Jérôme Long

Alexander N Selikhov

Ekaterina Mamontova

Konstantin A Lyssenko

Yannick Guari

Joulia Larionova

Alexander A Trifonov

Synthesis, structure, magnetic and luminescent properties of two dysprosium Single-Molecule Magnets based on phenoxide dye ligands

We report here two dysprosium complexes based on phenoxidebased emissive ligands 1-(2,4-dimetylphenylazo)-2-naphtholate (L1) and anthracenolate (L2). While 1 is a dinuclear homoleptic complex [Dy 2 (L1) 6 ]•2C 7 H 8 exhibiting a zero-field slow relaxation of the magnetization, 2 is a mononuclear complex [Dy(L2) 3 (py) 3 ] (py = pyridine) showing a field-induced relaxation.

Lanthanide-based complexes have been extensively studied for the last fifteen years due to their impressive magnetic properties that give rise to the appearance of a slow relaxation of the magnetization and so-called Single-Molecule Magnet (SMM) behaviour, which finds potential applications in highdensity information storage or quantum computing. [1][2][3][4] Yet, such expectations require to efficiently optimize the magnetic properties of the complexes by controlling the magnetic anisotropy through a modulation of the lanthanide ions' coordination environment. Indeed, in such systems, the high magnetic anisotropy of certain lanthanide ions may be exacerbated in a specific ligand environment/symmetry to provide a large crystal-field splitting and in turn generate a large anisotropic barrier, , separating two opposite magnetic states. Hence, the presence of dominated thermally activated relaxation (or Orbach relaxation) might result in the appearance of the SMM behaviour. One of the main challenges in the field is dedicated to increase the anisotropic barriers and the blocking temperatures. [5][6] Despite the fact that other processes, such as the Quantum Tunnelling of the Magnetization (QTM), direct or Raman relaxations may create their own relaxation pathways therefore decreasing the effective energy barrier, simple electrostatic considerations provide a synthetic strategy based on the optimal alliance between the electronic density of specific 4f lanthanide ions (oblate or prolate) and the crystal-field imposed by the ligand environment (axial or equatorial). 7 Following this approach, numerous lanthanide complexes with high anisotropic barriers of few thousands of wavenumbers have been recently reported, On the other hand, although exchange interactions between lanthanide ions in SMMs are commonly considered as weak with respect to the dominating single-ion anisotropy, their role is not yet clearly understood. 15 Different literature examples have demonstrated a decrease of the anisotropy barrier in polynuclear Dy 3+ based complexes due to the mixing of the states and/or QTM increase. 16 However, several cases show also a positive impact of the magnetic interactions caused by an exchange bias effect that shifts the usual zerofield QTM relaxation, [17][18][19][20] that could eventually lead to a large magnetic coercivity.

21

Obviously, dinuclear complexes represent therefore the simplest model to study the influence of exchange interactions over the slow relaxation dynamics. 22 In this sense, one strategy consists in employing bulky phenoxide ligands in the axial position that ensure Ln-O short distances, 8, 23-25 and potentially bridge lanthanide ions. In addition, such ligands may be appropriate to implement additional functionalities, such as luminescence. [26][27][28] With this in mind, numerous dyes incorporating phenoxide moieties may be used as ligands for the design of luminescent SMMs. Recrystallization from cold toluene resulted in the isolation of the complex as bright-orange moisture and air sensitive 1 crystals in 90% yield. Dissolution of 1 in THF does not lead to its dissociation and the dimeric structure of the complex remains retained after recrystallization from a mixture of THFhexane. Secondly, reaction of anhydrous DyCl 3 with potassium anthracenolate was carried out in a pyridine solution at 80 °C (Scheme 1). Yellow crystals of complex [Dy(L2) 3 (py) 3 ] (2) were obtained by slow diffusion of hexane into a solution of the complex in pyridine.

Scheme 1. Synthesis of complexes 1 and 2.

The single-crystal X-ray diffraction studies reveal that complex 1 crystallizes in the triclinic space group with 2 molecules of toluene (1•2C 7 H 8 ) as a solvate (Table S1). The asymmetric unit contains two crystallographically independent dinuclear complexes (one of those is shown in Fig. 1 -bridging oxygen with Dy1-O1B distance of 2.325(8) Å. The second crystallographic complex exhibits comparable distances and bond lengths with a O1C-Dy2-O1E angle of 147° and distances Dy2-O1C = 2.157(6) Å/Dy2-O1E = 2.160(6) Å in the axial direction, while the equatorial Dy2-O1D distance is equal to 2.365(9) Å (Fig. S1). In contrast, 2 could be described as a mononuclear complex with a unique crystallographic complex. This structure is similar to that of the previously described 2,6-diisopropyl-and 2,6-dimethylphenoxide complexes of Dy. 32 The Dy 3+ adopts a distorted octahedron geometry (Table S2) and is coordinated by three monoanionic anthracenolate ligands and three pyridine Noticeably, the two shortest Dy-O1 and Dy-O3 distances are located in the axial direction (in trans position) and forms an angle O1-Dy-O3 of 161.1(1)°. The lengths of the Dy-N bonds are largely longer with values ranging from 2.490(4) to 2.591(4) Å. For both complexes, the crystal packing analysis reveals that the shortest intermolecular Dy-Dy distances are equal to 13.328(3) and 9.2020(7) Å, for 1 and 2, respectively (Fig. S2). These values suggest that whereas the dinuclear complexes are relatively well isolated, the presence of intermolecular interactions may be expected for 2.

The magnetic properties were investigated in both, static (direct current (dc)) and dynamic (alternating current (ac)) modes by using a SQUID MPMS-XL. At room temperature, the T values of 27.84 and 13.30 cm 3 .K.mol 1 for 1 and 2, respectively, are in a relatively good accordance with the expected values of 28.34 cm 3 .K.mol 1 and 14.17 decrease of T due to the thermal depopulation of the m J levels (Fig. S3). Yet, the pronounced T decrease for 1 rather suggests antiferromagnetic interactions between the Dy 3+ ions.

The field dependence of the magnetization at 1.8 K reveals a rapid increase of the magnetization as a function of field with the values of 12.64 and 5.53 Nβ under a 70 kOe field for 1 and 2, respectively (Fig. S3). The occurrence of a slow relaxation of the magnetization was investigated by ac measurements. Under a zero-dc field, both 1 and 2 exhibit a significant out-of-phase susceptibility (") component but only 1 shows a maximum of " in our experimental conditions (Fig. S4). The frequency dependence of " for 1 reveals the presence of a temperature dependent single peak (Fig. 2). Despite the presence of two crystallographic dysprosium sites for 1, the fitting of the Cole-Cole plots (Fig. S5) with a generalized Debye model yields to relatively low values of the  parameter (i.e. < 0.3) at low temperature, indicating a moderate distribution of relaxation times (Table S3). From these data, the relaxation time, , could be extracted in order to analyse the relaxation dynamics. The ln vs. T plot (Fig. 3) reveals a deviation from the linearity at low temperature indicating a divergence from a thermally activated process. The whole data range could be modelled using the following equation:

 1 =  0 1 exp(/kT) + CT m +  1 QTM (Eq. 1)
. 33 The first term accounts for a thermally activated process, while the second and third ones stand for twophonon Raman and QTM, respectively. To avoid overparameterization, the m coefficient was fixed to different values until getting the best correlation coefficient. It turns out that the magnetization relaxes through a combination of these three relaxation processes with a modest  value of 24  5 cm 1 (Table 1). It could be noticed that a poor fitting in the low temperature range is observed with a large deviation for the QTM constant. This may be ascribed either to the presence of two dysprosium sites or the effect of the magnetic interactions between Dy 3+ ions. Besides, a strong contribution from the Raman process should be underlined. With the aim to shortcut the QTM, ac measurements were performed under static dc-fields for both complexes 1 and 2 (Fig. S6). The field dependence of the relaxation time for 2 (Fig. S7) could be modelled with the usual equation  1 = DH 4 T + B 1 /(1+B 2 H²) + K (Eq. 2), for which the first term accounts for the direct process (for Kramers-ion), the second one stands for the QTM, while the K constant accounts for the fieldindependent Raman and thermally activated processes (Table S4). For 1, the optimum field could be estimated at 500 Oe. However, it could be noticed that the relaxation time is almost field-independent up to 2000 Oe and it was not possible to model the field dependence of the relaxation time which may be related to the presence of two dysprosium sites. This fieldindependent character may also suggest that magnetic interactions could shortcut in some extent the QTM as observed in other dinuclear complexes. 17 On the contrary, Eq.

2 permits to nicely fit the  vs. H curve for 2 showing an optimal magnetic field between 1000 and 2000 Oe (Fig. S7).

The frequency dependence of the ac susceptibilities and related Cole-Cole plots under the corresponding optimal dc fields still show a single relaxation process for both complexes (Fig. S8-S9, Table S5-S6). The temperature dependence of  was also modelled by taking into account an additional direct process:  1 =  0 1 exp(/kT) + CT m + AT n (Eq. 4, Table 1, Fig. 3) giving the  values of 34 ± 1 and 30 ± 1 cm -1 for 1 and 2, respectively.

 / Hz

Further insights could be obtained by determining the direction of the anisotropic axes of the ground doublet in both complexes with the MAGELLAN package. 34 For the two dinuclear complexes in 1 and owing to the centrosymmetric character, the anisotropic axes of both Dy sites are found identical and are almost collinear to the two shortest Dy-O distances involving the terminal phenoxide moieties (Fig. S10) with a deviation of 8° from the shortest Dy1-O1 bond of 2.144(6) Å and 14° from the second shortest Dy1-O1A bond of 2.151(8) Å. The second dinuclear complex shows comparable deviation angle of 9.85 and 14.75°. Hence the electrostatic contributions are mainly dominated by these two negatively charged oxygens, while the bridging phenoxide oxygens located at longer distances (ranging from 2.311 to 2.375 Å) do not induce a strong deviation of the anisotropic axis. This results in the presence of an axial crystal-field suitable to stabilize the oblate electronic density of each Dy 3+ sites 7 and, probably with a non-detrimental or positive impact of the magnetic interactions between the two Dy 3+ ions that can explain the appearance of a slow magnetic relaxation in zerofield. Nevertheless, the absence of an opening in the hysteresis loop at 1.8 K in our experimental conditions (Fig. S11) for 1, suggests that the magnetic interactions between the Dy(III) ions are not strong enough to provide an exchange bias system that may greatly shortcut the zero-field QTM.

As regards 2, the anisotropic axis is almost collinear to the two shortest Dy-O3 and Dy-O1 bonds located in trans position (distances of 2.131 and 2.144 Å, Fig. S10) with deviation angles of 6.63 and 7.23°. On the other hand, the equatorial oxygen O2 is located almost perpendicularly (O1-Dy-O2 = 97.8° and O3-Dy-O2 = 98.8°) with a relatively short distance of 2.164 Å. This explains the occurrence of a transverse component leading to a strong QTM and in turn the need to use a static dc field to reveal the maximum of ". It could be however highlighted that despite the presence of relatively short Dy-O distances, the anisotropic barriers for 1 and 2 are found relatively weak, suggesting a major influence of the Raman relaxation process as deduced from the fitting of the temperature dependence of the relaxation time.

Moreover, phenolate-based ligands are also appropriate to implement luminescence properties in order to design luminescent SMMs. [26][27] At room temperature and solid-state, both, 1 and 2 exhibit a broad ligand-based emission band located at with different emissions located at 615 and 550 nm, respectively (Fig. S12). In summary, we have described in this communication two dysprosium complexes using different phenoxide-based dyes. Thus, in the homoleptic dinuclear complexes 1, the relatively short Dy-O distances in the axial position and a relatively wide O-Dy-O angle of 147-150° create an axial crystal-field for each Dy 3+ site. This promising axiality and the non-deleterious impact of the magnetic interactions between the two Dy 3+ are suitable to provide a zero-field slow relaxation of the magnetization observed for this compound and offers a nice example of a dinuclear Dy-based genuine SMM behaviour. On the other hand, the mononuclear complex shows only a fieldinduced SMM behaviour due to the presence of a phenolate ligand with the short Dy-O bond in the equatorial plane, which certainly decreases the axiality. In both cases, the influence of the Raman relaxation seems to be important and explain the presence of a relatively low energy barriers. Depending on the ligand's nature, a drastic change in the emission properties of the resulting complexes is observed. Consequently, the large tunability of such ligands in (steric and electronic) should provide facile experimental means to adjust and improve both the slow relaxation features and optical properties. The authors thank the University of Montpellier, CNRS for financial support, PAC of ICGM for magnetic measurements and the Analytical Center of The G.A. Razuvaev IOMC RAS for the X-ray study. The Russian co-authors thank the Russian

2 -

 2 and another in Fig S1, ESI) both located in the center of symmetry. Each dysprosium is coordinated by three N,O-κ 2 chelating naphtholate ligands; two of them are terminal, while one is μ bridging two Dy 3+ centers via phenolate oxygen. Hence, each Dy 3+ ion is seven-coordinated and surrounded by four naphtholate oxygen atoms of four ligands and three nitrogen atoms of -N=N-azo groups. Each dysprosium site was analyzed by the SHAPE 30 software that indicates a capped octahedron geometry (TableS2). The intramolecular Dy1-Dy1 and Dy2-Dy2 distances are equal to 3.8442(11) and 3.818(1) Å respectively, while the bridging angles Dy-O-Dy are 110.2(2)° and 109.0(2)°. These data are in a good agreement with the distances found in the previously described dinuclear phenolate dysprosium complexes with a similar structure and coordination environment.

17 , 31 Fig. 1 .

 17311 Fig. 1. a) Molecular structure of one of the two crystallographically independent dinuclear complexes in 1. b) View of one of the centrosymmetric dinuclear core in 1. c) molecular structure of the mononuclear complex 2 . Colour code: orange, Dy; blue, N; grey, C; Hydrogen atoms have been omitted for clarity.

  cm3 .K.mol 1

( 6 H

 6 15/2 ) for either two non-interacting Dy 3+ ions or a single Dy 3+ ion. Lowering the temperature results in a monotonous

Fig. 2 .

 2 Fig. 2. Frequency dependence of ' and " for 1 under a zero dc-field.

Fig. 3 .

 3 Fig. 3. Temperature dependence of the relaxation time for 1 (0 and 500 Oe) and 2 (1000 Oe) using the ac susceptibility data. The solid line represents the fit with Eq. 1 (0 Oe) and dashed lines with Eq. 3.

  Please do not adjust margins molecules. The aryloxy ligands and pyridine localization yields to the meridional isomer of the octahedral complex. The Dy-O bond lengths are 2.131(3), 2.144(3), 2.164(3) Å, with an average value of 2.146 Å, which is in good agreement with the values described for other Dy 3+ phenoxide derivatives.

	32

Table 1 :

 1 Fit parameters of the temperature dependence of the relaxation time for 1 and 2.

	Compound	 (cm 1 )	0 (s)	m	C (s 1 .K m )	QTM (s)	A (s -1 .K -1 )
	1 (0 Oe)	24 ± 5	(3 ± 2)  10 4	4.8*	0.00184  0.00004	0.08 ± 0.14	-
	1 (500 Oe) 34 ± 1	(4.6 ± 0.5)  10 6	6.5*	(6.4 ± 10 7 0.1) 	-	-
	2 (1000 Oe)	30 ± 1	(2.7 ± 0.4)  10 6	5*	0.0045 0.0001 	-	1 ± 1
	* fixed parameter

Please do not adjust margins

Science Foundation (grant 17-73-30036). The X-ray study of has been carried out in the framework of the Russian state assignment (Тheme № 44.2, Reg. № АААА-А16-116122110053-1) using the equipment of The Analytical Center of IOMC RAS.

Conflicts of interest

There are no conflicts to declare.

Notes and references


Julien Grand

Nicolas Barrier

Maxime Debost

Edwin B Clatworthy

Fabien Lainé

Philippe Boul- Lay

Nikolai Nesterenko

Jean-Pierre Dath

Jean-Pierre Gilson

Svetlana Mintova

Flexible Template-Free RHO Nanosized Zeolite for Selective CO2 Adsorption

Flexible small-pore zeolites are interesting candidates for flue and natural gas processing due to their high sorption capacity and selectivity. It is generally accepted that their high CO2/CH4 selectivity is due to cation gating behavior such as the "trapdoor" effect or cation-controlled molecular sieving. Herein, nanosized RHO-type zeolite containing only inorganic cations (Na + and Cs + ) has been prepared from a colloidal precursor suspension at 90 °C for 1 h without the use of expensive organic structure directing agents. The high Cs content significantly improved the thermal stability of the RHO nanocrystals up to 550 °C. The flexibility of the RHO cages upon water adsorption/desorption is demonstrated. The dehydration of the nanosized RHO zeolite resulted in two dehydrated RHO phases, the first one presents an enlargement of the cubic unit cell and a transformation from distorted to the more regular d8r units, and the second one presents a reduction of the cubic unit cell and an increase of the distortion of the d8r units. The 8-rings elliptical distortions of the as synthesized RHO (hydrated form) of 1.87 Å changed to 0.94 Å and 2.16 Å for the two dehydrated RHO forms. The flexibility of the nanosized RHO zeolite is due to the ability of the Cs + cation to displace out from and in to the D8R window sites which is of great importance for controlling the selective adsorption capacity of the RHO zeolite. The flexible RHO-type nanosized zeolite exhibited great selectivity towards CO2 over CH4. The adsorption capacity is retained after 10 cycles of CO2 adsorption/desorption and the crystalline structure is fully preserved.

■ INTRODUCTION

Zeolite materials are microporous crystalline solids possessing networks of channels and cages. As of 2019 the number of approved structure types is 249, of which 25 have been prepared with nanosized dimensions. Reducing the particle size of zeolites from the micro to the nanoscale leads to significant changes in their properties such as an increase in the external surface area and reduced diffusion path length, enhancing the accessibility of guest molecules to active sites within the micropores and cages. [START_REF] Mintova | Nanosized microporous crystals: emerging applications[END_REF] Several methods for the synthesis of nanosized zeolites have been developed in recent years including post-synthesis modification, one-step hydrothermal crystallization in the presence of mesopore modifiers, and synthesis with organic structure directing agents (OSDAs). [START_REF] Mintova | Nanosized zeolites: quo vadis?[END_REF] The synthesis of nanosized zeolites without OSDAs is an important research target because the post-synthetic removal of OSDAs requires additional energy, generates emissions (COx, NOx), and can reduce the crystallinity of the zeolite which negatively impacts their performance. [START_REF] Awala | Template-free nanosized faujasite-type zeolites[END_REF][START_REF] Ng | Capturing ultrasmall EMT zeolite from template-free systems[END_REF] Zeolite frameworks, composed of SiO4 4-and AlO4 5-tetrahedra, maintain charge neutrality by the presence of extra-framework cations, the most common of which are Na + and K + . In addition to providing charge neutrality, extra-framework cations have also been shown to influence the zeolite crystallization process; acting as inorganic structure directing agents. [START_REF] Shin | Rediscovery of the Importance of Inorganic Synthesis Parameters in the Search for New Zeolites[END_REF] For example, the presence of specific type and amount of cations, with or without the use of OSDAs, is required to facilitate the formation of RHO-type zeolites as reported by several groups. [START_REF] Ji | Steam-dealuminated, OSDA-free RHO and KFI-type zeolites as catalysts for the methanol-to-olefins reaction[END_REF][START_REF] Ke | Targeted synthesis of ultrastable high-silica rho zeolite through alkali metal-crown ether interaction[END_REF] In particular, it has been shown that not only is the presence of Cs + cations required, but a specific mixture of Na + and Cs + is necessary to avoid the formation of other zeolite phases such as FAU or ANA. [START_REF] Nearchou | Influence of alkali metal cations on the formation of zeolites under hydrothermal conditions with no organic structure directing agents[END_REF] Due to the change in global energy demands, there is an increasing need for porous materials able to separate the components of natural gas, mainly CH4 and CO2. In this context small pore zeolites are interesting candidates due to their high sorption capacity and CO2/CH4 selectivity. The RHO topology is constructed from d8r units that act as windows to lta cages resulting in two 3D interpenetrating but unconnected pore systems. [START_REF] Robson | Synthesis and Crystal Structure of Zeolite Rho-A New Zeolite Retlated to Linde Type A[END_REF] RHO-type zeolites have promising applications in catalysis and gas separation, [START_REF] Dusselier | Small-pore zeolites: synthesis and catalysis[END_REF] NOx removal, [START_REF] Ke | Targeted synthesis of ultrastable high-silica rho zeolite through alkali metal-crown ether interaction[END_REF] MTO reactions, [START_REF] Ji | Steam-dealuminated, OSDA-free RHO and KFI-type zeolites as catalysts for the methanol-to-olefins reaction[END_REF][START_REF] Ji | Organic-Free Synthesis of Small Pore Zeolite Catalysts[END_REF] and CO2/CH4 separation. [START_REF] Lozinska | Understanding carbon dioxide adsorption on univalent cation forms of the flexible zeolite Rho at conditions relevant to carbon capture from flue gases[END_REF][START_REF] Palomino | Zeolite Rho: a highly selective adsorbent for CO2/CH4 separation induced by a structural phase modification[END_REF][START_REF] Min | CO2 Adsorption in the RHO Family of Embedded Isoreticular Zeolites[END_REF] Furthermore, the synthesis of zeolites with high adsorption selectivity and nanoparticle morphology creates new opportunities in medical applications for the targeted delivery of therapeutic gases. [START_REF] Goldyn | Copper exchanged FAU nanozeolite as non-toxic nitric oxide and carbon dioxide gas carrier[END_REF] The origin of the high selectivity for CO2 is a combination of the narrow pore size (3.6 Å) and the presence of different extra-framework cations, such as Na + and Cs + , which sit in the pores and restrict access to the internal cages. [START_REF] Lozinska | Cation gating and relocation during the highly selective "trapdoor" adsorption of CO2 on univalent cation forms of zeolite rho[END_REF] The ability of extra-framework cations to selectively control the access of molecules to the zeolite channels is described as cation gating and may occur through different mechanisms: the "trapdoor" effect and molecular sieving. The proposed "trapdoor" effect [START_REF] De Baerdemaeker | Gas separation: trapdoors in zeolites[END_REF] occurs due to the ability of molecules which possess a relatively large quadrupole moment and polarizability, such as CO2 as compared to N2 or CH4, to displace extra-framework cations from the pore opening. However, other studies propose the interaction of CO2 with the zeolite and the amplitude of the thermal motion of the extra-framework cations is responsible for allowing CO2 into the cage while blocking N2 and CH4. [START_REF] Coudert | Molecular insight into CO2 "trapdoor" adsorption in zeolite Na-RHO[END_REF] Molecular sieving of gas mixtures by zeolite RHO occurs due to the influence of extra-framework cations on the framework structure resulting in changes to the pore size. In addition, temperature, the presence of water, and adsorption of CO2 have also been reported to affect zeolite RHO structure. [START_REF] Corbin | Flexibility of the zeolite RHO framework: in situ x-ray and neutron powder structural characterization of divalent cationexchanged zeolite RHO[END_REF][START_REF] Pera-Titus | Thermodynamic analysis of framework deformation in Na, Cs-RHO zeolite upon CO2 adsorption[END_REF][START_REF] Balestra | Understanding nanopore window distortions in the reversible molecular valve zeolite RHO[END_REF][START_REF] Guo | Thermodynamics of H2O and CO2 Absorption and Guest-Induced Phase Transitions in Zeolite RHO[END_REF] Herein, we present the first synthesis of nanosized flexible RHO-type zeolites without the use of expensive OSDAs or seeds. Highly crystalline nanoparticles of zeolite RHO were obtained using inorganic structure directing agents, Na + and Cs + cations, only. The zeolite RHO nanocrystals have a diameter of less than 70 nm and the framework structure and position of the cations have been determined by Rietveld refinement based on XRD patterns. The distortion of the d8r units as well as the displacement of the Na + and Cs + cations within the cages upon adsorption of water and CO2 was studied by temperature dependent in situ XRD.

■ EXPERIMENTAL

Synthesis. RHO-type nanosized zeolite was prepared by hydrothermal synthesis from a colloidal precursor suspension using the following chemicals without further purification: sodium aluminate (53% Al2O3 47% Na2O by mass, Sigma-Aldrich), colloidal silica LUDOX AS40 (40% by mass in water, Sigma-Aldrich), sodium hydroxide (99%, Sigma-Aldrich) and cesium hydroxide (98%, Alfa-Aesar). Doubly deionized water was used throughout the synthesis and post-synthesis treatments. Syntheses were carried out in 100 cm 3 polypropylene bottle (PP bottle) at autogenous pressure without agitation up to 100 °C and in autoclave above 100 °C. A clear colloidal suspension was prepared by mixing 516 mg of sodium aluminate in 3 g of dd H2O. 1.82 g of sodium hydroxide and 588 mg of cesium hydroxide were added to the clear suspension. During the mixing, the suspension was maintained at room temperature (i.e. 25 °C) while being stirred vigorously. The stirring at room temperature was continued for at least 2 h and afforded a clear (colloidal) aqueous alkali aluminate suspension. 5 g of LUDOX AS4O was added dropwise to the clear suspension. During the addition it was maintained at room temperature while being vigorously stirred. The pH of the resulting clear aqueous suspension was about 12. The resulting clear aqueous suspension with a molar composition, 10 SiO2: 0.8 Al2O3: 8 Na2O: 0.58 Cs2O: 100 H2O, was then aged on a magnetic stirrer for 14 h at room temperature. Then, the hydrothermal crystallization was conducted at 90 °C for 1 h so as nanocrystals RHO zeolite was synthesized. The solid was separated and recovered by high-speed centrifugation (20000 rpm, 10 min) and purified with hot double distilled water (heated at 100 °C for 30 min) until the pH of the decanting water was about 7.5. Further in the paper, the as synthesized nanosized RHO zeolite sample in hydrated form will be abbreviated as NaCs-RHO(H2O) and the dehydrated sample at high temperature (350 °C) will be abbreviated as NaCs-RHO. Characterization. The powder X-ray diffraction (PXRD) patterns were measured on a Bruker D8 Advance Vario1 diffractometer equipped with a primary germanium (111) Johansson monochromator (λKα1 = 1.5406 Å) and a LynxEye detector.

The PXRD patterns were collected at room temperature with variable divergent slits and constant illuminated sample length of 6 mm, between 5 and 120° 2θ with a step of ~0.014° 2θ. Temperature-dependent powder in situ X-ray diffraction (TD-PXRD) patterns were recorded on the same diffractometer with an Anton Paar HTK1200N chamber, under a flow of N2 (the first pattern was recorded at 350 °C with the same conditions as the previous one). Dehydration and hydration of the RHO samples were also performed between 30 and 300 °C in air. In situ CO2 adsorption was also followed by TD-PXRD. The samples were pre-treated in situ at 350°C under N2 atmosphere for 2 h to remove the adsorbed water. The CO2 flow (1 bar) was delivered at 350 °C for 3 h. Diffractograms were then recorded every 20 °C until 30 °C under a constant flow of CO2 (1 bar). The sample was maintained under CO2 for 10 hours to check any evolution of the amount of adsorbed CO2. The whole profile matching of the PXRD diagrams were performed using the JANA2006 program [START_REF] Petříček | Crystallographic computing system JANA2006: general features[END_REF] (the I-43m space group was used). The structures were solved using the ab initio approach with the SUPERFLIP program [START_REF] Palatinus | SUPERFLIP-a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions[END_REF] and the Rietveld refinements were performed with the JANA2006 program assuming fundamental approach for peak-shape fitting and crystallites size determination (Table S1). All the atomic positions and isotropic displacement parameters (Uiso) were refined (Table S2). Anisotropic atomic displacement parameters (ADPs) were taken into consideration for all the Cs1 atoms of the double eight-rings (D8Rs) except for the NaCs-RHO-2 structure; in the latter, the refinements of the ADPs gave unrealistic values. The crystal size and morphology of the samples were first characterized using a FEI Tecnai G30 (LaB6, Vacc = 300 kV) transmission electron microscope (TEM) on diluted colloidal suspensions of zeolite materials. High resolution transmission electron microscopy (HRTEM) was performed on a JEOL ARM 200F (cFEG, Vacc = 200 kV) TEM. Chemical composition was obtained by energy dispersive X-Ray spectroscopy (EDS) using a JEOL 2010 (LaB6, Vacc = 200 V) TEM. The chemical composition of the RHO samples was further confirmed by inductively coupled plasma (ICP) optical emission spectroscopy using a Varian ICP-OES 720-ES and by deconvolution of the 29 Si solid state MAS NMR signals using DMFIT software. The 29 Si solid state MAS NMR spectra were collected on a Bruker Avance III-HD 500 (11.7 T) spectrometer operating at 99.3 MHz, using 4-mm outer diameter zirconia rotors spun at 12 kHz. The 27 Al MAS NMR spectra were recorded on a Bruker Avance III-HD 500 (11.7 T) spectrometer operating at 130.3 MHz, using 4-mm outer diameter zirconia rotors spun at 14 kHz. N2 adsorption/desorption isotherms were measured at -196 °C and CO2 adsorption isotherms were measured at 25 °C using a Micrometrics Model ASAP 2020 volumetric adsorption analyzer. Samples were degassed at 300 °C under vacuum for 12 h prior to analysis. Thermogravimetric analysis (TGA/DTG) of samples was carried out on a SETSYS 1750 CS evolution instrument (SETARAM). The samples were heated from 25 °C to 800 °C with a heating ramp of 5 °C/min under nitrogen (flow rate: 40 mL/min). Cycles of CO2 adsorption/desorption were conducted 10 consecutive times by alternating between activation at 350 °C for 2 h under N2 flow (flow rate: 40 mL/min) and CO2 adsorption at room temperature (flow rate: 40 mL/min, 1 bar) for 2 h. CO2 and CH4 adsorption on RHO was studied by in situ IR spectroscopy using a self-supported pellet (10 mg.cm -2 ) of RHO; the transmission IR spectra were recorded with a Nicolet Avatar spectrometer. A room temperature IR-cell equipped with a heating device was used to activate the samples at 350 °C prior to the measurements. The cell was connected to a high vacuum line of 10 -5 Pa. The sample was activated at 100 °C for 0.5 h followed by heating at 350 °C for 3 h under high vacuum. All IR spectra were recorded at room temperature, CO2 cycling experiments were conducted under 1 bar of CO2. IR spectra were recorded on the sample under an atmosphere of equal volume, 1:1 of CO2:CH4 from 0 to 1 bar. The background spectra of the empty transmission cell was recorded under high vacuum at room temperature.

■ RESULTS AND DISCUSSION

A whole profile matching of the PXRD pattern of sample NaCs-RHO(H2O) was performed considering the I-43m space group. The refined unit-cell parameter was a = 14.8118(1) Å. All reflections were indexed confirming a single polycrystalline phase. The structure was solved and the location of the Na + and Cs + cations were identified by ab initio approach (see the experimental section). All the atomic positions and isotropic displacement parameters (Uiso) were refined and summarized in Table S2. Atomic displacement parameters (ADPs) were taken into consideration for the Cs1 atom. The oxygen atoms of the water molecules were localized from Fourier difference maps. The final reliability factors obtained after the last refinement cycle are: RFobs = 4.87% and Rwp = 4.80% (see Figure 1 for the corresponding PXRD fit). The average size of the diffracting domains of sample NaCs-RHO(H2O) calculated from the Rietveld refinement is 57(1) nm. The size and morphology of the crystals were further studied by TEM (see below). The chemical composition of the NaCs-RHO(H2O) sample determined by ICP analysis, EDS, and TGA are similar: Na13.4Cs6.0Si28.5Al19.5O96 36H2O (ICP/TGA), and Na13.7Cs6.5Si27.6Al20.4O96 36H2O (EDS/TGA). The slight differences in the chemical composition is attributed to the different sensitivity of each technique. The results from the Rietveld refinements show the preferential site of the Cs + cations is in the center of the D8R windows, while Na + and water molecules are located in the lta cages, outside the single six-ring (S6R) window site (Figure 1b). The ability of the Cs + to move along the "8-fold"axis of the D8R is observed. Indeed, the anisotropic displacement parameter is larger along the "8-fold"axis of the D8Rs (U11 = U// = 0.07335 Ų) compared to the perpendicular direction (U22 = U^ = 0.03114 Ų). The structure of the NaCs-RHO(H2O) was refined in the acentric I-43m cubic space group instead of the "ideal" centric Im-3m space group when the elliptical distortion of the D8Rs is taken into account. According to Parise et al. [START_REF] Parise | The structure of cesium-exchanged zeolite-RhO at 293K and 493K determined from high resolution neutron powder data[END_REF] , the elliptical distortion, or ellipticity, of the D8Rs can be evaluated by a parameter Δ, which corresponds to the O1-O2 distance projected on the (001) plane, i.e. the average of the difference between the longest and the shortest O-O distances within the cross-section of the D8R, vide infra. For NaCs-RHO(H2O), with a formula Na13.1Cs5.2(Si29.7Al18.3O96)(H2O)41, Δ is 1.87 Å. This value is slightly larger than the one reported by Baur Δ = 1.57 Å. [START_REF] Baur | Neutron powder diffraction study of a Na, Cs-Rho zeolite[END_REF] According to Corbin et al. [START_REF] Corbin | Flexibility of the zeolite RHO framework: in situ x-ray and neutron powder structural characterization of divalent cationexchanged zeolite RHO[END_REF] , the degree of distortion is related to the charge density of the cations located within the D8R windows. In this work, for NaCs-RHO(H2O) the occupancy factor of the Cs + cations sitting in the center of the D8R window site is larger than the one reported by Baur in agreement with a lower Si/Al ratio. [START_REF] Baur | Neutron powder diffraction study of a Na, Cs-Rho zeolite[END_REF] These extra-framework charge-compensating cations influence the distortion of the D8Rs, which is similar to the effect of temperature, ion-exchange or adsorption of different molecules in RHO-type zeolites. On the other hand, it has been shown that the high water content of the NaCs-RHO(H2O) sample is due to the higher Si/Al ratio which can also influence the RHO framework, i.e. inducing a degree of flexibility. Similarly, the flexible structure of MER-type has been recently reported. The authors showed that the zeolite structure tunes itself to coordinate extra framework cations when dehydrated or upon CO2 uptake. [START_REF] Georgieva | Triggered Gate Opening and Breathing Effects during Selective CO2 Adsorption by Merlinoite Zeolite[END_REF] In our case, the evolution of the unit cell parameter of NaCs-RHO(H2O) at room temperature under N2 flow was determined from the XRD data. It varies from 14.810 to 14.778 Å with a decrease in the unit cell volume from 3248.5 to 3227.2 Å 3 after 10 h of continuous exposure to N2 (Figure S1). The thermal stability of the as-synthesized NaCs-RHO(H2O) sample is studied. The PXRD patterns of as synthesized and calcined sample at 550 °C and 650 °C are shown in Figure 2a. Bragg peaks corresponding to the RHO-type framework only were present in all patterns of the samples, as-prepared and after the calcination (Figure 2). Partial amorphization of the NaCs-RHO(H2O) sample after treatment at 650 °C was observed; approximately 60% of the crystallinity was retained as determined from the XRD patterns obtained after calcination at 650 °C. Ionexchange with NH4Cl followed by thermal treatment at 400 °C resulted in the collapse of the structure, demonstrating that the presence of the Cs + cations improves the thermal stability of the zeolite (Figure S2). The distortion of the NaCs-RHO(H2O) zeolite was further investigated by NMR. 29 Si and [START_REF] Georgieva | Triggered Gate Opening and Breathing Effects during Selective CO2 Adsorption by Merlinoite Zeolite[END_REF] Al MAS NMR spectra of the sample NaCs-RHO(H2O) before and after calcination at 550 °C were found to be identical, confirming no removal of Al from the zeolite framework (Figure 2). Single peaks corresponding to tetrahedrally coordinated Al were observed at 60 ppm, and no peak at 0 ppm corresponding to octahedrally coordinated Al was present (Figure 2c). The Q 0 (4Al), Q 1 (3Al), Q 2 (2Al), Q 3 (1Al) and Q 4 (0Al) species at -84 ppm, -88 ppm, -92 ppm, -98 ppm and -102 ppm, respectively are present in the spectrum of NaCs-RHO(H2O) (Figure 2c). Using the 29 Si spectra, the Si/Al ratio of the as synthesized NaCs-RHO(H2O) zeolite is calculated to be 1.50. Additional EDS and ICP analyses confirmed the Si/Al ratio of the sample to be in the range [START_REF] Georgieva | Triggered Gate Opening and Breathing Effects during Selective CO2 Adsorption by Merlinoite Zeolite[END_REF] Al NMR and (c) 29 Si NMR NMR spectra of assynthesized NaCs-RHO(H2O) (black) and after calcination at 550 °C (red). of 1.4-1.5. TEM images reveal uniform nanosized crystals with a size of less than 100 nm in diameter (Figure 3). The as synthesized RHO nanocrystals have low microporosity due to the presence cations (Na + , Cs + ) that block access of N2 to the micropores (Figure S3). However, the comparatively high total pore volume is attributed to the interparticle mesoporosity. Nanosized crystals with a uniform size distribution and a high external surface area form regular interparticle mesopores when dried.

Flexibility of Nanosized RHO Zeolite upon Dehydration.

The flexibility of the nanosized RHO zeolite upon hydration/dehydration was studied using in situ XRD. The XRD patterns of sample NaCs-RHO(H2O) subjected to heating from 30 to 300 °C and subsequent cooling to 30 °C were collected in air and under N2 flow (Figure 4). Overall, fully reversible dehydration was observed for the NaCs-RHO(H2O) sample; a splitting of the peak at 8.45° 2θ under heating of the sample at 180 °C was measured, while during the cooling to 120 °C the splitting disappeared. The dehydration of the nanosized RHO-type zeolite is accompanied by an enlargement of the cubic unit cell and a transformation of the RHO structure from the distorted d8r units (I-43m acentric space group) to the more regular d8r units (Im-3m centrosymmetric space group). This phenomenon was observed previously for RHO-type zeolites subjected to very high pressure [START_REF] Lee | Phase transition of zeolite RHO at high-pressure[END_REF] or during the adsorption of CO2. [START_REF] Lozinska | Cation gating and relocation during the highly selective "trapdoor" adsorption of CO2 on univalent cation forms of zeolite rho[END_REF] The enlargement of the unit cell can be seen clearly in the PXRD patterns, i.e. the (101) reflection at 8.5° 2θ shifts towards higher degrees 2θ. In the case of the sample NaCs-RHO(H2O), the (101) reflection splits in two when the sample is heated above 120 °C (Figure 4). The splitting of the reflection remains present up to 300 °C. A complementary PXRD experiment performed until 440 °C showed that there is no evolution of the position and intensity of the split (101) reflection under heating from 270 to 440 °C (Figure S4). Initially, it was suspected that water was retained in the NaCs-RHO(H2O) zeolite sample up to 300 °C. However, the TGA results showed complete dehydration of the sample above 300 °C (Figure S5). Alternatively, it was considered that the splitting of the reflections could be due to a lowering of the symmetry. However, the diffraction peaks above 120 °C can only be indexed by two RHO cubic unit cells with To investigate this unexpected and completely reversible behavior, the structural analysis of the NaCs-RHO samples was conducted by assuming the existence of two phases (NaCs-RHO-1 and NaCs-RHO-2). The coexistence of two structural phases has been observed previously for NaCs-, K-and Cs-RHO in response to changes in applied the pressure of CO2. [START_REF] Palomino | Zeolite Rho: a highly selective adsorbent for CO2/CH4 separation induced by a structural phase modification[END_REF][START_REF] Lozinska | Cation gating and relocation during the highly selective "trapdoor" adsorption of CO2 on univalent cation forms of zeolite rho[END_REF] According to Parise et al. [START_REF] Parise | The structure of cesium-exchanged zeolite-RhO at 293K and 493K determined from high resolution neutron powder data[END_REF] the decrease of the unit cell parameter corresponds to an increase in the elliptical distortion of the D8Rs and the structure retaining the acentric I-43m symmetry. Conversely, with the enlargement of the unit cell, the elliptical distortion of the D8Rs decreases and the structure acquires the centric Im-3m symmetry. With these considerations, both structures were first solved and refined, considering the space groups Im-3m for the phase with the largest unit cell (NaCs-RHO-1), and I-43m for the phase with the smallest unit cell (NaCs-RHO-2). The obtained figures of merit (RFobs-1 = 6.38%; RFobs-2 = 6.32 % and Rwp = 5.37%) were further improved by considering the I-43m space group for both phases leading to the final result with: RFobs-1 = 4.39%; RFobs-2 = 4.31% and Rwp = 4.39%. The two different RHO frameworks related to NaCs-RHO-1 and NaCs-RHO-2 are shown in Figure 5. The chemical formulas determined from the Rietveld refinements for the phases NaCs-RHO-1 and NaCs-RHO-2 are Na7.4Cs11.0(Si29.6Al18.4)O96 and Na13.5Cs2.6(Si31.9Al16.1)O96 with the mass concentrations of 49(1) and 51(1)%, respectively. An increase of the elliptical distortion of the D8Rs from Δ = 1.87 Å for NaCs-RHO(H2O) to Δ = 2.16 Å for NaCs-RHO-2 is observed (Figure 6). This is comparable to the value calculated for pure Na-RHO reported earlier (Δ = 2.10 Å with a unit cell parameter of a = 14.4139(2) Å). [START_REF] Corbin | Flexibility of the zeolite RHO framework: in situ x-ray and neutron powder structural characterization of divalent cationexchanged zeolite RHO[END_REF][START_REF] Palatinus | SUPERFLIP-a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions[END_REF] The structure of NaCs-RHO-2 is similar to the hydrated NaCs-RHO(H2O) zeolite sample and similar to the dehydrated structure reported in the literature. [START_REF] Lozinska | Understanding carbon dioxide adsorption on univalent cation forms of the flexible zeolite Rho at conditions relevant to carbon capture from flue gases[END_REF] Both the Cs + and Na + cations can occupy the same locations as in the hydrated sample, i.e. inside the D8R window and outside of the S6R window respectively. However, the amount of Cs + in the sample NaCs-RHO-2 is reduced in comparison to the as synthesized hydrated NaCs-RHO(H2O) sample. The occupancy factor of Cs + in the D8R window site decreased from 0.863 to 0.323. A small amount of Cs + (0.080) is also present on the Na1 S6R window site. On the other hand, the unit cell parameter of sample NaCs-RHO-1 increased to 15.1603(3) Å, corresponding to a decrease in the elliptical distortion with Δ = 0.94 Å. To the best of our knowledge this is the largest unit cell parameter reported for the RHO type zeolite structure. The amount of Cs + increases from 5.2 in the as synthesized NaCs-RHO(H2O) to 11.0 in the NaCs-RHO-1 phase. Such a large amount of Cs + cannot stay in the central D8R window site. The Cs + cations move out from the D8R window site to reside in the lta cages, i.e. in front of the D8R. Again, we can notice the large anisotropic displacement of the Cs + cations along the "8-fold" axis of the D8Rs (Figure 5a) (U// = 0.291(6) Å 2 and U^ = 0.070(2) Å 2 ). The ability of the Cs + cations to be displaced in and out of the D8Rs is the basis for the change of the selective adsorption ability of RHO, which is described as "trapdoor" behavior. The opposite displacement was observed for Na + , from outside to inside of the S6R. It appears that all of the Cs + and Na + cations are inside the lta cages, replacing the water molecules, and thus stabilizing the zeolite framework. After rehydration of the sample at room temperature, the PXRD pattern was identical to that of the as-synthesized NaCs-RHO(H2O) nanosized zeolite. It is hypothesized that two RHO phases are present in the as synthesized sample, however, it is difficult to differentiate them due to the presence of water. This means that the structural model we proposed for NaCs-RHO(H2O) actually corresponds to an average structure between two hydrated structures with identical unit cell parameters, but with slightly different chemical compositions, and different Cs + /Na + distributions.

CO2 adsorption study on flexible RHO nanosized zeolite.

The CO2 adsorption process was also followed by in situ TD-PXRD (Figure S6). After dehydration under N2 atmosphere at 350 °C the nanosized zeolite sample was exposed to CO2 at 1 bar at 350 °C. This was followed by a decreasing of the temperature until 30 °C while maintaining the CO2 atmosphere. The PXRD patterns presented in Figure 7a clearly reveal the presence of the two zeolites, NaCs-RHO-1 and NaCs-RHO-2, after dehydration at 350 °C. Then, during the temperature decrease, the splitting of the reflection completely disappears below 110 °C, similar to the behaviour observed for the rehydration process. After cooling the sample to 30 °C under CO2 atmosphere a single NaCs-RHO(CO2) phase is obtained. The structure of NaCs-RHO(CO2) was refined by the Rietveld method from the PXRD pattern recorded at 30 °C, which was recorded immediately after cooling of the sample exposed to CO2 to avoid any moisture contamination. The substructure of the RHO framework was refined and the cationic positions determined in the I-43m space group, with a cubic unit-cell parameter of a = 14.6937(18) Å. However, the CO2 molecules could not be accurately located within the RHO cage. The Fourier difference map clearly revealed residues close to the Na + cations, however the atomic positions of the CO2 molecules could not be satisfactorily refined. The approximate positions of the CO2 molecules were retained in our model to take into account the electronic density present in the RHO cage. Thus, the positions C5, O5 and O6 of our model should be considered as approximate positions, since the C-O distances (dC1-O4 = 1.5626(1) Å, dC1-O5 = 1.2977 (1) Å) are longer than those expected for CO2 (1.16 Å). It is likely that the CO2 molecules are disordered around these positions, close to the Na + cations. The final reliability factors obtained after the last refinement cycle are: RFobs = 3.44% and Rwp = 6.02% (see Figure 7c for the corresponding PXRD fit). All the atomic positions are summarized in Table S1. The elliptical distortion parameter Δ = 1.97 Å of the D8Rs in the NaCs-RHO(CO2) structure has an intermediate value between those of the hydrated NaCs-RHO(H2O) and dehydrated NaCs-RHO-2 (Figure 6). This is in agreement with the unit-cell parameter a = 14.6937 (18) Å. This demonstrates that the flexible behaviour of the RHO framework is retained even during the adsorption of CO2. In the NaCs-RHO(CO2) structure the Cs + and Na + cations retain similar positions to those of the hydrated structure NaCs-RHO(H2O) (Figure 7b), and the cationic composition remains the same, Cs5.2Na13.1Si29.7Al18.3O96(CO2)18.

In addition to the structural refinement in the presence of CO2, the CO2 adsorption behaviour of the nanosized RHO-type zeolite was also investigated further by adsorption analysis, TGA, and in situ FTIR experiments. The CO2 isotherm recorded up to 900 mmHg, corresponding to 1.2 bar, is described with a Langmuir shape until P = 0.4 bar followed by a nearly linear trend up to P = 1.2 bar. At the highest absolute pressure (P = 1.2 bar, 25 °C) the RHO nanosized zeolite adsorbed 1.56 mmol CO2 per gram of zeolite (Figure S7). TGA confirmed that 1.37 mmol of CO2 (at 1 bar) per gram of RHO was adsorbed (Figure S8). The stability of the RHO nanosized zeolite under CO2 adsorption was confirmed by performing 10 consecutive cycles of adsorption and desorption under alternating atmospheres of N2 and CO2 (Figure 8a). In addition to the TGA, the stability of the RHO nanosized zeolite was investigated by in situ FTIR (Figure 8b). Following activation of the sample at 350 °C under vacuum (blue triangle), the adsorption of CO2 was monitored by the integration of the FTIR bands attributed to physi-and chemisorbed CO2. Multiple cycles of activation and adsorption clearly demonstrate that the adsorption capacity is maintained; the band areas (red circles) reached the same level in all cycles. In addition, the adsorption of CO2 appeared fully reversible as the band area after desorption (blue squares) always returned to the initial value (blue triangle). This result confirmed that the adsorption capacity of the RHO nanosized zeolite is preserved over the 10 cycles. Finally, XRD analysis revealed no loss of crystallinity of the sample after the 10 consecutive cycles of CO2 adsorption/desorption (Figure 8c). To investigate the selectivity of the RHO nanosized zeolite towards CO2 over CH4, in situ IR spectroscopy was performed using a mixture of CO2 and CH4 (1/1 in volume) gases (1 bar) delivered to the RHO self-supported pellet. The FTIR spectra collected at different concentrations are presented in Figure S9. The selective CO2 adsorption in the RHO zeolite is confirmed by the presence of the bands at 2650 cm -1 and 1650 cm -1 , corresponding to the physi-and chemisorption CO2 respectively (Figure S9). At the same time, no band in the region 2700-2900 cm -1 corresponding to CH4 adsorption was observed. The rovibrational bands observed centred at 3050 cm -1 are attributed to free CH4 molecules.

■ CONCLUSIONS

Flexible small-pore RHO-type nanosized zeolite has been prepared free of OSDAs with a size of individual particles below 100 nm. The RHO nanosized crystals have a Si/Al atomic ratio of 1.50 and high Cs + content that significantly improved the thermal stability up to 550 °C. Variable temperature in situ PXRD experiments reveal the flexibility of the RHO structure upon dehydration. Two different phases have been refined revealing different behavior upon dehydration according to their chemical compositions and different distributions of Cs + and Na + within the unit cell of RHO. The 8-rings elliptical distortions of the as synthesized RHO (hydrated form) of 1.87 Å changed to 0.94 Å and 2.16 Å to two dehydrated RHO forms. The Rietveld refinements of the RHO zeolite revealed the large anisotropic displacement parameters of the Cs + cations along the 8-fold axis of the D8R. The NaCs-RHO-1 structure shows the ability of the Cs + cation to displace out from and in to the D8R window site. This displacement is the basis of "trapdoor" behavior controlling the selective adsorption capacity of the RHO zeolite. The high stability and selectivity towards CO2 of the RHO-type nanosized zeolites is demonstrated. The integration of the flexible nanosized RHO zeolites in membranes is currently under investigation.

ASSOCIATED CONTENT

Supporting Information. Contains information pertaining to variable temperature PXRD analysis under N2 for 10 h, XRD pattern after ion-exchange and calcination, N2 adsorption analysis, in situ PXRD analysis in air up to 440 °C, TGA/DTG, CO2 adsorption analysis, in situ FTIR, PXRD data and Rietveld refinement details, atomic positions and Uiso-eq parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 1 .

 1 Figure 1. (a) Rietveld refinements from PXRD pattern of the nanosized NaCs-RHO(H2O) zeolite sample. (b) Views of the RHO framework with the specified localization of Cs + , Na + and O atoms of the water molecules. Ellipsoids are drawn at the 50% probability level.

Figure 2 .

 2 Figure 2. (a) PXRD patterns of as synthesized nanosized NaCs-RHO(H2O) (black), and after calcination at 550 °C (red) and at 650 °C (blue); (b)[START_REF] Georgieva | Triggered Gate Opening and Breathing Effects during Selective CO2 Adsorption by Merlinoite Zeolite[END_REF] Al NMR and (c)29 Si NMR NMR spectra of assynthesized NaCs-RHO(H2O) (black) and after calcination at 550 °C (red).

Figure 3 .

 3 Figure 3. TEM images of nanosized RHO-type zeolites at different magnifications. (a) TEM image showing the homogeneous crystal size. (b) and (c) HRTEM images revealing the high crystallinity of the sample, insets show the FFT from the areas encircled in white corresponding to [1-11] and [100] oriented crystals, respectively (SG: I-43m).

Figure 4 .

 4 Figure 4. In situ PXRD patterns of nanosized NaCs-RHO(H2O) zeolite recorded in air upon (a) heating from 30 to 300 °C (dehydration process), and (b) cooling from 300 to 30 °C (rehydration process); (c) Rietveld refinement of the PXRD pattern of nanosized NaCs-RHO zeolite collected at 350 °C under N2 atmosphere. different values for the lattice parameter, a = 15.1603(3) Å and a = 14.5179(5) Å. It appears that two different dehydrated RHO phases are present at high temperature (350 °C), compared to the presence of one structure after rehydration at lower temperature (< 150 °C).To investigate this unexpected and completely reversible behavior, the structural analysis of the NaCs-RHO samples was conducted by assuming the existence of two phases (NaCs-RHO-1 and NaCs-RHO-2). The coexistence of two structural phases has been observed previously for NaCs-, K-and Cs-RHO in response to changes in applied the pressure of CO2.[START_REF] Palomino | Zeolite Rho: a highly selective adsorbent for CO2/CH4 separation induced by a structural phase modification[END_REF][START_REF] Lozinska | Cation gating and relocation during the highly selective "trapdoor" adsorption of CO2 on univalent cation forms of zeolite rho[END_REF] According to Parise et al.[START_REF] Parise | The structure of cesium-exchanged zeolite-RhO at 293K and 493K determined from high resolution neutron powder data[END_REF] the decrease of the unit cell parameter corresponds to an increase in the elliptical distortion of the D8Rs and the structure retaining the acentric I-43m symmetry. Conversely, with the enlargement of the unit cell, the elliptical distortion of the D8Rs decreases and the structure acquires the centric Im-3m symmetry. With these considerations, both structures were first solved and refined, considering the space groups Im-3m for the phase with the largest unit cell (NaCs-RHO-1), and I-43m for the phase with the smallest unit cell (NaCs-RHO-2). The obtained figures of merit (RFobs-1 = 6.38%; RFobs-2 = 6.32 % and Rwp = 5.37%) were further improved by considering the I-43m space group for both phases leading to the final result with: RFobs-1 = 4.39%; RFobs-2 = 4.31% and Rwp = 4.39%. The two different RHO frameworks related to NaCs-RHO-1 and NaCs-RHO-2 are shown in Figure5. The chemical formulas determined from the Rietveld refinements for the phases NaCs-RHO-1 and NaCs-RHO-2 are Na7.4Cs11.0(Si29.6Al18.4)O96 and Na13.5Cs2.6(Si31.9Al16.1)O96 with the mass concentrations of 49(1) and 51(1)%, respectively. An

Figure 5 .

 5 Figure 5. Views of the RHO framework of the two structures (a) NaCs-RHO-1 and (b) NaCs-RHO-2 obtained after dehydration of the as synthesized NaCs-RHO(H2O) nanosized zeolite. Ellipsoids are drawn at the 50% probability level.

Figure 6 .

 6 Figure 6. 8-rings elliptical distortions from as synthesized NaCs-RHO(H2O) nanosized zeolite to the dehydrated forms NaCs-RHO-1 and NaCs-RHO-2 and the CO2-sorbed nanozeolite NaCs-RHO(CO2). The Δ parameter is the average of the difference between the two O-O distances delineated by dotted lines.

Figure 7 .

 7 Figure 7. CO2 adsorption process in the nanosized zeolite NaCs-RHO followed in situ by TD-PXRD measurements. (a) PXRD patterns of nanosized NaCs-RHO1-2 zeolites recorded in CO2 atmosphere at 1 bar upon cooling. (b) Views of the RHO framework of the NaCs-RHO(CO2) structure at room temperature after CO2 adsorption. Ellipsoids are drawn at the 50% probability level. (c) Rietveld refinement of the PXRD pattern of nanosized NaCs-RHO(CO2) zeolite collected at 30 °C under CO2 atmosphere.

Figure 8 .

 8 Figure 8. (a) TGA of NaCs-RHO under alternating atmospheres of N2 (350 °C) and CO2 (25 °C), (b) integrated band areas of physiand chemisorbed CO2 measured by in situ FTIR of NaCs-RHO under alternating vacuum and CO2 atmosphere (1 bar), activation (blue triangle), adsorption (red circle), desorption (blue square), (c) XRD patterns of NaCs-RHO(H2O) before and after TG cycling.

ACKNOWLEDGMENT

Financial support from CARNOT ESP, TOTAL and Industrial Chair ANR-TOTAL "Nanoclean Energy" is acknowledged, as well as from the Normandy Region through the RIN Recherche Program.

AUTHOR INFORMATION

Corresponding Authors nicolas.barrier@ensicaen.fr svetlana.mintova@ensicaen.fr

Notes

The authors declare no competing financial interest.


Rimeh Ismail

Jihene Arfaoui

Zouhaier Ksibi

Abdelhamid Ghorbel

Gérard Delahay

Keywords: Fe-ZrO 2 aerogel catalysts, Fe quantity, Toluene Oxidation

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Effect of the iron amount on the physicochemical properties of Fe-ZrO2 aerogel catalysts for the total oxidation of Toluene in the presence of water vapor

Rimeh Ismail, Jihene Arfaoui, Zouhaier Ksibi, Abdelhamid Ghorbel,

The emissions of volatile organic compounds (VOCs), such as toluene, aldehydes, alcohols, ketones, ethers, halogenated and aromatic compounds, are considered as one of the main sources of air pollution 1 . These substances can be issued from petroleum refineries; automotive, textile manufacturers, transportation, chemical industries and solvents using processes 2 .Their high concentration in the air may cause a serious human health and environmental problems. Therefore, it is necessary to minimize their emissions. Catalytic oxidation is generally used in many industrial processes for the control of VOCs emissions through their selective destruction at low temperature. Two groups of catalytic materials are usually employed for VOCs oxidation: (i) supported or unsupported noble metal catalysts (mostly Pt and Pd), and (ii) transition metal oxide-based catalysts. Noting that the noble metals are characterized by a good stability and high activity at low temperatures 3. Nevertheless, their high cost has limited their investigation and they have been replaced by transition metal oxide-based catalysts, such as copper oxides 4 , cobalt oxides 5 , iron oxides 6 , manganese oxides 7 and cerium oxides 8 . These systems have been extensively explored during the past years as effective and more economic catalysts than noble metal ones for the total oxidation of VOCs 9 . Only a few papers have been reported in the literature concerning the total oxidation of toluene, particularly in the presence of water vapor, over iron-based catalysts. In this framework: P. Djinović et al. 10 have showed that the Fe quantity greatly influences the catalytic performances of bimetallic CuFe catalysts in the oxidation of toluene when the reaction was realized in the absence of H 2 O. On the other hand, X. Liang et al. 11 have studied the effect of Fe amount and the presence of vapor H 2 O on the catalytic performance of Fe-Ti-PILC in the total oxidation of toluene. Their results revealed that the catalytic activity of the samples enhanced with increasing Fe content up to a value of 3.75%. The authors have also concluded that the prepared Fe-Ti-PILC catalysts showed high stability and superior activity at 250 and 350 °C in the presence of 50,000 ppm water vapor. In this work, a new xFe-ZrO 2 aerogel catalyst was developed, via the one step sol-gel method associated to supercritical drying process, for the total oxidation of toluene in the presence of water vapor. The effect of Fe quantity (x = 0; 1,25; 2,5, 5 and 10 wt.%) in the physicochemical properties and catalytic performances in the C 6 H 5 -CH 3 oxidation of aerogel materials was investigated.

ZrO 2 and xFe-ZrO 2 aerogel samples were synthesized via the single step sol-gel method using a similar procedure to that reported in our previous work for TiO 2 12 as following: First, Zirconium (IV) butoxide (Zr(OC 4 H 9 ) 4 , Sigma-aldrich, 80 %) was mixed, under stirring at room temperature, with anhydrous ethanol (C 2 H 6 O, Aldrich, 99.8 %) and Ethyl acetoacetate (C 6 H 10 O 3 , Fluka, > 99.5%). Then, the hydrolysis is favoured by addition of a HNO 3 aqueous solution (0.1 M). Etacac /Zr and H 2 O/Zr molar ratios were fixed to 1 and 10, respectively. The prepared gel was dried in the autoclave (at T = 243 °C and P = 63 bar) to obtain ZrO 2 support. For the xFe-ZrO 2 solids, an appropriate amount of iron (III) nitrate (Fe(NO 3 ) 3 , 6H 2 O), Sigma-aldrich, ≥ 99.95 %) was introduced in the mixture before the hydrolysis step. The ZrO 2 and x Fe-ZrO 2 aerogel samples were finally calcined at 550 °C for 3 h under O 2 flow (30 cc/min).

Toluene oxidation was carried out at atmospheric pressure in a fixed bed reactor using 0.02 g of catalyst, previously pretreated in situ (under air) at 200 °C for 1 h. The toluene concentration was adjusted by passing an appropriate He flow through a saturator, kept at 0.7 °C, containing toluene. The reaction was performed using a mixture of gas (total flow rate = 50 cm 3 /min) containing 0.01 vol % toluene (> 99.7 %), 3.4 vol % H 2 O (> 99.5 %), 8 vol. % O 2 ( > 99.9 %) and balance with He (> 99.9%) using a second gas line. The equivalent gas hourly space velocity (GHSV) obtained was of 60,000 h -1 . The catalytic activity was evaluated in terms of toluene conversion to CO 2 , in the 200-550 °C temperature range, with a ramp of 6 °C/min. The composition of the effluents was monitored continuously by sampling on line to a quadrupole mass spectrometer Pfeiffer Vacuum equipped with Faraday and SEM detectors (0-200 amu).

The toluene conversion (%) is expressed as:

X tol (%) =
Where [Toluene] in and [Toluene] out are the inlet and outlet concentrations of toluene, respectively.

Effect of the amount of iron on the physicochemical properties of new Fe-ZrO 2 aerogel catalysts for the total oxidation of Toluene in the presence of water

The concentration of CO 2 (ppm) was calculated as given below:

[CO 2 ] (ppm) =
Where [I] 0 , [I] 7000 and [I] t are the intensity of CO 2 signals at inlet, outlet and at any reaction temperature (t), respectively. 7000 ppm is the inlet concentration of CO 2 .

Textural properties of samples were examined by N 2physisorption, at 77 K, using a Micromeritics ASAP 2000. The samples were degassed at 250 °C before analysis. XRD patterns of solids were recorded from 2θ = 10 ° to 80 ° (with a step size of 0.02°) using a Brüker AXS D8 diffractometer with CuKα radiation (λ = 1.5406 Ǻ). UV-vis spectra were collected, in the 200-800 nm wavelength range, using a spectrometer Lambda 40 Perkin Elmer. Temperature programmed reduction was realized, in the 50-550 °C temperature range, using an AUTOCHEM 2920 (Micromeritics), with a mixture containing 5 vol. % H 2 in Ar (30 mL/min) and a heating rate of 10 °C/min. Prior to H 2 -TPR analysis, the catalyst (0.08 g) was pre-treated under 5 vol. % O 2 in He (flow rate = 30 mL/min) at 550 °C (ramp 10 °C/min) for 30 min, then cooled down to 50 °C. Acidity of catalysts was studied by NH 3 -TPD using an AUTOCHEM 2910 (Micromeritics). First, the solid was pre-treated at 550 °C (ramp 10 °C/min), during 2 h, under air flow (30 cm 3 /min). After, it was exposed to 5 vol. % NH 3 in He (flow rate = 30 cm 3 /min) for 30 min and then flushed with He (30 cm 3 /min) during 30 min in order to remove the physisorbed ammonia. Finally, The NH 3 desorption was done in helium flow (30 cm 3 /min) from 100 to 550 °C with a heating rate of 10 °C/min.

The C 6 H 5 -CH 3 conversion and CO 2 concentration as function of the reaction temperature over xFe-ZrO 2 aerogel catalysts are displayed in Figure . 1A and 1B, respectively.

Based on the Mars-Van Krevelen (MVK) mechanism, which has been widely used for describing the catalytic oxidation of VOC over metal oxides, [13][14] the reaction procedure includes two independent reactions steps: the oxidation of VOC molecules to CO 2 and H 2 O by surface oxygen species on catalyst (surface lattice oxygen and surface absorbed oxygen), and the reaction between the reduced sites and oxygen from gas phase to form surface oxygen species. Therefore, the surface oxygen species and the reducibility of catalyst (that is related to the release of surface oxygen species) are the key factors which govern its reactivity in the total oxidation of COVs. The surface acidity of catalysts is known to play also an important role in this reaction, as the presence of acid sites could facilitate both the sorption of COV at the catalyst surface and the breaking of carbon-carbon bonds which promotes the CO 2 production 11,15 As it can be seen in Fig 1A and1B, ZrO 2 support exhibits low C 6 H 5 -CH 3 conversion to CO 2 (50 % at 550 °C) which can be related to the low reactivity of its acidic sites in this reaction. When Fe species are introduced on the catalyst, the toluene conversion to CO 2 increases with both the reaction temperature and Fe content (until 5 % wt. Fe). This underlines the key role of iron species in the total oxidation of toluene. In fact, these species contribute to the growing of surface oxygen species and lead to the creation of new acidic and redox sites at Fe-ZrO 2 surface. This promotes the adsorption of toluene molecule and increases the active site amount. Similar results have been already reported by X. Liang et al. 11 who have demonstrated that the activity of Fe-Ti-PILCs solids, in the total oxidation of toluene (tested in the 250-350 °C temperature range), was initially enhanced with increasing Fe content up to a value of 3.75 %. Interestingly, a maximum of 96 % C 6 H 5 -CH 3 conversion at 550 °C was obtained over the novel 5Fe-ZrO 2 aerogel catalyst demonstrating its good stability and superior activity at high temperature (>450 °C) in the presence of 3.4 % water vapor. In general, the increase of specific surface area results in the enhancement of catalytic activity. Textural properties of solids are given in table 1. As it can be seen, all the samples calcined at 550 °C exhibit high specific area (³ 93 m 2 /g) and large pore volume (³ 0.26 cm 3 /g), indicating of their developed texture and good thermal stability. It should be that the surface area of xFe-ZrO 2 catalysts slightly decreases with the increase of iron content, but remains above 90 m 2 /g. The loss of the surface area can be explained by the blocking of some pores of the zirconia support by the iron oxide crystallites as explained earlier 16 .

Based on the N 2 adsorption-desorption isotherms at 77 K and pore size distribution curves of xFe-ZrO 2 aerogel catalysts, presented in Figures 2 and3, respectively, it can be concluded that all the samples, which are characterized by a type IV isotherms according to IUPAC classification and a hysteresis loops type H1, are mesoporous materials with a monomodal (case of ZrO 2 , 1,25Fe-ZrO 2 , 2,5Fe-ZrO 2 and 10 Fe-ZrO 2 ) or bimodal (for 5Fe-ZrO 2 ) pore size distributions. It is worthy to note that the pores of solids become larger by increasing the Fe loading (Figure . 3). This result can be explained by the existence of strong interactions between zirconia and iron which affect the porosity of catalysts 12 . of zirconia. It should be also mentioned that the XRD analysis did not reveal the presence of characteristic peaks of metallic Fe or its oxides, which is in good agreement with a previous study 17 and indicates the high dispersion of iron species at zirconia surface. DR UV-vis spectroscopy (Figure 5) is used to examine the nature and environment of Zr and Fe species. The ZrO 2 shows a typical absorption band in 200-250 nm range, which is attributed to O 2-→Zr 4+ charge transfer transition in zirconia 18 . The new bands observed at higher wavelengths in the UV-vis spectra of xFe-ZrO 2 solids correspond to iron species. Hence, the bands situated at λ< 300 nm can be attributed to Fe species in tetrahedral and octahedral coordinations 19 . Fe x O y aggregates or clusters display an absorption band at 300 <λ<400 nm 2 , whereas, the band located at λ> 400 nm indicates the presence of aggregates of aFe 2 O 3 iron oxide 2 .

It should be noted that the aFe 2 O 3 aggregates are obtained only in the case of 10Fe-ZrO 2 catalyst which demonstrates that the increase of the iron amount influences the nature of Fe species and consequently affects the catalytic performances of xFe-ZrO 2 in the total oxidation of toluene as confirmed by the catalytic test results. It is well established that the catalytic activity of metal oxide catalysts strongly depends on their acidic and redox properties [20][21] . Therefore, the reducibility and surface acidity of xFe-ZrO 2 were studied using H 2 -TPR and NH 3 -TPD techniques. The results are illustrated in Fig. 6 and Fig. 7, respectively.

As shown in the H 2 -TPR profiles of samples (Figure . 6), no H 2 consumption peaks are detected for ZrO 2 support since Zr 4+ is a non-reducible cation 22 . However, for the xFe-ZrO 2 solids, new peaks are appeared at different temperatures and are attributed to the reduction of various iron species. Therefore, the peaks at low temperatures (between 90 and 450 °C) can be ascribed to the reduction of Fe 2 O 3 to Fe 3 O 4 and the peaks at higher temperatures (> 450 °C) can be attributed to Fe 3 O 4 → FeO→ Fe 0 transformations 23 . This demonstrates that the addition of Fe contributes to the creation of redox sites at xFe-ZrO 2 surface. These sites were found active in the total oxidation of toluene and their nature and reactivity seem to be are affected by the amount of Fe. It can be also suggested, in line with the UV-vis results, and, that the amount of iron influences the nature, the dispersion and/or the oxidation state of iron species seeing that the position, the number and the intensity of H 2 consumption peaks change with increasing the iron quantity. In Fig. 7., ZrO 2 and xFe-ZrO 2 samples show a broad NH 3 desorption peak between 100 and 450 °C which can be attributed to NH 3 desorbed from weak (at T ≈ 200 ° C ) and medium ( at T ≈ 300-400 ° C) acid sites 24 . It should be underlined that the intensity of this peak is higher in case of the samples containing iron indicating the creation of new acidic sites at ZrO 2 surface by the Fe addition. It is known that acid sites provide protons or accepting electron pairs from the toluene molecule which is crucial for its further oxidation. In this work, new well-structured xFe-ZrO 2 aerogel catalysts with high surface area, large porosity and good thermal stability were successfully synthesized by the one step sol-gel method and tested in the total oxidation of toluene in the presence of H 2 O. It was found that the addition of iron affects the structural and textural properties of xFe-ZrO 2 catalysts, contributing to the creation of actives acidic and redox sites at their surfaces. This addition consequently enhances the activity of the total oxidation of toluene. Interestingly, high conversion of C 6 H 5 -CH 3 to CO 2 (96 % at 550 °C), in the presence of water vapor, was obtained over the novel 5Fe-ZrO 2 aerogel catalyst.

Figure 1 .

 1 Figure 1. (A) Toluene conversion and (B) CO2 concentration (ppm) as function of temperature reaction over the samples.

Figure 2 .

 2 Figure 2. N2 adsorption-desorption isotherms of samples.

Figure 3 .

 3 Figure 3. BJH pore size distribution curves of samples. The XRD analysis (Figure 4) reveals that both the diffraction peaks related to monoclinic and tetragonal phases of ZrO 2 (m-ZrO 2 and t-ZrO 2 ) are detected at 2θ = 24,38° (110), 28,26° (-111), 31,56° (111) , 41,07° (102) , 45,21° (-202), 54,09° (300), 55,60 (130), 65,68° (320) [PDF 89-9066] and 2θ = 30,33° (101) , 34,67° (002) , 50,37° (112), 60,09° (211), 62,57° (202) [PDF 79-1769], respectively, in the X-ray diffraction patterns of ZrO 2 . However, only the typical peaks of the t-ZrO 2 phase are observed in the case of xFe-ZrO 2 aerogel catalysts. This result, demonstrating a stabilization of ZrO 2 tetragonal phase, known by its metastability at low temperature, could indicate the existence of strong interactions between Fe and ZrO 2 which modify the structure

Figure 4 .

 4 Figure 4. XRD patterns of samples.

Figure 5 .

 5 Figure 5. DRUV-Vis spectra of samples.

Figure 6 .

 6 Figure 6. H2-TPR profiles of samples.

Figure 7 .

 7 Figure 7. NH3-TPD profiles of samples.

Table 1 .

 1 Textural properties of the samples