Selective Chemical Filters for VOF 3 : Tailoring MgF 2 Filter Selectivity through Surface Chemistry

: In order to synthesize chemical ﬁlters for the selective removal of volatile ﬂuorides, commercial magnesium ﬂuoride MgF 2 with high speciﬁc surface area (HSA) was investigated. The amount of -OH groups substituting ﬂuorine is not negligible, partly due to the high surface area, but also due to the synthesis route. These hydroxyl groups induce a Lewis basicity on the surface of metal ﬂuorides. The amount of these Lewis basic sites has been tailored using ﬂuorination with F 2 gas. The sorption of VOF 3 , used as model gas, onto these ﬂuorides was investigated. The versatility of surface chemistry as a function of a number of Lewis basic sites opens the way to ﬁlter selectivity mixture of volatile ﬂuorides depending on their Lewis acidity. HSA MgF 2 acts as a stable matrix towards the gas to be puriﬁed, and the selectivity may be achieved by a higher Lewis acidity of the gaseous impurity.


Introduction
Volatile fluorides are involved in numerous industrial applications, either as reagents or pollutants.Most of them are Lewis acids and react with a wide variety of compounds; for example, in organic synthesis, gaseous VOF 3 is often used for oxidative coupling of phenolic rings [1].Due to their ability to release atomic fluorine, volatile fluorides can also be used as fluorinating agents.Nanoparticles, thin films or atomic layers of tungsten and molybdenum metal are synthesized using WF 6 or MoF 6 as precursors [2,3]; metal deposition occurs through the reaction of MF 6 into M plus 3 F 2 .A reactant such as Si 2 H 6 is used together with MoF 6 to form Mo layers.Dense films of molybdenum oxide may be deposited by plasma-enhanced chemical vapor deposition using mixtures of MoF 6 , H 2 , and O 2 [4].
When the volatile fluorides are pollutants, however, chemical filters are needed to remove them.The most important application concerns UF 6 , which plays a key role in the nuclear industry.UF 6 , as a volatile uranium compound, allows uranium enrichment to 235 U whatever the process, i.e., gaseous diffusion, centrifugation, or laser excitation.This particular case highlights the necessity for selectivity of the chemical filter.To provide high-purity nuclear fuel, international standards have been established and the quantity of pollutants allowed in nuclear pellets is restricted.Pollutants originate from both uranium ore, the chemical agents used in the conversion process, and/or the fission products of spent uranium.All these elements are fluorinated during the synthesis of UF 6 and their volatility and solubility in UF 6 vary depending on the element [5].When the volatility of the impurity is close to that of UF 6 , its removal is complicated.Selective chemical filters that do not react with UF 6 are thus highly needed.Ca(OH) 2 , Mg(OH) 2 , KOH, either separately or in combination, cannot be used because they react easily with UF 6 [6].
Our strategy involves metal fluorides which contain just a small part of basic -OH groups.Considering the reactivity of the volatile fluorides which must be trapped, the criteria for an effective chemical filter are a high surface area, high porosity to increase the interface with the target gas, chemical stability in order to avoid their decomposition, but also enough Lewis basic sites on the surface to react with volatile fluorides as Lewis acids.To reach sufficient selectivity, the surface chemistry or porosity of the filter must be able to match the chemisorption or physisorption of the gas onto the filter surface.
To fulfil these criteria, our choice was to go towards fluorides with high specific surface area (HSA).Usually, such fluorides are obtained via the sol-gel route [7][8][9][10][11] or by microwave-assisted solvothermal synthesis [12][13][14][15][16][17].HSA metal fluorides are prepared using metal alkoxides in an organic medium or various solvents and metal precursors in an aqueous-HF medium.The synthesis conditions using microwave-assisted solvothermal routes, i.e., the choice of precursors, solvent, HF concentration and reaction temperature, strongly influence the formation of various networks with different chemical compositions.Magnesium difluoride MgF 2 , better known for its catalytic [18,19] and optical properties [20][21][22], has been selected as a chemical filter based on previous data on the removal of impurities such as fluorides of technetium [23], molybdenum [24], ruthenium, neptunium or plutonium [25].
The presence of weak basic sites was evidenced in MgF 2 prepared by sol-gel [26] and weak basic sites may prevent reaction with UF 6 .Basic sites coexist with a large amount of Lewis acid sites, which explains the unique catalytic properties of MgF 2 prepared by sol-gel [8].F − and O 2− are both intrinsically Lewis bases; however, fluorine atoms reduce the basicity of oxygen atoms.Moreover, the basicity of MgF 2 is much lower than that of MgO.We propose to adapt the OH content in order to reach the selectivity, VOF 3 versus UF 6 for instance, but in a general way by using the Lewis acidity differences of the gases in the mixture.Moreover, the aim is to design a chemical filter that may be regenerated; metal fluoride acts as the support and the OH content changes during reaction with the impurity to be removed.

Materials
In order to test the tuning of the OH content, three MgF 2−x (OH) x were used: a commercial MgF 2 received in pellet form (Nippon Puretec, Nagoya, Japan), which is used as either as received or after fluorination treatment, and a locally synthesized oxygen-free MgF 2 .The commercial sample was chosen for its ease of use, in pellet rather than powder form, for all the filtering and regeneration operations as well as for the quantities available for future industrial uses.VOF 3 was synthesized locally and used as is.

Filtering Capacities
All experiments were performed in polytetrafluoroethylene (PTFE) bottles sealed in a nitrogen dry box due to the hygroscopic nature of fluorides.MgF 2−x (OH) x pellets and VOF 3 powders were put in separate nickel baskets.The volatility of VOF 3 at 80 • C allowed the exposure of its gas on MgF 2−x (OH) x samples.After exposure to VOF 3 for 24 h, the MgF 2−x (OH) x pellets were crushed for characterization.The vanadium content on the filter surface was estimated by weight uptake and ICP analysis, both of which gave consistent data.

Fluorination
Pure molecular fluorine (Solvay, 99%+) was used.A chemical trap filled with soda lime scrubbed F 2 molecules from the exhaust in order to avoid their release into the atmosphere.The gas flow was set at 20 mL•min −1 .The treatment was performed at a fluorination temperature T F = 300 • C. The temperature profile of the treatment is a heating ramp of 5 • C • min −1 , with a stabilization of temperature for 4 h.After this, the reactor was flushed with nitrogen in order to remove all reactive gases.2.4.Characterization 2.4.1.X-ray Powder Diffraction X-ray powder diffraction patterns were recorded with a Panalytical X'Pert powder diffractometer in Θ-Θ Bragg Bentano geometry.The samples were transferred to a sealed cell to avoid exposure to moisture, with an aluminum sample holder for some.All patterns were recorded between 5 • and 70 • in 2Θ with a step of 0.015 • and a counting time of 60 min using a back graphite monochromated CuKα radiation (Kα1 = 1.54056Å and Kα2 = 1.54439Å).Profile matching refinements were performed using the FULLPROF software [27].

Adsorption/Desorption Isotherms
The nitrogen adsorption/desorption isotherms were measured using the Micromeritics ASAP 2020 instrument.Prior to each adsorption experiment, the samples were degassed at 473 K under primary vacuum and then under secondary vacuum.Pore volume, specific surface area, and pore size distribution were extracted from the N 2 adsorption/desorption isotherms at 77 K using the BET (Brunauer, Emmett and Teller) and BJH (Barrett, Joyner and Halenda) models for specific surface area (SSA) and pore size distribution for mesoporous materials, respectively.

Raman Spectroscopy
Raman spectra were collected at room temperature using a Bruker RFS 100/S apparatus with a Nd-YAG (aluminum-doped yttrium garnet) laser source at 1064 nm.A total of 500 scans were recorded between 4000 and 25 cm −1 Raman shift.Samples were prepared in a sealed fluorinated ethylene-propylene tube (FEP, La Mothe-aux-Aulnais, Saint Gobain) that resulted in the presence of additional Raman bands (marked on the spectra).For the Raman analyses, the MgF 2 single-crystal (Sigma Aldrich, 99%) was used as the reference for oxygen-free magnesium difluoride.

NMR Spectroscopy
Multinuclear 19 F, 1 H and 51 V NMR measurements were carried out with a Bruker Advance Spectrometer with working frequency of 282.2, 300.0 and 78.8 MHz, respectively.A Magic Angle Spinning (MAS) probe operating with 2.5 mm rotors was used allowing a 30 kHz spinning rate.A sequence with a single π/2 pulse duration of 4.0 µs was used.The 19 F, 1 H and 51 V NMR chemical shifts were externally referenced to CFCl 3 , tetramethylsilane (TMS) and solution of vanadium phosphate (1 M), respectively.

Fluorination to Tailor OH/F Ratio in MgF 2−x (OH) x
In order to test the tuning of the OH content, three MgF 2−x (OH) x were used.All the characterizations highlight the presence of -OH groups in the commercial sample, and synthesis via the sol-gel route is strongly suspected.The chemical composition may be written as MgF 2−x (OH) x .The aim of post-fluorination of the commercial source is to tailor its OH/F ratio and surface chemistry, but to obtain pure oxygen-free MgF 2 , the choice was made to fluorinate an oxygen-free source.

Conversion of MgB 2 into Oxygen-Free MgF 2
In order to select the precursor for the synthesis of oxygen-free MgF 2 via an etching during the fluorination of the element other than Mg, i.e., B, N; Si or P in MgB 2 (BF 3 evolution), Mg 3 N 2 (NF 3 ), Mg 2 Si (SiF 4 ) and Mg 3 P 2 , the following criteria were used: 1.
The toxicity of the gases released at the completion of the first reaction item.

3.
The presence of solid products other than MgF 2 .
According to (3) MgC 2 cannot be retained because fluorocarbons may be formed.MgH 2 results after fluorination in the narrowest pores and has not been selected because HF is also undesirable according to (2).A toxic gaseous mixture S 2 F 2 /SF 6 /F 2 is also formed from MgS, excluding this precursor according to (2).Mg 3 N 2 is not considered because a thermal post-treatment is necessary to remove NH 4 F from as-prepared MgF 2 , that should lead to decrease the surface area.A narrow pore size being preferred, MgB 2 is selected rather than Mg 3 P 2 .The fluorination of this MgB 2 precursor at 300 K and 1 atm occurred in two steps: for the addition of F 2 in between 0 and 4 moles, the initial precursor is totally consumed to form solid boron and MgF 2 ; when F 2 is further added, the boron is then fluorinated as gaseous BF 3 .From 4 moles of fluorine gas, the only solid product is MgF 2 and the gaseous mixture consists of BF 3 and F 2 .A solid reaction yield slightly higher than 100% is explained by the presence of the intermediate product Mg(BF 4 ) 2 found at the completion of the reaction.
While the XRD pattern of the final product (Figure 1a) reveals only magnesium difluoride MgF 2 [28,29] with traces of MgB 2 , the presence of the intermediate product Mg(BF 4 ) 2 is unambiguously revealed by FTIR spectroscopy (Figure 1b) considering the B-F bond vibration bands of at 1111, 1080, 461 cm −1 [30].
Surfaces 2023, 6, FOR PEER REVIEW 4 In order to select the precursor for the synthesis of oxygen-free MgF2 via an etching during the fluorination of the element other than Mg, i.e., B, N; Si or P in MgB2 (BF3 evolution), Mg3N2 (NF3), Mg2Si (SiF4) and Mg3P2, the following criteria were used: 1.The expected pore size, in accordance with the size of the released molecules (0.243, 0.320, 0.377 and 0.377 nm) for MgB2 (BF3), Mg3N2 (NF3), Mg2Si (SiF4) and Mg3P2 (PF5), respectively.2. The toxicity of the gases released at the completion of the first reaction item.3. The presence of solid products other than MgF2.
According to (3) MgC2 cannot be retained because fluorocarbons may be formed.MgH2 results after fluorination in the narrowest pores and has not been selected because HF is also undesirable according to (2).A toxic gaseous mixture S2F2/SF6/F2 is also formed from MgS, excluding this precursor according to (2).Mg3N2 is not considered because a thermal post-treatment is necessary to remove NH4F from as-prepared MgF2, that should lead to decrease the surface area.A narrow pore size being preferred, MgB2 is selected rather than Mg3P2.The fluorination of this MgB2 precursor at 300 K and 1 atm occurred in two steps: for the addition of F2 in between 0 and 4 moles, the initial precursor is totally consumed to form solid boron and MgF2; when F2 is further added, the boron is then fluorinated as gaseous BF3.From 4 moles of fluorine gas, the only solid product is MgF2 and the gaseous mixture consists of BF3 and F2.A solid reaction yield slightly higher than 100% is explained by the presence of the intermediate product Mg(BF4)2 found at the completion of the reaction.
While the XRD pattern of the final product (Figure 1a) reveals only magnesium difluoride MgF2 [28,29] with traces of MgB2, the presence of the intermediate product Mg(BF4)2 is unambiguously revealed by FTIR spectroscopy (Figure 1b) considering the B-F bond vibration bands of at 1111, 1080, 461 cm −1 [30].The bands related to the hydroxyl groups (3430 and 1647 cm −1 ) are absent contrary to the other MgF2 samples (main peak at 435 cm −1 associated with Mg-F vibration mode) [31].Two bands are identified in the 19 F NMR spectrum (Figure 1c).The main band at −196 ppm is assigned to Mg-F bonds.A small shoulder centered at −145 ppm is due to BF4 − present in very few amounts [32].BF4 − reveals the presence of an intermediate compound between MgB2 and BF3 following the mechanism: It is worthwhile to note that no shoulders indicating the presence of hydroxyl groups are observed in the final product.As a matter of fact, OH groups result in a change in electronic density around the 19 F nuclei in their neighboring and consequently a small band should appear at higher chemical shift due to the decrease in the Mg-F bond ionicity (increase in the covalence of the Mg-F bond).A band or shoulder is then observed in the −160/−180 ppm range for other MgF2 samples; its intensity is related to the synthesis and post-treatment but mainly to the nature and concentration of the hydroxyl groups The bands related to the hydroxyl groups (3430 and 1647 cm −1 ) are absent contrary to the other MgF 2 samples (main peak at 435 cm −1 associated with Mg-F vibration mode) [31].Two bands are identified in the 19 F NMR spectrum (Figure 1c).The main band at −196 ppm is assigned to Mg-F bonds.A small shoulder centered at −145 ppm is due to BF 4 − present in very few amounts [32].BF 4 − reveals the presence of an intermediate compound between MgB 2 and BF 3 following the mechanism: It is worthwhile to note that no shoulders indicating the presence of hydroxyl groups are observed in the final product.As a matter of fact, OH groups result in a change in electronic density around the 19 F nuclei in their neighboring and consequently a small band should appear at higher chemical shift due to the decrease in the Mg-F bond ionicity (increase in the covalence of the Mg-F bond).A band or shoulder is then observed in the −160/−180 ppm range for other MgF 2 samples; its intensity is related to the synthesis and post-treatment but mainly to the nature and concentration of the hydroxyl groups substituting fluorine.The higher the intensity of this band, the higher the content of hydroxyl groups, (Figure 1c).The 1 H NMR spectrum of MgF 2 without oxygen (Figure 1d) confirms the absence of hydroxyl groups.The objective of preparing oxygen-free MgF 2 is thus reached.The sample obtained by fluorination of the magnesium diboride precursor exhibits a type II profile for the N 2 isotherm (Figure 1e), typical of non-porous or macroporous materials.
The absence of micropores indicates that the lattice is totally rebuilt during the chemical etching and BF 3 evolution.One should note the SSA (specific surface area) of 35 m 2 • g −1 for the oxygen-free MgF 2 .BF 3 gaseous molecules which are produced during the fluorination of MgB 2 allow a relatively high specific surface area to be maintained, that is unusual with gas/solid fluorination synthesis of fluorides.

Tailoring of OH/F Ratio in Conventional MgF 2
Considering the 19 F NMR spectra (Figure 1c), the main band at −196 ppm is assigned to 19 F nuclei in the F-Mg-F groups.This chemical shift is in accordance with the literature data [32].A shoulder also appears for the raw compound and its intensity decreases after post-fluorination treatment.This shoulder is relative to the presence of hydroxyl groups (OH-Mg-F) in MgF 2 .As mentioned previously, the position of the band gives information on the fluorine-oxygen environment of magnesium in MgF 2 .By fitting the spectra using two Lorentzian lines, the amount of hydroxyl groups is obtained for each compound.The OH/F ratio is 0.14 for raw MgF 2 and 0.04 for the MgF 2 post-fluorinated at 240 • C. The composition of the two samples can be written MgF 1.75 (OH) 0.25 and MgF 1.925 (OH) 0.075 .Regarding 1 H NMR spectra (Figure 1d), only one band is observed (the others being related to the rotor cap) corresponding to the hydroxyl groups in MgF 2 .After fluorination, its intensity, i.e., the amount of OH groups, decreases, confirming the efficiency of the treatment in removing OH.
Conversely, oxygen-free MgF 2 exhibits the characteristics of a non-porous or macroporous compound, raw and post-fluorinated MgF 2 present a type IV hysteresis which is typical of a mesoporous material (Figure 1e).The BJH method indicates an average pore size of 8.3 and 20.3 nm for raw and post-fluorinated pellets of commercial MgF 2 , respectively.The increase in pore diameter is due to the coalescence phenomenon induced both by the fluorination temperature and by the release of OH groups.It is not possible to extract an average pore diameter using the BJH method for the sample obtained from the boride precursor because the BJH method is only suitable for mesoporous materials.The BET surfaces are 100 and 72 m 2 • g −1 for raw and post-fluorinated MgF 2 , respectively.The post-fluorination treatment decreases the surface area by substituting fluorine atoms for hydroxyl groups.

Sorption of VOF 3 in MgF 2−x (OH) x
At this step, three different kinds of magnesium difluoride with various contents of OH groups were available for VOF 3 sorption tests: commercial (MgF 1.75 (OH) 0.25 ), postfluorinated (MgF 1.925 (OH) 0.075 ) and oxygen-free MgF 2 (MgF 2 ).After exposure to VOF 3 , the relative quantity of vanadium trapped was 3.6 w.% for the raw MgF 2 , 2.1 w.% for the post-fluorinated sample and 1.3 w.% for the oxygen-free synthesized product.All weight uptakes were confirmed by ICP analysis (Table 1).The higher the OH/F ratio, the higher the sorption rate.The similitudes between the XRD patterns before and after exposure to VOF 3 indicate no new crystalline phase formed nor any change in crystallinity, the width of diffraction peak being the same as that of MgF 2 before the sorption of VOF 3 .
The FTIR spectra of the samples (Figure 2a) reacting with vanadium oxyfluoride reveal the occurrence of magnesium-fluorine and vanadium-oxygen bonds.The V=O and V-F stretching bands of vanadium oxyfluoride are identified as raw and treated commercial MgF 2 after sorption at 1000 cm −1 (V=O) and 820 cm −1 (V-O) on the IR spectra (Figure 2a) [33,34].The intensities of the vanadium-oxygen vibration bands are higher for raw MgF 2 than for treated MgF 2 in accordance with the amount of vanadium trapped (3.6 and 2.1 w.%, respectively); see Table 1.Only low bands are detected at 1000 cm −1 (V=O) and 720 cm −1 (V-F) in the case of oxygen-free samples.MgF2 than for treated MgF2 in accordance with the amount of vanadium trapped (3.6 and 2.1 w.%, respectively); see Table 1.Only low bands are detected at 1000 cm −1 (V=O) and 720 cm −1 (V-F) in the case of oxygen-free samples.Gas phase IR and pressure measurements were also performed to follow the nature of the gas generated or consumed during the sorption of VOF3 on MgF2.The pressure drops in a first sorption step as expected due to the trapping of VOF3 molecules onto the surface of the filter, but increases rapidly after, indicating the release of other gases into the IR chamber.The gas-phase FTIR spectra pointed out a massive group of bands between 4500 and 3500 cm −1 (Figure 2c).These bands are characteristic of gaseous HF.Their intensity increased during the sorption (Figure 2b).
HF is unambiguously the product of a chemical reaction occurring between MgF2−x(OH)x and VOF3.This constitutes another proof of a chemisorption process.V=O and V-F vibration bands characteristic of vanadium oxyfluoride were also detected with Raman spectroscopy (Figure 3).Gas phase IR and pressure measurements were also performed to follow the nature of the gas generated or consumed during the sorption of VOF 3 on MgF 2 .The pressure drops in a first sorption step as expected due to the trapping of VOF 3 molecules onto the surface of the filter, but increases rapidly after, indicating the release of other gases into the IR chamber.The gas-phase FTIR spectra pointed out a massive group of bands between 4500 and 3500 cm −1 (Figure 2c).These bands are characteristic of gaseous HF.Their intensity increased during the sorption (Figure 2b).
HF is unambiguously the product of a chemical reaction occurring between MgF 2−x (OH) x and VOF 3 .This constitutes another proof of a chemisorption process.V=O and V-F vibration bands characteristic of vanadium oxyfluoride were also detected with Raman spectroscopy (Figure 3).Data from the literature indicate that the first band at 1018 cm −1 is assigned to the vibration of the V=O bond in VOF3 whereas the second at 798 cm −1 is related to the V-F bond [35].As in the FTIR data of MgF2 exposed to VOF3, the peak intensity was higher for the raw MgF2 than for the post-fluorinated sample, once again in accordance with the sorption rates.No bands were observed with the oxygen-free crystalline MgF2 sample as the quantity of vanadium trapped (close to 1 w.%) was not sufficient to detect any sorption product.After sorption of VOF3, the 19 F NMR spectra show a single line due to Mg-F bonds (Figure 4a).The shoulder indicating the presence of hydroxyl groups disappears after the sorption.A chemical reaction occurs that removes OH − groups from the surface of MgF2 Data from the literature indicate that the first band at 1018 cm −1 is assigned to the vibration of the V=O bond in VOF 3 whereas the second at 798 cm −1 is related to the V-F bond [35].As in the FTIR data of MgF 2 exposed to VOF 3 , the peak intensity was higher for the raw MgF 2 than for the post-fluorinated sample, once again in accordance with the sorption rates.No bands were observed with the oxygen-free crystalline MgF 2 sample as the quantity of vanadium trapped (close to 1 w.%) was not sufficient to detect any sorption product.
After sorption of VOF 3 , the 19 F NMR spectra show a single line due to Mg-F bonds (Figure 4a).The shoulder indicating the presence of hydroxyl groups disappears after the sorption.A chemical reaction occurs that removes OH − groups from the surface of MgF 2 and involves the volatile compound.No further lines due to fluorinated vanadium compounds are observed in the final product.For the 1 H NMR spectra (Figure 4b), the peak intensity assigned to hydroxyl groups in MgF 2 decreases for another contribution at a chemical shift of +9 ppm.This band is assigned to the interaction between vanadium oxyfluoride and protons and confirms the chemical reaction between the OH − groups and VOF 3 . 51V NMR spectra recorded at different spinning rates in order to distinguish between isotropic and spinning bands (Figure 4c) reveal the presence of 3 isotropic bands for raw and post-fluorinated MgF 2 exposed to VOF 3 .The isotropic bands at −561, −615 and −791 ppm refer to VO 2 F 2 − , VOF 3 and VOF 4 − , respectively [36][37][38].The presence of VO 2 F 2 − ions evidences that VOF 3 reacts with the OH groups on the surface of the chemical filter to form MgF 2−x (VO 2 F 2 ) x .Assuming that the vanadium oxyfluoride anion coordinates Mg 2+ via oxygen and occupies a distorted tetrahedral site, the steric hindrance to hydroxyl groups is radically different and VO 2 F 2 − can substitute OH groups only at the surface and not in the rutile network.From an electronic point of view, V 5+ is a second-order Jahn-Teller ion exhibiting a strong polyhedral distortion.This implies that this molecular species can accommodate a high distortion which permits the stabilization of MgF 2−x (VO 2 F 2 ) x compositions [39].
The generated HF increases the pressure in the reactor and can react with the VOF 3 .The resulting product is HVOF 4 (trapped on the surface of the filter) in accordance with the isotropic band observed at −791 ppm.As expected, no isotropic band is detected for the oxygen-free MgF 2 because of the low quantity of vanadium trapped.VOF 3 sorption can be summarized as: In addition to this expected reaction on the metal fluoride surface (chemisorption), a physisorption mechanism may also occur because some of the vanadium is trapped by oxygen-free MgF 2 (1.3 w.%).
vibration of the V=O bond in VOF3 whereas the second at 798 cm −1 is related to the V-F bond [35].As in the FTIR data of MgF2 exposed to VOF3, the peak intensity was higher for the raw MgF2 than for the post-fluorinated sample, once again in accordance with the sorption rates.No bands were observed with the oxygen-free crystalline MgF2 sample as the quantity of vanadium trapped (close to 1 w.%) was not sufficient to detect any sorption product.After sorption of VOF3, the 19 F NMR spectra show a single line due to Mg-F bonds (Figure 4a).The shoulder indicating the presence of hydroxyl groups disappears after the sorption.A chemical reaction occurs that removes OH − groups from the surface of MgF2 Chemisorption involves the Lewis basicity of MgF 2 through OH groups.This basicity may be tailored both via the nature, i.e., the coordination number of OH/F anions, the content of OH groups and the cation associated with fluorine.To go further in the discussion, the combination of polarizable cations with low electronegativity (K + in KMgF 3 , Mg 2+ , Ca 2+ ) and OH − groups substituting F − ions is another route to control the strength and number of Lewis basic sites keeping in mind the Lewis acidity of the gas that must be removed.The surface concentration and strength of the Lewis basic sites of the filter can be fitted to the Lewis acidity of the target gas in a gaseous mixture.Considering the structural features of the fluoride series: CaF 2 with a fluorite-type structure and fluorine atoms coordinated fourfold to Ca 2+ , MgF 2 with a rutile-type structure with fluorine atoms coordinated threefold to Mg 2+ and KMgF 3 with a perovskite-type structure and fluorine atoms twice coordinated to Mg 2+ , a large variation in the concentration and strength of the Lewis basicity sites is expected.Furthermore, the concentration of OH − groups can be adjusted by post-fluorination.

Regeneration of the Chemical Filter
The hydroxyl groups were removed upon exposure to VOF 3 for MgF 2 .With the aim to regenerate the chemical filter a two-step process was investigated: the first step consisted of the removal of vanadium with a solvent for V species (cleaning), whereas the second involved a rinsing of the surface.The investigations were performed on a sample with V content of 11,000 ppm after exposure of VOF 3 (ICP data).The extraction rate is given as a function of the cleaning/rinsing agent pair (Figure 5a).A high yield extraction (95%) was achieved with HNO 3 as both cleaning and rinsing agent.It is to note that this removal occurred without significant losses of MgF 2 (only 4 w.%, Figure 5b).After removal of the vanadium, the number of basic sites (OH) must be recovered via the regeneration process.To reach this goal, MgF 2 may be treated with NaOH. 19F MAS NMR is once again a powerful tool to evidence and quantify the presence of OH groups (Figure 6b) in addition to XRD that proves the unchanged presence of the MgF2 structure without significant change in Mg(OH)2 (Figure 6a).The chemical compositions extracted from the fit of the NMR spectra according to the method before (Figure 6c) are: MgF1.79(OH)0.21,MgF1.72(OH)0.28,MgF1.65(OH)0.35as a function of the NaOH concentration, i.e., 0.25, 0.5 and 1 M, respectively.The number of basic sites can then be tailored.In order to nearly recover the initial O/F ratio of 0.25 (MgF1.6(OH)0.4), the concentration must be 1 M (O/F = 0.21); the duration of the treatment is 1 h at 60 °C.Two VOF3 filtering/regeneration cycles were carried out and the change in specific surface area was studied.Table 2 shows the slight decrease in the BET surface after a full filtering/regeneration cycle. 19F MAS NMR is once again a powerful tool to evidence and quantify the presence of OH groups (Figure 6b) in addition to XRD that proves the unchanged presence of the MgF 2 structure without significant change in Mg(OH) 2 (Figure 6a). 19F MAS NMR is once again a powerful tool to evidence and quantify the presence of OH groups (Figure 6b) in addition to XRD that proves the unchanged presence of the MgF2 structure without significant change in Mg(OH)2 (Figure 6a).The chemical compositions extracted from the fit of the NMR spectra according to the method described before (Figure 6c) are: MgF1.79(OH)0.21,MgF1.72(OH)0.28,MgF1.65(OH)0.35as a function of the NaOH concentration, i.e., 0.25, 0.5 and 1 M, respectively.The number of basic sites can then be tailored.In order to nearly recover the initial O/F ratio of 0.25 (MgF1.6(OH)0.4), the concentration must be 1 M (O/F = 0.21); the duration of the treatment is 1 h at 60 °C.Two VOF3 filtering/regeneration cycles were carried out and the change in specific surface area was studied.Table 2 shows the slight decrease in the BET surface after a full filtering/regeneration cycle.The chemical compositions extracted from the fit of the NMR spectra according to the method described before (Figure 6c) are: MgF 1.79 (OH) 0.21, MgF 1.72 (OH) 0.28, MgF 1.65 (OH) 0.35 as a function of the NaOH concentration, i.e., 0.25, 0.5 and 1 M, respectively.The number of basic sites can then be tailored.In order to nearly recover the initial O/F ratio of 0.25 (MgF 1.6 (OH) 0.4 ), the concentration must be 1 M (O/F = 0.21); the duration of the treatment is 1 h at 60 • C. Two VOF 3 filtering/regeneration cycles were carried out and the change in specific surface area was studied.Table 2 shows the slight decrease in the BET surface after a full filtering/regeneration cycle.Whereas the specific surface area decreased after the exposure to VOF 3 , the initial value was nearly recovered after regeneration with NaOH (1 M concentration, 1 h at 60 • C).Such characteristics prove the possibility of regeneration of the selective filter for both reuse and recovery of vanadium.When the selective filter is used for the purification of UF 6 , some uranium species will be present on the surface of the filter; the regeneration aims to remove these species too, underlining its primary importance.

Conclusions
Metal fluorides have been investigated as selective chemical filters for the removal of VOF 3 , a model gas for volatile fluorides.HSA MgF 2 containing different contents of OH − groups and an oxygen-free MgF 2 with a rather high specific area (35 m 2 • g −1 ) were tested.It is worth noting that such fluorination synthesis using MgB 2 precursor is reported for the first time.The sorption mechanism identified for MgF 2 consists of a chemical reaction between VOF 3 and Lewis basic sites, i.e., OH − groups.The higher the amount of OH − groups, the higher the quantity of vanadium trapped.Without hydroxyl groups (free-oxygen MgF 2 ), physisorption is possible but the amount of vanadium is lower than that of a raw and post-fluorinated commercial MgF 2 .It was expected that hydroxyl groups are involved in the reaction with VOF 3 but our data evidence physisorption too.Moreover, understanding the sorption mechanism using complementary techniques allowed us to select the most promising selective and regenerable filter.Both the amounts and strength of the Lewis basic sites may be tailored using a post-fluorination treatment with F 2 gas.This versatility opens the route for the selectivity of filtering for mixtures of volatile fluorides according to the Lewis acidity of the target gas, and the present materials can be considered for active materials of gas sensors [40][41][42].Since the reactions with the impurity to be removed occur at the surface of the HSA MgF 2 , the metal fluoride matrix is maintained and controlled regeneration of the hydroxyl groups with treatment in NaOH solution is possible.

Figure 2 .
Figure 2. FTIR spectra in the solid (a) and gaseous phase (c) of adsorbed species by MgF2−x(OH)x after exposure to VOF3, evolution of pressure in the gas chamber as a function of time (b) and XRD (d) patterns of resulting powders.

Figure 2 .
Figure 2. FTIR spectra in the solid (a) and gaseous phase (c) of adsorbed species by MgF 2−x (OH) x after exposure to VOF 3 , evolution of pressure in the gas chamber as a function of time (b) and XRD (d) patterns of resulting powders.

Figure 4 .
Figure 4. 19 F (a), 1 H (b) and 51 V (c) MAS NMR spectra (spinning rate of 30 kHz) before and after exposure to VOF3.Arrows mark the isotropic lines.

Figure 4 .
Figure 4. 19 F (a), 1 H (b) and 51 V (c) MAS NMR spectra (spinning rate of 30 kHz) before and after exposure to VOF3.Arrows mark the isotropic lines.

Figure 4 .
Figure 4. 19 F (a), 1 H (b) and 51 V (c) MAS NMR spectra (spinning rate of 30 kHz) before and after exposure to VOF 3 .Arrows mark the isotropic lines.

Figure 5 .
Figure 5. (a) Extraction rate of vanadium and (b) vanadium weight loss according to the cleaning and rinsing agents.

Figure 6 .
Figure 6.XRD patterns (a) and 19 F MAS NMR spectra (b) of raw MgF2 treated with NaOH solutions; (c) 19 F MAS NMR spectra of post-treated MgF2 exposed to VOF3, after cleaning (with HNO3) and rinsing (H2O) at 60 °C and regeneration with NaOH.

Figure 5 .
Figure 5. (a) Extraction rate of vanadium and (b) vanadium weight loss according to the cleaning and rinsing agents.

Surfaces 2023, 6 , 10 Figure 5 .
Figure 5. (a) Extraction rate of vanadium and (b) vanadium weight loss according to the cleaning and rinsing agents.

Figure 6 .
Figure 6.XRD patterns (a) and 19 F MAS NMR spectra (b) of raw MgF2 treated with NaOH solutions; (c) 19 F MAS NMR spectra of post-treated MgF2 exposed to VOF3, after cleaning (with HNO3) and rinsing (H2O) at 60 °C and regeneration with NaOH.

Figure 6 .
Figure 6.XRD patterns (a) and 19 F MAS NMR spectra (b) of raw MgF 2 treated with NaOH solutions; (c) 19 F MAS NMR spectra of post-treated MgF 2 exposed to VOF 3 , after cleaning (with HNO 3 ) and rinsing (H 2 O) at 60 • C and regeneration with NaOH.

Table 2 .
Change in the specific surface area during two filtering/regeneration sequences.