Letter Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10622−10626
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Iodine Adsorption in Metal Organic Frameworks in the Presence of Humidity Debasis Banerjee,† Xianyin Chen,‡ Sergey S. Lobanov,*,§ Anna M. Plonka,∥ Xiaojun Chan,∥ John A. Daly,‡ Taejin Kim,∥ Praveen K. Thallapally,*,† and John B. Parise‡,§ †
Physical and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Department of Chemistry, §Department of Geosciences, and ∥Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States
‡
S Supporting Information *
ABSTRACT: Used nuclear fuel reprocessing represents a unique challenge when dealing with radionuclides such as isotopes of 85 Kr and 129I2 due to their volatility and long half-life. Efficient capture of 129I2 (t1/2 = 15.7 × 106 years) from the nuclear waste stream can help reduce the risk of releasing I2 radionuclide into the environment and/or potential incorporation into the human thyroid. Metal organic frameworks have the reported potential to be I2 adsorbents but the effect of water vapor, generally present in the reprocessing off-gas stream, is rarely taken into account. Moisture-stable porous metal organic frameworks that can selectively adsorb I2 in the presence of water vapor are thus of great interest. Herein, we report on the I2 adsorption capacity of two microporous metal organic frameworks at both dry and humid conditions. Single-crystal X-ray diffraction and Raman spectroscopy reveal distinct sorption sites of molecular I2 within the pores in proximity to the phenyl- and phenol-based linkers stabilized by the I···π and I···O interactions, which allow selective uptake of iodine. KEYWORDS: MOF, porous structures, organic framework, used nuclear fuel, iodine, radioactive waste, single crystal X-ray diffraction, Raman spectroscopy
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products such as 85Kr, 127Xe, and 129I are formed, which must be efficiently captured and securely stored.5 For example, various radionuclides of I2, hydrogen iodide (HI), and alkyl halides possess serious health hazards due to their involvement in metabolic processes and long half-lives (e.g., 129I, t1/2 = 15.7 × 106 years);6−9 thus, there is a need for efficient capture of these iodine species. However, selective removal of iodine from water-saturated gas streams is very challenging due to its low concentration (∼22 ppm of I2) and even smaller concentrations of organic iodides (e.g., CH3I, C2H5I), HI, and HOI.5,10 Accordingly, a large number of materials and methods were developed over the years to selectively remove I2 and organic iodides with high decontamination factors.5 Among different
he total world energy demand is set to increase rapidly with the rise in global population and continuous economic growth of the developing world. In the meantime, the search for an efficient, reliable, low-emission energy source continues to be a global interdisciplinary research challenge.1 Solar and wind energy are often considered viable alternatives for our current fossil-fuel-based energy economy, but their intermittent nature of energy production leads to reliability concerns.2 As such, the quest for a reliable, nonfossil-fuel energy economy is becoming more important from a national security and economic development standpoint. Nuclear energy is a high-density, emission-free energy source often considered a cleaner option for continuous energy production.3 However, the successful mass production of nuclear power is associated with the implementation of an economically viable, industrialscale process to properly sequester and mitigate the fissionrelated highly radioactive waste (e.g., used nuclear fuel).4 During the fission of nuclear fuels, several gaseous radioactive © 2018 American Chemical Society
Received: February 12, 2018 Accepted: March 16, 2018 Published: March 16, 2018 10622
DOI: 10.1021/acsami.8b02651 ACS Appl. Mater. Interfaces 2018, 10, 10622−10626
Letter
ACS Applied Materials & Interfaces methods, caustic or acidic scrubbing solutions were used at a reprocessing facility for gaseous I2 control, but faced recyclability issues.5 Porous carbon, graphene-based nanomaterials, MoSx amorphous aerogel, and aluminophosphate zeolites were some of the materials tested for I2 adsorption.5,11,12 Silver exchanged zeolites (e.g., AgZ) were found to be very promising materials as solid adsorbents for I2 capture during reprocessing,5,11 yet the search for affordable porous solid-state adsorbents as alternatives with better adsorption capacity and kinetics is still ongoing.6 Among the next-generation of solid-state adsorbent materials, Hofmann-type structures, covalent organic frameworks, and metal organic frameworks (MOFs) are considered leading candidates for I 2 capture due to their tunable pore architectures.6−9,13−24 Previous studies showed exceptional total I2 uptake of MOFs in nonpolar solvents (e.g., hexane). For example, Nenoff and co-workers reported very high I2 adsorption properties in ZIF-8 (∼125 wt % I2), but the process is found to be irreversible because of its chemisorptive nature.7 Apart from the uptake capacity, reversibility, and cost, tolerance to water is one of the limiting factors for the choice of suitable material. Indeed, there is a significant amount of water vapor present in the off-gas stream and materials that selectively capture I2 over water can enable economic savings. Only several MOFs, however, have been tested for iodine uptake at condition of high humidity. Sava et al.25 have reported on a very high iodine uptake of Cu-BTC (∼175 wt %) in a humid gas stream. Also, Li and co-workers recently reported high organoiodide uptake in a postfunctionalized, thermally stable MOF at variable humidity.20 Here we build on these works, by evaluating materials with I2 adsorption capacity in the presence of water vapor. In this communication, we report on the iodine adsorption capacity of two microporous MOFs, SBMOF-1 and SBMOF-226,27 in the presence of water vapor, as both structures maintain crystallinity after exposure to 75% relative humidity (RH) for at least two months of storage.28 The mechanisms of adsorption were evaluated by means of single crystal and powder X-ray diffraction (XRD), Raman spectroscopy, and thermogravimetric analysis coupled with mass spectrometry. SBMOF-1 [Ca(sdb); sdb: 4,4′-sulfonyldibenzoate] and SBMOF-2 [Ca(tcpb); tcpb: 1,2,4,5-tetrakis(4-carboxyphenyl)benzene] were synthesized following the previously reported procedures.26,27 The diamond-shaped, one-dimensional channels within SBMOF-1 were built by v-shaped sdb organic linkers. SBMOF-2 was a microporous, three-dimensional MOF network with isolated CaO6 octahedra connected by linkers, possessing two distinct adsorption sites in channel of type-I and type-II. Similar to SBMOF-1, type-I channels in SBMOF-2 were built by phenyl rings with delocalized π-electron clouds and H atoms pointing into the channel, providing sorption sites for gas molecules. The half-deprotonated tcpb linkers in SBMOF-2 formed polar −OH groups in type II channel. Powdered samples of SBMOF-1 and SBMOF-2 were activated at 75° and 215 °C, respectively, and held in vacuum for 12 h before I2 adsorption experiments. The initial I2 loading experiments were performed by vapor-phase deposition technique as a function of time. Iodine adsorption into the MOFs was evident from the apparent change in color and mass with increasing exposure to I2 (Figure 1). The maximum I2 uptake on SBMOF-1 was 22.6(2) wt %, indicating that after ∼2 days SBMOF-1 reached the saturation. I2 loading on SBMOF-2 occurred similarly, with color change from light yellow to
Figure 1. I2 uptake curves and color changes for the investigated at room temperature (A) SBMOF-1 and (B) SBMOF-2.
brown and then dark purple. The I2 uptake curve of SBMOF-2 showed a faster adsorption kinetic than SBMOF-1, with a maximum sorption capacity of 42.7(2) wt % at around 15 h. After sorption, I2 may be evacuated from SBMOF-1 and SBMOF-2 at 200 °C in under 12 h. Visually, the MOFs remain intact over several consecutive loading−unloading cycles. Submersion in aqueous H2O slowly degrades SBMOF-1 and SBMOF-2, whereas aqueous acidic conditions rapidly dissolve these frameworks. Nonetheless, SBMOF-1 and SBMOF-2 are stable under high humidity even in the presence of HI produced upon iodine exposure to 43% RH. Although SBMOFs are stable at relatively high humidity, storage in aqueous solutions degrades crystallinity of the frameworks. To understand the effect of water on I2 adsorption, experiments were conducted using controlled humidity. For example, constant RH of 33% and 43% was obtained with saturated aqueous solution of calcium chloride and zinc nitrate in a closed vessel. The humidity chamber was kept at room temperature for ∼24 h to ensure uniform conditions before conducting the experiments. The uptake experiments were again performed with the vapor deposition technique. After 24 h, SBMOF-1 showed a ∼15 wt % uptake, whereas SBMOF-2 showed a much higher 35 wt % uptake. Thermogravimetricmass spectrometry analysis showed that HI was released upon heating at T ≈ 120−180 °C (Figure S1) with the total weight loss of 30.5 wt % for the sample loaded at 43% RH. This is lower than the iodine uptake in dry air (42.7 wt %) and can be attributed to a combined sorption of I2 and H2O and/or an 10623
DOI: 10.1021/acsami.8b02651 ACS Appl. Mater. Interfaces 2018, 10, 10622−10626
Letter
ACS Applied Materials & Interfaces
having phenyl rings in the channels of both SBMOF-1 and SBMOF-2 structures, the different structural configurations could explain the different uptake of I2: SBMOF-2 comprises hydrogen atoms directed into the channels providing potential I···H force, whereas in SBMOF-1, hydrogen atoms are directed along the wall. Interestingly, the sorption sites of iodine and water27 in SBMOF-2 are crystallographically very close (Figure S2). Thus, the observed preferential selectivity of I2 over H2O in SBMOF-2 may be due to the pore blocking by I2 in combination with its hydrophobic character, as has previously been suggested for Cu-BTC.25 Simulated powder XRD patterns derived from single crystal XRD were consistent with experimental powder XRD observations (Figure S3) and confirmed that the MOFs maintain structural topology after I2 loading at either dry or controlled humidity. Both frameworks retained their topologies and crystallinities after I2 removal by heating up to 290° and 320 °C for SBMOF-1 and SBMOF-2, respectively. Finally, we employed Raman spectroscopy to gain further insights into the mechanisms of I2 sorption in SBMOF-1 and SBMOF-2 (Figure 3). A single, strong, and sharp Raman band centered at 210 cm−1 was observed in I2@SBMOF-1 due to its homogeneous sorption environment with one type of channel.
increased concentration of adsorbed HI species. The release of HI at 150−250 °C has been documented for Cu-BTC loaded in ∼1:1 I2:H2O vapor; HI was formed upon desorption as revealed by IR spectroscopy.9 To understand the extraordinary iodine capacity and to gain insights into its adsorption mechanism, we performed single crystal X-ray diffraction (exposed to dry I2 for 4 and 2 days, respectively). Full crystallographic details are provided in Table S1. Positions of adsorbed molecules in I2@SBMOF-1 and I2@ SBMOF-2 were determined at -173 °C. The lattice dimension b increases from 10.7515(9) Å in the activated sample to 11.1129(5) Å in the I2@SBMOF-2, and the three angles of the unit cell expand resulting in a 1.9% volume change, whereas the change in the unit cell-volume of SBMOF-1 upon I2 adsorption is
