Structural and Dynamic Analysis of Sulphur Dioxide Adsorption in a Series of Zirconium‐Based Metal–Organic Frameworks

Abstract We report reversible high capacity adsorption of SO2 in robust Zr‐based metal–organic framework (MOF) materials. Zr‐bptc (H4bptc=biphenyl‐3,3′,5,5′‐tetracarboxylic acid) shows a high SO2 uptake of 6.2 mmol g−1 at 0.1 bar and 298 K, reflecting excellent capture capability and removal of SO2 at low concentration (2500 ppm). Dynamic breakthrough experiments confirm that the introduction of amine, atomically‐dispersed CuII or heteroatomic sulphur sites into the pores enhance the capture of SO2 at low concentrations. The captured SO2 can be converted quantitatively to a pharmaceutical intermediate, aryl N‐aminosulfonamide, thus converting waste to chemical values. In situ X‐ray diffraction, infrared micro‐spectroscopy and inelastic neutron scattering enable the visualisation of the binding domains of adsorbed SO2 molecules and host–guest binding dynamics in these materials at the atomic level. Refinement of the pore environment plays a critical role in designing efficient sorbent materials.


Synthesis of Compound (2)
To a stirred solution of 4-carboxybenzene boronic acid (20.0 g, 0.12 mol) in EtOH (500 mL) was added H2SO4 (6 mL). The reaction mixture was stirred at 90 °C for 18 h, after which it was cooled and reduced in volume in vacuo (ca. 200 mL). The white solid was crashed out of solution with the addition of water and was collected by filtration. The product was washed with excess water and dried in an oven (80 °C) overnight to yield a white solid (19.5 g, 84%). 1

Synthesis of Compound (3)
To a stirred solution of compound 2 (6.96 g, 35.9 mmol) and K2CO3 (4.34 g, 31.4 mmol) in water/toluene (v/v = 100 mL; 400 mL) was added compound 1 (5.15 g, 12.8 mmol). The reaction mixture was degassed at 60 °C for 1 h, after which was added tri-tert-butylphosphine (1 M in toluene, 3.8 mL) and Pd2(dba)3 (1.17 g, 1.3 mmol). The reaction mixture was stirred at 80 °C under an inert atmosphere for 5 h and then filtered whilst 4 hot. The product was extracted with CH2Cl2 and washed with water. The organic layer was separated, dried with MgSO4 and the solvent removed under reduced pressure. Recrystallization of the crude product from CH2Cl2/MeOH afforded the product as a yellow solid (5.05 g, 73%). 1

Synthesis of Compound (4)
Compound 3 (4.85 g, 8.42 mmol) was stirred in H2SO4 (2 M, 60 mL) and THF (245 mL) at 80 °C for 18 h. The reaction mixture was reduced in volume in vacuo until white precipitate was seen and excess water (ca. 500 mL) was added. The solid was filtered and washed with copious water (3.35 g, 96%). 1

Synthesis of Compound (5)
Compound 4 (3.35 g, 8.72 mmol) was added to a stirred solution of dibromobenzene (0.68 g, 2.90 mmol) and K2CO3 (2.0 g) in water/toluene (v/v = 65/ 260 mL). The reaction mixture was degassed at 60 °C for 1 h, after which was added tri-tert-butylphosphine (1 M in toluene, 1.7 mL) and Pd2(dba)3 (0.53 g, 0.56 mmol). The reaction mixture was stirred at 80 °C under an inert atmosphere for 5 h and then filtered whilst hot. The product was extracted with CH2Cl2 and the solution washed with water. The organic layer was separated, dried with MgSO4 and the solvent removed in vacuo. Recrystallization of the crude product from CH2Cl2/MeOH afforded the product as a white solid (1.61 g, 68%). 1

Synthesis of Compound (6)
Compound 5 (0.80 mg, 0.97 mmol) was added to a stirred solution of NaOH (2 M, 100 mL) and EtOH/THF (v/v = 1:1; 200 mL) and refluxed at 90 °C for 5 h. The organic solvent was removed in vacuo and the aqueous portion was acidified with HCl (2 M) until a white precipitate was seen. The fine solid was filtered, washed with water and dried in an oven to give a black solid, which was recrystallized from DMF/water to give a grey solid (530 mg, 79%). 1

SO2 safety
The hardware and piping involved in the supply, delivery and measurement of SO2 were rigorously leak tested and used only within range of a SO2 detection system with a sensitivity of 0.1 ppm.

Gas Adsorption Isotherms
Gravimetric sorption isotherms of SO2 were recorded on a Hiden Xemis system under ultra-high vacuum (10 -10 bar) using a turbo pumping system at 273, 278, 293 and 298 K, the temperature being maintained by a temperature-programmed water bath. Ultra-pure research grade (99.999%) SO2 was purchased from BOC. In a typical gas adsorption experiment, the acetone-exchanged MOF sample (50 mg) was loaded onto the Xemis system and activated at 393 or 573 K under dynamic high vacuum (10 -10 bar measured at pump) for 24 h to give fully desolvated MOF sample. Gravimetric sorption isotherms for N2 and CO2 were recorded on a Hiden Isochema IGA-003 system or a Hiden Xemis system under ultra-high vacuum (10 -10 bar) using a turbo pumping system at 273, 283, 293, 298 K, the temperature being maintained by a temperature-programmed water bath. Ultra-pure research grade (99.999%) N2 and CO2 were purchased from BOC or Air Liquide. In a typical gas adsorption experiment, 50 mg of acetone-exchanged MOF sample was loaded onto the Xemis/IGA system and activated at 393 or 573 K under dynamic high vacuum (10 -10 bar) for 24 h to give fully desolvated MOF sample.

Gas Separation by Breakthrough Experiments
Breakthrough experiments were performed on a Hiden Isochema IGA-003 with ABR attachments and a Hiden Analytical mass spectrometer to detect the gases as they break through the sample bed. Experiments were carried out in a 7 mm diameter fixed-bed of 120 mm length packed with MOF powder (particle size < 1 micron). The sample was pre-activated at 393 or 573 K under vacuum and the pre-activated sample was loaded to the column and re-activated under a flow of He for 12 h. The fixed-bed was cooled to room temperature (298 K) using a temperature programmed water bath and the breakthrough experiment performed with a series of gas mixtures at atmospheric pressure and room temperature. The flow rate of the entering gas mixture was maintained at 14-40 mL min -1 , and the gas concentration, C, of gases at the outlet determined by mass spectrometry and compared with the corresponding inlet concentration C0, where C/C0 =1 indicates complete breakthrough. For UiO-66 sample: 0.5 g sample was loaded onto the sample bed. For UiO-66-NH2 sample: 0.38 g sample was loaded onto the sample bed. For UiO-66-Cu II sample: 0.37 g sample was loaded onto the sample bed. For Zr-DMTDC sample: 0.5 g sample was loaded onto the sample bed. For Zr-bptc sample: 0.55 g sample was loaded onto the sample bed.

In situ Synchrotron X-Ray Powder Diffraction
High-resolution X-ray powder diffraction of SO2-loaded MOFs was carried out on beamline I11 of the Diamond Light Source. A high brightness monochromatic beam was produced by a Si(111) monochromator and double-bounce harmonic rejection mirrors. The beam was delivered to the main instrument hutch where five multi-analysing crystal-detectors (MAC) travel in an arc of 2θ around the sample. Measurements were carried out in capillary mode and the sample environment controlled using an Oxford Cryosystems open-flow N2 gas cryostat. The samples were ground to provide a uniform particle size, packed into a borosilicate capillary. The sample was activated under vacuum (1 x 10 -6 mbar) at 393 or 573 K for > 3 h to remove residual solvent molecules from the material. Diffraction data for the activated sample were collected and analysed to confirm that no residual solvent molecules are present in the pores. Wavelength and capillary details are tabulated in Tables S3.

Inelastic Neutron Scattering (INS)
INS spectra were recorded on the VISION spectrometer at Spallation Neutron Source, Oak Ridge National Laboratory (USA). VISION is an indirect geometry crystal analyser instrument that provides a wide dynamic range with high resolution. The sample of pre-activated Zr-bptc (573 K under vacuum) was loaded into a cylindrical vanadium sample container with an indium vacuum seal and connected to a gas handling system. The sample was degassed at 10 -7 mbar at 393 K for 1 day to remove any remaining trace guest water molecules. The temperature during data collection was controlled using a closed cycle refrigerator (CCR) cryostat (10 ± 0.1 K). The loading of SO2 was performed volumetrically at room temperature in order to ensure that SO2 was present in the gas phase when not adsorbed and also to ensure sufficient mobility of SO2 inside the crystalline structure of Zr-bptc. Subsequently, the temperature was reduced to below 10 K in order to perform the scattering measurements with minimum thermal motion for the framework host and adsorbed SO2 molecules. Background spectra [sample can plus bare Zr-bptc] were subtracted to obtain the difference spectra. 6

DFT Modelling and Simulation
Vibrational frequencies and polarization vectors were calculated using CP2K (http://www.cp2k.org) 7 , based on the mixed Gaussian and plane-wave scheme 8 and the Quickstep module 9 . The calculation used molecularly optimized Double-Zeta-Valence plus Polarization (DZVP) basis set 10 , Goedecker-Teter-Hutter pseudopotentials 11 , and the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional 12 . The plane-wave energy cutoff was 400 Ry. The DFT-D3 level correction for dispersion interactions, as implemented by Grimme et al 13 , was applied with a cut-off distance of 15 Å. The calculation was performed at the Gamma point only with no symmetry constraint. Structural optimization was performed using the Broyden-Fletcher-Goldfarb-Shannon (BFGS) optimizer, until the maximum force is below 0.00045 Ry/Bohr (0.011 eV/Å). Finite displacement method was used for the phonon calculation, with incremental displacement of 0.01 Bohr (0.0053 Å). The INS spectrum was then simulated using the OClimax software 14 .

In situ FT-IR Micro-spectroscopy
Synchrotron infrared micro-spectroscopy experiments were carried out at the Multimode Infrared Imaging and Microspectroscopy (MIRIAM) beamline at the Diamond Light Source, UK. The instrument is comprised of a Bruker Hyperion 3000 microscope in transmission mode, with a 15× objective and condenser and liquid N2 cooled MCT detector, coupled to a Bruker Vertex 80 V Fourier Transform IR spectrometer using radiation generated from a bending magnet source. Spectra were collected (256 scans) in the range 650-4000 cm -1 with 4 cm -1 resolution and an infrared spot size at the sample of about 25 × 25 μm.
Acetone-exchanged samples of UiO-66, UiO-66-NH2, UiO-66-Cu II and Zr-DMTDC were placed onto a ZnSe disk within a Linkam FTIR600 gas-tight sample cell equipped with ZnSe windows, a heating stage and gas inlet and outlets. The gases were dosed volumetrically into the sample cell using mass flow controllers, the total flow rate being maintained at 100 mL min -1 for all experiments. The gases were vented directly to an exhaust system and the total pressure in the cell was therefore 1 bar for all experiments. The sample was dehydrated under a flow of dry N2 at 100 mL min -1 and 393 K for 5 h. The sample was cooled to 298 K under a continuous flow of N2. Dry N2, CO2 and SO2 were dosed as a function of partial pressure. For the competitive binding studies with CO2 and SO2, the bare material was first equilibrated step by step to 1 bar of CO2, followed by sequential replacing of CO2 with SO2-containing mixture (i.e., tuning the SO2/CO2 mixture composition from 0/100 to 100/0 while maintaining a total pressure of 1 bar).

Analysis of the IAST Selectivity
Ideal adsorbed solution theory (IAST) was used to determine the selectivity factor, S, for binary mixtures from the pure component isotherm data. The selectivity factor, S, is defined according to the following Equation where x1 is the amount of component 1 adsorbed and y1 is the mole fraction of component 1 in the gas phase at equilibrium. The IAST adsorption selectivity was calculated for SO2/CO2 (1:99) and SO2/N2 (1:99), of compositions at 298 K and a total pressure of 1 bar.
Volumetric cryogenic N2, isotherms were performed on a Micromeritics 3Flex adsorption analyser using ultrahigh purity (99.999%), N2 at 77 K for void volumetric determination. The BET surface areas were calculated using the software integrated into the instrument. 7

Calculation of Isosteric Heats of Adsorption
To estimate the differential enthalpies (ΔHn) and (ΔSn) for SO2 adsorption, all isotherms at different temperatures were fitted to the van't Hoff isochore: where P is pressure, T is the temperature, R is the real gas constant. Selected linear fitting plots are shown in Figure S.36 and 42. All linear fittings show R 2 above 0.9, indicating consistency for the isotherm data. A plot of ln(P) versus 1/T at constant amount adsorbed allows the differential enthalpy of adsorption and also the isosteric enthalpy of adsorption (Qst) to be determined.

Conversion of Captured SO2
Activated Zr-bptc (168 mg) was dosed with SO2 for 1 h at 298 K to reach adsorption equilibrium (equivalent to 1.31 mmol SO2), then morpholin-4-amine (128.0 mg, 1.25 mmol) and CH3CN (3 mL) were added and stirred for 1 h. 4-Methoxy-aryldiazonium tetrafluoroborate (55.0 mg, 0.25 mmol) in CH3CN (1 mL) was added dropwise to the above suspension, and the mixture was stirred at room temperature for 1 h. The mixture was centrifuged, and the supernatant was evaporated. NMR spectroscopy and preparative thin layer chromatography (TLC) were used to quantify the conversion of 4-methoxy-aryldiazonium tetrafluoroborate and the yield of the sulfonamide.