Effect of Missing-Linker Defects and Ion Exchange on Stability and Proton Conduction of a Sulfonated Layered Zr-MOF

Intentionally introduced defects into solid materials create opportunities to control and tune their diverse physicochemical properties. Despite the growing interest in defect-engineered metal–organic frameworks (MOFs), there are still only a handful of studies on defective proton-conducting MOFs, including no reports on two-dimensional ones. Ion-conducting materials are fundamentally of great importance to the development of energy storage and conversion devices, including fuel cells and batteries. In this work, we demonstrate the introduction of missing-linker defects into a sulfonated proton conductive 2D zirconium-based MOF (JUK-14), using a facile post-synthetic approach and compare the stability and performance of the resulting materials, including proton conductivity, as well as adsorption of N2, CO2, and H2O molecules. We also discuss the associated presence of interlayer counterions and their effect on the properties and stability. Our approach to defect engineering can be extended to other layered MOFs and used for tuning their activity.


Details of physical measurements
Carbon, hydrogen and nitrogen were determined using an Elementar Vario MICRO Cube elemental analyzer.
Thermogravimetric analyses (TGA) were performed on a Mettler-Toledo TGA/SDTA 851e instrument with a heating rate of 10°C min -1 in a temperature range of 25 -900 °C (approx. sample weight of 10 mg). The measurements were performed at atmospheric pressure under air flow.
FTIR spectra were recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrophotometer equipped with an iD7 diamond ATR attachment.
Potentiometric titration was performed using a DMS Titrando 888 automatic titrator (Metrohm) set at the mode to collect the equilibrium pH. The initial samples (~ 50 mg) were ground and dispersed in NaNO3 (50 mL, 0.01 M), equilibrated overnight and continuously saturated with N2. Before the measurement, samples were acidified with HCl (0.10 M) to pH ~3.0 and then titrated with NaOH (0.10 M) to a pH of 11. The injection rate was 0.01 mL min -1 . Equivalence points were obtained as maxima on the first derivative curve of the collected data (the titration curve of pH as a function of volume of titrant added). The pKa values were determined as the pH in half of the titrant volume added to reach the equivalence point. 6 The derivative curves were smoothed using the Savitzky-Golay method with 60 points of window for JUK-14_MeOH and JUK-14_H2O and 180 points of window for JUK-14_HCl (OriginPro 2023 software).
UV-Vis spectra were recorded on Shimadzu UV-1800 UV spectrophotometer in the wavelength range 200 -800 nm with a 0.05 nm step. The measurements were carried out on 1.25•10 -5 M solutions of H2dsoa-2H in various concentrations of aqueous solutions of hydrochloric acid (from 0.001 to 12 M). 1 H NMR spectra were recorded on a Jeol 400 spectrometer. The chemical shifts (δ) are reported in parts per million (ppm) on a scale downfield from tetramethylsilane. The NMR spectra were referenced internally to the residual proton resonance in DMSO-d6 (δ 2.50 ppm).
Nitrogen (77 K), carbon dioxide (195 K) and water (293 K) adsorption/desorption studies were performed on an Autosorb iQ-C-XR-XR EPDM volumetric analyzer (Quantachrome Instruments). Prior to the physisorption measurements, samples were activated at 25 or 100 °C (specified for a given measurement in manuscript) under vacuum for 1 h. A temperature of 77 K was achieved by a liquid nitrogen bath and 195 K was achieved by dry ice/acetone bath Electrochemical Impedance Spectroscopy (EIS) measurements were performed using Hioki IM3570 impedance analyzer on pre-conditioned samples (see Samples preparation for EIS Measurement below) of a thickness of 0.017 -0.032 cm, pressed between two platinum electrodes (8.0 mm diameter with a pressure of 9 MPa) in a PTFE tube of a Fine Instruments PIP-260 probe (Elektronika Jądrowa Kraków, Poland), and kept in KK 115 TOP+ climatic chamber with an ultrasonic humidifier and a temperature control (the measurements with humidity control) or in a vacuum dryer Memmert, VO 200 (the measurement at the temperatures above 100 o C). The measurements were carried out in a quasi-four-probe setup in a frequency range from 4 Hz to 5 MHz, potential of 100 mV, alternating current and temperature range from 25 to 60 o C (for the measurements in 30 -90% RH) or from 100 to 160 o C (for the anhydrous measurement), and with pre-measurement sample conditioning for 1 h at each temperature. The proton conductivity (σ, S cm −1 ) of each sample was estimated by using the equation: where L (cm) is the sample thickness and A (cm 2 ) is the cross-sectional area of the measured pellet; R (Ω) is the resistance as the real part of the measured impedance.
IR measurements during the pyridine adsorption were carried out with a Bruker Vertex 70 spectrometer. The MOFs were dispersed on silicon wafer (1 mg×cm -2 ), which were placed in a quartz cell equipped with the CaF2 windows. Prior to the experiments, the samples were degassed at 50 o C for 60 min under vacuum (10 -5 bar) and cooled down to room temperature. Then upon admission of 10 Torr of pyridine (Py) the materials were heated gradually to 80 o C (with a rate of 1 o C/ min) and the spectra were collected. The spectra presented in the paper are difference spectra (from the spectrum after Py adsorption, the spectrum of the material before probe adsorption was subtracted). The spectra were normalized to the same mass of the sample.

Synthetic procedures
All chemicals and solvents (of analytical grade) were purchased from commercial sources (Merck, Avantor, Polmos) and were used without further purification. Ethanol (Polmos) contained water 8% by volume.
Synthesis of H2dsoa-2Cl precursor 4,4'-Oxybis [3-(chlorosulfonyl)benzoic] acid (H2dsoa-2Cl, Figure S2) was prepared according to the published method. 7 In the round bottom flask, the mixture of 4,4'-oxydibenzoic acid (5.00 g, 0.019 mol) and HSO3Cl (50.0 ml, 0.760 mol) was heated at 120 o C under reflux for 4 h. After that, the dark solution was poured on ice to destroy the excess of chlorosulfonic acid. The white precipitate formed was filtered off, washed with cold water (0 o C) until neutral pH was reached, and dried in a desiccator over CaCl2 overnight. The identity of H2dsoa-2Cl was verified as in reference. 8 Synthesis of H2dsoa-2H precursor 4,4'-Oxybis[3-(sulfonyl)benzoic] acid (H2dsoa-2H, Figure S2) was prepared was prepared according to the published method. 8 To the saturated Na2SO3 solution in H2O (5.0 mL) H2dsoa-2Cl (0.32 g, 0.70 mmol) acid was added. Then the 32% NaOH solution was added dropwise to reach pH of ca. 10 and the resulting solution was stirred overnight at RT. After that, the solution was placed in an ice bath and acidified with 25% HCl to reach pH of 1. The product formed was filtered off, washed with cold water and dried at 60 o C. The identity of H2dsoa-2H was verified as in reference. 8 Synthesis of JUK-14 JUK-14 was prepared according to a slightly modified (upscaled) published method. 8 ZrCl4 (274 mg, 1.18 mmol), H2dsoa-2Cl (683 mg, 1.50 mmol) were dissolved in DMF (25.0 mL) and CH3COOH (15.5 mL, 320 mmol), placed in a 100 ml glass bottle, sonicated (30 s) and heated at 120 C for 14 days. Colorless crystals of JUK-14 were filtered off, washed with DMF and dried in air at room temperature. The identity of JUK-14 was verified as in reference (IR, PXRD, EA). 8 Synthesis of JUK-14_MeOH 150 mg of JUK-14 was immersed in 10 mL of methanol. During the first hour of conditioning, the solvent was changed three times (at 15-minute intervals). In the last portion of the liquid the sample was conditioned for the next 23 hours. After that, white powder was filtered on a funnel (sinter G4), washed with 3 ml of methanol and dried in air at ambient conditions. For impedance and sorption measurements JUK-14_MeOH sample is firstly activated at an elevated temperature and reduced pressure, and then (for impedance measurements) conditioned in various humidity. To investigate the stability of the material in high humidity, the sample was subjected to a series of modifications, reproducing the conditions to which it is exposed prior to impedance measurements. Thus, JUK-14_MeOH was: 1) activated at 100 o C, 10 mbar for 2 h, 2) conditioned at 90% RH and 60 o C for 16 h, and finally 3) immersed in methanol for 2 h (to remove any possible non-bound linkers). During the conditioning, the solvent was changed three times and after that, the powder was filtered on a funnel (sintered glass funnel G4), washed with 3 ml of methanol and dried in air at ambient conditions.

Synthesis of JUK-14_H2O
The synthesis was carried out in a manner analogous to that described above, but instead of methanol, distilled water was used. In addition, the kinetics of JUK-14 defecting in water was investigated by performing soaking for 2, 6, 24 and 48 h.

Synthesis of JUK-14_HCl
The synthesis was carried out in a manner analogous to JUK-14_H2O, but instead of water, 1 M aqueous solution of HCl (pH = 0) was used. Additionally, after filtering, the powder was washed with three portion of distilled water, 3 ml each (to obtain a neutral pH).
After the aforementioned modifications, to investigate the stability and defectivity of the materials JUK-14_MeOH, JUK-14_H2O and JUK-14_HCl, elemental and thermogravimetric analysis measurements were carried out and powder diffraction patterns and IR spectra were recorded (see Characterization of JUK-14 derivatives section).
Sample preparation for EIS measurements Prior to each series of EIS measurements at a given relative humidity (RH), 20-30 mg of the JUK-14_MeOH, JUK-14_H2O or JUK-14_HCl was ground in a mortar and equilibrated in a humidity chamber for at least 16 hours at a specified RH (90, 75, 60, 45 or 30%) at 25 o C. Before equilibration, JUK-14_MeOH sample was additionally activated for 2 h in vacuum dryer (100 o C, 10 mbar) to remove methanol from the framework pores. Before the measurements in anhydrous environment, the same amount of sample was equilibrated in a vacuum dryer for at least 16 h at 100 o C (atmospheric pressure) to completely remove solvents from the framework pores.
Calculating the number of defects The number of defects was estimated using the thermogravimetric analysis. An exemplary plot of TG analysis was presented in Figure S1.

S-7
Using these values, the number of linkers was calculated with following equation:     Chemical formulas determined for materials discussed above:

JUK-14_MeOH after the aforementioned modifications (x + y = 4) {(Me2NH2)4(H3O)x[Zr6(μ3-O)4(μ3-OH)4(μ-dsoa)4(OH)4-y(H2O)4-x+y]•35H2O}n
S-11  Table S3. Rightcorresponding PXRD patterns of JUK-14_H2O . Chemical formulas determined for the materials discussed above:  Table S4. Rightcorresponding PXRD patterns of JUK-14_HCl . The numbers of defects were estimated according to the method specified in Calculating the number of defects section. For the calculation of the theoretical percent composition by mass (N, C, H, S) for JUK-14_H2O and JUK-14_HCl, an incomplete occupancy of the linkers, in the amount derived from the TG analysis, was assumed. In order to balance the framework charge and fill the coordination sites on zirconium cluster, one linker (2 x COO -) has been replaced by terminal ligands (2 x H2O and 2 x OH -). The proton compensating the negative charge of a sulfonate group is distributed over three possible sites -H3O + , OHand H2O. Further material characterization is presented for the samples soaked for 24 h.

C. Proton conduction properties of JUK-14 derivatives
For the measurements under controlled humidity conditions, an analysis of measurement uncertainties for activation energies (Ea) and conductivity values (σ) was carried out: 1) The measurement uncertainty of the activation energy was determined by multiplying the derivative of the activation energy with respect to the slope from the Arrhenius plot and the uncertainty of the determination of this slope: where: Eaactivation energy, aslope; ∆ differs for each Arrhenius plot, k -Boltzmann constant.
2) The measurement uncertainty of the conductivity values was determined as the sum of partial derivatives of the conductivity with respect to the sample thickness, resistance and pellet radius, multiplied by the uncertainties of the corresponding variables: where: σproton conductivity, Lsample thickness; ∆ = 0.001 , rpellet radius; ∆ = 0.01 , Rthe resistance as the real part of the measured impedance; The ∆ has been calculated according to the specification of the instrument, provided by the producer,

S-22
and varies depending on the resistance value and the frequency range over which the resistance has been measured. Overall, the average percentage ∆ was 3.5% of R.
The calculated measurement uncertainties were presented as error bars in Figure 4 in the main text. Figure S25. Temperature-dependent impedance plots for JUK-14_MeOH at 45, 60, 75 and 90% RH (specified in the plots). Temperature legend refers to all the plots.
S-27 Figure S32. Impedance plots for JUK-14_MeOH (grey) and JUK-14_HCl (orange) at 25 o C and different relative humidity; open circlesmeasured data, red linefitted curves (with the use of ZSimpWin software). The equivalent circuit used for fitting the AC impedance plots was depicted on the first graph and refers to all the plots.
S-28 Figure S33. Impedance plots for JUK-14_MeOH at 100, 130 and 160 o C; black circlesmeasured data, red linefitted curves (with the use of ZSimpWin software). The equivalent circuit used for fitting the AC impedance plots was depicted on the graph.