Romuald Coupan

Jean-Philippe Torre

Christophe Dicharry

Mehrdji Hemati

Fredeŕic Plantier

Kinetics of CO 2 Capture by Hydroquinone Clathrates

Organic clathrates formed by combining hydroquinone (HQ) and CO 2 could offer very interesting prospects in the near future, particularly in the field of CO 2 capture and storage. However, one of the main limitations hindering the large-scale deployment of this type of clathrate-based technology is the slow enclathration kinetics. Our experiments, performed at different pressures (1.5, 3.0, and 4.5 MPa) and temperatures (298, 323, and 348 K), with HQ in different forms (HQ powder, HQ pellets, and HQ-silica composites, each different in nature and in terms of pore size and HQ content) demonstrated that (i) an increase in both pressure and temperature enhances the enclathration rate, (ii) the textural properties of HQ significantly impact kinetics, and composite materials remain the most efficient for improving HQ clathrate formation kinetics.

■ INTRODUCTION

Carbon dioxide (CO 2 ) is one of the most common anthropogenic greenhouse gases in terms of its concentration in the atmosphere. If nothing is done to curb the amount of CO 2 currently released, global warming will continue, inducing dramatic changes in the global climate system. [START_REF] Nema | An Overview of Global Climate Changing in Current Scenario and Mitigation Action[END_REF] In the transition period from a fossil-fuel to a low-carbon world, one of the most critical technological challenges is to develop efficient, environmentally friendly, and low-cost CO 2 capture technologies. Despite the considerable academic and industrial R&D efforts already focused on this subject over the past decade, [START_REF] Míguez | Evolution of CO 2 Capture Technology Between 2007 and 2017 Through the Study of Patent Activity[END_REF] the high costs and energy penalty of many solutions (e.g., chemical absorption using amines) remain prohibitive, meaning that CO 2 capture processes still cannot be deployed on a large scale for industrial applications other than natural gas treatment or chemical production. Intensive research efforts must therefore be maintained to test innovative ideas and develop emerging technologies such as the use of clathrates for CO 2 separation.

Gas clathrates are supramolecular entities composed of a network of host molecules forming cages or channels capable of stabilizing different types of gas molecules in well-defined thermodynamic conditions. If the host molecule is water, clathrate hydrates are the host-guest compounds, commonly called gas hydrates. If the host molecule is organic, the hostguest compounds in this case are organic clathrates. Among the portfolio of new solutions on offer to separate CO 2 from gaseous compounds, technologies based on clathrate hydrates were initially presented as a gas separation technique potentially more advantageous than conventional separation methods, such as cryogenic fractionation, gas absorption, adsorption processes, or membranes. [START_REF] Dashti | Recent Advances in Gas Hydrate-Based CO 2 Capture[END_REF][START_REF] Tajima | Energy Consumption Estimation for Greenhouse Gas Separation Processes by Clathrate Hydrate Formation[END_REF] The hydrate-based process could be particularly advantageous from an economic point of view when the raw gaseous stream is already pressurized, typically for natural gas sweetening (i.e., CO 2 /CH 4 gas mixtures) or for precombustion CO 2 capture (i.e., CO 2 /H 2 gas mixtures). Various studies have already demonstrated its potential for separating CO 2 from gas mixtures. [START_REF] Linga | The Clathrate Hydrate Process for Post and Pre-Combustion Capture of Carbon Dioxide[END_REF][START_REF] Duc | CO 2 Capture by Hydrate Crystallization -A Potential Solution for Gas Emission of Steelmaking Industry[END_REF][START_REF] Castellani | Hydrate-Based Removal of Carbon Dioxide and Hydrogen Sulphide from Biogas Mixtures: Experimental Investigation and Energy Evaluations[END_REF][START_REF] Dabrowski | Purification of Natural Gases with High CO 2 Content Using Gas Hydrates[END_REF] However, although considerable improvements have been made in many respects, there are still many obstacles to overcome before the process can be deployed on a wide scale: (i) the slow formation rate of the gas hydrates crystals, (ii) the difficulties posed by handling a multicomponent and multiphase mixture in a gas separation unit, e.g., from the gas capture (hydrate formation) to the release section (hydrate dissociation), and (iii) insufficient hydrate selectivity toward CO 2 compared to other technologies such as adsorption or absorption, particularly for CO 2 /CH 4 separation. [START_REF] Ricaurte | CO 2 Removal from a CO 2 -CH 4 Gas Mixture by Clathrate Hydrate Formation using THF and SDS as Water-Soluble Hydrate Promoters[END_REF]10 The future directions and challenges for hydratebased gas separation, as well as information on the various types of chemical additives (promoters), innovative contactor designs, and new materials tested over the two past decades, were presented in recent reviews on the subject. [START_REF] Babu | A Review of the Hydrate Based Gas Separation (HBGS) Process for Carbon Dioxide Precombustion Capture[END_REF][START_REF] Linga | A Review of Reactor Designs and Materials Employed for Increasing the Rate of Gas Hydrate Formation[END_REF] In the same way as water can form hydrates in the presence of certain gases, hydroquinone (HQ) is also known to form organic clathrates with various gaseous species (e.g., CO 2 , H 2 S, CO, and SO 2 ) in well-specified thermodynamic conditions of pressure and temperature. [START_REF] Atwood | Encyclopedia of Supramolecular Chemistry[END_REF] These gases are retained in a network of self-associating molecules of HQ forming cavities, with an ideal stoichiometry corresponding to the ratio 3:1 (i.e., three molecules of HQ per guest molecule). [START_REF] Atwood | Encyclopedia of Supramolecular Chemistry[END_REF][START_REF] Steed | Core Concepts in Supramolecular Chemistry and Nanochemistry[END_REF] The HQ clathrates (β-HQ) are characterized by an occupancy factor (called clathrate occupancy), which is the proportion of cavities filled by guest molecules, ranging from 0 for guest-free clathrates to 1 for full clathrates. [START_REF] Steed | Core Concepts in Supramolecular Chemistry and Nanochemistry[END_REF][START_REF] Atwood | Structural Aspects of Inclusion Compounds Formed by Organic Host Lattices[END_REF] The formation of HQ clathrates is achieved in a two-step reaction, in which the native HQ (α-HQ) is put in contact with suitable guest molecules. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF][START_REF] Coupan | New Insights on Gas Hydroquinone Clathrates using in Situ Raman Spectroscopy: Formation/Dissociation Mechanisms, Kinetics and Capture Selectivity[END_REF] The gas molecules are first included in the cavities of α-HQ (in a phenomenon called "gas solubilization in the HQ αform"). [START_REF] Belosludov | Hydroquinone Clathrates and the Theory of Clathrate Formation[END_REF][START_REF] Conde | Revisiting the Thermodynamic Modelling of Type I Gas-Hydroquinone Clathrates[END_REF] Then after an induction period (corresponding to the time during which the α-HQ starts to transform into clathrates), the α/β phase transition takes place and the β-HQ is gradually filled.

It was recently shown that HQ clathrates could be used for the selective capture of CO 2 contained in CO 2 /H 2 , 20,21 CO 2 / N 2 , [START_REF] Lee | Preferential Occupation of CO 2 Molecules in Hydroquinone Clathrates Formed from CO 2 /N 2 Gas Mixtures[END_REF][START_REF] Lee | Separation of CO 2 from Flue Gases using Hydroquinone Clathrate Compounds[END_REF] and CO 2 /CH 4 17,20,24 mixtures. In addition, the equilibrium conditions of several HQ clathrates, and particularly CO 2 -HQ clathrates, were recently reported in literature [START_REF] Coupan | Experimental Determination of Phase Equilibria and Occupancies for CO 2 , CH 4 , and N 2 Hydroquinone Clathrates[END_REF] and demonstrated that HQ clathrates could be a very interesting alternative for gas storage and separation applications. Furthermore, it should be noted that if the HQ clathrates are entirely filled with CO 2 (i.e., a clathrate occupancy of 1), they can store approximately 94 L of CO 2 (STP) per liter of HQ. This is similar to the volume capacity of good adsorbents such as silica MCM-41 (81 L at 298 K), [START_REF] Belmabkhout | Adsorption of CO 2 from Dry Gases on MCM-41 Silica at Ambient Temperature and High Pressure. 2: Adsorption of CO 2 /N 2 , CO 2 /CH 4 and CO 2 /H 2 Binary Mixtures[END_REF] zeolite 13X (187 L at 298 K), [START_REF] Cavenati | Adsorption Equilibrium of Methane, Carbon Dioxide, and Nitrogen on Zeolite 13X at High Pressures[END_REF] zeolitic imidazolate framework ZIF-69 (83 L at 273 K), [START_REF] Banerjee | High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO 2 Capture[END_REF] and activated carbon MaxsorbAC (162 L at 298 K). [START_REF] Himeno | High-Pressure Adsorption Equilibria of Methane and Carbon Dioxide on Several Activated Carbons[END_REF] In addition, the storage capacity of the HQ clathrates is comparable to that of clathrate hydrates, i.e., around 170 L of CO 2 (STP) per liter of H 2 O (considering the stoichiometry of the hydrate-forming reaction of one molecule of CO 2 per 7.3 molecules of H 2 O). [START_REF] Uchida | Physical Data of CO 2 Hydrates[END_REF] However, the kinetics of HQ clathrate formation need to be much improved, particularly if a clathrate-based gas separation process is to be deployed on an industrial scale. Indeed, the gas capture rate has to be high enough to compete with adsorption and reactive absorption technologies. Previous studies have shown that enclathration kinetics are influenced by pressure and temperature conditions. The induction period is reduced at high pressure, [START_REF] Allison | Clathration by Phenol and Quinol[END_REF] and the enclathration kinetics are faster at high temperature. [START_REF] Lee | Preferential Occupation of CO 2 Molecules in Hydroquinone Clathrates Formed from CO 2 /N 2 Gas Mixtures[END_REF] Another parameter that significantly affects the clathrate formation kinetics is the gas-solid contact area: the larger the area, the faster the kinetics. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF][START_REF] Lee | Gas-Phase Synthesis and Characterization of CH 4 -Loaded Hydroquinone Clathrates[END_REF] Building on this knowledge, we recently developed an HQ-based composite material by impregnation of porous silica particles. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] In CO 2 capture experiments using this reactive medium at 3.0 MPa and 323 K, we demonstrated that (i) there was no need for an induction period, and (ii) the time needed to fill 50% of the clathrate cavities (known as t c 50 ) was much shorter compared to experiments involving powdered HQ (about 6 h instead of 9.1 days). Despite this progress, it is still essential for us to learn more about the key parameters controlling the enclathration kinetics so that further improvements can be made.

In this work, the HQ was conditioned in different ways in order to evaluate and quantify the impact of textural parameters on kinetics: it was ground to a powder, compacted to form pellets, or deposited on porous supports. The efficiency of the CO 2 gas capture kinetics of the media generated was evaluated by performing gravimetric measurements using a magnetic suspension balance. In our previous study, [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] the same series of techniques were used to compare HQ powder and composite materials, but at only one pressure (3.0 MPa) and temperature (323 K) value for two type of supports (spherical and irregular silica particles). The impact of the pore size of the support, and the effects of pressure and temperature conditions on the kinetics of CO 2 capture, were not investigated. This study therefore (i) assesses HQ pellets, an innovative way of conditioning HQ, for the very first time, (ii) evaluates the influence of both pressure and temperature conditions on the kinetics of CO 2 capture by HQ clathrates, (iii) analyzes the effect of the quantity of HQ loaded into the silica support, and (iv) assesses the influence of the pore size of the support.

■ EXPERIMENTAL SECTION Materials. The HQ (purity of 99.5 mol %) used in this work is provided by Acros Organics. The helium and CO 2 used for the experiments (minimum mole fraction purity of 99.995%) are purchased from Linde Gas SA. Absolute ethanol (purity greater than 99 mol %) is used as the solvent in the wetimpregnation method used to create the HQ composite materials. The porous supports employed are high-purity analytical-grade spherical silica particles (SiliaSphere, referred to as SS below) and high-grade irregular silica particles (SiliaFlash, referred to as SF below) provided by SiliCycle. The SS particles, of 200-500 μm in size, have pores of 50 nm. Three types of SF particles are used: (i) particles of 200-500 μm with a pore size of 38 nm (written hp-SF), (i) particles of 500-1000 μm with a pore size of 16 nm (noted mp-SF), and (iii) particles of 800-1200 μm with a pore size of 10 nm (written lp-SF).

Apparatus and Method. Conditioning the HQ. The HQ is conditioned in three different ways: (i) grinding, (ii) compaction, and (iii) deposition on supports. For the grinding, a mixer mill (model MM400 from Retsch) is used, producing powdered HQ with a particle size of no more than 100 μm. The compaction process is performed under 5 tons with a mechanical squeeze tool using evacuable pellet dies (from Specac) creating HQ pellets with an inner diameter of 13 mm and a thickness of 5 mm. The deposition of HQ on porous silica supports is performed by wet-impregnation (WI) based on a protocol previously detailed in literature. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] In short, after thermal pretreatment (i.e., drying at 423 K in a muffle furnace for 24 h), the porous supports are put to soak in an HQ in ethanol solution for 24 h at 308 K. The impregnated particles are then filtered and placed in the oven to dry for 24 h at 308 K. For the four particle batches, the impregnation solution used is a saturated HQ in ethanol solution prepared at 308 K based on the solubility data given in literature. [START_REF] Li | Solubility of Hydroquinone in Different Solvents from 276[END_REF] Additionally, for SS particles, 60% and 20% saturated HQ in ethanol solutions (at 308 K) are used. Although we had demonstrated that the dryimpregnation (DI) method in a fluidized bed produces an HQ-silica composite material that has slightly more efficient enclathration kinetics, [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] we preferred working with materials produced by wet impregnation in this study. This method is more suitable than the DI one when only a few grams of material are required (as the minimum load for a DI operation performed in our apparatus is 0.2 kg of silica particles per batch).

Physicochemical Characterizations. The samples are visualized by optical microscopy using a camera (Qioptiq) with a zoom system of ×16. The three-dimensional microstructure of HQ pellets is obtained by X-ray tomography using a Zeiss Xradia Versa 510. The morphology of the impregnated particles is observed thanks to a tabletop scanning electron microscope (SEM) (model TM3000 from Hitachi). This SEM is equipped with an energy dispersive X-ray spectrometer (EDX) detector (model SwiftED3000 from Oxford Instruments) that makes it possible to characterize the elemental composition of the surface analyzed. The mean particle diameters are evaluated using a laser diffraction particle size analyzer from Malvern Instruments (model Mastersizer 2000). The HQ content of the composite materials (ratio of the impregnated crystal mass to the support mass) is determined by thermogravimetric analysis (TGA) using a thermobalance analyzer (model Q600 from TA Instruments) that measures the mass loss of the sample exposed to a temperature ramp from ambient temperature to 773 K in an oxidizing atmosphere (air). The textural parameters (i.e., specific area, total pore volume, and pore size distribution (PSD)) are investigated by N 2 porosimetry analysis, which consists of adsorptiondesorption isotherms at 77 K using an automatic Micromeritics TriStar II 3020 system. This system can be used to evaluate mesoporous materials (i.e., materials with a pore size of 2 to 50 nm).

Kinetic Evaluation. The gas capture kinetics are measured by gravimetry using a Rubotherm magnetic suspension balance. The experimental apparatus and methodology are presented in detail in Coupan et al. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] and Khaddour et al., [START_REF] Khaddour | A Fully Consistent Experimental and Molecular Simulation Study of Methane Adsorption on Activated Carbon[END_REF] but we can say, briefly, that approximately 1 g of sample is used for each experiment. Prior to operations, the sample is purified at 323 K under vacuum until its mass is constant. The exact volume and mass of the sample are obtained before the gas capture experiment by a calibration step performed with helium. The measurement chamber is then pressurized with CO 2 at the target temperature. Experiments are performed at temperatures of 298, 323, and 348 K and pressures of 1.5, 3.0, and 4.5 MPa. The pressure is kept constant throughout the experiment by continuously adding feed gas. The release step then involves reducing the pressure in the system to 0.1 kPa and increasing the temperature to 348 K. On the basis of the previous study of Coupan et al., [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] we made sure that the excess mass measured (i.e., the mass measured by the magnetic suspension balance) differed from the total mass of gas captured by HQ clathrates by less than 0.1%. The reproducibility error of these experiments is evaluated at 7%.

■ RESULTS AND DISCUSSION

Conditioning and Characterization. The different conditioning processes produced powdered HQ, HQ pellets, and HQ-silica composite materials. The initial motivation for using pellets was principally to avoid the constraints related to the use of pulverulent material (in this case powdered HQ), essentially: (i) an excessive pressure drop in the process if a fixed bed reactor is chosen for gas-solid contacting, (ii) dispersal of very fine particles to other parts of the gas separation unit, and (iii) application of stringent safety rules and the use of personal protective equipment to prevent the operator from inhaling HQ dust. The characteristics of native HQ and powdered HQ are presented in Figure S1 of the Supporting Information. As we can see, grinding significantly reduced the size of HQ crystals. The mean particle sizes of native and powdered HQ were 163 and 25 μm, respectively. HQ pellets are shown in Figure 1.

The N 2 gas porosimetry measurements performed on native, powdered, and compacted HQ, indicated that negligible amounts of gas were adsorbed on these samples, proving that these media are neither micro-nor mesoporous materials (i.e., the samples do not have pores of less than 50 nm in diameter). Consequently, the specific area can be approximated to the geometrical surface of the crystals. In a first approximation, we estimated this surface using the particle size analyzer which gave results of 0.04 and 0.24 m 2 /g for native and ground HQ, respectively.

The HQ pellets (Figure 1a) were investigated using X-ray tomography. Three-dimensional scans with a voxel size of 1 μm revealed a dense microstructure exhibiting a large number of voids, essentially planar (Figure 1b). These planar voids correspond to the contact planes between different HQ crystals and to the compaction fractures within individual crystals. As compaction broke the crystals, but did not separate the fragments, the original monocrystals can still be identified at some locations, as illustrated by the blue contours in Figure 1c. However, the high degree of fragmentation makes it impossible to attribute each fragment to a crystal.

Coupan et al. ( 2017) [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] showed that HQ monocrystals exhibit macroporosity estimated to be less than 1%. When the HQ crystals are compacted into pellets, these macrovoids are crushed and the connected network of newly formed fracture planes produces additional void spaces and porosity. The X-ray data can be used to qualitatively assess this intercrystal porosity, which stands at around 4%, i.e., it is at least 4 times greater than that of the HQ monocrystals. But a quantitative assessment is not possible, as hairline fractures are invisible at the resolution of the scan.

The compaction-induced fragmentation also increases specific area. The visible specific area is 0.16 μm 2 /μm 3 (≈ 0.12 m 2 /g or 3 times greater than for native HQ), but the real value is undoubtedly significantly higher, as a surface texture of less than a few micron cannot be accurately determined from the X-ray data.

On the basis of the density of pure HQ (1.33 g/cm [START_REF] Dashti | Recent Advances in Gas Hydrate-Based CO 2 Capture[END_REF] ) and the apparent density of the HQ pellets (i.e., the ratio of the pellets' mass to the apparent volume), the average porosity calculated with six pellets is an estimated 4.7 ± 2.0%, confirming the previous determination by X-ray tomography.

Concerning the HQ-silica composite, we confirmed from previous work that the HQ impregnated on both the SS and SF particles did not undergo any chemical transformation. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] The different HQ-silica composite materials were analyzed by TGA. The HQ contents of the lp-SF-, mp-SF-, and hp-SF-based composite materials were 0.39 ± 0.02, 0.32 ± 0.02, and 0.44 ± 0.03 g HQ /g Support , respectively. For the SS-based composite materials synthesized from 100%-, 60%-, and 20%-saturated solutions of HQ, the HQ contents were 0.41 ± 0.01, 0.27 ± 0.04, and 0.10 ± 0.02 g HQ /g Support , respectively. Figure 2 shows SEM images of the SS-based composite materials obtained using the three HQ solutions: it clearly appears that HQ coats the external surface of the silica particles, as already reported in literature for other wet-impregnated particles. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] However, this coating is not uniform and depends on the concentration of HQ used in the impregnation solution: the higher the concentration, the more uniform the impregnation. The EDX analysis enabled us to check the surface of the impregnated particles where the target elements are carbon (from the HQ molecule) and silicon (from the silica support). These analyses revealed the presence of HQ not detected by the SEM. Indeed, despite the latter's high magnification, part of the silica surface appears to be uncovered, whereas the EDX detected carbon. It can therefore be affirmed that the pores are filled with HQ crystals.

The different HQ-based composite materials synthesized from SF particles were analyzed by N 2 gas porosimetry. SS particles were not included in this evaluation as part of their PSD exceeds the upper measuring limit of our equipment. The adsorption-desorption isotherms and the PSD of SF particles are shown in Figure S2 of the Supporting Information. According to the IUPAC classification, the isotherms are type IV and present type H1 hysteresis loops characteristic of mesoporous materials. The textural parameters obtained for native and impregnated particles are reported in Table S1 of the Supporting Information. The results obtained on native SF supports are consistent with supplier data to within 9% for porous volume and 4% for both pore diameter and specific area.

From the data obtained, it is clear that changes occurred after impregnation of the silica supports. Both the porous volumes and the specific areas of the three SF-based composite materials decreased by about 40% with respect to those of the native SF supports. However, the mean pore diameters of the impregnated SF particles were not affected by the HQ impregnation. By analyzing the PSD plots, we inferred that, in the case of the lp-SF-and mp-SF-based composite materials, about half of the pores were possibly totally filled or plugged by HQ crystals and the remaining pores were HQ-free. In the case of the impregnated hp-SF material, a second size class was observed, centered at about 28 nm and corresponding to 52% of the pore volume of the composite material. Accordingly, we concluded that half of the pores were partially filled with HQ crystals, and the remaining were HQ-free. This observation had already been made for hp-SF that had undergone dry impregnation in a fluidized bed in a previous study. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] Evaluating the Kinetics of Pure HQ. In this section the kinetics of pure HQ, conditioned in different forms (native, powder and compact), were evaluated in gas capture experiments. The amount of CO 2 captured (molar amount normalized by the mass of HQ) was tracked over time. To quantify the enclathration kinetics, we defined kinetic criteria considering that the clathrate formation mechanisms involve two stages (gas solubilization in the native α-HQ followed by enclathration by the β-HQ form) as was recently demonstrated. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF][START_REF] Coupan | New Insights on Gas Hydroquinone Clathrates using in Situ Raman Spectroscopy: Formation/Dissociation Mechanisms, Kinetics and Capture Selectivity[END_REF][START_REF] Allison | Clathration by Phenol and Quinol[END_REF] Two reaction rates could therefore be defined for each gas uptake: an initial reaction rate (r 0 ) for the gas solubilization in the native α-HQ, and a maximum reaction rate (r c ) for the enclathration step. The reaction rates were calculated by numerical differentiation based on the quantity of CO 2 captured versus time, considering a running interval of around 1 h. Characteristic times were measured: the induction time (t ind ), and a characteristic time (t c 50 ) at which the clathrate occupancy (x) was 50%. The induction time is the amount of time elapsed between the pressurization (gas-solid contacting at t = 0) and the onset of gas capture by enclathration. In these experiments, the gas capture rate versus time curve passed through a minimum value situated between the initial (r o ) and maximum (r c ) rates, due to two successive capture phenomena (the gas is first solubilized when the HQ is in α-form and then enclathrated when it is in β-form). More details on this calculation can be found in another publication on the subject. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] The clathrate occupancy was also recorded after 10 and 30 days of reaction. All the kinetic criteria obtained for native, powdered, and compacted HQ are listed in Table 1.

Effect of P/T Conditions on HQ Powder. First, only the powdered HQ was tested in order to investigate the effect of temperature and pressure conditions on the CO 2 enclathration kinetics. Experiments were conducted at 293 K/3.0 MPa, 323 K/3.0 MPa, and 323 K/4.5 MPa. We determined a reaction time of 1 month to allow sufficient time to discriminate the kinetics. Previous results have indeed shown that the enclathration kinetics are slow and that the system can only reach a pseudoequilibrium state (i.e., low mass variation over time) after 45 days. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] The amounts of gas captured by the powdered HQ over time for the given pressure and temperature conditions are shown in Figure 3. The influence of pressure and temperature was evaluated by considering increases of 1.5 MPa (experiments conducted at 3.0 and 4.5 MPa for 323 K) and of 30 K (experiments conducted at 293 and 323 K for 3.0 MPa). Analysis of the quantitative data reported in Table 1 reveals that, when pressure or temperature increases, the induction time is reduced and the CO 2 enclathration kinetics are enhanced. Allison and Barrer (1967) [START_REF] Allison | Clathration by Phenol and Quinol[END_REF] performed similar investigations for xenon (Xe) capture by HQ clathrates. They found that (i) induction times were nearly independent of temperature (experiments ranging from 233 to 283 K at a very low pressure of 0.072 MPa) but were highly dependent on pressure (experiments ranging from around 0.007 to 0.07 MPa at 273 K), and (ii) the enclathration rate increased with temperature (from 218 to 267 K at constant total pressure). One divergence, concerning the influence of temperature on the induction time, might be related to the temperature range covered in the two studies. At low temperature (i.e., the experimental conditions of Allison and Barrer), the effect of temperature, related to lower thermal agitation, on the induction time is negligible and hardly measurable. In addition, this difference could also be ascribed to the nature of the gas which was used (Xe instead of CO 2 ) and to the pressure level, which was 40 times lower in Allison and Barrer's experiments. Finally, our conclusions agree with those of Lee et al. (2011), [START_REF] Lee | Preferential Occupation of CO 2 Molecules in Hydroquinone Clathrates Formed from CO 2 /N 2 Gas Mixtures[END_REF] who also found that the initial rate of formation of the HQ clathrates with pure CO 2 increases with temperature. Consequently, in the conditions of this study, increasing pressure and temperature significantly enhanced the enclathration kinetics, as observed in typical gas-solid reactions. [START_REF] Baasel | Kinetics of a Typical Gas-Solid Reaction[END_REF][START_REF] Slycke | Thermochemical Surface Engineering of Steels[END_REF][START_REF] Szekely | Gas-Solid Reactions[END_REF] An increase in pressure effectively concentrates CO 2 molecules in the gas phase (and at the gas-solid interface) and enhances the probability of contact between CO 2 and the HQ surface. A temperature increase involves both a decrease of the density of the gas phase and the intensification of the degree of motion (i.e., thermal agitation) of the CO 2 molecules. Therefore, the effect of pressure and temperature on kinetics of HQ clathrates formed by direct gas-solid reaction can be well explained by the collision theory. [START_REF] Balint-Kurti | Theory of Molecular Collisions[END_REF] For the conditions investigated here, it appears that a 1.5 MPa increase in pressure has a more significant effect on the enclathration kinetics than a 30 K increase in temperature.

As described in literature, [START_REF] Allison | Clathration by Phenol and Quinol[END_REF] a linear fit generally links the logarithm of the induction time to the induction pressure (i.e., overpressure by reference to the thermodynamic equilibrium pressure) for a constant temperature, as given by eq 1:

= • + t a P b ln( ) ln( ) ind ind (1)
where t ind is the induction time expressed in hours, P ind is the induction pressure expressed in MPa, and a and b are constants. Consequently, on the basis of our experimental data for powdered HQ at a temperature of 323 K (see Table 1), and considering a clathrate equilibrium pressure of 0.27 MPa at the said temperature, [START_REF] Uchida | Physical Data of CO 2 Hydrates[END_REF] the coefficients a and b were estimated at -4.4 and 5.9. It appeared that the induction time could be greatly reduced by increasing pressure. As an example, we were able to predict, based on these data, that the induction time would be about 5 min for a total pressure of 7.0 MPa at 323 K.

To properly describe the kinetic law of enclathration when using powdered HQ, the induction time is deducted from the total time, as shown in Figure S3 of the Supporting Information. Drawing on the experimental data obtained with powdered HQ, the differential rate law was then used to determine the reaction order. From the linear plot of the reaction rate logarithm against the storage capacity logarithm (Figure S4 of the Supporting Information), the slope was found to be equal to 1.98, confirming a second-order reaction, as already suggested in literature. [START_REF] Allison | Clathration by Phenol and Quinol[END_REF] Consequently, on the reduced time scale (after removal of the induction time), the reaction kinetics could be defined by eq 2:

= - X t k X d d [1 ] 2 (2)
where k is the rate constant and X the conversion ratio defined by eq 3:

= X q q eq (3)
where q is the storage capacity at time t, and q eq is the storage capacity at an infinite time at which the equilibrium state is expected. Integrating eq 2 leads to eq 4, which gives the conversion ratio as a function of the reduced time:

= ′ + X kt 1 1 1 (4)
Considering the definition of the conversion ratio, it can be written as eq 5:

= ′ + q q kt q 1 1 1
eq eq

(5)

The latter can then be expressed as eq 6:

= ′ + q m t n 1 (6)
where m and n are respectively the slope and intercept of the linear fit of the storage capacity inverse as a function of the reduced time inverse. As shown in Figure S5 of the Supporting Information, the linear fit of the storage capacity inverse as a function of the reduced time inverse produces both the equilibrium storage capacity and the rate constant at the pressure and temperature conditions considered. The clathrate occupancy at equilibrium state (x eq ) is calculated based on equilibrium storage capacities. All these data are presented in Table 2.

Increasing both the temperature and pressure produced a higher rate constant. A 30 K increase doubled the rate constant and a 1.5 MPa increase quadrupled it. Furthermore, it was confirmed that the clathrate occupancies at equilibrium state were enhanced at low temperature [START_REF] Uchida | Physical Data of CO 2 Hydrates[END_REF] and high pressure. [START_REF] Conde | Revisiting the Thermodynamic Modelling of Type I Gas-Hydroquinone Clathrates[END_REF] From the results obtained at a pressure of 3.0 MPa for powdered HQ, and considering the Arrhenius equation (see Equations S1 to S3 in the Supporting Information), it was therefore possible to estimate that the activation energy of the CO 2 -HQ system was around 14 ± 1 kJ/mol CO2 . We are aware that only two data points were used to estimate the activation energy and that the result given in this study is considered as a rough estimation. The only data that can be used for comparison is the activation energy of 17.6 kJ/mol Guest found for the Xenon-HQ system at about 0.1 MPa over a temperature range of around 218-267 K. [START_REF] Allison | Clathration by Phenol and Quinol[END_REF] Effect of Conditioning and Recycling on Pure HQ. The native, powdered, and compacted forms of HQ were evaluated at 323 K and 3.0 MPa in two successive gas capture and gas release runs, performed on the same initial material. We have called cycle the combination of a formation step followed by a regeneration step: for example, cycle 1 is the first formation step followed by its regeneration step. Run 1 is therefore the stage in which HQ and gas react together for the first time, and run 2 (starting cycle 2) is the second formation step performed with the same material that has already undergone a gas capture and a gas release cycle (in cycle 1). These experiments allowed us to assess the influence that conditioning and recycling of the used reactive media has on kinetics and to study the variation of the induction time at this stage between the two runs. The first gas capture cycle lasted 1 month and the second 10 days. Figure 4 shows the amounts of CO 2 captured over time in the two successive gas capture runs for the three media used. In respect of the kinetic criteria obtained in Table 1, the clathrate formation kinetics were clearly improved by grinding and compacting the HQ crystals. It also appeared that compaction is more efficient than grinding for improving kinetics (Figure 4b,c). We are, however, aware that an extra fine HQ power, even though hard to handle and difficult to use for practical and safety reasons, could compete with HQ pellets in terms of performance. For example, when HQ pellets are used instead of powdered HQ, the induction time is reduced by a factor of ∼4 and the enclathration kinetics are enhanced (e.g., t c 50 divided by ∼2 and r c multiplied by ∼2.6). These results clearly reveal the importance of the contact area between the HQ and the gas: because of the high degree of fragmentation of the native crystals of HQ compacted in the pellet, this leads to numerous nucleation sites and a large surface area, which enhance both the clathrate initiation and formation processes. After one month's reaction with CO 2 , clathrate occupancies of 0.63 ± 0.04, 0.71 ± 0.04, and 0.80 ± 0.05 at 323 K and 3.0 MPa for native, powdered, and compacted HQ were found. These differences in clathrate occupancy suggest that the systems have not all reached their equilibrium state (i.e., the equilibrium plateau where the mass of CO 2 captured does not evolve over time) after 1 month. Note that the occupancy values found at this pseudoequilibrium state are in good agreement with literature data. [START_REF] Lee | Preferential Occupation of CO 2 Molecules in Hydroquinone Clathrates Formed from CO 2 /N 2 Gas Mixtures[END_REF][START_REF] Torre | CO2-Hydroquinone Clathrate: Synthesis, Purification, Characterization and Crystal Structure[END_REF][START_REF] Peyronel | On some new clathrates of hydroquinone[END_REF][START_REF] Mcadie | Thermal Decomposition of Molecular Complexes: IV. Further Studies of the β-Quinol Clathrates[END_REF] Looking at the second gas capture run, a marked improvement can be seen for the three media (i.e., a decrease in reaction times (t ind and t c 50 ) and an increase in reaction rates (r 0 and r c )) (see Figure 4 and Table 1). For example, comparing the first and second gas capture runs for native HQ, the t c 50 was divided by 2.3 and r c was multiplied by 5.9. Moreover, t ind appears to be considerably lower and was about 0.2 h for both native and powdered HQ. Interestingly enough, for the HQ pellets, no induction period can be observed. These improved kinetics in the second gas capture run are in line with the "preforming effect" recently highlighted by Coupan et al. [START_REF] Coupan | New Insights on Gas Hydroquinone Clathrates using in Situ Raman Spectroscopy: Formation/Dissociation Mechanisms, Kinetics and Capture Selectivity[END_REF] Indeed, it was demonstrated that after a first formation-dissociation cycle, there is a visible modification in the morphology of the crystal surfaces (the native HQ crystals have smooth, regular surfaces, and after one cycle, the same crystals exhibit a rough aspect with perforated surfaces), which results in a substantial increase in the solid-gas contact area.

Kinetic Evaluation of HQ-Silica Composite Materials. In this section, the gas capture experiments were conducted on the different HQ-based composites. The kinetic criteria obtained for these media are given in Table 3. As observed in our previous work, [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] the HQ-silica composite materials make it possible to couple adsorption on silica and enclathration through HQ. Additional adsorption measurements have shown that the different native silica supports reach the adsorption plateau after 6 ± 3 min. Considering this period, the amount of gas captured by adsorption on the HQ-silica composite materials can be graphically estimated. The adsorption capacities (q ADS ) measured for the different media are reported in Table 3. So, to study the effect of the impregnation and compare the results to those obtained with the other types of conditioning (i.e., grinding and compaction), the amount of CO 2 adsorbed on silica was deducted from the total amount of CO 2 captured by the composite materials. For this reason, only the amount of CO 2 captured by enclathration is considered in the section below.

Compared to native HQ, and despite the fact that grinding and compaction of HQ seem kinetically interesting, it is obvious that the kinetics of CO 2 enclathration using HQ-silica composite materials are much more efficient. Indeed, with these media, the system reached an equilibrium value of clathrate occupancy after approximately 1 or 2 days, whereas 1 month's reaction was insufficient to attain equilibrium state when using native, powdered, and compacted HQ. Moreover, it appeared that no induction period, or one of just a few minutes, was required when using HQ-silica composite materials. As an example, for the impregnated SS particles at 0.41 g HQ /g Support , r c was increased by a factor of 71 and t c 50 was decreased by a factor of 79 compared to native HQ at 323 K and 3.0 MPa. As discussed in literature, the improved kinetics and much shorter induction period can be ascribed to the significant increase in contact area between the HQ and the gas due to the presence of HQ crystallites in the mesoporosity of the particle. [START_REF] Coupan | Creating Innovative Composite Materials to Enhance the Kinetics of CO 2 Capture by Hydroquinone Clathrates[END_REF] Experiments with HQ-SS Composites. Using the SS-based composite materials with an HQ content of 0.41 ± 0.01 g HQ / g Support , we verified the effects of recycling, pressure and temperature. Figure 5 shows the amount of gas captured and released by SS-based composite materials over time for four successive cycles. In contrast to the experiments with pure HQ, the first gas capture run with the composite materials appeared to be slightly faster than the following ones (see Figure 5a). To explain this loss of efficiency in kinetics, we believe that the HQ starts to form a structure (i.e., the HQ crystallites agglomerate in the pores of the support) after the initial clathrate formation process, and this results in a slight decrease in the gas-solid contact area. Moreover, the first gas release phase was slower than the others (see Figure 5b): around 9.8 h were needed to release all the captured CO 2 in the first release phase, and about 5.5 h were needed in the following ones. In this case, the release kinetics were improved after the first cycle. At the moment, we have no clear explanation as to why the global dissociation kinetics were enhanced after the first run, but we can nevertheless say that the trend is clearly reproducible. After both the first formation and dissociation runs, it is worth noting that the kinetics remained almost unchanged in the following runs. Consequently, the HQ-based composite materials can be recycled without any noticeable degradation to achieve reversible CO 2 capture.

Regarding the experiments performed at 323 K and 1.5, 3.0, and 4.5 MPa (Figure 6a), the previous section on pure HQ confirmed the favorable influence of pressure on kinetics. In the case of the composite material, however, the improvement was less pronounced than for powdered HQ. It is worth noting that in the actual case study, the test at 323 K and 4.5 MPa using the SS-based composite material with an HQ content of 0.41 ± 0.01 g HQ /g Support gave the best kinetic criteria (i.e., r c of 35.8 mmol CO2 /kg HQ /min, and t c 50 of 1.9 h) and a clathrate occupancy of 0.99.

Concerning the experiments performed at 1.5 MPa and 298, 323, and 348 K (Figure 6b), we observed first of all that the curves are very close. In addition, for the experiments at 348 and 323 K, the curve profiles indicated a slight phenomenon of gas sorption corresponding to gas solubilization in the α-HQ. r 0 was therefore measured at 1.0 and 2.2 mmol CO2 /kg HQ /min. Because of the superposition of adsorption phenomena at the beginning of the cycle, it is sometimes hard to precisely evaluate the induction times. At 1.5 MPa, the induction time was less than 15 min for the three temperatures tested. As for the influence of temperature on the enclathration kinetics when using the HQ-based composite, the trend did not appear clearly on the plots of Figure 6b as error bars are superimposed on some parts of the curves. However, comparison of the t 50 values presented in Table 3 show a clear global trend which is consistent with the one given for pure HQ: the enclathration kinetics are improved by a temperature increase. For example, t 50 equals 92.9, 64.9, and 51.4 h at 298, 323, and 348 K, respectively. In addition, with reference to the experiment performed at 323 K and 1.5 MPa, it is obvious that a pressure increase of 1.5 MPa is preferable to a temperature increase of 25 K for improving global kinetics.

On the basis of the kinetic law (eq 5) given in the previous section for powdered HQ, calculations were performed for the HQ-based composite synthesized from SS particles to determine rate constants and gas storage capacities at equilibrium. Table 4 presents the results obtained and shows that the rate constants varied with the pressure and temperature conditions in the same way as discussed previously. Besides, the rate constants obtained with the HQ-based composite were generally higher than those calculated when using powdered HQ. It is worth noting, however, that the rate constant for HQ powder at 323 K and 4.5 MPa was slightly higher than that of the composite at 298 K and 1.5 MPa. From the results obtained at a pressure of 1.5 MPa for the HQ-based composite, it was then possible to calculate the activation energy of the CO 2 -HQ clathrate formation reaction (using Equation S2 in the

■ CONCLUSIONS

This study highlights the influence of the reactive medium and of pressure and temperature conditions on the kinetics of HQ clathrate formation by direct gas-solid reaction.

When using powdered HQ, it was demonstrated that high temperature, and more particularly high pressure, enhanced kinetics. Comparison of the results obtained in the CO 2 capture experiments using pure HQ in three forms (native, powdered, and compacted) as the reactive medium revealed that the enclathration kinetics can be improved by conditioning the HQ which increases the gas-solid contact area. Indeed, HQ pellets are more efficient than HQ powder, with a particle size of 25 μm, and native HQ. Our experimental results therefore demonstrated the important effect of textural parameters on kinetics. In addition, the preforming effect of HQ was confirmed after a first gas capture run, as significant improvements could be observed for native, powdered, and compacted HQ in the second gas capture run. This observation proved that HQ could be recycled to perform successive gas capture runs. In addition, in the specific case of the HQ pellets, there was no induction time as from the second gas capture run.

Owing to these experimental measurements, we proved that the enclathration of CO 2 by HQ is a second-order reaction. For the different P/T conditions, both the rate constants and clathrate occupancies at equilibrium state were determined. In the specific case of HQ powder at a pressure of 3.0 MPa and within a temperature range of 293-323 K, the activation energy of the CO 2 -HQ clathrate formation reaction was estimated to be around 14 ± 1 kJ/mol CO2 .

The HQ-silica composite materials used as a reactive medium gave the best results in terms of improving the HQ clathrate formation kinetics. Indeed, when HQ-silica composite materials are used, the induction period can be almost done away with and the enclathration rate increases substantially compared with the other reactive media. At 323 K and 3.0 MPa, the use of an HQ-based composite formed with SiliaSphere particles increased the enclathration rate by a factor of 71 and reduced the time needed to fill 50% of the clathrate cavities by a factor of 79 in contrast to pure native HQ. Moreover, for a test performed at 323 K and 4.5 MPa with the same composite material, the time needed to fill 50% of the clathrate cavities was reduced to 1.9 h and a clathrate occupancy of 0.99 was achieved. In view of the trends given in this study, these kinetic factors could be even higher when working at higher pressure. In addition, the HQ-silica composite materials can be recycled to achieve reversible CO 2 capture. However, to obtain efficient HQ-based composites, it is required (i) to have a support with a suitable pore size (>16 nm) to avoid plugging all the particle pores and (ii) to perform the impregnation process with a high concentrated HQ solution. These two points ensure that the pores of the support contain small HQ crystallites permitting a large developed HQ reactive area. Now, work is in progress to evaluate the viability and feasibility of using this type of reactive medium at pilot scale in clathrate-based gas separation processes.

■ ASSOCIATED CONTENT

* S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01462.

Aspect of native and powdered HQ; physicochemical characterizations; kinetic calculations; Arrhenius equation (PDF)

■ AUTHOR INFORMATION

Corresponding Author *Phone: +335 40 17 51 09. E-mail: jean-philippe.torre@univpau.fr.

ORCID

Jean-Philippe Torre: 0000-0001-5735-8626 Christophe Dicharry: 0000-0002-6318-3989

Figure 1 .

 1 Figure 1. (a) Full-size photo of the HQ pellets measuring 13 mm in diameter and 5 mm thick. (b) X-ray tomography of a central cylindrical region of 1.935 mm in diameter and 1.935 mm in height. The image shows voids in the pellet. (c) Cross-section through the X-ray data set. Black features represent voids, mostly corresponding to fracture planes. Gray regions correspond to solid HQ. The regions in blue represent fragments that initially belonged to the same HQ monocrystals.

Figure 2 .

 2 Figure 2. SEM images of (a) native SS particles and HQ-SS composite material with an HQ content of (b) 0.41 ± 0.01, (c) 0.27 ± 0.04, and (d) 0.10 ± 0.02 g HQ /g Support .

Figure 3 .

 3 Figure 3. Molar quantity of gas captured by enclathration as a function of time using powdered HQ normalized by mass of HQ. Experiments with CO 2 at (green line) 293 K/3.0 MPa, (yellow line) 323 K/3.0 MPa, and (red line) 323 K/4.5 MPa.

Figure 4 .

 4 Figure 4. Molar quantity of CO 2 captured by enclathration as a function of time at 323 K/3.0 MPa using (a) native HQ, (b) powdered HQ, and (c) compacted HQ normalized by mass of HQ. Dashed and solid lines represent the first and second gas capture runs, respectively.

Figure 5 .

 5 Figure 5. Molar quantity of CO 2 (a) captured by enclathration as a function of time at 323 K/3.0 MPa and (b) released at 348 K/0.1 kPa using SS particles impregnated at 0.41 g HQ /g Support , normalized by mass of HQ: (solid line) run 1, (dotted line) run 2, (dashed line) run 3, and (dotted/dashed line) run 4.

Figure 6 .

 6 Figure 6. Molar quantity of gas captured by enclathration as a function of time using SS particles impregnated at 0.41 g HQ /g Support normalized by mass of HQ. (a) Experiments performed at 323 K at different pressures: (dashed line) 1.5 MPa, (solid line) 3.0 MPa, and (dotted line) 4.5 MPa. (b) Experiments performed at 1.5 MPa at different temperatures: (dotted line) 298 K, (dashed line) 323 K, and (solid line) 348 K. The error bars on the data points correspond to a relative uncertainty of 7%.

Figure 7 .

 7 Figure 7. Molar quantity of gas captured by enclathration as a function of time at 323 K/3.0 MPa using impregnated SS particles, normalized by mass of HQ. Precursor content of (dashed line) 0.41 g HQ /g Support , (dotted line) 0.27 g HQ /g Support , and (solid line) 0.10 g HQ /g Support .

Figure 8 .

 8 Figure 8. Molar quantity of gas captured by enclathration as a function of time at 323 K/3.0 MPa using (dotted/dashed line) h-SF, (dotted line) m-SF, (solid line) l-SF particles, and (dashed line) SS particles impregnated at 0.41 g HQ /g Support normalized by mass of HQ.

Table 1 .

 1 Kinetic Data of the Gas Captured by HQ Clathrate Formation Using Native, Powdered, and Compacted HQ: (r 0 ) Initial Reaction Rate, (r c ) Maximum Reaction Rate, (t ind ) Induction Time, (t c

	50 ) Characteristic Time at Which the Clathrate

Table 2 .

 2 Equilibrium Storage Capacity, Equilibrium Clathrate Occupancy, and Rate Constant Obtained Using Powdered HQ

	P/T conditions	q eq (mol CO2 /kg HQ )	x eq	k (min -1 )
	293 K/3.0 MPa	2.83	0.93	0.55 × 10 -4
	323 K/3.0 MPa	2.64	0.87	0.93 × 10 -4
	323 K/4.5 MPa	2.65	0.88	3.87 × 10 -4

Table 3 .

 3 Kinetic Data of the CO 2 Captured by HQ Clathrate Formation Using HQ-Silica Composite Materials: (q TOT ) Global

	Gas Storage Capacity, (q ADS ) Adsorption Capacity, (r c ) Maximum Reaction Rate, (t c 50 ) Characteristic Time at Which the
	Clathrate Occupancy (x) is 50%
	support	τ (g HQ /g Supp. )	P (MPa)	T (K)	run no.	q TOT (mol/kg media )	q ADS (mol/kg media )	r c (mmol/kg HQ /min)	t c	50 (h)	x at 3 days
	lp-SF	0.39	3.0	323	1	1.77	1.40	6.8		64.9	0.38
	mp-SF	0.32	3.0	323	1	1.53	1.05	19.5		18.4	0.61
	hp-SF	0.44	3.0	323	1	1.47	0.73	28.2		4.8	0.93
	SS	0.10	3.0	323	1	0.52	0.31	1.8		16.6	0.81
		0.27	3.0	323	1	0.80	0.29	5.6		8.4	0.80
		0.41	1.5	298	1	0.78	0.26	2.5		92.9	0.57
		0.41	1.5	323	1	0.75	0.18	2.6		64.9	0.56
		0.41	1.5	348	1	0.63	0.14	2.6		51.4	0.56
		0.41	3.0	323	1	1.02	0.28	22.7		4.5	0.81
		0.41	3.0	323	2	0.90	0.28	12.1		17.8	0.71
		0.41	3.0	323	3	0.97	0.28	11.1		14.4	0.75
		0.41	3.0	323	4	1.00	0.28	21.8		13.9	0.79
		0.41	4.5	323	1	1.15	0.35	35.8		1.9	0.99

Table 4 .

 4 Equilibrium Storage Capacities, Equilibrium Clathrate Occupancies and Rate Constants Obtained Using the HQ-Based Composite Impregnated at 0.41 g HQ /g Support for the First Gas Capture Run

	P/T conditions	q eq (mol CO2 /kg HQ )	x eq	k (min -1 )
	298 K/1.5 MPa	2.53	0.83	3.31 × 10 -4
	323 K/1.5 MPa	2.26	0.75	6.12 × 10 -4
	348 K/1.5 MPa	2.06	0.68	9.20 × 10 -4
	323 K/3.0 MPa	2.51	0.83	53.45 × 10 -4
	323 K/4.5 MPa	3.03	1.0	60.00 × 10 -4

Supporting Information), estimated at around 18 ± 2 kJ/ mol CO2 .

Influence of the Precursor Content and the Silica Support. To better understand the rate at which the HQ-based composite materials capture the CO 2 , experiments were performed on composites with different HQ contents (SS particles impregnated at 0.41, 0.27, and 0.10 g HQ /g Support ) and formed from supports with different pore sizes (h-SF, m-SF, l-SF, and SS particles). Figures 7 and8 show the amount of gas captured by these different HQ-based composite materials over time.

Looking at Figure 7, it appears that the kinetics depend on the HQ content of the composite. Indeed, the kinetics of the composite with a high HQ content were faster. The r c were 1.8, 5.6, and 22.7 mmol CO2 /kg HQ /min for HQ contents of 0.10, 0.21, and 0.41 g HQ /g Support . These observations suggest that the pores contain more small HQ crystallites, the greater the HQ content of the composite materials. The developed area of the impregnated HQ is therefore larger for high HQ contents.

Looking at the CO 2 capture results presented in Figure 8, we can see that the kinetics were affected when HQ-based composite materials synthesized from different silica supports (i.e., impregnated lp-SF, mp-SF, hp-SF, and SS particles) were used. The differences between these composites mainly concerned pore size and specific area (Table S1 of the Supporting Information). Compared to Figure 7, we can assume here that the different precursor contents of the tested media (i.e., from 0.32 to 0.44 g HQ /g Support ) are not significant enough to explain the major differences observed between the kinetic profiles shown in Figure 8. The results highlight an improvement in kinetics when both the pore size increased and the specific area decreased (e.g., the r c of impregnated lp-SF, mp-SF, and hp-SF particles were measured at 6.8, 19.5, and 28.2 mmol CO2 /kgHQ/min, respectively). In addition, the clathrate occupancies of impregnated lp-SF, mp-SF, and hp-SF particles were measured at 0.38, 0.61, and 0.93, respectively, after 2 days' reaction (Table 3). We can therefore conclude that the smaller the pore diameter (within the range of those tested in this study), the lower the kinetic efficiency. This conclusion might imply that (i) the HQ impregnated in small pores (such as those present in lp-SF and mp-SF) does not react with the gas because the pores are plugged, or (ii) the amount of HQ in the pores is very low (or the pores contain no HQ), and the kinetic measurements are related to the HQ crystals at the particle surface explaining the very low values of occupancy obtained after 2 days' reaction. All these points are consistent with our previous conclusions drawn in the Conditioning and Characterization section: the PSD suggests that the pores of lp-SF and mp-SF are totally filled or plugged with HQ crystals. When comparing impregnated hp-SF and SS particles, there are no substantial kinetic dissimilarities. We can thus infer that both the size of the HQ crystallites in the pores and the developed area of the impregnated HQ are of the same order of magnitude for composites synthesized from both hp-SF and SS particles. On the basis of these experiments, it appears that the minimum pore size of the silica supports must be greater than 16 nm (i.e., pore size of native mp-SF) to ensure the efficiency of the enclathration process when using HQ-based composite materials.

Author Contributions

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

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We acknowledge the entire work group involved in the ORCHIDS project, as well as the Gas Solutions department of Total SA (E&P Division) and the Carnot Institute ISIFoR (Institute for the Sustainable Engineering of Fossil Resources) for their financial support. We thank Total for providing UMS 3360 DMEX with the Zeiss Xradia Versa 510T used to carry out the tomographic acquisitions described in this article; we also extend our warmest thanks to F. Guerton, P. Senechal, and P. Moonen for performing the tomographic scans and analyzing them.

■ ABBREVIATIONS USED HQ = hydroquinone DI = dry impregnation WI = wet impregnation


of ozone on strong Lewis acid sites and then its adsorption on weak acidic sites [START_REF] Brodu | Role of Lewis acid sites of ZSM-5 zeolite on gaseous ozone abatement[END_REF]. Nevertheless, there are still some doubts about the nature of chemical interactions between ozone and zeolite surface active sites.

On the other hand, the transition metals and their oxides have been found to be the most active substances for ozone decomposition [START_REF] Batakliev | Ozone decomposition[END_REF]. Zeolite is characterized by its ability to participate in ion-exchange reactions [START_REF] Weitkamp | Zeolites and catalysis[END_REF]. The incorporation of various transition metal ions, such as iron, copper, and cobalt ions, into these materials can significantly change the chemical properties of zeolites making them active and efficient catalysts for several redox processes [START_REF] Hartmann | Transition-Metal ions in aluminophosphate and silicoaluminophosphate molecular sieves: location, interaction with adsorbates and catalytic properties[END_REF]. The crystal structure of ZSM-5 is composed of two types of channel systems with similar size and 10-membered oxygen ring. Straight channels have an oval cross-section of 5.3 × 5.6 Å and sinusoidal channels with cross-section of 5.1 × 5.5 Å. These two types of channels are perpendicular to each other and generate microporous volume network of diameters of 8.9 Å. Na ZSM-5 zeolite is basic and a typical catalyst, but its catalytic ability is poor. Through ion exchange, that is, Na + from the framework is substituted by H + and forms HY zeolite which has unselective acid sites located on the external surface. Furthermore, the hydrothermal stability of HY zeolite is very poor. The presence of protons gives the ZSM-5 zeolite the ability to exchange with transition metal ions. Modification of ZSM-5 including incorporation of transition metal ions leads to good catalytic activity and good hydrothermal stability. The catalytic properties of transition metal zeolites are strongly influenced by the composition, location, and structures of the reactive metal species introduced into the microporous space [START_REF] Frising | Extraframework cation distributions in X and Y faujasite zeolites: A review[END_REF].

Among several methods, aqueous ion exchanges are the most commonly used for preparing zeolite with transition metals. The resulting material contains aqueous metals in the pores and cavities of the zeolite. Upon high temperature treatment, water is removed and the metal coordinates to oxygen's surface of the exchange sites. Several authors have studied the performance of the transition metals in zeolites or on other supports for different organic compounds' catalytic oxidation [20 22].

In this paper, we will focus firstly on the preparation of transition metals Cu and Fe loaded on zeolite ZSM-5 (commercial zeolites) by the conventional ion-exchange method. These catalysts are thus able to work at a low reaction temperature and both Fe and Cu were used as active components for ozone decomposition. The prepared catalysts were then characterized by Fourier transform infrared (FT-IR) techniques and scanning electron microscopy (SEM) assays. The second objective of this work was to compare the activity of parent zeolite ZSM-5 with the prepared catalysts Cu-ZSM-5 and Fe-ZSM-5 toward the decomposition of ozone. Understanding their structures and performance for ozone decomposition might lead to the development of novel active and selective oxidation catalysts.

Material and methods

Catalyst preparation

Commercial zeolite ZSM-5 with specific surface area of 400 m 2 /g and diameter particle of 1.2 2 mm was provided by Zeochem Co. and directly used as the parent zeolite in this study. The main characteristics of adsorbents are given in Table 1. The parent zeolite (40 g) was added to 500 mL of 0.1 M nitrate aqueous solutions of Cu(NO 3 ) 2 and Fe(NO 3 ) 3 and the ion exchange was then carried out under vigorous stirring for 48 h at room temperature. After the ion exchange, the zeolite was filtered, thoroughly washed with deionized water, dried overnight at 100°C, and then calcined at 600°C for 4 h in the air and denoted as Fe-ZSM-5 and Cu-ZSM-5.

Material characterization

Characterization of the parent (ZSM-5) and modified zeolites (Fe-ZSM-5 and Cu-ZSM-5) was performed by FT-IR spectroscopy (Perkin-ElmerSpectrum, USA) to describe the surface functional groups. The information about solid surface morphology (size, shape, and pore distribution) was given by SEM assays (QUANTA-200, Philips).

Experimental setup

Figure 1 shows the schematic diagram of the experimental setup; the pilot reactor was composed of a glass jacket column of 45 mm inner diameter with a height of 160 mm. The experimental conditions are summarized in Table 2. The reactor was filled with a fixed bed of 40 g zeolites. An air stream of ozone was introduced into the reactor bottom. Ozone gas was generated by electrical discharge of the air (Labo5LO Trailigaz ozoner kind). The feed pipes were Teflon and stainless steel. Sampling of the gas phase was performed in the inlet and outlet of the reactor and sent by a diaphragm pump to an UV ozone analyzer (MT 964 BT Messtechnik GmbH, Berlin Germany). The production of ozone is a function of the pressure (0.7 bar) and the air flow rate (Q), which was set at 0.18 m 3 /h with applied power of 90 W. Prior to the experiment, the ozone stream was passed for an hour to stabilize the ozone initial concentration. The inlet concentration of O 3 was adjusted to 13 g/m 3 . The output of the ozone generator and the ozone-containing air flow can be moistened with a sparging system in a water column. Ozone outlet concentration was recorded continuously by UV absorbance at 254 nm with an ozone analyser. The device was connected to a continuous data acquisition system with software (BMT link 964 V1.10).

The moisture content of the effluent could be measured by a portable hydrometer. The temperature was measured continuously at the inlet and outlet of the reactor by Pt100 temperature sensors used to monitor the temperature by digital displays (WEST 6010) fixed on the control cabinet. During each experiment, the zeolite bed was weighed at the end to calculate the difference in weight of zeolite. The exhaust gas stream was sent to an ozone destroyer before discharging into ambient air. The amount of ozone consumed was calculated by integrating the difference between the ozone amounts of column input and output during the experiment. Calculating the amount removed (g/g) is expressed by the following equation (Equation ( 1)):

q = t 0 (C 0 -C).Q v .d t m , (1)
where Q v is the volumetric flow rate (m 3 /min), C is the gas phase concentration at the reactor outlet (g/m 3 ), m is the mass of the zeolites (g), C 0 is the reactor inlet ozone concentration (g/m 3 ), and t is the time needed for the zeolite to reach saturation with ozone. The ozone removal efficiency (%) was calculated on the basis of equation (Equation ( 2)), where C refers to the concentration.

Removal efficiency (

%) = C inlet -C outlet C inlet × 100. (2)
3. Results and discussion

Fourier transform infrared spectrometry

Figure 2 depicts the FTIR spectra for the parent zeolite ZSM-5, Fe-ZSM-5, and Cu-ZSM-5. The absorption features of zeolite samples resulted in different absorption peaks at a range of 400 4000 cm -1 . Based on the literature, peaks at 3640 cm -1 can be assigned to O H groups attached to extra-framework alumina species [START_REF] Sedlmair | An in situ IR study of the NO x adsorption/reduction mechanism on modified Y zeolites[END_REF]. The peak at 3744 cm -1 represents the stretching silanol groups (Si OH) and the vibration at 1660 cm -1 can be assigned to the deformation band of adsorbed water [START_REF] Jing | Preparation and magnetic properties of fibrous gamma iron oxide nanoparticles via a nonaqueous medium[END_REF]. Hassan and Hameed [START_REF] Hassan | Oxidative decolorization of acid Red 1 solutions by Fe zeolite Y type catalyst[END_REF] reported that the region from 1600 to 3700 cm -1 is ascribed to the existence of zeolite water molecules. The zeolite framework also revealed peaks at a range of 1203 1077 cm -1 which are assigned to the complex of Si O Si asymmetric stretchings whereas the bands of 796 580 cm -1 could arise due to Si O Si symmetric stretchings. Moreover, the bands of 580 and 460 cm -1 could be assigned to Si O Si linkages of the alumino-silicates framework [START_REF] Yao | Magnetic field effects on selective catalytic reduction of NO by NH 3 over Fe 2 O 3 catalyst in a magnetically fluidized bed[END_REF]. The peak at 800 cm -1 could be ascribed to the vibration of Al O Si [START_REF] Madejova | FT-IR spectroscopic characterization of thermally treated Cu 2+ , Cd 2+ and Li + montmorillonites[END_REF]. Comparison between the band intensities of the individual samples before and after introducing Fe and Cu into zeolites allowed investigating the structural changes with respect to Fe-, Cu-, and Al-related acid sites. The coordination of Fe and Cu ions with the framework bridging oxygen atoms was documented by an intensity decrease in the stretching vibration band of OH-groups at 3640 cm -1 . This decrease in hydroxyls concentration is consistent with the removal of Al from the framework leading to the formation of an extra-framework Al species and condensation of silanol groups. The bands of Cu-ZSM-5 and Fe-ZSM-5 demonstrated a significant change in the frequency shift or a reduction in the intensity framework. Thus, Cu-ZSM-5 and Fe-ZSM-5 catalysts have a significant effect on the number of the zeolite framework bonds.

Scanning electron microscopy

To study the morphology of zeolites, the solid samples were characterized by SEM before and after the ion exchange treatment. The SEM photographs are shown in Figure 3. It can be observed that the parent ZSM-5 zeolites (Figure 3(A)) have coffin-shaped particles of an average size of about 3 5 µm. Similar morphology and particle size were observed for the modified zeolites; Fe-ZSM-5 and Cu-ZSM-5 (Figure 3(B) and (C), respectively). Therefore, it can be concluded that the morphology of ZSM-5 zeolite remained practically unchanged after the ion exchange treatment. The only detectable difference between these catalysts is that the modified zeolites have metallic (Fe Cu) particles dispersed as aggregate clusters located at the surface of ZSM-5 and the presence of these metals (Fe and Cu) on the modified zeolites is confirmed by the EDX analysis. The size of the metal particles varies at a range of 20 50 nm.

Catalytic performances of parent and transition metals-loaded zeolite

Figure 4 shows the catalytic activity of O 3 elimination over parent zeolite ZSM-5, Fe-ZSM-5, and Cu-ZSM-5.

The behavior of parent zeolite ZSM-5 toward the ozone decomposition was very dissimilar to that of Fe-ZSM-5 and Cu-ZSM-5. As shown in Figure 4, for the parent zeolite ZSM-5, a classical adsorption breakthrough curve was obtained. It was clear from the results that the physical adsorption of ozone took place onto the parent zeolite during the breakthrough time of 300 min, and its slope was increased as the ozone concentration increased. The presence of low Lewis acid sites' concentration could be explained by these results, where the molecular ozone could be adsorbed via coordinative bonding with weak Lewis acid sites. A similar observation was reported by Izumi et al. [START_REF] Izumi | Development on high performance Gas separation process using Gas adsorption[END_REF] who only observed reversible physical adsorption of ozone gas stream on different ZSM-5 for temperatures below 0°C due to the high aluminum content of ZSM-5. Furthermore, ozone could be transformed immediately after adsorption on strong Lewis acid sites and decomposed into O 2 and an active atom of oxygen that could participate in catalytic removal of ozone [START_REF] Brodu | Role of Lewis acid sites of ZSM-5 zeolite on gaseous ozone abatement[END_REF]. The structure of zeolite is another factor that could play a fundamental role in ozone removal. The ratio of ozone molecular size (5.8 Å) to zeolites pore diameter is defined as λ; it is 1.05 in the case of ZSM-5 and ozone diffusion could then be hindered [START_REF] Malherbe | Study of Fourier transform infrared-temperature programmed desorption of benzene, toluene and ethylbenzene from H-ZSM-5 and H-Beta zeolites[END_REF].

However, when Fe-ZSM-5 and Cu-ZSM-5 samples were used, a very high catalytic ozone conversion was observed after 30 min from the beginning of the reaction. Then, the catalyst activity decreased after 150 min and remained finally stable for the remaining duration of the reaction. In this case, the reaction started with very high activity for ozone conversion to oxygen due to the presence of free metals catalytic centers. These results could be related to the fact that the transition metals' ions preferably occupy exchange sites in six-membered oxygen rings where the transition metals coordinate with the oxygen atoms of Al tetrahedra. Moreover, removal of the extra-lattice oxygens during high temperature pre-treatments could result in auto-reduction. Oxidation of reduced transition metals' sites often results in the formation of highly reactive oxygen species [START_REF] Mortier | Compilation of extra framework sites in zeolites[END_REF].

When the ozone streams are made to flow over the catalyst and when the new free centers are accessible, the catalyst then converts the O 3 molecules. Therefore, Fe and Cu metals loaded to the parent zeolite ZSM-5 increase the number of decomposition-active sites by modifying the surface. The difference in zeolites' activities could be related to the formation of metals-containing active sites. According to the literature [31 32], ozone decomposition on the M-ZSM-5 catalyst (M: Cu or Fe) follows this mechanism:

O 3 + {M n+ -Z} O 2 + O {M n+1 -Z}, O 3 + O {M n+1 -Z} 2O 2 + {M n+ -Z}.
Transition metal oxides with several oxidation states could potentially show good catalytic performance in ozone decomposition. In the case of Cu-Z and Fe-Z catalysts, we have redox couples including Cu + Cu 2+ and Fe 2+ Fe 3+ . These couples could be represented by the following reaction mechanism:

O 3 + {Fe 2+ -Z} O 2 + O {Fe 3+ -Z}, O 3 + O {Fe 3+ -Z} 2O 2 + {Fe 2+ -Z}.
Panov et al. [START_REF] Parmon | Nitrous oxide in oxidation chemistry and catalysis: application and production[END_REF] reported the formation of the so-called αsites core in Fe-ZSM-5 by reacting the zeolite with N 2 O. Several studies evaluated the possible bi-nucleate metals' species formed in zeolite upon calcination in O 2 ; from the calculations of Yumura et al. [START_REF] Yumura | Effects of ZSM-5 zeolite confinement on reaction intermediates during dioxygen activation by enclosed dicopper cations[END_REF], both sideson peroxo and bis (μ-oxo) di-metal cores were found to be energetically favorable in ZSM-5. In previous study, many oxygen bridged bi-nuclear metal sites were reported in zeolite reaction with N 2 O [ 35 37]. In the work of Costa et al. [START_REF] Costa | Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu ZSM 5[END_REF] and Palomino et al. [START_REF] Palomino | Oxidation states of copper ions in ZSM-5 zeolites. A multitechnique investigation[END_REF] and Xamena et al. [START_REF] Xamena | Thermal reduction of Cu 2+ -mordenite and reoxidation upon interaction with H 2 O, O 2 , and NO[END_REF], an oxygen bridged metal dimer was proposed to be formed during the auto-reduction of two hydrated metal cores. The reaction speed of the ozone conversion decreased gradually due to deactivation of the active sites with time. Furthermore, it was noticed that the rate of O 3 decomposition by Fe-ZSM-5 catalysts was faster than that found by Cu-ZSM-5. This result suggested that Fe metal oxide could increase the catalytic activity of parent zeolite more than Cu metal oxide. These results could be explained by the higher activity of Fe ions than Cu ions according to the electrochemical series. Thus, Fe reacts with ozone faster than Cu, leading to ozone decomposition. Batakliev et al. [START_REF] Batakliev | Ozone decomposition[END_REF] investigated catalysts based on metals and their oxides in different oxidation levels and found that the catalytic activity of these metal oxides was increased by increasing the metal oxidation state. The catalytic activity degree of a number of elements to the decomposition of ozone was in the order Cu < Ag < Ni < Fe < Au < Pt. Similarly, Sugasawa and Ogata [START_REF] Sugasawa | Effect of different combinations of metal and zeolite on ozone-assisted catalysis for toluene removal[END_REF] found that the order of catalytic activity in the case of O 3 decomposition was Fe = Ni = Ag = Co = Mn > non-supported ZSM-5.

Generally, the reactivity of Fe-ZSM-5 and Cu-ZSM-5 toward O 3 decomposition is directly related to the presence of specific extra framework metal-containing cationic species in the micropores of ZSM-5 zeolite. Several studies demonstrated that the mononuclear oxygenated metal species for zeolites modified with Fe, Zn, Al, and Cu tend to self-organize into bi-nucleate oxygenated active cores [ 42 44].

Figure 5 shows the effect of metal oxide loaded onto zeolite ZSM-5 on the O 3 removal efficiency. It was showed that O 3 removal efficiency was obviously higher with Fe-ZSM-5 and Cu-ZSM-5 than that with the parent zeolite ZSM-5. In terms of O 3 removal efficiency, metal oxide catalysts could be ranked as follows: Fe > Cu. It was found that, O 3 removal efficiency reached 90% by Fe-ZSM-5 compared to zeolite ZSM-5 alone (40%); however, O 3 removal efficiency was 70% by Cu-ZSM-5. A similar observation was reported by Huang et al. [START_REF] Huang | Catalytic ozonation of gaseous benzene over MnOx/ZSM-5 at ambient temperature: prevention of catalyst deactivation and byproducts emission[END_REF] who found that Metals/ZSM-5 catalyst activity toward ozone decomposition was increased to 80% and kept stable after a long run. Kumar et al. [START_REF] Kumar | Ag-modified Hbeta, H-MCM-41 and SiO 2 : influence of support, acidity and Ag content in ozone decomposition at ambient temperature[END_REF] reported that for Ag-modified alumino-silicates, Beta and MCM-41 zeolites showed a very high decomposition of ozone (98% conversion for Ag/MCM-41 catalyst and almost 44% for Ag-modified Beta zeolite). Genov et al. [START_REF] Genov | Ozone decomposition over silver-loaded perlite[END_REF] also cited that Ag loaded onto Bulgarian natural zeolite allowed decomposition of O 3 to O 2 at room temperature with a relatively high activity of 85%.

Figure 6 shows the evolution of temperature difference (ΔT ) between the inlet and outlet of the adsorbent bed reactor as a function of time during ozone removal by Fe-ZSM-5, Cu-ZSM-5, and parent zeolite ZSM-5. The temperature profiles enabled us to explain the ozone removal mechanism. By using Fe-ZSM-5 and Cu-ZSM-5 catalysts, the concentration of ozone in the output decreased and the reaction temperature increased. The full decomposition of ozone was almost completely at nearly 150 min with a rapid increase in the temperature (Figure 6). After 150 min, the temperature slowly increased and the decomposition of ozone was less effective and the behavior of ozone output concentration was very similar. For the parent zeolite ZSM-5, the temperature at the first few minutes (10 min) was increased and at the second part of the curves, the temperature was approximately stable. The temperature profile of parent zeolite ZSM-5 is thus an indicator of the ozone adsorption mechanism. With all zeolite samples, the temperature was increased by ozone decomposition. The increase in temperature in both Fe-ZSM-5 and Cu-ZSM-5 was much higher than that found in case of the parent zeolite ZSM-5, indicating that the ozone decomposition is an exothermic reaction. The temperature was rapidly increased in case of parent zeolite ZSM-5, then stabilized, and finally it decreased gradually when the ozone concentration increased. According to this point, a significant oxidation of adsorbed compounds could be deduced. Ozone adsorption was predominant in the parent zeolite ZSM-5; however in the case of Fe-ZSM-5 and Cu-ZSM-5, the ozone decomposition was predominant. Similarly, Imamura et al. [START_REF] Imamura | Decomposition of ozone on a silver catalyst[END_REF] found that the oxides of transition metals exhibit higher catalytic activity for the decomposition of ozone; moreover the conversion degree of ozone and their catalytic activity was arranged as follows: MnO 2 (42%)> Co 3 O 4 (39%)> NiO (35%)> Fe 2 O 3 (24%)> Ag 2 O (21%)> Cr 2 O 3 (18%)> CeO 2 (11%)> MgO (8%)> V 2 O 5 (8%)> CuO (5%)> MoO 3 (4%).

Figure 7 shows that the quantity of ozone eliminated varies linearly from input ozone concentration for both Fe-ZSM-5 and Cu-ZSM-5. It may be supposed that in this case the decomposition was initiated by the Fe and Cu metals' oxides on the zeolite surface leading to a high ozone decomposition rate. In the case of parent zeolite ZSM-5, O 3 elimination was firstly increased gradually by increasing the quantity of input ozone, and then O 3 elimination was constant and reached a steady state indicating the saturation of the parent zeolite ZSM-5. The decomposition was initiated by the hydroxyls groups or by the local heterogeneity of the zeolite framework [START_REF] Monneyron | Regeneration of high silica zeolites via advanced oxidation processes-apreliminary study about adsorption reactivity towards ozone[END_REF]. During the ozone decomposition, very reactive species involved in some complex radical chain reactions leading to a total decomposition of the inlet ozone [START_REF] Dhandapani | Gas phase ozone decomposition catalysts[END_REF]. Investigation into the O 3 decomposition mechanism shows that the incorporation of metal into zeolite ZSM-5 reduces the activation energy for desorption of chemisorbed oxygen, then the rate-determining step, and thus improves the catalytic performance.

Conclusion

In this research, we found that transition metals Fe and Cu loaded onto zeolite ZSM-5 significantly enhanced the O 3 decomposition activity in comparison with the parent zeolite. As derived from FTIR spectra, the incorporation of Cu and Fe metals in zeolites causes a significant change in the number of zeolite framework bonds. It was found that, O 3 removal efficiency reached 90% by Fe-ZSM-5 compared to the parent zeolite ZSM-5 (40%); however, O 3 removal efficiency was 70% by Cu ZSM-5. These excellent properties of metal oxide catalysts could be attributed to the good dispersion of metals on the zeolite framework, the formation of framework Al O metal species, as well as the possible synergy between the active sites (zeolite and metal sites), making this catalyst a cost-effective candidate for catalytic O 3 elimination. Consequently, the deposition of transition metals in zeolite ZSM-5 improves the catalytic performance.

Figure 1 .

 1 Figure 1. The schematic diagram of the experimental setup.

Figure 2 .

 2 Figure 2. FTIR spectra for the parent zeolite ZSM-5, Fe-ZSM-5, and Cu-ZSM-5.

Figure 3 .

 3 Figure 3. SEM photographs of the parent ZSM-5 (A), Fe-ZSM-5 (B), and Cu-ZSM5 (C).

Figure 4 .

 4 Figure 4. Catalytic performances of parent zeolite ZSM-5, Fe-ZSM-5, and Cu-ZSM-5 samples toward ozone decomposition.

Figure 5 .

 5 Figure 5. Ozone removal efficiency of ZSM-5 (parent zeolite), Fe-ZSM-5, and Cu-ZSM-5 samples.

Figure 6 .

 6 Figure 6. Comparison of temperature difference (ΔT, °C) between the inlet and outlet of the adsorbent bed reactor during ozone decomposition by Fe-ZSM-5, Cu-ZSM-5, and parent zeolite ZSM-5.

Figure 7 .

 7 Figure 7. Quantity of ozone eliminated with the time using Fe-ZSM-5, Cu-ZSM-5, and parent zeolite ZSM-5.

Table 1 .

 1 Main characteristics of the parent zeolite ZSM-5 sample.

	Parameters	Parent zeolite ZSM 5
	Clay binder content (%)	20
	Crystalline framework	Interconnected channels
	Pore internal diameter (Å)	(5.7 * 5.1) and 5.4
	SiO 2 /Al 2 O 3 (mol/mol) Active porous volume (cm 3 /g)	1880 0.18
	Na 2 O (wt%) Bulk density (g/cm 3 )	<0.01 0.4 0.5
	Pellet diameter (mm)	1.5 2

Table 2 .

 2 Experimental conditions during the ozone decomposition by zeolite catalysts using the pilot reactor.

	Parameters	Values
	Reactor internal diameter (mm)	45
	Ozone air flow, Q (m 3 /h)	0.18
	Superficial velocity (m/s)	0.032
	Ozone concentration (g/m 3 )	1 3
	Bed mass (g)	40
	Ozone pressure (bar)	0.7
	Ozone generator power (W)	90
	Ozone flow (g/h)	0.9 3.6

Acknowledgements

The authors are grateful to the National Research Centre, Cairo, Egypt, and Toulouse University, INPT/ENSIACET, Toulouse, France, for providing laboratory facilities toward successful completion of this work.

Disclosure statement

No potential conflict of interest was reported by the authors.


Pierre Pouzet

Mohamed Maanan

Natalia Piotrowska

Agnès Baltzer

Pierre Stéphan

Marc Robin

Pouzet Pierre

Maanan Mohamed

Piotrowska Natalia

Baltzer Agnès

Stephan Pierre

Robin Marc

Review of the late-Holocene storm events along the European Atlantic coasts

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In Europe, a few Holocene paleostorm chronologies have already been conducted. The Mediterranean coast remains the main basin studied, with a good correlation shown between southern stormy events and cooling climatic periods. We focus here on the European Atlantic region, with two main objectives aimed : http://letg.univ-nantes.fr/

LETG-Nantes Géolittomer

Université de Nantes Campus du Tertre BP 81227 44312 NANTES CEDEX 3

Tél. 33 (0)2 53 48 76 76 Fax 33(0)2 53 48 76 50 geolitt@univ-nantes.fr

UMR 6554-CNRS

Littoral, Environnement, Télédétection, Géomatique

The first high resolution reconstruction of Western Central France mid to late Holocene stormy periods. We determine Yeu Stormy Period (YSP) by comparison of three stormy proxies measured in three different cores presented below : mean grain size, organic matter content and sandy proportions.

The review of late Holocene European Atlantic disturbances from previous projects found in bibliography, correlated with global climatic datas. We propose European Atlantic Stormy Events (EASE) : general periods of increasing storminess, extracted from the comparison of existing European Atlantic sedimentary studies.

Nine paleo-storm events identified into three Yeu Island deposition environments, including a 2100-1950 calBP deeply stormy-disturbed period and five others major impacted times : 600-500 calBP, 2850-2350 calBP, 3500-3270 calBP, 5400-5370 calBP and 6650-6510 calBP.

Six European Atlantic stormy periods stand out from the last 8000 years : 600-300 BP, 1700-1100 BP, 2900-2500 BP, and 3500-3300 BP, 5500-5100 BP and 7700-7100 BP globally corresponding to worlwide Holocene cold events.

(1)

(1) EASE 1 600 -300 BP EASE 2 1700 -1100 BP EASE 3 2900 -2500 BP EASE 4 3500 -3300 BP EASE 5 5600 -5200 BP EASE 6 7700 -7000 BP Analysis, operating coasts and field measurements were funded by the OR2C (part of the OSUNA program and the Pays-de-la-Loire region), and the Fondation de France (contract number 1503). We extend special thanks to the isle of Yeu town hall for helping us with fieldwork.

(1) (1) (2) (3) (1) (2) (3)

The isle of Yeu, Vendée, France : -A high coastal flooding exposed area -A semi-diurnal tidal regime, with 5.5 to 6m tidal ranges -A NE tilting with an W/SW incised shelf and a E/NE lower area -A NE coast dominated by sandy beaches Three marshes with low human impact

Study site

Storminess increased periods (SIP) from sedimentary stormy studies have been sorted on a chronological scale into the figure presented below. We visually crossed SIP exctracted from litterature to identify gray shaded periods where notable storminess increase concordance (EASE) can be observed beetween these projects.

The figure shows YSP detected in each core, and possible correlation between different marshes. YPS periods are assessed with radiocarbon dating. From eleven dated samples, we built an age depth model for each core to estimate datations for every centimeters of the three cores. Review of the late-Holocene storm events along the European Atlantic coasts

Storminess increased periods of mentioned paleostorm studies :

A : Iceland (Jackson et al., 2005); B : Brittany islands (Orme et al., 2016); C : Normandie (Sorrel et al., 2009); D : Bretagne (Baltzer et al., 2014& Van Vliet Lanoe et al., 2014); E : West-central France (this work); F : Aquitaine (Clarke et al., 2002); G : Central-Portugal (Clarke and Rendell, 2006).

We compare EASEs extracted with I : global high cold climatic periods (Wanner et al., 2011). These periods are determined by II : standardized ocean stacked Ice Rafted Debris records, with Bond's events also numbered above (Bond et al., 2001, 1997; Wanner et al., 2011). Cooling periods seem to be linked to storminess increases in European Atlantic coasts.