SUPPORTI G I FORMATIO. Enhanced CO 2 Adsorption in Metal-Organic Frameworks via Occupation of Open-Metal Sites by Coordinated Water Molecules

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1 SUPPORTI G I FORMATIO Enhanced CO 2 Adsorption in Metal-Organic Frameworks via Occupation of Open-Metal Sites by Coordinated Water Molecules A. Özgür Yazaydın, 1 Annabelle I. Benin, 2 Syed A. Faheem, 2 Paulina Jakubczak, 2 John J. Low, 2 Richard R. Willis, 2 Randall Q. Snurr 1 1 Chemical and Biological Engineering Department, Northwestern University, 2145 Sheridan Road, Evanston, IL, (USA) 2 UOP LLC, a Honeywell Company, 25 E. Algonquin Road, Des Plaines, IL (USA) 1. Density Functional Theory (DFT) Calculations DFT calculations were used for two purposes: first, to optimize the orientation of the hydrogen atoms of the water molecules and second, to calculate the partial charges of the Cu- BTC framework atoms which were used later in the GCMC simulations. All DFT calculations were performed with the Gaussian 03 1 software using the PBEPBE level of theory and 6-31+G* basis set. 1.a. Orientation of the hydrogen atoms of coordinated water molecules Positions of the oxygen atoms of coordinated water molecules were reported in experiments. 2 However, water hydrogens were missing. We considered two levels of hydration in which one and two coordinated water molecules were present per metal corner corresponding to a water loading of 4 wt% and 8 wt%, respectively. For a DFT calculation a metal corner was isolated from the reported crystal structure. This metal corner consisted of two copper atoms and four carboxylate groups each terminated by a methyl group bonded to the carboxylate carbon; plus one or two water oxygens, depending on the water loading. The water oxygen(s) were saturated with hydrogens and then the optimization calculation was performed. In this calculation only the hydrogens of the water molecule(s) were allowed to move, as we wanted to use the experimentally reported coordinates for the other atoms. At the end of the optimization calculations, hydrogens leaned towards the carboxylate oxygens, but in the case of 8 wt% in opposite directions for the two water molecules in the cluster. Figures S1

2 S1 and S2 are provided to aid in understanding the orientations of the water molecules in addition to the figures given in the main text (Figures 1a and 1b). Figure S1. Orientation of hydrogen atoms of water molecules in hydrated Cu-BTC (4 wt%) from two different perspectives. Oxygen atoms of water are marked by blue color. Figure S2. Orientation of hydrogen atoms of water molecules in hydrated Cu-BTC (8 wt%) from two different perspectives. Oxygen atoms of water are marked by blue color. 1.b. Partial atomic charges Partial atomic charges were calculated using the ChelpG 3 method. The calculations were done for isolated clusters extracted from unit cells of dry Cu-BTC, hydrated Cu-BTC (4 wt%) and hydrated Cu-BTC (8 wt%). These clusters included benzene-1,3,5-tricarboxylate (BTC) S2

3 and 3 metal corners, where each corner consisted of 2 copper atoms, a carboxylate group from the central BTC linker, and 3 additional carboxylate groups terminated with methyl groups. For hydrated Cu-BTC (4 wt%) one coordinated water molecule and for hydrated Cu-BTC (8 wt%) two coordinated water molecules were included for each metal corner. Figures S3, S4 and S5 show the unique atoms of Cu-BTC, hydrated Cu-BTC (4 wt%) and hydrated Cu-BTC (8 wt%), respectively, for which partial charges were calculated. Table S1 shows the calculated charges. Figure S3. Unique atoms for partial atomic charge calculations of dry Cu-BTC Figure S4. Unique atoms for partial atomic charge calculations of hydrated Cu-BTC (4 wt%) S3

4 Figure S5. Unique atoms for partial atomic charge calculations of hydrated Cu-BTC (8 wt%) Table S1. Partial atomic charges, q (e). Cu (a/b)* O(a/b)* Ca Cb Cc H Ow Hw Dry Cu-BTC NA NA Hydrated Cu-BTC (4 wt%) Hydrated Cu-BTC (8 wt%) / / / [*] In hydrated Cu-BTCs oxygen atoms of the MOF take two different charges (Oa or Ob) depending on whether they are close or far from the hydrogen of the water molecule. In hydrated Cu-BTC (4 wt%) only, cupper atoms take two different charges (Cua or Cub) depending on whether they are close or far from the water molecule. S4

5 2. Grand Canonical Monte Carlo (GCMC) Simulations 2.a. Potential and models The interaction energy between the atoms was computed through the Coulomb and Lennard-Jones (LJ) potentials V ij 12 6 σ ij σ ij qiq j = 4ε ij + r ij r ij 4ε 0r where i and j are interacting atoms, and r ij is the distance between atoms i and j. ε ij and σ ij are LJ well depth and diameter, respectively. q i and q j are the partial charges of the interacting atoms and ε 0 is the dielectric constant. LJ parameters between different types of sites were calculated using the Lorentz-Berthelot mixing rules. 2.a.1. Cu-BTC Model Partial charges for the atoms of Cu-BTC were calculated from DFT calculations as explained in section 1.b. LJ parameters for all Cu-BTC atoms were taken from the DREIDING 4 force field except copper, since this force field does not have LJ parameters for copper. Therefore, copper parameters were taken from the Universal Force Field (UFF). 5 Table S2 lists the LJ parameters for Cu-BTC atoms. ij Table S2. LJ parameters for Cu-BTC atoms. Cu(a,b) O(a,b) C(a,b,c) H σ (Å) ε/k B (K) a.2. Water Model Partial charges for the water molecule were calculated from DFT as explained in section 1.b. LJ parameters were taken from the TIP3P 6 water model. In this model hydrogen atoms interact with each other and other atoms through the Coulombic potential only. Table S3 lists the LJ parameters for water atoms. S5

6 Table S3. LJ parameters for water atoms. Ow Hw σ (Å) 3.15 NA ε/k B (K) NA 2.a.3. CO 2, CH 4 and 2 Models Partial charges and LJ parameters for CO 2, CH 4 and N 2 were taken from the Trappe-AA 7,8 force field. This force field has been fit to reproduce the vapor-liquid coexistence curves by Siepmann and co-workers. CO 2 molecule is modeled as a rigid and linear structure. CH 4 molecule is modeled with the united-atom representation where the effect of hydrogens is included in the central carbon atom. The united atom representation of CH 4 interacts with other atoms through the LJ potential only. N 2 molecule is modeled as a rigid and linear structure with an additional massless ghost atom placed at the center of mass. The ghost atoms interact with each other and other atoms through the Coulombic potential only. Tables S4, S5, and S6 show the LJ parameters and partial charges for CO 2, CH 4 and N 2, respectively. Table S4. LJ parameters and partial atomic charges for CO 2 atoms. O C σ (Å) ε/k B (K) q (e) Table S5. LJ parameters and partial atomic charges for CH 4 atoms. CH 4 σ (Å) 3.73 ε/k B (K) q (e) NA S6

7 Table S6. LJ parameters and partial atomic charges for N 2 atoms. M* σ (Å) 3.31 NA ε/k B (K) 36.0 NA q (e) [*] Massless ghost atom. 2.b. Simulation details All simulations were performed with the Monte Carlo simulation suite of the MUSIC 9 code and included a 2x10 7 step equilibration period followed by a 2x10 7 step production run. A cutoff distance of 12.8 Å was used for LJ interactions. Ewald sum technique was used to compute the electrostatic interactions. All simulations were performed at K and included random insertion, deletion and translation moves with equal probabilities. One unit cell was used during all simulations. S7

8 2.c. CO 2 adsorption isotherms Adsorbed amount (mg CO 2 /g) hydrated 4 wt% (Exp) dry (Exp) hydrated 8 wt% (Exp) hydrated 8 wt% (Sim) hydrated 4 wt% (Sim) dry (Sim) fully hydrated (Exp) Pressure (bar) Figure S8. Simulated and experimental adsorption isotherms of CO 2 at 298 K. S8

9 2.d. CH 4 and 2 adsorption isotherms 60 Adsorbed amount (mg CH 4 /g) hydrated (4 wt%) dry Pressure (bar) Figure S9. Simulated adsorption isotherms of CH 4 at 298 K. S9

10 50 Adsorbed amount (mg N 2 /g) hydrated (4 wt%) dry Pressure (bar) Figure S10. Simulated adsorption isotherms of N 2 at 298 K. S10

11 2.e. 2 Cu-BTC interaction energies Energy (kj/mol) Lennard-Jones (hydrated) Lennard-Jones (dry) Coulombic (hydrated) Coulombic (dry) CO 2 Loading (molecules / unit cell) Figure S11. Interaction energies between N 2 and dry Cu-BTC and hydrated Cu-BTC (4 wt%) from GCMC simulations at 298 K. S11

12 3. Experimental Methods Samples of Cu-BTC were prepared according to the procedure outlined in Rowsell et al., 10 except that an additional two dichloromethane washes were carried out in order to make certain that reaction solvent was removed as completely as possible. A TGA result for a sample activated at 175 o C in flowing nitrogen (Fig. S12) reveals very little mass loss until > 200 o C. This confirms samples activated at 175 o C are free from water or other contaminants. Fig. S12. TGA analysis of Cu-BTC previously activated at 175 o C in the TGA instrument. Note little mass loss until > 200 o C. Gas adsorption measurements Isotherm measurements were made on a high pressure volumetric system. This volumetric system uses the equation PV = cnrt to determine the amount of adsorbate adsorbed by the adsorbent. The c in the equation is the compressibility factor which is determined using the SRK equation for each temperature and pressure separately. The system is comprised of the following key parts (see Fig. S13): 1. feed gas isolation valves 2. reference volume vessel 3. pressure transducer S12

13 4. sample isolation valve 5. sample cell 6. constant temperature bath 7. thermocouples 8. vacuum pump 9. vacuum gauge 10. pneumatic valves The gas whose isotherm is to be measured is connected to two feed isolation valves. The third feed isolation valve is connected to He gas that is used to determine the void volume of the system Figure S13. Schematic of the high pressure volumetric gas adsorption system. Meanwhile, a known weight of the sample (~0.5 to 1g) was loaded into the sample cell. The sample loading is done in a glove box. The sample cell is equipped with a regulating ball valve to prevent exposure of the sample to ambient air while connecting to the system. After connecting the sample cell to the system, the system is evacuated. The vacuum in the system is read with the help of the vacuum gauge. Once a vacuum less than about 2 micron Hg has been achieved, the sample cell is placed in a heating mantle and heated to the desired S13

14 activation temperature for the appropriate length of time. The heater is then turned off and the sample cell is allowed to cool. When fully cooled, the heating mantle is removed and sample cell is immersed in the constant temperature bath. The void volume of the system is then determined using He gas, and the system is now ready for the isotherm measurement. Initially, the sample cell is isolated from the rest of the system by turning off the sample isolation valve. Feed gas is now slowly charged into the system (minus sample) to a desired pressure by opening the feed gas isolation valve. The pressure in the system is constantly monitored using the pressure transducer. The feed gas isolation valve is closed as soon as the target pressure is reached in the system. Temperature and pressure of the gas are monitored continuously until constant values are reached. The initial moles of feed gas are then calculated using the gas equation and SRK equation for compressibility factor determination. The sample isolation valve is then opened and feed gas is brought in contact with the sample. The drop in the pressure as a result of the adsorption is measured with the help of the pressure transducer and the temperature of the sample is measured with another thermocouple which is placed in the constant temperature bath in contact with the sample cell. The compressibility factor for each new pressure (and temperature) is calculated and this number is used to determine the current moles of gas utilized. From the difference in the number of moles between initial and current gas moles (corrected for the void volume) the loading is determined. These loading calculations are done automatically each time a new pressure and/or temperature are detected. The loading calculated at each step is added to the previously determined value and this process is continued until equilibrium is reached. After equilibration has been achieved, the sample isolation valve is closed and the sample cell is once again isolated from the system. The feed gas isolation valve is opened again and system is pressurized to the next level of desired pressure. This process of pressurization and adsorption is continued until the desired number of data points has been examined. The CO 2 isotherm on hydrated CuBTC sample was run in the same unit. The hydration of the sample, however, was done in a separate low pressure volumetric system. The low pressure unit is similar in working principle to the high pressure unit except for the fact that it uses the PV = nrt gas equation in its simple form without any compressibility factor determination. The low pressure unit is equipped with two pressure transducers (0-760 torr and 0-1 torr). This unit is mainly used to measure the adsorption isotherm for water and other solvents. Both units have a common sample cell. The following procedure was used for hydration and CO 2 isotherm measurement: 1. A known sample weight was loaded in the sample cell and activated in the high pressure unit. After activation and cooling, the void volume of the system with sample cell S14

15 was determined with helium gas. The system was then evacuated, and the valves exposing the sample and system to vacuum, as well as the ball valve on the sample cell, were closed. 2. The sample cell was removed from the high pressure unit and connected to the low pressure device and run for water isotherm until a desired water loading was achieved. For ~4 wt% water, a value of 0.65 torr pressure was utilized (see Fig. S14 for the water adsorption isotherm on Cu-BTC). At this point the ball valve on the sample cell was closed, and the sample cell was removed from the low pressure unit and once again connected to the high pressure unit. Since we had already determined the void volume for the sample, we did not have to go through helium pressurization and evacuation. Note: Even purging with helium may have removed some of the adsorbed water. 3. Before the start of the CO 2 isotherm determination, a minimum amount of vacuum was pulled on the sample to remove any water vapor left in the sample cell at end of the hydration step water, wt% wt% water at ~ 2.5 torr 5 4wt% water at ~ 0.65 torr pressure, torr Figure S14. Water isotherm on CuBTC at 25 o C after 175 o C activation. S15

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