Iodine Gas Adsorption in Nanoporous Materials: A Combined

Jan 30, 2017 - (7) Additionally, if Ag+ and I2 react in zeolite pores, the resulting AgI .... The in-house iodine adsorption unit (Figure 1) consists ...
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Iodine Gas Adsorption in Nanoporous Materials: A Combined Experiment−Modeling Study Dorina F. Sava Gallis,† Ivan Ermanoski,‡ Jeffrey A. Greathouse,§ Karena W. Chapman,∥ and Tina M. Nenoff*,⊥ †

Nanoscale Sciences Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States Materials, Devices, and Energy Technology Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States § Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States ∥ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, X-ray Science Division, Argonne, Illinois 60439 United States ⊥ Physical, Chemical and Nano Sciences Center, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States ‡

S Supporting Information *

ABSTRACT: Here, we present a combined experimental and Grand Canonical Monte Carlo (GCMC) modeling study on the adsorption of iodine in three classes of nanoporous materials: activated charcoals, zeolites, and metal−organic frameworks (MOFs). Iodine adsorption profiles were measured for the first time in situ, with a uniquely designed sorption apparatus. It was determined that pore size and pore environment are responsible for a dynamic adsorption profile, correlated with distinct pressure ranges. At pressures below 0.3 atm, iodine adsorption is governed by a combination of small pores and extra-framework components (e.g., Ag+ ions in the zeolite mordenite). At regimes above 0.3 atm, the amount of iodine gas stored relates with an increase in pore size and specific surface area. GCMC results validate the trends noted experimentally and in addition provide a measure of the strength of the adsorbate−adsorbent interactions in these materials.

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including fission gases4 and hydrocarbons.5 Chemical surface treatment via controlled heating under steam atmosphere is used to enhance adsorption properties.5 Charcoal adsorbs iodine (I2) gas readily; impregnation of the carbon with metals such as silver increases the affinity for organic−iodine gas adsorption.6 However, the holding capacity of iodine-loaded charcoal decreases with time because of physical or chemical “poisoning” by common environmental compounds like water and hydrocarbons. The second class of materials are zeolites, crystalline porous aluminosilicates with pores in the size range of ∼3−7 Å, surface areas of ∼100−500 m2/g, and a net negatively charged framework that is charge balanced by exchangeable extra framework cations. They are chemically, thermally, and mechanically stable materials. Over the past 8 years, Nenoff et al., have described both nanoscale chemistry and mechanisms of Ag−zeolite (Ag−Z = Ag−mordenite, Ag−MOR) capture of iodine gas (I2),7 and organic−iodides (eg., CH3−I)8 from complex streams (including H2O and NOx). This work is

INTRODUCTION There is a great need for the selective capture and storage of radiological fission gases created in alternative energy production cycles, such as aqueous nuclear fuel reprocessing, or in nuclear reactor accidents.1 One example is radiological iodine gas (I2) whose long-lived 129I isotope half-life is ∼17 million years. Safe and highly selective iodine isolation requires a detailed understanding of both the individual capture materials currently used and also those planned for the future. Nanoporous solid sorbents are vital to the selective chemical adsorption of fission gases.1 Prior work in the field of fission gas capture has focused on several key issues, including adsorption material types and selectivity factors that govern gas−sorbent chemical bonding mechanisms. Included in this class are activated charcoal/carbon, zeolites, and metal−organic frameworks (MOFs). Their exceptionally high selectivity and sorption capacity stem from pore size, chemical/physical adsorption mechanisms, and ionic charge. In general, they are chemically, radiologically, and mechanically robust. The first class of materials are activated charcoals (AC), mesoscale carbon-based materials with pore sizes from ∼4.5 to 60 Å2,3 and surface areas of up to 3000 m2/g. High surface area and widespread availability and/or ease of manufacturing makes them common adsorption materials for many types of gases, © 2017 American Chemical Society

Received: Revised: Accepted: Published: 2331

October 31, 2016 January 23, 2017 January 30, 2017 January 30, 2017 DOI: 10.1021/acs.iecr.6b04189 Ind. Eng. Chem. Res. 2017, 56, 2331−2338

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understanding of the strength, mechanism, and kinetics of each gas’ adsorption and desorption. These ultimately relate to the possible release into waste forms via processes such as pressure swing adsorption (PSA).23 The obvious method for understanding those factors is the use of adsorption isotherm testing. Unfortunately, because iodine is a corrosive gas, no commercial system is available for such an analysis. Various groups are working on addressing these limitations.2,24 Additionally, earlier work reports on the use of an adsorption test unit for iodine,2 in which a partial iodine pressure environment was established via mixed gas flows. However, to measure the iodine uptake, the adsorption isotherms were taking prohibitively long hold times during testing, and possible inconsistencies might have been encountered due to air exposure, as the sample was periodically removed from the apparatus and weighed in open air. More recently, controlled iodine adsorption kinetics into MOFs was measured using an iodine flow cell using argon as carrier gas.24 Herein, we present a study that elucidates and compares/ contrasts the individual adsorption profiles of nanoporous materials used in iodine gas capture. First, the design, build, and test parameters of our unique iodine adsorption unit are described. Second, candidates from three classes of nanoporous materials are investigated: (a) metal−organic frameworks (MOFs), ZIF-8 and Cu−BTC, (b) zeolites, Ag-exchanged mordenite (Ag+−MOR), and (c) two different activated charcoals, referred to here as AC1 and AC2. These materials have very distinct characteristics in terms of hydrophilic/ hydrophobic pore environments, along with specific pore apertures, surface areas, and pore size distribution. Third, the modeling and testing of various nanoporous materials in the adsorption unit with I2 gas is presented. Simulated I2 loadings obtained from Grand Canonical Monte Carlo (GCMC) methods are compared with the experimental results for the candidate materials. The complexity of AC complicates the use of a single molecular model that properly represents the variety of surfaces, active sites, and pore sizes found in these materials. Rather, we have chosen to use idealized carbon nanotube (CNT)25 models with no active adsorption sites as a simple model of AC. Trends in I2 adsorption can be examined as a function pore diameter, and the largest CNT model (pore diameter 22.3 Å) behaves much like a planar graphite sheet commonly used to model AC.26

considered a breakthrough due to its elucidation of the mechanism of iodine capture. Recent work confirms the mechanisms of Ag−-zeolite capture of iodine from organic iodides.9 Ag−MOR (surface area ∼300 m2/g) is the zeolite of choice for iodine capture during aqueous reprocessing.10 However, because it is generally a commercial product intended for catalytic operations, the silver is impregnated on (not ion exchanged into) the zeolite. This method of fabrication is a “wash” method, which produces partially ion-exchanged silver into the zeolite pores and partially bulk-deposited silver on the zeolite surface. The latter is easily oxidized to AgO, which is nonreactive to iodine gas.11 Our earlier published studies on the mechanisms of iodine capture indicate that either Ag+ in the pores or reduced Ag° metal (both in the pores and on the surface of the zeolite) will react with I2 to form chemisorbed AgI nanoparticles.7 Additionally, if Ag+ and I2 react in zeolite pores, the resulting AgI nanoparticles are also considered physisorbed by being physically trapped inside the pore. Subsequent thermal studies indicated that release of the iodine is only possible via two methods: (1) AgI melting at 558 °C or (2) I2 surface desorption due to lack of Ag bonding. A third class of nanoporous materials has been developed and applied to gas sorption and separations: metal−organic frameworks (MOFs).12 MOFs are porous crystalline structures that combine the connectivity of metal centers with the bridging ability of organic ligands and can afford very high surface areas (1000−6000 m2/g). The adsorption of gas molecules is tuned for maximum binding either to the metal center or the organic linker of the MOF framework. As a class of porous materials, they are being studied heavily for use in various separations and gas storage applications.12−15 There has been some preliminary work on the design, synthesis, and testing of metal−organic frameworks (MOFs) for I2 capture. Initially, size selectivity was thought to be best for I2 capture, and a pore-restricted ZIF-8 MOF (Zn(2methylimidazole)2·(DMF)·(H2O)3; 3.4 Å pore opening) framework was studied.16 While I2 readily adsorbed in large quantities into the MOF (up to 125 wt %), subsequent time on-stream studies indicated slow kinetics. Importantly, thermochemistry studies showed that the iodine adsorption into ZIF-8 was energetically much stronger than onto activated carbon, indicating much stronger selective bonding of I2 into the MOF.13 Further research into I2−ZIF-8 was also accomplished: chemi- and physisorption of iodine,17 amorphization for iodine retention,18 and incorporation into glass for long-term storage waste forms.19 In contrast, a larger pored MOF (eg., Cu−BTC, HKUST-1; copper benzene-1,3,5-tricarboxylate) was tested for iodine sorption.20 This MOF was tuned not for size selectivity, as it has large pore openings (∼9 Å), but for preferential chemisorption to the metal sites inside the pore. This proved successful, with up to 175 wt % I2 adsorbed into the MOF. Furthermore, it had preferential sorption for I2 from a complex stream of I2 and H2O. Importantly, these previous studies have also shown that the zeolites and MOFs of interest are stable in radiological fields, in particular, from gamma irradiation. Samples exposed to doses between 2218 Gy and 10 kGy and characterized by powder Xray diffraction showed no structural degradation.21,22 The use of nanoporous materials for the capture of iodine and other fission gases with extreme accuracy requires an

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EXPERIMENTAL SECTION

Materials. All materials were purchased from commercially available sources and used without further purification. Iodine (99.8% ACS) was purchased from Acros Organics. ZIF-8 and Cu−BTC powders were purchased from Sigma-Aldrich under the commercial names of Basolite Z1200 and Basolite C300, respectively. Sodium mordenite (Na+−MOR) was obtained from Honeywell UOP Corporation as LZM-5Na. Silverexchanged mordenite (Ag+−MOR) was synthesized by using a previously reported method.27 LZM-5Na (50.0 g) was soaked in an aqueous solution of AgNO3 (0.1 M, 500 mL) and held at 85 °C for 12 h with stirring. The solution was filtered in air, and the solids were mixed with a fresh AgNO3 solution (0.1 M, 500 mL) and held at 85 °C for a further 24 h, while stirring. This sample was then filtered and the solid was dried at 95 °C for 12 h. The dried sample was sieved after grinding to 75 μm particle size. Pelletized activated carbon samples (AC1 and AC2) were received from Kuraray under the commercial names of GC30 × 60 and 2GA-H2, respectively. 2332

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avoid I2 condensation, the connecting tubing is heated to Tc ∼ 120 °C during measurementswell above the temperatures in the rest of the system. Following evacuation of atmospheric gases, iodine vapor at a desired pressure (P) is created by setting the source temperature (T1). The vapor pressure was calculated from the known P(T) relationship for I2. Here, pI2(T) using the Antoine equation log 10(P) = A − (B/(T + C)), where the coefficients A, B, and C are based on previously published data.31 During measurements, T2 > T1 to avoid I2 condensation in the sample chamber. The mass gain onto the sorbent material during the adsorption process is measured in situ, using a vacuum- and temperature-compatible precision load cell (scale). The load cell digitally displays the weight gain, while the data is logged using a computer interface. The load cell (Transducer Techniques) has a 10 g range, 1 mg precision, and is compatible with operation between −55 and 93 °C and down to a high-vacuum environment of ∼10−4 Pa. I2 Gas Adsorption Measurements. For the experiments described in this study, the iodine uptake capacity was measured at a constant T2 = 75 °C. Here, T1 was gradually increased from 35 to 70 °C at a heating rate of 6 °C/h. At T2 = 75 °C, this T1 range corresponds to a relative pressure (P/P0) between 0.1 and 0.7. Here, P0 is the I2 (equilibrium) vapor pressure at 75 °C. Upon reaching the final source temperature (T1 = 70 °C) and corresponding P/P0 = 0.7, T1 was held at 70 °C for an additional hour. Simulations of Iodine Gas Adsorption. Simulated I2 adsorption isotherms and isosteric heats of adsorption were obtained from GCMC simulations using the Towhee code32 at temperatures of 27 and 75 °C for I2 pressures between 0.00040 and 1.17 bar. Atomic coordinates for the MOF structures were taken from the crystallographic coordinates with the solvent removed,33,34 while those of the Ag−MOR were taken from the literature,35 with extra framework sodium ions replaced by silver ions. Idealized CNT models were used to represent activated carbon. Three such models with van der Waals diameters of 5.0, 10.0, and 22.3 Å were generated using the nanotube builder in the Materials Studio software (BIOVIA, Inc.) as described previously.36 Simulation cells consisted of single unit cells for Cu−BTC, while 2 × 2 × 2 supercells were created for ZIF-8 and Ag− MOR. Framework atoms (including extra framework silver ions in mordenite) were held fixed at their crystallographic coordinates, and periodic boundary conditions were applied in all three dimensions to account for crystalline periodicity. Lennard-Jones interaction parameters for diatomic I2 molecules and atoms in the MOF structure were taken from the Universal Force Field.37 In the zeolite model, Lennard-Jones parameters were only included for oxygen atoms38 and extra framework silver cations.39 Parameters from a graphite model40 were used for carbon atoms in the CNT models. Lorentz−Berthelot combining rules41 were used to calculate the Lennard-Jones cross-parameters for all gas−framework interactions with a short-range cutoff radius of 12.5 Å. Electrostatic interactions were not taken into consideration. This modeling approach has been used successfully to simulate the dynamics of I2 in Cu− BTC20 and ZIF-816 as well as the adsorption of diatomic guest molecules in idealized CNT models.35 A total of 5 × 107 moves were performed in each simulation; the first half of these moves were used for equilibration and were not included when calculating gas loadings. Gas molecules

High-Energy Synchrotron Scattering and Pair Distribution Function (PDF) Analysis. X-ray scattering data suitable for diffraction and PDF analysis were collected at beamline 11-ID-B at the Advanced Photon Source at Argonne National Laboratory for AC1 and AC2 samples. For PDF analysis, high energy X-rays (11-ID-B, 58 keV, λ = 0.2114 Å) were used, in combination with a large amorphous silicon-based area detector, to collect data to high values of momentum transfer, Qmax = 22 Å−1. For diffraction analysis, the detector was placed a large distance (100 cm) from the sample to maximize the 2-theta resolution of the diffraction data. The two-dimensional images were reduced to one-dimensional scattering data within fit2d. The PDFs, G(r), were extracted within PDFgetX2,28 subtracting contributions from the background, Compton scattering, and fluorescence to the total scattering data as described previously.29 To separate the features in the PDF associated with the iodine-structure interactions, differential PDFs (dPDFs) were calculated, subtracting the pristine material from the PDF of the I2-loaded samples. The position and area of features of interest within the dPDF were quantified by fitting Gaussian functions within fityk.30 Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). SEM analyses were captured on a FEI NovaNano SEM 230, at various accelerating voltages between 1 and 20 kV. EDS analyses were collected on an EDAX Genesis Apex 2 with an Apollo SDD detector. Sample Activation and N2 Gas Adsorption Measurements. All samples were initially subjected to an activation protocol to ensure the highest surface area is accessed. ZIF-8 was thermally treated at 300 °C for 4 h, Cu−BTC at 180 °C for 10 h, Ag+−MOR at 180 °C for 5 h, and AC1 and AC2 at 200 °C for 5 h. Nitrogen gas adsorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 surface area and porosity analyzer. Nitrogen of ultrahigh purity (99.999%, obtained from Matheson Tri-Gas) was used in these experiments. Iodine Adsorption Unit. The in-house iodine adsorption unit (Figure 1) consists of two small vacuum chambers, for the

Figure 1. Schematic depiction of the iodine adsorption unit.

I2 source and for the sample, respectively. The vacuum chambers and connecting tubes utilize standard stainless steel ultrahigh vacuum (UHV) components. Nickel, nickel-plated copper, aluminum, and elastomer sealing gaskets were used wherever possible to avoid reactivity between standard copper gaskets and I2. The system is pumped by a single-stage rotary vane pump with a base pressure of 10 Pa. The chambers reside inside separate furnaces with temperature controllers. Iodine solid is introduced to the system at room temperature in a removable glass ampule attached to the source chamber. To 2333

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adsorbers under these conditions are the activated charcoals (∼117 wt % I2 for AC2; ∼105 wt % I2 for AC1), followed by ZIF-8 MOF (∼90 wt % I2), Cu−BTC MOF (∼46 wt % I2), and Ag+−MOR, (∼16 wt % I2). In addition to adsorption weight gain profiles, adsorption kinetics profiles were collected (Figure S3). The small pored Ag+−MOR sample reaches saturation fastest after ∼150 min due to increased chemisorptive affinity between the poreoccluded Ag and the iodine. Iodine adsorption in AC2 plateaus after ∼350 min and within this study’s pressure range at P/P0 = 0.7. It is important to note that equilibrium has not been reached with the 6 °C/h ramping for all samples studied except Ag+−MOR. Therefore, it was decided to hold the I2 source isothermally at 70 °C for an additional hour. Most of the samples have comparable adsorption kinetics and appear to reach saturation in ∼450 min, within the additional adsorption hold time. The maximum weight loadings reached after this additional 1 h treatment were ∼141 wt % I2 for AC1, ∼117 wt % I2 for AC2, ∼117 wt % I2 for ZIF-8, ∼76 wt % I2 for Cu− BTC, and ∼16 wt % I2 for Ag+−MOR. Both ZIF-8 and Ag+− MOR adsorption results are generally consistent with the I2 weight % loading levels we previously measured using the original iodine gas loading experiments in air.16,27 The trend is not held for the Cu−BTC structure, in which the I2 uptake is hindered under the vacuum conditions in this study; the original data shows a maximum of 175 wt % I2.20 The MOF and zeolite samples were analyzed by powder Xray diffraction and exhibited expected powder patterns (Figure S4). However, the activated charcoals have no long-range crystallinity and required a combination of surface area analysis, PDF and synchrotron, and GCMC modeling to develop a structure−property relationship between the material and the iodine sorption results. A molecular-level understanding of the pore environment in the AC samples can be obtained from the GCMC results of the three idealized CNT models with van der Waals pore diameters of 5.0, 10.0, and 22.3 Å. Snapshots from the highest loading pressure at 1.17 bar (Figure 5) show the strong interaction between I2 molecules and the pore walls. In the 5 Å CNT, the only adsorption site is characterized by I2 molecules in the center of the nanotube (Figure 5a) in axial orientation in an end-to-end arrangement (Figure 5d.) As the CNT diameter increases from 5 to 10 Å, I2 molecules remain near the pore walls to maximize attractive van der Waals interactions (Figure 5b). This trend continues with the 22 Å nanotube (Figure 5c), and interestingly, no adsorption is seen in the centers of the 10 and 22 Å CNTs. Adsorption results from GCMC simulations for all adsorbent materials are presented in Figure 6. From the adsorption isotherms (Figure 6a), it is observed that the highest initial I2 loading at low pressures (0.0008 bar) is in the smallest pores of various materials. When studying the CNTs, the adsorption of I2 in the 5 Å nanotube (22 wt %) is greater than the 10 Å nanotube (2 wt %); adsorption in the larger 22 Å nanotube is almost nonexistent at the lowest pressure (0.1 wt %). Among the MOF and Ag−MOR models, Cu−BTC has the largest initial I2 adsorption (0.7 wt %) followed by ZIF-8 (0.6 wt %) and last Ag−MOR (0.1 wt %). It should be noted that the I2− Ag−MOR potential parameters do not directly include electrostatic interactions or electronic (chemical bonding) effects such as in-pore AgI nanoparticle formation. Therefore, pore size versus iodine adsorption in Ag−MOR is skewed.

underwent insertion, deletion, translation, and rotation with equal frequency.

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RESULTS Materials characterization was undertaken for all the nanoporous sorbent materials studied in an effort to determine nanoscale structure−property relationships to bulk scale materials sorption performances. The BET specific surface area measured for Ag+−MOR was 236 m2/g. Among the MOFs, the surface area of ZIF-8 was 1733 m2/g, while for Cu− BTC the surface area measured was 1763 m2/g. These results are in line with the expected sample porosity.16,20 Particular emphasis was placed on the characterization of the two types of activated charcoals. These materials are wellknown and commonly used as radiological iodine adsorbers in the environmental communities.42,43 N2 gas adsorption properties were evaluated on these two samples (Figure 2); the BET

Figure 2. Nitrogen adsorption isotherms measured at 77 K for AC1 and AC2 samples.

surface area of AC1 is estimated to be 1323 m2/g, while for AC2 is 824 m2/g. The two AC samples studied here have similar carbon content (∼95 wt %) and impurities concentrations (Table 1). Table 1. Elemental Distribution in AC Samples As Determined via SEM-EDS AC sample

sample composition

AC1 AC2

C = 94.8%; O = 4.6%; trace %: Al, Si, P, S, Cl C = 94.9%; O = 4.8%; trace %: Si, S, Cl

Despite similar chemical compositions, the samples appear to have distinct structural features. Synchrotron X-ray diffraction data (Figure 3a) reveal that the lowest 2θ angle peak (at ∼1.75 Å) is sharper for the AC1 material than AC2. This, together with the PDFs for these pristine materials (Figure 3b), suggests better layer ordering (i.e., more graphitic character) in AC1 than in AC2. The differential PDFs (d-PDFs) (Figure 3c) were obtained by subtracting the PDFs of the as-received AC samples from the I2 loaded AC samples. A well-defined sharp feature at ∼2.7 Å is observed in both samples, representing guest I−I correlations. The I2 adsorption profiles were collected on all the nanoporous sorbent materials at 75 °C and up to P/P0 = 0.7 (Figure 4). The data indicates that the highest percentage 2334

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Figure 3. (a) Synchrotron X-ray diffraction data (17 keV, λ = 0.72768 Å). (b) PDF and (c) d-PDF analyses for AC1 and AC2 samples.

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DISCUSSION Herein, we report the I2 adsorption capacity, mechanisms, and kinetics in a variety of nanoporous materials. Experimentally, a newly designed vacuum-compatible I2 gas adsorption unit was used, which allows for continuous sample exposure to I2 and simultaneous in situ mass gain measurements over a wide range of pressure, temperature, and gas composition regimes. Importantly, this design can be adapted to co-adsorption measurements, enabling the characterization of sorbents in the presence of purposely chosen and well-controlled background gases. An important observation is that equilibrium was not achieved with the slowest heating rate of 6 °C/h, for the majority of the samples, with the exception of Ag+−MOR. Because of that, the experimental I2 adsorptions for the MOF and AC samples do not resemble the anticipated isotherm profiles. Notably, the GCMC-simulated I2 adsorption isotherms complement the experimental studies and confirm that all samples do in fact exhibit the expected type I adsorption isotherm, characteristic for all microporous materials. We anticipate that these data can be experimentally obtained with our designed apparatus, using a much slower ramping rate and allowing equilibrium to be reached at each data point. Significant information can still be inferred from the I2 adsorption profiles measured experimentally in these nanoporous materials under the conditions studied here. Data indicate that each of the samples studied has distinct I2 adsorption mechanisms associated with, and determined by, the distinct nanostructural features. As such, the Ag+−MOR sample adsorbed 16 wt % I2. There is a distinguishable I2 uptake in Ag+−MOR at the lowest relative pressures measured (P/P0 = 0.1) as compared to all other nanoporous materials studied. This is due to strong binding interactions between I2 and silver

Figure 4. Iodine uptake in ZIF-8, Cu−BTC, AC1, AC2, and Ag+− MOR measured at 75 °C.

Although Ag−MOR has small pores, I2 molecules interact weakly with the zeolite framework. As the I2 pressure is increased, loading in the sorbent materials with larger accessible volumes (ZIF-8, Cu−BTC, 22 Å CNT) continues to increase. Materials with limited accessible volume (Ag+−MOR, 5 and 10 Å CNTs) reach maximum loading at low pressures. These modeled adsorption trends are consistent with our experimental findings. The highest initial adsorption at 27 °C occurs in the 10 Å CNT (36 wt %), (Figure S6), while adsorption in the 5 Å CNT is unaffected by temperatures in this range. Corresponding modeled isosteric heats of adsorption are consistent with the adsorption isotherms and are discussed below (Figure 6b.) 2335

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Figure 6. Results from from I2−CNT GCMC simulations at 75 °C showing (a) adsorption isotherms and (b) isosteric heats of adsorption.

The two AC samples start adsorbing I2 at ∼ P/P0 = 0.2, at an intermediate pressure range as compared to Ag+−MOR (P/P0 = 0.1) and MOFs (P/P0 = 0.3), inferring moderate adsorbate− adsorbent interactions at pressures below P/P0 = 0.3. Due to their high surface areas, pore filling occurs at higher relative pressures, via a similar mechanism as that observed in the MOF samples. Comparison of the AC1 and AC2 I2 adsorption profiles shows variations in nanopore accessibility due to porosity structuring. The enhanced surface area in AC1 (∼37% higher as compared to AC2), infers more accessibility to the pores, based on the ordering observed in the XRD and PDF analyses. This translates in an overall higher saturation adsorption capacity in AC1, ∼141 wt % I2, vs AC2 ∼117 wt % I2. However, AC2 has faster adsorption kinetics, correlated with a higher affinity for iodine as compared directly to AC1. This behavior can be attributed to a less open pore network, resulting in stronger I2−carbon wall interactions. Additional information about the mechanism of I2 adsorption in these materials is provided by GCMC simulations. There is a very good correlation between the experimental and modeling results for the 75 °C I2 adsorption data. GCMC simulations reveal that materials with the largest number of favorable I2− sorbent interactions have the highest adsorption enthalpies (Figure 6b). As noted above, the only sample that does not follow the experimental trend studied here is Cu−BTC. On the basis of the modeling data, initial adsorption in the octahedral cavities of Cu−BTC is quite favorable (46 kJ/mol) due to the large number of favorable interactions between iodine atoms and framework atoms. However, adsorption in the larger cages is less favorable, so the adsorption enthalpy decreases once the octahedral cavities are filled. These GCMC results relate very

Figure 5. Snapshots from I2−CNT GCMC simulations at 27 °C and 1.17 bar for CNTs with VDW pore diameters of (a) 5.0 Å, (b) 10.0 Å, and (c) 22.3 Å. (d) Side-on view of the 5.0 Å CNT shows the linear arrangement of adsorbed I2 molecules.

ions via a chemisorptive process, with the formation of AgI in the zeolite pores. The adsorption rate is fast and correlates with easy accessibility to these strong binding sites. Very little additional I2 uptake is noted beyond P/P0 = 0.4, owing to relatively small pore sizes and surface area as compared to the MOF and AC samples. In both MOF materials studied here, ZIF-8 and Cu−BTC, no I2 gas is adsorbed at lower relative pressures, up to P/P0 = 0.3. Beyond this point, there is a gradual increase in the I2 adsorption as a function of pore size and framework characteristics. These findings indicate a distinct adsorption mechanism in both the larger pored MOFs as compared to that observed for the Ag+−MOR zeolite sample and are correlated with weaker I2−framework interactions (i.e., physisorption). Interestingly, we note slightly different behavior of the two MOF materials under the current experimental setup (vacuum). While I2 adsorption in ZIF-8 is consistent with the expected saturation capacity of ∼120 wt %, the maximum I2 uptake in Cu−BTC is vastly different (75 wt %), than previously measured, 175 wt %. This discrepancy can be attributed to weaker I2−framework interactions, in conjunction with large pore openings in Cu−BTC. As a result, the weaker bound I2 molecules desorb from the MOF easier under vacuum than in static air. 2336

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Industrial & Engineering Chemistry Research well with experimental findings from our previous experimental setup conducted at ambient conditions.20 Additionally, examining I2 adsorption in the CNT models allows us to consider only the effects of van der Waals interactions on I2 loading in activated carbons. GCMC indicates that molecules adsorbed in the 5 Å nanotube have only one possible (linear) orientation without experiencing repulsive interactions with pore walls. This linear orientation also maximizes the number of favorable interactions, resulting in an unusually high adsorption enthalpy of 100 kJ/mol. Accessible pore volume is limited, however, resulting in the maximum I2 loading of 27 wt % being achieved at the lowest pressure simulated. Doubling the nanotube diameter to 10 Å still results in a high adsorption enthalpy (56 kJ/mol), indicating that I2 molecules still have a favorable interaction with I2 atoms on the other side of the nanotube. For the 22 Å nanotube and the MOF models, the accessible surfaces are more planar with a consistent I2 adsorption enthalpy of 35 kJ/ mol, where the only interactions are occurring between I2 and carbon wall, rather than an additive effect of the I2···I2 interactions within the pore.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the office of NA-22, U.S. Department of Energy. Sandia National Laboratories is a multi-mission lab managed and operated by Sandia Corp., a wholly owned subsidiary of Lockheed Martin Corp., for the U.S. DOE’s NNSA under Contract No. DE-AC04-94AL85000. Work done at Argonne and use of the Advanced Photon Source, an Office of Science User Facility operated for the US DOE/Office of Science by Argonne National Laboratory, was supported by the U.S. DOE, Contract No. DE-AC0206CH11357.

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CONCLUSIONS Here, we reported for the first time the iodine adsorption profiles in distinct classes of nanoporous materials, measured via a newly designed system. It was determined that pore size and pore environment play the biggest roles in amount of I2 adsorbed in various pressure regimes. As such, at low relative pressures (P/P0 = 0.3), inferring a mainly physisorptive process. GCMC simulations correlate very well with the experimental data. Importantly, simulations of I2 adsorption in carbon nanotube models with various pore sizes confirm the experimental trend noted here. That is, the smallest 5 Å pores exhibit the highest adsorption enthalpy of 100 kJ/ mol and reach saturation at the lowest simulated pressures, whereas with increased pore sizes to 10 and 22 Å there is a decrease in the adsorption enthalpy, along with an increase in the overall I2 uptake. Together, all these findings establish the prerequisites for the synthesis of next generation made-to-order ef fective and selective I2 getters via a combination of (i) large pore size and high surface area and (ii) strong I2−in-pore bonding (extraframework cations or other polarizable species). Ongoing studies are focusing on the performance of these materials under competitive controlled mixed gases environments.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04189. Additional SEM-EDS and molecular modeling. (PDF)

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REFERENCES

(1) Riley, B. J.; Vienna, J. D.; Strachan, D. M.; McCloy, J. S.; Jerden, J. L., Jr. Materials and processes for the effective capture and immobilization of radioiodine: A Review. J. Nucl. Mater. 2016, 470, 307−326. (2) Bhatia, S. K.; Liu, F.; Arvind, G. Effect of pore blockage on adsorption isotherms and dynamics: anomolous adsorption of iodine on activated carbon. Langmuir 2000, 16, 4001−4008. (3) Laine, J.; Yunes, S. Effect of the Preparation Method on the Pore Size Distribution of Acivated Carbon from Coconut Shell. Carbon 1992, 30, 601−604. (4) Reyerson, L. H.; Cameron, A. E. The Sorption of Bromine and Iodine by Activated Charcoal. J. Phys. Chem. 1935, 40 (2), 233−237. (5) Ray, G. C.; Box, E. O., Jr. Adsorption of Gases on Activated Charcoal. Ind. Eng. Chem. 1950, 42 (7), 1315−1318. (6) Swarup, R.; Mishra, S. N.; Jauhari, V. P. Control and Sensing of Environmental Quality; Mittal Publications: New Delhi, India, 1992; Vol. 8. (7) Chapman, K. W.; Chupas, P. J.; Nenoff, T. M. Radioactive Iodine Capture in Silver-Containing Mordenites through Nanoscale Silver Iodide Formation. J. Am. Chem. Soc. 2010, 132, 8897−8899. (8) Nenoff, T. M.; Rodriguez, M. A.; Soelberg, N.; Chapman, K. A. Silver-Mordenite for Radiological Gas Capture from Complex Streams: Dual Catalytic CH3I Decomposition and I Confinement. Microporous Mesoporous Mater. 2014, 200, 297−303. (9) Chebbi, M.; Azambre, B.; Cantrel, L.; Koch, A. A Combined Drifts and Dr-Uv−Vis Spectroscopic in Situ Study on the Trapping of Ch3I by Silver-Exchanged Faujasite Zeolite. J. Phys. Chem. C 2016, 120, 18694−18706. (10) Soelberg, N. R.; Garn, T. G.; Greenhalgh, M. R.; Law, J. D.; Jubin, R.; Strachan, D. M.; Thallapally, P. K. Radioactive Iodine and Krypton Control for Nuclear Fuel Reprocessing Facilities. In Science and Technology of Nuclear Installations; Hindawi Publishing, 2013, p 702496. (11) Garino, T. J.; Nenoff, T. M.; Rodriguez, M. A. Optimization of Ag Addition to AgI-MOR GCMs to Suppress I2 Loss; FCRD-SWF-2014000286; Sandia National Laboratories, 2014. (12) MOFs are comprehensively reviewed in (a) The 2009 Metal− Organic Frameworks Issue. Chem. Soc. Rev. 2009, 38, 1203−1504. (b) The 2012 Metal−Organic Frameworks Issue. Chem. Rev. 2012, 112, 673−1268. (13) Nenoff, T. M. Hydrogen Purification, MOF Membranes Put to the Test. Nat. Chem. 2015, 7, 377−378. (14) Parkes, M. V.; Sava Gallis, D. F.; Greathouse, J. A.; Nenoff, T. M. The Effect of Metal in M3(btc)2 and M2(dobdc) MOFs for O2/N2 Separations: A Combined Density Functional Theory and Experimental Study. J. Phys. Chem. C 2015, 119, 6556.

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*E-mail: [email protected]. ORCID

Tina M. Nenoff: 0000-0002-7906-4810 2337

DOI: 10.1021/acs.iecr.6b04189 Ind. Eng. Chem. Res. 2017, 56, 2331−2338

Article

Industrial & Engineering Chemistry Research (15) Sava Gallis, D. F.; Parkes, M. V.; Greathouse, J. A.; Zhang, X.; Nenoff, T. M. Enhanced O2 Selectivity versus N2 by Partial Metal Substitution in Cu-BTC. Chem. Mater. 2015, 27, 2018−2025. (16) Sava, D. F.; Rodriguez, M. A.; Chapman, K. W.; Chupas, P. J.; Greathouse, J. A.; Crozier, P. S.; Nenoff, T. M. Capture of Volatile Iodine, a Gaseous Fission Product, by Zeolitic Imidazolate Framework-8. J. Am. Chem. Soc. 2011, 133, 12398−12401. (17) Hughes, J. T.; Sava, D. F.; Nenoff, T. M.; Navrotsky, A. Thermochemical Evidence for Strong Iodine Chemisorption by ZIF-8. J. Am. Chem. Soc. 2013, 135, 16256−16259. (18) Chapman, K. W.; Sava, D. F.; Halder, G. J.; Chupas, P. J.; Nenoff, T. M. Trapping Guests Within a Nanoporous Metal-Organic Framework Through Pressure-Induced Amorphization. J. Am. Chem. Soc. 2011, 133 (46), 18583−18585. (19) Sava, D. F.; Garino, T. J.; Nenoff, T. M. Iodine Confinement into Metal-Organic Frameworks (MOFs): Low Temperature Sintering Glasses to form Novel Glass Composite Material (GCM) Alternative Waste Forms. Ind. Eng. Chem. Res. 2012, 51, 614−620. (20) Sava, D. F.; Chapman, K. W.; Rodriguez, M. A.; Greathouse, J. A.; Crozier, P. S.; Zhao, H.; Chupas, P. J.; Nenoff, T. M. Competitive I2 Sorption by Cu-BTC from Humid Gas Streams. Chem. Mater. 2013, 25, 2591−2596. (21) Jubin, R. T. Report of the FY-11 Activities of the Off-Gas Sigma Team; FCRD-SWF-2011-000306; Oak Ridge National Laboratory2011. (22) Volkringer, C.; Falaise, C.; Devaux, P.; Giovine, R.; Stevenson, V.; Pourpoint, F.; Lafon, O.; Osmond, M.; Jeanjacques, C.; Marcillaud, B.; Sabroux, J. C.; Loiseau, T. Stability of metal-organic frameworks under gamma irradiation. Chem. Commun. 2016, 52, 12502−12505. (23) Banu, A.-M.; Friedrich, D.; Brandani, S.; Duren, T. A Multiscale Study of MOFs as Adsorbents in H2 PSA Purification. Ind. Eng. Chem. Res. 2013, 52, 9946−9957. (24) Munn, A. S.; Millange, F.; Frigoli, M.; Guillou, N.; Falaise, C.; Stevenson, V.; Volkringer, C.; Loiseau, T.; Cibin, G.; Walton, R. I. Iodine sequestration by thiol-modified MIL-53(Al). CrystEngComm 2016, 18 (41), 8108−8114. (25) Greathouse, J. A.; Teich-McGoldrick, S. L.; Allendorf, M. D. Molecular Simulation of Size-Selective Gas Adsorption in Idealised Carbon Nanotubes. Mol. Simul. 2015, 41, 1388−1395. (26) Liu, Y.; Wilcox, J. Molecular Simulation Studies of CO2 Adsorption by Carbon Model Compounds for Carbon Capture and Sequestration Applications. Environ. Sci. Technol. 2013, 47, 95−101. (27) Nenoff, T. M.; Garino, T.; Krumhansl, J. L.; Rademacher, D.; Jubin, R.; Haefner, D. Waste Form Development and Testing for I2; AFCI-WAST-PMO-MI-DV-2009-000156; Sandia National Laboratories, 2009. (28) Qiu, X.; Thompson, J. W.; Billinge, S. J. L. PDFgetX2: a GUIdriven program to obtain the pair distribution function from X-ray powder diffraction data. J. Appl. Crystallogr. 2004, 37, 678. (29) Chupas, P. J.; Qiu, X.; Hanson, J. C.; Lee, P. L.; Grey, C. P.; Billinge, S. J. L. Rapid-acquisition pair distribution function (RA-PDF) analysis. J. Appl. Crystallogr. 2003, 36, 1342−1347. (30) Wojdyr, M. Fityk: a general-purpose peak fitting program. J. Appl. Crystallogr. 2010, 43, 1126−1128. (31) Stull, D. R. Vapor Pressure of Pure Substances. Organic and Inorganic Compounds. Ind. Eng. Chem. 1947, 39 (4), 517−540. (32) Martin, M. G. MCCCS Towhee: A Tool for Monte Carlo Molecular Simulation. Mol. Simul. 2013, 39, 1212−1222. (33) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n. Science 1999, 283, 1148−1150. (34) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R. D.; UribeRomo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 10186−10191. (35) Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Positions of Cations and Molecules in Zeolites with the Mordenite-Type Framework: VIII Dehydrated Sodium-Exchanged Mordenite. Mater. Res. Bull. 1979, 14, 751−758.

(36) Greathouse, J. A.; Teich-McGoldrick, S. L.; Allendorf, M. D. Molecular Simulation of Size-Selective Gas Adsorption in Idealised Carbon Nanotubes. Mol. Simul. 2015, 41, 1388−1395. (37) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. UFF, a Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114, 10024−10035. (38) Di Lella, A.; Desbiens, N.; Boutin, A.; Demachy, I.; Ungerer, P.; Bellat, J. P.; Fuchs, A. H. Molecular Simulation Studies of Water Physisorption in Zeolites. Phys. Chem. Chem. Phys. 2006, 8, 5396− 5406. (39) Spezia, R.; Nicolas, C.; Archirel, P.; Boutin, A. Molecular Dynamics Simulations of the Ag+ or Na+ Cation with an Excess Electron in Bulk Water. J. Chem. Phys. 2004, 120, 5261−5268. (40) Shevade, A. V.; Jiang, S. Y.; Gubbins, K. E. Molecular Simulation Study of Water-Methanol Mixtures in Activated Carbon Pores. J. Chem. Phys. 2000, 113, 6933−6942. (41) Kong, C. L. Combining rules for intermolecular potential parameters. II. Rules for the Lennard-Jones (12−6) potential and the Morse potential. J. Chem. Phys. 1973, 59, 2464. (42) Hou, X.; Hansen, V.; Aldahan, A.; Possnert, G.; Lind, O. C.; Lujaniene, G. A review on speciation of iodine-129 in the environmental and biological samples. Anal. Chim. Acta 2009, 632, 181−196. (43) Jabbar, T.; Wallner, G.; Steier, P. A review on 129I analysis in air. A review on speciation of iodine-129 in the environmental and biological samples. J. Environ. Radioact. 2013, 126, 45−54.

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