Exceptionally High Perfluorooctanoic Acid Uptake in Water by a Zirconium-Based Metal–Organic Framework through Synergistic Chemical and Physical Adsorption

Perfluorooctanoic acid (PFOA) is an environmental contaminant ubiquitous in water resources, which as a xenobiotic and carcinogenic agent, severely endangers human health. The development of techniques for its efficient removal is therefore highly sought after. Herein, we demonstrate an unprecedented zirconium-based MOF (PCN-999) possessing Zr6 and biformate-bridged (Zr6)2 clusters simultaneously, which exhibits an exceptional PFOA uptake of 1089 mg/g (2.63 mmol/g), representing a ca. 50% increase over the previous record for MOFs. Single-crystal X-ray diffraction studies and computational analysis revealed that the (Zr6)2 clusters offer additional open coordination sites for hosting PFOA. The coordinated PFOAs further enhance the interaction between coordinated and free PFOAs for physical adsorption, boosting the adsorption capacity to an unparalleled high standard. Our findings represent a major step forward in the fundamental understanding of the MOF-based PFOA removal mechanism, paving the way toward the rational design of next-generation adsorbents for per- and polyfluoroalkyl substance (PFAS) removal.


Section 1. General methods and instruments
Nuclear Magnetic Resonance (NMR) spectroscopy. 1H NMR, 13 C NMR, and 19 F NMR spectra were obtained on a AVANCE NEO 400 spectrometer.

Thermogravimetric analysis (TGA)
TGA measurement was conducted on a Mettler-Toledo TGA/DSC 1 under nitrogen (N 2 ) atmosphere with a ramp rate of 5 °C/min from room temperature and 800 °C.

Scanning electron microscopy (SEM)
SEM was carried out using a FEI QUANTA 600 FE-SEM scanning electron microscope.The samples were dispersed over the slices of silicon wafer adhered to flat copper platform sample holders and then coated with gold using a sputter coater (ambient temperature, 85 torr pressure in a nitrogen atmosphere, puttered for 30s from a solid gold target at a current at 30 mA) before being submitted to SEM characterization.
Nitrogen (N 2 ) sorption measurement.N 2 adsorption-desorption measurement was performed on a Micromeritics ASAP 2020 system.Prior to the measurement, the as-synthesized PCN-999 sample was washed with N,N-dimethylformamide (DMF) to remove the unreacted starting materials, followed by the exchange with acetone for several times to remove the non-volatile DMF.The resulting sample was then activated under vacuum at 80 °C for 12 h.The N 2 adsorption-desorption isotherm was then measured at 77 K, from which the specific surface area was generated using the Brunauer-Emmett-Teller (BET) model.

Single-crystal X-ray Crystallography.
The single crystals of PCN-999 before and after perfluorooctanoic acid (PFOA) loading were directly transferred from the mother liquid to the oil, and then mounted onto a loop for single crystal X-ray diffraction measurements (SCXRD).The data were collected on a Bruker D8-Venture diffractometers equipped with Cu microfocus tubes (λ = 1.54178Å) and low temperature device.The single crystal structures were solved and refined using Olex2 software. [1]Both structures were solved by the direct method using the SHELXT program and refined by full-matrix least-squares method with SHELXL package. [2]All non-hydrogen atoms were refined with anisotropic displacement parameters, and the hydrogen atoms were positioned by geometrical calculation and then refined by riding.The free solvent molecules are highly disordered in MOFs and attempts to locate and refine the solvent peaks were unsuccessful.The diffused electron densities resulting from these solvent molecules were removed using the solvent MASK.Crystal data are summarized in Table S1 and the single crystal structures can be obtained free of charge from The Cambridge Crystallographic Data Centre with the CCDC number of S4 Calculation method.
The cp2k combined GFN1-xtb method was applied to optimize the model structures and calculate the binding energy (E binding ) using the equation of E binding = E MOF+PFOA (E MOF + E PFOA ), where E MOF+PFOA is the total energy of the MOF model compound binding with PFOA, E PFOA is the total energy of PFOA, and E MOF is the sum of the total energy of the MOF model compound.The more negative the value of E binding , the stronger the PFOA adsorption.Simulated PFOA adsorption isotherm was generated using the sorption module in Materials Studio 2020 software.

PFASs adsorption from aqueous solutions.
Prior to the PFASs adsorption experiments, the as-synthesized PCN-999 sample was washed with DMF and acetone several times, followed by vacuuming at 80 o C overnight.PFASs adsorption experiment was performed in aqueous solutions with a ratio of m/V = 1 (1 mg of MOF in 1 mL of solution).After shaking for certain time at room temperature, the supernatant was decanted, and the PFAS-loaded MOF (PFAS@PCN-999) sample was washed with water and dried.Subsequently, the PFAS@PCN-999 samples were fully digested using a mixture of DMSO-d 6 and deuterated sulfuric acid (50/3, v/v) by heating at 80 o C for 6 h.4-Trifluoromethyl benzoic acid was used as the internal standard for the quantification of adsorbed PFAS inside the MOF by integrating the area of the PFAS signal and the internal standard signal, respectively.

PFOA removal efficiency experiment.
PFOA removal efficiency experiment was performed by adding PCN-999 (3 mg) into the aqueous PFOA solution (1000 ppm, 0.6 mL, deuterium oxide).After shaking for 1 day at room temperature, the supernatant was detected by 19 F NMR using trifluoroethanol as the internal standard.

MOF regeneration and recycling experiment.
PCN-999 was regenerated from PFOA@PCN-999 samples by immersing in methanol for ~30 h at room temperature.The recycling experiment was conducted by repeating the PFOA adsorption and desorption experiments successively.

Selective adsorption of PFOA over different ionic species.
Selective PFOA adsorption experiment was performed in aqueous solutions with a ratio of m/V = 1 (1 mg of MOF in 1 mL of solution).The concentration of different species in the aqueous PFOA solutions is 250 ppm.After shaking for 3 days at room temperature, the supernatant was decanted, and the guestloaded PCN-999 sample was washed with water and dried.Subsequently, the guest-loaded PCN-999 samples were fully digested using a mixture of DMSO-d 6 and deuterated sulfuric acid (50/3, v/v) by heating at 80 o C for 6 h.4-Trifluoromethyl benzoic acid was used as the internal standard for the quantification of adsorbed PFOA inside the MOF. a 500-mL Schlenk flask charged with a stir bar.The flask was pumped under vacuum and refilled with N 2 three times before 250 mL degassed 1,4-dioxane was transferred to the system.The reaction mixture was refluxed for 72 h under a N 2 atmosphere.After the reaction mixture cooled to room temperature, the organic solvent was removed using a rotary evaporator, and the resulting mixture was poured into water and extracted with dichloromethane (3 × 100 mL).The combined organic layers were dried over anhydrous MgSO 4 , and then the solvent was removed under reduced pressure.After purification by column chromatography on silica gel using dichloromethane/hexane as eluent, compound 2 was obtained as yellow solid (2.09 g, yield: 56 %).

Found: 659. 1713 .Section 3 .
Figure S1.Design of the desymmetrized ligand L12 through the derivation from the D 2 h-symmetric ligand (H 4 ETTC) by changing the length of its two arms.

Figure S5 .
Figure S5.(a) The connection between L12 and two kinds of SBUs, Zr 6 and (Zr 6 ) 2 , to form PCN-999 network.(b) The alternative arrangement of (Zr 6 ) 2 and Zr 6 A SBUs, and the zig-zag layout of Zr 6 B clusters.(c) The scu topology of PCN-999.C, H, O, and Zr atoms are represented by gray, white, red, and cyan, respectively.

Figure S10. 19 F
Figure S10. 19F NMR spectrum of the digested PCN-999 after immersing in 1000 ppm PFOA solution for 3 days.Noted: 4-(Trifluoromethyl)benzoic acid was used as an internal standard.

Figure S11. 19 F
Figure S11. 19F NMR spectra of the digested PCN-999 samples after immersing in 1000 ppm PFOA solution for different time.Noted: 4-(Trifluoromethyl)benzoic acid was used as an internal standard.

Figure S12 .
Figure S12.Sorption kinetics of PFOA into PCN-999 with an initial concentration of 1000 ppm, fitted with a pseudo-second-order model.

Figure S13. 19 F
Figure S13. 19F NMR spectra of the digested PCN-999 samples after immersing in different PFOA solutions for 3 days.Noted: 4-(Trifluoromethyl)benzoic acid was used as an internal standard.

Figure S14 .Figure S15 .
Figure S14.Equilibrium PFOA adsorption capacity of PCN-999 as a function of equilibrium PFOA concentration (C e ) fitted with (a) Langmuir model and (b) Freundlich model.

Figure S16. 19 F
Figure S16. 19F NMR spectra of the digested PCN-999 samples after immersing in 5000 ppm PFOA solution for different cycles.Noted: 4-(Trifluoromethyl)benzoic acid was used as an internal standard.

Figure S18. 19 F
Figure S18. 19F NMR spectra of the digested PCN-999 samples after immersing in different PFOA solutions with the presence of different ions for 3 days.Noted: 4-(Trifluoromethyl)benzoic acid was used as an internal standard.

Figure S19. 19 F
Figure S19. 19F NMR spectra of the digested PCN-999 samples after immersing in different PFAS solutions with an initial concentration of 1000 ppm.Noted: 4-(Trifluoromethyl)benzoic acid was used as an internal standard.

Figure S25 .
Figure S25.Molecular electrostatic potential (MESP) distributions of (a) (Zr 6 ) 2 and (b) Zr 6 SBUs.Blue region represents the positive MESP value (e.g., nucleophilic centers), while the red region stands for the negative MESP value (e.g., electrophilic center).Protons in the structures are removed for clarity.

Figure S26 .
Figure S26.Molecular orbital calculation of (a) (Zr 6 ) 2 and (b) Zr 6 SBUs.Protons in the structures are removed for clarity.

Figure S27 .
Figure S27.Simulated adsorption of more PFOA molecules through chemical bonding within PCN-999.

Table S2 .
PFOA adsorption capacity of different reported adsorbents.