Opening the Gate: Framework Flexibility in ZIF-8 Explored by Experiments and Simulations

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1 Supporting Information for: Opening the Gate: Framework Flexibility in ZIF-8 Explored by Experiments and Simulations D. Fairen-Jimenez *, S. A. Moggach, M. T. Wharmby, P. A. Wright, S. Parsons and T. Düren Institute for Materials and Processes, School of Engineering, The University of Edinburgh, United Kingdom; School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, United Kingdom and School of Chemistry, University of St. Andrews, United Kingdom. *To whom correspondence should be addressed. Table of contents Section S1. Experimental and simulation procedures s1 Section S2. BET analysis......s2 Section S3. Adsorption snapshots s3 Section S4. XRD profiles s4 Section S5. References S4 Section S1. Experimental and simulation procedures. ZIF-8 was prepared following the method described by Huang et al. S1 N 2 adsorption isotherms were recorded at 77 K using a Micromeritics ASAP 2020 instrument. Prior to the measurements, the samples were degassed at 425 K using a heating rate of 5 K min -1 for 4 h. X-ray diffraction samples were introduced in quartz capillaries, and outgassed under high vacuum. Before being sealed, one of the samples was loaded with N 2 at a relative pressure of 0.4 P/P 0. In-situ XRD data at 77 K were collected on station I11 at the Diamond light source. Rietveld refinement was performed starting from the ambient pressure coordinates of Moggach et al. S2 Five N 2 molecules were located in different maps and refined. N-N distances were restrained to 1.15 Å. All sites were refined to their maximum occupancies and were therefore maximally loaded with N 2. The total number of N 2 molecules per unit cell based on this model equates to 52. The adsorption of N 2 was investigated using grand canonical Monte Carlo (GCMC) simulations S3 implemented in the multipurpose simulation code Music. S4 We used an atomistic model for both ZIF-8 structures, in which the framework atoms were kept fixed at the crystallographic positions. We used the standard Lennard-Jones (LJ) 12-6 potential to model the S1

2 interatomic interactions between the framework and N 2. Apart from the LJ, we included electrostatic interactions between N 2 molecules. The parameters for the framework atoms were obtained from the UFF force field. S5 N 2 was modeled using the TraPPE potential with charges placed on each atom and at the center of mass. S6 The Lorentz-Berthelot mixing rules were employed to calculate fluid/solid parameters. Interactions beyond 18 Å were neglected Monte Carlo steps were performed, the first 40% of which were used for equilibration, and the remaining steps were used to calculate the ensemble averages. To calculate the gas-phase fugacity we used the Peng-Robinson equation of state. S7 Section S2. BET analysis. The BET equation was applied to experimental and simulated N 2 isotherms using the consistency criteria suggested in the literature and carefully assuring that the BET constant remains positive (Table S1). S8 In the experimental isotherm, the BET equation was fitted to two different ranges: i) before the step (i.e. ZIF-8 I, in Table S1), and ii) in a broader range of pressures that included the step (i.e. ZIF-8 II) (Figure S1). Table S1. BET parameters for the experimental isotherm and the different structures using GCMC. N m, monolayer capacity [mmol/g] N m, monolayer capacity [molec/uc] C, BET constant BET Surface Area [m 2 /g] ZIF-8 I (Exp) ZIF-8 II (Exp) ZIF-8AP (Sim) ZIF-8HP (Sim) Accessible Surface Area [m 2 /g] Step in the N 2 isotherm Figure S1. (Left) BET representation of the N 2 isotherms in the low pressure range at 77 K on ZIF-8. (Right) Similar representation in a broader range of pressures (0 < P/P 0 < 0.15), where the step cannot be detected. Experimental, circles; and simulated data on ZIF-8, closed triangles and ZIF-8HP, open triangles. S2

3 Section S3. Adsorption snapshots. Snapshots during the N 2 adsorption process represent the position of all nitrogen molecules from a single configuration. Snapshots provide a visual impression about the degree of pore filling and where nitrogen molecules are located. ZIF-8 ZIF-8HP ZIF8-HP Figure S2. Snapshots of N 2 adsorption at 77 K (left) on ZIF-8 at 42 molecules per unit cell; (centre) on ZIF-8HP before the step; and (right) ZIF-8HP after the step. Top and bottom are different orientations to visualize the 4-ring windows (red circles) and the 6-ring windows (yellow circles), respectively. S3

4 Section S4. XRD profiles. Figure S3. Observed (blue), refined (red), and difference (grey) X-Ray diffraction profiles measured for (a), empty ZIF-8; (b) ZIF-8 loaded with N 2. Section S5. References. (S1) Huang, X. C.; Lin, Y. Y.; Zhang, J. P.; Chen, X. M. Angew. Chem. Int. Ed. 2006, 45, (S2) Moggach, S.; Bennett, T.; Cheetham, A., Angew. Chem. Int. Ed. 2009, 48, (S3) Frenkel, D.; Smit, B., Understanding Molecular Simulations: From Algorithms to Applications. 2 th ed.; Academic Press: San Diego, (S4) Gupta, A.; Chempath, S.; Sanborn, M. J.; Clark, L. A.; Snurr, R. Q. Molecular Simulation 2003, 29, S4

5 (S5) Rappé, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, (S6) Potoff, J. J.; Siepmann, J. I. AIChE Journal 2001, 47, (S7) Reid, R. C.; Pausnitz, J. M.; Poling, B. E., The properties of gases & liquids. 4 th ed.; McGraw- Hill Companies: New York, (S8) Rouquerol, J.; Rouquerol, F.; Sing, K. S. W., Adsorption by powders and porous solids. Academic Press: San Diego, S5