Exceptional Adsorption and Binding of Sulfur Dioxide in a Robust Zirconium-Based Metal–Organic Framework

We report a record-high SO2 adsorption capacity of 12.3 mmol g–1 in a robust porous material, MFM-601, at 298 K and 1.0 bar. SO2 adsorption in MFM-601 is fully reversible and highly selective over CO2 and N2. The binding domains for adsorbed SO2 and CO2 molecules in MFM-601 have been determined by in situ synchrotron X-ray diffraction experiments, giving insights at the molecular level to the basis of the observed high selectivity.


Physical Characterisation.
All reagents were used as received from commercial suppliers without further purification. Analyses for C, H and N were carried out on a CE-440 elemental analyzer (EAI Company). Thermal gravimetric analyses (TGA) were performed under air flow (100 ml/min) with a heating rate of 2 °C/min using a TA SDT-600 thermogravimetric analyzer (TA Company).

Synthesis of MFM-600
Benzoic acid (78.5 mmol, 9.60 g), zirconium (IV) chloride (1.33 mmol, 0.309 g) and H4L (0.31 mmol, 0.220 g) were dissolved in DMF (45 mL) and heated under reflux for 24 h. The suspension was then filtered and washed with DMF (3 x 30 mL) and acetone (3 x 30 mL). The yellow powder was then acetone-exchanged by suspending the as-synthesised sample in an excess of acetone for 1 week with frequent exchange of solvent. The activated sample was prepared using a Micrometrics Smart VacPrep by heating at 373 K for 10 h under ultra-high vacuum using a diaphragm and turbo pumping system.

Gas Adsorption Isotherms and Breakthrough
CO2, SO2, CH4 and N2 sorption isotherms were recorded at 273-303 K (temperature-programmed water bath from Hiden Company) on a Hiden XEMIS system at the University of Manchester under ultrahigh vacuum from a diaphragm and turbo pumping system. All gases used were ultra-pure research grade (99.999%) purchased from BOC or AIRLIQUIDE. In a typical gas adsorption experiment, ~80 mg of solvated sample was loaded into the XEMIS, and activated at 120 ˚C and high vacuum (10 -10 bar) for 1 day to give a fully desolvated sample.    Breakthrough experiments were carried out in a 7 mm diameter fixed-bed tube of 120 mm length packed with 0.76 g MFM-601 powder (particle size < 5 microns). The total volume of the bed was ca. 5 cm 3 .
The sample was heated at 120 °C under a flow of He for 12 hours for complete activation. The fixed bed was then cooled to room temperature (298 K) using a temperature programmed water bath and the breakthrough experiment was performed with a stream of 2500 ppm SO2 (diluted in 1:1 He:N2 and 1:1 He:CO2) at atmospheric pressure and room temperature. The flow rate of the entering gas mixture was maintained at 20 mL min -1 , and the gas concentration, C, of SO2, N2 and CO2 at the outlet was determined by mass spectrometry and compared with the corresponding inlet concentration C0, where C/C0 = 1 indicates complete breakthrough.  Table   Table S1. Comparison of the BET surface areas and total uptake of SO2 at 1 bar 298 K of several porous materials.

Calculation of IAST selectivity for gas adsorption.
To estimate the selectivity of SO2 over other gases at, isotherm data at 298 K was fitted using dual-site where is pressure in Pa, is the temperature, and is the ideal gas constant. Figure S19. Linear fitting of 1/T vs LnP at intervals of 0.1 mmol g -1 for CO2 and N2 in MFM-600 used to determine the isosteric heat of adsorption by the Van t'Hoff method. S18 Figure S20. Linear fitting of 1/T vs LnP at intervals of 0.1 mmol g -1 for CO2 and N2 in MFM-601 used to determine the isosteric heat of adsorption by the Van t'Hoff method.

Qst Calculations -Virial Method
For SO2 isotherm data was only recorded at two temperatures so the Van't Hoff method cannot be used to calculate Qst, therefore the virial method was used. This method involves fitting the isotherm data at the two temperatures to the virial expression: ln( ) = ln( ) + ( 1 ) ∑ =0 + ∑ =0 [4] Where in the above equation, [5] S19 Figure S21. Virial analysis of the adsorption data for SO2 in MFM-601. Figure S22. Virial analysis of the adsorption data for SO2 in MFM-600.

Scanning Electron Microscopic Imaging
Images were collected on a Hitachi TM1000 table top microscope. Figure S23. SEM micrograph of MFM-601 particles at a) 1000, b) 2000, c) 5000 and d) 10000 x magnification. The particle size is approximately at 40-60 microns.