Article pubs.acs.org/JPCC
Modeling Water and Ammonia Adsorption in Hydrophobic Metal− Organic Frameworks: Single Components and Mixtures Pritha Ghosh, Ki Chul Kim, and Randall Q. Snurr* Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: Capturing ammonia under humid conditions is difficult because chemical functionalities that attract ammonia tend to also attract water. In this work, three hydrophobic metal−organic frameworks (MOFs) were assessed for ammonia capture under dry and humid conditions. Using grand canonical Monte Carlo simulations, pure water isotherms were calculated and showed good agreement with experiment. The low heats of adsorption for water substantiate the hydrophobic nature of these MOFs. Simulated ammonia isotherms predict that hydrophobic MOFs adsorb similar amounts of ammonia under dry and humid conditions. These results suggest that hydrophobic MOFs could be appropriate candidates for ammonia capture under humid conditions. The simulations also provide detailed information on the behavior of water and mixtures of water and ammonia confined in hydrophobic pores.
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INTRODUCTION Toxic industrial chemicals (TICs), such as ammonia, sulfur dioxide, and ethylene oxide, are produced in a variety of industrial contexts.1 There is a need for adsorbents capable of capturing these chemicals to protect persons who may be at risk of exposure. The current standard is activated carbon, which is known to have a high affinity for organic molecules but is a less effective adsorbent for smaller, polar molecules.2 Because many TICs fall into this second class of molecules, there has been an effort to tailor activated carbons for use in TIC capture by adding metal and organic impregnates.3 Chemically tunable porous materials, such as metal−organic frameworks (MOFs), are an attractive alternative. MOFs are porous crystals composed of metal nodes connected by organic linkers, and they can be synthesized in a variety of topologies and chemical compositions. This great diversity suggests that MOFs have the potential to capture a range of TICs under ambient conditions. There have been some studies assessing TIC adsorption in MOFs.4−8 The majority of these studies focused on MOFs with unsaturated metal sites and found that these MOFs are able to capture TICs when exposed to the pure vapor. However, TIC capture is most critical under ambient conditions, where the adsorbent is exposed to a mixture of TIC vapor and other gases, including water vapor. Studies that assessed adsorption under humid conditions found that MOFs with open metal sites often experience a drastic decrease in performance in the presence of water vapor.9−12 This is believed to be due to occupation of the metal sites by water, thereby preventing the TIC from accessing its preferred adsorption site. Hydrophobic MOFs are an interesting alternative, because their hydrophobicity ensures that TICs will not have to compete against water for adsorption sites within the MOF. © 2013 American Chemical Society
In this work, we study the adsorption of TICs in hydrophobic MOFs under both dry and humid conditions. Ammonia is chosen as a representative TIC.13 The hydrophobic MOFs chosen are ZIF-8,14 a widely studied hydrophobic MOF; Zn4O (3,5-dimethyl-4-carboxy-pyrazolato)3,15 a hydrophobic MOF-5 analogue that we will refer to as Zn(pyrazole); and Al(OH)(1,4 naphthalenedicarboxylate),16 a MOF that is hydrophobic at low humidities. Using grand canonical Monte Carlo (GCMC) simulations, we first validate the accuracy of our model by calculating pure water isotherms and comparing to experimental results for each of these MOFs. We then predict both pure and mixture adsorption of ammonia in each MOF at two humidity conditions. We demonstrate that hydrophobic MOFs do not experience a decrease in ammonia adsorption under humid conditions.
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SIMULATION DETAILS Systems Studied. MOFs were chosen for this study based primarily on stability and hydrophobicity. Stability concerns ruled out the IRMOF series of MOFs, which are known to decompose under ambient conditions within a day.17,18 MOFs such as HKUST-1 were also excluded, because they have been shown to degrade when subject to a mixture of ammonia and water vapor.19,20 Requiring demonstrable hydrophobicity led to the exclusion of any MOF with unsaturated metal sites. By applying the criteria of stability and hydrophobicity, we ensure that the MOFs chosen are realistic candidates for TIC capture. Finally, to avoid computational complications, we focused on rigid MOFs, with no framework flexibility. Received: October 31, 2013 Revised: December 17, 2013 Published: December 19, 2013 1102
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Figure 1. Pure water isotherms at 298 K calculated using two different water models and compared to experiment.15,22 ZIF-8 on an (a) absolute and (b) relative pressure scale. Zn(pyrazole) on an (c) absolute and (d) relative pressure scale. The vertical dotted line indicates the experimental vapor pressure of H2O.
sodalite topology, with spherical cages of 11 Å connected by windows of 3.4 Å . The second MOF, Zn(pyrazole), has also demonstrated hydrophobicity and stability and, in fact, has been shown to successfully capture nerve agent analogues under humid conditions.15 This MOF has Zn4O6+ nodes connected by dimethyl-carboxypyrazole linkers and is topologically similar to the IRMOF series of MOFs, with pores of 6.5 Å connected through windows of 4 Å × 4 Å . The third MOF in this study, Al(NDC), is the least hydrophobic, showing appreciable uptake of water at roughly 45% relative humidity (RH). This MOF is made up of an aluminum backbone connected by napthalene dicarboxylate linkers; two sizes of square channels (7 Å × 7 Å and 3 Å × 3 Å) run parallel to the backbone of the MOF. Force Field. In developing a force field for these systems, we strove to develop a consistent methodology that could be applied to any hydrophobic MOF, without fitting to
While methods for measuring the hydrophobicity of surfaces are well established (for example, contact angle measurements), there is not yet a consensus on how to characterize hydrophobicity of micropores. In this study, the hydrophobicity of the MOF was determined on the basis of pure water vapor isotherms. Type III and type V isotherms21 are generally consistent with weak adsorbate−adsorbent interactions, and so MOFs with pure water isotherms of either of these types were considered hydrophobic. The hydrophobicity of a MOF can then be quantified by the pressure at which water condenses in the pores of the MOF, a higher pressure indicating a more hydrophobic MOF. Three MOFs were selected that fulfill the criteria of stability, hydrophobicity, and rigidity. The first MOF, ZIF-8, is the quintessential hydrophobic MOF. Made up of zinc atoms connected by methyl imidizaolate linkers, ZIF-8 is known to be hydrophobic as well as exceptionally stable.14,22 ZIF-8 has 1103
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Figure 2. Simulated results of water adsorption in Al(NDC) at 298 K using the TIP4P water model. (a) Comparison of pure water isotherms from experiment16 and simulated without any pore blocking (red) and with small channels blocked (orange). (b) Density plot of adsorbed water molecules at 3000 Pa with small channels blocked.
Mixture isotherms of ammonia and water started with an equilibrated pure water configuration (from a pressure corresponding to the relative humidity of interest), after which 10 million equilibration steps were run. After we checked that the system had equilibrated, 1 million production steps followed. The Peng−Robinson equation of state was used to relate fugacity and pressure, except for the pure water isotherms, where we assumed a fugacity coefficient of 1 (which is equivalent to using the Peng−Robinson equation of state for the pressures of interest45). Error bars are computed by dividing the simulation cycles into 5 blocks and calculating the standard deviation with a 95% confidence interval.
experimental results. A standard Lennard-Jones and Coulomb model was used. The Lennard-Jones parameters for the framework atoms were adopted from the DREIDING23 force field with the exception of aluminum, which was taken from the Universal Force Field (UFF).24 Framework charges were obtained from periodic DFT calculations; the electrostatic potential of each system was generated using the Vienna ab initio software package VASP,25−28 followed by REPEAT29 calculations to assign charges to the framework atoms. The model for ammonia was taken from the TraPPE set of force fields,30 which use an offset of 0.8 Å between the nitrogen and the M site, which is an additional charge site. There are a myriad of options available to model water. We tested two 4 site, nonpolarizable water models: TIP4P31 and TIP4P/2005.32 While TIP4P is much more popular than TIP4P/2005, there have been studies suggesting that TIP4P/2005 may be a better choice (albeit for the liquid phase).33−35 Both of these water models have been shown to work well in mixture simulations with TraPPE molecules, such as alcohols, dipropylene glycol dimethyl ether, and CO2.36−39 All Lennard-Jones potentials were truncated at a distance of 12.8 Å, and analytical tail corrections were implemented. All Coulomb interactions were computed using Ewald summations. GCMC Details. GCMC was used to simulate pure and mixture adsorption isotherms in the chosen MOFs at 298 K. GCMC moves were broken down as follows: 50% insertion/ deletion, 25% random reinsertion, and 12.5% each translation and rotation. For the mixture isotherms, there was also an identity change move that allowed a molecule of one species to be swapped for a molecule of the other species. In the case of the pure water isotherms, equilibration for each isotherm point took between 60 million and 2 billion steps. After it was confirmed that equilibrium had been reached, there were between 20 million and 600 million production steps, over which properties were averaged to calculate the loading and heat of adsorption. GCMC simulations of water in porous materials are notoriously difficult to equilibrate and regularly require upward of 10 million GCMC steps.38,40−45 Pure ammonia isotherms were equilibrated after a million GCMC steps, with another million steps used for production isotherms.
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RESULTS AND DISCUSSION Water Adsorption. Figure 1 compares simulated water isotherms with experimental results for the ZIF-8 and Zn (pyrazole) MOFs. Both the simulated and the experimental results show no appreciable adsorption of water in either of the MOFs until near the vapor pressure. There are two sets of simulated results, one using TIP4P and the other using TIP4P/ 2005. Plotted on an absolute pressure scale (Figure 1a and c), the TIP4P/2005 results show good agreement with experiment, while the TIP4P results are shifted toward higher pressures. However, when plotted on a relative pressure scale (Figure 1b and d), the TIP4P results show excellent agreement with experiment. Experimental water isotherms are regularly reported in terms of relative pressure, and when comparing with simulation, it is more consistent to use the saturation vapor pressure as predicted by the water model. While the experimental saturation vapor pressure of water at room temperature is 3.2 kPa,46 the vapor pressure of TIP4P water is roughly 4.1 kPa, and TIP4P/2005 predicts a vapor pressure of around 0.9 kPa.33,38 Because of this difference in predicted vapor pressure, we chose TIP4P for all mixture simulations in this study. Figure 2 illustrates water adsorption in Al(NDC). Notably, Figure 2a shows that the simulated loading differs from experimental results at relative pressures above 0.5. While our results do correctly identify the pressure at which water condenses in the pores of the MOF, we predict a much higher 1104
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Figure 3. Isosteric heats of adsorption for water in (a) ZIF-8, (b) Zn(pyrazole), and (c) Al(NDC) calculated using fluctuations over the course of the simulation (eq 1), red ●, and the derivative of the potential energy with respect to loading (eq 2), blue ■.
saturation loading (Figure 2a, red ●). This MOF features two sets of 1D square channels: large channels with a cross section of 7 Å × 7 Å and small channels with a cross section of 3 Å × 3 Å . Density plots from our simulations indicate that the larger channels fill first, followed by the smaller channels at higher pressures. Reported kinetic diameters for water range between 2.6547 and 3.2 Å,48 suggesting that the small channel may not be accessible to water molecules. To investigate this further, the simulation was rerun with the small channels blocked (Figure 2a, orange ●). By blocking the small channels, the simulated isotherm does plateau at a lower loading, but it is still higher than the experimental results. Figure 2b shows the density distribution of water in this MOF at 3000 Pa with the small channels blocked. We obtain a loading of 12 molecules per large channel, in contrast to 9 molecules per large channel as reported in the experiments of Comotti et al.16 This difference may be explained by some pore blockage in the experimental sample; the original synthesis paper reports that it was not possible to measure nitrogen uptake in this MOF.
It is interesting to compare our methodology for water adsorption in Al(NDC) with that of Paranthaman et al.42 In both cases, the force field parameters used for the framework atoms are from DREIDING with the exception of the aluminum, which is nominally taken from UFF. However, there is a significant difference in the values reported for the aluminum atom; in our study σAl = 4.008 Å and εAl/kB = 254.1 K; however, they report values of 4.39 Å and 156.1 K, respectively. Additionally, while this study uses TIP4P, the Paranthaman study uses TIP4P-Ew.32 Perhaps most importantly, our charges are calculated from periodic QM calculations of Al(NDC). In contrast, their charges are adapted from another MOF, MIL-47, and then scaled to fit the experimental results for Al(NDC). Comparing these two sets of charges, it is clear that the trend in the charges is identical, but our charges are greater in magnitude than those of Paranthaman. This difference may explain why our results show a higher loading than their results, because water adsorption is highly sensitive to electrostatics. 1105
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Isosteric heats of adsorption were calculated in two equivalent ways.49 The first averages the fluctuations of the potential energy over the course of the Monte Carlo cycles for each pressure point: Q st = RT −
⟨VN ⟩ − ⟨V ⟩⟨N ⟩ ⟨N 2⟩ − ⟨N ⟩2
(1)
where V is the total potential energy of the system, N is the number of molecules, and the angled brackets indicate an ensemble average. The heats of adsorption calculated by this method are shown in Figure 3 as red ●. However, because of the large fluctuations inherent in simulating water adsorption (as evidenced by the billion Monte Carlo steps necessary for equilibration), the error bars on these values are quite large. After the entire isotherm had been obtained, we also calculated the isosteric heats of adsorption using the derivative of the total potential energy: ⎛ ∂⟨V ⟩ ⎞ Q st = RT − ⎜ ⎟ ⎝ ∂⟨N ⟩ ⎠T
Figure 4. Histogram of the number of hydrogen bonds for bulk liquid water and water adsorbed in ZIF-8, Zn(pyrazole), and Al (NDC) at P/ P0 = 0.95 . Hydrogen bonding was determined using geometric criteria.
(2)
The results of this method are shown in Figure 3 as blue ■. Values obtained by this method have more reasonable error bars, while generally agreeing with the values calculated from the fluctuations. Because the errors in the loading and the potential energy of the system are small, propagation of these errors leads to small errors bars for the heats of adsorption as well. For all of the MOFs, at low loadings, the heats of adsorption are quite low, and only as water molecules begin to condense do the heats of adsorption rise. This is to be expected for hydrophobic MOFs; at low loadings, the only interactions are between the water and the framework, and these are weak interactions. However, as the loading increases, the number of water−water interactions also increases, leading to heats of adsorption slightly above the heat of vaporization for bulk water. This is in stark contrast to the heats of adsorption for functionalized MOFs, where the heats of adsorption at low loading are often much greater than those at high loadings.50,51 Experimental data for water in the Zn(pyrazole) and Al(NDC) MOFs are limited to pure vapor isotherms; however, heats of adsorption and Henry’s constants have been measured for water in ZIF-8. Saint Remi et al. report a low coverage heat of adsorption of 35.2 kJ/mol and a Henry’s constant of 8.5 × 10−6 mol/kg/Pa at 130 °C.52 Figure 3 shows a range of low coverage heats of adsorption from 15 to 35 kJ/mol for loadings between 0.0001 and 0.01 mmol/g. Thus, our results are consistent with the experiment, while suggesting that at even lower coverage the experimental heat of adsorption may be lower still. To calculate Henry’s constant at 130 °C, we used the Widom insertion method53 and obtained a Henry’s constant of 5.3 × 10−7 mol/kg/Pa, which (consistent with the findings for the heat of adsorption) is somewhat lower than the experimental value. To understand the differences between water confined in hydrophobic MOFs and bulk water, we looked at the hydrogenbond network of water in these MOFs at P/P0 = 0.95. Hydrogen bonds were determined using a geometric criterion: the oxygen−oxygen distance between two water molecules must be less than or equal to 3.5 Å, and the OHO angle must be no less than 150°.54 The results are shown in Figure 4. In all cases, more than 70% of water molecules have at least 3 or 4 hydrogen bonds. However, in bulk water, most water molecules have 4 hydrogen bonds, while most adsorbed water molecules
have 3 hydrogen bonds. This indicates that, as expected, most water molecules in the MOFs are interacting with the walls of the pore and have less opportunity to hydrogen bond with other water molecules compared to the bulk liquid. It is also interesting to compare ZIF-8 to Al(NDC). The pores of ZIF-8 are spherical with a diameter of around 11 Å connected via narrow windows roughly 3.4 Å in diameter; on the other hand, Al(NDC) has square channels with a cross section of 7 × 7 Å. While 45% of water molecules in both of these MOFs have 3 hydrogen bonds, there is a 10% difference in the number of water molecules with 4 hydrogen bonds. The channels of Al(NDC) allow for uninterrupted hydrogen bonding along the length of the channel, leading to a higher percentage of water molecules with 4 hydrogen bonds. Ammonia Adsorption. Figure 5 shows predicted ammonia isotherms under dry, moderately humid, and extremely humid conditions for ZIF-8, Zn(pyrazole), and Al(NDC). Moderately humid conditions in this Article are defined as 36% RH, while extremely humid conditions are 80% RH. Humidity was simulated by fixing the partial pressure of water across the isotherm at 1500 or 3300 Pa, corresponding to 36% RH and 80% RH, respectively, based on the vapor pressure predicted by TIP4P. Most experimental studies of TIC adsorption in MOFs under humid conditions take place at 80% RH. In general, the three MOFs do not show a large change in the loading of ammonia when comparing humid conditions to dry conditions. There is significant adsorption of water in the Zn(pyrazole) and Al(NDC) MOFs at 80% RH and low partial pressures of ammonia; this corresponds with the pure water isotherms for these MOFs, both of which experience water condensation in the pores at 0.8 P/P0. These results suggest that, unlike MOFs with open metal sites, hydrophobic MOFs do not experience a large decrease in TIC adsorption capacity under humid conditions. We had speculated that as the adsorption of ammonia progressed, the environment within the pores might become less hydrophobic, thereby encouraging water adsorption. However, this does not seem to occur to a significant extent. In ZIF-8, there is a small increase in water adsorption as ammonia adsorption increases 1106
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Figure 5. Ammonia isotherms for (a) ZIF-8, (b) Zn(pyrazole), and (c) Al(NDC) at 298 K under dry (0% RH), moderately humid (36% RH), and humid (80% RH) conditions.
comparable to those of pure ammonia mixtures.30 Because of both the low concentration of ammonium ions and their negligible effect on solvation structure, we neglect the presence of ammonium ions in these simulations. Figure 6 shows density plots of ammonia and water in Zn(pyrazole) at 80% RH. These density plots illustrate how the siting of molecules in the pores changes when the partial pressure of ammonia increases from 0.1 to 1 bar. At 0.1 bar, small amounts of ammonia are concentrated near the metal nodes of the MOF, while water molecules are scattered about the pore at a constant density. However, at 1 bar, the walls of the pore are lined with a high concentration of ammonia molecules, while the center of the pore is a mix of ammonia and water molecules. The radial distribution functions (RDFs) associated with these two pressures, Figure 7, corroborate what is shown in the density plots. While the position of the peak for each RDF does not shift between the lower and higher ammonia pressures, the intensity of the peak increases at the higher pressure, indicating a more ordered organization of the adsorbates. This is most pronounced for gOO(r); the loading of water decreases from 95 molecules per unit cell to 8 molecules per unit cell. However, the higher intensity peak illustrates that these fewer water molecules cluster together more tightly.
above 1 bar. However, there is no decrease in ammonia adsorption as compared to the dry isotherm. In Al(NDC) at 36% RH, there is a large increase in water adsorption around 0.05 bar partial pressure of ammonia. This MOF does show a slight decrease in ammonia adsorption under humid conditions, with a difference of 2 mmol/g between dry and extremely humid conditions at 0.5 bar partial pressure of ammonia. It is well-known that ammonia can form ammonium ions in the presence of water; however, in these simulations we did not let ammonia and water react. In other words, we neglect the presence of ammonium ions. To check the validity of this assumption, we estimated the concentration of ammonium ions using the dissociation constant of ammonia. The estimated number is 1 ammonium ion per 500 ammonia molecules (calculations shown in the Supporting Information). This value is an overestimation, because in these systems there is less water than ammonia and dissociation constants are calculated for dilute aqueous solutions, where there is an abundance of water molecules available. Additionally, even if a few ammonium ions were present, it is unlikely that they would disrupt the solvation structure of the adsorbed ammonia−water mixtures; it is known that the ammonia−ammonia radial distribution functions for a binary mixture of water and ammonia in the presence of an ammonium ion55 are 1107
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low loading may explain the results of breakthrough studies of ammonia capture in ZIF-8 under dry and humid conditions, where ZIF-8 is shown to have a very low capture capacity for ammonia.9 The Zn(pyrazole) and Al(NDC) MOFs adsorb around 7 and 6 mmol/g of ammonia at 1 bar, respectively; the significant capacity and high selectivity of these MOFs suggest that they are potential candidates for ammonia capture under humid conditions.
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CONCLUSIONS We simulated isotherms for water in three MOFs of varying hydrophobicities and have found the TIP4P water model to be superior to TIP4P/2005 at predicting water adsorption in these hydrophobic MOFs. The water isotherms show good agreement with experimental isotherms, especially with respect to the pressure at which water condenses in the pores of the MOF. We also calculated the heats of adsorption using two different methods; these results further confirmed the hydrophobic nature of these materials. Additionally, we analyzed the hydrogen bonding between water molecules at saturation loading in these MOFs. We then assessed the adsorption of ammonia in these MOFs under dry and humid conditions. We find that hydrophobic MOFs do not experience a decrease in ammonia capture capacity under humid conditions as compared to dry conditions, unlike other MOFs previously studied for TIC capture. While the hydrophobic MOFs studied all showed high selectivity for ammonia over water, ZIF-8 does not show a large capacity for ammonia capture at 1 bar, because it only adsorbs roughly 2 mmol/g of NH3. However, the Zn(pyrazole) and Al(NDC) MOFs are both capable of adsorbing significant amounts of ammonia at 1 bar under both dry and humid conditions.
Figure 6. Density plots in Zn(pyrazole) at 80% RH. Top row from left to right: density of ammonia at 0.1 and 1 bar partial pressure of ammonia; loadings of 1.4 and 41 molecules per unit cell, respectively. Bottom row from left to right: density of H2O at 0.1 and 1 bar partial pressure of ammonia; loadings of 95 and 8 molecules per unit cell, respectively. Brighter colors indicate higher densities.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Force field parameters, details for calculation of framework charges, additional water isotherms for Al(NDC), and details for isosteric heat of adsorption calculations. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 7. Radial distribution functions (RDFs) in Zn(pyrazole) at 80% RH. Water−water RDF (gOO(r)), water−ammonia RDF (gNO(r)), and ammonia−ammonia RDF (gNN(r)) compared at 0.1 and 1 bar partial pressure of ammonia.
ACKNOWLEDGMENTS We thank the Defense Threat Reduction Agency (HDTRA110-1-0023) for financial support and Northwestern University’s high performance computing system, QUEST, for computational resources. Thanks also to Dr. David Fairen-Jimenez for many fruitful discussions and Olga Karagiaridi and Zachary Brown for their help regarding the Al(NDC) MOF.
Ideally, a MOF for the capture of ammonia should have a high selectivity for ammonia over water, and all three of these MOFs demonstrate high selectivities, over 100, for ammonia over water. (Selectivities for each of the MOFs at both humidity conditions can be found in the Supporting Information.) However, capacity is equally important. ZIF-8, for example, shows a high selectivity for ammonia over water, but at 1 bar only adsorbs roughly 2 mmol/g of ammonia. This
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