Actinide-based MOFs: a middle ground in solution and solid-state structural motifs

Ekaterina A. Dolgopolova , Allison M. Rice and Natalia B. Shustova *
Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208, USA. E-mail: shustova@sc.edu

Received 22nd December 2017 , Accepted 6th February 2018

First published on 6th February 2018


In this review, we highlight how recent advances in the field of actinide structural chemistry of metal–organic frameworks (MOFs) could be utilized towards investigations relative to efficient nuclear waste administration, driven by the interest towards development of novel actinide-containing architectures as well as concerns regarding environmental pollution and nuclear waste storage. We attempt to perform a comprehensive analysis of more than 100 crystal structures of the existing An (U,Th)-based MOFs to establish a correlation between structural density and wt% of actinide and bridge structural motifs common for natural minerals with ones typically observed in the solution chemistry of actinides. In addition to structural considerations, we showcase the benefits of MOF modularity and porosity towards the stepwise building of hierarchical material complexity and the capture of nuclear fission products, such as technetium and iodine. We expect that these facets not only contribute to the fundamental science of actinide chemistry, but also could foreshadow pathways for more efficient nuclear waste management.


Introduction

The development of new actinide-containing architectures is essential in light of fundamental understanding of actinide chemistry as well as the acquisition of fundamental knowledge of actinide-containing hybrid materials, which could enhance the efficiency of current radionuclide management and disposal.1–3 In the past couple of decades, there has been a significant focus on the structural aspects of actinide chemistry,4–12 since it could lead to new facets in nuclear waste administration, as well as predictions of material stability and long-term performance.

Uranium has emerged as the most studied actinide, due to its rich structural and coordination chemistry, as well as its use in the nuclear fuel cycle.13,14 Thorium, on the other hand, is much less explored, but is gaining increasing interest due to recent advances in its redox chemistry as well as its potential as a nuclear fuel.15

The current research, mainly driven by the quest for novel actinide-containing structures and their correspondence to material properties, has recently shifted towards metal–organic frameworks (MOFs) as a versatile platform for the investigation of actinide behavior, owing to their synthetic diversity and structural tunability.16–18 Since MOFs have shown promising results in the realms of catalysis,19,20 sensing,21,22 storage,23 and separation,24 the efforts to construct hybrid porous materials could be prolific in expanding the benefits of MOFs towards nuclear waste administration as well. MOFs not only offer the potential for radionuclide-containing species in the pores, but also present the opportunity to integrate actinides through cation exchange inside the metal nodes, chelation to organic linkers, or metal node extension as shown in Fig. 1.25 Additional benefits for An-MOF preparation come from the solvothermal approach that relies on moderate temperatures, thus preventing formation of volatile radioactive species in contrast to An-containing borosilicate glass.26 However, properties of An-frameworks are largely underexplored despite that there have been many structural reports of An-MOFs.4 Therefore, a shift from structural and topological studies towards establishment of fundamental structure–property relationships is essential to reveal the full potential of An-MOFs.


image file: c7cc09780h-f1.tif
Fig. 1 A schematic representation of a MOF, in which potential places for An integration are shown in red.

As a start in this direction, we attempt to highlight the structural patterns observed for An-MOFs in comparison with actinide-containing minerals or molecular complexes to shed light on potential tendencies in MOF chemical behavior. We want to demonstrate that MOF versatility allows adaptation of either mononuclear metal nodes (i.e., one actinide ion per the secondary building unit (SBU)) typically observed in natural minerals or multinuclear ones, more common for discrete complexes. Thus, we are attempting to bridge structural aspects typical for solid-state An-based extended compounds with those of molecular derivatives through a MOF platform. Another benefit of MOFs, their porosity, will be discussed with focus on the capture of nuclear fission products, including technetium and iodine.27–33

To summarize, this review will outline the structural aspects of An-MOFs reported to date in comparison with those of An-derivatives observed in solid-state as well as solution. The unprecedented modularity of MOFs, allowing the possibility to build stepwise hierarchical complexity in An-MOFs, will also be discussed. Recent progress for radionuclide sequestration inside framework cavities will be examined, highlighting their potential for applications such as selective membranes for An separation, efficient actinide sensors, or porous materials for An storage, ultimately leading to more efficient nuclear waste administration.

Structural diversity of U- and Th-MOFs

In this review, we focus on the classification of U- and Th-based MOFs depending on the nature and nuclearity of actinide-bearing structural units. The majority of prepared An-MOF structures have mononuclear units isolated from each other through organic linkers.4,5 Control over the formation of desirable motifs is an ongoing challenge, since mechanistic studies are rarely available and, in general, the synergistic effect of factors such as pH, concentration, solvent, and temperature on MOF growth is poorly understood.4

To date, An-MOF chemistry reveals a wide diversity of actinide-bearing SBUs, which ensemble and geometry will be discussed in comparison with structural units previously observed in minerals or molecular complexes. For instance, as in solid-state extended structures, where actinide-containing species can form chains or sheets connected through higher valence cations, MOFs could exhibit similar patterns of actinide-units connected by organic linkers. Meanwhile, multinuclear SBUs of MOFs could contain oxo and/or hydroxyl bridges, typically observed in solution chemistry.6,34

One of the most studied actinides is uranium due to its importance in nuclear fuels. Usually, it exists as a linear dioxo-cation, UO22+, giving rise to three typical coordination environments: hexa-coordinated tetragonal pyramid, hepta-coordinated pentagonal bipyramid, and octa-coordinated hexagonal bipyramid, which could be distorted through coordination to organic ligands in MOFs.4,7 Uranyl-containing frameworks usually possess chain-like or layered topology, while three-dimensional MOF structures are still rare. Their preparation is challenging and requires, in most cases, presence of the second functional group on the organic linker, for instance, different from –COOH (e.g., pyridyl). The majority of reported uranium-based MOFs were structurally characterized without any analysis of their chemicophysical properties.

In comparison to uranium, thorium chemistry is even less explored due to only one dominant oxidation state (+4), and a previously limited interest in thorium as nuclear fuel.35 Consequently, there have only been a few reports of Th-MOFs to date, which structural motifs will be discussed below. However, the possibility to obtain thorium in low oxidation states such as +3 or +2 were shown on examples of soluble molecular complexes, which slightly open the door to a very unexplored part of Th chemistry.36 For other actinides (e.g., Np, Pu), even structural information is limited by very few examples of formate or oxalate extended structures.37

Mononuclear motifs

The reactivity of U- and Th-precursors with organic linkers functionalized with carboxylate, pyridyl, and phosphoryl groups has resulted in a number of structures with mononuclear SBUs (nuclearity = a number of actinide centers involved in the formation of SBUs). There are 17 minerals, which structures contain isolated or infinite chains of mononuclear uranium polyhedral, according to an analysis performed by Burns and co-workers.34 Notably, the number of structural types is constantly growing due to the persistent interest in novel architectures and motifs, which are crucial to manage nuclear waste storage in a more efficient fashion.

Despite the fact that U-containing units in MOFs are separated from each other through organic linkers, topological similarities with minerals in the organization of structural units can still be found (Fig. 2–4). For instance, Loiseau and co-workers prepared two MOFs, in which U-based bipyramids are connected through carboxylate groups of organic linkers (Fig. 2).38 The same structural pattern for uranium polyhedra was observed in the natural mineral, walpurgite ((BiO)4(UO2)(As2O4)2·3H2O), in which uranyl units are separated by arsenates (AsO43−).39 Interestingly, walpurgite is known to be non-fluorescent, while UO2(1,4-BDC) (H2BDC = 1,4-biphenyldicarboxylic acid) exhibits green emission typical for uranyl-containing compounds.38


image file: c7cc09780h-f2.tif
Fig. 2 Similar structural motifs observed in (a) walpurgite, (b) UO2(1,4-BDC), and (c) UO2(4,4′-ADC), containing isolated mononuclear uranium-based polyhedra (1,4-BDC2− = 1,4-biphenyldicarboxylate, 4,4′-ADC2− = 4,4′-azobenzenedicarboxylate).38,39

image file: c7cc09780h-f3.tif
Fig. 3 Structural pattern in (a) adolfpateraite and (b) UO2(OH)(PYCA) MOF, containing chains of vertice sharing polyhedra.40,42

image file: c7cc09780h-f4.tif
Fig. 4 Structural comparison of studtite (a) and MOF (b), containing chains of edge-sharing polyhedra.41,43

Another structural motif observed in U-based minerals is infinite chains of uranyl polyhedra. Chains can be organized in two different fashions: sharing vertices between polyhedra or sharing of polyhedron edges as shown in Fig. 3 and 4.40,41 However, chain-type topology is observed in only 10 natural minerals.34 One of them is adolfpateraite, K(UO2)(SO4)(OH)(H2O), which crystal structure has pentagonal bipyramids that form chains by sharing vertices (Fig. 3).42 Bipyramids possessing the same connectivity were observed in a MOF structure prepared in a hydrothermal reaction between uranyl acetate and pyrazine-2-carboxylic acid (PYCA).40 The obtained framework exhibits very intense green emission, attributed to ligand-to-metal charge transfer, which cannot occur in minerals consisting of only inorganic units.

A rare example of uranium-containing peroxide mineral is studtite, [(UO2)(O2)(H2O)2]·H2O, which could be prepared in an oxidizing environment.43,44 The interest towards studtite is mainly driven by its presence as a phase in nuclear waste. The edge sharing hexagonal bipyramids observed in studtite could be replicated in MOF structures by utilization of relatively flexible aliphatic dicarboxylate linkers (e.g., suberic and azelaic acids) as shown in Fig. 4.41 The observed structural similarity between studtite and the MOF could be used as a foundation for new architectures of future nuclear wasteforms.

Recently, Farha and co-workers presented an outstanding example of a uranium-based MOF with the largest unit cell and triangular uranium nodes, [UO2(RCOO)3], were connected by a tricarboxylate ligand, 5′-(4-carboxyphenyl)-2′,4′,6′-trimethyl-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid.45 Such coordination was previously observed in uranyl-containing carbonate minerals containing [UO2(CO3)3]4− clusters.34 In minerals, a uranyl hexagonal bipyramid shares three edges with carbonate triangles, and all equatorial oxygen atoms are a part of the carbonate groups. In the MOF structure, all equatorial oxygen atoms are part of tetratopic linkers, giving rise to an extended porous structure.45 Due to the large pore aperture, the presented anionic framework can be useful not only for encapsulation of cationic dyes (e.g., methylene blue), but also proteins (e.g., cytochrome c).

Zhang and co-workers demonstrated the possibility to vary framework dimensionality beyond 1D or 2D materials as a function of the ligand design by utilization of flexible carboxylic linkers (e.g., 4,4′-[[2-[(4-carboxyphenoxy)ethyl]-2-methylpropane-1,3diyl]dioxy]dibenzoic acid and hexakis[4-(carboxyphenyl)oxamethyl]-3-oxapentane).46 This approach resulted in the preparation of the first examples of two interpenetrated 3D U-MOF structures. In one of them, two uranium based pentagonal bipyramids form the SBU, while in the second MOF, the SBU consists of three uranium-based structural units: one square bipyramid and two pentagonal bipyramids.46

A mononuclear Th-containing framework with 8-coordinated polyhedra was prepared by the O’Hare group.47 The obtained hexagonal MOF structure had an interesting arrangement of organic linkers, which was described as “double walled” pores. The BTC3− ligands (H3BTC = 1,3,5-benzenetricarboxylic acid) were aligned parallel to each other, resulting in this unprecedented double wall organization, which is not common for aromatic hydrocarbons. The prepared framework possesses selectivity in gas sorption studies resulting in almost no adsorption of N2, while possessing enhanced CO2 capacity. Such behavior was attributed to the difference in interaction of gas molecules with the pore surface.47

In addition to linkers containing exclusively carboxylate groups, tris-(4-carboxylphenyl)phosphineoxide (H3TPO) was used for preparation of An-MOFs.15,48 In a solvothermal reaction with uranyl nitrate, a 3D framework composed of a mononuclear uranium SBU connected by three TPO3− linkers was obtained (Fig. 5).48 In this case, the monodentate terminal –P[double bond, length as m-dash]O group does not participate in coordination to uranyl ions and is available for coordination of metal ions from solution. This feature was used for successful sequestration of Th4+ species from aqueous solutions, providing a novel pathway towards synthesis of bi-actinide compounds.


image file: c7cc09780h-f5.tif
Fig. 5 (a) Crystal structure of an interpenetrated 3D framework composed of a mononuclear uranium SBU connected by TPO3− ligands. Green, purple, red, and grey represent U, P, O, and C, respectively. Hydrogen atoms were omitted for clarity. (b and c) Coordination modes of TPO3− linker.15,48

The same H3TPO linker was utilized for the formation of a Th-containing framework by its heating in the presence of thorium nitrate using DMF as a media.15 However, in this case, the –P[double bond, length as m-dash]O group was involved in coordination with thorium cations (Fig. 5). In addition, coordination of –COOH groups resulted in two types of metal nodes as illustrated in Fig. 5. These simple examples can be used as a good illustration of structural differences between Th- and U-MOFs.

One of the examples demonstrating the unusual coordination environments for U and Th cations inside the extended structures of MOFs was investigated by Wang and co-workers.8,9 They demonstrated a significant distortion in the equatorial plane of uranium-based building units, in which the O[double bond, length as m-dash]U–O bond angles significantly deviated from 90°, resulting in an unprecedented umbrella shape geometry.9 This type of distortion resulted in energetic instability shown by linear transit calculations performed for U-MOFs. More surprisingly, the same umbrella-like distortion was observed in the ThO6Cl3 units of Th-MOFs.8 This example showed the possibility for low valent actinide ions (Th) to adopt a coordination environment typical for high valent actinide ions (U(VI)). Such coordination flexibility of Th allowed for the synthesis of the first bi-actinide containing frameworks with a U to Th ratio from 1 to 9.8

Multinuclear motifs

Despite a number of known An-MOF structures, mechanistic details of their formation are typically not well understood. For instance, although a number of frameworks contain polynuclear motifs, the majority of An-MOF structures have mononuclear SBUs, and factors causing this phenomenon are not well studied.

To shed light on mechanistic aspects, the Loiseau research group performed systematic studies on how reaction conditions (e.g., water content in the reaction mixture and temperature) influenced the structural motifs of U- and Th-MOFs.49,50 They showed that the controlled addition of water drastically changes the coordination environment of uranium in MOFs. For instance, absence of water in the reaction mixture led to crystallization of U2Cl2(BDC)3(DMF)4.49 The addition of water into the reaction at relatively low reaction temperatures (110–130 °C), resulted in the formation of a hexanuclear SBU.49 Further increase of both water and temperature resulted in formation of uranium dioxide, UO2.49 The similar tendency in the formation of the hexanuclear clusters was also observed for thorium, however, appearance of thorium dioxide was not detected.50

Similar studies of the SBU formation as a function of pH were performed on the uranium system, in which U-SBUs are connected by an imidazolium linker.51 The pH was varied by the addition of different amounts of NaOH. As a result, a strong correlation between the nuclearity of SBU and pH was established. Thus, discrete uranyl centers were formed under acidic conditions, while formation of 1D chains were observed for pH > 3. These studies could offer hints of how to achieve control over SBU nuclearity in such complex system as a MOF.

Binuclear motifs. The first member of the multinuclear SBU family is an actinide-bearing unit with two actinide ions. These dimeric units sharing edges or corners of actinide monomers is a common SBU in uranium coordination chemistry (Fig. 6).34
image file: c7cc09780h-f6.tif
Fig. 6 Structural patterns in the (a) deliensite and (b) U-based MOF, containing binuclear uranyl-containing units.52,53

The binuclear geometry of the SBU was observed by the Loiseau group, who heated uranium(IV) chloride in the presence of mellitic acid (H6MEL), which resulted in the formation of U2(OH)2(H2O)2(MEL).52 In the obtained structure, the SBU consists of two pentagonal prisms with a common edge (Fig. 6). An identical uranium-based unit was found in a phosphuranylite-type mineral, deliensite.53 However, deliensite has a typical uranium-based mineral layered topology, while the prepared MOF structure belongs to a class of 3D materials (Fig. 6).

An interesting example of a binuclear Th-based framework was obtained through the reaction of thorium nitrate with 3,5-pyridinedicarboxylic acid.54 In this case, thorium oxyfluoride polyhedra were connected through the corners and resulted in a chain structure. The similar pattern was also observed in a series of thorium or uranium fluorides.55,56

Trinuclear motifs. Trinuclear-based SBUs are one of the most unexplored types of building units (Fig. 7).4 Three uranium-containing trigonal-prismatic polyhedral, connected by common edges forming a trinuclear core, was found in a honeycomb-like MOF possessing 1D channels (Fig. 7).57 A similar motif was also observed in uranium(IV) complexes with Schiff base ligands, tBu-Calix[4, 5, and 6]arenes and oxo-alkoxide complexes.58,59
image file: c7cc09780h-f7.tif
Fig. 7 (a) Schematic representation of a honeycomb-like U-MOF with 1D channels containing trinuclear-based SBUs. (b and c) Crystal structures of trinuclear complexes.57

Multinuclear SBUs are even less common for Th-based frameworks in comparison with U-MOFs. Recently, a mesoporous 3D cationic MOF was synthesized through the reaction of thorium nitrate with H3BPTC (H3BPTC = [1,1′-biphenyl]-3,4′,5-tricarboxylic acid, Fig. 8).60 The resulting SBU of the Th-MOF had Th4+ in an atypical 10-coordinated environment described as a triangular cupola, which was previously observed for trivalent lanthanides.60 Such connectivity resulted in the formation of a framework possessing the highest surface area reported for Th-MOFs to date.60 The prepared framework also showed sorption capacity towards ReO4, a commonly used surrogate for TcO4.


image file: c7cc09780h-f8.tif
Fig. 8 (a) A trinuclear Th-based SBU and (b) crystal structure of a cationic Th-based framework. Blue, red, and grey colors correspond Th, O, and C, respectively. Hydrogen atoms were omitted for clarity.60
Tetranuclear motifs. The next group of polynuclear uranium-based MOFs is composed of four uranium ions in one SBU, which is a frequently encountered motif in the solution chemistry of actinides, but has never been reported for any natural or synthesized solid-state extended structures. Six different types for this class of SBUs have been reported. The types I–III (Fig. 9) are the most common moieties, while the other three tetranuclear motifs (IV–VI) are unique and have only one example of each case in the literature.4,61–66
image file: c7cc09780h-f9.tif
Fig. 9 Structural motifs consisting of tetranuclear secondary building units in U-based MOFs.61–66
Hexanuclear motifs. Formation of MOFs with a hexanuclear core was demonstrated by the Loiseau research group through the solvothermal reaction of a tetravalent uranium salt with dicarboxylate linkers, such as 4,4′-biphenyldicarboxylate, 2,6-naphthalenedicarboxylate, 1,4-benzenedicarboxylate, or fumarate.67 All prepared MOFs possess UiO-type (UiO = University of Oslo) topology, in which metal nodes are connected through the maximum possible amount of –COOH groups, 12. A similar hexanuclear core was observed for actinide hydrolysis products in aqueous solutions (Fig. 10).6
image file: c7cc09780h-f10.tif
Fig. 10 (top) Crystal structure of hexanuclear U- and Th-based complexes formed with monodentate formic acid. (bottom) Crystal structure of U- and Th-based MOFs formed with bidentate terephthalic acid, containing hexanuclear SBUs. The green, blue, red, and grey colors indicate U, Th, O, and C, respectively. Hydrogen atoms were omitted for clarity.6,67

Zeller and co-workers showed the formation of the same uranium-based An6O4(OH)x structural unit after the reaction of uranyl nitrate with sodium terephthalate and sodium glutarate.68 The authors explained the observed U(VI)-to-U(IV) reduction through a slow photoreduction of uranyl ions in an alcohol solution with their further stabilization by glutarate linkers.

The first example of a Th-based MOF with a hexanuclear unit was made by utilization of terephtalic acid as an organic linker, which resulted in the formation of a porous framework with a surface area of 730 m2 g−1.50

Framework modularity

Due to the unprecedented modularity, MOFs are ideal platforms for the postsynthetic step-wise integration of actinides inside their structures. Using a sequential multistep approach combining the modularity and versatility of MOFs, Shustova and co-workers devised several strategies for actinide incorporation including: (i) modification of the metal node through cation exchange and/or metal node extension, (ii) modification of organic linkers with anchoring groups for selective actinide capture, and (iii) capture of guest molecules in MOF cavities with subsequent linker installation, preventing release of guest species (Fig. 11).25 These approaches were feasible to apply due to the initial preparation of An-containing frameworks with “unsaturated” metal nodes (i.e., the number of organic linkers is less than maximum possible such as 12) for the first time, allowing for hierarchical complexity to be built stepwise. On the examples of Zr- and Th-based frameworks possessing “unsaturated” metal nodes, they showed the possibility of metal node extension through uranium cation coordination, and therefore, structural node modification, while the parent MOF topology was preserved. Another way to incorporate two different actinides into one structure was achieved through a postsynthetic cation exchange. On the example of a U-based MOF, it was shown that almost complete U-to-Th transmetallation (95%) can be achieved.25
image file: c7cc09780h-f11.tif
Fig. 11 A schematic representation of framework modularity for An integration utilizing An- and Zr-MOFs as precursors. The integrated An-containing species are shown in red.25 Reproduced from ref. 25 with permission from the American Chemical Society, copyright 2017.

Similar to a metal node, an organic linker can also play a crucial role for actinide integration through its functionalization with anchoring groups.25 Furthermore, organic linkers can be utilized as caps (“capping” ligands), which installation occurred postsynthetically for preventing potential actinide leaching from the framework pores. For instance, Zr- or Th-based MOFs have already been probed for capping linker coordination.25 As a result of simultaneous capping linker installation and actinide capture, a material with 52 wt% of Th was obtained (based on inductively coupled plasma atomic emission spectroscopy data). It was also possible to achieve simultaneous capping linker installation and An-guest inclusion using bimetallic An-MOFs, prepared through transmetallation. Thus, there are three synthetic strategies combined in the described An-MOF: capping linker installation, cation exchange, and guest inclusion. This stepwise combination resulted in preparation of a framework with an overall content of actinides of 67 wt%. It is possible to further increase the actinide content of An-based MOFs through the concurrent heating of the sample in the presence of a Th-containing species, which was integrated as guests inside the MOF pores.

In summary, the stepwise building of hierarchical complexity in An-MOFs is possible through unprecedented framework modularity, which can lead to development in the fundamental understanding of the processes involved in the incorporation of actinides in extended structures.

MOF porosity

The phenomenal porosity of MOFs can be an additional key for efficient incorporation of actinide-containing guests inside the framework, in addition to actinide utilization as building units. Due to their high surface area and small structural density, these materials can serve as an effective platform in terms of actinide content versus structural density. Based on single-crystal X-ray data analysis, the structural density of An-MOFs is low due to the porous nature of MOFs, which opens the potential for the incorporation of actinide-containing guests. Based on our structural analysis of 100 U-MOFs and 27 Th-MOFs as shown in Fig. 12, incorporation of actinides inside metal nodes for majority of MOFs results in an actinide content >20 wt%. However, this value can be easily surpassed by utilization of framework porosity, which renders MOFs as an upcoming class of sorbent materials for radionuclide sequestration.27,69 Moreover, MOFs could potentially lead to a higher sorption capacity and faster kinetics, even in comparison with commonly used sorbents such as resins, dendrimers, or pure inorganic materials.27,69 Furthermore, MOFs possess a dual nature for actinide exchange: through size exclusion or selective binding within the pores or channels. Therefore, along with the potential of the pores, the utilization of either a rationally designed organic linker with a specific binding site or even binding occurring at the SBU could be explored in MOFs, for example, radionuclide extraction from seawater.29–32,70,71
image file: c7cc09780h-f12.tif
Fig. 12 Weight percent of (a) uranium and (b) thorium in MOFs as a function of 1/d (d = structural density determined from single-crystal X-ray data).

In this section of the review, we will exclusively focus on the utilization of MOF pores for iodine and technetium capture, alongside the exploration of actinide and lanthanide separations, highlighting the potential of MOFs for nuclear waste remediation.

Iodine capture

One of the highly volatile gases produced from nuclear fission, which possesses significant concerns in nuclear waste management, is iodine.28 It is usually present in the environment or nuclear waste in the forms of molecular I2, I, IO3, and/or organo-I.28 The difficulties associated with iodine capture arise from the high mobility of its species and low adsorption capacities. In addition, it possesses low solubility in vitreous waste forms and is highly volatile at the processing temperatures.72 Therefore, the search for an efficient and cost-effective material is crucial for efficient iodine capture and storage. Due to high MOF porosity and framework tunability, MOFs have already been probed as iodine adsorbents.73–77

Over the last decade, Nenoff and co-workers performed a range of systematic studies to determine the key parameters influencing sorption of molecular I2.78–86 The high adsorption of I2 was achieved in the ZIF-8 MOF through application of a size-selective approach (Fig. 13).79 The obtained results suggested that there are strong interactions between molecular iodine and 2-methylimidazole linker that resulted in up to 125 wt% sorption.79 Moreover, this framework exhibits high absorption capacity even in a pellet form. Furthermore, the same framework was also applied towards engineering of an electrical readout device as one of the first MOF-based sensors in nuclear fuel recycling.84 The observed changes in the electrical response were used for the development of a real-time sensor for I2, which showed high selectivity and direct detection, even in the presence of competing analytes.84


image file: c7cc09780h-f13.tif
Fig. 13 A schematic representation of iodine loading inside ZIF-8 showing interactions of iodine molecules with imidazole linkers. The orange, blue, purple, and grey colors indicate Cu, N, I, and C, respectively. Hydrogen atoms were omitted for clarity.79

A very recent study also highlights the possibility to construct a MOF sensor setup that exhibited a rapid and linear response to the concentration of I2 (Fig. 14).77 The drastic increase in conductivity (∼7 orders of magnitude) was achieved through the formation of an I⋯I2⋯I arrangement between {Cu4I4}n SBUs of the framework.


image file: c7cc09780h-f14.tif
Fig. 14 A schematic representation of iodine loading inside ZIF-8 showing interactions of iodine molecules with imidazole linkers. The orange, blue, purple, and grey colors indicate Cu, N, I, and C, respectively. Hydrogen atoms were omitted for clarity.77

Nenoff and co-workers also probed iodine capture using the well-known Cu3(BTC)2 framework possessing unsaturated metal sites, which are suitable for analyte binding.85 As a result, up to 175 wt% of I2 was successfully captured from the mixed gas steam of iodine and water.

To conclude, systematic studies clearly demonstrated that the combination of pore size, surface area, and/or the presence of cationic species inside the framework pores could be applied for successful iodine capture.78

Another approach included the visualization of iodine binding inside a framework through single-crystal X-ray diffraction was shown by Murugesu and co-workers.87 In their work, a Zn-based MOF constructed from zinc nodes and a 2,4,6-tris(4-pyridyl)-1,3,5-triazine ligand was utilized to study the iodine adsorption process by X-ray diffraction, which can identify preferred binding motifs throughout the uptake process. These studies demonstrated that MOFs can integrate the combination of chemisorption (binding to open metal sites or functional groups) and physisorption (guest uptake inside pores) resulting in enhancement of iodine capture.

Very recently, Li and co-workers proposed a novel approach towards the optimization of the capture of radioactive organic iodides (ROIs, such as methyl or ethyl iodides).88,89 Through a postsynthetic modification of MIL-101-Cr with different tertiary amines (e.g., triethylenediamine, hexamethylenetetramine, or N,N-dimethyethylenediamine) grafted to binding sites within a framework, a record-high value for CH3I capture (71 wt%) was achieved.89 It was also found that the CH3I adsorption follows both chemisorption and physisorption mechanisms, where chemisorption occurs at the amine functionalized sites, and the physisorption depends on the porosity of framework. Loiseau and co-workers also studied the influence of the MOF topology towards adsorption of gaseous CH3I.74 Since adsorption involves weak van der Waals type interactions, the best adsorption capacity was found in MOFs with similar pore diameter to the size of CH3I. This observation was confirmed by theoretical studies performed on several MOFs with a wide range of pore volumes.

Other studies involved the investigation of sorption kinetics of I2 in cyclohexane using Al-MOFs.75 In this case, functionalization of the framework with a –NH2 group can lead to a significant increase in iodine uptake through formation of a charge transfer complex between the amino group and iodine.

Technetium capture

In addition to iodine, another major long-lived fission product is 99Tc, usually present in nuclear waste in the form of anionic pertechnetate (TcO4).27 The pertechnetate ion has an extremely high mobility and noncomplexing nature, meanwhile technetium is volatile and can leach through glass, interfering with the nuclear waste vitrification process. Nowadays, ion exchange resins, molecular and supramolecular anionic receptors are designed to remove TcO4 from the waste stream, but these materials possess low loading capacities as well as slow uptake kinetics. Ionic MOFs, specifically cationic ones, were shown as materials which can potentially capture anionic radioactive pollutants.27 Up until now, cationic MOFs have been tested for sequestration of ReO4,90 CrO42−,90–93 and MnO4,90,93 as surrogates for pertechnetate ion.69,93

Oliver and co-workers described a new methodology to capture oxo-anion pollutants (ReO4, CrO42−, and MnO4) by a cationic framework.90 During the anion uptake by the material, its structure changed by replacing anions present in the structure (1,2-ethanedisulfonate) by anions of interest. This approach resulted in a permanent trapping of anions, which is crucial for radioactive waste treatment. A similar approach was shown recently for selective immobilization of ReO4/TcO4 inside the framework lattice even in the presence of an excess of nitrate ions (Fig. 15).94 Furthermore, ReO4 uptake was performed through a single-crystal-to-single-crystal transformation, where each ReO4 was bound to unsaturated silver sites, foreshadowing a potential way for immobilization of TcO4 into future wasteforms.


image file: c7cc09780h-f15.tif
Fig. 15 Crystal structure of Ag-based framework with ReO4 bound to unsaturated silver sites. The purple, grey, blue, red, and green colors indicate Ag, C, N, O, and Re, respectively. Hydrogen atoms omitted for clarity.94

Thallapally and co-workers showed that the amino-functionalized MOF, UiO-66-NH2, can be used for efficient extraction of ReO4 from water,95 while the Gosh research group probed the anion exchange approach for rapid removal of MnO4 and Cr2O72−, as model anions for TcO4.93

Very recently, the first example of a cationic Ag-MOF (SCU-100, SCU = Soochow University) was directly tested with radioactive TcO4 (Fig. 16).96,97 Based on preliminary studies using the surrogate ReO4, SCU-100 was shown to selectively capture TcO4 in the presence of competing anions.97 As a further study, a novel Ni-based framework was prepared.96 This material could selectively remove TcO4 from an aqueous solution even with a low concentration of pertechnetate. Incorporation of TcO4 inside the channels of the framework did not influence the MOF crystallinity, and the TcO4 position was determined by single-crystal X-ray diffraction. Thus, MOFs provide an effective way to remove TcO4 from streams prior to the vitrification process, however further development of cationic frameworks is necessary to enhance framework capacity as well as selectivity.


image file: c7cc09780h-f16.tif
Fig. 16 Crystal structure of SCU-100 before (left) and after (right) incorporation of ReO4 (right). The purple, gray, blue, red, and green colors indicate Ag, C, N, O, and Re, respectively. Hydrogen atoms were omitted for clarity.97

Element separation

Effective conversion of radioactive mixtures and the further separation of elements from the fission process (a waste stream can consist of up to 40 elements)98 is another problem associated with nuclear waste generation. For instance, separation of thorium from rare-earth elements is a challenging task due to their chemical similarities including small differences in their solubility and oxidation states. The shift from traditional solvent extraction is necessary due to its disadvantages demonstrated beyond the laboratory scale such as generation of a large amount of waste, complex multistep procedures, as well as utilization of expensive mixer-settlers or centrifugal contactors.98 Based on these considerations, it has become imperative to develop a high-performance porous adsorbent, such as MOFs, with selective binding sites. For instance, derivatization of UiO-66 MOFs with carboxylic groups led to formation of a series of highly porous and highly stable MOFs (e.g., UiO-66-COOH and UiO-66-(COOH)2), which showed great selectivity toward Th4+ over a wide range of competing cations (e.g., Zn2+, Co2+, Ni2+, Sr2+, Yb3+, Nd3+, Sm3+, Gd3+, and La3+).99 Furthermore, derivatization of MOFs with –COOH groups resulted in enhanced Th4+ uptake of 236 mg per gram of the framework in comparison with non-functionalized UiO-66 (17 mg per gram). However, the remaining challenge in this case is associated with MOF integrity since the structures of these frameworks were partially collapsed during MOF reuse.

Recently, Long and co-workers demonstrated that porous aromatic frameworks (BPP-7), prepared through polymerization of 1-nonyl terephthalate ester and functionalized with –COOH groups, could be applied towards Ln/An separation.98 For instance, preferential binding of neodymium, even at a low concentration versus iron was shown. Such selectivity was mainly attributed to a size exclusion effect of the cavity engaged in binding.98

Sun and co-workers reported a selective Th4+/Ln3+ separation through utilization of a Zn-MOF possessing unsaturated metal sites located on the N,N′-bis(salicylidene) ethylenediamine linker, which were created through the postsynthetic removal of Mn(III) ions.100 The framework with the “demetalated” ligand was applied to a solution consisting of Eu3+, Lu3+, and Th4+. As a result, adsorption capacity for Th4+ (46.3 mg of Th per gram) was higher in comparison with competing cations. Thus, this work demonstrated a potential towards a more rational treatment of fission products.

Conclusion

As highlighted in this review, it is crucial to develop new actinide-containing architectures in order to facilitate current efforts in nuclear waste administration. Although understanding the properties of actinides has a great fundamental importance, its development still falls behind most of the other elements. Moreover, MOFs can be seen as a remarkably powerful tool to address the fundamental questions dealing with the chemical behavior of An-based structures, including uranium and thorium. The understanding of An-MOF structural aspects could be based on analysis of patterns observed in actinide-containing minerals or organic complexes, since MOFs could be considered as a bridge between solid-state and solution chemistry of actinides. This idea has become feasible through recent advancements in the fields of both the structural chemistry of An-based materials as well as MOFs.

MOFs not only offer the benefit to study structural trends in An-containing extended structures, but also offer the potential to serve as efficient adsorbents for radionuclides. For instance, the sequestration of fission products, such as technetium and iodine, which are mobile hazardous species possessing environmental and human health threats, can be addressed through MOF usage. However, to efficiently utilize MOFs for capture of volatile radionuclide species, it would require mechanistic studies of adsorption/desorption kinetics, as well as development of synthetic routes for modification of pore microenvironment to enhance radionuclide-binding affinity. Furthermore, despite almost limitless choices in metal ions and organic linkers, the poor thermochemical stability of many MOF-based materials could cause significant challenges for anion exchange.27

Nevertheless, MOFs can offer size exclusion or selective binding within its pores, that may not be realized in more conventional materials such as resins. The current reports of TcO4 capture using MOFs are still in its rudimentary phases, but the potential in the recent reports should fuel further studies.

Another aspect which requires immediate attention is a study of MOF stability towards ionizing radiation.101–105 Even though frameworks were shown to maintain their integrity in radiation environments, important aspects, which should be further elucidated, include behavior of the organic building blocks under radiation, effect of different metals on stability, as well as the behavior of actinide-containing guests inside the pores of the radionuclide-containing framework. While there have been only a few reports in this direction, these studies demonstrated the great potential of metal–organic materials in comparison with their organic analogs (e.g., stilbene- or anthracene-containing compounds).102–104

The nature of metals utilized for MOF synthesis is also a key parameter affecting the interaction of a MOF with, for instance, gamma radiation. Thus, due to metal's lower absorption cross-section, Al-based MOFs have the highest resistance towards gamma irradiation in comparison with similar systems made from transition metals (e.g., Cu or Zn).105 At the same time, in Hf- and Zr-based MOFs, a metal-oxo cluster could be used as an antenna for radiation absorption, which was further released as ligand emission.104

The incorporation of radiation responsive structural units (e.g., scintillating or photochromic organic linkers)102,104,106 could expand the opportunity of porous MOFs in the field of actinide detection as novel radiation dosimeters. A novel radiation sensor with enhanced stability and efficiency could be designed through incorporation of known scintillation materials as organic linkers inside the framework.102,104 Moreover, MOFs not only offer increased radiation stability in comparison to pure organic components, but they also allow for the enhancement of photoluminescence and radioluminescence lifetimes through control over chromophore environment, leading to lower detection limits.102 Furthermore, integration of photochromic components or counterparts, capable of switching upon X-ray irradiation, could lead to the development of turn-on sensors allowing detection to be performed by the naked eye.101,106

As a method of action, it would be ideal to synergistically improve synthetic efforts alongside computational modeling for the rational construction of high-performance MOF-based adsorbents. Another aspect, which should be considered to realize the full potential of MOFs as porous adsorbents, is their processability (e.g., pellets vs. powders or crystals).

Since structural studies are abundant for An-based frameworks, this review can serve as the initial foundation for the comprehensive analysis of the current trends in the field, however, deeper fundamental knowledge of structure–function relationships is key for future progress in the An-MOF sector.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported as part of the Center for Hierarchical Wasteform Materials (CHWM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award DE-SC0016574. N. B. S. acknowledges the support from the Sloan Research Fellowship provided by Alfred P. Sloan Foundation and the Cottrell Scholar Award from the Research Corporation for Science Advancement.

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