This journal is©The Royal Society of Chemistry 2018 Chem. Commun.

Cite this:DOI: 10.1039/c7cc09780h

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

Ekaterina A. Dolgopolova, Allison M. Rice and Natalia B. Shustova *

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.

IntroductionThe development of new actinide-containing architectures isessential in light of fundamental understanding of actinidechemistry as well as the acquisition of fundamental knowledge ofactinide-containing hybrid materials, which could enhance theefficiency of current radionuclide management and disposal.1–3

In the past couple of decades, there has been a significant focuson the structural aspects of actinide chemistry,4–12 since it couldlead to new facets in nuclear waste administration, as well aspredictions of material stability and long-term performance.

Uranium has emerged as the most studied actinide, due toits rich structural and coordination chemistry, as well as its usein the nuclear fuel cycle.13,14 Thorium, on the other hand, ismuch less explored, but is gaining increasing interest due torecent advances in its redox chemistry as well as its potential asa nuclear fuel.15

The current research, mainly driven by the quest for novelactinide-containing structures and their correspondence tomaterial properties, has recently shifted towards metal–organicframeworks (MOFs) as a versatile platform for the investigationof actinide behavior, owing to their synthetic diversity andstructural tunability.16–18 Since MOFs have shown promisingresults in the realms of catalysis,19,20 sensing,21,22 storage,23 andseparation,24 the efforts to construct hybrid porous materialscould be prolific in expanding the benefits of MOFs towards

nuclear waste administration as well. MOFs not only offer thepotential for radionuclide-containing species in the pores, butalso present the opportunity to integrate actinides throughcation exchange inside the metal nodes, chelation to organiclinkers, or metal node extension as shown in Fig. 1.25 Additionalbenefits for An-MOF preparation come from the solvothermalapproach that relies on moderate temperatures, thus pre-venting formation of volatile radioactive species in contrast toAn-containing borosilicate glass.26 However, properties ofAn-frameworks are largely underexplored despite that therehave been many structural reports of An-MOFs.4 Therefore, ashift from structural and topological studies towards establish-ment of fundamental structure–property relationships is essentialto reveal the full potential of An-MOFs.

As a start in this direction, we attempt to highlight thestructural patterns observed for An-MOFs in comparison with

Fig. 1 A schematic representation of a MOF, in which potential places forAn integration are shown in red.

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

DOI: 10.1039/c7cc09780h

rsc.li/chemcomm

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actinide-containing minerals or molecular complexes to shedlight on potential tendencies in MOF chemical behavior. Wewant to demonstrate that MOF versatility allows adaptation ofeither mononuclear metal nodes (i.e., one actinide ion per thesecondary building unit (SBU)) typically observed in naturalminerals or multinuclear ones, more common for discretecomplexes. Thus, we are attempting to bridge structural aspectstypical for solid-state An-based extended compounds with thoseof molecular derivatives through a MOF platform. Anotherbenefit of MOFs, their porosity, will be discussed with focuson the capture of nuclear fission products, including technetiumand iodine.27–33

To summarize, this review will outline the structural aspectsof An-MOFs reported to date in comparison with those ofAn-derivatives observed in solid-state as well as solution. Theunprecedented modularity of MOFs, allowing the possibility tobuild stepwise hierarchical complexity in An-MOFs, will alsobe discussed. Recent progress for radionuclide sequestrationinside framework cavities will be examined, highlighting theirpotential for applications such as selective membranes for Anseparation, efficient actinide sensors, or porous materials forAn storage, ultimately leading to more efficient nuclear wasteadministration.

Structural diversity of U- and Th-MOFsIn this review, we focus on the classification of U- and Th-basedMOFs depending on the nature and nuclearity of actinide-bearing structural units. The majority of prepared An-MOF struc-tures have mononuclear units isolated from each other throughorganic linkers.4,5 Control over the formation of desirable motifsis an ongoing challenge, since mechanistic studies are rarelyavailable and, in general, the synergistic effect of factors such aspH, concentration, solvent, and temperature on MOF growth ispoorly understood.4

To date, An-MOF chemistry reveals a wide diversity ofactinide-bearing SBUs, which ensemble and geometry will bediscussed in comparison with structural units previously observedin minerals or molecular complexes. For instance, as in solid-stateextended structures, where actinide-containing species can formchains or sheets connected through higher valence cations, MOFscould exhibit similar patterns of actinide-units connected byorganic linkers. Meanwhile, multinuclear SBUs of MOFs couldcontain oxo and/or hydroxyl bridges, typically observed in solutionchemistry.6,34

One of the most studied actinides is uranium due to itsimportance in nuclear fuels. Usually, it exists as a linear dioxo-cation, UO2

2+, giving rise to three typical coordination environ-ments: hexa-coordinated tetragonal pyramid, hepta-coordinatedpentagonal bipyramid, and octa-coordinated hexagonal bipyramid,which could be distorted through coordination to organicligands in MOFs.4,7 Uranyl-containing frameworks usually possesschain-like or layered topology, while three-dimensional MOFstructures are still rare. Their preparation is challenging andrequires, 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 MOFswere structurally characterized without any analysis of theirchemicophysical properties.

In comparison to uranium, thorium chemistry is even lessexplored due to only one dominant oxidation state (+4), anda previously limited interest in thorium as nuclear fuel.35

Consequently, there have only been a few reports of Th-MOFs todate, 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 com-plexes, which slightly open the door to a very unexplored part ofTh chemistry.36 For other actinides (e.g., Np, Pu), even structuralinformation is limited by very few examples of formate or oxalateextended structures.37

Mononuclear motifs

The reactivity of U- and Th-precursors with organic linkersfunctionalized with carboxylate, pyridyl, and phosphoryl groupshas resulted in a number of structures with mononuclear SBUs(nuclearity = a number of actinide centers involved in theformation of SBUs). There are 17 minerals, which structurescontain isolated or infinite chains of mononuclear uraniumpolyhedral, according to an analysis performed by Burns andco-workers.34 Notably, the number of structural types is con-stantly growing due to the persistent interest in novel architec-tures and motifs, which are crucial to manage nuclear wastestorage in a more efficient fashion.

Despite the fact that U-containing units in MOFs are separatedfrom each other through organic linkers, topological similaritieswith minerals in the organization of structural units can still befound (Fig. 2–4). For instance, Loiseau and co-workers preparedtwo MOFs, in which U-based bipyramids are connected throughcarboxylate groups of organic linkers (Fig. 2).38 The samestructural pattern for uranium polyhedra was observed in thenatural mineral, walpurgite ((BiO)4(UO2)(As2O4)2!3H2O), in whichuranyl units are separated by arsenates (AsO4

3").39 Interestingly,walpurgite is known to be non-fluorescent, while UO2(1,4-BDC)(H2BDC = 1,4-biphenyldicarboxylic acid) exhibits green emissiontypical for uranyl-containing compounds.38

Another structural motif observed in U-based minerals isinfinite chains of uranyl polyhedra. Chains can be organized intwo different fashions: sharing vertices between polyhedra orsharing of polyhedron edges as shown in Fig. 3 and 4.40,41

However, chain-type topology is observed in only 10 naturalminerals.34 One of them is adolfpateraite, K(UO2)(SO4)(OH)(H2O),which crystal structure has pentagonal bipyramids that formchains by sharing vertices (Fig. 3).42 Bipyramids possessing thesame connectivity were observed in a MOF structure prepared in ahydrothermal reaction between uranyl acetate and pyrazine-2-carboxylic acid (PYCA).40 The obtained framework exhibits veryintense green emission, attributed to ligand-to-metal chargetransfer, which cannot occur in minerals consisting of onlyinorganic units.

A rare example of uranium-containing peroxide mineral isstudtite, [(UO2)(O2)(H2O)2]!H2O, which could be prepared in an

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oxidizing environment.43,44 The interest towards studtite ismainly driven by its presence as a phase in nuclear waste.The edge sharing hexagonal bipyramids observed in studtitecould be replicated in MOF structures by utilization of relatively

flexible aliphatic dicarboxylate linkers (e.g., suberic and azelaicacids) as shown in Fig. 4.41 The observed structural similaritybetween studtite and the MOF could be used as a foundationfor new architectures of future nuclear wasteforms.

Recently, Farha and co-workers presented an outstandingexample of a uranium-based MOF with the largest unit cell andtriangular uranium nodes, [UO2(RCOO)3]", were connected bya tricarboxylate ligand, 50-(4-carboxyphenyl)-20,40,60-trimethyl-[1,10:30,100-terphenyl]-4,400-dicarboxylic acid.45 Such coordinationwas previously observed in uranyl-containing carbonate mineralscontaining [UO2(CO3)3]4" clusters.34 In minerals, a uranyl hexa-gonal bipyramid shares three edges with carbonate triangles,and all equatorial oxygen atoms are a part of the carbonategroups. In the MOF structure, all equatorial oxygen atoms arepart of tetratopic linkers, giving rise to an extended porousstructure.45 Due to the large pore aperture, the presented anionicframework can be useful not only for encapsulation of cationicdyes (e.g., methylene blue), but also proteins (e.g., cytochrome c).

Zhang and co-workers demonstrated the possibility to varyframework dimensionality beyond 1D or 2D materials as afunction of the ligand design by utilization of flexible carboxyliclinkers (e.g., 4,40-[[2-[(4-carboxyphenoxy)ethyl]-2-methylpropane-1,3diyl]dioxy]dibenzoic acid and hexakis[4-(carboxyphenyl)-oxamethyl]-3-oxapentane).46 This approach resulted in thepreparation of the first examples of two interpenetrated 3DU-MOF structures. In one of them, two uranium based pentagonalbipyramids form the SBU, while in the second MOF, the SBUconsists of three uranium-based structural units: one squarebipyramid and two pentagonal bipyramids.46

A mononuclear Th-containing framework with 8-coordinatedpolyhedra was prepared by the O’Hare group.47 The obtainedhexagonal MOF structure had an interesting arrangement oforganic 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 unprece-dented double wall organization, which is not common foraromatic hydrocarbons. The prepared framework possesses selec-tivity in gas sorption studies resulting in almost no adsorptionof N2, while possessing enhanced CO2 capacity. Such behaviorwas attributed to the difference in interaction of gas moleculeswith the pore surface.47

In addition to linkers containing exclusively carboxylategroups, tris-(4-carboxylphenyl)phosphineoxide (H3TPO) was usedfor preparation of An-MOFs.15,48 In a solvothermal reaction withuranyl nitrate, a 3D framework composed of a mononuclearuranium SBU connected by three TPO3" linkers was obtained(Fig. 5).48 In this case, the monodentate terminal –PQO groupdoes not participate in coordination to uranyl ions and is availablefor coordination of metal ions from solution. This feature wasused for successful sequestration of Th4+ species from aqueoussolutions, providing a novel pathway towards synthesis ofbi-actinide compounds.

The same H3TPO linker was utilized for the formation of aTh-containing framework by its heating in the presence ofthorium nitrate using DMF as a media.15 However, in this case,the –PQO group was involved in coordination with thorium

Fig. 2 Similar structural motifs observed in (a) walpurgite, (b) UO2(1,4-BDC),and (c) UO2(4,40-ADC), containing isolated mononuclear uranium-basedpolyhedra (1,4-BDC2" = 1,4-biphenyldicarboxylate, 4,40-ADC2" = 4,40-azobenzenedicarboxylate).38,39

Fig. 3 Structural pattern in (a) adolfpateraite and (b) UO2(OH)(PYCA) MOF,containing chains of vertice sharing polyhedra.40,42

Fig. 4 Structural comparison of studtite (a) and MOF (b), containing chainsof edge-sharing polyhedra.41,43

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cations (Fig. 5). In addition, coordination of –COOH groupsresulted in two types of metal nodes as illustrated in Fig. 5.These simple examples can be used as a good illustration ofstructural differences between Th- and U-MOFs.

One of the examples demonstrating the unusual coordina-tion environments for U and Th cations inside the extendedstructures of MOFs was investigated by Wang and co-workers.8,9

They demonstrated a significant distortion in the equatorial planeof uranium-based building units, in which the OQU–O bondangles significantly deviated from 901, resulting in an unprece-dented umbrella shape geometry.9 This type of distortion resultedin energetic instability shown by linear transit calculations per-formed for U-MOFs. More surprisingly, the same umbrella-likedistortion was observed in the ThO6Cl3 units of Th-MOFs.8 Thisexample showed the possibility for low valent actinide ions (Th) toadopt a coordination environment typical for high valent actinideions (U(VI)). Such coordination flexibility of Th allowed for thesynthesis of the first bi-actinide containing frameworks with a Uto Th ratio from 1 to 9.8

Multinuclear motifs

Despite a number of known An-MOF structures, mechanisticdetails of their formation are typically not well understood. Forinstance, although a number of frameworks contain polynuclearmotifs, the majority of An-MOF structures have mononuclearSBUs, and factors causing this phenomenon are not well studied.

To shed light on mechanistic aspects, the Loiseau researchgroup 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 Theyshowed that the controlled addition of water drastically changesthe coordination environment of uranium in MOFs. For instance,absence of water in the reaction mixture led to crystallization ofU2Cl2(BDC)3(DMF)4.49 The addition of water into the reaction atrelatively low reaction temperatures (110–130 1C), resulted in theformation of a hexanuclear SBU.49 Further increase of both waterand temperature resulted in formation of uranium dioxide, UO2.49

The similar tendency in the formation of the hexanuclear clusterswas also observed for thorium, however, appearance of thoriumdioxide was not detected.50

Similar studies of the SBU formation as a function of pHwere performed on the uranium system, in which U-SBUs areconnected by an imidazolium linker.51 The pH was varied by

the addition of different amounts of NaOH. As a result, a strongcorrelation between the nuclearity of SBU and pH was estab-lished. Thus, discrete uranyl centers were formed under acidicconditions, while formation of 1D chains were observed forpH 4 3. These studies could offer hints of how to achieve controlover SBU nuclearity in such complex system as a MOF.

Binuclear motifs. The first member of the multinuclearSBU family is an actinide-bearing unit with two actinide ions.These dimeric units sharing edges or corners of actinide mono-mers is a common SBU in uranium coordination chemistry(Fig. 6).34

The binuclear geometry of the SBU was observed by theLoiseau group, who heated uranium(IV) chloride in the presenceof mellitic acid (H6MEL), which resulted in the formation ofU2(OH)2(H2O)2(MEL).52 In the obtained structure, the SBU con-sists of two pentagonal prisms with a common edge (Fig. 6). Anidentical uranium-based unit was found in a phosphuranylite-type mineral, deliensite.53 However, deliensite has a typicaluranium-based mineral layered topology, while the preparedMOF structure belongs to a class of 3D materials (Fig. 6).

An interesting example of a binuclear Th-based frameworkwas obtained through the reaction of thorium nitrate with3,5-pyridinedicarboxylic acid.54 In this case, thorium oxyfluoridepolyhedra were connected through the corners and resulted in achain structure. The similar pattern was also observed in a seriesof thorium or uranium fluorides.55,56

Trinuclear motifs. Trinuclear-based SBUs are one of themost unexplored types of building units (Fig. 7).4 Three uranium-containing trigonal-prismatic polyhedral, connected by commonedges forming a trinuclear core, was found in a honeycomb-likeMOF possessing 1D channels (Fig. 7).57 A similar motif was alsoobserved in uranium(IV) complexes with Schiff base ligands,tBu-Calix[4, 5, and 6]arenes and oxo-alkoxide complexes.58,59

Multinuclear SBUs are even less common for Th-basedframeworks in comparison with U-MOFs. Recently, a meso-porous 3D cationic MOF was synthesized through the reactionof thorium nitrate with H3BPTC (H3BPTC = [1,10-biphenyl]-3,40,5-tricarboxylic acid, Fig. 8).60 The resulting SBU of the

Fig. 5 (a) Crystal structure of an interpenetrated 3D framework composedof a mononuclear uranium SBU connected by TPO3" ligands. Green, purple,red, and grey represent U, P, O, and C, respectively. Hydrogen atoms wereomitted for clarity. (b and c) Coordination modes of TPO3" linker.15,48

Fig. 6 Structural patterns in the (a) deliensite and (b) U-based MOF,containing binuclear uranyl-containing units.52,53

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Th-MOF had Th4+ in an atypical 10-coordinated environmentdescribed as a triangular cupola, which was previously observedfor trivalent lanthanides.60 Such connectivity resulted in theformation of a framework possessing the highest surface areareported for Th-MOFs to date.60 The prepared framework alsoshowed sorption capacity towards ReO4

", a commonly usedsurrogate for TcO4

".Tetranuclear motifs. The next group of polynuclear

uranium-based MOFs is composed of four uranium ions inone SBU, which is a frequently encountered motif in thesolution chemistry of actinides, but has never been reportedfor 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 theother three tetranuclear motifs (IV–VI) are unique and haveonly one example of each case in the literature.4,61–66

Hexanuclear motifs. Formation of MOFs with a hexanuclearcore was demonstrated by the Loiseau research group throughthe solvothermal reaction of a tetravalent uranium salt withdicarboxylate linkers, such as 4,40-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 connectedthrough the maximum possible amount of –COOH groups, 12.A similar hexanuclear core was observed for actinide hydrolysisproducts in aqueous solutions (Fig. 10).6

Zeller and co-workers showed the formation of the sameuranium-based An6O4(OH)x structural unit after the reactionof uranyl nitrate with sodium terephthalate and sodiumglutarate.68 The authors explained the observed U(VI)-to-U(IV)reduction through a slow photoreduction of uranyl ions in analcohol solution with their further stabilization by glutaratelinkers.

The first example of a Th-based MOF with a hexanuclearunit was made by utilization of terephtalic acid as an organiclinker, which resulted in the formation of a porous frameworkwith a surface area of 730 m2 g"1.50

Framework modularityDue to the unprecedented modularity, MOFs are ideal plat-forms for the postsynthetic step-wise integration of actinidesinside their structures. Using a sequential multistep approachcombining the modularity and versatility of MOFs, Shustovaand co-workers devised several strategies for actinide incor-poration including: (i) modification of the metal node throughcation exchange and/or metal node extension, (ii) modificationof organic linkers with anchoring groups for selective actinidecapture, and (iii) capture of guest molecules in MOF cavitieswith subsequent linker installation, preventing release of guestspecies (Fig. 11).25 These approaches were feasible to apply dueto the initial preparation of An-containing frameworks with‘‘unsaturated’’ metal nodes (i.e., the number of organic linkersis less than maximum possible such as 12) for the first time,allowing for hierarchical complexity to be built stepwise. On theexamples of Zr- and Th-based frameworks possessing ‘‘unsatu-rated’’ metal nodes, they showed the possibility of metal nodeextension through uranium cation coordination, and therefore,structural node modification, while the parent MOF topologywas preserved. Another way to incorporate two different acti-nides into one structure was achieved through a postsyntheticcation exchange. On the example of a U-based MOF, it wasshown that almost complete U-to-Th transmetallation (95%)can be achieved.25

Similar to a metal node, an organic linker can also play acrucial role for actinide integration through its functionaliza-tion with anchoring groups.25 Furthermore, organic linkerscan be utilized as caps (‘‘capping’’ ligands), which installationoccurred postsynthetically for preventing potential actinideleaching from the framework pores. For instance, Zr- or Th-basedMOFs have already been probed for capping linker coordination.25

As a result of simultaneous capping linker installation and actinidecapture, a material with 52 wt% of Th was obtained (based oninductively coupled plasma atomic emission spectroscopy data).It was also possible to achieve simultaneous capping linkerinstallation and An-guest inclusion using bimetallic An-MOFs,

Fig. 7 (a) Schematic representation of a honeycomb-like U-MOF with 1Dchannels containing trinuclear-based SBUs. (b and c) Crystal structures oftrinuclear complexes.57

Fig. 8 (a) A trinuclear Th-based SBU and (b) crystal structure of a cationicTh-based framework. Blue, red, and grey colors correspond Th, O, and C,respectively. Hydrogen atoms were omitted for clarity.60

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prepared through transmetallation. Thus, there are three syn-thetic strategies combined in the described An-MOF: cappinglinker installation, cation exchange, and guest inclusion. Thisstepwise combination resulted in preparation of a frameworkwith an overall content of actinides of 67 wt%. It is possibleto further increase the actinide content of An-based MOFsthrough the concurrent heating of the sample in the presenceof a Th-containing species, which was integrated as guestsinside the MOF pores.

In summary, the stepwise building of hierarchical com-plexity in An-MOFs is possible through unprecedented frameworkmodularity, which can lead to development in the fundamentalunderstanding of the processes involved in the incorporation ofactinides in extended structures.

MOF porosityThe phenomenal porosity of MOFs can be an additional key forefficient incorporation of actinide-containing guests inside theframework, in addition to actinide utilization as building units.Due to their high surface area and small structural density, thesematerials can serve as an effective platform in terms of actinidecontent versus structural density. Based on single-crystal X-raydata analysis, the structural density of An-MOFs is low due to theporous nature of MOFs, which opens the potential for the incor-poration of actinide-containing guests. Based on our structuralanalysis of 100 U-MOFs and 27 Th-MOFs as shown in Fig. 12,incorporation of actinides inside metal nodes for majority ofMOFs results in an actinide content 420 wt%. However, thisvalue can be easily surpassed by utilization of framework porosity,which renders MOFs as an upcoming class of sorbent materialsfor radionuclide sequestration.27,69 Moreover, MOFs could poten-tially lead to a higher sorption capacity and faster kinetics, evenin comparison with commonly used sorbents such as resins,dendrimers, or pure inorganic materials.27,69 Furthermore, MOFspossess a dual nature for actinide exchange: through size exclu-sion or selective binding within the pores or channels. Therefore,along with the potential of the pores, the utilization of either arationally designed organic linker with a specific binding site oreven binding occurring at the SBU could be explored in MOFs, forexample, radionuclide extraction from seawater.29–32,70,71

In this section of the review, we will exclusively focus on theutilization of MOF pores for iodine and technetium capture,alongside the exploration of actinide and lanthanide separa-tions, highlighting the potential of MOFs for nuclear wasteremediation.

Iodine capture

One of the highly volatile gases produced from nuclear fission,which possesses significant concerns in nuclear waste manage-ment, is iodine.28 It is usually present in the environment or

Fig. 10 (top) Crystal structure of hexanuclear U- and Th-based complexesformed with monodentate formic acid. (bottom) Crystal structure of U- andTh-based MOFs formed with bidentate terephthalic acid, containing hexa-nuclear SBUs. The green, blue, red, and grey colors indicate U, Th, O, and C,respectively. Hydrogen atoms were omitted for clarity.6,67

Fig. 9 Structural motifs consisting of tetranuclear secondary building units in U-based MOFs.61–66

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nuclear waste in the forms of molecular I2, I", IO3", and/or

organo-I".28 The difficulties associated with iodine capturearise from the high mobility of its species and low adsorptioncapacities. In addition, it possesses low solubility in vitreouswaste forms and is highly volatile at the processing tempera-tures.72 Therefore, the search for an efficient and cost-effectivematerial is crucial for efficient iodine capture and storage. Dueto high MOF porosity and framework tunability, MOFs havealready been probed as iodine adsorbents.73–77

Over the last decade, Nenoff and co-workers performed a rangeof systematic studies to determine the key parameters influencingsorption of molecular I2.78–86 The high adsorption of I2 was achievedin the ZIF-8 MOF through application of a size-selective approach(Fig. 13).79 The obtained results suggested that there are stronginteractions between molecular iodine and 2-methylimidazolelinker that resulted in up to 125 wt% sorption.79 Moreover, thisframework exhibits high absorption capacity even in a pellet form.Furthermore, the same framework was also applied towards engi-neering of an electrical readout device as one of the first MOF-basedsensors in nuclear fuel recycling.84 The observed changes in the

electrical response were used for the development of a real-timesensor for I2, which showed high selectivity and direct detection,even in the presence of competing analytes.84

A very recent study also highlights the possibility to con-struct a MOF sensor setup that exhibited a rapid and linear

Fig. 11 A schematic representation of framework modularity for An integration utilizing An- and Zr-MOFs as precursors. The integrated An-containingspecies are shown in red.25 Reproduced from ref. 25 with permission from the American Chemical Society, copyright 2017.

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).

Fig. 13 A schematic representation of iodine loading inside ZIF-8 showinginteractions of iodine molecules with imidazole linkers. The orange, blue,purple, and grey colors indicate Cu, N, I, and C, respectively. Hydrogenatoms were omitted for clarity.79

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response to the concentration of I2 (Fig. 14).77 The drasticincrease in conductivity (B7 orders of magnitude) was achievedthrough the formation of an I"! ! !I2! ! !I" arrangement between{Cu4I4}n SBUs of the framework.

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

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

Another approach included the visualization of iodine bind-ing inside a framework through single-crystal X-ray diffractionwas shown by Murugesu and co-workers.87 In their work, aZn-based MOF constructed from zinc nodes and a 2,4,6-tris(4-pyridyl)-1,3,5-triazine ligand was utilized to study the iodineadsorption process by X-ray diffraction, which can identifypreferred binding motifs throughout the uptake process. Thesestudies demonstrated that MOFs can integrate the combinationof chemisorption (binding to open metal sites or functionalgroups) and physisorption (guest uptake inside pores) resultingin enhancement of iodine capture.

Very recently, Li and co-workers proposed a novel approachtowards the optimization of the capture of radioactive organiciodides (ROIs, such as methyl or ethyl iodides).88,89 Through apostsynthetic modification of MIL-101-Cr with different tertiaryamines (e.g., triethylenediamine, hexamethylenetetramine, orN,N-dimethyethylenediamine) grafted to binding sites within aframework, a record-high value for CH3I capture (71 wt%) wasachieved.89 It was also found that the CH3I adsorption followsboth chemisorption and physisorption mechanisms, wherechemisorption occurs at the amine functionalized sites, andthe physisorption depends on the porosity of framework.Loiseau and co-workers also studied the influence of the MOFtopology towards adsorption of gaseous CH3I.74 Since adsorp-tion involves weak van der Waals type interactions, the bestadsorption capacity was found in MOFs with similar porediameter to the size of CH3I. This observation was confirmedby theoretical studies performed on several MOFs with a widerange of pore volumes.

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

Technetium capture

In addition to iodine, another major long-lived fission productis 99Tc, usually present in nuclear waste in the form of anionicpertechnetate (TcO4

").27 The pertechnetate ion has an extremelyhigh mobility and noncomplexing nature, meanwhile technetiumis volatile and can leach through glass, interfering with the nuclearwaste vitrification process. Nowadays, ion exchange resins, mole-cular and supramolecular anionic receptors are designed toremove TcO4

" from the waste stream, but these materials possesslow loading capacities as well as slow uptake kinetics. Ionic MOFs,specifically cationic ones, were shown as materials which canpotentially capture anionic radioactive pollutants.27 Up untilnow, cationic MOFs have been tested for sequestration ofReO4

",90 CrO42",90–93 and MnO4

",90,93 as surrogates for pertech-netate ion.69,93

Oliver and co-workers described a new methodology tocapture oxo-anion pollutants (ReO4

", CrO42", and MnO4

") by acationic framework.90 During the anion uptake by the material,its structure changed by replacing anions present in the struc-ture (1,2-ethanedisulfonate) by anions of interest. This approachresulted in a permanent trapping of anions, which is crucial forradioactive waste treatment. A similar approach was shownrecently for selective immobilization of ReO4

"/TcO4" inside the

framework lattice even in the presence of an excess of nitrateions (Fig. 15).94 Furthermore, ReO4

" uptake was performedthrough a single-crystal-to-single-crystal transformation, whereeach ReO4

" was bound to unsaturated silver sites, foreshadow-ing a potential way for immobilization of TcO4

" into futurewasteforms.

Thallapally and co-workers showed that the amino-functionalizedMOF, UiO-66-NH2, can be used for efficient extraction of ReO4

" fromwater,95 while the Gosh research group probed the anion exchangeapproach for rapid removal of MnO4

" and Cr2O72", as model anions

for TcO4".93

Fig. 14 A schematic representation of iodine loading inside ZIF-8 showinginteractions of iodine molecules with imidazole linkers. The orange, blue,purple, and grey colors indicate Cu, N, I, and C, respectively. Hydrogenatoms were omitted for clarity.77

Fig. 15 Crystal structure of Ag-based framework with ReO4" bound to

unsaturated silver sites. The purple, grey, blue, red, and green colors indicateAg, C, N, O, and Re, respectively. Hydrogen atoms omitted for clarity.94

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Very recently, the first example of a cationic Ag-MOF (SCU-100,SCU = Soochow University) was directly tested with radioactiveTcO4

" (Fig. 16).96,97 Based on preliminary studies using thesurrogate ReO4

", SCU-100 was shown to selectively capture TcO4"

in the presence of competing anions.97 As a further study, a novelNi-based framework was prepared.96 This material could selec-tively remove TcO4

" from an aqueous solution even with a lowconcentration of pertechnetate. Incorporation of TcO4

" inside thechannels of the framework did not influence the MOF crystallinity,and the TcO4

" position was determined by single-crystal X-raydiffraction. Thus, MOFs provide an effective way to remove TcO4

"

from streams prior to the vitrification process, however furtherdevelopment of cationic frameworks is necessary to enhanceframework capacity as well as selectivity.

Element separation

Effective conversion of radioactive mixtures and the furtherseparation of elements from the fission process (a waste streamcan consist of up to 40 elements)98 is another problem asso-ciated with nuclear waste generation. For instance, separationof thorium from rare-earth elements is a challenging task dueto their chemical similarities including small differences intheir solubility and oxidation states. The shift from traditionalsolvent extraction is necessary due to its disadvantages demon-strated beyond the laboratory scale such as generation of a largeamount of waste, complex multistep procedures, as well asutilization of expensive mixer-settlers or centrifugal contactors.98

Based on these considerations, it has become imperative todevelop a high-performance porous adsorbent, such as MOFs,with selective binding sites. For instance, derivatization of UiO-66MOFs with carboxylic groups led to formation of a series ofhighly porous and highly stable MOFs (e.g., UiO-66-COOH andUiO-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, derivati-zation of MOFs with –COOH groups resulted in enhanced Th4+

uptake of 236 mg per gram of the framework in comparisonwith non-functionalized UiO-66 (17 mg per gram). However, theremaining challenge in this case is associated with MOFintegrity since the structures of these frameworks were partiallycollapsed during MOF reuse.

Recently, Long and co-workers demonstrated that porousaromatic frameworks (BPP-7), prepared through polymerization of1-nonyl terephthalate ester and functionalized with –COOHgroups, could be applied towards Ln/An separation.98 For instance,preferential binding of neodymium, even at a low concentrationversus iron was shown. Such selectivity was mainly attributed to asize exclusion effect of the cavity engaged in binding.98

Sun and co-workers reported a selective Th4+/Ln3+ separationthrough utilization of a Zn-MOF possessing unsaturated metalsites located on the N,N0-bis(salicylidene) ethylenediaminelinker, which were created through the postsynthetic removalof Mn(III) ions.100 The framework with the ‘‘demetalated’’ligand was applied to a solution consisting of Eu3+, Lu3+, andTh4+. As a result, adsorption capacity for Th4+ (46.3 mg of Th pergram) was higher in comparison with competing cations. Thus,this work demonstrated a potential towards a more rationaltreatment of fission products.

ConclusionAs highlighted in this review, it is crucial to develop newactinide-containing architectures in order to facilitate currentefforts in nuclear waste administration. Although understandingthe properties of actinides has a great fundamental importance, itsdevelopment still falls behind most of the other elements. More-over, MOFs can be seen as a remarkably powerful tool to addressthe fundamental questions dealing with the chemical behavior ofAn-based structures, including uranium and thorium. The under-standing of An-MOF structural aspects could be based on analysisof patterns observed in actinide-containing minerals or organiccomplexes, since MOFs could be considered as a bridge betweensolid-state and solution chemistry of actinides. This idea hasbecome feasible through recent advancements in the fields of boththe structural chemistry of An-based materials as well as MOFs.

MOFs not only offer the benefit to study structural trends inAn-containing extended structures, but also offer the potential toserve as efficient adsorbents for radionuclides. For instance, thesequestration of fission products, such as technetium and iodine,which are mobile hazardous species possessing environmentaland human health threats, can be addressed through MOF usage.However, to efficiently utilize MOFs for capture of volatile radio-nuclide species, it would require mechanistic studies of adsorp-tion/desorption kinetics, as well as development of syntheticroutes for modification of pore microenvironment to enhanceradionuclide-binding affinity. Furthermore, despite almost limit-less choices in metal ions and organic linkers, the poor thermo-chemical stability of many MOF-based materials could causesignificant challenges for anion exchange.27

Nevertheless, MOFs can offer size exclusion or selectivebinding within its pores, that may not be realized in more con-ventional materials such as resins. The current reports of TcO4

"

capture using MOFs are still in its rudimentary phases, but thepotential in the recent reports should fuel further studies.

Another aspect which requires immediate attention is a studyof MOF stability towards ionizing radiation.101–105 Even though

Fig. 16 Crystal structure of SCU-100 before (left) and after (right) incorpora-tion 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

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frameworks were shown to maintain their integrity in radiationenvironments, important aspects, which should be furtherelucidated, include behavior of the organic building blocksunder radiation, effect of different metals on stability, as wellas the behavior of actinide-containing guests inside the pores ofthe radionuclide-containing framework. While there have beenonly a few reports in this direction, these studies demonstratedthe great potential of metal–organic materials in comparisonwith their organic analogs (e.g., stilbene- or anthracene-containingcompounds).102–104

The nature of metals utilized for MOF synthesis is also a keyparameter 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 towardsgamma irradiation in comparison with similar systems madefrom 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 asan antenna for radiation absorption, which was further releasedas ligand emission.104

The incorporation of radiation responsive structural units(e.g., scintillating or photochromic organic linkers)102,104,106 couldexpand the opportunity of porous MOFs in the field of actinidedetection as novel radiation dosimeters. A novel radiation sensorwith enhanced stability and efficiency could be designed throughincorporation of known scintillation materials as organic linkersinside the framework.102,104 Moreover, MOFs not only offerincreased radiation stability in comparison to pure organic com-ponents, but they also allow for the enhancement of photolumi-nescence and radioluminescence lifetimes through control overchromophore environment, leading to lower detection limits.102

Furthermore, integration of photochromic components orcounterparts, capable of switching upon X-ray irradiation,could lead to the development of turn-on sensors allowingdetection to be performed by the naked eye.101,106

As a method of action, it would be ideal to synergisticallyimprove synthetic efforts alongside computational modelingfor the rational construction of high-performance MOF-basedadsorbents. Another aspect, which should be considered torealize the full potential of MOFs as porous adsorbents, is theirprocessability (e.g., pellets vs. powders or crystals).

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

Conflicts of interestThere are no conflicts to declare.

AcknowledgementsThis research was supported as part of the Center for Hier-archical Wasteform Materials (CHWM), an Energy FrontierResearch Center funded by the U.S. Department of Energy,

Office of Science under Award DE-SC0016574. N. B. S. acknowl-edges the support from the Sloan Research Fellowship providedby Alfred P. Sloan Foundation and the Cottrell Scholar Awardfrom the Research Corporation for Science Advancement.

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