Maëva Leloire

Philippe Nerisson

Frédérique Pourpoint

Marielle Huve

Jean- François Paul

Laurent Cantrel

Thierry Loiseau

Christophe Volkringer

Capture and immobilization of gaseous ruthenium tetroxide RuO 4 in UiO-66-NH 2 Metal-Organic Framework

Keywords: RuO4 capture, Metal-Organic Frameworks, radionuclides mitigation

SYNOPSIS. UiO-66-NH2 is a very efficient porous materials for the capture and the immobilization of gaseous RuO4, a highly toxic molecule produced during a nuclear accident.

INTRODUCTION.

Ruthenium is an important fission product generated in a power plant nuclear reactor containing uranium oxide or mixed plutonium-uranium oxides as fuel. 1 Whereas this element remains as stable metallic aggregate within the nuclear fuel (>100 kg for a 900 MWe reactor), an exposition to oxidizing atmosphere gives rise to the production of multiple stoichiometries of ruthenium oxides (RuOn), which are susceptible to be much more volatile than its metallic form. 2 Among them, ruthenium tetroxide RuO4 has paid a special attention for the radioprotection context. Indeed, in addition to the chemical toxicity of ruthenium, the species RuO4 is the only ruthenium oxide occurring at the gaseous state under standard conditions, 2 and is able to disseminate radiotoxic isotopes, mainly 103 Ru (T1/2 = 369 days) and 106 Ru (T1/2 = 39.3 days).

Among the nuclear accidents that have occurred in the last 40 years, the events of Chernobyl (Ukraine) in 1986 (>200 TBq of Ru) and the undeclared major nuclear release in 2017 from Mayak (Russia) (from 100 to 300 TBq) have produced the highest amounts of radioactive Ru, that were disseminated in the atmosphere and contaminating numerous countries. Recent studies confirmed that volatile and highly reactive RuO4 was the origin of the release of the radioactive pollution. [2][START_REF] Masson | Proc. Nati. Acad. Sci[END_REF][START_REF] Cooke | Proc. Nati. Acad. Sci[END_REF] While specific porous adsorbent media, like activated charcoals or silver exchanged zeolites, have been used for decades for the capture of radiotoxic gaseous iodine ( 129/131 I), [START_REF] Riley | [END_REF] the development of specific process remains much less explored for the immobilization or the separation of gaseous RuO4. Typically, in the spent fuel reprocessing, gaseous RuO4 precipitates on cold surfaces as a black nonvolatile RuO2 deposit, which makes difficult the separation from the other radioactive wastes. 6 In the case of a nuclear accident, RuO4 can be trapped in by pool scrubbing in alkaline solution and solubilized into ruthenates (RuO4 2-) and perruthenates (RuO4 -). 7 Moreover, sand beds and metallic filters usually mounted on Filtered Containment Venting System (FCVS) equipping nuclear power plants, 8 are not efficient enough for the RuO4 trapping for operating conditions up to 100°C. 7 However, above 120°C, this filtration setup shows a different behavior, involving the precipitation of gaseous RuO4 into solid RuO2 at the surface of the sand bed and the metallic filters.

The same temperature dependence and behavior were also observed with zeolites. [9][10][11] In the recent research for innovative trapping compounds, Metal-Organic Frameworks (MOFs) are a class of hybrid crystalline porous solids synthesized from a variety of organic ligands and metallic cations (Zn 2+ , Al 3+ , Zr 4+ , etc). 12 Thanks to their very high porosity and specific surfaces (up to 7000 m 2 /g), these materials found many applications in several fields like gas sorption, catalysis or drug release. 13 So far, one notices already one industrial utilization of MOFs for the storage and safe delivery of hazardous dopant gases (arsine, phosphine, boron trifluoride), operated in electronics manufacturing. [START_REF] Tom | 21st International Conference on Ion Implantation Technology (Iit)[END_REF] In the nuclear domain, MOF compounds have shown their efficiency for the capture and the immobilization of various radionuclides, either as solubilized cations ( 79 Se, 90 Sr, 99 Tc, 137 Cs, 232 Th, 238 U) [START_REF] Xiao | [END_REF]16 or as gaseous species ( 131 I, 222 Rn). 17,18 For these latter, MOF solids usually exhibit comparable or even better sorption properties than materials typically used in the nuclear industry, such as silver doped zeolites or activated charcoals for molecular iodine, for instance. Among the different radioactive species, pertechnetate anion TcO4 -exhibits a tetrahedral coordination environment very similar to RuO4. Several cationic MOFs have already shown excellent decontamination factor for this species, even in the presence of competitive anions. [19][20][21] These good porosity performances are not altered under the accidental context, since some MOFs show great resistance under drastic conditions encountered during a nuclear accident, involving radiations, 22 relative high temperature and steam. 23 This is the case of the zirconium aminoterephtalate MOF called UiO-66-NH2, which is stable in severe nuclear accidental conditions and without the release of trapped radioactive iodine 131 I. 18 This good affinity for molecular iodine was assigned to the presence of amino groups attached to the hybrid framework and enhancing stable charge transfer complex with iodine. 24 Whereas DFT calculations indicate that porous MOF compounds could be good candidates for the trapping of gaseous RuO4, 25 no investigation has been experimentally carried out to evaluate their efficiency (as well as other porous materials) for the filtration of this particular gas. This lack of knowledge can be attributed to the difficulty to generate a stable flow without the decomposition of RuO4, as well as to quantify and characterize ruthenium species trapped within the porous matrix.

In this study, we report for the first time the capture and the immobilization of gaseous RuO4 within a porous Metal-Organic Framework. Our choice orientated to the Zr-based MOF UiO-66-NH2, 26 due to its great stability under severe conditions as mentioned above, 23 a very good affinity for iodine, its straightforward synthesis, but also its structural organization, that we suppose well adapted for the capture and the confinement of RuO4. Indeed, UiO-66-NH2 (Figure 1) is built from the assembly of Zr-centered oxo/hydroxo hexanuclear clusters with aminoterephtalate ligands, giving rise to two types of cavities (octahedral or tetrahedral shape), which are accessible through windows slightly larger (5 Å) than the estimated diameter of RuO4 (4.5 Å). As already proved with gaseous iodine (∼3.3 Å) trapped within ZIF-8 (pore aperture of 3.4 Å), 27,28 this structural feature should allow the diffusion of the gaseous species (in our case RuO4) within the pores while limiting its releasing. Furthermore, the presence of amino functional group within the pore could improve the immobilization of RuO4 within the pores through hydrogen bonding. 25 For this work, we used an experimental setup devoted to RuO4 dynamic filtration test at 50°C.

After the filtration step, the loaded UiO-66-NH2 sample will be deeply characterized by means of numerous techniques of characterization (X-ray diffraction, solid state NMR, electronic microscopy, gas sorption, etc.), in order to determine the quantity and the behavior of ruthenium within the pores.

MATERIALS AND METHODS.

Materials and Chemicals.

Zirconium chloride (ZrCl4, 99.5%, from Alfa Aesar), 2-aminoterephtalic acid (H2BDC-NH2, 99.0%, from Alfa Aesar), formic acid (HCOOH, 99.9%, from Acros Organics), dimethylformamide (DMF, 98%, from Fisher Scientific), methanol (98%, VWR), RuO2•xH2O (Acros Organics 54%w Ru).

Synthesis and characterization of UiO-66-NH2.

UiO-66-NH2 was synthesized in a 1L Schott DURAN ® pressure bottle under. 4 g of ZrCl4 (17.16 mmol) and 6 g of H2BDC-NH2 were added to a mixture of DMF (650 ml) and formic acid (32 ml).

The solution was heated under static conditions at 120°C during 24h, then cooled to room temperature. The resulting solid was collected by centrifugation and redispersed in 1L of fresh DMF during 24h (3 times) and then in fresh methanol during 24h (3 times). After a centrifugation step, the solid was dried at 100°C during 1 h and then at 150°C during 8 h. Before RuO4 filtration, the solid was placed in a closed vial and stored in a O2/H2O free glove box (Ar).

Pristine and RuO4 impregnated UiO-66-NH2 were thoroughly characterized by means of several techniques. The crystallinity of the solids was characterized at room temperature by powder x-ray diffraction (PXRD, D8 advance A25 Bruker apparatus equipped with Bragg-Brentano geometry), ranging from 3 to 50° (2𝜃𝜃), a step length of 0.02° (2𝜃𝜃) and a counting time of 0.5 sec/step. The specific surface area was determined by nitrogen adsorption at 77 K using a Micromeritics ASAP 2020 apparatus. Specific surface area was calculated in the p/p0 range: 0.015-0.3 using the Brunauer-Emmett-Teller (BET) method. Before the sorption measurements, the samples were outgassed at 150°C under secondary vacuum during 15 h. IR spectra were measured on a Perkin-Elmer Spectrum Two spectrometer, equipped with a diamond attenuated total reflectance (ATR) accessory between 4000 and 400 cm -1 .

The thermal stability of pristine and RuO4 loaded UiO-66-NH2 was analyzed by themogravimetric analysis (TGA 92 Setaram, under air, 5°C/min) and thermodiffraction. X-ray thermodiffractometry was performed under 5 L.h -1 air flow in an Anton Parr HTK1200N of a D8 Advance Bruker Diffractometer (CuK𝛼𝛼) equipped with a Vantec 1 linear position sensitive detector. Each powder pattern was recorded in the range 5-60° (2𝜃𝜃) (at intervals of 20 to 800 °C), with a 0.5 s/step scan, corresponding to a duration of approximatively 30 min. The temperature ramps between two patterns were 0.08°C -1 to 800°C. TEM measurements were conducted on the TEM FEI TITAN Themis 300 (300 kV). It is equipped with a probe corrector for resolution of 0.7 Å in STEM mode, a Super-X quad EDS detector for elemental analysis. The powder was crushed and dropped in the form of alcohol suspension on carbon supported copper grids, followed by evaporation under ambient condition.

Solid state NMR experiments were carried out on a Bruker Advance III 9.4 T spectrometer (400 MHz and 100.6 MHz for 1 H and 13 C respectively) equipped with a 4 mm double resonance probe spinning at a frequency of νR=12.5 kHz. 13 C spectra was recorded using a cross polarization experiment (CPMAS). The first π/2 1 H rf-field (radio-frequency) amplitude was 58 kHz. During the contact, a 13 C rf-field amplitude of 70 kHz and a linear ramp from 29 to 58 kHz on the 1 H channel were used. The contact time was tcp = 1500 ms. A recycling delay of 1 s and a number of scans of 2048 and 51200 for the UiO-66-NH2 and the RuO4 loaded UiO-66-NH2 were used, respectively. A SPINAL-64 decoupling sequence of 58 kHz was used during the acquisition. 29 The 13 C chemical shifts were referenced using the CH group of adamantane at 29.47 ppm. by a colored line, red for RuO4(g) generation (1), green for filtration in UiO-66-NH2 (2) and blue for quantification (3).

The experimental bench used in this study (Figure 2) is partitioned into three modules, corresponding respectively to the generation (1), the filtration (2) and the trapping of RuO4 for quantification (3).

The generation of gaseous RuO4(g) ( 1) is carried out in a glass column (height: 10 cm; diameter: 2 cm), containing commercial black powder of RuO2⋅xH2O (≈ 550 mg, Merck manufacturer data 54% wt of Ru), and delimited by two sintered glass filters at each end of the column. RuO4(g) is obtained by the ozonation RuO2 according to equation 1.

RuO2 (s) + 2/3 O3 (g) → RuO4 (g) (1)
Ozone is produced by a specific generator (Anseros brand ozonator) from oxygen. With an O2 flow rate equal to 0.06 nL.min -1 , the ozone concentration is stabilized at [O3] = 75 g.m -3 , at room temperature.

The resulting orange gaseous stream contains a mixture of RuO4(g), oxygen and ozone, is directed to the filtration device heated at 50°C. In this module corresponding to a glass rod (length: 10 cm, diameter: 0.5 cm), powdered UiO-66-NH2 is placed on a piece of rock wool as an inert support.

95 mg of dried UiO-66-NH2 were used for this study. Upstream the filtration step, the addition of an inert carrier gas (Ar) allows to adjust and stabilize the stream velocity around 10 cm.s -1 , which corresponds to the expected maximum velocity in a filtration device, in nuclear reactor accidental situation.

After the filtration step, the resulting flow is directed to the three successive bubblers (Figure 3) containing sodium hydroxide aqueous solution (NaOH, 0.05 M), for the trapping of RuO4(g) not having be adsorbed by UiO-66-NH2. In the basic solution, RuO4 is dissolved into perruthenate (RuO4 -) and ruthenate (RuO4 2-) species according to the equations ( 2) and (3):

2 RuO4 + 2 NaOH → 2 Na + + 2 RuO4 -+ ½ O2 + H2O (2) 2 RuO4 + 4 NaOH → 4 Na + + 2RuO4 2-+ O2 + 2 H2O (3)
The first bubbler is equipped with a double jacket allowing it to be cooled by water at 5°C for an efficient trapping of RuO4(g). 30 The following bubblers are at room temperature and without double jacket and ensure a full and safe trap of any residual RuO4(g) by bubbling, before discharge to the fume hood. The solubilization of RuO4(g) and its chemical trapping is confirmed by the color change of the solution, from colorless to brown-yellow (Figure 3a).

Samples (5 mL) of solution from each bubbler are taken during the experiment and then analyzed by ICP-AES (Perkins) to determine decontamination factor and the quantity of ruthenium trapped by the UiO-66-NH2 compound. By this method, we confirmed the linear production of RuO4(g) (Figure 3b) during the experimental time used for the filtration test (135 min) and the calculated average production of RuO4 generated by the setup is approximately 45 mg of RuO4 per hour.

In this setup, these concentrations are not easily accessible simultaneously during a single test.

Therefore, a multi-steps procedure was carried out to calculate accurately the DF.

The first step involves a blank test, under the desired experimental conditions and without the filtration element (i.e. without MOF) within the glass column. The second step corresponds to the "real" test, under strictly identical conditions to the blank test, but including the powdered MOF within the glass column. Therefore, the decontamination factor of RuO4(g) is calculated using equation 5 (measurements downstream the glass column).

DF (RuO4(g)) = [RuO4(g)]blank / [RuO4(g)]real (5)
The total gas flow rate being the same during blank and "real" test, DF (RuO4(g)) at a given time can also be calculated as the ratio of total amounts of RuO4 cumulated at this time, for these two tests.

Theoretical calculations.

The evaluation of interaction energies between RuO4 and the UiO-66 has been performed at the Density Functional Theory (DFT) level, using the Vienna Ab-initio Simulation Package (VASP). 31,32 The electron-ion interactions are described using the Projector Augmented Wave (PAW) method of Blöchl 33 , adapted by Kresse and Joubert. 34 The calculations were done at the Γpoint only due to the large size of the unit cells. The gradient corrected exchange correlation functional of Perdew, Burke and Ernzerhof (PBE) was employed. 35 The DFT-D3 correction method of Grimme and co-workers 36 was used in the present work, as implemented in VASP by Moellmann et Grimme, 37,38 to estimate efficiently the dispersion forces. The wavefunction has been expanded in a plane wave basis set using a cutoff energy of 550 eV. The convergence parameters were set to 10 -6 eV for the total energy and to 0.02 eV/Å for the residual forces on atoms. To improve electronic convergence, a Gaussian smearing with σ = 0.2 eV has been used.

The frequencies have been determined numerically by diagonalization if the hessian matrix that is computed by a double displacement of the atoms in the three x, y, z, directions (∆x = ±0.015 Å).

We have used our recently optimized cell 25 with the following parameters: α = β = γ = 90° and a = b = c = 20.9 Å to model the unit cell of UiO-66.

RESULTS.

UiO-66-NH2 characterization.

The production of UiO-66-NH2 has a yield of 60% based on the inorganic reactant ZrCl4. The yellowish powder is well crystallized and does not contain traces of impurity based on PXRD analysis. 1 H NMR performed on the digested solid (Figure S1) allows to quantify the amount of BDC-NH2 and formate ligands constituting the porous framework, and gives the followed formula for the dried solid Zr6O4(OH)4(BDC-NH2)4,55(Form)2.90 (BDC-NH2 = 2-aminoterephtalate; Form = formate). The occurrence of formate ligands within the structure indicates that some aminoterephtalate linkers are substituted by formates. This difference compared to the ideal calculated structure Zr6O4(OH)4(BDC-NH2)6 is a common feature in this family of MOFs and usually leads to a better pore accessibility compared to the no-defect structure. 39 The solid synthesized in this study exhibits a specific surface area (BET model) of 825 m 2 .g -1 and a pore volume 0.31 m 3 .g -1 . These values (>800 m 2 /g) are in agreement with those usually reported in the literature. 40 Filtration test and calculation of the decontamination factor.

When UiO-66NH2 is placed in the filtration column under RuO4(g) flow, the breakthrough is observed between 75 and 105 min and characterized by the color change of the first bubbler, turning from colorless to yellow. The value of the DF is 569 (± 97) after 30 min (Table 1), and the maximal value is measured before breakthrough after 75 min. At this time, DF is equal to 5745 (± 977). Experiments realized on the same experimental setup and under the same conditions with sand, silver-doped zeolites and activated charcoal have shown a very low decontamination below 4. 7,41 So, UiO-66-NH2 is a much more promising material for the filtration of RuO4. After 135 min, the generation of RuO4 is stopped. At the end of the experiment, the saturation of the filter media after 135 min is also highlighted by the black coloration of UiO-66-NH2, initially pale yellow (Figure 4a).

Characterization of the loaded-ruthenium MOF: Ru@UiO-66NH2.

After the filtration study (135 min), the ruthenium impregnated MOF noted Ru@UiO-66-NH2

was characterized by means of several techniques, to analyze the crystallinity of the composite material as well as the ruthenium chemistry within the pores. PXRD analysis (Figure 4b) measured at room temperature confirms the structural integrity of the UiO-66-NH2 matrix and does not indicate the presence of additional crystallized phases in the sample like ruthenium oxides (RuOx). Furthermore, the typical octahedral-shaped of UiO-66-NH2 crystal was not affected by RuO4(g) flow, confirming the excellent stability of the selected MOF under the experimental conditions. Whereas ruthenium oxide nanoparticles are used to precipitate easily at the surface of MOFs or other solids, 42 high resolution high angle annular dark field imaging does not show any traces of such particles or aggregates on the surface of the crystal.

RuO2 is the unique decomposition product from RuO4, with a characteristic black color. Since it is the only ruthenium oxide stable and non-gaseous/volatile at room temperature, we argue that RuO2 is the species trapped within the pore of UiO-66-NH2. Furthermore, the conditions used in this study (relative low temperature and presence of water from RuO4 precursor) and the precipitation of amorphous RuO2, fit very well with the syntheses and the structural properties of ruthenium dioxide hydrates RuO2.xH2O. 43 This claim will be confirmed later in the manuscript by transmission electron microscopy (no liberation of volatile species under vacuum) or NMR (large amount of water, presence of paramagnetic tetravalent ruthenium, instead of octavalent ruthenium in initial RuO4 gas).

The homogeneous insertion of RuO2 within the UiO-66-NH2 crystallites is confirmed by Scanning Transmission Electron Microscopy-Energy Dispersive X-Xray spectroscopy mapping (STEM-EDS), since ruthenium and zirconium signals are overlapped (Figure 5). The deduced average composition gives an averaged molar ratio Ru/Zr close to 2 (estimated out of hundred UiO-66-NH2 crystallites), in accordance with the following formula Zr6O4(OH)4(BDC-NH2)4,55(Form)2.90•(RuO2)12.

Based on this formula, the trapping capacity of RuO2 into UiO-66-NH2 (Ru uptake) can be estimated at 0.985 g(RuO2)/g(UiO-66-NH2) or 0.748 g(Ru)/g(UiO-66-NH2). This value is slightly overestimated comparing to the estimation of Ru production during the test, based on ICP-AES measurements: 0.833 g(RuO2)/g(UiO-66-NH2) or 0.633 g(Ru)/g(UiO-66-NH2). This gap can be explained by a slightly higher RuO2 concentration at the top of the UiO-66-NH2 bed, where the TEM analyzed sample would come from. TEM images of Ru-loaded UiO-66-NH2 exhibit crystallized nanodomains characterized by fringes (2.1 and 2.95 Å), which are also observed in the as-made MOF and that do not fit with crystallized UiO-66-NH2. Therefore, these domains cannot be attributed to RuO2 which should occur within the framework as an amorphous ruthenium dioxide hydrate, 43 but correspond to interplanar spacing of ZrO2 fluorite structure (d111 = 2.956 Å and d112 = 2.09 Å). 44 The crystallization of zirconia is related to the local decomposition of the hybrid framework under the electron beam.

The structural integrity of Ru@UiO-66-NH2 as well as the occurrence of paramagnetic Ru IV from hydrated RuO2 was confirmed by solid state NMR. Whereas 1 H spectrum is not exploitable due to a very large amount of trapped water molecules associated to RuO2.xH2O or trapped within the pores hiding the structural bands (Figure S2), 13 C analysis is not affected by this high hydration rate. The 13 C CPMAS NMR spectrum of the pristine UiO-66-NH2 is characterized by six resonances associated to BDC-NH2 linker and one to the formate ligand (Figure 6). 45 The signal at 171 ppm is assigned to the two carboxylates functions C7 and C8 which are connected to the alpha carbons C6 and C3, resonating at 123 and 138 ppm, respectively. The C-H groups C2, C4 and C5 are characterized by the resonances at 117, 123 and 131 ppm, respectively.

The last signal associated to the ligand corresponds to the carbon atom (C1) linked to the amino groups and resonates at 150 ppm. The presence of formate ligand (Cform) signal is emphasized by the signal at 160 ppm. The fact that some signals associated to benzene rings are unchanged whereas others are removed confirms the stability of the structure, but indicates the proximity of a paramagnetic species (e.g. RuO2) alongside specific carbon atoms (C4 and C5). This close interaction with paramagnetic RuO2 is known to induce significant shift (several hundred ppm) for the nearest atoms. 46 The same explanation can be used to explain the disappearance of the peak associated to formate ligand (Cform). However, the departure of this single carbon-based molecule and its oxidation into CO2 cannot be ruled out, based on NMR results. This mechanism is analogous to that of the decomposition of formic acid activated by ruthenium-catalysts (RuO2 or RuCl3). [47][48][49] Thermal behavior of ruthenium-loaded UiO-66-NH2

To confirm the presence of stable RuO2 within the framework, instead of a volatile species such as RuO4, we analyzed the thermal resistance of Ru@UiO-66-NH2 solid. Whereas pristine UiO-66-NH2 is stable up to 260°C under air (Figure 7a), the introduction of ruthenium oxide within the solid decreases the thermal stability down to 220°C (Figure 7b). In this last case, the thermal degradation leads to the crystallization of ZrO2 (pdf file : 04-013-3441) and RuO2 (PDF file 04-009-8496), separately. The different behavior is even more obvious by thermal gravimetric analysis (TGA) (Figure 7c). Pure UiO-66-NH2 shows three distinctive weight losses, successively assigned to the departure of trapped solvent (from RT to 100°C), the thermal decomposition of the organic ligand (from 250°C to 400°C) and the dehydroxylation process (from 500°C to 600°C), respectively. Only one step is observed for the Ru@UiO-66-NH2 sample, occurring from RT to 250°C. This single event is ascribed to the departure of organic ligand as well as water molecules (from solvent trapped into the pores, MOF dihydroxylation and/or hydrated RuO2) and leads to a final plateau corresponding to a mixture of crystallized ZrO2 and RuO2 (Figure S3). Owing to an unclear weight loss distinction between the different constituents of the Ru@UiO-66-NH2, no attempt for calculation has been made.

The quicker decomposition of the loaded UiO-66-NH2 is associated to the presence of RuO2 which acts as an oxidative catalyst for the decomposition of the hybrid compound. This behavior is very similar to that of organic compounds transformation into gases (CO2, CH4, H2), catalyzed by RuO2 at high temperature in supercritical water. 50

RuO4 behavior after trapping within UiO-66-NH2

The different methods of characterization used for this study indicate that ruthenium is trapped as stable form within UiO-66-NH2. Since RuO4 is a highly volatile species, which could decompose into nonvolatile RuO2, 51 we suppose that this last oxide is predominant inside the UiO-66-NH2 pores. Whereas other stable ruthenium oxides could exist (RuO, RuO3), these species are only stable as gaseous form at very high temperature (>1000°C), 52 incompatible with the classic MOF stability. 2 As indicated by NMR spectroscopy, the introduction of RuO4 within the pores leads to the disappearance of 13 C NMR signals (C4 and C5) of aminoterephtalate ligands. This change is associated to the presence of paramagnetic Ru(IV) oxide close to the accessible carbons atoms around the benzene ring.

The proximity between aminoterephtalate ligand and RuO4 is easily explained by a classical 3+2 Diels-Alder reaction (5) (Figure 8). This type of reaction has been studied for OsO4 eV). The most suitable position of the proton is on the carboxylic group, since the catechol configuration is less stable (0.42 eV). The RuO2 formed during the reaction, is coordinated to the two OH groups which enhances their acidity and favors the displacement of the protons on the carboxylic acid. This mechanism is in perfect agreement with the solid-state NMR characterization, assuming the proximity of C4 and C5 with ruthenium atom.

To confirm the formation of the protonated carboxylate function, we computed the vibrational frequencies of the linker before and after oxidation. In our model, the wave numbers of C=O As expected, a blue shift is observed on the experimental infrared spectrum of Ru@UiO-66-NH2 by a new band visible at 1868 cm -1 , involving a shift of 288 cm -1 (Figure S4). The difference between the experimental and the calculated values is assigned to the presence of RuO2 aggregates within the pores (and not isolated RuO2 species as calculated), that could modify structural vibrations.

CONCLUSION.

For the first time, a Metal-Organic Framework (UiO-66-NH2) compound was used for the capture of gaseous ruthenium tetroxide RuO4, which may exhibit radiotoxic isotopes in case of power plant nuclear accident. Since gaseous RuO4 is not commercially available, we developed a specific installation to produce this gas as well as the quantification of RuO4 trapped within the filtering medium. UiO-66-NH2 has a remarkable affinity for RuO4 capture, since this MOF exhibits the worldwide highest RuO4 decontamination factor (DF up to 5745), hundreds of times higher than the DF values of sorbents daily used by the nuclear industry (zeolites or activated charcoal).

The efficiency of UiO-66-NH2 is explained by its pores diameter well adapted to the capture and the immobilization of RuO4, as well as its conversion into stable hydrated ruthenium dioxide (RuO2.xH2O) within the pores. This conversion corresponds to the reactivity of RuO4 with the MOF organic sub-network, leading to the decomposition of formate ligands as well as the oxidation of terephthalate ligands. As proved by powder X-ray diffraction or NMR, these modifications do not decompose the MOF structure. However, the presence of catalytic RuO2 within the pores slightly accelerates the thermal decomposition of this solid, when increasing temperature from 220°C (instead of 260°C for the pristine UiO-66-NH2 phase). As a future work, additional techniques like X-ray Photoelectron Spectrometry (XPS) or X-ray absorption spectroscopies (EXAFS/XANES) would be used to analyze the oxidation state of ruthenium within the MOF.

Whereas UiO-66-NH2 exhibits the best decontamination factor for RuO4, the efficiency of this MOF needs to be confirmed under more relevant accidental conditions, involving high temperature, steam or radiation. Moreover, the resistance of UiO-66-NH2 versus beta emitter like 103/106-Ru needs to be confirmed. A very recent publication about the excellent resistance of Albased MOF303 56 is very promising for the capture of such isotopes.

Figure 1 .

 1 Figure 1. Structure of UiO-66-NH2 and representation of aminoterephtalate ligand. Green and purple

Figure 3 .

 3 Figure 3. Monitoring the dissolution of RuO4(g) in a bubbler in an aqueous solution of NaOH (0.05 M) at

Figure 4 .

 4 Figure 4. Photography of the filtering cell containing pristine UiO-66-NH2 and after filtration of RuO4

Figure 5 .

 5 Figure 5. Ruthenium-loaded UiO-66-NH2: a-c) TEM images (a, b). Enlargement showing crystallized nanodomains with interfringe spacing characteristic of ZrO2. High-angle annular dark field (HAADF) image and corresponding EDS maps of Zr (e), Ru (f) and Ru+Zr (g).

Figure 6 .

 6 Figure 6. 13 C solid-state CPMAS NMR spectra of UiO-66-NH2 (top) and ruthenium-loaded UiO-66-

Figure 7 .

 7 Figure 7. Evolution of the powder X-ray diffraction patterns under air of pristine UiO-66-NH2 (a) and rutheniumloaded UiO-66-NH2 (b) as a function of temperature. Circle, square and star represent diffraction peaks of Al2O3 sample holder, ZrO2 and RuO2, respectively. Thermogravimetric curves (c) of UiO-66-NH2 and ruthenium-loaded UiO-66-NH2 under air atmosphere (5°C/min).

  stretching mode are increased by 160 cm -1 compared to the non-protoned linker. The addition of proton on the carbonyl group increases the C-OH distance by 0.07 Å (1,34 vs. 1,27 Å) and a reduction of the second C=O bond by 0.04 Å (1,23 Å vs. 1,27 Å), which explains the evolution of the wave numbers.

Figure 8 .

 8 Figure 8. Mechanisms of reactivity of RuO4 with aminoterephtalate ligands highlighted by structural

Table 1 .

 1 Decontamination factor (DF) for RuO4 filtered by UiO-66-NH2 at different contact times.

	Time (min)	DF (RuO4(g))
	30	569 ±97
	45	1586 ± 270
	60	3556 ± 605
	75	5745 ± 977
	105	1216 ± 207
	115	216 ± 37
	125	77 ± 13
	135	35 ± 6

  53,54 or RuO455 on simple aromatic systems or double bonds. DFT calculation confirms the good affinity RuO4 and the amines are broken during the process. This transformation leads to the reduction of Ru(VIII) into Ru(IV) and is related to the oxidation of benzene ring into a catecholate group,

	between the ditopic ligand of UiO-66-NH2 and RuO4, through strong hydrogen bonding between
	the porous framework (from NH2 groups connected to neighboring ligands) and the gaseous
	species in intermediate 1.
	The 3+2 Diels-Alder reaction (2) is endothermic (∆rE=0,63 eV) as the hydrogen bond between
	the

connected to RuO2 in intermediate 3. In the following step, protons are displaced from the tetrahedral carbon to restore the aromaticity of the benzene ring (intermediate 4). The displacement of the two protons is exothermic (∆rE=-1.46 eV and (∆rE=-1.43 eV for the first and second displacement, respectively). The oxidation of the aromatic ring is strongly exothermic (∆rE=-2,26

Experimental setup for the production and the filtration of gaseous RuO4. Additional PXRD, infrared and 1 H NMR analysis.

AUTHOR INFORMATION

Corresponding Author

Christophe VOLKRINGER -Univ. Lille, CNRS, Centrale Lille, UMR 8181 -UCCS-U ité de Catalyse et Chimie du Solide, Lille F-59000, France Email : christophe.volkringer@centralelille.fr

Author Contributions

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