Nisrine Hammi
email: nisrine.hammi@univ-lille.fr

Mickaele Bonneau

Abdelkrim El Kadib

Susumu Kitagawa

Thierry Loiseau

Christophe Volkringer

Sébastien Royer

Jérémy Dhainaut
email: jeremy.dhainaut@univ-lille.fr

Enhanced Gas Adsorption in HKUST-1@Chitosan Aerogels, Cryogels, and Xerogels: An Evaluation Study

Keywords: metal-organic framework, chitosan beads, drying processes, shaping, gas adsorption

This study investigates the use of chitosan hydrogel microspheres as a template for growing an extended network of the MOF-type HKUST-1. Different drying methods (supercritical CO2, freezedrying, and vacuum drying) were used to generate three-dimensional polysaccharide nanofibrils embedding MOF nanoclusters. The resulting HKUST-1@Chitosan beads exhibit uniform and stable loadings of HKUST-1 and were used for the adsorption of CO2, CH4, Xe, and Kr. The maximum adsorption capacity of CO2 was found to be 1.98 mmol.g -1 at 298 K and 1 bar, which is significantly higher than most MOF-based composite materials. Based on Henry's constants, thusprepared HKUST-1@CS beads also exhibit fair selectivity for CO2 over CH4 and Xe over Kr, making them promising candidates for capture and separation applications.

INTRODUCTION

Gas purification is a significant challenge to face growing air pollution resulting from gas emissions from numerous industrial sources. [START_REF] Li | Porous Metal-Organic Frameworks for Gas Storage and Separation: Status and Challenges[END_REF] Different materials, such as zeolite and activated carbons, [START_REF] Cavenati | Adsorption Equilibrium of Methane, Carbon Dioxide, and Nitrogen on Zeolite 13X at High Pressures[END_REF][START_REF] Balou | Synergistic Effect of Nitrogen Doping and Ultra-Microporosity on the Performance of Biomass and Microalgae-Derived Activated Carbons for CO2 Capture[END_REF] have been extensively studied for gas capture, including primary greenhouse gases like carbon dioxide (CO2) and methane (CH4), or noble gases such as krypton (Kr) and xenon (Xe). [START_REF] Jiang | Recent Advances in Metal-Organic Frameworks for Gas Adsorption/Separation[END_REF] Metal-Organic Framework (MOF) materials, known for their large porosity, reactive metal sites, defective ligands, and framework functionalization, [START_REF] Zhou | Introduction to Metal-Organic Frameworks[END_REF] are also widely used in gas adsorption and separation. [START_REF] Abdelhamid | Removal of Carbon Dioxide Using Zeolitic Imidazolate Frameworks: Adsorption and Conversion via Catalysis[END_REF] However, their applications are limited by factors such as poor thermal and chemical stability, metal site reactivity, recycling difficulties, and handling issues. [START_REF] Bazer-Bachi | Towards Industrial Use of Metal-Organic Framework: Impact of Shaping on the MOF Properties[END_REF][START_REF] Yeskendir | From Metal-Organic Framework Powders to Shaped Solids: Recent Developments and Challenges[END_REF] The limitations of MOFs have impeded their industrial applications, and it is necessary to explore alternative strategies, specifically focusing on the shaping of MOFs, to overcome these challenges. [START_REF] Abramova | Extrusion-Spheronization of UiO-66 and UiO-66_NH2 into Robust-Shaped Solids and Their Use for Gaseous Molecular Iodine, Xenon, and Krypton Adsorption[END_REF][START_REF] Kitagawa | Future Porous Materials[END_REF] The development of MOF-based gels has greatly alleviated these problems.

The gel part of MOF-based hydrogels and aerogels can support the structure of MOFs to a certain extent, which improves the stability of the MOFs and enhance their adaptability to different environments (Table S1 provides a survey of the reported MOF-based gels up to now). In this line, integrating MOFs within polysaccharide aerogels has proven to be an efficient method with significant engineering implications. [START_REF] El Hankari | Biopolymer@Metal-Organic Framework Hybrid Materials: A Critical Survey[END_REF][START_REF] Hammi | Polysaccharide Templated Biomimetic Growth of Hierarchically Porous Metal-Organic Frameworks[END_REF][START_REF] Hammi | Shaping MOF Oxime Oxidation Catalysts as Three-Dimensional Porous Aerogels through Structure-Directing Growth inside Chitosan Microspheres[END_REF] Especially, combining MOFs with pliable polymers, such as cellulose and alginate, can greatly enhance their flexibility, recyclability, and processability. [START_REF] El Hankari | Biopolymer@Metal-Organic Framework Hybrid Materials: A Critical Survey[END_REF][START_REF] Ding | Improving MOF Stability: Approaches and Applications[END_REF] While chitosan itself offers unique benefits due to the basicity of its amino groups, [START_REF] Fan | Chitosan Derived Nitrogen-Doped Microporous Carbons for High Performance CO2 Capture[END_REF] its nonporous structure and low specific surface area (SBET  0.3 m 2 .g -1 ) result in low CO2 adsorption capacity of only 0.1 mmol.g -1 ) at 25 °C and 1 atm. [START_REF] Chagas | Characterization, and CO2 Uptake of Adsorbents Prepared by Hydrothermal Carbonization of Chitosan[END_REF] To tackle such low performance, researchers have developed chitosan aerogel composites incorporating materials such as SBA-15, polyvinyl alcohol, polymer (PVA), clay, and graphene oxide that exhibit improved maximum CO2 adsorption capacities ranging from 0.6 to 0.8 mmol.g -1 . [START_REF] Oliveira | CO2 Capture over MCM-41 and SBA-15 Mesoporous Silicas Impregnated with Chitosan[END_REF][START_REF] Alhwaige | Carbon Aerogels with Excellent CO2 Adsorption Capacity Synthesized from Clay-Reinforced Biobased Chitosan-Polybenzoxazine Nanocomposites[END_REF] The combination of MOFs with chitosan can not only overcome these uptake limitations but also provide shaped adsorbents with enhanced adsorption capacity.

For example, Fan et al. synthesized chitosan/ZIF-8 by annealing, resulting in a higher CO2 capture capacity up to 0.99 mmol.g -1 with interesting recycling abilities, owing to the carbonization of chitosan and appropriate pore regeneration during thermal treatment. [START_REF] Fan | Structuring ZIF-8-Based Hybrid Material with Hierarchical Pores by in Situ Synthesis and Thermal Treatment for Enhancement of CO2 Uptake[END_REF] However, a part of the initial MOF porosity is lost during the pyrolysis process, lowering its potential performance.

While the physico-chemical properties of MOF beads towards gas sorption have been relatively well studied, few investigations have focused on the impact of the preparation pathway and the nature of drying treatment on the beads. Indeed, the gas sorption capacities of activated MOFs are significantly influenced by the synthesis route and further handling of as-synthesized samples. [START_REF] Bueken | Gel-Based Morphological Design of Zirconium Metal-Organic Frameworks[END_REF] Recent reports indicated that supercritical CO2 treatment and freeze-drying can activate MOFs and result in significantly higher gas adsorption capacities compared to thermal or room-temperature evacuation methods. [START_REF] Park | A Highly Porous Metal-Organic Framework: Structural Transformations of a Guest-Free MOF Depending on Activation Method and Temperature[END_REF][START_REF] Ma | Freeze Drying Significantly Increases Permanent Porosity and Hydrogen Uptake in 4,4-Connected Metal-Organic Frameworks[END_REF] This phenomenon is attributed to the prevention of mesopores and macropores collapse and breathing effects within the framework, resulting in increased accessibility to micropores. Our previous work involved the templating of microporous MOFs inside porous chitosan microspheres to create MOF@CS aerogel composites with hierarchical micro-, meso-, and macro-porous networks using a supercritical CO2 drying process. [START_REF] Hammi | Shaping MOF Oxime Oxidation Catalysts as Three-Dimensional Porous Aerogels through Structure-Directing Growth inside Chitosan Microspheres[END_REF] However, further researches are necessary to systematically investigate the impact of various parameters on the gas capture performance of MOF@CS composites, as well as to apply this strategy to better performing MOFs such as HKUST-1. HKUST-1 (Cu(II) benzene-1,3,5-tricarboxylate) is still considered to be one of the most promising adsorbents for CO2 capture due to its high surface area, simple and low-cost synthesis, and high density of adsorption sites. Hence, HKUST-1 powder can adsorb more than 6 mmol.g -1 of CO2 at 25 °C. On the other hand, this MOF structure is fragile and difficult to maintain after shaping, reducing its potential use. [START_REF] Yeskendir | From Metal-Organic Framework Powders to Shaped Solids: Recent Developments and Challenges[END_REF] In this study, we developed a simple and environmentally-friendly in-situ synthesis route to create HKUST-1 beads with varying surface properties. We first formed a composite material by coordinating the copper salt precursor with chitosan, which was then bridged with trimesic acid as the organic linker to produce HKUST-1@CS microspheres with a hierarchical micro-/meso-porous structure and HKUST-1 loadings capacity of up to 63 wt.%. Emphasis was given to investigating the effects of the metal/NH2 ratio as well as the activation process (CO2 supercritical drying, freezedrying, and vacuum drying) on the textural properties and adsorption capacity of the prepared composites. In this study the HKUST-1 crystals embedded in the HKUST-1@CS composites will offer high surface area for gas adsorption, while the chitosan will provide a robust skeleton to prevent the MOF architecture from collapsing under harsh adsorption conditions. Such material with a hierarchical porosity can be advantageously used for different gases such as CO2, CH4, Xe, and Kr.

EXPERIMENTAL SECTION

Chemicals

Chemicals used are: Cu(NO3)2•2.5H2O, trimesic acid, absolute ethanol, acetic acid (99%), sodium alginate (CAS number 9005-38-3) and chitosan with a high molecular weight (310000-375000 Da) and deacetylation degree of >75% (CAS number 9012-76-4). All reagents and solvents were obtained from Sigma Aldrich without further treatment.

Typical synthesis of HKUST-1@CS beads

1.5 g of high molecular weight chitosan powder were dissolved in 73 mL acetic acid aqueous solution (1 wt.%) and stirred for 24 h. A selected mass of Cu(NO3)2•2.5H2O with different Cu:CS molar ratios (1:1, 2:1, 1:2) with respect to chitosan NH2 groups (5.8 mmol.g -1 ) was added to the transparent solution. Of note, a 3:1 ratio did not yield stable beads. The resulting mixture was magnetically stirred for 2 h at room temperature until complete homogenization. Dropping this solution through a 0.8 mm syringe needle into a NaOH (300 mL) bath (1 M) using a syringe induces spontaneous chitosan gelation into porous beads. After 5 h in the alkaline solution, the beads were washed with distilled water until the aqueous phase approached neutral pH, and further dehydrated by immersion for 15 min in a series of successive ethanol-water 200 mL baths (10-90, 30-70, 50-50, 70-30, 90-10, 100-0 in volume). [START_REF] Valentin | Supercritical CO2 Dried Chitosan: An Efficient Intrinsic Heterogeneous Catalyst in Fine Chemistry[END_REF] The beads were next placed in an alcoholic solution (20 mL) containing trimesic acid (H3BTC) with a Cu:H3BTC molar ratio of 1:2 at 80 °C for 48 h. The diffusion of the H3BTC ligand within the beads and its further condensation with the previously entrapped copper cations promotes the growth of the HKUST-1 crystals within the resulting composite. Indeed, typical octahedrally-shaped crystals of HKUST-1 are clearly observed in the beads (Fig. 1). For now, such strategy represented in Scheme 1 can only be applied with MOFs whose metal clusters or ions are stable under strongly basic pH, and whose linker can be dissolved in conditions where the stability of the chitosan beads is maintained.

The solvent was then removed from the wetted HKUST-1@CS scaffolds following three different drying techniques, namely: a-Supercritical CO2 drying, which was carried at 73 bar and 35 °C. The given aerogels are named HKUST-1@CS1:1A. b-Vacuum drying, which was done in a standard vacuum oven at room temperature for 24 h.

The obtained xerogels are named HKUST-1@CS1:1X. c-Freeze drying, involving the rapid freezing of alcogel beads using liquid nitrogen. These frozen beads are then placed in a freeze-dryer. The resulting cryogels are known as HKUST-1@CSx:yC, where "x:y" represents the Cu:CS molar ratio employed.

For comparison, pure chitosan beads, denoted as CS, were also prepared similarly and dried under freeze-drying conditions. Besides, in order to unveil any possible role of nitrogen, nitrogen-free beads were prepared by following the same procedure except that chitosan was replaced by sodium alginate (SA). The obtained sample was denoted HKUST-1@SA2:1C. The HKUST-1 material was synthesized according to the procedure outlined in the literature, resulting in a high-quality and pristine product. 24

Procedures for HKUST-1@CS gas adsorption

Before measurement, about 50 mg of the sample was degassed for 8 h at 100 °C under a primary vacuum. CO2 (99.995, Air Liquide), Xe (99.997, Linde Gas), and Kr (99.9999 Linde Gas) singlecomponent gas sorption isotherms were collected with a Micromeritics ASAP 2020 automated gas sorption system. The desired sorption temperature (273, 298, or 323 K) was controlled with a thermostatic glycol bath (Huber, Pilot one). High-pressure CH4 (>99, Air Liquide) sorption isotherms were measured with a BEL Japan BELSORP-max volumetric adsorption instrument at 298 K and up to 70 bar.

Materials characterization

Nitrogen (N2) physisorption isotherms were recorded at 77 K on a Micromeritics Tristar II automated gas sorption system. Before analysis, the samples were outgassed under a dynamic vacuum at 120 °C overnight. The specific surface area was determined using the multipoint B.E.T. algorithm in the p/p0 range from 0.10 to 0.25 and the total pore volume is determined on the adsorption branch at p/p0 = 0.98. The pore diameters and the pore size distributions were calculated from the desorption branch using the Barrett-Joyner-Halenda (BJH) and non-local density functional theory (NLDFT) methods.

A Micromeritics Autopore IV 9500 penetrometer was used to probe the mesopores and macropores by mercury intrusion. A contact angle of 130° was assumed in the pore size calculations.

X-ray diffraction (XRD) was performed using a Bruker X-ray AXS D8 Advance diffractometer in Bragg-Brentano configuration and equipped with a LynxEye Super Speed detector. XRD patterns were recorded with Cu Kα radiation (λ = 0.154 nm, 40 kV, 30 mA) in the 10-80° 2θ range with a 0.02° 2θ step. Phase identification was made by comparison with the ICDD database.

Fourier transform infrared spectroscopy (FTIR) spectra were monitored using a NICOLET iS10 spectrometer.

A KRATOS Axis Ultra spectrometer operated under ultrahigh vacuum conditions, using a twin Al X-ray source (1486.6 eV) at pass energy of 40 eV, was used for the X-ray photoelectron spectroscopy (XPS) analyses. The solid, in the form of a pellet, was fixed on a copper holder with copper tape. The binding energy values were estimated, positioning the C 1s peak of contaminant carbon at a B.E. of 285.0 eV. The Casa XPS software package was used for data analysis.

Thermogravimetric curve profiles were obtained on TA instruments SDT2960 under the constant flow of simulated air (20 vol% O2-80 vol% N2) in the range of 25-900 °C with a 5 °C/min heating rate.

Scanning electronic microscopy (SEM) images were obtained using a JEOL JSM-7800F LV After the sample covering with 10 Å of the carbon layer.

A Vinci Technologies Versatile Crushing Strength Tester was used to perform crushing tests on single solids, using a set of flat anvil and a hammer. The load applied was recorded by the force sensor as a function of the displacement of the punch at a constant speed of 0.6 mm/min until failure, happening at the ultimate crushing strength of the solid. For each sample, 10 representative solids were crushed and the resulting values, given in Newton, were averaged.

RESULTS AND DISCUSSION

The synthesis of HKUST-1@CS beads was achieved through an in-situ method as illustrated in Scheme 1. The initial formation of the copper-chitosan complex is facilitated by the favored coordination between the Cu 2+ cations and the amino groups of chitosan. [START_REF] Hammi | Polysaccharide Templated Biomimetic Growth of Hierarchically Porous Metal-Organic Frameworks[END_REF] Further dropwise addition of this complex into a NaOH bath results in the generation of Cu(OH)2chitosan alcogels due to the insolubility of chitosan in alkali solutions.

Scheme 1. Illustration of the multistep preparation procedure for HKUST-1@CS beads. The interplay between the biopolymer, the metal precursor and the ligand is presented along these steps. The digital photos show: A) Cu 2+ -Chitosan solution; B) HKUST-1@CS alcogel; C) HKUST-1@CS aerogel; D) HKUST-1@CS cryogel, E) HKUST-1@CS xerogel.

The as-prepared HKUST-1@CS alcogel composites were dried by different techniques (supercritical CO2, freeze-drying, and vacuum drying). The use of carbon dioxide is privileged for the supercritical treatment as it reaches its supercritical state at a relatively low temperature of 31 °C, compared to 373 °C required for water and 513 °C for ethanol, making it suitable to dry soft, labile materials without decomposition, as it would be the case of polysaccharides. As a result, the HKUST-1@CS alcogel microspheres can be converted into highly porous aerogels without any noticeable damage or structural collapse. This successful preservation of the microsphere dimensions during the transition from alcogels to aerogels confirms the absence of any severe collapsing or densification of the chitosan matrix. Scheme 1 and Figure S1 provide clear visual evidence of the preserved shape of the microspheres in the dry samples. However, it is important to highlight that the xerogels, with an average size of 1.1 mm, demonstrate considerably greater shrinkage compared to both the cryogels and aerogels, which have both an average size of 1.5 mm, while the initial size of the hydrogels before drying was 1.8 mm.

The chemical structures of the different HKUST-1@CS composites were analyzed using FT-IR (Fig. S2). All HKUST-1@CS beads exhibit the typical absorption peaks of chitosan, including the carbonyl band (C=O-NHMe) at 1646 cm -1 and the amine band (NH2) at 1589 cm -1 . [START_REF] Hammi | Shaping MOF Oxime Oxidation Catalysts as Three-Dimensional Porous Aerogels through Structure-Directing Growth inside Chitosan Microspheres[END_REF][START_REF] El Kadib | Nanosized Vanadium, Tungsten and Molybdenum Oxide Clusters Grown in Porous Chitosan Microspheres as Promising Hybrid Materials for Selective Alcohol Oxidation[END_REF] In comparison to pure chitosan beads, new absorption bands corresponding to metal-organic coordination (Cu-O-C) and metal-chitosan chelation (H2N→Cu and HO→Cu) are observed. Specifically, the absorption band at 727 cm -1 is attributed to Cu-O stretching, [START_REF] Hammi | Polysaccharide Templated Biomimetic Growth of Hierarchically Porous Metal-Organic Frameworks[END_REF] resulting from either metal-organic coordination in MOFs or chelating bonds between the metal and hydroxyl groups of chitosan. Additionally, a new band at 1371 cm -1 , derived from C=C stretching in the aromatic ring of the linker, is observed. [START_REF] Jagódka | On the HKUST-1/GO and HKUST-1/RGO Composites: The Impact of Synthesis Method on Physicochemical Properties[END_REF] Furthermore, the NH2 band is shifted towards lower wavenumbers in the spectrum of HKUST-1@CS beads (1554 cm -1 for HKUST-1@CS1:2C, 1560 cm -1 for HKUST-1@CS1:1C, and 1550 cm - 1 for HKUST-1@CS2:1C), indicating electron density donation from NH2 to the copper center (NH2→Cu). Finally, the visible bands at 454-477 cm -1 are characteristic of the Cu-N stretching band. [START_REF] Hammi | Polysaccharide Templated Biomimetic Growth of Hierarchically Porous Metal-Organic Frameworks[END_REF][START_REF] Park | A Highly Porous Metal-Organic Framework: Structural Transformations of a Guest-Free MOF Depending on Activation Method and Temperature[END_REF] Powder XRD measurements (Fig. S3a-b) revealed typical diffraction Bragg peaks related to the crystalline HKUST-1 phase in HKUST-1@CS1:1C and HKUST-1@CS2:1C samples, even if slightly broader lines are observed compared to those of the pristine HKUST-1 (at 11.6° 2 -(222) plane, FHWM of HKUST-1, HKUST-1@CS2:1C and HKUST-1@CS1:1C is 0.156°, 0.158° and 0.168°, respectively) (Fig. S3c). As expected, the Cu:CS ratio thus has an impact on the final size of the crystals, with larger Cu loadings facilitating the crystals growth. The powder XRD diagram of HKUST-1@CS1:2C composite exhibited a partially amorphous phase, indicated by the broad bump in the 15-40° (2) range (Fig. S3a), together with less intense Bragg peaks attributed to the HKUST-1 phase. This may arise from the strong bonding of copper ions by amine groups emanating from chitosan biopolymer in excess, impeding their further coordination with H3BTC ligands. To evaluate the role of NH2 from chitosan, it is worth noting that alginate was utilized as a reference giving the material HKUST-1@SA1:1C. An almost fully amorphous XRD pattern was observed, with a single weak reflection at around 11.6° (2), indicating the occurrence of a very small amount of HKUST-1. This may be attributed to the competitive binding of alginate with the copper ions or organic H3BTC ligands during the self-assembly process of HKUST-1.

Additionally, it is possible that the formation of complexes during the self-assembly process could interfere with the formation of HKUST-1, leading to the observed amorphous pattern, representative of the polymeric polysaccharide structuration. When comparing the same sample dried via different methods, it could be observed that XRD patterns of HKUST-1@CS1:1C, HKUST-1@CS1:1A, and HKUST-1@CS1:1X composites displayed similar Bragg reflections, as shown in Figure S3b, indicating that the crystalline HKUST-1 structure was preserved despite the drying method used. This result is consistent with the literature. [START_REF] Denning | Metal-Organic Framework HKUST-1 Promotes Methane Hydrate Formation for Improved Gas Storage Capacity[END_REF] The SEM analyses of the HKUST-1-chitosan composite series provide valuable insights into the internal structure of the chitosan microspheres (Fig. 1,S4a). On the external surface of the aerogel beads, truncated particles with an octahedral shape were observed (Fig. 1a-c), while smaller HKUST-1 particles grown inside the polysaccharide network became visible upon halving the beads (Fig. 1d,e, S4a). Notably, there was a significant difference in particles size between those formed on the surface and those grown internally. For instance, HKUST-1@CS1:1A exhibited an average particle size of 14 µm on the surface (Fig. 1b,c, S4b) compared to approximately 2 µm in the bulk (Fig. S4a). This disparity can be attributed to two factors: (i) the confinement effect within the microspheres, where the nitrogencontaining functional groups could impede the growth of HKUST-1 particles, leading to their smaller size; (ii) the formation of larger HKUST-1 particles due to the presence of metal ions near the outer surface, which offers more freedom and space for network expansion and crystal growth. Furthermore, irregular spherical pores with an estimated size of 2.55 μm were observed in HKUST-1@CS1:1A (Fig. S4). This observation further supports the feasibility of creating porous aerogels, which can be achieved through hydrogel swelling or by modifying the hydrogen bonding between adjacent biopolymer fibrillar chains. These alterations likely occur during the solvent exchange or removal process, which is a crucial step in drying polysaccharides. [START_REF] Hammi | Shaping MOF Oxime Oxidation Catalysts as Three-Dimensional Porous Aerogels through Structure-Directing Growth inside Chitosan Microspheres[END_REF] Additionally, in HKUST-1@CS1:1C, a macroscopic porosity was observed (Fig. 1d, S4), which can be attributed to the formation of ice crystals. These ice crystals create skeletal walls with large pores at the upper level. [START_REF] Dhainaut | Freezing-Induced Ordering of Block Copolymer Micelles[END_REF] As expected, the particles size attributed to the presence of crystalline HKUST-1 increased with the ratio HKUST-1:CS (Fig. 1e-f and S4), while increasing the amount of chitosan enhanced the coating of the particles as taking place within HKUST-1@CS1:2C. This indicates that the polysaccharide acts as both a structure-directing agent and a growth inhibitor during the MOF crystallization. Mapping the location of carbon and nitrogen versus the copper reveals the uniform distribution of these elements within the microspheres and thus the homogeneous dispersion of HKUST-1 particles (Fig. 1g,h and S4). The thermal stability of these nanocomposites, as well as their MOF loadings, were further assessed using thermogravimetric analysis under airflow (Fig. S5). HKUST-1 MOF powder shows two distinct weight losses. The first weight loss at 150 °C is attributed to the removal of solvent and metal-coordinated water. Subsequently, a narrow temperature range starting at ≥ 300 °C shows a second weight loss, which corresponds to the decomposition of the trimesate linker. Chitosan (CS) also shows two weight loss steps: one from 150 to 350 °C, corresponding to the dehydration of glycosidic rings as well as initial thermal depolymerization, and a second one exothermic from 350 to 550 °C and attributed to the decomposition of deacetylated and acetylated units. The composite beads, being a mixture of both materials, present all of these features. While native chitosan microspheres are completely burnt off from a temperature of 580 °C, the resulting hybrid materials display a mineral residue varying from 11 wt.% to 25 wt.% up to 800 °C. This residual solid can be attributed to the conversion of HKUST-1 into copper oxide (CuO). Based on this residue, the HKUST-1 loadings for HKUST-1@CS1:2C, HKUST-1@CS1:1C, HKUST-1@CS2:1C

and HKUST-1@SA1:1C are recalculated as 40 wt.%, 58 wt.%, 63 wt.% and 28 wt.%, respectively (Table S2). Excepted for HKUST-1@SA1:1C, these values closely approximate the theoretical values. Under ideal conditions, it can be assumed that the copper precursor is fully incorporated, indicating that the added copper and linker are used for the formation of HKUST-1 (Table S2).

The textural properties of the different HKUST-1@CS composites were first characterized using N2 adsorption-desorption measurements shown in Figure S6 and summarized in Table 1. For HKUST-1@CS2:1C and HKUST-1@CS1:1C composites, Type IV isotherms with a sharp increase in the micropore and the large mesopore regions and type H2 hysteresis loops are observed, which suggests that these beads present a hierarchical pores network. This shape is very typical of chitosan cryogels presenting moderate surface area, 71 m 2 .g -1 in our case (Table 1), [START_REF] Chartier | Tuning the Properties of Porous Chitosan: Aerogels and Cryogels[END_REF] and for which the entanglement of the fibrils leads to forming large mesopores connected to macropores. The specific surface areas of HKUST-1@CS2:1C, HKUST-1@CS1:1C and HKUST-1@CS1:2C are 502, 527 and 249 m 2 .g -1 , respectively.

Significantly increasing the amount of chitosan in HKUST-1@CS1:2C decreases the specific surface area. This phenomenon is mostly ascribed to the dilution of the HKUST-1 within the less porous biopolymer as well as partial pore blockage by the polymer chains.

Of note, HKUST-1@CS1:1C presents intercrystalline mesopores in the 2 to 50 nm region (Fig. S7), which are the result of smaller crystals size and their interactions with the chitosan matrix. Their presence affects the calculation of the specific surface area, resulting in a value that is higher than for HKUST-1@CS2:1C with a lower MOF loading. However, the microporous surface and volume as determined by the BJH method are in accordance with the MOF loading. Finally, nitrogen sorption of HKUST-1@SA1:1C reveals that the material is dense and devoid from any appreciable open framework since no N2 sorption could be performed. As stated above, this material is mostly amorphous with copper mostly interacting with alginate and H3BTC species entrapped within the polymer.

The effect of the drying method on the porous structure was further investigated. The surface area of the HKUST-1@CS1:1X xerogel (460 m 2 .g -1 ) was lower than the HKUST-1@CS1:1A aerogel (531 m 2 .g -1 ) and HKUST-1@CS1:1C cryogel (527 m 2 .g -1 ), which suggests the limited ability to maintain the porosity via vacuum drying. In addition, HKUST-1@CS1:1X

xerogel exhibited a narrower BJH pore size distribution (7.4 nm) (Fig. S7, Table 1) than the aerogel and cryogel counterparts (> 14 nm). These results are consistent with the absence of capillary forces and surface tension effect during supercritical CO2 and freeze-drying. [START_REF] Nelson | Supercritical Processing as a Route to High Internal Surface Areas and Permanent Microporosity in Metal-Organic Framework Materials[END_REF] The micropore size distribution (Fig. S7) further shows the presence of 1.2 and 1.7 nm diameter pores in all samples, consistent with the contribution of HKUST-1 microporosity. This size distribution remains constant with varying metal/NH2 ratio, pledging for neglectable MOF pores filling by the biopolymer. Notably, HKUST-1@CS1:1A shows a remarkably higher total pore volume (1.71 cm 3 .g -1 ) than those of HKUST-1@CS1:1C (0.8 cm 3 .g -1 ) and HKUST-1@CS1:1X (0.63 cm 3 .g -1 ), which is ascribed to complete pore evacuation via supercritical CO2 drying. Crucially, both the size and volume of the mesopores obtained in this study vary with drying conditions.

To further investigate the presence of meso-and macroporosity within the beads, mercury intrusion measurements were performed (Fig. S8). The pore size distributions of HKUST-1@CS1:1A and HKUST-1@CS1:1C exhibit a bimodal feature: except for a main peak centred at 13 nm and 8 nm, respectively, indicating the prevalence of mesopores, a relatively weak and broad distribution with peaks centred at 0.240 µm and 5 µm, respectively, can also be observed, indicating the existence of a various range of macropores. In contrast, for HKUST-1@CS1:1X, only macropores in the range of 4-6 µm are retained, in line with the densification of the beads after activation. Thus HKUST-1@CS features a multimodal porous structure consisting of interconnected pores ranging from micro-, meso-to macropores. Such hierarchical porosity may be beneficial to adsorb or separate gas.

Table 1. Textural properties for the HKUST-1@CS beads and the pristine HKUST-1

Samples SBET [a] (m 2 g -1 )

Sµ [b] (m 2 g -1 )

Vtot [c] (cm 3 g -1 )

Vµ [d] (cm 3 g -1 )

Vmes [e] (cm 3 g -1 )

Dµ [f] (nm)

Dmes [ One of the major issues of highly porous solids is their mechanical stability. Here, the mechanical stability of HKUST-1@CS1:1A towards compression was evaluated through crushing tests (Fig. S9). An average crush strength of 12.35 N was measured over 10 beads.

For comparison, commercial catalysts for refining processes present a crush strength of 20 N or higher, demonstrating that such produced HKUST-1@CS beads are well-suited for practical applications. [START_REF] Yeskendir | From Metal-Organic Framework Powders to Shaped Solids: Recent Developments and Challenges[END_REF] Insight into the chemical composition was gained by comparative XPS analysis of HKUST-1 and HKUST-1@CS1:1A (Fig. S10). N 1s spectra were decomposed into two peaks with binding energies at 397.6 ± 0.2 eV and 399.0 ± 0.2 eV, which can be attributed to free NH2 groups and the coordinated NH → Cu, respectively. This further corroborates the intimate interaction of Cu species with nitrogen. [START_REF] Huang | Facile Method To Prepare a Novel Biological HKUST-1@CMCS with Macroscopic Shape Control for the Long-Acting and Sustained Release[END_REF] A close look at the XPS spectra of Cu 2p3/2 for the two materials HKUST-1 and HKUST-1@CS1:1A reveals intense broad lines at 934.5 eV for HKUST-1@CS1:1A accompanied by satellites centred at 939-945 eV corresponding to the Cu 2+ state. [START_REF] Huang | Facile Method To Prepare a Novel Biological HKUST-1@CMCS with Macroscopic Shape Control for the Long-Acting and Sustained Release[END_REF][START_REF] Basumallick | Hydrothermally Derived Water-Dispersible Mixed Valence Copper-Chitosan Nanocomposite as Exceptionally Potent Antimicrobial Agent[END_REF] The similarities between the XPS profiles of HKUST-1 and HKUST-1@CS1:1A indicate that most of the copper entrapped in the chitosan microspheres are coordinated into HKUST-1 phase, which excludes the predominance of nanosized copper clusters or molecular copper that could be linked to chitosan. 33

CO2 uptake by HKUST-1@CS beads

The adsorption capacity of HKUST-1@CS materials typically depends on the physicochemical nature of the sorbents, and in this specific case, on the potential synergistic effects arising from CS-MOF interactions. Especially, the surface area and micropore volume, and the presence of basic N atoms can significantly affect the adsorption performance. In this context, the effect of the drying method over the CO2 uptake of HKUST-1@CS beads has first been investigated. Figure 2a shows the CO2 adsorption capacity of the three samples (HKUST-1@CS1:1A, HKUST-1@CS1:1C, and HKUST-1@CS1:1X) at 273 K up to 1 bar.

In line with the measured specific surface areas, the CO2 uptakes of all HKUST-1@CS adsorbents are significantly higher than that of the neat chitosan (0.083 mmol.g -1 ) (Fig. S11), with maximum capacities of HKUST-1@CS1:1C, HKUST-1@CS1:1A, and HKUST-1@CS1:1X reaching 2.68 mmol.g -1 , 2.54 mmol.g -1 and 2.28 mmol.g -1 , respectively. Thus, for the same material, the CO2 capacity at equilibrium is mainly governed by the available micropore volume. [START_REF] Vrtovec | Structural and CO2 Capture Properties of Ethylenediamine-Modified HKUST-1 Metal-Organic Framework[END_REF] Especially, the CO2 uptake of the HKUST-1@CS1:1C composite, with a HKUST-1 loading of only 58 wt.%, reached 2.68 mmol.g -1 . In contrast, HKUST-1@CS1:1A achieved a CO2 uptake of 2.54 mmol.g -1 . It is hypothesized that this difference in CO2 adsorption between HKUST-1@CS1:1A and HKUST-1@CS1:1C may be attributed to the quicker diffusion for CO2 molecules within the cryogel structure due to the presence of radial macropores. Remarkably, HKUST-1@CS1:1C's CO2 uptake value corresponds to 42 % of the CO2 adsorption capacity exhibited by pure HKUST-1, which has a capacity of 6.44 mmol.g -1 (Fig. S12). It is worth noting that the reported literature value for pure HKUST-1 is typically up to 6.85 mmol.g -1 . [START_REF] Xu | Ultrafast Room Temperature Synthesis of GrO@HKUST-1 Composites with High CO2 Adsorption Capacity and CO2/N2 Adsorption Selectivity[END_REF] Thus, it can be concluded that the adsorption capacity attributed to the inclusion of HKUST-1 in the composite was effectively preserved, despite a slight reduction caused by variations in pore accessibility and differences in pore size distribution among the composites. Figure 2b shows a comparison of the adsorption behaviour using cryogels with different HKUST-1 loadings at 298 K up to 1 bar. The adsorption capacity for the sorbent HKUST-1@CS2:1C containing 63 wt.% of HKUST-1 is 1.98 mmol.g -1 while the capacities of HKUST-1@CS1:1C and HKUST-1@CS1:2C beads are 1.91 mmol.g -1 and 0.99 mmol.g -1 , respectively. These results are in accordance with the lower HKUST-1 loading in the composite materials, and the related decrease in specific surface area and total pore volume.

The effect of temperature on CO2 uptake using HKUST-1@CS1:1C hybrid sorbent at 273, 298, and 323 K was also investigated (Fig. 2c). In line with the facilitated desorption of CO2 molecules at higher temperatures, [START_REF] Yang | Facile Preparation of N-Doped Porous Carbon from Chitosan and NaNH2 for CO2 Adsorption and Conversion[END_REF] the CO2 maximum capacity decreases with increasing the temperature: from 2.68 mmol.g -1 at 273 K to 1.03 mmol.g -1 at 323 K.

To the best of our knowledge, the CO2 uptake reported for HKUST-1@CS1:1C composite presented in this work at 298 K is twice as high as previously reported chitosan-based shaped sorbents (Table 2), and competes with liquid amines while being easily reusable. [START_REF] Singh | Emerging Trends in Porous Materials for CO2 Capture and Conversion[END_REF] For instance, Hsan and coworkers reported a CO2 uptake around 0.257 mmol.g -1 at 1 bar and 298 K, for a CS grafted graphene oxide aerogels. [START_REF] Hsan | Chitosan Grafted Graphene Oxide Aerogel: Synthesis, Characterization and Carbon Dioxide Capture Study[END_REF] In another study, Fan et al. synthesised chitosan/ZIF-8 by an annealing process to enhance CO2 capture to 0.99 mmol.g -1 , which was higher than that of as-synthesized ZIF-8/CS spheres (0.679 mmol.g -1 ). [START_REF] Fan | Structuring ZIF-8-Based Hybrid Material with Hierarchical Pores by in Situ Synthesis and Thermal Treatment for Enhancement of CO2 Uptake[END_REF] We also recently reported the preparation of ZIF-67/CS microspheres, with CO2 uptakes at 298 K up to 0.76 mmol.g -1 . 39 For practical applications, not only should the CO2 uptake at 298 K be around 2 mmol.g -1 or higher, but the isosteric heats of adsorption (Qst) should also be below 50 kJ.mol -1 for facile sorbent regeneration. [START_REF] Singh | Emerging Trends in Porous Materials for CO2 Capture and Conversion[END_REF] Several reported data obtained for amine-modified porous substrates showed that their values of adsorption heat range from 20-50 kJ/mol, which indicates weak chemisorption. [START_REF] Alhwaige | Carbon Aerogels with Excellent CO2 Adsorption Capacity Synthesized from Clay-Reinforced Biobased Chitosan-Polybenzoxazine Nanocomposites[END_REF][START_REF] Xia | Superior CO2 Adsorption Capacity on N-Doped, High-Surface-Area, Microporous Carbons Templated from Zeolite[END_REF] Primo et al. [START_REF] Primo | From Biomass Wastes to Highly Efficient CO2 Adsorbents: Graphitisation of Chitosan and Alginate Biopolymers[END_REF] reported that the Qst values of CO2 adsorption using CS-derived spherical carbons were in the range of 19-35 kJ mol -1 . Very recently, we found that the values of Qst for CO2 adsorption using ZIF-67/CS2:1 beads were in the range of 11.6-44.1 kJ mol -1 . [START_REF] Hammi | Hierarchically Porous ZIF-67/Chitosan Beads with High Surface Area and Strengthened Mechanical Properties: Application to CO2 Storage[END_REF] Herein, the Qst values of HKUST-1@CS1:1C were calculated from the CO2 sorption isotherms measured at different temperatures (Fig. 2c) using the Clausius-Clapeyron equation as plotted in Figures S13-S14. [START_REF] Liu | Carbon Dioxide Capture by Functionalized Solid Amine Sorbents with Simulated Flue Gas Conditions[END_REF] The maximum value of Qst (~22.6 kJ mol -1 ) indicates that weak interactions are involved, making HKUST-1@CS1:1C a promising candidate for CO2 capture storage and reutilization (CCUS).

High-pressure CH4 uptake and CO2/CH4 selectivity

HKUST-1 has been extensively investigated for methane storage and release in automotive applications, as it outperforms commercial activated carbons. In this case, both gravimetric and volumetric uptakes should be considered. The total CH4 adsorption capacities of the different studied samples at 298 K are plotted for various pressures in Figure 2d. At low pressures, HKUST-1@CS1:1C shows the highest CH4 uptakes supported by the MOF microporosity, followed by nearly identical CH4 uptakes for the HKUST-1@CS1:1A beads.

However, in the high-pressure regime where pore volume effects are important and the energy and affinity-driven adsorption sites are occupied, the meso/macroporous nature of the HKUST-1@CS beads becomes evident. This distinctive structure facilitates enhanced accessibility to additional adsorption sites, leading to a substantial adsorption capacity of 4.70 mmol.g -1 at 50 bar. The much lower CH4 adsorption capacity with HKUST-1@CS1:1X beads and saturation from 40 bar are unexpected and might be related to their densification during drying. These findings highlight the importance of considering both microporous and meso/macroporous structures when designing high-performance adsorbents for gas storage and separation applications.

It is worth noting that the adsorption capacity of these composite beads is comparable to that of the previously reported HKUST-1 3D-printed solids, which exhibited an adsorption capacity of 5.18 mmol.g -1 at 298 K and 50 bar. [START_REF] Dhainaut | Formulation of Metal-Organic Framework Inks for the 3D Printing of Robust Microporous Solids toward High-Pressure Gas Storage and Separation[END_REF] However, it should be emphasized that the 3D-printed solids had a significantly higher surface area (SBET = 1200 m 2 .g -1 ). [START_REF] Dhainaut | Formulation of Metal-Organic Framework Inks for the 3D Printing of Robust Microporous Solids toward High-Pressure Gas Storage and Separation[END_REF] However, the reported adsorption capacity of pure HKUST-1 powder at 298 K and 35 bar, which is 10.13 mmol.g -1 surpasses that of the composite beads. [START_REF] Spanopoulos | Reticular Synthesis of HKUST-like Tbo-MOFs with Enhanced CH4 Storage[END_REF] One way to valorize CO2 is through its use as a reagent. Notably, methanation allows to transform CO2 into CH4 (CO2 + 4H2 → CH4 + 2H2O) for energy applications. [START_REF] Tripodi | Carbon Dioxide Methanation: Design of a Fully Integrated Plant[END_REF] Thus, it is of practical importance to develop selective sorbents to purify the gas flow downstream of the reaction. To evaluate the CO2 separation performance of HKUST-1@CS1:1C, single component CO2, and CH4 isotherms were collected at 1 bar at 298 K. The adsorption capacity of HKUST-1@CS1:1C at 298 K for CH4 is 0.48 mmol.g -1 , in comparison with 1.91 mmol.g -1 for CO2 (Table S3). Figure S15a shows that the CO2 and CH4 adsorption isotherms have initial slopes of 1.93 and 0.44, respectively. By calculating the ratio of Henry's constants, the HKUST-1@CS1:1C beads present a predicted CO2/CH4 selectivity of 4.4 at 298 K, indicating preferential adsorption of CO2. This selectivity further improves to 6.8 at 273 K and 1 bar (Fig. S15b), suggesting that the material is more selective towards CO2 at lower temperature and pressure conditions.

Furthermore, the adsorption capacity of HKUST-1@CS1:1X at 298 K for CH4 is 0.19 mmol.g -1 , in comparison with 1.33 mmol.g -1 for CO2. Figure S15c shows that the CO2 and CH4 adsorption isotherms have initial slopes of 1.33 and 0.16, respectively. By calculating the ratio of Henry's constants, the HKUST-1@CS1:1X beads present a predicted CO2/CH4 selectivity of 8 at 298 K, indicating even more preferential adsorption of CO2.

Moreover, it has been reported that HKUST-1 may be functionalized to further improve this selective adsorption, for example by incorporating calcium ions in the framework, which is beyond the scope of this study. [START_REF] Abid | CH4, and H2 Adsorption Performance of the Metal-Organic Framework HKUST-1 by Modified Synthesis Strategies[END_REF] Overall, these findings indicate that HKUST-1@CS1:1C may be used for CO2 separation owing to the stronger attraction of CO2 by the polar functional groups of HKUST-1@CS1:1C.

Xe/Kr capture by HKUST-1@CS beads

As a way to showcase the material's versatility for gas separation, we also performed adsorption tests with xenon and krypton noble gases. This application is of growing interest for the valorisation of gaseous fission products. [START_REF] Abramova | Extrusion-Spheronization of UiO-66 and UiO-66_NH2 into Robust-Shaped Solids and Their Use for Gaseous Molecular Iodine, Xenon, and Krypton Adsorption[END_REF] The experiments conducted at 273 K and 1 bar using HKUST-1@CS1:1C revealed an adsorption capacity of 1.61 mmol.g -1 for Xe, compared to only 0.29 mmol.g -1 for Kr (Fig. S16). This leads to a predicted Xe/Kr selectivity of 6.2 based on Henry's constants (Fig. S17), which is slightly lower than that of the pristine HKUST-1 (8.1). The reduction in selectivity observed in the composite can be attributed to the presence of chitosan, a constituent of the composites, which indiscriminately adsorbs both gases. [START_REF] Sikora | Thermodynamic Analysis of Xe/Kr Selectivity in over 137 000 Hypothetical Metal-Organic Frameworks[END_REF][START_REF] Banerjee | Metal-Organic Framework with Optimally Selective Xenon Adsorption and Separation[END_REF] Of note, under a flow of Xe and Kr diluted in air, pure HKUST-1 demonstrates a cumulated adsorption capacity of 3.18 mmol.g -1 for Xe and 1.92 mmol.g -1

for Kr. [START_REF] Liu | Metal-Organic Frameworks for Removal of Xe and Kr from Nuclear Fuel Reprocessing Plants[END_REF] The influence of chitosan under these conditions would be of interest to evaluate.

CONCLUSION

HKUST-1@CS composite beads with different compositions were successfully prepared using different drying methods and then assessed for gas (CO2, CH4, Xe, Kr) capture by adsorption at different operating conditions. The results presented herein together show increased gas sorption capacity when dried by treatment with supercritical CO2 or by freeze-drying rather than by thermal evacuation methods, giving well-preserved macrostructures and enhanced porosity.

The adsorbents exhibit high surface areas (up to 502 m 2 g -1 ) with excellent adsorption performance (CO2 adsorption capacity in the range of 0.99 -1.98 mmol g -1 at 298 K at 1 bar) and relatively low heats of adsorption (below 22.6 kJ mol -1 ). Importantly, HKUST-1@CS1:1C beads showed high equilibrium selectivities for CO2/CH4 separation (up to 6.8), suggesting that they are potential adsorbents for CO2 separation. Finally, HKUST-1@CS1:1C also exhibited interesting performances for Xe separation from Kr with a predicted Xe/Kr selectivity up to 6.2 at 273 K and 1.0 bar, which should be further looked upon.

Supporting Information

General bibliography; additional characterization of samples by SEM, XRD, XPS, EDS, FTIR, TGA, mercury intrusion porosimetry and N2-physisorption isotherms; tables with additional experimental data; additional CO2 adsorption isotherms and thermodynamic calculations.

Acknowledgment

The Chevreul Institute is thanked for its help in the development of this work through the ARCHI-CM project supported by the "Ministere de l'Enseignement Superieur de la Recherche et de l'Innovation", the region "Hauts-de-France", the ERDF program of the European Union and the "Metropole Europeenne de Lille. The region "Hauts-de-France" is also thanked for its support through the REGCAT project and UPCAT projects. The authors would like to thank Martine

Table of Contents

Fig. 1 .

 1 Fig.1. SEM images of HKUST-1@CS1:1A, HKUST-1@CS2:1C and HKUST-1@CS2:1C. (a-c,f)

Fig. 2 .

 2 Fig. 2. (a) Effect of the drying method on CO2 uptake of HKUST-1@CS beads; (b) Effect of

  BET surface area; [b] micropore surface area; [c] total pore volume obtained at P/Po = 0.98; [d] micropore volume with t-plot method; [e] Mesopore volume is determined by subtracting the micropore volume from the total volume; [f] NLDFT pore diameters, with secondary pore distributions noted in brackets; [g] BJH pore diameter calculated on desorption branch; NA: not applicable.

								g]
								(nm)
	CS	71	NA	0.13	NA	NA	NA	9.6
	HKUST-1	1337	1145	0.77	0.60	0.17	1.2	-
	HKUST-1@CS1:1C	527	323	0.8	0.13	0.67	1.2(1.7)	14.2
	HKUST-1@CS1:1A	531	337	1.71	0.16	1.55	1.2(1.7)	14.2
	HKUST-1@CS1:1X	460	290	0.63	0.11	0.52	1.2(1.7)	7.4
	HKUST-1@CS1:2C	249	137	0.23	0.05	0.18	1.2(1.5)	4/19
	HKUST-1@CS2:1C	502	420	0.32	0.16	0.16	1.2(1.5,1.7) 4.1/43
	HKUST-1@SA2:1C	NA	NA	NA	NA	NA	NA	NA
	[a]

Table 2 :

 2 Comparison of the CO2 adsorption of related porous aerogels.

	Sorbent	Conditions	Types of material	Shape	SBET	CO2 uptake	Ref
					(m 2 g -1 )	mmol.g -1
	CSGO	25°C, 1 bar	CS grafted graphene	Aerogels	33	0.26	38
			oxide
	ZIF-67/CS2:1	25°C, 1bar	Chitosan/ZIF-8	Beads	1197	0.76	39
	CTS-GO-5%	25°C, 1bar	Chitosan and grapheme	Aerogels	<40	0.60	40
			oxide
	PIL-chitosan	25°C, 1 bar	Poly(ionic liquid)s	Aerogels	449	0.70	41
	CS-(4-	25°C, 5bar	Chitosan with 4-	Gel	52.78	0.85	42
	formyltriphenylamine)		formyltriphenylamine
	CS/ZIF-8	25°C	Chitosan/ MOF	Beads	626	0.99	19
	GZA8	25°C, 1bar	Graphene/ZIF-8	Aerogel	1099	0.99	43

Trentesaux for her help in carrying out the work on the Raman Spectroscopy Analysis, Laurence Burylo for the powder X-Ray diffraction and diffusion facility, Anne-Marie Blanchenet for SEM analyses, Pardis Simon for XPS analyses and Ridvan Yildiz from CERAMATHS laboratory for mercury intrusion.


Le Pape Olivier

Amour Brind'

Anik

Bégout Marie-Laure

Olivier Le Pape
email: olivier.le_pape@agrocampus-ouest.fr

Flatfish ecology: Advances from the 10th international flatfish symposium

C C E P T E D M A N U S C RI P
Since their first edition, the flatfish symposia have provided a platform for all flatfish scientists to share their interest and knowledge in the biology and ecology of this fish group. These symposia are organized every three years, and always stimulate interactions between various scientific disciplines (systematics, morphology, physiology, genetics, ecotoxicology, technology, modeling, population dynamics, ecology, fisheries and aquaculture science, etc.). The last meeting on which this special issue of Journal of Sea Research is based, was entitled "Ten International Flatfish Symposia and 30+Years of Advanced Research: Flatfish Ecology in 2017". It was held from 11 th to 16 th November 2017 in Saint-Malo (France) and aimed to:

-Present emerging research on the biology and ecology of flatfish species worldwide, -Quantify linkages between flatfishes, atmospheric, oceanographic, and other abiotic and biotic components of the ecosystem, -Explore management tools for assessing and improving the sustainability of flatfish populations and related human activities (fisheries and aquaculture). This 10 th symposium welcomed 130 attendees from across the globe. In total 5 continents and 19 countries were represented. A selection of 13 papers based on the 8 keynote presentations, 67 talks and 33 posters are published here.

The main insights of the symposium and the related published papers were:

ACCEPTED MANUSCRIPT A C C E P T E D M A N U S C RI P T
-Ongoing progress concerning biology, ecology, biotechnology and flatfish aquaculture.

Four related papers were published as summarised here. Van der Veer et al. described life history characteristics of poorly known flatfish species in the Caribbean and did not find the expected patterns of lower growth rate in tropical, temperate and subtropical species. Poiesz et al.

analysed the link between food availability and growth of juvenile flounder and plaice in the North Sea. They evidenced that flounder, an epibenthic predator, is less limited by a decrease in prey availability in summer in the North Sea than plaice, an infauna benthic feeder. Joh and Wada (2018) focused on spatio-temporal patterns in timing of spawning and planktonic larval stage duration of yellow striped flounder in Northern Japan and showed the influence of water temperature on both. Tomiyama et al. demonstrated that the optimal temperature for growth of juvenile marbled flounder changes ontogenetically, in accordance with an in situ increase in ambient temperature after settlement in Japanese waters in spring.

-Large improvements in knowledge of flatfish movements and behaviour at different life stages.

New tagging tools and improvements in analysis of natural tracers (otoliths, genetics, stable isotopes, etc) have allowed enhanced multi-tracer research activities. Accordingly, knowledge on spatial connectivity at different life stages has progressed. The accuracy of stocks as management units was also investigated, with metapopulation dynamics of flatfish revealing both small-scale (below stock unit size) spatial structure and exchange between populations, with consequences on effectiveness and appropriateness of management measures. Three related papers were

  published in this issue as follows. Randon et al. used patterns in growth and asynchrony in cohort densities to evidence spatial structure in the stock of common sole in the Eastern Channel. Sato et al. estimated the genetic structure and demographic connectivity of the marbled flounder in Tokyo Bay, and focused on local segregation at small scales among subpopulations. Lower and Soderlung used pop-up archival transmitting tags on the Pacific halibut, and demonstrated connectivity between the Salish Sea and offshore populations in Canada and Alaska. -Deeper enquiries of anthropogenic pressures impacting flatfish, with a large panel of accounted pressures, i.e., granulate extraction, beach nourishment, wind farms, xenobiotics, eutrophication, direct and indirect effects of fishing, including fishing practice (pulse trawling). New approaches were presented to infer on local impacts (e.g., exposure to xenobiotics) at population scale. A first set of 3 papers focused on the impacts of global change. Yeung and Yang analysed spatial variation in habitat quality for juvenile flatfish in the south-eastern Bering Sea, and demonstrated that warm-cold environmental phases affect habitat usage, but that the northern drift may not increase productivity. Cheung and Oyinlola proposed a worldwide overview of the vulnerability of flatfish and their fisheries and mariculture to climate change, highlighting regions and species that are expected to be the most impacted. Andrade et al. developed laboratory experiments to analyse the behavioural responses of the speckled sanddab to elevated CO 2, and evidenced their relative resilience. Three other papers focused on other anthropogenic pressures. Frisk et al. assessed the drivers of the collapse of the winter flounder throughout the southern New England/mid-Atlantic region. They highlighted the main influence of fishing mortality and the lack of evidence of environmental drivers. Eggleston et al. evidenced the impact of bottom trawling on the diet of 3 of the 4 studied flatfish species in the North Sea. Wilber et al. focused on the impact of wind farms on 7 flatfish species before and during their construction, and then during operation time, without evidencing an attraction effect nor negative consequences. Steering committee: Janet Duffy-Anderson (NOAA, USA), Josianne Gatt-Stottrup (DTU-Aqua, DK), Elisabeth Fairchild (UNH, USA), Henk van der Veer (NIOZ, NL), Satoshi Katayama (Tohoku U., Japan), IPHC, USA), Richard Nash (IMR, Norway), Jan-Jaap Poos (WUR, NL), Henrique Cabral (Lisbon U., PT), Olivier Le Pape (ACO, France). Organizing committee: Marie Laure Bégout and Anik Brind'Amour (Ifremer, France), Catherine Le Penven, Amélie Régimbart and Olivier Le Pape (ACO, France), Ingrid de Raad (NIOZ, NL). Sponsors: NIOZ, ICES, Saint Malo Agglomération, Région Bretagne, Ifremer, Starr Oddi, Thelma Biotel, Vemco-Anhydre, Biologging solutions, Biolog Id, France Filière Pêche, CNPMEM, Cobrénord, Pêcheurs d'Aquitaine, OPPAN.