Mohamad El-Roz

Igor Telegeiev

Natalia E Mordvinova

Oleg I Lebedev

Nicolas Barrier

Asma Behlil

Moussa Zaarour

Louwanda Lakiss

Valentin Valtchev

Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity Under Visible Light

Keywords: Sub-nanoparticles, Silver clusters, Zeolite, Photocatalysis, Formic acid reforming, H2

Sub-nanometer silver clusters that exhibit discrete electronic structure with molecular-like properties are highly desirable in various technologies. However, the methods for their preparation suffer from limitations related with the reproducibility and particles uniformity, and/or the possibility of the scaling-up. Another critical drawback is that free sub-nanometer silver clusters tend to aggregate into larger particles. In this work, a new approach that successfully overcomes the above limitations is developed. It allows, for the first time, an ultrafast preparation of sub-nanometer silver particles with high abundance, uniformity (7 Å) and stability into the cages of nanosized zeolite crystals. The new method consists in UV excitation of a water suspension of nano-zeolite containing photoactive vanadate clusters in the presence of ethanol (as an electron donor) and silver precursor. The characteristic features of sub-nanometer silver particles are presented and the mechanism of their formation is discussed. Sub-nanometer Ag clusters exhibit exceptional photocatalytic activity and selectivity in the reforming of formic acid to H2 and CO2 under visible light.

Introduction

It is well known that the properties of sub-nanometer silver clusters differ strongly from those of the bulk materials and conventional nanoparticles. [START_REF] Rycenga | Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications[END_REF][START_REF] Xu | Size-Dependent Catalytic Activity of Supported Metal Clusters[END_REF][START_REF] Corma | Exceptional Oxidation Activity with Size-Controlled Supported Gold Clusters of Low Atomicity[END_REF][START_REF] Desireddy | Ultrastable Silver Nanoparticles[END_REF] Silver nanoparticles exhibit a significant surface plasmon resonance absorption due to a coherent oscillation of the conduction band electrons induced by light (electro-magnetic field). [START_REF] Cuong | Theoretical Modeling of Optical Properties of Ag8 and Ag14 Silver Clusters Embedded in an LTA Sodalite Zeolite Cavity[END_REF] In contrast, sub-nanometeric silver clusters (<1 nm) show a discrete electronic structure with molecular-like properties (HOMO-LUMO gap). Thus, size-dependent fluorescence is often observed in the tiny metal clusters. [START_REF] Corma | Exceptional Oxidation Activity with Size-Controlled Supported Gold Clusters of Low Atomicity[END_REF][START_REF] Shang | Silver Nanocluster-Based Fluorescent Sensors for Sensitive Detection of Cu(Ii)[END_REF] Their unique electronic and optical properties render them potential candidates for application in various fields ranging from heterogeneous catalysis to biomedical application. [START_REF] Shang | Silver Nanocluster-Based Fluorescent Sensors for Sensitive Detection of Cu(Ii)[END_REF][START_REF] De Cremer | Optical Encoding of Silver Zeolite Microcarriers[END_REF][START_REF] Wang | Fluorescent Gold Nanoclusters as a Biocompatible Marker for In Vitro and In Vivo Tracking of Endothelial Cells[END_REF][START_REF] Gobin | Near-Infrared Resonant Nanoshells for Combined Optical Imaging and Photothermal Cancer Therapy[END_REF][START_REF] Royon | Silver Clusters Embedded in Glass as a Perennial High Capacity Optical Recording Medium[END_REF][START_REF] Guo | Fluorescent Ag Clusters via a Protein-Directed Approach as a Hg(II) Ion Sensor[END_REF] Metallic nanoparticles with size between 1-12 nm are widely used in catalysis. The control of their preparation at an atomic scale is crucial for tuning their properties. There is a continuous quest for developing highly efficient methods of size-selected metal clusters synthesis and their integration in appropriate solid carriers. However, the instability of the clusters due to their tendency to aggregate irreversibly into larger nanoparticles is a recurrent problem. The use of confining scaffolds, such as the regular system of cages in crystalline zeolite-type material, could be an efficient solution to overcome this problem. Silver-zeolite composites are often employed as catalysts, [START_REF] Patterson | Nanoclusters of Silver Doped in Zeolites as Photocatalysts[END_REF] adsorbents, [START_REF] Hutson | Mixed Cation Zeolites: LiAg-X as a Superior Adsorbent for Air Separation[END_REF] molecular sieves, [START_REF] Seoung | Irreversible Xenon Insertion into a Small-Pore Zeolite at Moderate Pressures and Temperatures[END_REF] and recently as highly luminescent materials. [START_REF] Coutino-Gonzalez | Determination and Optimization of the Luminescence External Quantum Efficiency of Silver-Clusters Zeolite Composites[END_REF][START_REF] Coutino-Gonzalez | Thermally Activated LTA(Li)-Ag Zeolites with Water-Responsive Photoluminescence Properties[END_REF][START_REF] Fenwick | Tuning the Energetics and Tailoring the Optical Properties of Silver Clusters Confined in Zeolites[END_REF][START_REF] Altantzis | Direct Observation of Luminescent Silver Clusters Confined in Faujasite Zeolites[END_REF][START_REF] Weckhuysen | Porous Materials: Zeolites Shine Bright[END_REF] The usual wet-chemistry method for preparing supported metal clusters involves anchoring of well-defined precursors to a suitable support. [START_REF] Jia | Colloidal Metal Nanoparticles as a Component of Designed Catalyst[END_REF][START_REF] Ledo-Suárez | Facile Synthesis of Stable Subnanosized Silver Clusters in Microemulsions[END_REF] The ligands are then removed by postsynthesis treatments, which usually results in clusters of high polydispersity. [START_REF] Turner | Selective Oxidation with Dioxygen by Gold Nanoparticle Catalysts Derived from 55-Atom Clusters[END_REF][START_REF] Liu | Efficient and Selective Epoxidation of Styrene with TBHP Catalyzed by Au25clusters on Hydroxyapatite[END_REF][START_REF] Xie | Enhancement in Aerobic Alcohol Oxidation Catalysis of Au 25 Clusters by Single Pd Atom Doping[END_REF][START_REF] Zou | Controlled Growth of Silver Nanoparticles in a Hydrothermal Process[END_REF][START_REF] Wu | One-Step Hydrothermal Synthesis of Silver Nanoparticles Loaded on N-Doped Carbon and Application for Catalytic Reduction of 4-Nitrophenol[END_REF] Soft landing of monodisperse metal clusters grown in a gas phase and sorted in size by mass spectrometry is an efficient method, however, it requires sophisticated equipment and the scaling up of the process is difficult. [START_REF] Lee | Selective Propene Epoxidation on Immobilized Au 6-10 Clusters: The Effect of Hydrogen and Water on Activity and Selectivity[END_REF][START_REF] Schröder | Ruthenium Nanoparticles inside Porous [ Zn 4 O ( Bdc ) 3 ] by Hydrogenolysis of Adsorbed [ Ru ( Cod )( Cot )]: A Solid-State Reference System for Surfactant-Stabilized Ruthenium Colloids[END_REF] Here, we report the formation of sub-nanometer silver clusters with an enhanced narrow size distribution using uniform in size cages of FAU-type zeolite as a scaffold. A suspension of nanosized zeolite Na-X (FAU-type), denoted as ZX, was employed in this study. The extra small zeolite nanocrystals (10-30 nm) were synthesized without organic template. The small size of zeolite scaffold is indispensable for the uniform incorporation and stabilization of the photoactive sites during the insitu synthesis as well as for the silver reduction, which requires a rapid ionic exchange ability. Sub-nanometer silver clusters in ZX (Ag@ZX-V) samples were prepared in two consecutive steps. The first step involves in-situ incorporation of vanadate clusters (ZX-V), which are introduced to initiate the formation of Agm d+ (with m>d) sub-nanometer clusters in zeolite cages. [START_REF] El-Roz | Ultra-Fast Framework Stabilization of Ge-Rich Zeolites by Low-Temperature Plasma Treatment[END_REF] The second step comprises photocatalytic reduction of silver ions using a suspension of ZX-V to form Ag@ZX-V (SI-Section 2, Figure S1-S6). Ag@ZX-V(HT) prepared by a conventional hydrothermal method [START_REF] Wang | Silver-Nanoparticles-Modified Biomaterial Surface Resistant to Staphylococcus: New Insight into the Antimicrobial Action of Silver[END_REF] is used as a reference. The main objective of this study is to selectively generate uniform sub-nanometer silver clusters that exhibit photocatalytic activity under visible light and can be used as a catalyst for hydrogen production.

Results and discussion

The evolution of the absorbance spectra of AgNO3/ZX-V suspensions as a function of exposure time to UV irradiation is reported in Figure 1. For comparison, the UV-visible spectrum of ZX suspension prepared under the same conditions and subjected to a longer irradiation time is also included. There is no reduction of silver in vanadium-free zeolite (ZX) suspension, while new UV-visible bands at 275, 300, 325, 371 and 414 nm appear after 30 s of UV irradiations of the vanadium-containing sample (ZX-V). The intensities of these bands increase with the irradiation time. In addition new bands at 432 and 562 nm attributed to the formation of silver nanoparticles with a particle size >12 nm and >20 nm, respectively, appear in the spectrum after 30 seconds of treatment. [START_REF] Cuong | Theoretical Modeling of Optical Properties of Ag8 and Ag14 Silver Clusters Embedded in an LTA Sodalite Zeolite Cavity[END_REF][START_REF] Zou | Controlled Growth of Silver Nanoparticles in a Hydrothermal Process[END_REF][START_REF] Gurin | Metal Clusters and Nanoparticles Assembled in Zeolites: An Example of Stable Materials with Controllable Particle Size[END_REF][START_REF] Elemike | Green Synthesis of Ag/Ag2O Nanoparticles Using Aqueous Leaf Extract of Eupatorium Odoratum and Its Antimicrobial and Mosquito Larvicidal Activities[END_REF][START_REF] Mock | Shape Effects in Plasmon Resonance of Individual Colloidal Silver Nanoparticles[END_REF][START_REF] Severance | Evolution of Silver Nanoparticles within an Aqueous Dispersion of Nanosized Zeolite Y: Mechanism and Applications[END_REF] The later bands originate from the formation of silver nanoparticles on the external surface of some of zeolite crystals (Figure S7). However, the major part of these particles is eliminated by a simple washing as demonstrate the UV-visible spectra of the washed samples (Figure S8). The bands between 275 and 380 nm are characteristic of the electronic transitions of reduced Ag species (Agm d+ ) with sub-nanometer particles size. [START_REF] Mulvaney | Long-Lived Nonmetallic Silver Clusters in Aqueous Solution: A Pulse Radiolysis Study of Their Formation[END_REF][START_REF] Treguer | Fluorescent Silver Oligomeric Clusters and Colloidal Particles[END_REF][START_REF] Loginov | Photoabsorption of AgN(N-6-6000) Nanoclusters Formed in Helium Droplets: Transition from Compact to Multicenter Aggregation[END_REF][START_REF] Ozin | Cryophotoclustering Techniques for Synthesizing Very Small, Naked Silver Clusters Agn of Known Size (Where n = 2-5). The Molecular Metal Cluster-Bulk Metal Particle Interface[END_REF] The presence of narrow UV bands at specific wavelength indicates that the silver clusters have well-defined size and charge. For the samples prepared by conventional hydrothermal process under similar reaction conditions, only a broad visible band at 430 nm in both ZX and ZX-V samples is observed (Figure S9). This is a strong evidence for the formation of polydisperse silver particles with size larger than 12 nm. A direct evidence of the formation of sub-nanosized silver clusters in zeolite cages was obtained by HAADF-STEM analysis. The size (10-30 nm) and the morphology of ZX nanoparticles remain intact with respect to the untreated sample (Figure S1), with clearly expressed faceting with <111> and <100> planes (Figure 2a). Since HAADF-STEM contrast is proportional to atomic number (~Z 2 ) and thickness, the heavier Ag (Z=47) sub-nanoparticles appear as bright white dots within the pores. The dark spots correspond to surrounding ZX framework which is not clearly resolved due to the low density and large difference in atomic number between Ag and silica-alumina zeolite framework (ZSi=14; ZAl=13, ZO=8). It should be noted that the framework of Ag-free ZX crystallites is visible in HAADF-STEM images (Figure S10). Despite the high sensitivity of the Ag@ZX-V particles to the electron beam and tendency to agglomerate in larger Ag NPs (supporting information, Section III-A, Figure S11), it was possible to acquire almost an intact high resolution image along the [011] zone axis of a Ag@ZX-V crystallite (Figure 2c) using low doses and weak e-beam conditions. [START_REF] Wiktor | Transmission Electron Microscopy on Metal-organic Frameworks -a Review[END_REF] The corresponding Fast Fourier Transform (FFT) pattern confirms the presence of Ag sub-nanoparticles within the pores of FAU-type structure. The Ag sub-nanoparticles appear in projection to have close to square shape with side lengths about 0.7 nm, sharing common corners and forming checkerboard pattern reflecting the FAU structure symmetry. Their size is compatible with the diameter of the 12-member ring window of FAU super cage, as shown in Figure 2c The diameter of these needles is around 0.7 nm and perfectly fits with the entrance window diameter of the supercages. These needles can explain the visible absorbance of the silver sub-nanoparticles (Figure 1). However, the HAADF-STEM image and corresponding intensity plot profile reveals different degree of loading among the channels; some of them being completely free of Ag particles (Figure 2d). This result is attributed to a non-uniform distribution of photoactive vanadate clusters in the zeolite crystals and/or to an internal shielding of some of the crystals during the UV irradiation. The evolution of faujasite crystals prior to (ZX-V) and after Ag loading (Ag@ZX-V) was studied by PXRD (Figure S12). For ZX-V, Rietveld refinement showed the following chemical formula Na82Al73Si119O384(H2O)139. The projection of the ZX-V structure along the [011] axis is shown in Figure 3a. In this projection, the visible channels result from the alignment along [011] directions of the supercage "windows" and sodalite cages overlapping within the FAU framework. Both the sodalite (Fig- ure 3c) and the supercage of faujasite (Figure 3e) are occupied by sodium cations and water molecules. The amount of sodium in sodalite and super cages are 21 and 55 out of 82 Na + cations per formula unit, respectively. In contrast, the double six-member rings (D6R) are only occupied by Na + cations (6 on 82 Na + cations). For Ag@ZX-V the chemical formula calculated from the Rietveld refinement was Ag27Na55Al73Si119O384(H2O)120 with the stoichiometric sum of the Ag d+ and Na + cations equal to 82, as previously observed for ZX-V.

The projection of Ag@ZX-V structure along [110] axis is shown in Figure 2b. Notably, faujasite framework is preserved after the Ag loading. The comparison of the two structures, ZX-V and Ag@ZX-V, revealed that sodalite cages are occupied by Na + ions (32 out of 56 Na + sites) and H2O molecules. After iexchange with Ag + , Na + located in proximity to the D6R is completely replaced by Agm d+ ions (Figure 3g). Only a few Na + ions and water molecules remain in the supercage. 24 Na + cations are localized in the supercages of Ag@ZX-V, which is significantly lower than the 55 Na + found in the supercages of ZX-V before Ag loading. Agm d+ cations are also present at the border of the supercages in the vicinity of the 6 members ring. It should be noted that the positions of the Ag cations in Ag@ZX-V are consistent with those observed in other silvercontaining faujasites. [START_REF] Altantzis | Direct Observation of Luminescent Silver Clusters Confined in Faujasite Zeolites[END_REF][START_REF] Calestani | Structural Study of Zeolite X Exchanged with 'f' Transition Elements. I. Crystal Structure of Reference Hydrated Na-X[END_REF][START_REF] Butikova | Crystal-Structure of Ag-Ion Exchanged Modification of X-Zeolite[END_REF][START_REF] Gellens | Oxidation and Reduction of Silver in Zeolite Y: A Structural Study[END_REF] The Ag metal atoms that do not occupy zeolite crystallographic sites and form sub-nanometer silver clusters could not be localized from the PXRD data. However, difference Fourier maps were investigated during Rietveld refinement process and no residues attributable to Ag clusters were found. This suggests no ordering of silver clusters inside the 3D lattice of the faujasite. This result is fully consistent with the HAADF-STEM studies, which show sub-nanometer silver clusters randomly distributed inside the pores of the zeolite. Such a distribution produces only a weak diffuse scattering signal instead of XRD diffraction peaks. To underline the fact that after Ag exchange the supercage in Ag@ZX-V are partially filled, the Na positions in the supercages occupied at 25% are drawn with a reduced radius (Figure 3b). This figure illustrates an average of ions/water distribution showing that a part of supercages is filled with silver atoms while the others remain occupied by Na + ions and water molecules. It should be noted that the characteristic XRD peaks of vanadate are not detected due to their low concentration (< 0.2 wt. %) . FTIR spectroscopy of adsorbed CO provides valuable information on the oxidation state of silver nanoparticles. The FTIR spectra of CO adsorbed on Ag-exchanged ZX-V samples display broad absorption bands in the range 2170-2200 cm -1 corresponding to linear carbonyls Ag + -CO (Figure S13). [START_REF] Pestryakov | Active Electronic States of Silver Catalysts for Methanol Selective Oxidation[END_REF][START_REF] Bechoux | Infrared Evidence of Room Temperature Dissociative Adsorption of Carbon Monoxide over Ag/Al2O3[END_REF] The overlapping bands at 2180 and 2185 cm -1 are assigned to the CO adsorbed on Ag + ions located on the external surface and inside the pores, respectively. [START_REF] Gellens | Oxidation and Reduction of Silver in Zeolite Y: A Structural Study[END_REF] For Ag@ZX-V sample, the band at 2185 cm -1 is more intense pointing out that the Ag ions are mainly located inside the pores. In addition, this sample displays a broad band around 2140 cm -1 characteristic of adsorbed CO on reduced silver clusters. It is known that CO does not adsorb on the Ag 0 at room temperature. [START_REF] Kolobova | Formation of Silver Active States in Ag/ZSM-5 Catalysts for CO Oxidation[END_REF] Therefore, the low intensity of the band at 2140 cm -1 is rather induced by the weak adsorption ability of Agm d+ clusters due to their low effective charge rather than their low concentration. It should be underlined that at room temperature no signal was observed in Na-ZX and the ZX-V samples. This result points out that silver clusters possess partial positive charge (not completely metallic) and that they are accessible to CO (the pores are not completely filled).

The sub-nanometeric silver species represent the missing link between atomic and nanometer sized silver particles. The small size of these particles (<1nm) render them photoluminescent with a molecular behavior. Their fluorescence emission is an undisputable evidence of this feature. The fluorescence spectra of the Ag@ZX-V suspension are reported in (Figure S14). An emission band centered at 550 nm can be clearly observed after the excitation of the sample at 270 or 320 nm, but no fluorescence signal was observed for Ag + /ZX-V suspension (Figure S15). This result confirms the molecular behavior of the subnanometer particles absorbing at 270 and 320 nm. The excess of energy received by these particles is then released as radiative emission in visible region. In contrast, no emission is observed for the Ag@ZX-V suspension after excitation with 370 or 435 nm. Therefore, the silver species that absorb at these wavelengths are nanoparticles with size larger than 2 nm (Figure 1). [START_REF] Kolobova | Formation of Silver Active States in Ag/ZSM-5 Catalysts for CO Oxidation[END_REF] The absorption band at >370 nm is attributed to needle silver nanoparticles connected through the window of supercages as demonstrated by the TEM analysis (Figure 2). The band at 435 nm originates from a few large particles attached to the external surface of zeolite crystals (Figure S16)

In order to shed light on the mechanism of silver cluster generation, series of experiments under different wavelength irradiations were performed. As mentioned, no silver photoreduction was observed in vanadium-free zeolite (ZX). This reveals the crucial role of vanadate species in the generation of Ag clusters. On the other hand, the concentration of vanadium is very low (< 0.2 wt. %), which precludes the uniform and abundant presence of vanadium species in zeolite cages. The use of long wave ultraviolet light (UV-A or UV-B) did not result in Ag cluster formation. This is mainly due to the selective absorbance of vanadate species that takes place in the short-wave region of ultraviolet light (UV-C). [START_REF] Zaarour | Photochemical Preparation of Silver Nanoparticles Supported on Zeolite Crystals[END_REF] In the following experiment the sample was shortly (30 s) irradiated with polychromatic light (UV A -C), which irradiated that initiates the formation of silver clusters as revealed by the UV-visible spectrum of the suspension (Figure S17,a). The excitation of this sample with UV-B (365 nm) or visible (>390 nm) light, where only the silvers clusters absorb, led to an increase in the intensity of the characteristic bands of the sub-nanometers reduced silver clusters (Figure S17, b-c). These results unambiguously demonstrate that the vanadium species initiate the process of silver clusters formation. After this initial stage the obtained silver clusters generate new silver particles under UV-B-visible light excitation. The clusters grown until filling the zeolite cage and become immobile. The second stage of the process can be described as autocatalytic process where sliver clusters generation continues until exhausting the available silver pool. The self-production of Ag clusters explains their formation in neighboring supercages of FAU-type zeolite and the fact that large number of zeolite crystals exhibit rows of sub-nanometer Ag clusters (Scheme 1).

The photocatalytic activity of Ag@ZX-V was tested in the reforming of formic acid (FAc) to H2 and CO2 under visible and UV irradiations. This reaction offers an efficient route for hydrogen generation and thus attracts tremendous interest lately. [START_REF] Celaje | A Prolific Catalyst for Dehydrogenation of Neat Formic Acid[END_REF] However, the elaboration of an efficient catalyst remains a challenge. [START_REF] Jiang | A Hierarchical Z-Scheme α-Fe2O3/g-C3N4 Hybrid for Enhanced Photocatalytic CO2 Reduction[END_REF][START_REF] Zhou | Boron Carbon Nitride Semiconductors Decorated with CdS Nanoparticles for Photocatalytic Reduction of CO2[END_REF] Namely, the dehydration reaction of FAc must be avoided in order to generate H2 without CO poisoning the catalyst in the fuel cells. [START_REF] Cheng | Controlling Catalytic Dehydrogenation of Formic Acid over Low-Cost Transition Metal-Substituted AuPd Nanoparticles Immobilized by Functionalized Metal-organic Frameworks at Room Temperature[END_REF] During past decades, various catalysts combining noble metals and different supports have been explored to obtain a suitable catalyst for the selective dehydrogenation of FAc. [START_REF] Wen | Plasmonic Au@Pd Nanoparticles Supported on a Basic Metal-Organic Framework: Synergic Boosting of H 2 Production from Formic Acid[END_REF][START_REF] Masuda | PdAg Nanoparticles Supported on Functionalized Mesoporous Carbon: Promotional Effect of Surface Amine Groups in Reversible Hydrogen Delivery/Storage Mediated by Formic Acid/CO2[END_REF] The Ag@ZX-V has been tested in the photocatalytic reforming of formic acid in liquid phase. The reaction conditions are reported in the supplementary information. For comparison, silver exchanged Ag + /ZX-V and Ag@ZX-V(HT) prepared with by in-situ hydrothermal synthesis are used as references. The results are reported in (Figure 4). Ag@ZX-V exhibits 3 times higher activity than that of Ag@ZX-V(HT) with a very high selectivity (more than 99%) toward H2/CO2 formation. The silver content in Ag@ZX-V(HT) is substantially higher than in Ag@ZX-V. Nevertheless, the Ag@ZX-V shows impressively higher activity due to the uniform, sub-nanosized and stable silver particles generated by the new approach reported in the present study. There is no significant deactivation of the material after 16h of reaction as proves the second cycle of the photocatalytic test (Figure S18). This demonstrates the high stability of the photocatalyst obtained by the new synthetic approach.

It should be noted that no significant activity forAg@ZX-V was observed under UV-light excitation (Figure S19). Therefore, the silver particles excited by UV-light release their gain of energy as radiative emission (fluorescence) while the excitation under visible light promotes the plasmonic/photocatalytic activity. The most plausible explanation of this phenomenon is the presence of needle-like silver providing the high activity under UV-light excitation, while the fluorescence emission is related with the isolated sub-nanometer clusters in the FAU cages. H2, CO2 and CO content determined after 5h of visible-light irradiation of formic acid solution (0.35 ml in 10 ml of can; mcatl=50 mg) in the presence of Ag@ZX-V(HT) and Ag@ZX-V as photocatalysts.

Conclusions

In summary, a new approach for encapsulation of sub-nanometer silver clusters in the pores of zeolite nanoparticles was developed. The presence of vanadate clusters inside the zeolite pores, introduced by in-situ synthesis, is crucial for the formation of the first Ag clusters. Further, the self-photoreduction of silver is the dominant mechanism leading to extensive formation of sub-nanometer sized silver clusters encapsulated in zeolite cages. A characteristic feature of the process is the size uniformity of the generated sub-nanosized Ag clusters. The obtained materials exhibit a florescence emission and impressive photocatalytic activity in the reforming of formic acid (FAc) to H2 and CO2. The high performance of the new material in photocatalysis under visible light makes it a very promising candidate for renewable energy applications. In addition, the photoluminescent behavior makes the composite attractive for optical applications such as sensors for UV emissions as well as biomedical, electrocatalytic and imagery applications.

ASSOCIATED CONTENT

Supporting Information

The following data and results are available as Supporting information: experimental section including the characterization techniques and the synthesis procedures used in this work. Supplementary XRD, SEM, TEM, and IR results obtained for the different samples. Details of PXRD data and Rietveld refinements for ZX-V and Ag@ZX-V. This material is available free of charge via the Internet at http://pubs.acs.org. Ag@ZX-V

Figure 1 .

 1 Figure 1. UV-visible spectra of ZX-V-1 suspensions after (a) 30 (b) 120 and (c) 300 s of UV irradiation in presence of 5x10 -3 M of AgNO3. UV visible spectrum of ZX suspension after 10 minutes of UV irradiation in the presence of 5x10 -3 M of AgNO3 (d). Inset: Optical images of the corresponding suspensions.

  insert, where the [011] FAU&Ag structural model is laid over the experimental image. HAADF-STEM image and the corresponding structural model of FAU&Ag projected on (112) plane are shown in Figure 2d. Large area of zeolite nanocrystals are almost completely filled with Ag (bright white contrast needles).

Figure 2 .

 2 Figure 2. TEM analysis of the Ag@ZX-V sample: (a) Low magnification HAADF-STEM images of different Ag@ZX-V nanoparticles along [011]FAU zone axis. Enlargement of the edge of Ag@ZX-V nanoparticle is given as inset (a) and shows well defined single Ag cluster with a characteristic size of around 0.7 nm. (b) Triangle shaped Ag@ZX-V nanoparticle (outlined) imaged along [011]FAU. (c) High resolution HAADF-STEM of Ag@ZX-V nanoparticle along [011]FAU direction and corresponding FFT pattern confirm the presence of Ag nanoparticles within ZX-V material. Enlarged image with an overlaid structural model of Ag@ZX-V is given as inset. (d) HAADF-STEM image viewed along the channel close to [112]FAU zone axis. Enlargement and corresponding intensity line profile show channels with different levels of Ag filling: from high (maximum intensity) to almost empty (marked with red arrows). Corresponding structural model is in the right bottom corner.

43

 43

Figure 3 .

 3 Figure 3. Schematic drawing of the faujasite structure along the [110] direction (a) before and (b) after Ag loading. (c) Sodalite cage, (d) D6R cage, (e) supercage of ZX-V before Ag loading. (f) Sodalite cage, (g) D6R cage, (h) Supercage of Ag@ZX-V after Ag loading.

Scheme 1 .

 1 Scheme 1. Illustration of the photocatalytic process leading to the formation of silver clusters in the super cage of FAU-type zeolite: (I) silver photoreduction by excitation of the vanadate clusters (VC) with l<300 nm (D); (II) photoreduction Ag 0 formed in the stage (I) by exicitation of silver clusters with l>320 nm (D).

Figure 4 .

 4 Figure 4. (A)Evolution of the formic acid conversion versus the visible irradiation (l>390 nm) time over: (a) Ag@ZX-V, (b) Ag@ZX-V(HT), and (c) ZX-V. Inset: figure correspond to the in-situ evolution of the CO2 IR band of headspace of the corresponding samples. (B) H2, CO2 and CO content determined after 5h of visible-light irradiation of formic acid solution (0.35 ml in 10 ml of can; mcatl=50 mg) in the presence of Ag@ZX-V(HT) and Ag@ZX-V as photocatalysts.

ACKNOWLEDGMENT

This work was supported by the European Union (Feder SIRCO project) and the Normandy Region (RHIN, RAPHYD project). The authors are grateful to Dr. Jaafar Elfallah and Dr. Sarah Komaty and Mr. Adrien Lanel for their help in performing SEM analyses, zeolite synthesis, and for creating the new in-situ IR reactor for liquid-gas photocatalysis, respectively.

Funding Sources

No funds was used to support the research of the manuscript.

AUTHOR INFORMATION

Corresponding Author * mohamad.elroz@ensicaen.fr. * valentin.valtchev@ensicaen.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. / SYNOPSIS TOC Ag 0 @ZX-V HCOOH H 2 + CO 2 0.7 nm Ag 0


Global estimation based on indicators factorization

nicolas.bez@ird.fr june 2021

1 Abstract

In the context of highly 0-inflated skewed data, a non linear approach is recommended. However a full disjunctive model is probably too demanding. An intermediate approach consists in using indicators to code the data through a set of disjunctive indicator variables (one by class of density including the class of null densities). However, this gets the following two drawbacks. First, the dimension of the model is increased (moving from mono-variate to multivariate which notably wight up the kriging system). Second, the mathematical coherence of the model is not guaranteed for indicators variables. These two drawbacks were solved by using MAFs. This makes the variables orthogonal both locally and for small distances. Assuming full orthogonality, the new variables are factorized and handleable one by one with their corresponding appropriate variograms. By the way, punctual uncorrelation as produced by standard PCA is not sufficient to handle variables separately. While MAFs are linear combination of the input indicator variables, one can back-transform all the outputs and produce relevant estimations with their estimation variance.

Method

Discretization of Z(x)

The random function is decomposed into N disjunctive classes of densities.

Y (x) = φ Z(x) = N 1 m i 1 i (x) N 1 1 i (x) = 1 m i = E Z(x)1 i (x) 1
p i = E 1 i (x) m = N 1 m i p i 2.

Closed form

Being a closed formalization,

1 N (x) = 1 - N -1 i=1 1 i (x)
the decomposition of Z(x) into N indicator variables should be reduced to N -1:

Y (x) = N -1 1 m i 1 i (x) + m N 1 - N -1 1 1 i (x) = N -1 1 m i -m N 1 i (x) + m N
that is finally wrote as:

Y (x) = m N + N -1 1 mi 1 i (x)
Given their disjunctive characteristic, the indicator variables are locally correlated:

cov 1 j (x), 1 k (x) = -p j p k < 0
They are also not spatially uncorrelated:

cov 1 j (x), 1 k (x + h) = 0
Given the above considerations, the modelization of the indicator variables should be done jointly (multivariate geostatistics). However, rigorous multivariate models for indicator variables do not have explicit mathematical characterization. Even if such a model would exist, the dimension of the coKriging system would drastically slow down computations, especially when global kriging is concerned. A factorization of the system permits, without lose of information and generality, to reduce the N-dimensional question into N separate monovariate systems much easily handleable.

MAFactorization

The factorization (see below) works on centered variables. Denoting:

O i (x) = 1 i (x) -p i
We have:

Y (x) = m + N -1 1 mi O i (x)
There is N -1 spatially correlated variables that we would like to transform into N -1 spatially uncorrelated variables. A sequence of two PCAs (also called Min-max Autocorrelation Factors -MAFs [START_REF] Switzer | Min/Max Autocorrelation Factors for Multivariate Spatial Imagery[END_REF]) allow computing factors that are uncorrelated locally (h = 0) and at a short distance to be chosen (h ref ). As all PCAs, the factors F j (x), j = 1, ..., N -1, are nothing but a linear combination of the input variables. In turns, the input variables are also linear combination of the factors:

O i (x) = N -1 j=1 a j,i F j (x)
Cautious with the order of the subscripts: a j,i corresponds to the j th line of the i th column of matrix A. In matrix notation this amounts to:

O = F • A        • • • . . . .
         ↓ • • ↓ • • ↓ • •  
Given the factors are Min-max Autocorrelation Factors (MAFs), we have by construction that:

cov F j (x), F k (x) = 0 cov F j (x), F k (x + h ref ) = 0
The discrete version of Z(x) finally writes down:

Y (x) = m + N -1 i=1 mi N -1 j=1 a j,i F j (x)
which can be simplified into:

Y (x) = m + N -1 i=1 α i F i (x)
where (be care full with the order of the subscripts):

α i = N -1 i=1 mi N -1 j=1 a i,j
In matrix notations, this amounts to:

α = A • m   • • •   =   → → → • • • • • •     ↓ ↓ ↓  

Hypothesis

We now assume that the factors are uncorrelated for all distances, i.e. not only for h = 0 and for h = h ref :

cov F j (x), F k (x + h) = 0 ∀h

Punctual Kriging: mapping

Being spatially uncorrelated, the cokriging of the factors reduced to their monovariate krigings [START_REF] Chilès | Geostatistics: Modeling spatial uncertainty[END_REF], so that:

Ŷ (x 0 ) = m + N -1 1 α i F K i (x 0 )
One can also estimate the indicator variables, that is the probabilities to be in a class of density:

Ôi (x) = N -1 j=1 a j,i F K j (x) so that 1i (x) = p i + N -1 j=1 a j,i F K j (x), for i = 1, ..., N -1 1N (x) = 1 - N -1 i=1 1i (x)

Global Kriging: estimation of global sA

Thanks to the factorization, the global estimation can be readily deduced:

Ŷ (v) = m + N -1 1 α i F i (v) with Ŷ (v) = m + N -1 1 α i F K i (v)
and

σ 2 E = var( Ŷ (v) -Y (v)) = N -1 1 α 2 i σ 2 E,i (v)
where

σ 2 E,i (v) = var( Fi (v) -F i (v))
As in standard estimation, the coefficient of variation is :

CV E = σ E Ŷ (v)
3 Discussion

Assumptions

The factorization is not complete strictly speaking. The MAF are uncorrelated at 0 and, on average for distances falling in the interval used for their elaboration. While building recursive MAFs of larger and larger interval do not solve the problem, the most appropriate way of doing is to enlarge the interval. By doing so, we loose interpretability of the factors that are produced, but we gain in terms of absence of correlation. It is thus possible to generate MAFs whose spatial cross-correlation are on average null over a large distance interval.

As for any global approach, the model is considered relevant for all geographical distances at play. This means that order 2 stationarity is assumed.

We produced the estimation of Y (v) i.e. the discretized version of Z(v). This is not the estimation of Z(v) which is not accessible with reasonable assumptions. This is a change of paradigm that must be acknowledged strongly. It brings the idea that we can estimate the of biomass

Practical considerations

Some practical questions can be addressed in real cases:

• Should we truncate to 0 the negative estimation ? How ? When in the process ?

• Should we use survey by survey thresholds or should we use fixed threshold e.g. powers of 10 with possible empty class in some surveys ?


Tadeja Birsa

Matjaž Mazaj

Nathalie Guillou

Erik Elkaım

Mohamad El Roz

∥ Fredericthibault-Starzyk

∥ Gregormali

⊥ Mojcarangus

Tomazč ̌endak

Vencešlavkaucič

Natasǎ Zabukovec Logar

Study of hydrothermal stability and water sorption characteristics of 3-dimensional Zn-based trimesate

Keywords: Metal-organic frameworks, hydrothermal stability, zinc trimesate, water sorption, MAS NMR spectroscopy

A zinc trimesate Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O with the three-dimensional framework contains [Zn 2 O 6 (OH) 2 (H 2 O)] chains with the arrangement of ZnO 2 (OH) 2 tetrahedra and ZnO 4 (OH)(H 2 O) octahedra corner-shared through µ 3 -OH group. Inorganic chains are linked with 1,3,5-benzenetricarboxylates forming two types of parallel channels (open and closed) containing adsorbed water in different environments. Closed channels are occupied by free water molecules connected through strong hydrogen-bonds with coordinated water, whereas open-type channels contain water molecules with partially occupied oxygen atom sites. The dynamics of water adsorption/desorption was well evaluated by complementary techniques of TG, IR, water sorption and different MAS NMR techniques. The removal of water from open channels occurs below 100 o C, whereas hydrogen-bonded water molecules and coordinated water are expelled at higher temperatures. 2 H MAS NMR was employed to prove that the removal of water from closed channels is not entirely simultaneous and that adsorbed water begins to diffuse at slightly lower temperatures than the coordinated one. The investigated material shows high hydrothermal and withstands 40 cycle-hydrothermal stability tests without any significant loss of structure integrity. It also shows complete structural reversibility upon dehydration/rehydration process at 200 o C. The reason for high chemical resistivity to water mainly lies in the presence of the adsorbed water molecules interacting with the coordinated ones (framework water) by hydrogen bonds. The material also exhibits notable sorption capacity for water (208 mg•g -1 ) adsorbed in stepwise process.

 The development of MOFs is

particularly focused for separation, catalysis and gas storage applications, due to their welldefined pore architectures, high surface areas, low framework densities and high concentration of active sites represented by free metal sites or organic functional groups.

Great potential for the above mentioned applications that this group of materials offers is in large extent limited by their un-stability in humid conditions. [START_REF] Greathouse | The Interaction of Water with MOF-5 Simulated by Molecular Dynamics[END_REF][START_REF] Kondo | Adsorption of water on three-dimensional pillared-layer metal organic frameworks[END_REF][START_REF] Castillo | Understanding Water Adsorption in Cu-BTC Metal-Organic Frameworks[END_REF][START_REF] Küsgens | Characterization of metal-organic frameworks by water adsorption[END_REF][START_REF] Paranthaman | Water adsorption in hydrophobic MOF channels[END_REF][START_REF] Cychosz | Water Stability of Microporous Coordination Polymers and the Adsorption of Pharmaceuticals from Water[END_REF][START_REF] Hauptvogel | Flexible and Hydrophobic Zn-Based Metal-Organic Framework[END_REF] Knowledge of MOF performance under humid conditions is crucial before elevating these materials to the applied level. However, the parameters that contribute to the sensitivity to water are still relatively poorly understood. [START_REF] Liang | CO 2 Adsorption, Selectivity and Water Tolerance of Pillared-Layer Organic Frameworks[END_REF][START_REF] Schoenecker | Effect of Water Adsorption on Retention of Structure and Surface Area of Metal-Organic Frameworks[END_REF][START_REF] Jasuja | Kinetic Water Stability of an Isostructural Family of Zinc-Based Pillared Metal-Organic Frameworks[END_REF][START_REF] Han | MOF Stability and Gas Adsorption as a Function of Exposure to Water, Humid Air, SO 2 , and NO 2[END_REF][START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF] It was already shown that pyrazolates and imidazolates in general exhibit higher chemical resistivity to water than carboxylate-based MOFs. Which is related to higher basicity (pK a ) of the nitrogen-based ligand and consequently stronger metal-ligand bonds in comparison with oxygen-based carboxylate ligands. [START_REF] Liang | CO 2 Adsorption, Selectivity and Water Tolerance of Pillared-Layer Organic Frameworks[END_REF][START_REF] Schoenecker | Effect of Water Adsorption on Retention of Structure and Surface Area of Metal-Organic Frameworks[END_REF][START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF][START_REF] Choi | Hydrogen Storage in Water-Stable Metal-Organic Frameworks Incorporating 1,3-and 1,4-Benezenedipyrazolate[END_REF][START_REF] Colombo | High Thermal and Chemichal Stability in Pyrazolate-Bridged Metal-Organic Frameworks with Exposed Metal Sites[END_REF][START_REF] Li | Gas Adsorption and Storage in Metal-Organic Framework MOF-177[END_REF][START_REF] Kaye | Impact of preparation and handling on the hydrogen storage properties of Zn 4 O(1,4-benzenedicarboxylate) 3 (MOF-5)[END_REF] Despite the fact that carboxylate-based MOFs tend to exhibit lower structure stability in water than pyrazolate or imidazolates, there are quite a few MOF carboxylates that withstand exposure to water even at elevated temperatures without significant losses of their structure integrities. The most frequently investigated water-resistant MOF members are MIL-53(Al, Cr), MIL-101(Cr), MIL-100(Cr), UiO-66(Zr) and MIL-125(Ti). [START_REF] Serre | Very Large Breathing Effect in the First Nanoporous Chromium(III)-Based Solids: MIL-53 or Cr III (OH)•{O 2 C-C 6 H 4 -CO 2 }•{HO 2 C-C 6 H 4 -CO 2 H} x •H 2 O[END_REF][START_REF] Loiseau | A Rationale for the Large Breathing of the Porous Aluminum Terephthalate (MIL-53) Upon Hydration[END_REF][START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF][START_REF] Cavka | A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability[END_REF][START_REF] Dan-Gardi | A New Photoactive Crystalline Highly Porous Titanium(IV) Dicarboxylate[END_REF] This exceptions suggest that structure stability in water is not only governed by the basicity of the ligand and thus the strength of the metal-ligand bond but it is related to many other independent factors such as metal type, coordination state and framework dimensionality. [START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF][START_REF] Kang | Chemical and Thermal Stability of Isotypic Metal-Organic Frameworks: Effect of Metal Ions[END_REF][START_REF] Ma | Tuning the Moisture Stability of Metal-Organic Frameworks by Incorporating Hydrophobic Functional Groups at Different Positions of Ligands[END_REF] MOFs with good hydrothermal stability and specific structural properties such as high specific surface areas, high affinities and adsorption capacities for water and possibility to tune the hydrophilic nature of their frameworks during the synthesis are appropriate candidates for water sorption based energy applications (heat storage, heat pumps processes).

Although possibilities of MOFs for water sorption applications are still relatively unexplored, several MOF systems were already examined for this purpose: DUT-4(Al) [START_REF] Paranthaman | Water adsorption in hydrophobic MOF channels[END_REF] , MOF-74(Mg) [START_REF] Schoenecker | Effect of Water Adsorption on Retention of Structure and Surface Area of Metal-Organic Frameworks[END_REF] , UiO-66(Zr) [START_REF] Schoenecker | Effect of Water Adsorption on Retention of Structure and Surface Area of Metal-Organic Frameworks[END_REF] , DMOF-1(Zn) [START_REF] Schoenecker | Effect of Water Adsorption on Retention of Structure and Surface Area of Metal-Organic Frameworks[END_REF] , UMCM-1(Zn) [START_REF] Schoenecker | Effect of Water Adsorption on Retention of Structure and Surface Area of Metal-Organic Frameworks[END_REF] , MIL-101(Cr) [START_REF] Seo | Energy-Efficient Dehumidification over Hierarchically Porous Metal-Organic Frameworks as Advanced Water Adsorbents[END_REF][START_REF] Henninger | MOFs for Use in Adsorption Heat Pump Processes[END_REF][START_REF] Ehrenmann | Water Adsorption Characteristics of MIL-101 for Heat-Transformation Applications of MOFs[END_REF][START_REF] Khutia | Water Sorption Cycle Measurements on Functionalized MIL-101Cr for Heat Transformation Application[END_REF][START_REF] Akiyama | Effect of Functional Groups in MIL-101 on Water Sorption Behavior[END_REF] , MIL-100(Fe) [START_REF] Seo | Energy-Efficient Dehumidification over Hierarchically Porous Metal-Organic Frameworks as Advanced Water Adsorbents[END_REF][START_REF] Henninger | MOFs for Use in Adsorption Heat Pump Processes[END_REF] , MIL-53(Al) [START_REF] Henninger | MOFs for Use in Adsorption Heat Pump Processes[END_REF] , HKUST-1(Cu) [START_REF] Henninger | MOFs for Use in Adsorption Heat Pump Processes[END_REF] , ISE-1(Ni) [START_REF] Henninger | MOFs for Use in Adsorption Heat Pump Processes[END_REF] and ZIF-8(Zn) [START_REF] Henninger | MOFs for Use in Adsorption Heat Pump Processes[END_REF] . Among these materials MIL-101 (Cr) shows the best performance for heat transfer applications. It exhibits high hydrothermal stability, huge water sorption uptakes up to 1.4 g•g -1 of sorbent at 25 °C and high desorption rates below 80 °C. On the other hand Zn-based carboxylates (DMOF-1, UMCM-1, IRMOFs) undergo significant loss of crystallinity upon water exposure, therefore they are in general evaluated to be unsuitable for such applications. [START_REF] Cychosz | Water Stability of Microporous Coordination Polymers and the Adsorption of Pharmaceuticals from Water[END_REF][START_REF] Schoenecker | Effect of Water Adsorption on Retention of Structure and Surface Area of Metal-Organic Frameworks[END_REF][START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF][START_REF] Li | Gas Adsorption and Storage in Metal-Organic Framework MOF-177[END_REF] In the present study we report on the solvothermal synthesis of a Zn-trimesate, Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O material containing tetrahedrally and octahedrally coordinated Zn centres which are connected through BTC linkers to the extended 3-dimensional structure with two types of parallel channels. [START_REF] Fu | Two Novel Transition Metal-Organic Frameworks Based on 1,3,5-Benzenetricarboxylate Ligand: Syntheses, Structures and Thermal Properties[END_REF] We showed that material exhibits an exceptional hydrothermal stability, complete reversibility upon dehydration-rehydration process and notable water sorption capacity. The reason for high hydrothermal stability mainly lies in the stabilization of inorganic chains by hydrogen-bonded water molecules, which was evaluated by complementary techniques of solid state NMR, XRD, in-situ IR and water sorption measurements. High water resistance even at elevated temperatures, which is a rare feature among Zn-based MOFs, together with promising water sorption properties implies that investigated Zn-trimesate is a potential material for water sorption based energy applications.

EXPERIMENTAL SECTION

Synthesis. For the synthesis of Zn

2 (BTC)(OH)(H 2 O)•1,67H 2 O
we modified the procedure published by Fu et al. [START_REF] Fu | Two Novel Transition Metal-Organic Frameworks Based on 1,3,5-Benzenetricarboxylate Ligand: Syntheses, Structures and Thermal Properties[END_REF] Solvothermal synthesis was performed under autogenous pressure from a mixture of zinc acetate dehydrate (98 % Zn(CH 3 CO 2 ) 2 •2H 2 O, Aldrich), 1,3,5benzentricarboxylic acid (95 % BTC, Aldrich), ethanol (99.8 %, Aldrich) and distilled water.

The molar ratios of starting reagents were Zn(ac) 2 •2H 2 O : BTC : 100 H 2 O : 70 EtOH. Zinc acetate was dissolved in distilled water and an ethanolic solution of trimesic acid was prepared separately. White gel obtained immediately after solutions were combined was stirred for 5 minutes, transferred into Teflon-lined stainless-steel 25 mL autoclave and treated at 175 °C for 24 hours. White crystalline powder was thoroughly rinsed with ethanol and/or water and dried at room temperature. The comparison of the measured powder X-ray diffractogram with the calculated one confirmed that the material contained pure

Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O phase (see Supporting Information). 37
General characterization methods and sorption measurements. Thermogravimetric (TG/DTG) analysis was performed on a SDT 2960 Thermal Analysis System (TA Instruments, Inc.). The measurement was carried out in static air with the heating rate of 10 o C/min. Thermal stability was monitored by high-temperature XRD measurements in vacuum employing a PANalytical X'Pert PRO MPD diffractometer from 5 to 55 o 2q using step of 0.013 o per 100 s in the temperature range between 25 and 700 °C. Structural changes and hydrothermal stability were monitored by powder X-ray diffraction measurements on a PANalytical X'Pert PRO high-resolution diffractometer with Alpha1 configuration (CuKα 1 λ = 1.5406 Å radiation) using continuous scanning mode in 2θ range from 5 to 35° with the step of 0.016° per 100 s.

1 H (Magic-Angle Spinning) and 1 H- [START_REF] Paranthaman | Water adsorption in hydrophobic MOF channels[END_REF] C CPMAS (Cross-Polarization Magic-Angle Spinning) NMR spectra were recorded on a 600 MHz Varian NMR system equipped with a 3.2 mm Varian probehead. Larmor frequencies for 1 H and 13 C were 599.72 MHz and 150.82 MHz, sample rotation frequencies were 20 kHz and 16 kHz, and the numbers of scans were 64 and 600, respectively. Repetition delays between scans were 5 s in all experiments. 1 H 90° excitation pulse was 2 µs. 1 H-13 C CPMAS experiment employed RAMP [START_REF] Metz | Ramped-Amplitude Cross Polarization in Magic-Angle-Spinning NMR[END_REF] during CP block and high-power XiX 39 heteronuclear decoupling during acquisition. Chemical shifts of 1 H and temperature 2 H MAS NMR spectra were acquired on the same spectrometer using a 5 mm Doty probehead and sample rotation frequency of 12 kHz. 2 H Larmor frequency was 92.05 MHz, 90° excitation pulse was 6 µs, repetition delay was 5 s and number of scans was 128.

Before the 2 H NMR measurements the sample was evacuated at 200 °C for two hours and subsequently a drop of 2 H 2 O was added to it. Variable-temperature NMR measurement started when the drop was completely absorbed.

For in-situ infrared spectroscopy powders were pressed (~10 7 Pa) into self-supported discs (2 cm 2 area, 9-11 mg cm -2 ). They were placed in a cell equipped with KBr windows. A movable quartz sample holder allowed placing the pellet in the infrared beam for recording spectra and moving it into a furnace at the top of the cell for thermal treatment. The cell was connected to a vacuum line for evacuation (P~ 10-5 torr), and a gauge controls the pressure of the vapors of H 2 O (or D 2 O) introduced into the cell. A Nicolet 6700 IR spectrometer equipped with a MCT detector and an extended-KBr beam splitter was used for the acquisition of spectra recorded in the 400-5500 cm -1 range. Spectra were recorded at 4 cm -1 and 128 scans were co-added for each spectrum.

Water sorption characteristics of Zn-trimesate were determined by an IGA-100 gravimetric analyzer (Hiden Isochema Ltd). Water sorption isotherms were obtained at 30 o C by setting equal pressure intervals of 1.6 mbar between vacuum and 40 mbar (saturation vapor pressure of 42.5 mbar at 30 °C) with an equilibrium time of 80 s. Before adsorption measurements, the sample was outgassed to a constant weight under ultra high vacuum (<10 -5 mbar) at 200 o C for three hours. The hydrothermal stability of the material was additionally evaluated with cycle test measurements consisting of 40 cycles between designated temperatures of 40 and 140 °C in a helium gas flow with 75 % relative humidity. The relative humidity was controlled by varying the ratio of dry and saturated helium via two mass flow controllers.

RESULTS AND DISCUSSION

Structure description, thermal properties and water removal studies.

Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O

The chain is running along c-axis bridged through carboxylate groups (left). A view of Zn

2 (BTC)(OH)(H 2 O)•1,67H 2 O
structure along c-direction with the unit cell edges revealing closed and opened types of channels. Opened channels contain water molecules with partially occupied oxygen atoms sites (Ow3, Ow4) (right). Blue circles -zinc atoms, red dots and circles -oxygen atoms, black circles -carbon atoms, grey circles -hydrogen atoms. As it will be described later in the text, it was found that in this study investigated Zntrimesate exhibit high hydrothermal stability and the arrangement of water moieties within the framework seems to be one of the key factors which contributes to the chemical resistivity towards water of this material. Thermogravimetric analysis was already briefly described by Fu et al. [START_REF] Fu | Two Novel Transition Metal-Organic Frameworks Based on 1,3,5-Benzenetricarboxylate Ligand: Syntheses, Structures and Thermal Properties[END_REF] , however to gain more knowledge about the type and the location of water molecules within the structure, TG analysis was performed again and additionally supported by the detailed in-situ IR and NMR discussions. The thermogravimetric (TG/DTG) curves In order to support the above proposed arrangement of water molecules, in-situ IR measurements were performed. The complexity of the IR profile depends on the hydroxylation degree of the surface. Figure 3 The broad IR-bands at 5380-4820 cm -1 and 3762-2644 cm -1 are assigned to the free water molecules within the pores and coordinated water to Zn atoms (Ow1), respectively. [START_REF] Vimont | Investigation of Acid Sites in a Zeotypic Giant Pores Chromium(III) Carboxylate[END_REF][START_REF] Tan | Stability and Hydrolyzation of Metal Organic Frameworks with Paddle-Wheel SBUs upon Hydration[END_REF] After activation, the IR bands assigned to H 2 O disappeared in comparison with the original IR spectrum indicating that physisorbed and coordinated water are totally removed from the material. In the spectrum of the as-prepared Zn-trimesate we can distinguish three contributions at approximately 7.7 ppm, 4.4 ppm and 1.6 ppm. The three signals can be assigned to the hydrogen atoms in the benzene ring, in the water molecules, and in the hydroxyl groups, respectively. Thermal treatment of the material at 100 °C and 200 °C reduces the intensity of the 4.4 ppm peak, but hardly affects the intensities of the 7.7 and 1.6 ppm peaks. The latter peak is suppressed only when the sample is heated to 300 °C. These thermally induced changes, firstly, confirm the assignments of the 4.4 ppm peak to water molecules and the 1.6 ppm peak to hydroxyl groups, and secondly, support the observations of the thermal analysis, showing that water molecules are largely expelled from the pores of the Zn-trimesate up to the temperature of about 250 °C. It is interesting to note that the 4.4 ppm 1 H NMR signal becomes asymmetric when the material is heated to 100 °C, suggesting that the signal is actually composed of more than one contribution. Since the intensity of the water signal at 100 °C is already reduced as compared to the intensity of the water signal of the as-prepared sample and since it is proposed that water molecules from the open channels are already desorbed at this temperature, the asymmetry of the 4.4 ppm 1 H NMR signal indicates that water molecules that are coordinated to Zn atoms and water molecules that are hydrogen bonded to the framework resonate at slightly different chemical shifts. When the sample is further heated to 200 °C, the 1 H NMR signal of the water molecules is still lowered and apparently shifted to smaller isotropic chemical shift, confirming that one of the contributions to the 1 H NMR signal of water molecules was removed. It seems that the removal of the hydrogen-bonded water molecules and the removal of the coordinated water molecules, which according to the thermal analysis both take place in the temperature interval between approximately 100 °C and 200 °C, are not entirely simultaneous and that most probably slightly higher temperatures are needed to completely expel the coordinated water molecules from the closed channels. At 300 °C the signal of the hydroxyl groups reduces significantly and the signal belonging to water molecules finally becomes negligible. water molecules exhibiting relatively slow dynamics and can thus be assigned to the coordinated water molecules. Differently, the broad single central peak could belong to water molecules that undergo intermediately fast dynamic processes, because of which the quadrupolar sideband pattern gets reduced and thus limited to the central peak only (and perhaps to few weak sidebands that cannot be resolved because of their large linewidth) and because of which the centreband (and individual sidebands) get 'motionally broadened'. [START_REF] Kristensen | Investigation of multiaxial molecular dynamics by 2 H MAS NMR spectroscopy[END_REF] This observation suggests that upon heating the sample from 40 °C to 90 °C the dynamics of the previously hydrogen-bonded water molecules become notably faster and that the hydrogen bonds between them most probably break apart. When the temperature is further increased to 120 °C the 'free' water molecules enter the fast dynamics regime; the centreband and the first-rank sidebands become 'motionally narrowed' [START_REF] Kristensen | Investigation of multiaxial molecular dynamics by 2 H MAS NMR spectroscopy[END_REF] , while the quadrupolar sideband pattern gets significantly reduced. The shape of the broad quadrupolar pattern belonging to the coordinated water molecules does not change, but its intensity drops slightly. Heating the sample to 150 °C drastically reduces the intensity of the central peak, showing that most of the uncoordinated water gets removed from the pores. The signal of the water coordinated to zinc is still present but lowered even further. This means that also the coordinated water molecules are being expelled from the material, but the temperature for this process needs to be higher or the thermal treatment needs to last longer than for the removal of the hydrogenbonded molecules.

(Figure 2) shows that Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O
The location of water moieties and their dynamics during the heating process was well evaluated and the next step was to monitor structural changes of the framework during the thermal treatment. The changes were monitored in-situ by high-temperature XRD and ex-situ Very similar information can be deduced also from the 1 H-13 C CPMAS NMR spectra (Figure 5). The spectrum of the as-prepared material exhibits very narrow signals indicating that the material is very well crystalline. The contributions of the three crystallographically distinct carboxyl carbon atoms are clearly resolved one from another, and also the carbon nuclei within the benzene ring detect slightly different local environments and resonate at slightly different frequencies. Heating the sample up to 200 °C only slightly changes the 1 H- O is very little affected by the removal of water suggests that the material could be efficiently rehydrated. Indeed, as shown by the in-situ variable-temperature XRD measurements in air flow in Figure 6, the dehydration-rehydration process is completely reversible if the material is exposed to air for more than one hour. The reversibility of the process is confirmed also by the comparison of the 1 H-13 C CPMAS NMR spectra recorded before and after the dehydration-rehydration cycle (inset in Figure 6). Furthermore, the Zn-trimesate material is hydrothermally stable and its structure can withstand different water treatment procedures -water adsorption, treatment in boiling water, hydrothermal cycling from 40 to 140 o C at constant humidity and water adsorption after cycling procedure. As can be seen in Figure 7, it is apparent that any kind of water exposure procedure does not affect the crystal structure significantly. These XRD patterns will be discussed in more details later in the text. The removal of adsorbed water from open channels should yield microporous framework with one-dimensional parallel channels with the diameter of 5 Å. Therefore it is expected that pores of the activated material are accessible for gas molecules. However, sorption measurements with different gases (N 2 , CO 2 , H 2 ) on degassed Zn-trimesate did not show any notable uptakes. On the other hand, the material exhibits better uptakes for water. The water adsorption isotherm recorded at 30 o C on Zn-trimesate is shown in Figure 8. The maximum water uptake is 208 mg per gram of material at a relative pressure p/p 0 of 0.95. It is apparent that the shape of the water adsorption isotherm is of type I in the IUPAC classification scheme, indicating that the investigated material has hydrophilic character and that interactions between water and the framework are strong. In the lower pressure range from p/p 0 = 0.02 -0.3 stepwise water adsorption can be observed indicating the presence of two energetically different processes. Firstly, water molecule coordinates to the free zinc metal sites via coordination bonds since Zn-trimesate contain open-metal site after heating the sample at 200 °C. Interestingly, similar adsorption on the available metal sites in the lower pressure region of adsorption process was previously reported for HKUST-1 and MIL-100(Al, Fe, Cr) type compound. [START_REF] Küsgens | Characterization of metal-organic frameworks by water adsorption[END_REF][START_REF] Akiyama | Highly Porous and Stable Coordination Polymers as Water Sorption Materials[END_REF][START_REF] Jeremias | MIL-100(Al, Fe) as water adsorbents for heat transformation purposes -a promising application[END_REF] Additional filling of the pores on hydrophilic sites takes place and finally also remaining open channels are filled with water molecules. In the higher pressure region above p/p 0 = 0.9, additional adsorption occurs which is the result of condensation of water in pores. Although the adsorption and desorption branches are quite similar, the hysteresis can be observed. The notable hysteresis in the low pressure region indicates that some water molecules are very strongly bonded to the framework of The isosteric heat of adsorption, q st was estimated by applying Dubinin-Raduschevich equation [START_REF] Dubinin | The potential theory od adsorpton of gases and vapors for adsorbents with energetically nonuniform surfaces[END_REF] using the sorption data at 30 o C. Heat of adsorption values were calculated in different relative pressure ranges of the isotherm (Supporting information Figure S2, Table S4). At very low relative pressure the isosteric heat value is high (~57 kJ•mol -1 ) since the sample is dry and energetically favorable metal sites are free to be occupied. In the following adsorption steps the value slowly decreases to approximately 47 kJ•mol -1 , which is very close to the molar evaporation enthalpy of water at 30 °C (~ 44 kJ•mol -1 ). The trend obtained and the isosteric values are comparable with those previously reported. [START_REF] Akiyama | Highly Porous and Stable Coordination Polymers as Water Sorption Materials[END_REF][START_REF] Jeremias | MIL-100(Al, Fe) as water adsorbents for heat transformation purposes -a promising application[END_REF] The adsorption of water into activated Zn-trimesate was also investigated by IR. An in-situ IR study has been performed by introducing small amounts of water to the dried sample (rehydration process). The results are presented in Figure 9. The introduction of water causes the reappearance and progressive increase of the IR bands at 5235 cm -1 and 3615 cm -1 which were disappeared after activation (Figure 9 a). Figure 9 b presents the evolution of the combination band area (ν+δ(H 2 O)) at 5235 cm -1 and the intensity of the IR band at 3615 cm -1 as a function of the amount of introduced water. For the amount of water lower than 2.5 µmol, an increase of the ν(OH) bands of the coordinated water at 3615 cm -1 is observed. This is not the case for the ν+δ(H 2 O) band at 5235 cm -1 which slowly increases for 2.5 < n(H 2 O) < 10 µmol and starts to increase more rapidly for n(H 2 O) > 10 µmol. It is accompanied with an increase of the broad IR band at 3315 cm -1 assigned to the vibration modes of hydrogenbonded physisorbed water molecules. These results show that water molecules are adsorbed preferentially on the free metal sites and the excess of water is stoked in the channels as physisorbed water. than almost all zinc MOFs and also some other MOF materials (MOF-5, MOF-69, HKUST-1). [START_REF] Greathouse | The Interaction of Water with MOF-5 Simulated by Molecular Dynamics[END_REF][START_REF] Küsgens | Characterization of metal-organic frameworks by water adsorption[END_REF][START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF] The comparison of water adsorption isotherms (inset in Figure 8) of Zn-trimesate phase before and after the hydrothermal cycling treatment indicate that the adsorption capacity for water at high relative pressures remains the same after the hydrothermal cycling treatment under humid conditions. However in the low relative pressure region the water adsorption process is slower than in the initial material. Adsorption is still stepwise but the steps are much less pronounced. Interestingly, even though the inflection point between two steps is shifted towards higher p/p o values (0.1 and 0. Dimensionality of the metal-containing SBU and its connectivity in MOF structure is another important factor which affects stability in water. 1-D chain-like SBUs tend to contribute to higher water resistance of the MOFs than 0-D clusters. However, in the case of structures with 1-D SBUs, the type of connections of the individual polyhedra to chain-like units is the factor that should also be considered. Edge-sharing polyhedral SBUs are more robust and seem to exhibit relatively higher water resistance than less rigid SBUs with corner-shared polyhedra.

Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O.
4) Metal-oxygen bond strength. Stronger interactions between metal cation and oxygen atoms should obviously contribute to higher hydrothermal stability of the framework, since ligand displacement or exchange with water is expected to be less favorable as in the structures with weaker metal-oxygen bonds. [START_REF] Greathouse | The Interaction of Water with MOF-5 Simulated by Molecular Dynamics[END_REF][START_REF] Cychosz | Water Stability of Microporous Coordination Polymers and the Adsorption of Pharmaceuticals from Water[END_REF][START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF][START_REF] Tan | Stability and Hydrolyzation of Metal Organic Frameworks with Paddle-Wheel SBUs upon Hydration[END_REF][START_REF] Davies | Anionic zinc-trimesic acid MOFs with unusual toplogies: reversible hydration studies[END_REF][START_REF] Park | Exceptional chemical and thermal stability of zeolitic imidazolate frameworks[END_REF] Taking into account all the structural features described in the previous paragraph, relatively high water resistance of Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O can be explained by several structural features that this framework possesses. Firstly, the structure is composed of tetrahedral and octahedral Zn-centered chains, where stability in water media should be indeed higher than in the presence of 0-dimensional cluster SBUs. Secondly, polyhedra are bridged through h 1 carboxylate nodes, thus the ligand displacement process is not as favorable as in the case of h 2 carboxylate node configuration. 50 % of all Zn atoms are in octahedral environment which also may have some contribution to the higher stability in water. The benzene rings stacked along c-axis at the average distance of 3.47 Å forms p-p interactions which additionally stabilizes the framework. [START_REF] Kostakis | Interpenetrated networks from a novel nanometer-sized pseudopeptidic ligand, bridging water, and transition metal ions with cds topology[END_REF] The last and in our opinion most pronounced feature which contributes to the water stability is the presence of the hydrogen-bonded water

  consists of one-dimensional chains which are composed of ZnO 2 (OH) 2 tetrahedral and ZnO 4 (OH)(H 2 O) octahedral units connected through corner-shared µ 3 -hydroxy bridging groups. (Figure 1 left). Octahedral and tetrahedral entities are additionally linked by carboxylate groups of BTC moieties. In this way the [Zn 2 O 6 (OH)(H 2 O)] n chains are connected into the extended three-dimensional structure with two types of parallel channels which can be denoted as closed and opened channels (Figure 1 right). Closed channels are occupied by water molecules (Ow1) coordinated to octahedral Zn atoms and have no available free space. The opened channels with the free space diameter of 5 Å contain adsorbed water molecules on partially occupied sites Ow3 and Ow4. There is another water molecule (Ow2, full occupancy) located between the Ow1 vertices of two nearby ZnO 4 (OH)(H 2 O) octahedra (Figure 1 left). This water molecule interacts with the Zncoordinated water through hydrogen bonds (Ow1…Ow2 distance of 2.794(13) Å).

Figure 1 .

 1 Figure 1. Polyhedral presentation of 1-dimensional [Zn 2 O 6 (OH) 2 (H 2 O)]n chain with carbon

  loses weight in three steps. In the first step the loss of approximately 3 wt. % of the total mass could be attributed to the release of adsorbed water located within the open channels. The detected loss corresponds to the removal of 0.7 water molecules (Ow3, Ow4) per one asymmetric unit. Removal of adsorbed water at temperatures below 100 o C can be explained by the fact that the open channels have a relatively hydrophobic nature and are big enough for unobstructed and easy water diffusion from the material. The second step with the weight loss of 9.1 wt.% could be attributed to the simultaneous desorption of the remaining hydrogen-bonded water molecules (Ow2) located within the closed channels and the coordinated water molecules (Ow1). Both water molecules fully occupy their crystallographic positions and theoretically the removal of these water moieties would correspond to the loss of 9.0 wt.% of the total mass. The reason for the delayed release of the water located on the Ow2 sites compared to the water molecules adsorbed within the open channels can be explained by the presence of the above mentioned strong hydrogen bonding and also by the fact that these water molecules are located within narrower channels from which the diffusion is expected to be slower. Weight loss of 3.1 wt.% in the third step could be attributed to the removal of μ 3 -bridging OH groups. Since the theoretical weight loss for the removal of these groups is bigger (4.3 wt.%) than the measured one, we can conclude that OH groups are not completely removed in the temperature range between 250 and 350 o C within the 10 minutes time period (heating ramp = 10 o C/min).Removal of the remaining OH groups takes place during the final framework decomposition.

Figure 2 .

 2 Figure 2. Thermogravimetric analysis of Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O material. TG -

  shows the comparison of IR spectra of Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O recorded at ambient conditions and after heating at 200 °C.

Figure 3 .

 3 Figure 3. IR spectra of Zn-trimesate under air (a) and after activation at 200°C under vacuum

1

  H and 2 H MAS NMR spectroscopy put an additional light on the information about environment of water molecules and the dynamics of its removal from the Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O material. 1 H MAS spectra of the as-prepared material and of the materials thermally treated (ex-situ) at 100 °C, 200 °C and 300 °C are shown in Figure 4a.

Figure 4 .

 4 Figure 4. 1 H (a) and 2 H (b) MAS NMR spectra of Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O. Thermal

by 1 H 5 .

 15 -13 C CPMAS NMR and the obtained diffraction patterns and spectra are shown in Figure We can see that the removal of adsorbed water from the open channels (up to 75 o C) does not affect the framework structure. Structural changes begin to occur only above 100 o C, when the processes of remaining water desorption from the closed channels and removal of coordinated water start. Changes in peak positions in the temperature range between 100 and 150 o C indicate a certain degree of framework flexibility, which is required if the framework is to withstand the removal of the coordinated water. Most of the peaks at higher angles are broadened and significantly lowered due to the anisotropic effects or structural defects that may occur during the thermal treatment. The second structural change caused by the removal of the μ 3 -bridging OH groups begins at 250 °C. Bridging OH groups connect the ZnO 2 (OH) 2 and ZnO 4 (OH)(H 2 O) polyhedra into the inorganic chain, and their removal causes the collapse or reorganization of the chains and leads to the substantial decrease in crystal structure ordering. In the temperature range between 400 and 500 o C the framework of Zn-trimesate finally decomposes and transforms to wurtzite ZnO phase.

Figure 5 .

 5 Figure 5. HT-XRD patterns of Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O thermally treated at given

13 C

 13 CPMAS spectrum. The individual signals become somewhat broader and therefore start to overlap one another more intensely, yet all the above-described contributions can still be distinguished. The signal positions do not change because of the thermal treatment at 200 °C, showing that the organic part of the framework is practically unaffected by the removal of water molecules. 1 H-13 C CPMAS spectrum changes drastically only when the sample is heated to 300 °C. At this temperature the signals in the NMR spectrum get severely broadened and thus indicate that the framework becomes significantly less ordered. Stability in water and water sorption properties. The fact that the framework of the Zn 2 (BTC)(OH)(H 2 O)•1,67H

Figure 6 .

 6 Figure 6. HT-XRD patterns of Zn-trimesate during the heating up to 200 o C and cooling

Figure 7 .

 7 Figure 7. XRD patterns of as-synthesized material (a), after water sorption isotherm (b), after

Figure 8 .

 8 Figure 8. H 2 O vapor adsorption and desorption isotherms for degassed Zn-trimesate (black

Figure 9 .

 9 Figure 9. (a) Evolution of the IR spectra of activated Zn-trimesate during the water

  2 before and after hydrothermal cycling treatment, respectively), it occurs at approximately the same water loading amounts (cca. 120 mg•g -1 ). At relative pressure of 0.4 the water loading reaches saturation and adsorption branch above p/p o = 0.4 becomes comparable with the isotherm of Zn-trimesate before cycling treatment. The slower adsorption processes of the material after hydrothermal cycling in both steps at low pressure range can be explained by slight structural changes which may occur during the long-term hydrothermal cycling treatment. The framework changes can cause partial blocking of the 1-dimensional channels which means that adsorption sites become more obstructed and less accessible for water molecules. Thus higher pressures or longer time are needed for water to coordinate back to Zn(II) cations. Partial structural changes are indicated by XRD pattern analysis of the sample after cycling treatment where the peaks are broadened and slightly shifted toward higher angles (Figure7 d) if compared with the XRD pattern of the sample after first isotherm measurement (Figure7 b). At relative pressures between 0.4 and 1.0, where water capacity reaches the values of the original (degassed as-synthesized) sample, the Zn-trimesate framework seems to be completely recovered. The assumption of structure recovering can be again supported by XRD analysis. XRD pattern of the sample measured after hydrothermal cycling and subsequent water isotherm treatment shows sharpening of the peaks, increasing of intensities and shifting back towards lower angles (Figure7 e). We can conclude that even though the long-term hydrothermal cycling between 40 and 140 o C at constant humidity slightly affects the Zn-trimesate framework and its water loading capacity, the structure of this material can be completely recovered by exposure to humid conditions at ambient temperatures.

  interacting with the framework (coordinated) water molecules. As it was clearly shown by NMR and HT-XRD analyses the dehydrated material (heated at 200 °C) is completely recovered back to Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O material upon exposure to the moisture.Additional investigations by water sorption measurements indicated that hydrogen-bonded water molecules occupy their positions first with relatively high heat of adsorption values (> 57 kJ•mol -1 ). Dehydrated material obviously has high tendency to adsorb hydrogen-bonded water molecules (Ow2) to interact with the framework (coordinated) water. This plays a key role in the stabilization of inorganic chains and increases the chemical stability in hydrothermal conditions. The exceptional hydrothermal stability is a rarity in Zn-based MOF materials. Relatively high stability in water has been reported for imidazolate framework (ZIF-8), but to our knowledge the investigated material is beside MOF-74, which possesses chains of Zn octahedra, the only carboxylate-based Zn-MOF to exhibit such properties.

  2

  [START_REF] Park | Exceptional chemical and thermal stability of zeolitic imidazolate frameworks[END_REF][START_REF] Rosi | Rod Packings and Metal-Organic Frameworks Constructed from Rod-Shaped Secondary Building Units[END_REF] CONCLUSION. We presented the studies of water stability and water sorption behavior of 3dimensional Zn-trimesate material Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O. Structure contains inorganic SBU, described as [Zn 2 O 6 (OH)(H 2 O)] n , and composed of ZnO 2 (OH) 2 tetrahedra and ZnO 4 (OH)(H 2 O) octahedra which are corner-shared through µ 3 -OH group. Chains are connected through BTC linkers forming two types of parallel channels which contain adsorbed water in different environments. Dynamic of water adsorption/desorption was thoroughly investigated by TG, IR, water sorption and different MAS NMR techniques. 2 H MAS NMR elucidated the sequence of removal of coordinated water and hydrogen-bonded water molecules, whereas those processes could not be resolved by TG analysis. Thus, the 2 H MAS NMR turned out to be a useful technique for distinguishing the dynamics of molecular species during the heating process, where those species occur in similar chemical environment. Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O show surprisingly high hydrothermal stability for Zn-based MOFs since it is not significantly affected by water exposure even after 40-cycle hydrothermal treatment and shows structural reversibility upon dehydration/rehydration process up to 200 o C. We showed that adsorbed water molecules interacting with coordinated (framework) water by hydrogen bonds play a key role in stabilizing the framework structure and in large extent contribute to the high stability of the material in water. The material also shows notable water adsorption capacity. Exceptional stability in water is a rarity for Znbased MOFs and to our knowledge Zn 2 (BTC)(OH)(H 2 O)•1,67H 2 O represents the only zincbased carboxylate beside MOF-74 to exhibit such properties. Further developments of isoreticular materials with hydrothermal stability comparable to this Zn-trimesate can be achieved by using bulkier tridentate ligands and could expand the applicability of MOFs for water sorption-based storage applications. Calculated and measured XRD patterns, scanning electron micrograph of Zn trimesate crystals, N 2 adsorption isotherm, DR plots and hydrothermal cycle test. This material is available free of charge via the Internet at http://pubs.acs.org.

	ASSOCIATED CONTENT
	Supporting Information.

ACKNOWLEDGMENT This work was supported by the Slovenian Research Agency research programme P1-0021 and BI-FR/11-12-PROTEUS-011 project. ABBREVIATIONS MOFs, metal-organic frameworks; SBU, secondary building unit; BTC, 1,3,5benzenetricarboxylic acid; NMR, nuclear magnetic resonance; ICP-AES, inductively coupled plasma atomic emission spectrometer; XRD, X-Ray diffraction; CPMAS, cross-polarization magic angle spinning; TG, Thermogravimetric analysis; DTG, Derivative Thermogravimetric analysis; ZIF, zeolitic imidazolate framework; HKUST, Hong Kong University of Science and Technology; MIL, Materials of Institut Lavoisier.

AUTHOR INFORMATION

Corresponding Author

* Email: matjaz.mazaj@ki.si. Phone: +38614760215. Fax.: +38614760300

Notes

The authors declare no competing financial interest.