Selective sulfur dioxide adsorption on crystal defect sites on an isoreticular metal organic framework series

The widespread emissions of toxic gases from fossil fuel combustion represent major welfare risks. Here we report the improvement of the selective sulfur dioxide capture from flue gas emissions of isoreticular nickel pyrazolate metal organic frameworks through the sequential introduction of missing-linker defects and extra-framework barium cations. The results and feasibility of the defect pore engineering carried out are quantified through a combination of dynamic adsorption experiments, X-ray diffraction, electron microscopy and density functional theory calculations. The increased sulfur dioxide adsorption capacities and energies as well as the sulfur dioxide/carbon dioxide partition coefficients values of defective materials compared to original non-defective ones are related to the missing linkers enhanced pore accessibility and to the specificity of sulfur dioxide interactions with crystal defect sites. The selective sulfur dioxide adsorption on defects indicates the potential of fine-tuning the functional properties of metal organic frameworks through the deliberate creation of defects.


Supplementary Note 1
We investigated the relevant siting of SO 2 by studying 6 different configurations in 1. Once these calculations were performed, three configurations were further considered for the case of 1@KOH considering variations of the local structure in connection to the missing linker. The case of SO 2 incorporation in 1@Ba(OH) 2 was studied with the two more stable configurations. Since it was shown above that the metal hydroxide cluster -ligand vacancy -extra-framework cation complexes are different for K + and Ba 2+ . It would be expected that the interaction with the initially arriving SO 2 molecules would be different. The calculations reveal it, but surprisingly the way SO 2 interacts with the defective solids is quite different (Supplementary Figure 15 and Supplementary Table 7). While the preferential interaction of the SO 2 molecule with 1@Ba(OH) 2 is directly through the extraframework cation (Ba), in the case of K the stabilization of the SO 2 molecule is produced via monodentate interaction with extra-framework cation (K) supplemented with a hydrogen bonding with a hydroxide cation of the metal cluster (Supplementary Figure 15 and Supplementary Table 7). These high energies are indicative of strong interactions between the framework and the adsorbed SO 2 molecules.

Supplementary Methods
All the general reagents and solvents were commercially available and used as received.
Elemental Analysis: Elemental Analysis was carried out on a Thermo Finniganon Flash EA1112 Series CHNS/O Analyzer using 2-5 mg of samples. Metal content of the samples were determined on previously acid digested and dissolved samples in aqueous solution by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) NEXION 300D instrument. FT-IR measurements: FT-IR spectroscopy measurements were performed on a Tensor 27 Bruker Spectrometer using a Platinum Diamond crystal Attenuated Total Reflectance accessory, with 4 cm -1 resolution and scan range from 4000 to 400 cm -1 . Thermal Gravimetric Analysis: Thermogravimetric analyses were performed, in air atmosphere, on a Shimadzu-TGA-50H equipment, at a heating rate of 20 K min −1 . XRPD analysis: XRPD data were obtained on a D2 PHASER Bruker diffractometer using Cu Kα radiation (λ = 1.5418 Å) by means of a scan in the 2 θ range of 5-50° with 0.02°/1s. The compounds were manually grounded in an agate mortar and then deposited in the hollow of a zero-background silicon sample holder.

Adsorption measurements
Adsorption isotherms were measured using a Micromeritics Tristar 3000 volumetric instrument under continuous adsorption conditions. Brunauer-Emmet-Teller (BET) and Langmuir analyses were used to determine the total specific surface areas for the N 2 and CO 2 isotherms, at 77 K and 273K respectively. All the samples were activated at 423K and outgassed for 12 hours prior measurements Electron Microscopy -Energy Dispersive X-ray diffraction measurements: VP-SEM (Variable Pressure Scanning Electron Microscopy) analysis was performed on samples with Zeiss SUPRA40VP instrument with accelerating voltage 5 to 20kV. All the samples were measured by dispersing the material onto a sticky carbon surface attached to a flat aluminium sample holder. The samples were then, carbon coated using a BAL-TEC MED-020 Sputter at ambient temperature in inert atmosphere. High Resolution Transmission Electron Microscopy (HRTEM). The samples were prepared as follows: materials were grounded and 1 mg was suspended in 1 mL of absolute ethanol by sonication, for 20 minutes to disperse the nanoparticles into the solution and subsequently the materials were hold using a copper Holey Carbon type grid by dipping the grid into the solution for 20 times and finally they were dried overnight before analysis. Samples were analyzed using a HAADF FEI TITAN G2 instrument with an accelerating voltage in the range 50 to 300kV and magnification up to 1,5MX.

Synthesis of ligands 4,4'-benzene-1,4-diylbis(1H-pyrazole) and derivatives 4,4'-benzene-1,4-diylbis(1H-pyrazole)
The first intermediate, 1,4-bis(1-dimethylamino-3dimethylimonio-prop-1-en-2-yl)benzene bis(perchlorate), was prepared according to methods reported 2 44.8 mL (0.48 mol) POCl 3 were added drowpwise to 180 mL of DMF at 5-10 °C with constant stirring. The mixture was stirred for an additional hour at room temperature. Then 15.52 g (80.0 mmol) solid p-phenylenediacetic acid was added at once and the clear solution formed was stirred for 4 hours at 90-95 °C and then at room temperature overnight. The resulting black mixture was poured on 400 g crushed ice. After decomposition of the excess Vilsmeyer reagent a saturated solution of 60.0 g NaClO4 was added with stirring. The resulting nearly white crystalline deposit of the bis(trimethinium) diperchlorate was filtered and washed with two 30 mL portions of water. (35 g, yield 84 %). 1  For the preparation of derivatives amino-and hydroxyl-4,4'-benzene-1,4-diylbis(1H-pyrazole) the following methods were used: 2-Nitro[1,4-bis(1H-pyrazol-4-yl)benzene] (H 2 BDP_NO 2 ). 1,4-Bis(1H-pyrazol-4-yl)benzene (1.000 g, 4.76 mmol) was added in portions to concentrated sulphuric acid (10 mL) while keeping the reaction mixture cold with an ice bath. To the solution was then added 70% nitric acid (0.255 mL, 5.71 mmol) dropwise while maintaining the reaction mixture cold. The ice bath was then removed, and the solution was left at room temperature under stirring for 1 h. Next, 10 g of crushed ice was added, and the precipitate was filtered off and washed with 10 mL (2 × 5 mL) of water. The precipitate was neutralized with aqueous NaHCO3, and the resulting product was collected by filtration and washed with 10 mL of water (2 × 5 mL), affording the pure ligand as a yellow solid (1.23 g, yield 98%).. 1  . To a suspension of H 2 BDP_NO 2 (0.9 g, 3.528 mmol) in DMF (15 mL) was added ammonium formate (1.11 g, 17.619 mmol) at room temperature. The reaction mixture was then heated to 120 °C, and Pd/C (5%, 90 mg) was added in small portions. The final mixture was then kept under stirring at 100 °C for 2 h. After the reaction mixture became clear, it was filtered through Celite pad. The Celite pad was then washed with a small amount of DMF, and the filtrate was diluted with crushed ice (30 g). The obtained precipitate was then filtered off and washed with water (2 × 15 mL), affording a white powder of pure H 2 BDP_NH 2 product (0.75 mg, yield 96%). 1H NMR (DMSO-d6): δ 4.77 (s, 2H), 6.84 (dd, 1H), 6.97 (d, 1H), 7.15 (d, 1H), 7.86 (br s, 4H), 12.88 (br s, 2H). Anal. Calc. for C 12 H 11 N 5 (MW = 225.2 g/mol. 2-Hydroxo[1,4-bis(1H-pyrazol-4-yl)benzene] (H 2 BDP_OH). One gram (4.444 mmol) of H 2 BDP_NH 2 was dissolved in 5 mL of sulfuric acid. The mixture was stirred until a thick paste was formed. To this was added about 3 g of crushed ice, and the mixture was then kept in an ice bath. In a separate beaker, NaNO 2 (0.440 g, 5.176 mmol) was dissolved in 4 mL of water. This solution was cooled and added dropwise, with constant stirring, to the acid amine solution. In a separate flask, a solution of H 2 SO 4 (3 mL) and water (3 mL) was heated to 110 °C, and the entire diazonium salt solution was added dropwise. After the addition was over, the solution was allowed to boil for another 30 min. It was then cooled with an ice bath, and the precipitate was filtered off and suspended in a solution of NaHCO 3 in water and stirred for 2 h at 80 °C. The yellowish precipitate was then filtered off, washed with water (2 × 5 mL), and dried under vacuum (0.9 g, yield 90%). 1

Synthesis of materials 1, 2 and 3
The synthesis of MOFs samples 1-3 were prepared according to the procedure reported by our group 3 , with subtle changes as follows: in a typical synthesis, 631mg (3mmol) of 4,4'-benzene-1,4diylbis(1H-pyrazole) were dissolved in 160 mL of N,N'-dimethylformamide and 992 mg (4mmol) of Ni(CH 3 COO) 2 4 H 2 O were dissolved in 40 mL of H 2 O. The two solutions were mixed and refluxed for 12 h under stirring. The solid obtained was filtered off and washed with N,N'-dimethylformamide, ethanol and diethyl ether, yielding the corresponding MOF 1-3. Prior to use or characterization of materials, 500mg of as-synthesized solids were solvent exchanged with 100ml of dichloromethane, with stirring at room temperature for 2 h.

Preparation of materials 1@KOH,2@KOH and 3@KOH
The postsynthetical modification of 1 , 2 and 3 materials were done according to previously reported procedure by our group 4 , with activation of as synthesized MOFs thermally at 423 K and outgassed to 10 −1 Pa for 12h, in order to obtain solvent-free porous matrix. Afterwards, 0.055 mmol of each material was suspended in 0.35 M KOH absolute ethanol solution (5.5 mL). The resulting suspensions were stirred overnight under an inert N 2 atmosphere, filtered off and washed copiously with absolute ethanol yielding the corresponding compounds 1@KOH, 2@KOH, 3@KOH.
Preparation of materials Ba 0.5 (Ni 8 (OH) 3 (2@Ba(OH) 2 ) and Ba 0.5 (Ni 8 (OH) 3 The materials 1@KOH , 2@KOH , 3@KOH were used as prepared without previous activation as follow, 100 mg of the 1@KOH-3@KOH materials were suspended in 12 mL of a 0.1M aqueous solution of the Ba(NO 3 ) with stirring for 72h at room temperature. The postmodified materials were subsequently filtered off, washed with water and ethanol and dried in air. Later, the solids (~50mg) as obtained were suspended in 50ml of water for 4 hours in order to remove the eventual absorbed ion pairs. The materials 1@Ba(OH) 2 , 2@Ba(OH) 2

Breakthrough and Pulse Gas Chromatography Experiments
Adsorption isotherm of SO 2 on 1@Ba at 303K Adsorption isotherm was measured at 303K, point by point using breakthrough experiments with total flow of 30 mL min -1 He/SO 2 variable gas mixtures from (97.5/2.5) to (25/75). The MOFs is activated at 423K for 24 hours before first chemisorption cycle, and for 2 hours between further cycles. Each gas mixture is measured twice in order to assure the physisorption process. The adsorbed amount was calculated using the same procedure as for routine breakthrough experiments. It should be noted that all isotherm points are measured on the same prepared column and the material characterized after this adsorption cycles maintain crystallinity and porosity. The desorption branch was unable to measure with this procedure.

Breakthrough Experiments for Gas Separation
For these measurements, the PSM materials used were carefully handled avoiding possible chemisorption of CO 2 from air and the 20-cm chromatographic column, that was prepared employing a stainless steel 20 cm-column (0.4 cm internal diameter) packed with ca. 0.5 g of the studied materials (1-3, 1@KOH-3@KOH and 1@Ba(OH) 2 -3@Ba(OH) 2 . The column was activated under a pure He flow (20 mL min -1 ) at 423 K overnight and for two hours between successive breakthrough cycles. The desired gas mixture (20 mL min -1 ) was prepared via mass flow controllers. For instance, N 2 /SO 2 (97.5 : 2.5), N 2 /CO 2 /SO 2 (83.5 : 14 : 2.5), and N 2 /H 2 O/SO 2 (94.1 : 3.4 : 2.5) gas mixtures were prepared in order to simulate the emission of flue gas from a power plant. The breakthrough experiments were carried out, at 303 K, by step changes from He to N 2 /SO 2 , N 2 /CO 2 /SO 2 , and N 2 /H 2 O/SO 2 flow mixtures. The subsequent breakthrough cycles were measured with prior sample reactivation under a pure He flow (20 mL min -1 ) at 423 K during 2 h. The relative amounts of gases passing through the column were monitored on a Mass Spectrometer Gas Analysis System (Pfeiffer Vacoon) detecting ion peaks at m/z 64 (SO 2 ), 44 (CO 2 ), 28 (N 2 ), 18 (H 2 O) and 4 (He). The adsorbed amounts of SO 2 for the different materials are summarized in Supplementary Table 3.
Variable temperature pulse gas chromatography Gas-phase adsorption at zero coverage surface was studied using the pulse chromatographic technique 5 employing a gas chromatograph and stainless steel 20 cm-column (0.4 cm internal diameter) packed with ca. 0.5 g of the studied materials (1-3, 1@KOH-3@KOH and 1@Ba(OH) 2 -3@Ba(OH) 2 ). It should be noted that all the measurements were done on the materials after SO 2 chemisorption (after breakthrough experiments) in order to ensure the thermodynamic equilibrium. Prior to measurement, samples were heated overnight at 423 K in a He flow (30 mL min -1 ). Later on, an equimolecular gas mixture composed of H 2 , N 2 , CO 2 , SO 2 gases (0.4 mL) was injected at 1 bar and the separation performance of the chromatographic column was examined at different temperatures (403 K-433 K) by means of a mass Spectrometer Gas Analysis System (Pfeiffer Vacoon), detecting the corresponding masses. The dead volume of the system was calculated using the retention time of hydrogen as a reference. The zero-coverage thermodynamic parameters of the adsorption process for SO 2 and CO 2 are gathered in Tables 1 and S4, respectively. These values were calculated using a van't Hoff type analysis employing isothermal chromatographic measurements. Error! Bookmark not defined. The retention volumes were corrected taking into account the volume expansion of the gas entering the capillary due to the temperature increase according to V S = (t R -t m )F a (T/T a )j where V S = net retention volume (mL); t R = retention time (min); t m = dead time (min); F a = volumetric flow-rate measures at ambient temperature (ml min -1 ); T= column temperature (K); T a = ambient temperature (K); the James-Martin gas compressibility correction j= (3(p i /p 0 ) 2 -1)/(2(p i /p 0 ) 3 -1) where p i = pressure of gas applied to the chromatogram and p 0 = pressure of gas at outlet. Once these corrections where applied, the van't Hoff plot of the equation lnV S = ln(RTn s ) + S/R -H diff /(RT) was used to calculate the thermodynamic parameters of each analyte taking into account that the term ln(RTn s ) is usually small and can be neglected in the determination of S. In addition to the H diff value obtained from the van't Hoff plot the isosteric heat of adsorption (H iso ) was also determined according to the relation H iso  = H diff  + RT average . The  SO2/CO2 partition coefficients have been calculated from the henry constants ratio

Characterization of materials after SO 2 chemisorption
The materials were characterised after SO 2 chemisorption process by means of EA, XRPD, FTIR, TEM-EDX, TGA-FTIR and N 2 adsorption isotherms in order to know the effect of the chemisorption process on the structural integrity of the material. The results are indicative that the crystal phase of both MOF and Ba(OH) 2 cocrystals are maintained and only a slight diminution on surface area is observed as a probable consequence of the formation of BaSO 3 nanoclusters.  X-ray Crystallography for Le Bail profile fitting X-ray powder diffraction patterns of activated samples were collected at Phillips Analytical B.V. Instrument at ambient temperature and pressure, in reflectance Bragg-Brentano geometry employing Ni filtered CuKα lines focused radiation (1.54059 Å, 1.54439 Å) at 45kV power and 40mA current, 1/4°divergence slit, 1/2° antiscatter slit. For Le Bail fitting the patterns of all samples were collected. The Le Bail fitting was carried out using Fullprof software package via the Winplotr interface 6 . To define the pattern profile, a triple Pseudo-Voight function was employed. For background file a 4-coefficient expression correction was used. The instrumental parameters were determined by refining a profile from a standard calcite sample. The following parameters have been allowed to refine: a lattice cell parameter, zero shift and profile parameters. For the patterns of 1@Ba(OH) 2 , 2@Ba(OH) 2 and 3@Ba(OH) 2 it was necessary to fit with three different phases of Barium hydroxide, the crystalline phases were:

Computational details of periodic DFT calculations. Study of the adsorption of SO 2 molecules on defective MOFs
A theoretical study of 1, 1@KOH, and 1@Ba(OH) 2 was carried out using density functional theory, as implemented in the VASP program 7 . The calculations were performed with a cut-off energy of 500 eV and PAW potentials. 8 The PBE exchange-correlation functional 9 , with corrected van der Waals interactions introduced via the D2 Grimme scheme 10 was used. Due to the large sizes of the unit cells, only the gamma point was used. No symmetry constrains were used and both the atomic coordinates and the cell parameters were allowed to vary. Calculations on 1, 1@KOH, 1@Ba(OH) 2 were performed with a cubic primitive cell containing 166 and 306 framework atoms for 1, and for 1@KOH and 1@Ba(OH) 2 respectively. The initial cubic cell has the cell parameters equal to a = 25.38 Å. Due to the presence of local defects, the optimized cells depart from the ideally cubic structure, as it is common in defective porous materials. 11 This is not a problem in the present study, as our interest lies on the local host-guest interactions. What it is important is the accurate description of the local structure and the host-guest interactions. Note that indeed both bond lengths and angles are in agreement with reported crystal data.

Computational study of the adsorption of SO 2 molecules
The initially high symmetry of the material allows us to model the defective solid with a relative small number of configurations. In this context, three different configurations were considered for the extraction of the missing linkers and the introduction of K + cations. The dangling metal-N bonds were capped with OH. In the case of the replacement of two K + cations by one Ba 2+ one, four configurations were taken into account. As mentioned above, the local structure is strongly affected by the presence of defects, as can be seen in Supplementary Figure 14 and Supplementary Table 6. It is observed that larger distortions result from the exchange of K + by Ba 2+ cations, as a consequence of the two times larger polarizing power of Ba 2+ .