to prepare high-performance PV MMMs.
structures and similar particle size distributions (1-1.2 μm), indicating that the original crystal structure of MOF-808 was not damaged in the postmodification process.It is worth noting that the 3F-MOF-808(D) prepared by the de novo synthesis method showed an irregular block form and could not keep good crystallinity.Therefore, PM-MOF-808 will inherit the ordered pores, excellent dispersibility, and high stability of MOF-808, which can effectively promote molecular transport in Zr-MOFbased MMMs.In addition, the energy-dispersive X-ray spectroscopy (EDX) element mapping images of PM-MOF-808 and 3F-MOF-808(D) showed a uniform distribution of F elements (Figure2d, S1, Supporting Information).The fluorine content