Xinran
Zhang
a,
John
Maddock
a,
Tina M.
Nenoff
b,
Melissa A.
Denecke
ac,
Sihai
Yang
*a and
Martin
Schröder
*a
aSchool of Chemistry, University of Manchester, Manchester, M13 9PL, UK. E-mail: sihai.yang@manchester.ac.uk; m.schroder@manchester.ac.uk
bMaterials, Physics and Chemical Sciences Center, Sandia National Laboratories, Albuquerque, NM 87185, USA
cDivision of Physical and Chemical Science, Department of Nuclear Applications, International Atomic Energy Agency, Vienna International Centre, PO Box 100, 1400 Vienna, Austria
First published on 1st April 2022
Nuclear power will continue to provide energy for the foreseeable future, but it can pose significant challenges in terms of the disposal of waste and potential release of untreated radioactive substances. Iodine is a volatile product from uranium fission and is particularly problematic due to its solubility. Different isotopes of iodine present different issues for people and the environment. 129I has an extremely long half-life of 1.57 × 107 years and poses a long-term environmental risk due to bioaccumulation. In contrast, 131I has a shorter half-life of 8.02 days and poses a significant risk to human health. There is, therefore, an urgent need to develop secure, efficient and economic stores to capture and sequester ionic and neutral iodine residues. Metal–organic framework (MOF) materials are a new generation of solid sorbents that have wide potential applicability for gas adsorption and substrate binding, and recently there is emerging research on their use for the selective adsorptive removal of iodine. Herein, we review the state-of-the-art performance of MOFs for iodine adsorption and their host–guest chemistry. Various aspects are discussed, including establishing structure–property relationships between the functionality of the MOF host and iodine binding. The techniques and methodologies used for the characterisation of iodine adsorption and of iodine-loaded MOFs are also discussed together with strategies for designing new MOFs that show improved performance for iodine adsorption.
Metal–organic frameworks (MOFs) are a class of crystalline porous materials that possess high surface areas, tuneable structures and can have high chemical stability.13 The majority of current research is focused on the use of MOFs for gas adsorption and storage.14 However, they also exhibit potential for toxic waste elimination, including removal of corrosive gases,15 separation of noble gases16 and adsorption of heavy metals.17 Recently, MOFs have been shown to adsorb a wide range of radionuclides,18,19 including barium,20 uranium,21 thorium,22 iodine and triiodide (I3−).23 More recently, novel MOF materials have also been synthesised based on actinides metal clusters.24,25 The application of MOFs for iodine adsorption has not been systematically reviewed previously, and herein, we discuss the latest progress on the adsorption and binding of iodine in robust MOF materials. The crystalline nature of MOFs enables investigations of the host–guest binding interactions for iodine-loaded materials at crystallographic and molecular resolution. The dynamics of iodine adsorption in MOFs are also discussed to afford insights for the design of future systems with improved properties. Furthermore, we explore the potential utilisation of iodine-loaded MOFs, for example in heterogeneous catalysis.
Typical MOR-type zeolitic frameworks include MxAl2Si10O24·7H2O (M = Ca, x = 1; M = Na, K, x = 2) that comprise of 12-membered rings with a pore diameter of 7.0 × 6.5 Å and 8-membered rings incorporating windows of 5.7 × 2.6 Å. These features facilitate the diffusion of iodine vapour through the structure. Ion-exchange reactions afford zeolites with high Ag(I) ion loading, and these sorbents exhibit excellent stability when exposed to a stream of iodine. The mechanism of host–guest binding has been investigated by pair distribution function (PDF) analysis,30 and two different forms of AgI can be generated by reaction of iodine with these Ag-loaded zeolites: α-AgI is retained within the pores, while larger nanoparticles of γ-AgI reside on the surface of the zeolite. Recently, a novel hydrophobic all-silica zeolite HISL (hydrophobicity intensified silicate) has been developed for iodine adsorption. Its narrow channels (5.5 × 5.1 Å) are advantageous for selectively trapping iodine, the kinetic diameter of which is 4 Å, and an uptake of 0.53 g g−1 has been obtained for HISL under humid conditions. The location of five independent binding sites of adsorbed iodine molecules in the channels of HISL have been confirmed by single crystal diffraction,31 which confirms that the adsorbed iodine molecules are stabilised by a combination of strong host–guest interactions to the electron-rich pore wall and stabilised further by inter-molecular guest–guest interactions between adjacent iodine molecules with an average molecular separation of 3.7–4.1 Å. Furthermore, compared to the bare zeolite, I2@HISL shows eight-orders of magnitude increase in electron conductivity to 2.0 × 104 S m−1, indicating potential in application as semiconductors. These results demonstrate that zeolites are practical candidates for I2 capture, and further improvements in uptake capacities will boost greatly their potential application.
Chalcogenide aerogels have been most widely applied to the adsorption of iodine due to their high affinity to soft binding sites based upon on Pearson's hard–soft–acid–base principles. Chalcogen-based phases such as GeSx, CdSe and PbS are usually formed as aerogels using thiolysis, condensation or chemical linkage methods, and a novel aerogel denoted Cg-5C has been synthesised by mixing (CH3)4NGe4S10 and K2PtCl4 in an aqueous solution to enhance gelation.35 The resultant material shows a large pore volume (up to 2.3 cm3 g−1), high surface area (typically ∼1200 m2 g−1), and exhibits high iodine capacity (up to 2.39 g g−1). Moreover, a removal efficiency of 99% can be achieved using a flow of dry air leaving a residual iodine concentration of 4.2 ppm. Another chalcogel, denoted as ZnSnS, shows a high iodine uptake of 2.25 g g−1 due to its unique structural features based upon a polarisable and electron rich pore surface.37
Aerogel composites have also been widely investigated for iodine adsorption. Using aluminosilicate aerogels as scaffolds, Ag-based crystallites have been incorporated into the aerogel matrix via the wetness impregnation method.36 The resultant Ag-functionalised aerogel shows an iodine uptake of 0.52 g g−1, which is four times higher than that of the pristine aerogel. The enhancement of iodine adsorption is again due to the formation of AgI particles within the pores of the Ag-loaded material. Graphene-containing aerogels have also been successfully synthesised using hydrothermal methods.38 Thus, by combining a solution of graphene-oxide and aerogel a homogenous aerogel phase was formed and this shows an iodine uptake of 0.51 g g−1.
These studies indicate that aerogels can be utilised for iodine adsorption owing to their high uptakes. However, their amorphous structures render studies of the host–guest interactions challenging, if not impossible, which is prohibitive to the informed design of further improved materials.
An imine-based COF has been synthesised by employing a [C3 + C2] topology (Fig. 3)41 to form hexagonal-shaped channels with a pore diameter of 3.3 nm, a surface area of 1927 m2 g−1 and pore volume of 1.28 cm3 g−1. This material shows an extremely high iodine adsorption of 6.26 g g−1 and shows excellent stability as no notable loss of the iodine capacity was observed after five cycles of iodine adsorption/desorption. Another imine-based COF has been designed by using a similar strategy, and the as-prepared COF also displays high and reversible iodine adsorption of 5.43 g g−1.42
Fig. 3 Design strategy based on the [C3 + C2] topologies for the construction of 2D hexagonal COF: TPB-DMTP. This figure has been reproduced from ref. 41 with permission from Elsevier, copyright 2018. |
A study into the effect of conjugation on the uptake of iodine in COFs has been reported as an efficient strategy to optimise these materials.43 The π⋯π conjugated structure in COF-LZU1 shows a higher uptake of iodine (5.3 g g−1) compared to the corresponding π⋯π and p⋯π conjugated structure in TpPa1 (2.45 g g−1) indicating that the choice of conjugated system plays a key role in iodine binding. Another recent study investigated the impact of porosity on overall iodine uptake.44 It was shown that a mesoporous COF, Meso-COF-3, exhibits, as expected, a higher iodine uptake than two related microporous systems, Micro-COF-1 and Micro-COF-2 (4.0 g g−1 compared to 2.9 g g−1 and 3.5 g g−1, respectively). However, a marked drop-off in iodine uptake was noted for extremely large pores (from 4.0 g g−1 to 3.3 g g−1) even though a higher uptake was predicted. It was rationalised that the presence of fewer adsorption interactions for iodine molecules in the highly porous COF materials was responsible for the observed reduction in iodine uptake as the porosity increases, although a degree of interpenetration or entanglement of the network might also lead to reduced uptake.
COFs tend to show higher iodine capacities compared to other interpenetrated/crosslinked networks due to the formation of wide-open channels that facilitate iodine diffusion. Moreover, variation of the organic building blocks enables the rational design for COF materials with desirable pore size, although experimental investigations on the host–guest binding mechanism in COFs remain a major challenge due to their often poor crystallinity.
Cyanuric chloride is a commonly used precursor and a variety of triazine-based POPs have been constructed via Friedel–Crafts polymerisation.7,50,51 A novel POP has been designed based on this strategy using cyanuric chloride as the backbone and this exhibits a high iodine capacity of 4.9 g g−1.51 Strong peaks at 170 cm−1 have been observed in the Raman spectrum of iodine-loaded POPs, assigned to the formation of the V-shaped pentaiodide I5− within the pores. This suggests that charge-transfer between the guest iodine molecules and electron-rich hosts facilitates the formation of charged species, leading to the high overall iodine adsorption. Linking cyanuric chloride with triazine and triphenylamine groups affords heteroatom-rich fluorescent conjugated microporous polymer which also shows high iodine uptakes of 4.9 g g−1.7
Another strategy to construct POPs uses the Sonogashira–Hagihara cross-coupling reaction to inter-connect terminal alkyne and aryl halide groups.52–58 Using this methodology a series of porous aromatic framework (PAF) materials have been reported by constructing a charged tetrahedral lithium tetrakis(4-iodophenyl)borate linker with various alkyne monomers.52 The charged PAFs provide multiple binding sites (e.g., ionic bonds and phenyl rings), which result in an iodine adsorption up to 2.76 g g−1. The diversity of synthetic strategies to the synthesis of POPs enables the rational design of molecular building blocks to place and control functionality within the material to maximise iodine uptake.
The templating procedure is less common and involves iodine being added during MOF synthesis where it acts as a structural modulator or structure-directing agent.65 It is a synthetic challenge to introduce iodine guests into MOFs using this technique due to the limited stability of some MOFs under these conditions, and the resultant poor crystallinity of the products makes them structurally difficult to analyse by diffraction methods. For example, an iodine-encapsulated MOF [Cu6(pybz)8(OH)2]·I5−·I7− (Hpybz = 4-pyridyl benzoic acid) has been successfully synthesised using iodine as a template.66 The existence of both I5− and I7− chains within a cationic bilayer structure was confirmed by single crystal diffraction (Fig. 4). It was also noted that this structure had good stability under both acidic and alkaline solutions. Recently, a series of lanthanide-copper bimetallic MOFs, [Ln2Cu5(OH)2(pydc)6(H2O)8]·I8 (Ln = Sm, Eu, Gd, Tb; H2pydc = 2,5-pyridinedicarboxylic acid) have been synthesised (Fig. 4).67 In contrast to [Cu6(pybz)8(OH)2]·I5−·I7−, in which the polyiodides are disordered in zigzag chains, in [Sm2Cu5(OH)2(pydc)6(H2O)8]·I8 they form highly ordered linear chains. The complex shows good performance for photocatalytic H2 evolution as well as good stability under basic and alkaline solutions.
Fig. 4 Views of the crystal structures of iodine-loaded MOFs: (a) [Sm2Cu5(OH)2(pydc)6(H2O)8]·I8 (H2pydc = 2,5-pyridinedicarboxylic acid).67 (b) BOF-1·(I3)4.79 (c and d) [Cu6(pybz)8(OH)2]·I5−·I7− (pybz = 4-pyridylbenzoic acid)85 (C: grey, O: red, N: blue, I: pink, Ni: green, Cu: Turquoise, Gd: light blue; hydrogen atoms are omitted for clarity). |
Fig. 5 View of host–guest binding of iodine in MOFs. (a) ZIF-8·0.65I261 (b) MIL-53-SH(Al)·0.35I269 and MIL-53-NH2(Al)·0.16I268 (c) [Tb3(Cu4I4)3(ina)9]n·1.5I2 (Hina = isonicotinic acid)73 (d) MFM-300(Sc)·2.62I294 (C: grey, O: red, N: blue, I: pink, Cu: turquoise, Tb: light orange, Sc: teal, Zn: sky blue; hydrogen atoms are omitted for clarity). |
Another interesting strategy to improve iodine binding is to introduce iodide containing groups. The complex [Tb(Cu4I4)(ina)3(DMF)]·1.5I2 (Hina = isonicotinic acid) incorporates [Cu4I4] moieties and possesses channels of 9.4–9.7 Å diameter,71 and these are an ideal size for the assembly of I42− species within the structure.71,72 The channels thus facilitate the formation of tetraiodide anions (I42−) via interactions between iodine molecules and the [Cu4I4] groups through the formation of I−⋯I2⋯I− interactions with a short intermolecular distance of 3.34 Å. This result was confirmed by single crystal diffraction (Fig. 6).73 The interaction of iodine molecules and the framework phenyl rings has also been observed involving a I2⋯ring centroid interaction at of 4 Å. Owing to its high framework density, this MOF only shows a moderate iodine adsorption of 0.28 g g−1. Formation of a similar I42− assembly has been observed in activated [(ZnI2)3(TPT)2] (TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine) on exposure to iodine vapour.74 In this material, the adsorbed iodine molecules initially form [I4]2− moieties stabilised by interaction with accessible iodide ions from the ZnI2 centres within the framework. With increasing loading, [I4]2− convert to less energetic I3− groups that accommodate additional iodine molecules inside the pores. This system shows a high iodine capacity of 1.73 g g−1 (Fig. 7).
Fig. 6 View of the linear I42− bridge constructed in [Tb(Cu4I4)(ina)3]. This figure has been reproduced from ref. 73 with permission from Elsevier, copyright 2017. |
Fig. 7 Views of X-ray crystal structure of [(ZnI2)3(TPT)2] (TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine). Guest molecules of nitrobenzene are sequentially exchanged with I2 molecules after 3, 6 and 15 h of exposure to I2 vapor. This figure has been reproduced from ref. 74 with permission from The Royal Society of Chemistry, copyright 2017. |
Another recent report uses acid treatment of a a Zr-based MOF, UPC-158, to increase the overall iodine uptake.75 The treatment involves soaking the MOF in an aqueous acidic solution (pH = 3) containing HX (X = F, Cl, Br, I) for two days. This process results in the functionalisation of the MOF with halide ions and protonated imidazolate ligands. The protonation produces different levels of fluorescence as well as changing the BET surface area and pore size, and this combination increases the iodine uptake of UPC-158 from 1.78 g g−1 to 2.92 g g−1 in the case of the HCl-treated MOF. Treatment with ethanol leads to desorption of trapped iodine molecules, but the strong interactions between iodine and the framework highlights the potential for this material to be used for long-term iodine storage.
Recently, a series of iso-reticular Zr-based MOFs have been constructed using the extended form of UiO-66 where the elongated ligands contain unsaturated alkene, alkyne and units as bridges (Fig. 8).76 These exhibit high surface areas ranging from 2650–3850 m2 g−1, coupled with high pore volumes of 1.2–1.7 cm3 g−1. These Zr-MOFs are stable to iodine dosing with uptakes of 1.1–2.8 g g−1. In particular, [Zr6O4(OH)4(peb)6] [H2peb = 4,4′-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid] shows a pore volume of 1.16 cm3 g−1, a surface area of 2650 m2 g−1, and contains a highly elongated organic building block to give a large pore size (14.2 Å). The length of the ligand results in the formation of an interpenetrated structure that contributes to a high iodine uptake of 2.8 g g−1 due to the higher density of aromatic groups and metal clusters within the structure. Another benchmark MOF, HKUST-1, with a pore volume of 0.74 cm3 g−1 also shows a high iodine uptake of 1.75 g g−1.77 These values compare favourably with other benchmark solid sorbents, such as PAF-24 (2.76 g g−1),52 the conjugated microporous polymer TTPB77 (TTPB = triazine and triphenylamine-based fluorescent conjugated microporous polymer) (4.43 g g−1), and the hydrogen-bonded organic framework HcOF-178 (2.9 g g−1).
Fig. 8 View of alkyne-functionalised ligands used in building Zr-MOFs. This figure has been reproduced from ref. 76 with permission from Elsevier, copyright 2016. |
It is worth noting that high surface area and porosity are not always pre-requisites for high iodine uptake in MOFs because other factors, such as pore geometry/shape, can also affect adsorption significantly. The complex [Zn2(tptc)(apy)] (H4tptc = triphenyl-3,3′′,5,5′′-tetracarboxylic acid, apy = aminopyridine) shows a high iodine uptake (2.16 g g−1), albeit with a relatively low surface area (∼168 m2 g−1) and pore volume (0.46 cm3 g−1) (Fig. 9).79 This is due to the combined effects of the conjugated π-electron aromatic system, halogen bonding, and electron-donating amine groups. This contrasts with a thorium MOF, Th-SINAP-13, that has a significantly higher surface area of 3396.5 m2 g−1, but shows a lower iodine uptake of 0.6 g g−1 but with a rapid rate of adsorption rate due to its high porosity.80 While higher surface area and pore volume of a MOF do contribute to producing higher iodine uptakes (Fig. 10), the presence of functional groups that tailor the pore environment provide an ideal platform for iodine capture.
Fig. 9 Views of the structure of [Zn2(tptc)(apy)] (H4tptc = triphenyl-3,3′′,5,5′′-tetracarboxylic acid, apy = aminopyridine); (a) the asymmetric unit, (b) small pore (9.9 Å × 17.0 Å), (c) large pore (18.8 Å × 24.7 Å). This figure has been reproduced from ref. 79 with permission from American Chemical Society, copyright 2016. |
The overall correlation of the porosity of a MOF material with its adsorption capacity for iodine is summarised in Fig. 10, Tables 1 and 2. Vapor diffusion of iodine into MOFs with high surface areas and pore volumes generally exhibit high iodine capacities, thus affording an approximate linear relationship between the porosity and iodine capacity. There is though clearly significant scatter in these data. In particular, solution-based adsorption processes have uncertainties owing to the presence of competitive adsorption between free solvent and iodine molecules; such competitive processes require further study.
MOF | BET surface area (m2 g−1) | Pore volume (cm3 g−1) | Iodine uptake (g g−1) | Ref. |
---|---|---|---|---|
a Studies were conducted on a single crystal MOF and no BET surface area or pore volume was reported. b Study was carried out on a doped MOF.L1 = 4-amino-3,5-bis(4-pyridyl-3-phenyl)-1,2,4-triazole; L2 = 4,4′-di(1H-1,2,4-triazol-1-yl)-1,1′-biphenyl; L3 = 5,5′,5′′-(2,4,6-triethylbenzene-1,3,5-triyl)tris(2-(pyridin-3-yl)-1,3,4-oxadiazole); H2L4 = 4,4′-stilbene dibenzoic acid; H2L5 = 4,4′-(ethyne-1,2-diyl)dibenzoic acid; H2L6 = 4,4′-(buta-1,3-diyne-1,4-diyl)-dibenzoic acid; H2L7= 4,4′-[1,4-phenylenebis(ethyne-2,1-diyl)]-dibenzoic acid; H3BTC = trimesic acid (benzene-1,3,5-tricaroxylic acid); TIB = 1,3,5-tris(imidazol-1-ylmethyl)benzene; Bitmb = 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene; H44pba = 4-(4-pyridyl)benzoic acid; H2sdb = 4,4′-sulfonyldibenzoate; H4tcpb = 1,2,4,5-tetrakis(4-carboxyphenyl)-benzene; TMBP = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole; H2DL-lac = lactic acid; Hpybz = 4-pyridylbenzoic acid; H4ao2btc = dioxygenated form of 3,3′,5,5′-azobenzenetetracarboxylic acid; pbix = 1,4-bis(imidazol-1-ylmethyl)benzene; H4tptc = terphenyl-3,3′′,5,5′′-tetracarboxylic acid; TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine; apy = aminopyridine. | ||||
[Cd(L1)2](ClO4)2a | — | — | 0.46 | 98 |
[Cd3(BTC)2(TIB)2]a | — | — | 0.03 | 123 |
[Zn3(BTC)2(TIB)2]a | — | — | 0.04 | 123 |
[Cu2(bitmb)2Cl4]a | — | — | 0.31 | 124 |
{[(Me2NH2)2]·[Cd3(5tbip)4]·2DMF}n | — | — | 1.63 | 125 |
[Ni(L2)2Cl2] a | — | — | 0.22 | 106 |
[Ni(44pba)2]a | — | — | 1.10 | 126 |
TIF-1a | — | — | 0.54 | 127 |
[Zn(C6H8O8)]·2H2Oa | — | — | 0.16 | 128 |
[(ZnI2)3(TPT)2] | — | — | 1.73 | 74 |
Cu-BTC | 1850 | 0.74 | 1.75 | 77 |
[Cu4I4(L3)] | 641 | 0.31 | 0.14 | 107 |
[Fe3(HCOO)6] | 385 | 0.15 | 0.49 | 129 |
MIL-53-SH(Al) | 324 | 0.07 | 0.33 | 69 |
Ca(sdb) | 145 | 0.62 | 0.26 | 100 |
Ca(tcpb) | 195 | 0.84 | 0.43 | 100 |
TMBP·CuI | 520 | 0.12 | 0.64 | 96 |
ZIF-8 | 1630 | 0.66 | 1.25 | 130 |
[Zn3(DL-lac)2(pybz)2] | 763 | 0.41 | 1.01 | 92 |
[Zn2(μ4-ao2btc)(μ-pbix)2] | 78 | 0.07 | 0.20 | 131 |
Zn2(tptc)(apy) | 168 | 0.46 | 2.16 | 79 |
[Zr6O4(OH)4 (L4)6] | 2900 | 1.33 | 1.07 | 76 |
[Zr6O4(OH)4 (L5)6] | 3280 | 1.39 | 1.80 | 76 |
[Zr6O4(OH)4 (L6)6] | 3850 | 1.70 | 1.80 | 76 |
[Zr6O4(OH)4 (L7)6] | 2650 | 1.16 | 2.79 | 76 |
MFM-300(Sc) | 1250 | 0.50 | 1.54 | 94 |
MFM-300(In) | 1050 | 0.41 | 1.16 | 94 |
MFM-300(Fe) | 1192 | 0.46 | 1.29 | 94 |
MFM-300(Al) | 1370 | 0.37 | 0.94 | 94 |
MFM-300(VIII)a | — | — | 1.42 | 82 |
MFM-300(VIV)a | — | — | 1.25 | 82 |
UPC-158 | 2170 | 0.93 | 1.78 | 75 |
UPC-158-HF | 2137 | 0.96 | 2.19 | 75 |
UPC-158-HCl | 2289 | 0.99 | 2.92 | 75 |
UPC-158-HBr | 2151 | 0.93 | 2.75 | 75 |
UPC-158-HCl | 1954 | 0.85 | 2.59 | 75 |
SION-8a | — | — | 0.25 | 132 |
MOF-808 | 1930 | 0.82 | 2.18 | 133 |
NU-1000 | 2126 | 1.27 | 1.45 | 133 |
MOF-867 | 2403 | 1.12 | 0.88 | 133 |
UiO-66 | 1170 | 0.3 | 1.17 | 134 |
UiO-66-FA | 1705 | 0.73 | 2.25 | 134 |
UiO-67 | 2638 | 1.17 | 0.53 | 133 |
PCN-333(Al) | 2935 | 2.97 | 4.42 | 86 |
IL@PCN-333(Al)b | 1635 | 1.40 | 7.35 | 86 |
(ZnI2)3(tpt)2a | — | — | 0.38 | 135 |
[Cd(pbica)2]·1.5DMF·2CH3OH | 1073 | — | 0.66 | 136 |
MBM | 62 | 0.624 | 0.98 | 137 |
HKUST-1@PES | 1250 | — | 0.376 | 116 |
HKUST-1@PEI | 990 | — | 0.348 | 116 |
HKUST-1@ PVDF | 1100 | — | 0.225 | 116 |
Cu@MIL-101b | 418 | 0.19 | 3.42 | 85 |
Th-SINAP-13 | 3396.5 | — | 0.60 | 80 |
MOF | Solution media | BET surface area (m2 g−1) | Pore volume (cm3 g−1) | Iodine uptake (g g−1) | Ref. |
---|---|---|---|---|---|
a Study was carried out on a doped MOF. b Studies were conducted on a single crystal MOF and no BET surface area or pore volume was reported. c Langmuir surface area since BET surface areas of these MOFs were not reported.L1 = 4-amino-3,5-bis(4-pyridyl-3-phenyl)-1,2,4-triazole; HL2 = 2-(1-hydroxyethyl)-1H-benzo[d]imidazole-5-carboxylic acid; HL3 = 2-vinyl-1H-benzo[d]imidazole-5-carboxylic acid; H4L4 = N-phenyl-N′-phenyl bicyclo[2,2,2]oct-7-ene-2,3,5,6-tetracarboxdiimide tetracarboxylic acid; H2L5 = pyridine-3,5-bis(phenyl-4-carboxylic acid); H3BTC = trimesic acid (benzene-1,3,5-tricarboxylic acid); TIB = 1,3,5-tris(imidazol-1-ylmethyl)benzene; H2btz = 1,5-bis(5-tetrazolo)-3-oxapentane; Hina = isonicotinic acid; H2BDC = benzene-1,4-dicarboxylic acid; 4-bpmh = N,N-bis-pyridin-4-yl-methylene-hydrazine; H2TMBD = tetrakis(methylthio)-1,4-benzenedicarboxylic acid; Hebic = 2-ethyl-1H-benzo[d]imidazole-5-carboxylic acid; H2PYDC = 2,5-pyridinedicarboxylic acid; H4ao2btc = dioxygenated form of 3,3′,5,5′-azobenzenetetracarboxylic acid; pbix = 1,4-bis(imidazol-1-ylmethyl)benzene. | |||||
{[WS4Cu4(4,4′-bpy)4][WS4Cu4I4(4,4′-bpy)2]}b | CCl4 | — | — | 0.20 | 95 |
[Cd3(BTC)2(TIB)2]nb | Hexane | — | — | 0.16 | 123 |
[Cd(L1)2(ClO4)2]b | Hexane | — | — | 0.19 | 98 |
TMU-16b | Hexane | — | — | 0.22 | 130 |
TMU-16-NH2b | Hexane | — | — | 1.28 | 130 |
[Zn3(BTC)2(TIB)2]nb | Hexane | — | — | 0.21 | 123 |
[Zn3(L2)2(μ2-OH)2]nb | Hexane | — | — | 0.28 | 138 |
[Zn3(L3)2(μ2-OH)2]nb | Hexane | — | — | 0.26 | 138 |
JLU-Liu14b | Ethanol | — | — | 0.16 | 139 |
[CuII(btz)]nb | Cyclohexane | — | — | 0.47 | 97 |
JLU-Liu32b | Cyclohexane | — | — | 0.29 | 130 |
[Tb3(Cu4I4)3(ina)9]nb | Cyclohexane | — | — | 0.28 | 73 |
TMU-15b | Cyclohexane | — | — | 1.30 | 59 |
[Zn7(L4)3]n·[Zn5(L4)3]nb | Cyclohexane | — | — | 0.46 | 140 |
[Cd(bdc)(4-bpmh)]n | Hexane | 36 | 0.09 | 0.15 | 141 |
[Cd(2-NH2bdc)(4-bpmh)]n | Hexane | 30 | 0.13 | 0.28 | 141 |
Cu2TMBD | Hexane | 197 | 0.15 | 0.18 | 142 |
IFMC-10 | Hexane | 185 | 0.09 | 0.04 | 143 |
IFMC-15 | Hexane | 138 | 0.07 | 1.10 | 144 |
BOF-1 | DMSO/H2O | 138c | 0.09 | 0.93 | 81 |
Cu(H2L5) | Cyclohexane | 646 | 0.35 | 0.66 | 145 |
[Co(ebic)2]n | Cyclohexane | 42c | 0.07 | 0.75 | 146 |
MIL-53-NH2(Al) | Cyclohexane | 735 | 0.35 | 0.18 | 68 |
MIL-101-NH2(Al) | Cyclohexane | 2100 | 1.12 | 0.31 | 68 |
JLU-Liu31 | Cyclohexane | 1700 | 0.85 | 0.25 | 147 |
UiO-66-PYDC | Cyclohexane | 1030 | 0.43 | 1.25 | 148 |
[Zn(ebic)2]n | Cyclohexane | 50c | 0.08 | 0.74 | 146 |
[Zn2(μ4-ao2btc)(μ-pbix)2]n | Cyclohexane | 78 | 0.07 | 0.18 | 131 |
UiO-66 | Cyclohexane | 1970 | 0.898 | 0.667 | 148 |
AgNPs@UiO-66a | Cyclohexane | 700 | 0.41 | 1.260 | 149 |
IL@PCN-333(Al)a | Hexane | 1635 | 1.40 | 3.40 | 86 |
[DMA][In(TDC)2] | Cyclohexane | 384.21 | — | 0.1 | 150 |
Th-TATABb | Cyclohexane | — | — | 0.075 | 151 |
{[Zn2(α-bptc)(H2O)4]·(pra)}n | Methanol | 8.94 | 0.0221 | 0.085 | 152 |
Ag-MSHC-6a | H2O | — | — | 0.077 | 62 |
[Cd(pbica)2]·1.5DMF·2CH3OH | Cyclohexane | 1073 | — | 1.00 | 136 |
MIL-125-NH2@chitosan | H2O | 965.8 | — | 0.019 | 63 |
MBM | H2O | 62 | 0.624 | 0.88 | 137 |
HKUST-1@PES | Cyclohexane | 1250 | — | 0.376 | 116 |
HKUST-1@PEI | Cyclohexane | 990 | — | 0.348 | 116 |
HKUST-1@ PVDF | Cyclohexane | 1100 | — | 0.225 | 116 |
Ag2O-Ag2O3@ZIF-8a | H2O | 369.9 | 0.14 | 0.23 | 153 |
Ag@MIL-101a | H2O | 1045 | 1.54 | 2.14 | 84 |
Another example uses redox-active vanadium centres in MFM-300(VIII) for iodine adsorption.82 MFM-300(VIII) shows a reversible uptake of iodine of 1.42 g g−1, with the adsorbed iodine molecules binding in two domains to form helical chains within the MOF. Interestingly, the adsorption of iodine results in an increase in conductivity by a magnitude of 106, and this makes MFM-300(VIII) an excellent candidate for detecting iodine. The increase in conductivity is caused by the host–guest charge transfer interactions as a result of the partial oxidation of VIII to VIV and the formation of I3− within the pore. The presence of I3− was confirmed using a combination of X-ray photoelectron spectroscopy and Raman spectroscopy.
PCN-333(Al) can be doped with an ionic liquid to give an iodine uptake of 7.35 g g−1 from vapour and 3.4 g g−1 from hexane solutions.86 These results are some of the highest recorded uptakes in vapour or solution, and originate from interactions between iodine and the halide, in this case bromide present in the ionic liquid located inside the pores of the MOF. This technique was replicated with MIL-101(Cr) to produce an increase in iodine uptake from 0.39 g g−1 to 0.96 g g−1 on doping with an ionic liquid.
Fig. 11 The mass loss associated with I2 release from the crystalline and amorphized ZIF-8 based by TGA of the as-loaded (left) and annealed (right) samples. This figure has been reproduced from ref. 87 with permission from the American Chemical Society, copyright 2011. |
A systematic study was conducted to further investigate the pressure-induced process using a series of ZIFs (ZIF-4, ZIF-69, ZIF-mnIm) as hosts.88 By ball-milling, all of these sorbents experience a similar amorphisation process as the ZIF-8 material on iodine adsorption, but they maintain the short range order of their structures. ZIF-mnIm exhibits the highest level of retention of iodine (up to 0.25 g g−1) of the three tested samples, and this stems from the methyl-functionalisation controlling and hindering loss of adsorbed iodine species. This simple mechanical modification provides new insights for control of nanoscale sorption, opening up possibilities for future applications in interim storage and controlled release of radioactive iodine and other substrates.
The colour of MOFs typically darkens on uptake of iodine molecules. The gradual colour change of [Zn3(DLlac)2(pybz)2]n (H2DL-lac = lactic acid, Hpybz = 4-pyridylbenzoic acid) on immersion into an iodine–cyclohexane solution has been investigated in detail (Fig. 12).92 The adsorbent changed from colourless to brown with concomitant change of the dark brown solution to pale red, consistent with iodine is being trapped by the MOF host. UV/Vis spectroscopy was used to monitor the concentration of iodine in solution to calculate the concentration of iodine adsorbed by the MOF together with the kinetics of iodine uptake and release. The release of iodine from I2@[Zn3(DLlac)2(pybz)2]n in ETOH was then followed spectroscopically and confirmed that the release takes place linearly over time. The release of iodine is governed by the homogenous host–guest interaction; however, I2@[Zn3(DLlac)2(pybz)2]n requires more than 11 days to reach equilibrium. This is significantly longer than zeolite 13× and commercial activated carbons, which typically take only a few hours to reach equilibrium.
Fig. 12 (a) Visual color change of single crystals of [Zn3(DL-lac)2(pybz)2] when immersed in iodine containing cyclohexane solution (0.1 M L−1). (b) Progress of I2 enrichment of crystals of I2@[Zn3(DL-lac)2(pybz)2] in cyclohexane solution. This figure has been reproduced from ref. 92 with permission from the American Chemical Society, copyright 2010. |
A gravimetric method76 can also be used to measure iodine uptakes in materials that chemisorb iodine or have low thermal stability. This method has the advantage of being non-destructive and can be used to investigate kinetics of adsorption. A similar method to adsorption of iodine from vapour is used, and the MOF sample is removed for weighing at set times. The change in mass upon iodine adsorption is then compared to the mass of the original sample to calculate the total iodine uptake. The results plotted over time clearly present the rate at which iodine is adsorbed and the point of saturation. The level of iodine uptake can also be confirmed using elemental analysis.
Another method to quantify iodine adsorption is to record the sorption isotherm of iodine from the vapour phase. Whilst this is a conventional technique for volatile gases (e.g., CO2, CH4, N2), it is highly challenging to measure isotherms for iodine uptake due to the need to control the pressure of iodine vapour at a given temperature. A purpose-built rig has been developed to enable in situ measurements of iodine uptake (Fig. 13).94 This apparatus is built from standard stainless steel and nickel sealing gaskets as protection from the highly reactive iodine. The whole system can be retained at 120 °C to avoid condensation of iodine, while the target pressure of vapour can be controlled accurately by heating the iodine reservoir at various temperatures to dial-up the appropriate vapour pressure. A study of a series of nanoporous materials, including activated charcoals, zeolites and MOFs, has been reported using this system and good agreement with previously reported results was obtained.94 For example, an iodine uptake of 0.16 g g−1 was observed for the silver-containing zeolite mordenite [Ag(I)-MOR] at low pressure (P/P0 = 0.1). This was attributed to the strong interaction between iodine and Ag(I) ions through a chemisorption process to form AgI clusters within the pores. Very low additional iodine uptake was observed beyond P/P0 = 0.4 owing to the relatsively small pore size and surface area of this zeolite. In contrast, negligible amounts of iodine vapour were adsorbed below a relative pressure of P/P0 = 0.3 in two benchmark MOFs, ZIF-8 and HKUST-1, indicating the absence of any chemisorption process with either material. Beyond this pressure, a gradual increase in adsorption of iodine was observed as a function of pressure, and the final adsorption equilibrium was achieved at 1 bar reflecting a physisorption process. This contrasts with the chemisorption process observed for Ag(I)-doped zeolites; MOF systems rely typically upon weak and long-range iodine-framework supramolecular interactions, while doped Ag(I)-MOR samples exhibit strong interactions via chemisorption.
Fig. 13 Schematic view of an iodine adsorption unit. This figure has been reproduced from ref. 93 with permission from the American Chemical Society, copyright 2017. |
Single crystal diffraction is the most straightforward method to monitor phase changes of host materials on iodine adsorption. The introduction of functional groups into porous MOFs is an important approach to tune the iodine capacity by providing additional binding sites. To date, only a few crystal structures have been reported to confirm the formation of X–I⋯I–I⋯I–X complexes (X = Zn, Cu), as discussed above.73,74,94 An unusual system built around W and Cu clusters and 4,4′-bpy ligands within a diamond-type network89 shows trapped iodine molecules associated with the coordinated iodide group on the MOF host. The adsorbed iodine molecules are accommodated between the iodide ions of adjacent bridging clusters to form the polyiodide I42− anions which lie parallel to the 4,4′-bpy bridges. Many reported crystal structures of iodine-loaded MOFs exhibit host–guest interactions between iodine molecules and the aromatic rings in the organic bridging ligands. For example, [Co1.5(bdc)1.5(H2bpz)] (H2bdc = 1,4-benzenedicarboxylic acid, H2bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole) shows two types of rectangular 5.7 × 3.2 Å and 5.7 × 4.5 Å channels.99 It was confirmed that adsorbed iodine molecules reside linearly within these channels with I⋯H–C interactions to the phenyl –CH groups on the channel walls observed. A I⋯H contact of 3.14 Å suggests a significant host–guest interaction in this system. The calcium-based MOF, SBMOF-2, comprises of isolated CaO6 octahedra bridged by 1,2,4,5-tetrakis(4-carboxyphenyl)benzene linkers.100 The resultant channels are decorated with phenyl rings which act as sorption sites for iodine molecules. Indeed, the trapped iodine molecules are highly ordered and point to the centre of the phenyl ring with a short I⋯phenyl ring distance of 3.47 Å. This is supplemented by additional hydrogen bonding (I⋯H = 3.35 Å), thus providing key crystallographic evidence for the mechanism of host–guest binding in this system.
Recently, a family of highly rigid and iso-structural MOFs, MFM-300(M) (M = Al, Sc, Fe, In) has been reported to show high iodine uptakes (0.94–1.54 g g−1), with MFM-300(Sc) showing the highest uptake (1.54 g g−1).93 Advanced structural analysis using synchrotron radiation confirms the presence of intermolecular interactions between iodine molecules (I2⋯I2 distances of ∼3–4 Å), resulting in the formation of aggregated iodine chains within the pores of the MOF. The disordered iodine chains in the MFM-300 materials afford a high iodine packing density of 3.08 g cm−3, ∼63% of that of solid iodine (4.93 g cm−3 at 298 K). The combination of suitable pore size (6–8 Å), shape/geometry of channels and functional groups (i.e., pendant hydroxyl bridges) provide a unique platform to induce and stabilise the formation of a complex assembly of molecular iodine, resulting in highly efficient packing and exceptional storage density.
Another important technique to investigate iodine-loaded materials is synchrotron X-ray pair distribution function (PDF) analysis.101,102 This collects total scattering data to give structural information within the local region of the host–guest system. Differential (d-) PDF enables the study of species inside a nanoporous framework (e.g., I2 in ZIF-8) by subtraction of data for the framework from data for the substrate-loaded framework.30,60,77,87,93 For example, d-PDF analysis [augmented by density functional theory (DFT), Grand Canonical Monte Carlo (GCMC) analysis] has been used to understand the occupancy of iodine sites within the ZIF-8 framework;60 this has enabled the Rietveld refinement of the corresponding X-ray diffraction data.60 ZIF-8 possesses two type of cages: the small cage constructed from four-membered rings are too constrained to allow diffusion of guest iodine molecules, while the larger cage of 11.6 Å in diameter is connected by six-membered rings and accommodates adsorbed iodine molecules. Two independent binding sites were located, both of which are in the middle of the pore to form a specific host–guest interaction between iodine and the HmIm (HmIm = 2-methylimidazole) linkers of ZIF-8, with a I2⋯imidazole distance of 4.12 Å (Fig. 5). By subtracting the reference PDF data measured for pristine ZIF-8, a differential analysis of I⋯I and I⋯framework interactions were obtained. With changes of peak intensity and positions, the incremental d-PDFs provide detailed insights into the process of adsorption and binding of iodine within the pore.
PDF analysis has also been used to confirm that the local order of HKUST-1 was preserved on adsorption of iodine, although loss of Bragg peaks was clearly observed.77 From the analysis, the characteristic peaks in the PDF data correlated with framework Cu–O (∼2 Å) bonds and guest I2⋯I2 (∼2.7 Å) interactions. Moreover, it enabled the quantification of the ratio of Cu/I in the iodine-loaded system, which was found to be in good agreement with the results obtained from TGA analysis.
Vibrational spectroscopy has been used widely in the analysis of iodine-adsorbed MOFs. The I–I vibration is Raman active with a distinct band at 180 cm−1 for solid iodine. The interaction of adsorbed iodine and the framework polarises the I–I bond and leads to blue shifts of the Raman band of typically 5–15 cm−1.68,93
Fig. 14 Simulated iodine adsorption isotherms at 298 K for selected MOFs. This figure has been reproduced from ref. 104 with permission from Elsevier, copyright 2014. |
Fig. 15 Impedance response for sensing applications. (left) Non-MOF coated IDE with high impedance (|Z| > 1011 Ω at 10 mHz) and highly capacitive character (θ ≈ −90°). In a thin film of ZIF-8, the low frequency impedance phase angle barely changes. Upon exposure to I2 gas in air, a large change is produced for both the impedance and phase angle at low frequencies. (right, top to bottom) IDE, MOF film on IDE, I2@MOF film on IDE. This figure has been reproduced from ref. 108 with permission from the American Chemical Society, copyright 2017. |
MOF | Conductivity of bare MOF (S cm−1) | Conductivity of iodine-loaded MOF (S cm−1) | Conductivity enhancement (x) | Ref. |
---|---|---|---|---|
Value of electrical conductivity for solid iodine is 1 × 10−9 S cm−1.H2pdt = pyrazine-2,3-dithiol; Hpybz = 4-pyridylbenzoic acid; H2bdc = benzene-1,4-dicarboxylic acid; bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole; Hebic = 2-ethyl-1H-benzo[d]azole-5-carboxylic acid; H3L1 = biphenyl-3,4′,5-tricarboxylate; H2-5-tbip = 5-tert-butylisophthalic acid; Hina = isonicotinic acid; H2DL-lac = lactic acid; Hpybz = 4-pyridylbenzoic acid; Hebic = 2-ethyl-1H-benzo[d]imidazole-5-carboxylic acid. | ||||
Cu[Ni(pdt)2] | 1 × 10−8 | 1 × 10−4 | ∼104 | 111 |
[Cu6(pybz)8(OH)2](I−)2 | 8.04 × 10−9 | 8.11 × 10−7 | ∼100 | 66 |
[Co1.5(bdc)1.5(H2bpz)] | 2.59 × 10−9 | 1.56 × 10−6 | ∼1000 | 99 |
[Co(ebic)2]n | 2.46 × 10−9 | 2.21 × 10−7 | ∼90 | 146 |
[Eu(L1)] | 8.27 × 10−7 | 2.71 × 10−5 | ∼33 | 154 |
IFMC-15 | 2.59 × 10−9 | 2.07 × 10−7 | ∼80 | 144 |
{[(Me2NH2)2]·[Cd3(5-tbip)4]}n | 1.71 × 10−8 | 1.30 × 10−6 | ∼76 | 125 |
MET-3 | 0.77 × 10−4 | 1 × 10−3 | ∼13 | 155 |
[Tb3(Cu4I4)3(ina)9]n | 5.72 × 10−11 | 2.16 × 10−4 | ∼108 | 73 |
[Zn3(DL-lac)2(pybz)2]n | — | σ ‖ = 3.4 × 10−3 | — | 92 |
σ ⊥ = 1.7 × 10−4 | ||||
[Zn(ebic)2]n | 4.33 × 10−9 | 3.47 × 10−7 | ∼80 | 146 |
Mn(F4TCNQ)(py)2 | 5 × 10−10 | 1.4 × 10−4 | ∼105 | 156 |
MFM-300(V) | 1.7 × 10−10 | 1.16 × 10−4 | ∼106 | 121 |
A two orders of magnitude increase in electrical conductivity was observed on loading [Zn3(DL-lac)2(pybz)2] (H2DL-lac = lactic acid, Hpybz = 4-pyridylbenzoic acid) with iodine, and this has been attributed to the interaction between the iodine and the aromatic rings of the ligands.92 More interestingly, introduction of iodine molecules into the redox-active Cu[Ni(pdt)2] (H2pdt = pyrazine-2,3-dithiolate) results in an ∼10000 fold increase in electrical conductivity.111 Very recently, [Tb(Cu4I4)(ina)3(DMF)] (Hina = isonicotinic acid) has been reported to show seven orders of magnitude enhancement in conductivity ongoing from the pristine (5.72 × 10−11 S cm−1) to the iodine-loaded material (2.16 × 10−4 S cm−1).73 This dramatic increase was assigned to the presence of I−⋯I2⋯I− interactions inside the pore, as confirmed by single crystal diffraction studies (Fig. 6).
A single crystal iodine sensor based upon HKUST-1 has also been developed.112 Though there is an initial report of MOF crystallized films in direct electrical readout sensors,167 most require the prepartion of a powder sample for use in the sensor. In this case the powder must be compressed, which can cause structural collapse or framework amorphisation that can affect the reproducibility of results between batches of samples. The single crystalline sample produced consistent and reproducible results; however, it can still be challenging to produce single crystals of MOFs that are stable to iodine.
MOFa | Iodine uptake (g g−1) | Stability | Reversibility | Ref. |
---|---|---|---|---|
a H2DL-lac = lactic acid; Hpybz = 4-pyridylbenzoic acid; H44pba = 4-(4-pyridyl)benzoic acid; H2pydc = 2,5-pyridinedicarboxylic acid; TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine; H3BTC = trimesic acid (benzene-1,3,5-tricaroxylic acid); H4tptc = terphenyl-3,3′′,5,5′′-tetracarboxylic acid; apy = aminopyridine; H2peb = 4,4′-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid. | ||||
[Zn3(DL-lac)2(pybz)2] | 1.01 | Stable | Fully reversible | 92 |
[Ni(4,4′-pba)2] | 1.10 | Stable | Fully reversible | 126 |
IFMC-15 | 1.10 | Stable | Fully reversible | 144 |
ZIF-8 | 1.25 | Stable up to 0.7 g g−1 | No | 60 |
UiO-66-PYDC | 1.25 | Stable | Fully reversible | 148 |
TMU-16-NH2 | 1.28 | Stable | Fully reversible | 130 |
TMU-15 | 1.30 | Stable | Fully reversible | 59 |
MFM-300(Sc) | 1.54 | Stable | Fully reversible | 94 |
[(ZnI2)3(TPT)2] | 1.73 | Stable | N/A | 74 |
CuBTC | 1.75 | Stable | Fully reversible | 77 |
Zn2(tptc)(apy) | 2.16 | Stable | Fully reversible | 79 |
[Zr6O4(OH)4(peb)6] | 2.79 | Stable | No | 76 |
The adsorption of iodine in porous materials can be affected by a complex combination of many factors. In general vapour diffusion of iodine into sorbents with high surface areas and large pore volumes exhibit iodine capacities that generally increase in a linear manner with respect to the porosity of the material (Fig. 10). Covalent organic frameworks (COFs) and porous organic polymers (POPs) can exhibit higher iodine capacities compared with MOFs and aerogels with similar (or even higher) porosity (Table 5). Key factors for high iodine adsorption also include the presence of electron-rich surfaces that play a critical role in surface adsorption and the efficient packing of adsorbed iodine molecules within the pores. Thus, frameworks incorporating infinite 1D channels can absorb iodine very effectively owing to more efficient packing of extended iodine-chains within the pores.
Category | Name | BET surface area (m2 g−1) | Pore volume (cm3 g−1) | Iodine uptake (g g−1) | Ref. |
---|---|---|---|---|---|
a Studies were conducted for a single crystal and no BET surface area or pore volume was reported.TPB = triphenylbenzene; DMTP = dimethoxyterephthaldehyde; TTA = 4,4′,4′′-(1,3,5-triazine-2,4,6-triyl)trianiline; TTB = 4,4′,4′′-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde); H2L1 = 4,4′-(ethyne-1,2-diyl)dibenzoic acid; H2L2 = 4,4′-(buta-1,3-diyne-1,4-diyl)-dibenzoic acid; H2L3 = 4,4′-[1,4-phenylenebis(ethyne-2,1-diyl)]-dibenzoic acid; TPT-DHBD = 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine; DHBD = 3,3′-dihydroxybenzidine.AC = activated carbon. | |||||
Activated carbon | Activated Carbon | — | — | 4.35 | 157 |
Uassis-PC800 | 3053 | 1.67 | 2.25 | 158 | |
KOH-AC | 1973 | 1.15 | 3.76 | 55 | |
AC | 1292 | 0.74 | 2.42 | 55 | |
AC | 820 | 0.5 | 0.76 | 159 | |
Aerogels | Cg-5C | 1200 | 2.30 | 2.39 | 35 |
Cg-5P | 957 | 1.62 | 0.87 | 35 | |
MoSx | 370 | 0.93 | 1.00 | 6 | |
NiMoS | 490 | 1.39 | 2.25 | 37 | |
CoMoS | 360 | 0.50 | 2.00 | 37 | |
SbSnS | 240 | 1.16 | 2.00 | 37 | |
ZnSnS | 400 | 0.77 | 2.25 | 37 | |
KCoS | 350 | 1.17 | 1.60 | 37 | |
Porous organic polymers | PAF-23 | 82 | 0.04 | 2.71 | 52 |
PAF-24 | 136 | 0.10 | 2.76 | 52 | |
PAF-25 | 262 | 0.20 | 2.60 | 52 | |
AzoPPN | 400 | 0.68 | 2.90 | 136 | |
PSIF-5 | 574 | 1.41 | 4.85 | 137 | |
SCMP-II | 120 | 0.62 | 3.45 | 56 | |
TTPB | 222 | 0.13 | 4.43 | 7 | |
TTPPA | 512 | 0.30 | 4.90 | 51 | |
TatPOP-2 | 36.5 | 0.18 | 4.50 | 116 | |
Zeolites | HISL | — | — | 0.53 | 31 |
Ag@4A | 23.62 | 0.077 | 0.160 | 160 | |
Bi5@Mordenite | 412 | 0.27 | 0.538 | 161 | |
Mordenite | 305 | 0.19 | 0.275 | 161 | |
ZIF-67@MCF | 1148 | 0.76 | 1.78 | 162 | |
Covalent organic frameworks | HcOFa | — | — | 2.90 | 77 |
TPB-DMTP | 1927 | 1.28 | 6.26 | 41 | |
TTA-TTB | 1733 | 1.01 | 4.95 | 41 | |
COF-DL229 | 1762 | 0.64 | 4.7 | 138 | |
TPT-DHBD | 109 | 0.30 | 5.43 | 42 | |
SIOC-COF-7 | 618 | 0.41 | 4.81 | 8 | |
COF-LZU1 | 858 | 0.46 | 5.30 | 163 | |
TpPa1 | 765 | 0.48 | 2.45 | 163 | |
Micro-COF-1 | 816 | 0.59 | 2.9 | 45 | |
Micro-COF-2 | 1056 | 0.71 | 3.5 | 45 | |
Meso-COF-3 | 982 | 0.84 | 4.0 | 45 | |
Meso-COF-4 | 926 | 1.01 | 3.3 | 45 |
Recent research has explored methods of overcoming the challenges posed by the practical application of MOFs for the capture of radionuclides. A major concern when MOFs are discussed for industrial applications is that the reactions used to synthesis them require high temperatures, high pressures, long synthesis times and hazardous solvents. These problems have been alleviated using synthesis techniques such as electrosynthesis113 and microwave synthesis,114 and these methods can be scaled up to make MOFs quickly using continuous flow processes. The use of green solvents has also shown promise in removing the need for hazardous solvents.115 Because MOFs are typically produced as powders this makes them impractical for use in industrial scale processes due to problems with handling, contamination and transport. To overcome these problems MOF-polymer composite beads have been fabricated.116–118 The beads can be readily handled and have also been shown to increase the iodine capture performance in the case HKUST-1 and PES composites when compared to bulk MOF powder.116 Investigations into the stability of MOFs as well as the ability to adsorb and retain iodine under conditions expected during reprocessing and nuclear accidents have also been investigated with promising results. A recent paper confirms that UiO-66-NH2 is able to retain iodine under high radiation, temperature and humidity conditions and that the structure was unaffected.119 The stability of MOFs when exposed to high levels of radiation120–122 has highlighted the potential for them to be used, not only in the capture of radioactive iodine, but also for long-term storage.
MOFs can be considered as promising sorbents for a wide range of small molecules. In this review, we have discussed the emerging understanding of iodine adsorption in MOFs gained from investigations of adsorption methods, MOF design and host–guest chemistry of iodine-loaded systems. We also demonstrate useful strategies for enhancing the sequestration of iodine via materials engineering, e.g., glass sintering. Recent research confirms the potential of utilising MOFs in the field of adsorptive capture of radioactive iodine from nuclear fission waste products. However, this area remains largely unexplored due to challenges around the reactive nature of iodine and requirements for MOF stability and difficulties in characterisation of the host–guest systems.
Looking towards the future, the emerging properties of iodine-loaded MOFs also hold great promise for additional practical applications, for example, in catalysis and sensing. Of note is the recent report of the use of MOFs has a carrier of iodine for medical and microbial applications.164 ZIF-8 has been immobilised on titanium and can actas a carrier and release agent of iodine for antibacterial therapy in orthopaedics, the release of iodine being triggered by near ir radiation.165 Likewise, the photochemical release of dichromate by iodine sorption in a water stable system166 reflects the huge potential that MOFs have in developing new polyfunctional and targeted technologies.
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