Understanding the Effects of Preadsorbed Perfluoroalkanes on the

Jan 14, 2015 - Porous materials play an important role in the capture of toxic compounds from the atmosphere. ... the framework atoms were taken from ...
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Understanding the Effects of Preadsorbed Perfluoroalkanes on the Adsorption of Water and Ammonia in MOFs Peyman Z. Moghadam, Pritha Ghosh, and Randall Q. Snurr* Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: The effects of preadsorbed perfluoroalkanes on water and ammonia adsorption in a well-known metal−organic framework, Cu−BTC, were investigated using molecular simulations. It was found that perfluoroalkane molecules have little effect on water or ammonia adsorption in the Henry region. At higher loading, an analysis of the hydrogen bonds for water revealed that the presence of perfluoroalkanes reduces water clustering inside Cu−BTC. However, despite the expected hydrophobicity of perfluoroalkanes, their presence was not found to induce a more hydrophobic behavior in Cu−BTC, and the water condensation step was not significantly shifted. Ammonia uptake in Cu−BTC was not enhanced by perfluoroalkanes under dry or humid conditions.

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INTRODUCTION Porous materials play an important role in the capture of toxic compounds from the atmosphere. Impregnated activated carbons have been used as adsorbents to remove low vapor pressure compounds from air, but they are not effective in filtering some gases such as ammonia, NOx, and formaldehyde.1 The lack of effective filtration devices for these toxic gases necessitates the development of novel adsorbents. Advances in nanotechnology have enabled the development of novel materials through a targeted molecular design approach. In particular, porous coordination polymers or metal−organic frameworks2 (MOFs) are promising materials for filtration, as their pore size, shape, and surface chemistry can be tuned for specific adsorption separations.3−5 An inherent challenge in the capture of toxic compounds from the atmosphere is the competitive adsorption of water.6 Studies have shown that the adsorption capabilities of MOFs, especially those with open metal sites, are often adversely affected when exposed to water vapor.7−9 This suggests that the adsorbent should not only possess high capacity and selectivity for a targeted chemical but also be rather hydrophobic and stable in the presence of water. Recently, Decoste et al.10 reported that Cu−BTC (also known as HKUST-111) treated with a plasma-enhanced chemical vapor deposition of perfluorohexane (PFH) could withstand high humidity and even submersion in liquid water much better than the parent MOF. This improved stability was partially attributed to increased hydrophobicity of the MOF. It was also argued that the presence of PFH impedes the formation of water clusters in the pores. Thermogravimetric analysis (TGA) indicated that PFH constituted 16.1 wt % of the treated sample. In addition to their experimental studies, Decoste et al. performed grand canonical Monte Carlo (GCMC) simulations of PFH in Cu−BTC, which showed © 2015 American Chemical Society

that PFH molecules can adsorb in the windows and act as struts, thereby providing another mechanism for stabilizing the framework. They also reported that the treated MOF showed enhanced ammonia adsorption.10 In other work, the hydrophobicity of fluorinated compounds has been studied on perfluoroalkane surfaces12 and in MOFs with fluorinated cavities.13 To investigate the hypotheses put forth by Decoste et al., we performed molecular simulations of water and ammonia adsorption in Cu−BTC with and without preadsorbed perfluoroalkanes. We used the simulations to see if preadsorbed perfluoroalkanes enhance ammonia adsorption, if they suppress water uptake, and if they reduce clustering of water molecules. Ammonia adsorption results under dry and humid conditions were compared with experiments where available. To assess the generality of the results, the Henry’s constants of ammonia and water were calculated in 50 hypothetical MOFs with different loadings of preadsorbed PFH.

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GCMC SIMULATION DETAILS Water and ammonia adsorption isotherms were calculated using grand canonical Monte Carlo (GCMC) simulations14 implemented in the RASPA code.15 The Monte Carlo moves used were insertions and deletions attempted with twice the probability of reinsertions, displacements, and rotation moves. In the case of binary mixtures, we also used identity swap moves for faster convergence. For both pure component and mixture simulations, we used 5 × 105 cycles for equilibration and another 5 × 105 cycles to collect ensemble averages, where Received: November 26, 2014 Revised: January 11, 2015 Published: January 14, 2015 3163

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a cycle is the maximum of 20 or the number of molecules in the system at the beginning of the cycle. All simulations were run at 298 K. We used one unit cell of Cu−BTC with dimensions of 26.34 × 26.34 × 26.34 Å with the framework atoms fixed at their crystallographic positions.11 To mimic the experimental plasma-enhanced chemical vapor deposition of PFH, we added a fixed number of fully flexible perfluoroalkane molecules in our simulations, which were allowed to move inside the pores (i.e., perfluoroalkanes were only grown, displaced, and rotated in the pores). All adsorbate−adsorbate and adsorbate−adsorbent interactions were modeled with a Lennard−Jones (LJ) plus Coulomb potential with a LJ cutoff distance of 12.8 Å. The LJ parameters for the framework atoms were taken from the universal force field (UFF).16 Several water models exist in the literature that reproduce the experimental structural and thermodynamic properties of water with reasonable accuracy.17 On the basis of our previous study18 where we used the TIP4P19 model and obtained good agreement between simulated and experimental water adsorption isotherms in a number of MOFs, we used this model in the current work. For ammonia the TraPPE20 model was used, where positive charges are placed on the hydrogen atoms and a compensating negative charge site is placed 0.08 Å from the nitrogen atom. A single LJ site is also placed on the nitrogen. Perfluoroalkanes were modeled as fully flexible using the united atom model from Cui et al.21 Lorentz−Berthelot mixing rules were used to calculate all cross interactions. The partial charges for the Cu−BTC framework atoms were taken from Yazaydin et al.,22 who derived them from DFT calculations. The extended charge equilibrium (EQeq) method23 was used to obtain the partial charges for the hypothetical MOFs. Ewald summation14 was used to calculate electrostatic interactions for both adsorbent−adsorbate and adsorbate−adsorbate interactions. All force field parameters and atomic partial charges for Cu−BTC, water, ammonia, and perfluoroalkanes can be found in the Supporting Information.

Figure 1. Comparison between experimental26,27 and simulated water adsorption isotherms in Cu−BTC at 298 K.

this paper we are more interested in PFH effects on water adsorption at pressures above 40% relative humidity in Cu− BTC, so the agreement with experiment was deemed satisfactory. To study the influence of perfluoroalkanes on water adsorption, perfluorohexane (PFH) and perfluorodecane (PFD) molecules were preadsorbed at loadings of four and eight molecules per unit cell of Cu−BTC, which correspond to 13 and 25 wt % for PFH and 20 and 40 wt % for PFD, respectively. GCMC simulation results for the adsorption of water in the neat Cu−BTC and in the presence of PFH and PFD are shown in Figure 2, which shows that the presence of PFH and PFD chains does not influence the water uptake in the Henry’s region of the isotherms. However, their presence significantly reduces the steepness of the condensation step occurring between 20% and 40% relative humidity. The change in the condensation step becomes more prominent as the perfluoroalkane loading increases. However, the pressure at which condensation begins is not significantly shifted by the presence of perfluoroalkanes. At higher pressures, the saturation uptake is reduced for both PFH and PFD chains. This reduction is more prominent for the longer and bulkier PFD, due to the larger decrease in available pore volume. A similar reduction in the steepness of the water condensation step has been observed experimentally in a Zr−oxide-based MOF, NU1000, functionalized with perfluoroalkane carboxylates as the perfluoroalkane chain length increases.28 It should be noted that in the case of NU-1000, the perfluoroalkanes are coordinated to the metal sites. This means that in contrast to bare NU-1000, water molecules cannot fully access the metal nodes when the framework is functionalized. Even with the attached perfluorohexanes, only modest enhancement of hydrophobicity in the structures was observed. In this study, perfluoroalkanes are not coordinated to Cu−BTC and are free to move inside the pores. Therefore, water molecules can access favorable adsorption sites even in the presence of the perfluoroalkanes. To examine the energetics of water adsorption in Cu−BTC in the presence of perfluoroalkane molecules, we broke down the total potential energy into Cu−BTC−water and water− water energies for the cases of zero, four, and eight PFH per unit cell (Figure 3a). At relative humidities up to 20%, the Cu− BTC−water potential energy is dominant, as water molecules interact strongly with the framework and are adsorbed around hydrated Cu atoms (see simulation snapshots in Figure S1, Supporting Information). At higher loadings, water−water

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RESULTS AND DISCUSSION To validate our simulation results against experiment, we first calculated the single-component water adsorption isotherms in Cu−BTC using GCMC simulations. It is known that traditional classical force fields do not correctly describe interactions with the exposed copper sites in Cu−BTC and using a dry framework (i.e., no water molecules on Cu sites) significantly underpredicts water uptake through the entire pressure range.24 To better predict water adsorption behavior in Cu−BTC, we fixed one water molecule on each Cu atom of the framework using water oxygen positions from the hydrated material synthesized by Chui et al.11 (water hydrogen atoms were added using software). Previously, it has been shown both experimentally and in simulations that CO2 adsorption is increased by the presence of water molecules coordinated to open Cu sites in Cu−BTC.25 As shown in Figure 1, using the hydrated Cu−BTC, we found a reasonably good agreement between our simulated and experimental water isotherms from the literature26,27 at pressures higher than 30% relative humidity (RH). However, at low pressure, our simulated water isotherm underpredicts the amount adsorbed compared to experiments. At water loadings up to 4 mol/kg (i.e., 50 molecules per Cu− BTC unit cell) the water molecules are adsorbed near the hydrated Cu sites and at the windows connecting large and small cavities (see Figure S1, Supporting Information) but the condensation occurs at a higher relative humidity of ∼30%. In 3164

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Figure 2. Simulated water adsorption isotherms in Cu−BTC with and without preadsorbed (a) perfluorohexane (PFH) and (b) perfluorodecane (PFD) at 298 K.

Figure 3. (a) Breakdown of potential energy into Cu−BTC−water and water−water components for water adsorption without and with four or eight perfluorohexanes per Cu−BTC unit cell at 298 K. (b) Additional details about the breakdown of the potential energy for water adsorption with eight perfluorohexanes per Cu−BTC unit cell.

Figure 4. Simulation results with eight perfluorohexane molecules per unit cell of CuBTC. (a) Simulation snapshot at 80% RH (oxygen, red CPK representation; hydrogen, white CPK representation; perfluorohexane, orange VDW representation). Framework atoms are shown in VDW representation (Cu, blue; all other atoms, gray). (b) Radial distribution functions between framework Cu atoms and the oxygen atom in water (Ow) and the CF3 united atom site in perfluorohexane for 10% and 80% RH.

BTC (up to three water molecules per cavity) as illustrated in Figure S1, Supporting Information. At low loadings of water, the presence of PFH molecules has little effect on the

interactions become the dominant energy contributor, as water molecules start to cluster in the large octahedral pores. Above 40% RH, water molecules start to fill the small cavities in Cu− 3165

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Figure 5. (a) Distribution of the number of hydrogen bonds for different levels of relative humidity (RH) in Cu−BTC. (b) Distribution of the number of hydrogen bonds for water at 80% RH with and without eight perfluorohexane or perfluorodecane compared with bulk TIP4P liquid water.29

Figure 6. Simulated ammonia adsorption isotherms in Cu−BTC with and without preadsorbed (a) perfluorohexane (PFH) and (b) perfluorodecane (PFD) at 298 K.

cell), the first peak for water occurs at a distance of 3.4 Å and corresponds to water molecules interacting strongly with the hydrated Cu sites. The next two peaks correspond to next nearest neighbors of water molecules adjacent to Cu sites. At 80% RH (255 molecules per unit cell) the first peak is only slightly shifted by about 0.3 Å toward larger distances and the height of the second peak decreases. This means water is less localized in the second nearest peak to Cu sites at saturation loading. For PFH, the first peak for CF3 occurs around 6 Å for both 10% and 80% RH. The second peak for CF3 at 80% RH is shifted toward larger distances by 0.5 Å compared with the lower relative humidity of 10%. This further indicates that the bulky PFH molecules sit preferentially closer to the large pore center, while water molecules are able to interact with the metal clusters. Broader peaks for PFH also indicate less localization compared to the water molecules. To investigate how perfluoroalkanes affect water clustering inside Cu−BTC pores, we calculated the distribution of hydrogen bonds at different relative humidities averaged over the production cycles. To calculate the number of hydrogen bonds, we used a geometric criterion described by Xu et al.29 In these calculations, a pair of water molecules is considered hydrogen bonded if the O−O distance is closer than 3.5 Å and simultaneously the O−H···O angle is greater than 150°.30 Using this criterion we obtained the hydrogen bond distributions for different water loadings in Cu−BTC shown in Figure 5a. At relative humidities less than 20%, water molecules are far apart and form zero or, at most, one hydrogen bond. At higher relative humidities, water molecules start to cluster and begin to form two or more (dominantly three)

energetics, but at higher loadings there is a slight increase in the magnitude of Cu−BTC−water interactions and a slight decrease in the water−water interactions. Figure 3b shows more details about the energetics for the case of eight PFH molecules per unit cell. It can be seen that PFH molecules interact very weakly with each other and with the water molecules. At very low loadings of water, PFH interacts with the pore walls with an energy of ca. −10 kJ/mol, which is much lower in magnitude than the water−Cu−BTC interactions, which are ca. −50 kJ/mol. As the number of water molecules increases, the PFH interaction with the pore walls becomes smaller as more water molecules interact with the framework and with each other, indicating that PFH molecules are forced toward the center of the large pores. To examine how the presence of PFH molecules affects the arrangement of water molecules near the Cu atoms in Cu− BTC, we also looked at simulation snapshots and radial distribution functions (RDF). Figure 4a shows the results at 80% RH with eight PFH molecules per unit cell. Similar to adsorption in bare Cu−BTC, water molecules are adsorbed close to the Cu atoms even in the presence of PFH molecules. PFH molecules cannot access the small tetrahedral pores in Cu−BTC and sit either inside the larger octahedral pores or extend toward the framework windows that connect the large cavities. The siting of PFH molecules is similar to what Decoste et al. found in their pure component GCMC simulations of PFH in Cu−BTC.10 Figure 4b shows the radial distribution functions between framework Cu atoms and either the oxygen atom in water or the CF3 united atom site in PFH for 10% and 80% RH. It can be seen that at 10% RH (54 molecules per unit 3166

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Figure 7. (a) Breakdown of potential energy into Cu−BTC−ammonia and ammonia−ammonia components for adsorption with and without four or eight perfluorohexanes per Cu−BTC unit cell at 298 K. (b) Breakdown of potential energy for ammonia and eight perfluorohexanes per unit cell.

ammonia−ammonia interactions decrease when PFH is present. Figure 7b further compares the energy contributions when there are eight PFH molecules per unit cell. Similar to what was observed for water, PFH molecules interact very weakly with each other and with the ammonia molecules. PFH−Cu−BTC interactions decrease in magnitude as ammonia loading increases, suggesting that PFH molecules are displaced by ammonia from their preferred adsorption sites and pushed toward the center of the larger pores. As the ammonia loading increases, the PFH interaction with the pore walls become smaller and more ammonia molecules interact with the framework atoms and with each other. Comparing Figures 3 and 7, it can be seen that the water−water interactions are much stronger than the ammonia−ammonia interactions. The isosteric heats of adsorption for ammonia in Cu−BTC are shown in Figure 8 as a function of loading; they are calculated from the fluctuations of the potential energy over the production run of the GCMC simulations.31 The adsorption heats are greatest at low loadings of ammonia, and as the ammonia uptake increases, the favorable adsorption sites (i.e.,

hydrogen bonds per water molecule. It should be noted that the reason we occasionally observe water molecules with five hydrogen bonds is due to the geometric criterion used in defining a hydrogen bond. Figure 5b compares the distribution of hydrogen bonds for bulk liquid water and water in Cu−BTC at 80% relative humidity with and without eight PFH or PFD molecules preadsorbed. In liquid water (red), molecules construct mainly four hydrogen bonds, forming tetrahedral conformations. In the confined space of Cu−BTC, water molecules tend to form dominantly three hydrogen bonds (blue). This reduction in the number of hydrogen bonds from four in the liquid phase to three in the adsorbed phase was also observed in other MOFs.18 It is also observed that in the presence of either PFH or PFD there is a decrease in the probability of three and four hydrogen bonds and an increase in the probability of one or two hydrogen bonds (green and orange). This indicates that water molecules become more spread out in the pore space, and water clustering becomes less probable with perfluoroalkanes present in the pores. We also studied possible effects of perfluoroalkanes on ammonia adsorption in Cu−BTC. Figure 6 compares the adsorption isotherms for ammonia with and without (Figure 6a) PFH and (Figure 6b) PFD in Cu−BTC. Similar to the water adsorption isotherms, at low pressures, the perfluoroalkanes do not significantly affect the ammonia uptake. Above 0.2 bar, ammonia molecules start to fill the small cavities in Cu−BTC (Figure S2, Supporting Information). At pressures higher than 0.5 bar, the uptake is reduced as the available pore space is partly occupied by the perfluoroalkane molecules. The ammonia capacity predicted by simulations in pure Cu−BTC at 1 bar is ∼14.4 mol/kg. The breakdowns of the total potential energies of ammonia adsorption in Cu−BTC with and without PFH are compared in Figure 7a. At low ammonia loadings, ammonia interacts much stronger with Cu−BTC than with other ammonia molecules and is adsorbed around hydrated Cu atoms as shown in the simulation snapshots in Figure S2, Supporting Information. In the presence of PFH molecules, no significant effects were observed in the ammonia−ammonia and Cu−BTC−ammonia interactions at low loadings of ammonia. At 1 bar, there is a slight increase in Cu−BTC−ammonia interactions while

Figure 8. Isosteric heats of adsorption for ammonia at different pressures and 298 K with and without the presence of four or eight perfluorohexanes per unit cell of Cu−BTC. 3167

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Figure 9. Mixture simulations of ammonia and water at 1 bar of ammonia pressure and 298 K with and without preadsorbed PFH at (a) 80% and (b) 40% relative humidity.

Figure 10. Henry’s constants for 50 hypothetical MOFs for (a) water and (b) ammonia at 298 K with and without perfluorohexane.

hydrated Cu sites) become saturated (see Figure S2, Supporting Information) causing the heats of adsorption to fall to ∼30 kJ/mol at 1 bar. PFH molecules have minimal effects on the heats of adsorption throughout the pressure range studied. To provide another perspective on water and ammonia interactions with PFH, we also performed quantum mechanical calculations at the MP2/6-311+G(d,p) level to estimate binding energies between these molecules (see Supporting Information for details). The strongest calculated binding energies for water and ammonia with PFH are −8.3 and −8.8 kJ/mol, respectively. From the magnitude of the binding energies, it is evident that neither ammonia nor water binds strongly with perfluorohexane. As expected, the obtained interaction energies are much lower than the binding energies reported for water32−35 and ammonia33 interacting with Cu paddlewheel units, which have binding energies of ca. −50 and −78 kJ/mol, respectively. This supports the idea that water and ammonia molecules prefer to sit in the immediate vicinity of Cu sites even in the presence of PFH. As mentioned above, Decoste et al.10 hypothesized that one reason for the ability of Cu−BTC treated with PFH plasma to maintain its crystallinity in the presence of water or ammonia is due to PFH molecules acting as struts to prevent pore collapse. PXRD patterns reported by Decoste et al. show that neat Cu− BTC is not water stable and its framework undergoes major structural changes when immersed in water. On the other hand, Cu−BTC treated with PFH shows no change in the PXRD pattern under similar conditions. The GCMC simulations of Decoste et al. for single-component PFH in Cu−BTC showed

that these molecules adsorb across the windows connecting the large pores in Cu−BTC. In agreement with their finding, our mixture simulations of ammonia with PFH also revealed that PFH molecules are adsorbed mainly inside the larger pores and extend toward the windows as shown in the simulation snapshot in Figure S3, Supporting Information. Similar siting was also observed for mixtures of water and PFH (see Figure 4a); in both cases PFH molecules are too bulky to access the small pores (Figure S3, Supporting Information). To investigate the competitive adsorption of ammonia and water, mixture simulations were carried out at high (80%) and moderate (40%) relative humidities at 1 bar of ammonia with and without PFH in the pores. When there are no perfluoroalkanes present, the ammonia uptake is ∼13.2 mol/ kg at 80% relative humidity. This value is much higher than those obtained experimentally, which are on the order of 6−9 mol/kg at saturation.36 As discussed previously, irreversible partial loss of porosity is observed in experiments when the structure is exposed to water or ammonia in the absence of perfluorohexane, leading to a much lower saturation loading than in our simulations, where the framework atoms are kept fixed at their crystallographic positions. As shown in Figure 9, similar to pure component isotherms, the amount adsorbed for both ammonia and water decreases as PFH loading increases in Cu−BTC. Again, this is not surprising since the porosity is reduced when large PFH molecules are in the pores. The value obtained for ammonia adsorption at 80% RH and four PFH molecules per unit cell of Cu−BTC is 11.3 mol/kg and in very good agreement with breakthrough experimental results of 11.8 mol/kg of ammonia capacity for Cu−BTC treated with a PFH 3168

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plasma. These simulation results, along with results above, suggest that the observed experimental increase in ammonia uptake in Cu−BTC with perfluoralkanes is related to stabilization of the framework by the perfluoroalkanes and not due to preferential ammonia−perfluoroalkane interactions or increased hydrophobicity of the pores. To study the effects of perfluoroalkanes on ammonia and water adsorption on a wider range of MOFs, we used the Widom insertion37 method to calculate Henry’s constants (KH) for water and ammonia in 50 hypothetical MOFs38 with various pore sizes (see Supporting Information for the list of structures). KH is the slope of the isotherm at low pressure and describes the adsorbate−adsorbent interaction strength. It can serve as a useful metric to compare a large number of MOFs because it can be calculated very quickly. The KH results for ammonia and water with and without PFH molecules were compared in these MOFs, providing insight into how perfluoroalkanes affect water and ammonia uptake within the pore environment. Our simulation cell consisted of eight unit cells for each structure, and we inserted eight PFH molecules in the simulation cell. During these simulations we first carried out 30 000 cycles of Monte Carlo moves in the canonical ensemble to equilibrate the system (only for the PFH molecules), followed by another 30 000 cycles for production and Widom particle insertion while allowing the PFH molecules to move in the pores. Figure 10 shows the parity plots between KH with PFH and KH without PFH for water (Figure 10a) and ammonia (Figure 10b). It can be seen that in the majority of the structures, KH is only slightly lower when PFH is present in the pores. For structures with higher affinities toward H2O and NH3 (i.e., KH > 10−4 mol/(kg Pa)), the values differ more prominently when PFH is present in the pores. This is because some of these structures have smaller pores and PFH molecules can block the intrinsic interaction sites of the MOFs and hinder ammonia or water interaction with the framework. For structures with smaller KH (e.g., the structures with larger pores), there is no significant change in the KH values for either ammonia or water when PFH is in the pores at the specified loadings. In general, the small variations in KH values for water and ammonia observed in Figure 10 are in agreement with the results for Cu−BTC where changes in the uptake at low loading were minor in the presence of PFH and PFD (Figures 2 and 6).

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ASSOCIATED CONTENT

S Supporting Information *

Force field parameters and atomic partial charges for Cu−BTC, water, ammonia, and perfluoroalkanes; simulation snapshots; lowest energy configurations of water and ammonia interacting with perfluorohexane; corresponding ammonia and water binding energies with perfluorohexane; Henry’s constant calculations for 50 hypothetical MOFs; hypothetical MOF IDs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s): R. Q. Snurr has a financial interest in the start-up company NuMat Technologies, which is seeking to commercialize metal−organic frameworks.

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ACKNOWLEDGMENTS We thank the Army Research Office (grant W911NF-12-10130) for financial support. Computational work was partly supported by Northwestern University’s shared computer system, Quest (project P20261).

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REFERENCES

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CONCLUSIONS Molecular simulations indicate that the presence of preadsorbed perfluoroalkanes has little effect on ammonia and water uptake in Cu−BTC in the Henry’s region. However, due to the bulky nature of perfluoroalkanes, they reduce the accessible pore volume leading to lower saturation capacity for both compounds. The distribution of hydrogen bonds revealed less water clustering when perfluoroalkanes were present in the pores. However, no significant shift in the water condensation step, and hence no enhanced hydrophobicity in internal Cu− BTC cavities, was observed. Furthermore, the potential energy calculations showed that water and ammonia molecules interact more strongly with the hydrated copper sites and with each other than with perfluorohexane molecules. The results suggest that the observed experimental increase in ammonia uptake in Cu−BTC upon plasma treatment with perfluorohexane is mainly due to improved stability of the framework prompted by the presence of perfluoroalkane species in the pores. 3169

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DOI: 10.1021/jp511835d J. Phys. Chem. C 2015, 119, 3163−3170