Impact of the flexible character of MIL-88 iron(iii) dicarboxylates on the adsorption of n-alkanes Naseem A. Ramsahye 1, Thuy Khuong Trung 1, Lorna Scott, 2 Farid Nouar 2, Thomas Devic 2, Patricia Horcajada 2, Emmanuel Magnier 2, Olivier David, 2 Christian Serre 2, Philippe Trens 1* 1 Institut Charles Gerhardt, UMR CNRS 5253, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l'ecole Normale, 34296 Montpellier cedex 5, France 2 Institut Lavoisier, UMR CNRS 8180, Université Versailles Saint Quentin, 45 avenue des Etats-Unis, 78035 Versailles, France. Supporting information Experimental details The vapor adsorption/desorption experiments have been performed with a home-made apparatus already described elsewhere. 1 This set-up is based on manometric measurements (with two capacitative pressure gauges (0-10 mbar and 0-1000 mbar). The sample cell can be disconnected from the system to undergo a thermal treatment up to 500K (depending on the stability of the sample) under a vacuum of 10-3 mbar. The thermal stability of the samples was tested prior to the thermal treatment by X-ray thermodiffraction. The n-alkanes used as adsorbates (provided by Aldrich, purity > 99.9%) were outgassed and stored over activated 3Å molecular sieve. This set-up allows for the choice of the pressure of the adsorbate to be introduced. The amount of sample was usually around 150 mg. Vapor adsorption was performed at 313 K, each sorption experiment being performed with fresh samples in order to ensure the same initial state of activation of the samples. A duration of 300 s at the same pressure in the sample cell was chosen as criterion for the thermodynamic equilibrium. 2 Longer times of equilibration gave the same sorption isotherms. Depending on the relative pressure, and therefore the sorption process, different times of adsorption could be observed (see later).
Computational Methods The MIL-88B-2CF 3 and -4CH 3 structures were built using the initial coordinates already published by Horcajada et al, 3 and imposing the unit cell parameters obtained from ex-situ x- ray diffraction experiments on samples loaded with n-hexane. These structures represent the case where the MOF is already open to its maximum extent. They were minimised using the same strategy as Horcajada et al 3 and Devic et al, 4 using the Materials Studio Forcite software 5 and the Universal Force Field (UFF) 6 with the cell parameters held fixed. Thus we obtained structures with which Monte Carlo simulations were performed in order to examine the n-hexane conformers within the pores. Configurational Bias Grand Canonical Monte Carlo simulations were then performed using the CADSS simulation code and at least 107 iterations. The forcefield parameters and models used were the same as those used in our previous work. 7 Firstly, the atomic partial charges of the system were all set to zero, since the alkane framework and alkane-alkane interactions are governed by van der Waals interactions rather than electrostatics. Therefore, Lennard Jones parameters were used to describe these interactions. The potentials for the framework were taken from the Universal Force Field.6 The n-hexane molecules were modelled using a united atom model, where each carbon atom and its associated hydrogen atoms are treated as one Lennard-Jones interaction site. These sites were described using parameters from the TraPPE forcefield. 8 Ex-situ X-ray powder diffraction. XRPD data have been collected in a Bruker D8 diffractometer (λ=1.5406å). MIL-88A, MIL- 88B, MIL-88B_2CF 3 and MIL-88B_4CH 3 were first dispersed into glass capillaries of 1 mm
diameter and then activated during 4 hours at 423 K under primary vacuum. Back to adsorption temperature (313 K), the materials were exposed to vapors of n-pentane or n- hexane during 3 hours at relative pressures corresponding to the respective saturation plateaus. The XRPD patterns were then processed and unit cell assessed using the Fullprof software package and its graphical interface Winplotr. 9-10 The corresponding unit cell parameters and pattern matchings are reported in the SI (table S1 and figures S10 to S12). Synthesis and Activation MIL-88A: Fe3O[C2H2(CO2)2]3X nh2o (X= Cl, OH) Synthesis: FeCl 3.6H 2 O (2.70 g, 10 mmol), fumaric acid (1.16g, 10 mmol) and NaOH (0.32 g, 8 mmol) were dispersed in 50 ml of dimethylformamide (DMF) and loaded into a 250 ml round bottom flask and heated to 100 C for 16 hours under continuous stirring. The solid was then recovered by filtration and washed with dimethylformamide. Activation: The powder was suspended in 200 ml of ethanol for 16 hours. The final activated product was recovered by filtration and dried at 100 C. (see SI for XRPD and TGA data) MIL-88B: Fe3O[C6H4(CO2)2]3X nh2o (X= Cl, OH) Synthesis and activation conditions used are described elsewhere.3 MIL-88C: Fe3O[C10H6(CO2)2]3X nh2o (X= Cl, OH) Synthesis: A mixture of FeCl 3.6H 2 O (12.80 g, 47.6 mmol), 2,6-naphthalenedicarboxylic acid (10.40 g, 34.2 mmol) in 500 ml of dimethylformamide was loaded into a 1 L round bottom flask and heated to 130 C under continuous stirring for 18 hours. The resulting powder was then recovered by filtration. Activation: The solid was refluxed with 1 L of ethanol for 15 hours under stirring and recovered by filtration. (see XRPD and TGA data further down).
MIL-88B-Br: Fe3O[C6H3Br(CO2)2]3X nh2o (X= Cl, OH) Synthesis: FeCl 3.6H 2 O (1.53 g, 5.66 mmol), 2-bromoterephthalic acid (1.25g, 5.10 mmol) and NaOH ( 0.11 g, 2.83 mmol) were mixed in 50 ml dimethylformamide and 1 ml H 2 O and loaded to a 100mL round bottom flask. The mixture was then heated under continuous stirring to 100 C for 20 hours. The solid was then recovered by filtration and washed with dimethylformamide. Activation: The recovered solid was soaked in 50 ml of dimethylformamide for 3 hours, filtrated and placed in a vacuum oven for 15 hours at 150 C. The dried powder was then soaked in methanol (100 ml) for 3 hours. The activated product ( ~ 0.7g) was finally recovered by filtration and placed in an oven at 100 C for 1 hour. (XRPD and TGA can be found in SI). MIL-88B-2CF 3 : Fe3O[C6H2(CF 3 ) 2 (CO2)2]3X nh2o (X= Cl, OH) Synthesis and activation conditions can be found elsewhere.3 MIL-88B-4CH 3 : Fe3O[C6(CH 3 ) 4 (CO2)2]3X nh2o (X= Cl, OH) Synthesis: A mixture of FeCl 3.6H 2 O (4.05 g, 15 mmol), 2,3,5,6-tetramethylterephthalic acid 16 (3.33 g, 15 mmol) and NaOH (0.3g, 7.5 mmol) in 72 ml of dimethylformamide and 3 ml of H 2 O was loaded into a 250 ml round bottom flask and heated to 100 C under continuous stirring for 16 hours. The product was then recovered by centrifugation. Activation: In a first step, the solid was soaked in 150 ml of water for 15 hours under stirring and recovered by centrifugation. In a second step, the solid was dispersed in 75 ml of
methanol for 15 hours. About 3.2 g of dried solid was finally recovered by centrifugation (see XRPD, surface area determination and TGA data further down). X-ray powder diffraction: XRPD data have been collected in a Siemens D5000 diffractometer (CuKα; λcu=1.5406å). Thermogravimetric analyses: Data was collected on a PerkinElmer STA 6000 apparatus under O 2 atmosphere from room temperature to 600 C (heating rate 1-2 C/min). Accessible surface area measurement: Experiments were performed on a BelsorpII mini BEL apparatus. Nitrogen sorption isotherms were recorded at 77K after sample activation. MIL88(Fe)-A: Fe3O[C2H2(CO2)2]3X nh2o (X= Cl, OH) X-Ray Powder Diffraction: The data collected confirms the successful synthesis and activation of the compound (Figure S1). 11 15 14 13 12 11 10 9 8 7 6 d - Scale Figure S1: XRPD patterns of the as-synthesized MIL88(Fe)-A form (black) compared to the activated MIL88(Fe)-A form (red). TGA: The mass loss steps are within expected range (Figure S2). 11
120 100 Weight loss (%) 80 60 40 20 0 100 200 300 400 500 600 Temperature ( C) Figure S2: TGA of the activated MIL88(Fe)-A. The first steps correspond to ethanol and water molecules departure respectively (~27%) and the second step is attributed to the ligands (~37%). MIL-88(Fe)-C: Fe3O[C10H6(CO2)2]3X nh2o (X= Cl, OH) X-Ray Powder Diffraction: The data collected confirms the successful activation of the compound (Figure S3). 11 10 20 30 2-Theta - Scale Figure S3: XRPD patterns of the activated form of MIL88(Fe)-C.
TGA: The mass loss steps are within expected range confirming the successful activation of the compound (Figure S4). 11 100 Weight loss (%) 80 60 40 20 0 100 200 300 400 500 600 Temperature ( 0 C) Figure S4: TGA of the activated MIL88(Fe)-C. The first steps correspond to ethanol and water molecules departure respectively (~15%) and the second step is attributed to the ligands (~65%). MIL88(Fe)-B-Br: Fe3O[C6H3Br(CO2)2]3X nh2o (X= Cl, OH) X-Ray Powder Diffraction: The data collected confirms the flexibility of the compound and the successful activation (Figure S5).3
20 10 d - Scale 5 Figure S5: XRPD patterns of the as-synthesized MIL88(Fe)-B-Br form (black) compared to the activated MIL88(Fe)-B-Br form (red). TGA: The mass loss steps are within expected range (Figure S6).3 100 Weight loss (%) 80 60 40 20 0 100 200 300 400 500 600 Temperature ( 0 C) Figure S6: TGA of the activated MIL88(Fe)-B-Br. The first step loss corresponds to water molecules (~5.5%) and the second mass loss step is attributed to the ligands (~68%). MIL-88(Fe)-B-4CH 3 : Fe3O[C6(CH 3 ) 4 (CO2)2]3X nh2o (X= Cl, OH)
X-Ray Powder Diffraction: The data collected confirms the successful synthesis and activation of the compound. The bulky methyl groups render the structure less flexible (Figure S7).3 20 10 d - Scale 5 Figure S7: XRPD patterns of the as-synthesized MIL88(Fe)-B-4CH 3 form (black) compared to the activated MIL88(Fe)-B-4CH 3 form (red) TGA: Weight loss steps are well within expected range confirming the successful activation (Figure S8).3 100 Weight loss (%) 80 60 40 20 0 100 200 300 400 500 600 Temperature ( 0 C) Figure S8: TGA of the activated MIL88(Fe)-B-4CH 3. The first step loss corresponds to ethanol molecules and coordinated water molecules within the pores (~24%) and the second mass loss step is attributed to the ligands (~53%).
Accessible surface area measurement: A nitrogen sorption isotherm was recorded on the activated form of MIL88(Fe)-B-4CH 3 after degassing at 130 C for 15 hours. A specific surface area of about 1100 m 2 /g was obtained (BET model) (Figure S9).3 300 Amount adsorbed cm 3 /g 200 100 0.0 0.2 0.4 0.6 0.8 1.0 P/P 0 Figure S9: Nitrogen sorption isotherm (adsorption in blue and desorption in red) for the activated MIL-88(Fe)-B-4CH 3 Phase/Cell paramers a(å) b(å) c(å) Beta( ) Cell volume(å 3 ) MIL-88A_C5 11.56(1) 11.56(1) 14.64(1) -------- 1695.1(1) 10.24(2) 10.24(2) 14.70(2) 1335.2(1) MIL-88B_4CH 3 13.28(1) 14.82(1) 16.62(1) 14.82(1) 17.69(1) 16.62(1) 122.53(1) -------- 3293.2(1) 3162.3(1) MIL-88B_2CF 3 13.10(1) 13.48(1) 13.10(1) 13.48(1) 18.06(1) 18.01(1) -------- -------- 2692.6(1) 2837.5(2) Table S1 : unit cell parameters of MIL-88 samples loaded with various alkanes. Space group P-62c (n 190) P-62c (n 190) P2 1 /n (n 14) P-62c (n 190) P-62c (n 190) Cell parameters of MIL-88A and MIL-88B_4CH 3 have been obtained from the Dicvolgv program and the Winplotr interface 12 while those of MIL-88B_2CF 3 were deduced through analogy with other MIL-88 solids filled with solvents bearing similar peak positions. Pattern matchings have been realised from XRPD and the Fullprof program 13 through its graphical interface Winplotr. Note that in the case of MIL-88A and MIL-88B_2CF 3, a second phase has been refined corresponding to a lower degree of pore opening. Note also that, as reported previously for such flexible MOFs, the higher the contraction of the pores, the higher the anisotropic peak broadening of the XRPD.
Figure S10 : Rietveld plot of the activated MIL-88A sample exposed to vapor of pentane (λcu 1.5406Å).
Figure S11 : Rietveld plot of the activated MIL-88B_4CH 3 sample exposed to vapor of hexane (λcu 1.5406Å). Figure S11b : Rietveld plot of the activated MIL-88B_4CH 3 sample exposed to vapor of hexane (λcu 1.5406Å) (hexagonal symmetry). Figure S12 : Rietveld plot of the activated MIL-88B_2CF 3 sample exposed to vapor of hexane (λcu 1.5406Å).
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