Supporting Information

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1 Supporting Information Thermochemistry of Zeolitic Imidazolate Frameworks (ZIFs) of Varying Porosity James T. Hughes, Thomas D. Bennett, Anthony K. Cheetham, Alexandra Navrotsky Peter A. Rock Thermochemistry Laboratory, NEAT ORU, University of California at Davis, Davis, California 95616, United States Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ (UK) SI-1: Synthesis SI-2: Calorimetry SI-3: Oxygen - Nitrogen Coordination Change and Proton Transfer Calculations SI-4: Cobalt Coordination Correction SI-5: Thermogravimetric Confirmation ZIF Activation SI-6: Thermogravimetric Determination of DMF Content in ZIF-4 DMF S1

2 SI-1: Synthesis Crystalline samples of ZIF-1, ZIF-4 and CoZIF-4 were prepared following the experimental methodology already in the literature. s1-3 They were also evacuated using the procedures outlined there, though one sample of ZIF-4 was left with pore-occupying N,N-dimethylformamide (DMF) molecules intact (TGA characterization in SI-5). ZIF-zni was prepared as previously reported, and does not contain any pore-occupying solvent. ZIF-7 was prepared and evacuated using previously outlined procedures. s1b ZIF-8 was obtained from Sigma-Aldrich, under the name Basolite. A sample of a m ZIF-4 was synthesized according to the methodology set out in. s6 TGA curves of degassed samples are provided in SI-5. SI-2: Calorimetry Solution enthalpies were measured with a CSC 4400 isothermal microcalorimeter operating at 298 K. For each measurement, 25.0 g of 5.00 M HCl was placed inside a 50 ml PTFE cell that is a part of a removable apparatus that allows placement of the solvent cell into the center of the calorimeter chamber. The calorimetric cell was connected to the outside environment by a 5 mm inner diameter silica glass tube, approximately 1 m in length. At the beginning of each experiment the solution cell was cleaned and re-assembled with fresh solvent, then equilibrated with the isothermal block for at least three hours before performing an enthalpy measurement. A calorimetric measurement consisted of dropping a 5-15 mg pellet into the temperature equilibrated 5.00 HCl solvent. Upon contact with the solution all pellets dissolved rapidly. Mechanical stirring at approximately ½ Hz was applied to all experiments. The mass of each sample pellet was performed by a Mettler MT-5 microbalance with an accuracy of 1 μg. Calibration of the calorimeter was performed by dissolving 15 mg of KCl (NIST standard reference material 1655) in 25.0 g of deionized water at 298 K. The enthalpy of solution of KCl in water at 298 K was calculated using the known solution enthalpy at a reference concentration of (mol kg -1 ) and enthalpy of dilution measurements. s4 Liquid DMF was injected through a Teflon tube that extended into the silica connection tube until approximately 2 cm above the solvent surface. The mass of injected DMF was determined by weighing the syringe assembly before and after DMF injection. The dry masses of the syringe and Teflon tube were known, allowing the amount of DMF injected into the calorimetric cell to be calculated. All the weight measurements involving DMF were done on a Mettler microbalance with an accuracy of 10 μg. All uncertainties presented in this report, calorimetric or otherwise, are the 95 % standard error of the measurement average. S2

3 SI-3: Oxygen - Nitrogen Coordination Change and Proton Transfer Calculations The exothermic ZIF heat of formation reaction ( H rxn ) from metal oxide and the proper imidazole linker (equation 1) contains several key physical changes, an endothermic transition to a more porous structure and an exothermic change in local bonding. The bonding change in heat of formation reaction contains two processes: a proton transfer from the imidazole linker to oxygen in the formation of water, which was found to only be mildly favorable (Table S1), and a change of the tetrahedral coordination environment of the metal from oxygen to nitrogen exothermic (Table S2). which is strongly Table S1. Physical properties: free energies of imidazole, 2-methyl-imidazole, benzimidazole and water at 298 K. Proton transfer reaction from imidazole base organic to water. Compound Abbreviation pka ΔG (kj mol -1 ) ΔG trasfer (kj mol -1 ) Imidazole H-Im ± ± methylimidazole H-mIm Benzoimidazole H-bIm ± ± 0.6 Water H 2 O H-Im (cr) + OH - H 2 O (l) + Im - (aq) = [ ΔG 1 ΔG 2 ] = ΔG transfer = kj mol -1 per H-Im proton 2 H-mIm (cr) + Zn(Ac) 2 2H 2 O (cr) Zn(mIm) H-Ac (l) + 2H 2 O (l) (S1) This change in coordinating ligand from oxygen to nitrogen is considered to be the major reason for the exothermic ZIF formation from zinc oxide and imidazole linker. The energetic effect from this bonding change was estimated to be similar to the enthalpy of transformation of imidazole and zinc acetate dihydrate into ZIF-8, acetic acid and water (Reaction S1). Zinc acetate was chosen as a reference because it contains zinc-oxygen bonds very similar to those in ZnO and importantly it has a framework density similar to the ZIFs we seek to correct. The thermodynamic cycle (Table S2) determined the enthalpy of reaction (S1) to be ± 0.95 (kj mol -1 ). This value was subtracted from H rxn to eliminate the exothermic contribution of the bonding change and obtain H trans, allowing the examination of the destabilizing effects of increasing porosity of the studied ZIF frameworks (Figure 2). Table S2. Energetics of Zn 2+ metal center transfer from T d oxygen environment (2 acetate and 2 water) to T d nitrogen (imidazolate) environment, measured by room temperature solution calorimetry in 5 M HCl Reaction Scheme Enthalpy Measurement H s (kj mol -1 ) Zn(Ac) 2 2H 2 O (cr) Zn 2+ (aq) + 2 H 2 O (aq) H 1 = H s (Zn(Ac) 2 2H 2 O) ± 0.01 s8 H-mIm (cr) mim - (aq) + H + (aq) H 2 = 2 H s (H-mIm) ± 0.21 Zn(mIm) 2 (cr) Zn 2+ (aq) + 2 mim - (aq) H 4 = - H s (ZIF-8) ± 0.57 H-Ac (l) Ac - (aq) + H + (aq) H 5 = -2 H s (H-Ac) ± 0.01 s8 H 2 O (aq) H 2 O (l) H 6 = -2 H dil (H 2 O) H IM (cr) + Zn(Ac) 2 2H 2 O (cr) ZIF-zni (cr) + 2 H-Ac (l) H corr = H 1 + H 2 + H 3 + H 4 + H 5 S3

4 SI-4: Cobalt Coordination Correction CoZIF-4, the only CoZIF examined in this study, has an H rxn of ± 1.71 kj (mol of Co) -1, a destabilization of 6.21 kj (mol of Co) -1 with respect to ZIF ± 1.76 kj (mol of Zn) -1. The difference between the Zn and Co ZIFs can likely be ascribed to the change in the coordination environment of Co in the formation reaction, from octahedral in dense reference rocksalt CoO to tetrahedral in the open framework. A similar transformation of Co coordination within the oxide environment gives a comparable energetic shift, having been shown to destabilize CoO in the wurtzite structure by 8.4 kj mol -1 relative to the rocksalt form. s9,s10 Accounting for this endothermic transformation, CoZIF-4 has similar framework destabilization relative to its Zn counterpart. SI-5: Thermogravimetric Confirmation ZIF Activation Thermogravimetric analysis (TGA) was performed for all porous crystalline ZIF samples, ZIF-zni, ZIF-1, ZIF-4, CoZIF-4 and ZIF-8 (Not enough material of ZIF-7 was available to perform TGA). TGA was done to confirm activation by the absence of solvent in the framework prior to calorimetry. All samples were studied using a Setaram LabSys Evo TGA/DSC. Each sample was heated at 5 ( C min -1 ) to a final temperature of 650 C. S4

5 Figure S1. ZIF-zni Figure S2. ZIF-1 S5

6 Figure S3. ZIF-4 Figure S4. CoZIF-4 S6

7 Figure S5. ZIF-8 SI-5: Thermogravimetric Determination of DMF Content in ZIF-4 DMF The amount of DMF contained in the as-synthesized ZIF-4 (ZIF-4 DMF) was determined from an average of three TGA runs, each performed at 2 ( C min -1 ) to 650 C. The Gradual slope of the TG curve after the initial solvent removal step ( C) meant it was not possible to identify the fully desolvated framework using TG alone. The onset of ZIF-4 combustion is identified to occur slightly before 300 C, as shown by the DSC curve. Using this point of reference the mass loss between it and the final zinc oxide product was taken to be the decomposition of the imidazolate linkages. The variation in mass percentage for this process between the three experiments was in remarkable agreement: 35.8 ± 0.3 %. Using this value, it was possible to determine the molecular weight of ZIF-4 DMF, and hence the amount of DMF solvent within the framework. ZIF-4 DMFwas found to contain 1.8 ± 0.01 DMF molecules per Zn(IM) 2 unit. S7

8 Fig. S6. TGA/DSC of ZIF-4DMF References (S1) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R. D.; Uribe-Romo, F. J.; Chae, H. K.; O'Keeffe, M.; Yaghi, O. M., Proceedings of the National Academy of Sciences of the United States of America 2006, 103 (27), (S2) Bennett, T. D.; Goodwin, A. L.; Dove, M. T.; Keen, D. A.; Tucker, M. G.; Barney, E. R.; Soper, A. K.; Bithell, E. G.; Tan, J. C.; Cheetham, A. K., Phys Rev Lett 2010, 104 (11), (S3) Bennett, T. D.; Keen, D. A.; Tan, J. C.; Barney, E. R.; Goodwin, A. L.; Cheetham, A. K., Angew Chem Int Edit 2011, 50, (S4) Parker, V. B. Thermal Properties of Aqueous Uni-univalent Electrolytes; National Bureau of Standards: U.S. Government Printing Office, April 1, 1965, (S5) Catalan, J.; Claramunt, R. M.; Elguero, J.; Laynez, J.; Menendez, M.; Anvia, F.; Quian, J. H.; Taagepera, M.; Taft, R. W., Journal of the American Chemical Society 1988, 110 (13), (S6) Martin, R. B., Proceedings of the National Academy of Sciences 1974, 71 (11), (S7) Perrin, D. D., J. Chem. Educ. Journal of Chemical Education 1983, 60 (5). (S8) Hughes, J. T.; Navrotsky, A., The Journal of Chemical Thermodynamics 2011, 43 (6), (S9) Navrotsky, A.; Kleppa, O. J., Journal of Inorganic and Nuclear Chemistry 1967, 29 (11), (S10) DiCarlo, J.; Navrotsky, A., Journal of the American Ceramic Society 1993, 76 (10), S8