Dylan Chaillot

Simona Bennici
email: simona.bennici@uha.fr

Jocelyne Brendlé

Layered Double Hydroxides and LDH-derived materials in chosen environmental applications: a review

Keywords: Hydrotalcite, layered double hydroxides, materials, synthesis, applications, industrial, environmental

With increasing global warming awareness, layered double hydroxides (LDHs), hydrotalcites and their related materials are key components to reduce the environmental impact of human activities. Such materials can be synthesized quickly with high efficiency by using different synthesis processes. Moreover, their properties tunability is appreciated in various industrial processes. Regarding their physical and structural properties, such materials can be applied in environmental applications such as the adsorption of atmospheric and aqueous pollutants, the hydrogen production, or the formation of 5-Hydroxymethylfurfural (5-HMF). After a first part dedicated to the synthesis processes of hydrotalcites, the present review reports on specific environmental applications chosen as example in various fields (green chemistry and depollution) that have gained increasing interest in the last decades, enlightening the links between structural properties, synthesis route and application using the lamellar materials.

Introduction

Layered double hydroxides, also called LDHs or anionic clays, are particular lamellar materials made of octahedral sheets composed of metallic cations in the center of the octahedra. These materials can be found in nature, and present the brucite (Mg(OH) 2 ) structure with divalent cations M II (Mg 2+ or Cu 2+ for example) partly substituted by trivalent cations M III (such as Al 3+ or Fe 3+) . This partial substitution induces a positive charge deficit that is compensated by anions, such as carbonates, located in the interlayer space along with water molecules. The easily accessible interlayer is one of the sources of the anion exchange capacity of the compounds, and of its ability to capture and exchange anions. This characteristic is almost unique for inorganic materials, indeed clay minerals exhibit cation exchange capacity. Thanks to these characteristics, they are appreciated for their versatility, simplicity of tailoring and low cost. The general formula of LDHs can be described as follows:

[M II 1-x M III x (OH) 2 ] x+ [A m- x/m .nH 2 O] x-
where M II and M III respectively represent the metallic divalent (Mg 2+ , Cu 2+ , Zn 2+ ) and trivalent (Al 3+ , Fe 3+ ) cations , A the compensating anion, and x and m depend on the substitution rate between M II and M III cations.

Hydrotalcites are particular LDHs (characterized by a Mg:Al molar ratio of 3:1) in which magnesium is partly substituted by aluminum . Specific properties such as porosity and surface area can be tailored by varying the synthesis method and conditions, leading to a wide range of materials adapted to targeted applications. The structure can also be modified by changing the combination of the cations and the interlayer anions, in order to confer specific properties, [START_REF] Homsi | Steam reforming of ethanol for hydrogen production over Cu/Co-Mg-Al-based catalysts prepared by hydrotalcite route[END_REF][START_REF] El Rouby | Efficient water decontamination using layered double hydroxide beads nanocomposites[END_REF][START_REF] Sheng | Adsorption of phosphorus by using magnetic Mg-Al-, Zn-Al-and Mg-Fe-layered double hydroxides: comparison studies and adsorption mechanism[END_REF]Wu et al. 2019a).

LDHs can be synthesized by various techniques depending on the final requirements such as high crystallinity, low time and/or energy consuming reaction, and phase purity. The most common methods used to synthesize these materials are hydrolysis reaction, co-precipitation, hydrothermal synthesis, steam activation, microwave irradiation, and sol-gel. These fast and environmental-friendly synthesis processes, combined to high sorption capacities, allow the use of hydrotalcites and derived materials in various applications, such as adsorption of various molecules (organic and inorganic pollutants or dyes for example) or catalytic processes (also thanks to the presence of strong basic sites). In particular, these materials have been intensively studied as catalysts and catalyst supports based on their specific physicochemical features.

Mixed oxides, obtained from the calcination of the parent material, are most generally used in catalytic and environmental applications, due to the high temperatures required. The composition of the obtained oxides depend on the initial combination of M II and M III cations [START_REF] El Rouby | Efficient water decontamination using layered double hydroxide beads nanocomposites[END_REF][START_REF] Rahmanian | Synthesis and characterization of citrate intercalated layered double hydroxide as a green adsorbent for Ni2+ and Pb2+ removal[END_REF][START_REF] Sheng | Adsorption of phosphorus by using magnetic Mg-Al-, Zn-Al-and Mg-Fe-layered double hydroxides: comparison studies and adsorption mechanism[END_REF]Wu et al. 2019a), but also on the heating temperature and the targeted reaction (dehydration, dehydroxylation, removal of the interlayer anion, and oxide reformation). Considering the rising interest in using synthesized LDHs for various applications, this paper reviews the most common synthesis methods for their preparation and some chosen environmental applications (Fig1). Due to the wide panel of applications involving layered double hydroxides and LDH-derived materials, the list of applications here reported is not exhaustive. Moreover, certain applications have been already extensively reported in the literature and they could represent alone the subject of dedicated review articles (for example in the case of biodiesel production [START_REF] Helwani | Technologies for production of biodiesel focusing on green catalytic techniques: A review[END_REF][START_REF] Atadashi | The effects of catalysts in biodiesel production: A review[END_REF])).

Synthesis techniques for hydrotalcite-like compounds

As previously explained, hydrotalcite (and more generally LDHs) can be of natural origin (alteration minerals) or synthetic. The advantages in synthesizing LDHs are the ability to finely tune the mineral properties in the view of a specific application, by finding the appropriate combination of reactants and the adapted M II /M III molar ratio. The most important factors to be considered are the nature of the metallic cations, the pH, the temperature, the ageing time, and the preparation method. Several synthesis methods have been developed

during the last decades, but this review summarizes only those most commonly used and described in the literature.

Co-precipitation method

Co-precipitation is the most common method to synthesize LDHs. It consists in dissolving the inorganic salts in alkaline media, at constant or increasing pH. This reaction allows to control the morphology and particle size, depending on the supersaturation of the solution.

Feitknecht reported for the first time in 1942 on the LDH synthesis by co-precipitation. [Mg-Al-CO 3 ] LDH was synthesized starting from diluted solutions of the reactants. The coprecipitation synthesis process has been then studied years later by [START_REF] Gastuche | Mixed Magnesium-Aluminium Hydroxides I. Preparation and Characterization of Compounds Formed in Dialysed Systems[END_REF] and [START_REF] Miyata | The Syntheses of Hydrotalcite-like Compounds and their Structures and Physicochemical Properties I. The Systems Mg2+-Al3+-NO3-, Mg2+-Al3+-Cl-, Mg2+-Al3+-ClO4-, Ni2+-Al3+-Cl-and Zn2+-Al3+-Cl[END_REF][START_REF] Miyata | Physico-Chemical Properties of Synthetic Hydrotalcites in Relation to Composition[END_REF] by modifying parameters such as the concentrations of the reactants and the washing conditions. They also introduced the control of the pH as a key parameter for the formation of LDHs. Several studies have been performed later, focusing on the substitution of the type of metallic cations in the structure of the LDHs, as shown by Sato et al. (1988), in order to obtain several LDH with different structural formulas (M II = Mg 2+ , Ni 2+ or Co 2+ and M III = Al 3+ or Fe 3+ ). Co-precipitation allows to finely tune the structure of the synthesized materials by controlling the M II /M III molar ratios, the type of interlayer anion, the synthesis duration, the temperature, and the pH of the solution [START_REF] Thevenot | PREPARATION AND CHARACTERIZATION OF A1-RICH Zn-A1 HYDROTALCITE-LIKE COMPOUNDS[END_REF][START_REF] Ulibarri | Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity[END_REF][START_REF] Zhao | Synthesis, characterization, and application of hydotalcites in hydrodesulfurization of FCC gasoline[END_REF][START_REF] Kloprogge | The effects of synthesis pH and hydrothermal treatment on the formation of zinc aluminum hydrotalcites[END_REF][START_REF] Sharma | Effect of Hydrothermal Conditions on Structural and Textural Properties of Synthetic Hydrotalcites of Varying Mg/Al Ratio[END_REF][START_REF] Klemkaite | Synthesis and characterization of layered double hydroxides with different cations (Mg, Co, Ni, Al), decomposition and reformation of mixed metal oxides to layered structures[END_REF]).

Co-precipitation can be performed at low or high supersaturation conditions depending on the desired crystallization state, as long as the initial concentration of the reactants is above the saturation state of the solution (supersaturation conditions). In any case, the pH of the solution must be controlled: a pH value too low does not allow the complete precipitation of all metallic ions, while a too high value leads to the leaching of one or more metallic ions.

Basic media are preferred in most cases, but the optimal pH depends mainly on the metal cations used. Fig2. presents the steps involved in the co-precipitation synthesis process. The co-precipitation method performed at low supersaturation conditions has been firstly documented in the '90s by [START_REF] Kannan | Synthesis and physicochemical properties of cobalt aluminium hydrotalcites[END_REF]. This technique is performed by the slow addition of solutions containing the divalent and trivalent cations in the appropriate molar ratio, before adding an aqueous solution of the chosen interlayer anion [START_REF] Kannan | High-temperature transformations of Cu-rich hydrotalcites[END_REF][START_REF] Climent | Increasing the basicity and catalytic activity of hydrotalcites by different synthesis procedures[END_REF][START_REF] Kloprogge | The effects of synthesis pH and hydrothermal treatment on the formation of zinc aluminum hydrotalcites[END_REF]Kovanda et al. 2005a;[START_REF] Perez-Lopez | Effect of composition and thermal pretreatment on properties of Ni-Mg-Al catalysts for CO2 reforming of methane[END_REF][START_REF] Sharma | Effect of Hydrothermal Conditions on Structural and Textural Properties of Synthetic Hydrotalcites of Varying Mg/Al Ratio[END_REF][START_REF] Wang | Mg-Al-carbonate layered double hydroxides as a novel catalyst of luminol chemiluminescence[END_REF][START_REF] Klemkaite | Synthesis and characterization of layered double hydroxides with different cations (Mg, Co, Ni, Al), decomposition and reformation of mixed metal oxides to layered structures[END_REF][START_REF] Balsamo | Synthesis and Characterization of Metal Mixed Oxides from Layered Double Hydroxides[END_REF]). An alkaline solution is then added in order to fix the pH during the reaction and to promote the coprecipitation of the two metal salts. This synthesis process has been well described by a schematic flow chart edited by El [START_REF] El Rouby | Efficient water decontamination using layered double hydroxide beads nanocomposites[END_REF]. The advantage of this method is the possibility to control the charge density by keeping the pH constant under low supersaturation conditions by the addition of a mixture of NaOH and NaHCO 3 or Na 2 CO 3 [START_REF] Sharma | Effect of Hydrothermal Conditions on Structural and Textural Properties of Synthetic Hydrotalcites of Varying Mg/Al Ratio[END_REF][START_REF] Wang | Mg-Al-carbonate layered double hydroxides as a novel catalyst of luminol chemiluminescence[END_REF][START_REF] Klemkaite | Synthesis and characterization of layered double hydroxides with different cations (Mg, Co, Ni, Al), decomposition and reformation of mixed metal oxides to layered structures[END_REF][START_REF] Balsamo | Synthesis and Characterization of Metal Mixed Oxides from Layered Double Hydroxides[END_REF][START_REF] Sheng | Adsorption of phosphorus by using magnetic Mg-Al-, Zn-Al-and Mg-Fe-layered double hydroxides: comparison studies and adsorption mechanism[END_REF]Li et al. 2019a). Materials obtained by co-precipitation at low supersaturation present a higher crystallinity than those obtained at high supersaturation conditions thanks to the continuous regulation of the pH of the synthesis solution. In the high supersaturation condition, the concentration of the solutions is continuously increased or the solution containing the dissolved salts is added to a solution containing a small excess of alkali bicarbonates. This process leads to the formation of less crystalline materials than low supersaturation method, but gives rise to a high number of small aggregates due to the interlayer confinement effect of the carbonate ions [START_REF] Zhang | Preparation of Ni(II)/Ti(IV) layered double hydroxide at high supersaturation[END_REF][START_REF] Abelló | Perturbing the properties of layered double hydroxides by continuous coprecipitation with short residence time[END_REF][START_REF] Solovov | Synthesis of Ni(II)-Ti(IV) Layered Double Hydroxides using Coprecipitation at High Supersaturation Method[END_REF].

Co-precipitation synthesis can be considered as an efficient method to obtain crystalline materials, particularly thanks to the long synthesis duration time (several days), the controlled synthesis pH, and the addition of post treatments such as hydrothermal treatment for example [START_REF] Kannan | Synthesis and physicochemical properties of cobalt aluminium hydrotalcites[END_REF][START_REF] Kloprogge | The effects of synthesis pH and hydrothermal treatment on the formation of zinc aluminum hydrotalcites[END_REF]). However, due to the large dimension of the obtained particles the specific surface area values (between 50-100 m².g -1 ) are lower than those obtained by sol-gel synthesis (150-200 m².g -1 ), and discussed later in this review [START_REF] Aramendía | Comparative Study of Mg/M(III) (M=Al, Ga, In) Layered Double Hydroxides Obtained by Coprecipitation and the Sol-Gel Method[END_REF][START_REF] Park | Influence of the preparation method on the catalytic activity of Mg Al hydrotalcites as solid base catalysts[END_REF]). The substitution with other metallic cations, such as Fe, Zn, Cu, and Ni, as demonstrated by [START_REF] Gevers | Comparison of nano-structured transition metal modified tri-metal MgMAl-LDHs (M = Fe, Zn, Cu, Ni, Co) prepared using coprecipitation[END_REF], add the possibility to modify the crystallite dimensions and influence the thermal stability of LDHs. Thermal treatments, such as hydrothermal treatment or microwave irradiation, can be performed to improve crystallinity and/or to reduce the synthesis time. Similar structures are then obtain even with shorter synthesis time [START_REF] Climent | Increasing the basicity and catalytic activity of hydrotalcites by different synthesis procedures[END_REF]. This progress was reported by [START_REF] Tichit | Comparison of the structural and acid-base properties of Ga-and Al-containing layered double hydroxides obtained by microwave irradiation and conventional ageing of synthesis gels[END_REF] by comparing microwave and hydrothermal treatments in co-precipitated hydrotalcites. The authors did not find any significant differences in the structural properties of the final material, despite the shorter synthesis time that represent a clear advantage. The thermal treatment presents a significant impact on the structural properties of the materials: [START_REF] Othman | Effects of thermal treatment on the micro-structures of co-precipitated and sol-gel synthesized (Mg-Al) hydrotalcites[END_REF] compared the effect of the hydrothermal treatment on hydrotalcites prepared by coprecipitation and sol-gel methods. They showed that the specific surface area and pore sizes significantly increased in both cases, with a stronger impact on the samples synthesized by sol-gel.

Urea hydrolysis

Urea is a weak Brönsted base highly soluble in water. It can be used as a precipitating agent instead of sodium hydroxide to increase the pH through its thermal decomposition. The slow decomposition, starting around 90°C, leads to an increase of the pH up to 10 [START_REF] Costantino | New Synthetic Routes to Hydrotalcite-Like Compounds -Characterisation and Properties of the Obtained Materials[END_REF][START_REF] Yang | Urea method for the synthesis of hydrotalcites[END_REF]). Since urea decomposes slowly, it leads to a lower degree of supersaturation. Moreover, prolonged hydrolysis results in the formation of CO 3 2-anions in basic medium that act as interlayer anions. Therefore, urea enables an efficient pH control and the formation of mono-dispersed LDH materials with high crystallinity through a low-cost method [START_REF] Adachi-Pagano | Synthesis of Al-rich hydrotalcite-like compounds by using the urea hydrolysis reaction-control of size and morphology[END_REF][START_REF] Yang | Urea method for the synthesis of hydrotalcites[END_REF]Zeng et al. 2009a;[START_REF] Xu | Preparation Behavior of the Mg-Fe Hydrotalcite by Urea Method and Its Cr(VI) Sorption Property[END_REF][START_REF] Park | Influence of the preparation method on the catalytic activity of Mg Al hydrotalcites as solid base catalysts[END_REF]. The purity and crystallinity of hydrotalcite phases can also be optimized by tuning the M II :M III molar ratio, the urea concentration, and the ageing time [START_REF] Costantino | New Synthetic Routes to Hydrotalcite-Like Compounds -Characterisation and Properties of the Obtained Materials[END_REF]Zeng et al. 2009a;[START_REF] Berber | Uniform nanoparticles of hydrotalcite-like materials and their textural properties at optimized conditions of urea hydrothermal treatment[END_REF][START_REF] Montañez | Nickel catalysts obtained from hydrotalcites by coprecipitation and urea hydrolysis for hydrogen production[END_REF][START_REF] Zhang | Mg-Al hydrotalcites as solid base catalysts for alcoholysis of propylene oxide[END_REF][START_REF] Bian | Ni-based catalyst derived from Ni/Al hydrotalcite-like compounds by the urea hydrolysis method for CO methanation[END_REF][START_REF] Xu | Preparation Behavior of the Mg-Fe Hydrotalcite by Urea Method and Its Cr(VI) Sorption Property[END_REF][START_REF] Park | Influence of the preparation method on the catalytic activity of Mg Al hydrotalcites as solid base catalysts[END_REF]. As an exemple, [START_REF] Berber | Uniform nanoparticles of hydrotalcite-like materials and their textural properties at optimized conditions of urea hydrothermal treatment[END_REF] identified the optimal synthesis conditions to obtain uniform and highly crystalline hydrotalcite particles by using a urea/metallic cation molar ratio of 2, and 12h of ageing time at 120°C. Secondary phases have been observed with molar ratios lower or greater than 2, leading to lower crystallinity and specific surface areas of the materials. The main steps involved in hydrotalcite synthesis via urea hydrolysis are summarized in Fig3.

Most of the time, this technique is combined to others (in which urea is used only as a precipitating agent), such as co-precipitation or hydrothermal synthesis. The combination of urea hydrolysis and hydrothermal treatment has been well-documented by [START_REF] Berber | Uniform nanoparticles of hydrotalcite-like materials and their textural properties at optimized conditions of urea hydrothermal treatment[END_REF] and [START_REF] Benito | Uniform Fast Growth of Hydrotalcite-like Compounds[END_REF] in order to obtain uniform particles, but also by [START_REF] Rao | Hydrothermal synthesis of Mg-Al hydrotalcites by urea hydrolysis[END_REF] to find optimal Mg:Al molar ratios of 1:1 and 2:1 with uniform and small particles' size. In the first article, microwave irradiation was used as heating source in the synthesis process, strongly shortening the synthesis time and the formation of pure and well-crystallized hydrotalcite phases presenting high specific surface areas. Further studies were performed by [START_REF] Montañez | Nickel catalysts obtained from hydrotalcites by coprecipitation and urea hydrolysis for hydrogen production[END_REF] by comparing the properties of mixed oxides obtained by calcination of a series of Ni-Al-Mg hydrotalcites synthesized, either by co-precipitation or by urea hydrolysis. Enhanced redox properties, larger pore sizes and smaller particles (interesting features in catalytic applications) were obtained on calcined samples prepared with urea hydrolysis. In general, the use of microwave irradiation allows to reduce the preparation time without to impact on the structural and physical properties. This methodology is of a great interest for the industrial scaling-up [START_REF] Yang | Microwave Synthesis of Hydrotalcite by Urea Hydrolysis[END_REF][START_REF] Benito | Microwave-Assisted Homogeneous Precipitation of Hydrotalcites by Urea Hydrolysis[END_REF]). Hibino and Ohya.

(2009) combined urea hydrolysis with hydrothermal treatment in order to remove certain byproducts (i.e. hydrated magnesium carbonate hydroxide phases). Further studies have been performed by [START_REF] Kloprogge | Characterization of Intercalated Ni/Al Hydrotalcites Prepared by the Partial Decomposition of Urea[END_REF] to show the evolution of the intercalated species depending on the hydrothermal treatment: (NH 2 )COO -tends to directly form after precipitation, but slowly transforms into carbonated species during the hydrothermal treatment.

As a partial conclusion, urea hydrolysis is a slow reaction that leads to a low degree of supersaturation during the precipitation. It presents the advantage to provide large and thin platelets with a narrow particle size distribution in a shorter ageing time than co-precipitation [START_REF] Naseem | Comparison of transition metal (Fe, Co, Ni, Cu, and Zn) containing tri-metal layered double hydroxides (LDHs) prepared by urea hydrolysis[END_REF]. However, the decomposition of urea leads to the formation of CO 2 that reacts with water to form carbonates and acting as compensating anions [START_REF] Thomas | Mechanochemical synthesis of layered hydroxy salts[END_REF]. In this case, a further anion exchange may be required depending on the targeted application [START_REF] Bish | Anion-exchange in takovite : applications to other hydroxide minerals[END_REF][START_REF] Iyi | Deintercalation of Carbonate Ions from a Hydrotalcite-Like Compound: Enhanced Decarbonation Using Acid-Salt Mixed Solution[END_REF][START_REF] Inayat | The urea method for the direct synthesis of ZnAl layered double hydroxides with nitrate as the interlayer anion[END_REF].

Sol-gel method

The sol-gel method is appreciated for its low cost, low energy and limited time consuming, for the possibility to obtain high purity materials, and for the possibility to tune their composition [START_REF] Lopez | Synthesis and Characterization of Sol-Gel Hydrotalcites. Structure and Texture †[END_REF][START_REF] Paredes | Sol-gel synthesis of hydrotalcite -like compounds[END_REF][START_REF] Sharma | Effect of Hydrothermal Conditions on Structural and Textural Properties of Synthetic Hydrotalcites of Varying Mg/Al Ratio[END_REF][START_REF] Ramos-Ramírez | Adsorption isotherm studies of chromium (VI) from aqueous solutions using sol-gel hydrotalcite-like compounds[END_REF][START_REF] Lee | Simple preparation method for Mg-Al hydrotalcites as base catalysts[END_REF]. This process allows to control the structural properties of the final materials by simply varying the chemical nature of the reactants, the ageing time, and by removing/adding reactant species. This technique consists in dissolving the desired metal sources (inorganic salts or metal organic compounds) in water, at room temperature (Fig4.).

The concentration of the metal cations can be varied to tune the substitution ratio of M II and M III cations and to obtain a wide range of materials [START_REF] Corma | Hydrotalcites as Base Catalysts: Influence of the Chemical Composition and Synthesis Conditions on the Dehydrogenation of Isopropanol[END_REF][START_REF] Valente | Preparation and Characterization of Sol-Gel MgAl Hydrotalcites with Nanocapsular Morphology[END_REF][START_REF] Valente | Physicochemical Study of Nanocapsular Layered Double Hydroxides Evolution[END_REF][START_REF] Sharma | Effect of Hydrothermal Conditions on Structural and Textural Properties of Synthetic Hydrotalcites of Varying Mg/Al Ratio[END_REF]). The appropriate amount of acid or base is then added to the reactants' solution during hydrolysis to favor the condensation reaction. The solution is finally aged during several hours or days, at room temperature or lightly heated up to 100°C. Prince et al. ( 2009) proposed a general sol-gel method for the preparation of LDHs that can be adapted to obtain materials containing specific metallic cations and possessing a defined morphology in the view of a specific application. Ramos-Ramirez et al. (2009) also evidenced the formation of small amounts of brucite phase, in addition to hydrotalcite, with a Mg/Al molar ratio of 2 and explained this behavior by the similar structure of the two materials.

Synthetic hydrotalcites can also be calcined and rehydrated without losing their lamellar structure. During regeneration, they show a "memory effect", as evidenced by [START_REF] Climent | Activated hydrotalcites as catalysts for the synthesis of chalcones of pharmaceutical interest[END_REF] by using rehydrated sol-gel hydrotalcites for pharmaceutical applications.

Several parameters can be varied to improve the crystallinity (Kovanda et al. 2005b;[START_REF] Valente | Physicochemical Study of Nanocapsular Layered Double Hydroxides Evolution[END_REF]) of the particles, to modify their morphology [START_REF] Valente | Preparation and Characterization of Sol-Gel MgAl Hydrotalcites with Nanocapsular Morphology[END_REF][START_REF] Valente | Physicochemical Study of Nanocapsular Layered Double Hydroxides Evolution[END_REF][START_REF] Valente | Comprehending the Thermal Decomposition and Reconstruction Process of Sol-Gel MgAl Layered Double Hydroxides[END_REF], and to enhance the surface area [START_REF] Corma | Hydrotalcites as Base Catalysts: Influence of the Chemical Composition and Synthesis Conditions on the Dehydrogenation of Isopropanol[END_REF][START_REF] Lopez | Synthesis and Characterization of Sol-Gel Hydrotalcites. Structure and Texture †[END_REF]Lopez et al. , 1997a;;[START_REF] Climent | Activated hydrotalcites as catalysts for the synthesis of chalcones of pharmaceutical interest[END_REF][START_REF] Paredes | Sol-gel synthesis of hydrotalcite -like compounds[END_REF][START_REF] Valente | Preparation and Characterization of Sol-Gel MgAl Hydrotalcites with Nanocapsular Morphology[END_REF]Prince et al. 2009). Parameters, as the composition of the aqueous media, the pH, and the ageing time, were considered [START_REF] Corma | Hydrotalcites as Base Catalysts: Influence of the Chemical Composition and Synthesis Conditions on the Dehydrogenation of Isopropanol[END_REF]Kovanda et al. 2005b). Lopez et al. (1997b) studied the impact of different alkoxides, such as magnesium (Mg(OEt) 2 ) and aluminum (Al(NO 3 ) 3 ), used as reactants, on the thermal stability of a series of hydrotalcites. Most of the time, the comparison between the synthesis techniques and the parameters evidences the interest of using one or the other specific method, generally sol-gel or co-precipitation [START_REF] Jitianu | Comparative Study of Sol-Gel and Coprecipitated Ni-Al Hydrotalcites[END_REF][START_REF] Prinetto | Synthesis and characterization of sol-gel Mg/Al and Ni/Al layered double hydroxides and comparison with co-precipitated samples[END_REF][START_REF] Bolognini | Mg/Al mixed oxides prepared by coprecipitation and sol-gel routes: a comparison of their physico-chemical features and performances in mcresol methylation[END_REF][START_REF] Othman | Effects of thermal treatment on the micro-structures of co-precipitated and sol-gel synthesized (Mg-Al) hydrotalcites[END_REF][START_REF] Valente | Preparation and Characterization of Sol-Gel MgAl Hydrotalcites with Nanocapsular Morphology[END_REF][START_REF] Valente | Comprehending the Thermal Decomposition and Reconstruction Process of Sol-Gel MgAl Layered Double Hydroxides[END_REF][START_REF] Smalenskaite | A comparative study of co-precipitation and sol-gel synthetic approaches to fabricate cerium-substituted Mg Al layered double hydroxides with luminescence properties[END_REF]. [START_REF] Smalenskaite | A comparative study of co-precipitation and sol-gel synthetic approaches to fabricate cerium-substituted Mg Al layered double hydroxides with luminescence properties[END_REF] compared sol-gel and co-precipitation for the preparation of cerium-substituted Mg-Al LDHs; [START_REF] Othman | Effects of thermal treatment on the micro-structures of co-precipitated and sol-gel synthesized (Mg-Al) hydrotalcites[END_REF] studied the impact of the thermal treatment on the structure of similar materials. In many cases, the sol-gel approach is preferred for its simplicity and the high quality of the materials obtained [START_REF] Ramos | Thermal stability of sol-gel hydrotalcites[END_REF][START_REF] Othman | Effects of thermal treatment on the micro-structures of co-precipitated and sol-gel synthesized (Mg-Al) hydrotalcites[END_REF][START_REF] Valente | Comprehending the Thermal Decomposition and Reconstruction Process of Sol-Gel MgAl Layered Double Hydroxides[END_REF][START_REF] Lee | Simple preparation method for Mg-Al hydrotalcites as base catalysts[END_REF][START_REF] Smalenskaite | A comparative study of co-precipitation and sol-gel synthetic approaches to fabricate cerium-substituted Mg Al layered double hydroxides with luminescence properties[END_REF].

The sol-gel synthesis method is appreciated for its high reproducibility, the high homogeneity and purity of the obtained compounds, the small size of the particles (nanoscale)

and their high specific surface area (Prince et al. 2009;[START_REF] Smalenskaite | A comparative study of co-precipitation and sol-gel synthetic approaches to fabricate cerium-substituted Mg Al layered double hydroxides with luminescence properties[END_REF]Valeikiene et al. 2019). However, lower crystallinity is generally observed through this synthesis route [START_REF] Smalenskaite | A comparative study of co-precipitation and sol-gel synthetic approaches to fabricate cerium-substituted Mg Al layered double hydroxides with luminescence properties[END_REF]. Often and depending on the final application of the materials, additional steps such as sonication, microwave irradiation [START_REF] Benito | Uniform Fast Growth of Hydrotalcite-like Compounds[END_REF] or hydrothermal treatment are required.

Hydrothermal treatment

As previously reported, the synthesis of LDHs by co-precipitation, sol-gel method or urea hydrolysis can be optimized by hydrothermal treatment in order to improve the crystallinity and the size of the crystallites [START_REF] Kloprogge | The effects of synthesis pH and hydrothermal treatment on the formation of zinc aluminum hydrotalcites[END_REF][START_REF] Sharma | Effect of Hydrothermal Conditions on Structural and Textural Properties of Synthetic Hydrotalcites of Varying Mg/Al Ratio[END_REF]). The treatment generally occurs under mild hydrothermal conditions, up to 200°C, autogenous pressure, with durations between hours to several days. Fig5. displays the hydrothermal synthesis process.

Besides the improvement of the crystallinity, hydrothermal synthesis can be directly used as a synthesis method, in order to form LDH phases with increased particle sizes. It consists in dissolving the metallic salts into a solvent (generally water) before putting the solution into a stainless-steel reactor and heating under hydrothermal conditions, generally under medium temperatures (30-300°C) and steam pressure, with different synthesis times (few hours to several days) [START_REF] Xu | Hydrothermal Synthesis of Layered Double Hydroxides (LDHs) from Mixed MgO and Al 2 O 3 : LDH Formation Mechanism[END_REF][START_REF] Liao | Hydrothermal synthesis of Mg-Al layered double hydroxides (LDHs) from natural brucite and Al(OH)3[END_REF][START_REF] Bontchev | Synthesis, Characterization, and Ion Exchange Properties of Hydrotalcite Mg 6 Al 2 (OH) 16 (A) x (A') 2 -x •4H 2 O (A, A' = Cl -, Br -, I -, and NO 3 -, 2 ≥ x ≥ 0) Derivatives[END_REF][START_REF] Bankauskaite | The hydrothermal synthesis of hydrotalcite by using different partially soluble and insoluble in water mangesium and aluminium components[END_REF][START_REF] Lin | Preparation of CuZnAl hydrotalcite-like catalysts for AsH3 abatement at low temperatures[END_REF][START_REF] Budhysutanto | Pre-treatment of raw materials for the hydrothermal synthesis of hydrotalcite-like compounds[END_REF][START_REF] Roelofs | On the Structure of Activated Hydrotalcites as Solid Base Catalysts for Liquid-Phase Aldol Condensation[END_REF][START_REF] Jang | Hydrothermal Synthesis of K 2 CO 3 -Promoted Hydrotalcite from Hydroxide-Form Precursors for Novel High-Temperature CO 2 Sorbent[END_REF]. Several reactants such as inorganic salts and minerals can be used as metal sources.

Most commonly, metal oxides and hydroxides [START_REF] Xu | Hydrothermal Synthesis of Layered Double Hydroxides (LDHs) from Mixed MgO and Al 2 O 3 : LDH Formation Mechanism[END_REF][START_REF] Budhysutanto | Pre-treatment of raw materials for the hydrothermal synthesis of hydrotalcite-like compounds[END_REF][START_REF] Jang | Hydrothermal Synthesis of K 2 CO 3 -Promoted Hydrotalcite from Hydroxide-Form Precursors for Novel High-Temperature CO 2 Sorbent[END_REF][START_REF] Labuschagné | Green synthesis of hydrotalcite from untreated magnesium oxide and aluminum hydroxide[END_REF] or nitrate salts [START_REF] Roelofs | On the Structure of Activated Hydrotalcites as Solid Base Catalysts for Liquid-Phase Aldol Condensation[END_REF][START_REF] Bontchev | Synthesis, Characterization, and Ion Exchange Properties of Hydrotalcite Mg 6 Al 2 (OH) 16 (A) x (A') 2 -x •4H 2 O (A, A' = Cl -, Br -, I -, and NO 3 -, 2 ≥ x ≥ 0) Derivatives[END_REF][START_REF] Lin | Preparation of CuZnAl hydrotalcite-like catalysts for AsH3 abatement at low temperatures[END_REF]) are used as reactants, but other sources can be used: [START_REF] Ogawa | Hydrothermal Synthesis of Layered Double Hydroxide-Deoxycholate Intercalation Compounds[END_REF] used for example natural minerals (brucite and gibbsite) as starting materials to intercalate organic guests. [START_REF] Liao | Hydrothermal synthesis of Mg-Al layered double hydroxides (LDHs) from natural brucite and Al(OH)3[END_REF] also used natural brucite and Al(OH) 3 as synthesis reactants to study the morphology and structural properties of hydrotalcites obtained under different synthesis conditions.

Like for the other synthesis techniques, the Mg:Al molar ratio and the interlayer anions can be modified in order to obtain a wide range of hydrotalcites and LDHs with different structural properties. [START_REF] Bontchev | Synthesis, Characterization, and Ion Exchange Properties of Hydrotalcite Mg 6 Al 2 (OH) 16 (A) x (A') 2 -x •4H 2 O (A, A' = Cl -, Br -, I -, and NO 3 -, 2 ≥ x ≥ 0) Derivatives[END_REF] studied the impact of several interlayer anions (Cl -, Br -, I -, NO 3 -) on the physico-chemical characteristics of hydrotalcites. Moreover, this study showed the possibility to mix two different types of compensating anions to finely tune the properties of these compounds. The hydrothermal synthesis of hydrotalcites with different soluble and insoluble magnesium and aluminum reactants in water has been studied by [START_REF] Bankauskaite | The hydrothermal synthesis of hydrotalcite by using different partially soluble and insoluble in water mangesium and aluminium components[END_REF], with a fixed Mg:Al molar ratio of 3:1. The non-formation of hydrotalcite phases was observed in the presence of certain reactants. It was demonstrated that alumina and magnesium aluminum hydroxide were preferentially formed when using The advantages of using this synthesis process as additional treatment are the higher LDH crystallinity, crystallites dimension, and purity of the samples (Kovanda et al. 2005b;[START_REF] Sharma | Effect of Hydrothermal Conditions on Structural and Textural Properties of Synthetic Hydrotalcites of Varying Mg/Al Ratio[END_REF]). In general, a higher content of hydrotalcite is observed by increasing the preparation temperature and the time of the hydrothermal treatment (Kovanda et al. 2005b).

Unfortunately, the energy required to heat-up the solution and the long synthesis time (from hours to days) represent drawbacks for the industrial implementation of the hydrothermal treatment.

The recent advancements and progresses on the synthesis methodologies have been reviewed in this section. Moreover, each synthesis method has been detailed in order to enlighten the advantages and drawbacks in terms of structural and textural properties. The modulation of these features is crucial to drive the performances towards specific environmental and industrial applications. Table 1 lists the positive and negative impacts of the various synthesis routes on the structural and textural properties of LDHs.

Use of hydrotalcites and derived materials in environmental applications

The depletion of fossil fuels and their environmental impacts (global warming, pollution, (2011). Moreover, due to their composition (mainly metallic cations), LDHs can be calcined to form mixed oxides presenting enhanced catalytic properties. Even if much more environmental related processes can be carried out in presence of lamellar materials, only few of them are discussed in this review. Some specific environmental application covering various technological fields have been chosen to be reported in the present review article such as the adsorption/decomposition of gaseous pollutants (CO 2 , NOx, and SOx adsorption, and elimination of VOCs), the adsorption of pollutants in aqueous media (as the adsorption of heavy metals and dyes), and the production of green energy vectors and/or chemical building blocks (hydrogen production and formation of 5-hydroxymethylfurfural).

Adsorption of atmospheric pollutants

CO 2 adsorption -Capture and storage

Among the possible uses of hydrotalcite-like compounds and their derived mixed-oxides, CO 2 adsorption gains interest. Methods for safely control and dispose of carbon dioxide emissions are widely studied in the last few years, mainly due to the increase of consciousness in climate changes [START_REF] Wang | A Review of Layered Double Hydroxides as Intermediatetemperature CO2 Adsorbents[END_REF]). Among all the emitted greenhouse gases, the contribution of carbon dioxide to global warming has been estimated to more than 60%. For example, steam reforming of hydrocarbons [START_REF] Marquevich | Hydrogen production via steam reforming of sun¯ower oil over Ni/Al catalysts from hydrotalcite materials[END_REF][START_REF] Ashok | Hydrotalcite structure derived Ni-Cu-Al catalysts for the production of H2 by CH4 decomposition[END_REF][START_REF] Li | Steam reforming of tar from pyrolysis of biomass over Ni/Mg/Al catalysts prepared from hydrotalcite-like precursors[END_REF], that is the most suitable process for hydrogen production, releases high amounts of carbon dioxide. Considering the large field of materials tested for CO 2 capture and storage, zeolites, hydrotalcite and LDH-derived mixed oxides are widely studied thanks to their high surface area, pore structure, and charge density [START_REF] Mao | SYNTHESIS A N D CO2 ADSORPTION FEATURES OF A HYDROTALCITE-LIKE COMPOUND OF THE Mg2+-A13+-Fe(CN)6 4-SYSTEM WITH HIGH LAYER-CHARGE DENSITY[END_REF][START_REF] Tsuji | Hydrotalcites with an extended Al3+-substitution: Synthesis, simultaneous TG-DTA-MS study, and their CO2 adsorption behaviors[END_REF][START_REF] Yong | Adsorption of carbon dioxide at high temperature-a review[END_REF][START_REF] Wang | The Effect of Trivalent Cations on the Performance of Mg-M-CO3 Layered Double Hydroxides for High-Temperature CO2 Capture[END_REF][START_REF] Dantas | CO 2 Adsorption on Modified Mg-Al-Layered Double Hydroxides[END_REF][START_REF] Yang | A review on strategies to LDH-based materials to improve adsorption capacity and photoreduction efficiency for CO2[END_REF]. Several reviews on CO 2 adsorption explain that an appropriate adsorbent should satisfy several criteria such as (1) low-cost materials, (2) fast kinetics, (3) high adsorption capacity and selectivity, and (4) thermal and chemical stability towards several cycles [START_REF] Yong | Adsorption of carbon dioxide at high temperature-a review[END_REF][START_REF] León | Adsorption of CO 2 on Hydrotalcite-Derived Mixed Oxides: Sorption Mechanisms and Consequences for Adsorption Irreversibility[END_REF][START_REF] Wang | A Review of Layered Double Hydroxides as Intermediatetemperature CO2 Adsorbents[END_REF][START_REF] Yang | A review on strategies to LDH-based materials to improve adsorption capacity and photoreduction efficiency for CO2[END_REF]. The adsorption of CO 2 requires materials with large specific surface area, but also a high number of accessible basic sites. Regarding the synthesis method adapted to this application, a high number of active sites and large particles are required for better adsorption efficiency. Thus, the co-precipitation method represents the best solution to reach these requirements, especially when the LDHs structure is modified by the addition of metal cations. The higher the energy of active sites, the stronger CO 2 interacts with the sites and remains trapped. Recently, zeolites are reported as efficient CO 2 adsorbents thanks to their size, charge density, and tunable chemical composition [START_REF] Jiang | Simultaneous biogas purification and CO2 capture by vacuum swing adsorption using zeolite NaUSY[END_REF][START_REF] Yu | Enhancing CO 2 Adsorption and Separation Properties of Aluminophosphate Zeolites by Isomorphous Heteroatom Substitutions[END_REF]. Hydrotalcite-based materials are thereby interesting alternative materials to zeolites, as their chemical composition can also be finely tuned. CO 2 adsorption performed on these materials is a natural phenomenon, as hydrotalcite can adsorb carbon dioxide thanks to the reaction of atmospheric CO 2 with the compensating ions present in the interlayer space.

This has been evidenced by [START_REF] Ishihara | Dynamic Breathing of CO 2 by Hydrotalcite[END_REF] by intercalating 13 C carbonate anions in the hydrotalcite structure. They were exchanged with carbonate anions derived from atmospheric CO 2 within 1 day, showing the dynamism of the carbon cycle in nature. Moreover, these materials can be regenerated several times, without losing their adsorption capacity and selectivity towards CO 2 , are hydrothermally stable, and lead to fast sorption kinetics. A huge amount of studies has been performed on these materials in order to find the most efficient preparation route for CO 2 capture, such as impregnation of commercial hydrotalcites [START_REF] Bhatta | Enhancement in CO 2 Adsorption on Hydrotalcite-based Material by Novel Carbon Support Combined with K 2 CO 3 Impregnation[END_REF], synthesis of tunable Mg/Al hydrotalcites [START_REF] Tsuji | Hydrotalcites with an extended Al3+-substitution: Synthesis, simultaneous TG-DTA-MS study, and their CO2 adsorption behaviors[END_REF][START_REF] Moreira | Adsorption of CO 2 on Hydrotalcite-like Compounds in a Fixed Bed[END_REF][START_REF] Dantas | CO 2 Adsorption on Modified Mg-Al-Layered Double Hydroxides[END_REF], and mixed oxides derived from hydrotalcites [START_REF] León | Adsorption of CO 2 on Hydrotalcite-Derived Mixed Oxides: Sorption Mechanisms and Consequences for Adsorption Irreversibility[END_REF][START_REF] Gao | Comprehensive investigation of CO2 adsorption on Mg-Al-CO3 LDH-derived mixed metal oxides[END_REF][START_REF] Radha | Energetics of CO 2 Adsorption on Mg-Al Layered Double Hydroxides and Related Mixed Metal Oxides[END_REF][START_REF] Colonna | CO2 adsorption and desorption properties of calcined layered double hydroxides: Effect of metal composition on the LDH structure[END_REF]. Table 2 summarizes the synthesis conditions, the specific surface area, and the CO 2 adsorption capacity of the LDHs and related mixed oxides. It shows that the surface area is strongly linked to the CO 2 adsorption capacity: a higher surface area is related to an increase in the sorption capacity.

The preliminary studies of CO 2 adsorption by hydrotalcite, layered double hydroxides and related materials have been performed by [START_REF] Mao | SYNTHESIS A N D CO2 ADSORPTION FEATURES OF A HYDROTALCITE-LIKE COMPOUND OF THE Mg2+-A13+-Fe(CN)6 4-SYSTEM WITH HIGH LAYER-CHARGE DENSITY[END_REF] by varying the M(II)/M(III) molar ratio and consequently the layer charge of the synthesized hydrotalcites.

The composition tuning resulted in a wide range of materials capable of chemically adsorb CO 2 with different adsorption capacities (from 0.4 to 1.5 mmol.g -1 ). The same year, [START_REF] Tsuji | Hydrotalcites with an extended Al3+-substitution: Synthesis, simultaneous TG-DTA-MS study, and their CO2 adsorption behaviors[END_REF] Few years after [START_REF] Tsuji | Hydrotalcites with an extended Al3+-substitution: Synthesis, simultaneous TG-DTA-MS study, and their CO2 adsorption behaviors[END_REF], the studies about CO 2 adsorption were mainly focused on industrial applications and enlightened the influence of structural parameters derived from the thermal treatment of LDHs, leading to the formation of related mixed oxides. Ram [START_REF] Reddy | Layered Double Hydroxides for CO 2 Capture: Structure Evolution and Regeneration[END_REF] showed that, depending on the applied treatment temperature, the samples exhibited different sorption capacities and reversibility. The highest sorption capacities were observed on samples pretreated at 400°C and a sorption temperature of 200°C, leading to CO 2 adsorption of 0.486 mmol.g -1 . The pretreatment conditions showed also an impact on the regeneration behavior (cycling studies). Two years later, the same team (Ram Reddy et al.

2008) studied high-temperature CO 2 adsorption in industrial conditions. An increase of the sorption efficiency was observed, from 0.61 to 0.71 mmol.g -1 , in presence of dry and wet CO 2 respectively, with more than 90% of the original capacity recovered by regeneration. Dadwahl et al. ( 2008) reported on the influence of the particle size on CO 2 adsorption, and analyzed the formation of large particles by co-precipitation. Mixed oxides generally exhibit higher sorption capacities due to the higher specific surface area than the fresh samples, but the analysis conditions (sorption temperature, mixture of gas, etc…) affect the CO 2 adsorption capacities in the same way than on non-calcined samples, as demonstrated by [START_REF] León | Adsorption of CO 2 on Hydrotalcite-Derived Mixed Oxides: Sorption Mechanisms and Consequences for Adsorption Irreversibility[END_REF]. The obtained capacities, in the 0.58-1.15 mmol.g -1 range at 50°C, and 0.40-0.84 mmol.g -1 range at 100°C, depend on the pretreatment temperature, 450 or 700°C respectively.

Regarding the reversibility of these materials, the co-precipitation method at high supersaturation conditions is more adapted than that at low supersaturation, due to the formation of weaker basic sites for the latter. As previously explained, Wang et 2013) discovered the optimal parameters to obtain hydrotalcites with improved CO 2 adsorption capacity: synthesis by co-precipitation method, at high supersaturation conditions, with a Mg/Al molar ratio between 3 and 3.5, and a pre-calcination temperature of 400°C (The CO 2 adsorption measurements were performed during 5 hours at 200°C). The operating conditions (temperature and pressure) impact the adsorption capacity of CO 2 too: Ramirez-Moreno et al.

(2014) stated that high pressures (up to 4350 kPa) and temperatures (30-350°C) are related to higher CO 2 adsorption levels, with a maximum value of 5.76 mmol.g -1 measured at 300°C and 3450 kPa. Higher values can be reached at higher temperatures and pressures, and related to structural changes in the LDHs framework (formation of magnesium oxides and periclase for example).

The most recent researches focus on modifications of the structure of LDHs synthesized by co-precipitation in order to increase their CO 2 adsorption capacity. Similar to simple ionexchange reaction, the potassium impregnation consists in putting the synthesized materials into a solution with a defined concentration of potassium salt during few hours, followed by the washing of the salt excess. K 2 CO 3 impregnation seems to increase the number of surface active sites despite a decrease in the specific surface area. This impregnation leads to high adsorption capacities, as demonstrated by [START_REF] Bhatta | Enhancement in CO 2 Adsorption on Hydrotalcite-based Material by Novel Carbon Support Combined with K 2 CO 3 Impregnation[END_REF]: values up to 1.10 mmol.g -1 at 300°C for the initial sorption cycle, and of 0.42 mmol.g -1 for the following 9 cycles of adsorption/desorption were measured. Polymers addition can modify the textural properties of layered materials and contribute to the enhancement of the CO 2 sorption capacity, as showed by [START_REF] Dantas | CO 2 Adsorption on Modified Mg-Al-Layered Double Hydroxides[END_REF] by the addition of a co-polymer that increased the CO 2 uptake from 0.72 to 1.36 mmol.g -1 . 2018) obtained a series of calcined teraethylenepentamine (TEPA)supported hydrotalcites. At first, increasing the TEPA loading led to higher adsorption capacities, explained by the availability of more basic sites with a higher affinity for CO 2 . A too high TEPA content resulted in the decrease of the sorption capacity due to the steric hindrance of TEPA that blocks the access of CO 2 to the sorption sites.

Among the large panel of industrial applications, sorption enhanced reactions processes (SERP) gained huge interest in the last decade, especially thanks to novel modifications introduced into LDHs structure. SERPs have been developed for the efficient conversion of methane to hydrogen. The main drawback of this process is the formation and release of large amounts of CO 2 , then materials with high carbon dioxide sorption capacity are crucial for this application. The first amine modifications of LDHs for CO 2 adsorption performance towards SERP applications have been performed by Wang et al. (2012b, a) via two different routes: anionic surfactant-mediated synthesis and exfoliation. In the first case, low CO 2 adsorption capacities (up to 0.6 mmol.g -1 ) have been recorded, mainly due to the presence of Sodium Dodecyl Sulfate (SDS) in the interlayer space. SDS can be then removed using monoethylamine extraction (ion exchange reaction), leading to higher sorption values (between 1.15 and 1.4 mmol.g -1 ) (Fig6.). On the opposite, the exfoliation route in the presence of toluene allows to graft amine groups on the surface of LDHs giving high sorption capacities towards CO 2 (around 1.7 mmol.g -1 at 80°C). The two studies underline the importance of the interlayer space in LDHs: sodium dodecyl sulfate and aminosilanes molecules present in this area cover the main part of the basic sites involved in the CO 2 adsorption, consequently lowering the sorption capacity. It is important to consider the steric hindrance when considering lamellar materials. Most of the time, K-promoted hydrotalcites are interesting adsorbents in SERP, as the potassium presence leads to sorption capacities higher than 1 mmol.g -1 , as demonstrated by [START_REF] Wu | K-Promoted Hydrotalcites for CO 2 Capture in Sorption Enhanced Reactions[END_REF]. Moreover, the adsorption capacity and kinetics were not affected even after 10 cycles, which is interesting for SERP Table 3 andFig7. display the CO 2 adsorption capacities of various LDHs reported in the literature. Non-modified materials exhibit values between 0.5 and 1 mmol.g -1 , while modified LDHs present much higher values, up to 6.1 mmol.g -1 .

As demonstrated by various studies, LDHs are interesting alternatives to zeolites and activated carbons for CO 2 adsorption and storage. Adsorption capacities between 0.5 and 6 mmol.g -1 could be measured depending on the structural modifications, the pre-treatments performed prior the adsorption, and the configuration of the adsorption experiments (operating temperature and pressure, gas composition, time of adsorption). Older papers stated on the optimal synthesis conditions and co-precipitation performed at highsupersaturation seems to be the most adapted method. More recent researches focus on sorption-enhanced reaction processes, due to the needs of minimize the release of CO 2 while producing hydrogen from methane. Regarding the synthesis process, co-precipitation allows to obtain large and crystalline particles, increasing the amount of basic sites that are required to reach large amounts of CO 2 adsorbed. The calcination of such materials leads to the formation of mixed oxides with higher surface areas, and consequently an increased number of basic sites. One of the most important parameter to take into account in this application is the specific surface area that is strongly related to the CO 2 adsorption capacity. Moreover, Table 2 evidences slightly higher adsorption capacities for the materials synthesized by coprecipitation at high supersaturation conditions than for those prepared at low supersaturation conditions, except in the samples modified with TEPA (up to 6 mmol.g -1 ), as shown by [START_REF] Thouchprasitchaia | Optimization of CO2 adsorption capacity and cyclical adsorption/desorption on tetraethylenepentamine-supported surface-modified hydrotalcite[END_REF]. However, the commercial hydrotalcite tested by Wu et al.

(2013) exhibits similar adsorption capacities than the modified LDHs.

NOx and SOx adsorption

Among the wide family of atmospheric pollutants, nitrogen and sulfur-based molecules are emitted from power plants, petroleum refinery, and vehicle engines [START_REF] Yang | Layered double hydroxide (LDH) derived catalysts for simultaneous catalytic removal of soot and NOx[END_REF].

Nitrogen oxides (NOx) and Sulfur oxides (SOx, a mixture of SO 2 and SO 3 ) are released into the atmosphere with exhausted gases, causing environmental and climatic concerns such as acid rains, photochemical smog and human diseases (asthma for example) [START_REF] Corma | Simultaneous Catalytic Removal of SOxand NOxwith Hydrotalcite-Derived Mixed Oxides Containing Copper, and Their Possibilities to Be Used in FCC Units[END_REF][START_REF] Yu | Adsorption/Desorption Studies of NO x on Well-Mixed Oxides Derived from Co-Mg/Al Hydrotalcite-like Compounds[END_REF][START_REF] Yang | Layered double hydroxide (LDH) derived catalysts for simultaneous catalytic removal of soot and NOx[END_REF][START_REF] Wang | NOx Removal by Selective Catalytic Reduction with Ammonia over a Hydrotalcite-Derived NiFe Mixed Oxide[END_REF]. The control of these emissions is becoming mandatory due to the increased awareness towards the climate change. Moreover, several laws and strict regulations have been put into place in the last decade, and encourage researches on more efficient ways to reduce them. Several depollution processes have been developed with good results, such as the direct decomposition [START_REF] Yokomichi | Fundamental study on the NOx direct decomposition catalysts[END_REF][START_REF] Imanaka | Advances in direct NOx decomposition catalysts[END_REF][START_REF] Gunugunuri | Removal of antimony(V) and antimony(III) from drinking water by coagulation-flocculation-sedimentation (CFS)[END_REF][START_REF] Gunugunuri | Removal of antimony(V) and antimony(III) from drinking water by coagulation-flocculation-sedimentation (CFS)[END_REF], adsorption [START_REF] Mahzoul | Experimental and mechanistic study of NOx adsorption over NOx trap catalysts[END_REF][START_REF] Ch | Elementary steps of NOx adsorption and surface reaction on a commercial storage-reduction catalyst[END_REF][START_REF] Atribak | Effect of NOx adsorption/desorption over ceria-zirconia catalysts on the catalytic combustion of model soot[END_REF]), Selective Catalytic Reduction (SCR) [START_REF] Lukyanov | Comparison of Catalyzed and Homogeneous Reactions of Hydrocarbons for Selective Catalytic Reduction (SCR) of NOx[END_REF][START_REF] Forzatti | New "Enhanced NH 3 -SCR" Reaction for NOx Emission Control[END_REF][START_REF] Gao | Fast SCR" reaction over Sm-modified MnOx-TiO2 for promoting reduction of NOx with NH3[END_REF][START_REF] Gramigni | Catalyst systems for selective catalytic reduction + NO x trapping: from fundamental understanding of the standard SCR reaction to practical applications for lean exhaust after-treatment[END_REF]Wu et al. 2019b;Li et al. 2019b), and NOx Storage and Reduction (NSR) [START_REF] Castoldi | Simultaneous removal of NOx and soot on Pt-Ba/Al2O3 NSR catalysts[END_REF][START_REF] Breen | Insight into the key aspects of the regeneration process in the NOx storage reduction (NSR) reaction probed using fast transient kinetics coupled with isotopically labelled 15NO over Pt and Rh-containing Ba/Al2O3 catalysts[END_REF][START_REF] Sakano | Synthesis Of Hydrotalcite-Like Compounds And Their Physico-Chemical Properties[END_REF][START_REF] Umeno | NOx storage reduction catalyst for purifying exhaust gas and exhaust gas purification method using said catalyst Valeikiene L[END_REF]. LDHs are interesting due to their efficiency in a wide range of application, as catalysts and adsorbents. In recent years, hydrotalcite-like compounds have exhibited good NOx and SOx adsorption capacities. This chapter is dedicated to the adsorption and removal of these atmospheric pollutants by LDHs and related mixed oxides; the synthesis conditions of each LDH are reported in the Table 4 with the associated structural modifications.

The most important part of the emitted NOx comes from the Fluid Catalytic Cracking (FCC) method employed by refinery processes to convert heavy distillates into lighter ones, such as gasoline and diesel. In this process, the catalyst gets progressively covered with coke and needs to be reactivated by burning and oxidizing the coke in a regenerator unit, NOx is here produced [START_REF] Chaparala | Reaction Mechanism for the Formation of Nitrogen Oxides (NO x ) During Coke Oxidation in Fluidized Catalytic Cracking Units[END_REF]. The catalysts are reactivated by reduction at 750°C, so most of the studies are focused on experiments under these conditions. The storage and catalytic reduction of NOx compounds on the surface of LDH catalysts occurs firstly on the catalytic sites to form nitrites and nitrates, as shown in Fig8, and according to the following reactions [START_REF] Guo | High capacity potassium-promoted hydrotalcite for CO2 capture in H2 production[END_REF]: The stored NOx is finally converted to N 2 and H 2 O by reacting with H 2 at 350°C:

NO + Co 3+ -O 2-→ Co 2+ -(ONO -) formation of nitrite NO 2 + Co 3+ -O 2-→ Co 2+ -(ONO 2
2NO 3-+ 5H 2 → N 2 + O 2-+ 5H 2 O 2NO 2-+ 3H 2 → N 2 + O 2-+ 3H 2 O
The first catalytic removal of NOx using copper-containing LDHs-derived mixed oxides has been reported in the late 90s by [START_REF] Corma | Hydrotalcites as Base Catalysts: Influence of the Chemical Composition and Synthesis Conditions on the Dehydrogenation of Isopropanol[END_REF]. The possibility to incorporate the catalyst in the FCC units was explored, because NOx can be decomposed by the catalyst reduced under a reductive atmosphere at 550°C, conditions close to those present in the regenerator unit of a FCC plant. The simultaneous removal of NOx and SOx in conditions close to those of the regenerator unit was investigated: Cu-mixed oxides derived from hydrotalcite showed to be able to remove both SO 2 (via an oxidation or reduction reaction)

and NO (via reduction and/or decomposition) at the same time in the presence of small amounts of O 2 (between 0 and 1.5%). Doping the catalysts with transition metals, such as cobalt, is of great interest on the storage capacity of NOx in hydrotalcites; Palomares et al.

(1999) studied the catalytic activity of CoMgAl mixed oxides derived from hydrotalcites.

Besides increasing the surface area by addition of cobalt, the authors concluded that in presence of O 2 (up to 1%) the catalyst is able to efficiently remove the atmospheric pollutants.

In presence of oxygen in the feed and at 750°C, cobalt-containing materials showed to be more selective towards NO reduction than Cu-based compounds. Almost 100% of NO conversion was reached in less than 50s and remained stable during time. [START_REF] Yu | Adsorption/Desorption Studies of NO x on Well-Mixed Oxides Derived from Co-Mg/Al Hydrotalcite-like Compounds[END_REF] prepared a range of Co x Mg 3-x /Al hydrotalcite-like compounds, where x varied from 0 (pure MgAl hydrotalcite) to 3 (pure CoAl hydrotalcite). Despite the decrease of the specific surface area while increasing the cobalt content, the results showed that higher storage capacities of NOx were reached at low temperatures with mixed oxides of intermediate Co concentrations (0.22 mmol.g -1 at 100°C for x = 0.5). The pure phases exhibited the lowest storage capacities, around 0.14 mmol.g -1 for MgAl and CoAl hydrotalcites. These results are explained by the increase in the pore size, from 11.6 nm in the pure MgAl mixed oxides to 24.8 nm for the samples with the highest cobalt concentration. Similar experiments were performed the next year by the same team [START_REF] Yu | Effective NO x Decomposition and Storage/Reduction over Mixed Oxides Derived from Layered Double Hydroxides[END_REF]) with two catalysts: Ca 2 CoAl-oxide and Ca 2 CoLa 0.1 Al 0.9 -oxide, derived from their LDH precursors. NOx storage capacities up to 0.65 mmol.g -1 at 250°C were measured. Noble metals such as platinum, palladium and ruthenium have also been intensively studied towards the storage of NOx due to their high catalytic redox properties. [START_REF] Cheng | NOx storage-reduction over Pt/Mg-Al-O catalysts with different Mg/Al atomic ratios[END_REF] Concerning SOx removal for industrial applications, several patents are related to the use of hydrotalcites in FCC units [START_REF] Pinnavaia | Proposed General Sol-Gel Method to Prepare Multimetallic Layered Double Hydroxides: Synthesis, Characterization, and Envisaged Application[END_REF][START_REF] Pinnavaia | Proposed General Sol-Gel Method to Prepare Multimetallic Layered Double Hydroxides: Synthesis, Characterization, and Envisaged Application[END_REF][START_REF] Vierheilig | Functionalizing biochar with Mg-Al and Mg-Fe layered double hydroxides for removal of phosphate from aqueous solutions[END_REF]. These compounds are emitted during the oxidative regeneration process of the catalysts at 700-800°C. The general FCC process applied to the SOx removal is schematized in Fig10. Regarding the process conditions, several requirements are needed to trap SOx [START_REF] Mathieu | Adsorption of SOx by oxide materials: A review[END_REF]): (1) a high thermal stability of the adsorbent under FCC conditions, (2) the ability to oxidize SO 2 to SO 3 , (3) the possibility to regenerate the adsorbent under a reductive flow of H 2 to release sulfur as H 2 S, (4) the possibility to strongly chemisorb SOx in the regenerator unit, and (5) the absence of negative impact of the adsorbent on the conversion and selectivity of FCC products. LDHs and hydrotalcites can answer to these requirements: they have the ability to form mixed oxides of the desired elements, stable at high temperatures, and are able to be regenerated in water. The catalytic activity of such compounds strongly depends on the surface area (related to the number of active sites) and on the type of metallic cation employed (for their ability to react with SOx compounds) [START_REF] Cantú | SOx Removal by Calcined MgAlFe Hydrotalcite-like Materials: Effect of the Chemical Composition and the Cerium Incorporation Method[END_REF] unstable at these temperatures, leading to the release of sulfate species in the regenerator [START_REF] Yoo | NOx decomposition, storage and reduction over novel mixed oxide catalysts derived from hydrotalcite-like compounds[END_REF]. Three basic reactions are involved in the removal of SOx in the FCC regenerators [START_REF] Yoo | NOx decomposition, storage and reduction over novel mixed oxide catalysts derived from hydrotalcite-like compounds[END_REF]):

(1) SO 2 + O 2 → SO 3

(2) SO 3 + MgO → MgSO 4 The performances of mixed oxides derived from MgAlFeCe hydrotalcites were compared to those of a commercial catalyst. Hydrotalcites showed a lower deactivation rate and the necessity to use 33% less material than the commercial catalyst. However, only very few recent papers report on the adsorption and removal of SOx by LDHs-based materials. The most recent researches focus on catalysts capable of simultaneously adsorb SOx and NOx [START_REF] Kameda | Removal of HCl, SO2, and NO by treatment of acid gas with Mg-Al oxide slurry[END_REF](Kameda et al. , 2019a[START_REF] Kameda | Simultaneous treatment of HCl-SO2-NOx gas with Mg-Al layered double hydroxide intercalated with CO32-and its recycling process[END_REF].

NOx and SOx are dangerous compounds formed during the regeneration process of the catalysts employed in the FCC units. Due to their toxicity, their removal has been widely studied. Hydrotalcites and mixed oxides derived from LDHs, in particular those containing transition metals (copper or cerium) or noble metals (platinum, palladium or ruthenium) have been considered for this application. High specific surface area, large pore sizes and improved catalytic properties have been obtained by the addition of metallic cations, leading to interesting results in NOx and SOx adsorption in regeneration conditions (around 750°C under a reducing atmosphere). Co-precipitation was the most widely used synthesis method for LDHs, allowing to obtain high specific surface areas, pore sizes and a high number of active sites. Higher adsorption capacities were reached with co-precipitated LDHs (0.10-0.65 mmol.g -1 ), compared to commercial samples (up to 0.06 mmol.g -1 ), as shown by the Table 3.

The specific surface areas remain in the same range of values (between 100 and 250 m².g -1 )

thanks to the modifications performed on the commercial LDHs, which evidences that the specific surface area is not the only parameter to be taken into consideration for increasing the performances in this application. Some additional studies have been performed on the simultaneous adsorption of NOx and SOx in order to obtain a polyvalent compound stable under the conditions of the FCC regenerator units [START_REF] Kameda | Removal of HCl, SO2, and NO by treatment of acid gas with Mg-Al oxide slurry[END_REF](Kameda et al. , 2019a[START_REF] Kameda | Simultaneous treatment of HCl-SO2-NOx gas with Mg-Al layered double hydroxide intercalated with CO32-and its recycling process[END_REF].

VOCs total decomposition

Volatile Organic Compounds (VOCs) give origin to an important class of organic pollutants, such as toluene, ethanol, methanol, acetone and ethylene [START_REF] Bahranowski | LDH-derived catalysts for complete oxidation of volatile organic compounds[END_REF][START_REF] Carpentier | Characterisation of Mg/Al hydrotalcite with interlayer palladium complex for catalytic oxidation of toluene[END_REF]Gennequin et al. 2010a;[START_REF] Mrad | Degradation of VOCs and NOx over Mg(Cu)-AlFe mixed oxides derived from hydrotalcite-like compounds[END_REF][START_REF] Kamal | Catalytic oxidation of volatile organic compounds (VOCs) -A review[END_REF]. They are emitted from various industrial processes and transport vehicles and represent a serious environmental problem. VOCs are the major contributors to air pollution due to their direct toxic and carcinogenic properties, or indirect contribution to ozone formation. The decomposition of VOCs is thereby crucial for an environmental point of view. VOCs removal requires technologies working under hard conditions (temperatures up to 1100°C and presence of toxic products). The possibility to eliminate VOCs at temperatures lower than those reached during classical combustion is of great interest. Catalytic total combustion of VOCs is then one of the most effective and economically attractive treatment explored in the last decades [START_REF] Bahranowski | LDH-derived catalysts for complete oxidation of volatile organic compounds[END_REF][START_REF] Mikulová | Calcined Ni-Al layered double hydroxide as a catalyst for total oxidation of volatile organic compounds: Effect of precursor crystallinity[END_REF][START_REF] Dula | Mn-containing catalytic materials for the total combustion of toluene: The role of Mn localisation in the structure of LDH precursor[END_REF][START_REF] Tanasoi | New Cu-based mixed oxides obtained from LDH precursors, catalysts for methane total oxidation[END_REF][START_REF] Palacio | Total oxidation of toluene over calcined trimetallic hydrotalcites type catalysts[END_REF][START_REF] Kamal | Catalytic oxidation of volatile organic compounds (VOCs) -A review[END_REF]. Most catalysts used for the total decomposition of VOCs contain noble metals, such as palladium and gold. Mixed oxides based on transition metals have been also successfully tested with similar activity, as cheaper alternative. The catalysts must present a high thermal stability and a large number of active sites. Coprecipitation is once again the most adapted method to obtain the desired properties. It permits to easily substitute cations in the structure and to promote the formation of large platelets with a high number of active sites. The synthesis conditions of all the samples cited in this part have been listed in Table 5.

Similarly to CO 2 adsorption, the total decomposition of VOCs by hydrotalcites and LDHs showed to be the most efficient [START_REF] Mrad | Degradation of VOCs and NOx over Mg(Cu)-AlFe mixed oxides derived from hydrotalcite-like compounds[END_REF]. [START_REF] Dula | Mn-containing catalytic materials for the total combustion of toluene: The role of Mn localisation in the structure of LDH precursor[END_REF] performed a similar study on MgAlMn-oxide (obtained from calcination of the parent LDH). The catalyst was more active than MnO 4 -impregnated MgAl LDH for the combustion of toluene under air.

This behavior can be explained by the enhanced presence of redox sites on the surface of the as-generated mixed oxide particles. Methane is a bit more difficult to oxidize due to its high inertness: its complete oxidation occurs at much higher temperature than toluene and ethanol, as studied by [START_REF] Tanasoi | New Cu-based mixed oxides obtained from LDH precursors, catalysts for methane total oxidation[END_REF], with 50% of methane decomposition at 480-490°C over a calcined Cu-containing MgAl-LDH catalyst. This work shows that the catalytic activity is dependent of the reducible amount of copper in the catalyst. [START_REF] Aguilera | Cu-Mn and Co-Mn catalysts synthesized from hydrotalcites and their use in the oxidation of VOCs[END_REF] performed a more complete study on the total oxidation of 3 different VOCs: butanol, ethanol and toluene by copper-manganese and cobalt-manganese catalysts derived from the parent LDHs.

The catalytic activity depended on the VOC type, and the activity scale was the following for both catalyst: butanol < ethanol < toluene. Some other studies [START_REF] Chmielarz | Total oxidation of selected mono-carbon VOCs over hydrotalcite originated metal oxide catalysts[END_REF])

focused on mono-carbon VOCs (methane, methanol, and formic acid). The calcination temperature had a direct impact on the complete oxidation: the presence of cobalt phases, obtained at high calcination temperatures (up to 800°C), induced higher catalytic activities in conversions of methanol and formic acid.

Other modifications, such as the type of interlayer anion or the use of a support, can be introduced in the catalyst conception. [START_REF] Carpentier | Characterisation of Mg/Al hydrotalcite with interlayer palladium complex for catalytic oxidation of toluene[END_REF] demonstrated that the substitution of carbonate anions by palladium complexes in the interlayer space of Mg/Al hydrotalcites improves the catalytic activity. Kovanda and Jirátová (2011a, b) supported several mixed oxides, derived from hydrotalcites, on Al 2 O 3 /Al or calcined alumina, and resulted in interesting catalytic activities with the production of side products such as acetaldehyde. Spinel phases identified in the calcined samples are also linked to the high conversion values observed.

Similar to CO 2 adsorption, the total elimination of VOCs remains a very important environmental issue to be solved. Most of the LDHs used in this field are synthesized by coprecipitation method that makes possible the structural substitution of metallic cations, such as chromium or copper, in order to increase the catalytic activity. Differently than for the samples used in NOx and SOx adsorption, the materials prepared for VOCs elimination, and that contains metal cations in the structure, show similar specific surface area values (between 10 and 250 m².g -1 ) when obtained at low or high supersaturation conditions (Table 5). The main differences in the samples comes essentially from the inserted metallic cation and the thermal treatment (temperature and duration). According to the literature, the total elimination of VOCs by LDH-based materials is very promising.

2.1.Adsorption of aqueous pollutants

Nowadays the pollution of water is mainly deriving from several industrial effluents (papermaking, textiles and dyeing for example) and agricultural activities [START_REF] Chuang | Removal of 2-Chlorophenol from Aqueous Solution by Mg/Al Layered Double Hydroxide (LDH) and Modified LDH[END_REF][START_REF] Tzompantzi | Improved Photocatalytic Degradation of Phenolic Compounds With ZnAl Mixed Oxides Obtained from LDH Materials[END_REF][START_REF] Ali | New Generation Adsorbents for Water Treatment[END_REF]. Some of the pollutants released in nature can move into soils and plants. Their high mobility provokes a global concern due to their significant impact on the human health, biotoxicity and carcinogenic effects [START_REF] Lv | Factors influencing the removal of fluoride from aqueous solution by calcined Mg-Al-CO3 layered double hydroxides[END_REF][START_REF] Chuang | Removal of 2-Chlorophenol from Aqueous Solution by Mg/Al Layered Double Hydroxide (LDH) and Modified LDH[END_REF]. As water resource is a fundamental resource on earth and remains rare in some regions, the need to find cheap and efficient depollution processes has increased worldwide.

The wide family of aqueous pollutants includes inorganic pollutants (toxic heavy metals such as As, Pb, Cd, Ni, Zn and Cr for example [START_REF] Lehmann | Removal of metal Ions from dilute aqueous solutions: A comparative study of inorganic sorbent materials[END_REF][START_REF] Yang | Removal of Trace Levels of Arsenic and Selenium from Aqueous Solutions by Calcined and Uncalcined Layered Double Hydroxides (LDH)[END_REF][START_REF] El-Sayed | Adsorption of heavy metals from aqueous solutions by Mg-Al-Zn mingled oxides adsorbent[END_REF]Wan et al. 2017)), organic pollutants (mainly dyes from paper and textile industries [START_REF] Tzompantzi | Improved Photocatalytic Degradation of Phenolic Compounds With ZnAl Mixed Oxides Obtained from LDH Materials[END_REF][START_REF] Ahmed | Adsorption study of anionic reactive dye from aqueous solution to Mg-Fe-CO3 layered double hydroxide (LDH)[END_REF][START_REF] De Sá | Effect of pH on the adsorption of Sunset Yellow FCF food dye into a layered double hydroxide (CaAl-LDH-NO3)[END_REF][START_REF] Shan | Magnetic Fe3O4/MgAl-LDH composite for effective removal of three red dyes from aqueous solution[END_REF]), but also drugs and pharmaceuticals [START_REF] Chuang | Removal of 2-Chlorophenol from Aqueous Solution by Mg/Al Layered Double Hydroxide (LDH) and Modified LDH[END_REF][START_REF] Zhao | Adsolubilization of 2,4,6-trichlorophenol from aqueous solution by surfactant intercalated ZnAl layered double hydroxides[END_REF][START_REF] Yu | Layered double hydroxide intercalated with aromatic acid anions for the efficient capture of aniline from aqueous solution[END_REF]). Many approaches have been proposed for water treatment, including coagulation (Zou et al. 2016a, b), precipitation [START_REF] Barrera-Díaz | A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction[END_REF][START_REF] Cheng | Hexavalent chromium removal using metal oxide photocatalysts[END_REF], sedimentation [START_REF] Rubí | Pollutant removal from oily wastewater discharged from car washes through sedimentation-coagulation[END_REF]Guo et al. 2009), sorption [START_REF] Karickhoff | Organic Pollutant Sorption in Aquatic Systems[END_REF][START_REF] Fosso-Kankeu | A comprehensive study of physical and physiological parameters that affect bio-sorption of metal pollutants from aqueous solutions[END_REF][START_REF] Alila | Sorption potential of modified nanocrystals for the removal of aromatic organic pollutant from aqueous solution[END_REF], and ion exchange [START_REF] Leinonen | Purification of nickel and zinc from waste waters of metalplating plants by ion exchange[END_REF][START_REF] Mazur | Design of a fixed-bed ion-exchange process for the treatment of rinse waters generated in the galvanization process using Laminaria hyperborea as natural cation exchanger[END_REF]. Regarding the literature, sorption is the most versatile process for removing pollutants at relatively low concentrations, and it is environmentally friendly. This process includes the use of lamellar materials such as clay minerals (Milutinović-Nikolić et al. 2014, p. 99;[START_REF] Wang | Interaction mechanism of Eu(III) with MX-80 bentonite studied by batch, TRLFS and kinetic desorption techniques[END_REF][START_REF] Yang | Competitive sorption and selective sequence of Cu(II) and Ni(II) on montmorillonite: Batch, modeling, EPR and XAS studies[END_REF] and

LDHs [START_REF] Yang | Removal of Trace Levels of Arsenic and Selenium from Aqueous Solutions by Calcined and Uncalcined Layered Double Hydroxides (LDH)[END_REF][START_REF] Chuang | Removal of 2-Chlorophenol from Aqueous Solution by Mg/Al Layered Double Hydroxide (LDH) and Modified LDH[END_REF][START_REF] Ahmed | Adsorption study of anionic reactive dye from aqueous solution to Mg-Fe-CO3 layered double hydroxide (LDH)[END_REF][START_REF] Zou | Controllable Synthesis of Ca-Mg-Al Layered Double Hydroxides and Calcined Layered Double Oxides for the Efficient Removal of U(VI) from Wastewater Solutions[END_REF]), but also activated carbons [START_REF] Beita-Sandí | Removal of N -nitrosodimethylamine precursors with powdered activated carbon adsorption[END_REF][START_REF] Bedin | KOH-activated carbon prepared from sucrose spherical carbon: Adsorption equilibrium, kinetic and thermodynamic studies for Methylene Blue removal[END_REF], graphene [START_REF] Li | Uranium(VI) adsorption on graphene oxide nanosheets from aqueous solutions[END_REF][START_REF] Song | Plasma-induced grafting of polyacrylamide on graphene oxide nanosheets for simultaneous removal of radionuclides[END_REF][START_REF] Sun | Interaction of sulfonated graphene oxide with U(VI) studied by spectroscopic analysis and theoretical calculations[END_REF], and metal oxides [START_REF] Tzompantzi | Improved Photocatalytic Degradation of Phenolic Compounds With ZnAl Mixed Oxides Obtained from LDH Materials[END_REF][START_REF] Cheng | Hexavalent chromium removal using metal oxide photocatalysts[END_REF][START_REF] Zou | Controllable Synthesis of Ca-Mg-Al Layered Double Hydroxides and Calcined Layered Double Oxides for the Efficient Removal of U(VI) from Wastewater Solutions[END_REF]). The main obstacle using these materials is their relatively low adsorption capacity, so the majority of the studies is focused on increasing their efficiency.

This part of the review discusses about the last advances in the use of LDHs and derived mixed oxides in the adsorption and removal or aqueous pollutants in the last decades. The synthesis parameters of the materials described in this part are listed in the Table 6.

Inorganic pollutants are mainly constituted of heavy metals such as arsenic, lead, cadmium, nickel, zinc and chromium. Their toxicity has been well documented, and their removal gained large focus in the last decades [START_REF] Barrera-Díaz | A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction[END_REF]). Thanks to their accessible and expansible interlayer space, LDHs are interesting adsorbents and ion exchangers of these pollutants. Their use has been firstly documented in the late '90s and the performances often compared to those of other inorganic sorbents, such as activated carbons and metal oxides [START_REF] Lehmann | Removal of metal Ions from dilute aqueous solutions: A comparative study of inorganic sorbent materials[END_REF]. Pure MgAl hydrotalcites generally exhibit interesting removal capacities, up to 50% of Cr(VI) and 100% Zn(II) at pH 4, and are able to act as anionexchangers (in the interlayer space) and proton-acceptors (presence of carbonate ions CO 3

2-

).

Moreover, an increase of the solution pH leads to a partial dissolution of the LDH structure and causes the precipitation of cationic metal species increasing the overall removal of inorganic pollutants. These materials are also efficient towards the removal of trace levels of pollutants, such as arsenic and selenium for example, as described by [START_REF] Yang | Removal of Trace Levels of Arsenic and Selenium from Aqueous Solutions by Calcined and Uncalcined Layered Double Hydroxides (LDH)[END_REF] by using calcined and uncalcined MgAl hydrotalcites. The adsorption values of uncalcined samples are strongly influenced by the pH of the solution, while no influence of pH was measured on calcined samples (Fig12). In both cases, the desorption of As(V) and Se(IV) strongly depended on the anion species, with maximum amounts of 100ppm in presence of HPO 4 2-and 1000ppm with SO 4 2-

, and with desorption yields close to 100%. Several parameters influence the removal capacity by LDHs, such as the pH of the solution, the initial pollutant concentration, and the adsorbent dosage [START_REF] Lv | Factors influencing the removal of fluoride from aqueous solution by calcined Mg-Al-CO3 layered double hydroxides[END_REF]. Similar to other applications previously discussed, the addition of metallic cations in the structure of LDHs and derived mixed oxides increases the specific surface area and the pore volume of these materials, leading to better sorption capacities. [START_REF] El-Sayed | Adsorption of heavy metals from aqueous solutions by Mg-Al-Zn mingled oxides adsorbent[END_REF] reported the efficiency of MgAlZn mixed oxides derived from calcined hydrotalcites and observed adsorption capacities up to 1.98 and 1.19 mmol.g -1 for Co(II) and Ni(II), respectively. A more recent study showed the positive impact of incorporating LDHs onto a matrix of other nature. Once incorporated into biochar [START_REF] Xue | High efficiency and selectivity of MgFe-LDH modified wheat-straw biochar in the removal of nitrate from aqueous solutions[END_REF], MgFe LDHs can uptake nitrates with values up to 0.4 mmol.g -1 , which are higher than those obtained on MgFe LDHs alone (0.08 mmol.g -1 ) or on activated carbons (0.15 mmol.g -1 ). Chitosan has also been tested due to the possibility to adsorb large amounts of metal ions. Indeed, TiFe LDHs modified with chitosan can exhibit high removal capacities (close to 100%) for cadmium and phosphate in mono-and multiplepollutant solutions [START_REF] Mahmoud | Comparative Study of Single and Multiple Pollutants System Using Ti-Fe Chitosan LDH Adsorbent with High Performance in Wastewater Treatment[END_REF]. The use of hierarchical SiO 2 @LDHs on SiO 2 spheres has been studied by [START_REF] Yang | Rational design and synthesis of monodispersed hierarchical SiO 2 @layered double hydroxide nanocomposites for efficient removal of pollutants from aqueous solution[END_REF] to remove uranium (U(VI)) from aqueous solution. Thanks to the complexations and electrostatic interactions with the abundant oxygen-containing functional groups, a maximum sorption capacity of 1.27 mmol.g -1 was reported. Similar adsorption capacities have been reported by [START_REF] Zou | Controllable Synthesis of Ca-Mg-Al Layered Double Hydroxides and Calcined Layered Double Oxides for the Efficient Removal of U(VI) from Wastewater Solutions[END_REF] using CaMgAl

LDHs and derived mixed oxides, with values depending on the calcination temperature of the LDH: from 0.55 (for uncalcined LDH) up to 2.04 mmol.g -1 (for calcined LDH at 500°C).

Another family of pollutants regroups the organic aqueous pollutants (pharmaceuticals and molecules with high molecular mass) that are released in nature by chemical industries, creating serious environmental issues due to their high toxicity, even at low concentrations [START_REF] Chuang | Removal of 2-Chlorophenol from Aqueous Solution by Mg/Al Layered Double Hydroxide (LDH) and Modified LDH[END_REF][START_REF] Tzompantzi | Improved Photocatalytic Degradation of Phenolic Compounds With ZnAl Mixed Oxides Obtained from LDH Materials[END_REF]. The interest in finding new sorbent for removing such compounds from water increased in the last ten years, even if the toxicity and the possibility to adsorb these pollutants by natural materials is known since the '80 [START_REF] Zepp | Effects of equilibration time on photoreactivity of the pollutant DDE sorbed on natural sediments[END_REF][START_REF] Karickhoff | Organic Pollutant Sorption in Aquatic Systems[END_REF]. Sorption is one of the most common treatment to remove organic pollutants from nature; LDHs and derived mixed oxides are great candidates thanks to the high sorption capacity in the interlayer region, the large specific surface area and the presence of reactive surfaces [START_REF] Pavan | Adsorption of sodium dodecylsulfate on a hydrotalcite-like compound. Effect of temperature, pH and ionic strength[END_REF][START_REF] Pavan | Sorption of Anionic Surfactants on Layered Double Hydroxides[END_REF][START_REF] Seki | Paraquat adsorption onto clays and organoclays from aqueous solution[END_REF][START_REF] Chuang | Removal of 2-Chlorophenol from Aqueous Solution by Mg/Al Layered Double Hydroxide (LDH) and Modified LDH[END_REF]. Phenolic compounds are the most common organic water pollutants and their adsorption has been widely studied. [START_REF] Tzompantzi | Improved Photocatalytic Degradation of Phenolic Compounds With ZnAl Mixed Oxides Obtained from LDH Materials[END_REF] reported on the fast photocatalytic degradation of phenols in the presence of ZnAl mixed oxides derived from LDHs. The photodegradation rate is maximized thanks to the electron transfer from Zn 2+ to Al 3+ . The removal of phenol molecules by photocatalysis has also been reported by [START_REF] Seftel | Photocatalytic removal of phenol and methylene-blue in aqueous media using TiO2@LDH clay nanocomposites[END_REF] using TiO 2 deposited on various LDHs matrices presenting different cation compositions (Zn 2+ , Cu 2+ , Al 3+ , Fe 3+ and Ti 4+ ). The specific surface area depended on the cations (from 5 m 2 .g -1 with Cu-substituted LDH to 199 m 2 .g -1 for Fe-substituted LDH), and almost 90% efficiency towards phenol adsorption by all the TiO 2 -modified LDHs was obtained after 5h of UV and visible light irradiation.

Concerning dyes elimination from aqueous solutions, a number of technologies were recently developed, such as the adsorption by activated carbon (Özacar and Şengil 2002) or the nanofiltration [START_REF] Koyuncu | Reactive dye removal in dye/salt mixtures by nanofiltration membranes containing vinylsulphone dyes: effects of feed concentration and cross flow velocity[END_REF][START_REF] Lau | Polymeric nanofiltration membranes for textile dye wastewater treatment: Preparation, performance evaluation, transport modelling, and fouling control -a review[END_REF]. However, the formation of sludges in large quantities or the high operation cost make them not suitable for industrial and largescale applications [START_REF] Ahmed | Adsorption study of anionic reactive dye from aqueous solution to Mg-Fe-CO3 layered double hydroxide (LDH)[END_REF]. The low cost and regeneration properties of LDHs make the use of such materials interesting, as demonstrated by [START_REF] Ahmed | Adsorption study of anionic reactive dye from aqueous solution to Mg-Fe-CO3 layered double hydroxide (LDH)[END_REF] on the removal of Crystal Red, an anionic dye. Fig13 shows the adsorption of the dye;

after only 15min, 97% of the dye is removed when starting from a solution with a concentration of 100 mg.L -1 . Moreover, LDH can be regenerated and cycled several times.

Similar experiments were also performed by [START_REF] De Sá | Effect of pH on the adsorption of Sunset Yellow FCF food dye into a layered double hydroxide (CaAl-LDH-NO3)[END_REF] with the Sunset Yellow FCF, a synthetic dye used to color food. The increase of the interlayer distance was observed, indicating that the adsorption takes place by simple ion exchange in this area. Magnetic materials can also be great sorbents towards organic dyes, as studied by [START_REF] Shan | Magnetic Fe3O4/MgAl-LDH composite for effective removal of three red dyes from aqueous solution[END_REF] by using magnetic Fe 3 O 4 /MgAl hydrotalcites to remove three different red dyes (reactive red, congo red, and acid red 1). The uptake equilibrium was reached after 30 min without significant impact on the solution pH. The different results reported all along this review show that the morphology of the LDHs has a great impact on the specific surface area and plays a role on the adsorption capacity. A recent study performed by [START_REF] Li | Different dye removal mechanisms between monodispersed and uniform hexagonal thin plate-like MgAl-CO32--LDH and its calcined product in efficient removal of Congo red from water[END_REF] focused on the adsorption of congo red by MgAl hydrotalcite. The specific surface area increased from 24.74 to 165.07 m 2 .g -1 upon calcination of the parent hydrotalcite, due to the enhancement of the microporosity deriving from the dehydroxylation of the layers and the decomposition of carbonate anions. However, only a slight increase of the adsorption capacity (from 0.186 to 0.206 mmol.g -1 ) was observed. Indeed, the adsorption mechanism implies the anion exchange in the interlayer space and the sorption on the external surface, while the internal surface related to microporosity is not involved in the process due to accessibility problems.

The adsorption of aqueous pollutants, inorganic (toxic heavy metals) and organic (dyes and heavy molecules), by LDHs and derived mixed oxides has been discussed. Due to the increase of industrial and agricultural activities, these dangerous and harmful compounds can be found in high concentrations and need to be removed from aqueous media. Their sorption by lamellar materials, more specifically LDHs, is one of the most investigated solutions. As the adsorption of these pollutants takes place in the interlayer space and on the external surfaces, LDHs and derived mixed oxides with large external specific surface area are required. The coprecipitation method is the most employed because of its ability to form a wide range of LDHs with high crystallinity and reactive surfaces, mainly by substituting magnesium and/or aluminum by other metallic cations in the structure at different molar ratios. According to Table 6, similar specific surface areas were obtained with the samples synthesized by coprecipitation either at low or high supersaturation conditions (between 5 and 230 m².g -1 ).

Other synthesis methods such as the ball milling [START_REF] Mahmoud | Comparative Study of Single and Multiple Pollutants System Using Ti-Fe Chitosan LDH Adsorbent with High Performance in Wastewater Treatment[END_REF] or the ethanol-water mediated solvothermal method [START_REF] Li | Different dye removal mechanisms between monodispersed and uniform hexagonal thin plate-like MgAl-CO32--LDH and its calcined product in efficient removal of Congo red from water[END_REF]) are also interesting alternatives to obtain materials with high surface areas (around 150 m².g -1 ). Based on the existing literature, the adsorption of aqueous pollutants on LDHs and their derived materials reveals to be a promising alternative.

carbonaceous materials (also named coke) that brings to the progressive deactivation of the active sites, as shown by [START_REF] De Souza | Synthesis Gas Production by Steam Reforming of Ethanol over M-Ni-Al Hydrotalcite-type Catalysts; M=Mg, Zn[END_REF] in the Boudouard reaction: 2CO → CO 2 + C (s) . The synthesis conditions of the LDHs and related mixed oxides discussed in this part are summarized in Table 7.

The first experiments using LDHs to produce hydrogen were performed, in the last decade, by steam reforming. [START_REF] Marquevich | Hydrogen production via steam reforming of sun¯ower oil over Ni/Al catalysts from hydrotalcite materials[END_REF] studied the feasibility with sunflower oil, as feedstock, and Ni/Al mixed oxides derived from calcined LDHs, as catalyst. More recently, steam reforming was studied in the presence of hydrocarbons such as ethanol, methane or ethylene glycol. For each source, the key-point for process development is to identify the most efficient catalysts and synthesize it by varying the synthesis conditions or the chemical composition. [START_REF] Contreras | Catalysts for H 2 production using the ethanol steam reforming (a review)[END_REF] reported in a review article the various catalysts tested for hydrogen production by steam reforming. Ni-containing catalysts show very good catalytic activity for the conversion of ethanol and ethylene glycol, as shown by [START_REF] He | Co-Ni Catalysts Derived from Hydrotalcite-Like Materials for Hydrogen Production by Ethanol Steam Reforming[END_REF][START_REF] He | Approaching Sustainable H 2 Production: Sorption Enhanced Steam Reforming of Ethanol †[END_REF] and [START_REF] Cesar | Hydrogen production from ethylene glycol reforming catalyzed by Ni and Ni-Pt hydrotalcite-derived catalysts[END_REF], respectively. In the first case, the catalytic activity of Co-Ni catalysts decreased by increasing the nickel content. In presence of ethylene glycol, [START_REF] Cesar | Hydrogen production from ethylene glycol reforming catalyzed by Ni and Ni-Pt hydrotalcite-derived catalysts[END_REF] showed that Pt-containing catalysts are the most active and selective.

Finally, a complete study on the steam reforming process, carried out with methane as feedstock, has been reported by Halabi et al. (2012a, b), using calcined K 2 CO 3 -promoted Mg/Al hydrotalcite with in-situ CO 2 capture. A similar study has been performed by [START_REF] Chanburanasiri | Simulation of Methane Steam Reforming Enhanced by in Situ CO 2 Sorption Using K 2 CO 3 -Promoted Hydrotalcites for H 2 Production[END_REF] by comparing the impact of different commercial K 2 CO 3 on the sorption efficiency and H 2 production. The co-precipitation method revealed to be the most adapted for the easy substitution of the cations in the structure. In some syntheses, coprecipitation was performed in the presence of urea in order to improve the crystal size and the number of active sites.

Methane decomposition is the second process largely studied for hydrogen production.

Methane is firstly adsorbed on the surface of the catalysts and then decomposed on the active sites. Graphite-like structure or coke can accumulate on the active sites and block them, Hydrogen can be produced by various processes, such as steam reforming and methane decomposition. Highly active and selective catalysts exist and are listed in Table 8. The possibility to tune the catalysts composition is fundamental to form mixed oxides with high catalytic activity. Most of the studies discussed in this section are focused on the coprecipitation method for the possibility of operating structural substitutions, particularly to add nickel cations in the LDH structure. Table 7 also shows the interest in using coprecipitated LDHs instead of commercial samples; the specific surface areas of the synthesized samples is higher than that of the commercial ones (between 69 and 156 m².g -1 instead of 15-104 in the commercial hydrotalcites). Urea is also introduced during preparation in order to obtain a high number of active sites and increase the size of the platelets.

Moreover, the hydrogen production can be combined with CO 2 adsorption with interesting results. Combining CO 2 capture and H 2 formation in the same process is promising for future industrial applications.

5-Hydroxymethylfurfural formation

Thanks to its natural abundance, biomass is a promising sustainable feedstock:

carbohydrates are the largest natural carbon source constituting up to 75% of the annual biomass production, with promising applications in chemistry, food, paper, and pharmaceutical industries [START_REF] Rosatella | 5-Hydroxymethylfurfural (HMF) as a building block platform: Biological properties, synthesis and synthetic applications[END_REF][START_REF] Zakrzewska | Ionic Liquid-Mediated Formation of 5-Hydroxymethylfurfural-A Promising Biomass-Derived Building Block[END_REF]. Carbohydrates are rich in oxygen and can be easily reduced and dehydrated into a large spectrum of chemicals such as 5-Hydroxymethylfurfural (5-HMF) or levulinic acid. The US Department of Energy listed 5-HMF among the top 10 value-added chemicals; it is generally used as an intermediate to synthesize a wide range of chemicals nowadays derived from petroleum. Unfortunately, 5-HMF is not extensively produced at industrial scale due to the high production cost [START_REF] Van Putten | Hydroxymethylfurfural, A Versatile Platform Chemical Made from Renewable Resources[END_REF]. Fructose and glucose conversion are well-documented reactions that occur at relatively low temperature (below 150°C). These mild reaction conditions allow the use of clays, hybrids and other lamellar materials as catalysts. The dehydration of fructose leads to the formation of 5-Hydroxymethylfurfural (5-HMF), the main reaction product, and other byproducts, such as levulinic acid and formic acid, that are formed by degradation of 5-HMF, as explained in Fig15. The catalysts used in this reaction must present a high specific surface area in order to maximize the catalytic surface and react with the fructose dissolved in the solvent (generally water). These requirements can be fulfilled by LDHs synthesized by the sol-gel method or any other method implying a hydrothermal treatment, and that present high specific surface areas and an improved thermal stability. The synthesis parameters of all the cited samples are reported in Table 9.

The main drawback in using clays, hybrids and other lamellar materials in catalytic reactions is their thermal stability. In typical natural clays, dehydration occurs below 200°C

with the removal of interlayer and surface water, while irreversible dehydroxylation with formation of oxides starts already around 350°C, depending on the type of metallic cations in the starting material. Several catalytic reactions occur above 300°C, thus the majority of the studies performed with LDHs is based on mixed oxide catalysts obtained by their calcination.

However, few experiments have been performed on modified clay minerals such as montmorillonite; thanks to their expandable layers and swelling properties, these minerals can be easily modified by ion-exchange and pillaring in order to modify their structural properties. [START_REF] Lourvanij | Dehydration of glucose to organic acids in microporous pillared clay catalysts[END_REF] studied the partial dehydration of glucose to organic acids on iron-, chromium-, aluminum-, and non-pillared montmorillonite. Their large pore size (higher than 10Å) allows the diffusion of glucose molecules (equal to 8.6Å) in the interlayer space, as demonstrated by the Fig16. Depending on the reaction path, coke formation, leading to their progressive deactivation, has been observed. Further studies have been performed by [START_REF] Fang | Efficient conversion of carbohydrates into 5-hydroxymethylfurfural catalyzed by the chromium-exchanged montmorillonite K-10 clay[END_REF] with a commercial montmorillonite (K-10) to compare the impact of the reaction solvent: dimethyl sulfoxide (DMSO) and butyl-3-methylimidazole chloride ([BMIM]Cl) were used. The results are shown in Fig17. and point out the higher conversion of glucose in presence of higher concentrations of [BMIM]Cl in DMSO. The efficient recycling of the solvent (up to six times) without significant loss of the catalytic activity was also observed.

Beside the well-known basicity of hydrotalcites, LDHs and related materials, only few studies on the dehydration of fructose in 5-Hydroxymethylfurfural have been performed on these materials. However, recent researches pointed out the promising application of these materials in the transformation of biomass-derived monomers [START_REF] Chheda | An overview of dehydration, aldol-condensation and hydrogenation processes for production of liquid alkanes from biomass-derived carbohydrates[END_REF][START_REF] Li | Steam reforming of tar from pyrolysis of biomass over Ni/Mg/Al catalysts prepared from hydrotalcite-like precursors[END_REF][START_REF] Climent | Conversion of biomass platform molecules into fuel additives and liquid hydrocarbon fuels[END_REF]. Zeolites are widely employed for such applications, but mixed oxides represent a great alternative thanks to their easy synthesis by sol-gel method or by simple thermal treatment of LDHs and hydrotalcites.

In the case of glucose catalysis, three different reactions can be expected: dehydration, isomerization, and retro-aldol condensation. The first reaction generally allows to form HMF and levulinic acid in the presence of acid catalysts, while the other reactions are favored in the presence of basic materials to form fructose or lactic acid. The bifunctional acid-base properties of Al-Zr mixed oxides bring to different reaction products: HMF and levulinic acid are formed by dehydration on the acid sites, while lactic acid and fructose are obtained by isomerization and retro-aldol condensation in the basic sites.

As explained in this part, the catalytic formation of 5-HMF by dehydration of glucose with the use of hydrotalcites or LDHs is not much documented, but the potential use of their related mixed oxides shows interesting results that may be promising for future researches as zeolite substitutes and needs further investigations. Even if the specific surface areas of the LDHs reported in the Table 9 are similar to those of modified montmorillonites, the catalytic activity in this application is related to the surface acidity, limiting the potential use of LDHs.

Moreover, the sol-gel method or hydrothermal treatment can be easily performed to obtain LDHs adapted to this application.

Conclusion

LDHs, hydrotalcites and their related materials gained huge interest in the last decades thanks to their simple synthesis processes and numerous applications. Even if they are mainly synthesized by co-precipitation or hydrothermal methods, the possibility to combine different M(II) and M(III) cations and interlayer anions in their structure allows to tailor the chemical composition, as well as their structural properties (surface areas, pore sizes, number of active sites), making them potential catalysts in several industrial and environmental processes.

Moreover, their memory effect permits to reuse them many times, which is interesting at industrial scales.

In this review, the impact of synthesis methods on the structural and textural properties of LDHs has been discussed in order to identify the most suitable synthesis process for a chosen environmental application. The adsorption of pollutants (atmospheric and aqueous) and hydrogen production are important environmental applications in which LDHs can find their place. The key parameter for implementing the use of LDHs and derived materials is the choice of the appropriate synthesis process. The adsorption of pollutants requires high amount of basic sites and strong thermal stability that can be obtained using the co-precipitation method to synthesize LDHs. On the other side, the formation of 5-HMF takes place on the acid sites that can be potentially obtained by sol-gel or hydrothermal synthesis. Finally, the intercalation, impregnation or addition of appropriate elements (metals) with uniform dispersion is important for hydrogen production to increase the catalytic activity of hydrotalcites (Fig18.).

Fig3. Scheme of the synthesis of hydrotalcites via urea hydrolysis.

Fig4. Scheme of the synthesis of hydrotalcites by sol-gel process.

Fig5. Hydrothermal synthesis process applied to hydrotalcite synthesis.

Table 1. Main structural and textural impacts of different synthesis processes on LDHs.

Synthesis method Advantages Drawbacks

Co-precipitation Fig7. Comparison between non-modified (red) and modified (blue) hydrotalcites and derived materials on the CO 2 sorption capacity, according to Table 3. Fig18. Relations between structural properties and applications of hydrotalcites and related materials.

-

  Abstract ...................................................................................................................................... 1 Introduction ................................................................................................................................ 2 1. Synthesis techniques for hydrotalcite-like compounds ....................................................... 4 1.1. Co-precipitation method .............................................................................................. 4 1.2. Urea hydrolysis ............................................................................................................ 7 1.3. Sol-gel method ............................................................................................................. 9 1.4. Hydrothermal treatment ...........................................................................................

4MgCO 3 .

 3 Mg(OH) 2 .5H 2 O, even after 24h of hydrothermal treatment at 200°C. The choice of the appropriate metal salts is the most important parameter to be considered. More recently, the impact of the synthesis temperature (from 35 to 140°C) has been tested by[START_REF] Lin | Preparation of CuZnAl hydrotalcite-like catalysts for AsH3 abatement at low temperatures[END_REF] to synthesize CuZnAl hydrotalcite-like catalysts for arsine abatement. The authors concluded on a significant influence of the synthesis conditions on the arsine removal efficiency without clear explanation. The problem comes from the multiplicity of the parameters varied during the synthesis of the catalysts (temperature, calcination, time, molar ratios), making the results difficult to interpret. The most recent studies focus on the green synthesis of hydrotalcites and derived materials for environmental concerns, as demonstrated by[START_REF] Labuschagné | Green synthesis of hydrotalcite from untreated magnesium oxide and aluminum hydroxide[END_REF] by using untreated magnesium oxide and aluminum hydroxide for hydrothermal dissolution/precipitation in water.

  health concerns…) are currently boosting the studies on alternative solutions and clean synthetic routes for the production of non-fossil carbon sources. Recent international conventions agreed to the Kyoto Protocol and intended to reduce by 2050 the global emissions up to 50-60% (of those measured in 2006). The necessity in finding alternative environmental applications based on easy-to-use materials becomes more than urgent. Several reactions can be performed on layered materials, but their low thermal stability makes their implementation difficult. Clay minerals are generally stable up to 200°C due to the loss of the interlayer water and the consequent irreversible degradation of the structure at temperatures higher than 200°C. The presence of specific metal elements in the structure of the LDHs can enhance their catalytic properties, depending on the applications, as mentioned byXu et al.

  synthesized various LDHs by changing the M(II) cation nature. The selectivity towards carbon dioxide adsorption depended on the cation in the following order: Cu-Al ~ Zn-Al < Co-Al < Mg-Al < Ni-Al, and was related to the thermal behavior of the various compounds. Moreover, the sorption capacities of these materials have been drastically increased by removing the carbonate counterions in the Mg-Al and Co-Al LDHs. Several years later Wang et al. (2010) studied substitution of divalent metal cations M(II) by trivalent metal cations M(III) (Al 3+ , Ga 3+ , Fe 3+ and Mn 3+ ) in CO 2 adsorption at high temperature, performed on calcined samples.

  applications.Coenen et al. (2017b, a) also published about K-promoted hydrotalcites in a series of publications for sorption-enhanced water-gas shift. This article points out the importance of chemisorbed CO 2 and H 2 O, strongly linked to the specific adsorption sites. The presence of four different adsorption site types was identified: two site types with high affinity for CO 2 , one for H 2 O and one on which CO 2 and H 2 O can competitively adsorb. The CO 2 adsorption sites can be easily regenerated in presence of N 2 , and the cycling is favored at high temperatures. In the most recent study of the same group (2018), the adsorption processes were studied by infrared spectroscopy, showing once again the presence of the four types of adsorption sites. The presence of water molecules enables the decomposition of the strong bonds with carbonate sites, ameliorating the cycling behavior. The comparison between LDHs and zeolites has been recently performed by[START_REF] Megías-Sayago | CO2 Adsorption Capacities in Zeolites and Layered Double Hydroxide Materials[END_REF] and the importance of the surface area was evidenced. While zeolites exhibited surface areas around 360 m².g -1 and adsorb CO 2 up to 1.142 mmol.g -1 , MgAl and CaAl LDHs presented lower specific surface areas (50 and 21 m².g -1 ) and adsorb carbon dioxide up to 0.68 and 0.10 mmol.g -1 , respectively.

-)

  formation of nitrate Nitrites are unstable on the active sites and are then oxidized to nitrate in the presence of O 2 or decomposed via the following reactions: (ONO -) → Co 3+ -O 2-+ NO by decomposition 2Co 2+ -(ONO -) → 2Co 3+ -O 2-+ N 2 + O 2 by decomposition

( 3 )

 3 MgSO 4 + 4H 2 → MgO + H 2 S + 3H 2 OBased on these equations, several oxides derived from hydrotalcites and LDHs have been tested in the last decade. The most studied samples were prepared by addition of transition metals similar to those used in the NOx removal processes.[START_REF] Cantú | SOx Removal by Calcined MgAlFe Hydrotalcite-like Materials: Effect of the Chemical Composition and the Cerium Incorporation Method[END_REF] reported for example on mixed oxides derived from MgAlFeCe hydrotalcites. Despite the lower surface area and low SO 2 adsorption capacity, the oxides containing iron and cerium presented an interesting resistance to deactivation. The addition of cerium increased the adsorption rates and reduced the saturation times, while a limited amount of iron improved the catalytic regeneration of SOx adsorbents. A similar study was then performed by[START_REF] Valente | Novel SOx removal catalysts for the FCC process: Manufacture method, characterization, and pilot-scale testing[END_REF] in partial and full combustion conditions (used for regeneration).

  has been studied on various catalysts presenting different substitutions, and in different analytical conditions. The tests were mainly focused on the total decomposition of toluene or ethanol. The first experiments focused on the elimination of VOCs by mixed oxides derived from LDHs have been performed in the late 90s, even if the reaction was already known since many decades. Moderated temperatures, around 200°C, are required for this catalytic reaction.The catalysts are mixed oxides and amorphous phases obtained by calcination of LDHs at high temperature.[START_REF] Bahranowski | LDH-derived catalysts for complete oxidation of volatile organic compounds[END_REF] reported on the impact of Cu(II) and Cr(III) cations in mixed oxides derived from parent LDHs on the total oxidation of toluene and ethanol performed at various temperature. The authors observed that the purity and crystallinity of the materials depends on the initial Cu:Cr molar ratio, and that the catalyst derived from the CuCr hydrotalcite, with an initial Cu:Cr molar ratio of 2, gave the best catalytic performances in both toluene and ethanol oxidations. A similar study, focused on the crystallinity of the LDH precursors, was performed few years later byMikulova et al. (2007) on calcined NiAl LDHs. The analyses showed slightly lower catalytic activities than for CuCr hydrotalcites-derived oxides synthesized by[START_REF] Bahranowski | LDH-derived catalysts for complete oxidation of volatile organic compounds[END_REF] towards toluene oxidation. In ethanol oxidation, the formation of reaction intermediates, such as acetaldehyde, could be avoided by increasing the reaction temperature. Cobalt-containing hydrotalcite precursors with different compositions were tested by[START_REF] Gennequin | Co-Mg-Al Hydrotalcite Precursors for Catalytic Total Oxidation of Volatile Organic Compounds[END_REF]Gennequin et al. ( , 2010b)). They showed interesting activities in toluene oxidation, especially the sample containing the highest amount of cobalt, that presented the highest specific surface area, well dispersed oxides and lower presence of cobalt aluminate species. Fig11. shows the impact of the calcination temperature of Co-containing hydrotalcites on the toluene conversion. The same research group also performed studies focused on the memory effect of hydrotalcite(Gennequin et al. 2010a), their ability to regenerate their structure by simple wetting or impregnation. In this case, cobalt impregnation led to the partial regeneration of the LDH structure and improved the toluene oxidation activity.[START_REF] Palacio | Total oxidation of toluene over calcined trimetallic hydrotalcites type catalysts[END_REF] studied the impact of different trimetallic hydrotalcites (ZnCuAl and MnCuAl), after calcination at 450°C, over the oxidation of toluene. The MnCuAl catalysts

  leading to a progressive deactivation of the catalyst. The presence of transition metals (Ni, Co) in the catalyst structure positively contributes to methane decomposition at low temperatures and with high yields. By varying the Cu/Al molar ratios (maintaining fixed Ni content) of calcined Ni/Cu/Al hydrotalcites, Ashok et al. (2008) identified the ideal composition (Ni/Cu/Al=60/25/15) to obtain the highest hydrogen yield. The amount of Ni in Ni/Mg/Al hydrotalcites can also be modulated; García-Sancho et al. (2017) investigated the influence of the nickel content on hydrogen production. Fig14. shows the methane conversion and the hydrogen production in the presence of hydrotalcites of various nickel content during temperature-programmed tests. 55% of methane conversion with 100% hydrogen selectivity was measured on a catalyst containing 46% at of nickel in the structure. The conversion was not improved by increasing the nickel content, due to the presence of inactive isolated Ni particles among the spinel phases. One year later, the same team (García-Sancho et al. 2018) compared the impact of adding nickel during the synthesis of hydrotalcites prior to the calcination and to form bulk Ni-mixed oxides, or after the calcination of the hydrotalcites (to produce Ni-supported mixed oxides). The two kind of catalysts show similar activities, and only slight differences in the reduction temperatures of the catalysts were observed. The impact of the calcination temperature was studied by Sikander et al. (2018) on Mg/Ni/Al hydrotalcites with different nickel content. The sample containing 40% at nickel showed the best performances after calcination at 500 and 750°C. The presence of spinel-like structures on the catalyst surface favors the diffusion of the deposited carbon, enhancing the overall catalytic activity.

  Synthesis parameters of the LDHs and mixed oxides cited in the part "2.1.1. CO 2 adsorption -Capture and storage". change of the M(II) cations: Mg 2+ , Ni 2+ , Zn 2+ , Co 2+ .

Fig9.

  Fig9. NOx storage capacity at different temperatures of mixed oxides derived from MnMgAl

Fig10.

  Fig10. Simplified scheme of the Fluid Catalytic Cracking (FCC) process adapted to SOx

Fig11.

  Fig11. Toluene conversion versus temperature for Co-containing hydrotalcites previously

Fig12.

  Fig12. Effect of pH on the uptake of As(V) and Se(IV) on (a) calcined and (b) uncalcined

Fig17.

  Fig17. Catalytic conversion of glucose into 5-HMF at various [BMIM]Cl concentrations in

  .. 10 2. Use of hydrotalcites and derived materials in environmental applications ...................... 13 2.1. Adsorption of atmospheric pollutants ........................................................................ 14 2.1.1. CO 2 adsorption -Capture and storage ............................................................... 14 2.1.2. NOx and SOx adsorption ................................................................................... 21 2.1.3. VOCs total decomposition ................................................................................. 27 2.1. Adsorption of aqueous pollutants .............................................................................. 30 2.2. Other applications ...................................................................................................... 36 2.2.1. Hydrogen production .......................................................................................... 36 2.2.2. 5-Hydroxymethylfurfural formation .................................................................. 39 Conclusion ................................................................................................................................ 42 References ................................................................................................................................ 43

  al. (2010) studied the influence of M(III) cation type towards high-temperature CO 2 adsorption on calcined samples at different temperatures, from 250 to 400°C. Different phases have been observed depending on the calcination temperature: MgFe 2 O 4 and MgMnO 4 with coexistence of MgO with the appropriate M(III) cations, whereas only MgO phases have been observed in MgAl and MgGa LDHs. Moreover, the M(III) cation nature did not affect the sorption capacity (around 0.4 mmol.g -1 ), except for the calcined MgGa LDH that gave lower values of CO 2 sorption capacity due to its lower thermal stability. Gao et al. (

  [START_REF] Tang | New Insights into CO2 Adsorption on Layered Double Hydroxide (LDH)-Based Nanomaterials[END_REF] increased the basal spacing of LDHs by exchanging

anions with dodecylsulfonate ions in order to chemically graft (3-aminopropyl)triethoxysilane (APS) and enhance the CO 2 sorption capacity (up to 2.09 mmol.g -1 at 50°C). The presence of amino groups in the interlamellar space strongly contributed to the capture of carbon dioxide through the zwitterion mechanism and weak bonding. By optimizing the synthesis parameters,

Thouchprasitchaia et al. (

  Al molar ratio in the initial hydrotalcites, the stored amount of NOx at 350°C was enhanced. Moreover, the addition of platinum as active phase greatly improved the NOx storage capacity (from 0.06 to 0.50 mmol.g -1 ). NOx are stored as nitrates that are

	observed, but the removal of NO 2 was possible, and stable at around 50% removal degree,
	during 3 cycles.
	more stable over Pt/Mg oxides than over Pt/MgAl oxides. A similar approach has been
	performed by Silletti et al. (2006) with Pd/MgAl oxides. Additional adsorption sites were
	created, bringing to an increase of 70% in the adsorption capacity, when compared to the
	MgAl oxides (according to the reaction PdO (s) + NO 2 → Pd(NO 3 ) (s) ). Finally, RuMgAl oxides
	have been reported by Li et al. (2007) and exhibited good performances, with a maximum
	storage capacity of 0.22 mmol.g -1 at 350°C.
	novel NiMnTi mixed oxide, showing high surface area and strong reducibility, resulting in
	strong surface acidity. Moreover, manganese, nickel and titanium give interesting redox
	properties to LDHs and gained interest in the last years (Wang et al. 2018; Wu et al. 2019b; Li
	et al. 2019b). The recent study performed by Kameda et al. (2019) presents another approach
	on the adsorption of NOx. The employed catalysts were fresh MgAl hydrotalcites that could
	be simply recycled by anion desorption in a solution of Na 2 CO 3 . Low NO removal rate was

worked on Pt/MgAl oxides: MgAl oxides supports with different molar ratios were obtained by calcination of hydrotalcites, while platinum cations were incorporated by incipient wetness impregnation. The authors concluded that by increasing the Mg/

The most recent researches focus on low temperature storage and release of NOx between 150 and 300°C, as reported by

[START_REF] Cui | High Performance of Mn-Doped MgAlOx Mixed Oxides for Low Temperature NOx Storage and Release[END_REF]

with manganese-doped mixed oxides derived from hydrotalcite precursors. The incorporation of manganese provokes changes in the crystal phases with formation of Mn 3 O 4 and MgMn 2 O 4 spinel besides brucite. These changes result in the increase of the surface area (from 108 to 139 m².g -1 ) and of the pore size (from 14 to 23 nm), while the presence of manganese favors the oxidation of NO during adsorption. The largest adsorption capacity of 0.426 mmol.g -1 at 300°C was obtained in the presence of 15 wt% of Mn, as reported in Fig9. The efficiency of LDHs containing various metallic elements towards the adsorption of NOx pollutants has also been reported by

Li et al. (2019b)

on a

  MgO forms stable MgSO 4 compounds and Al 2 O 3 forms Al 2 (SO 4 ) 3 that is very

;[START_REF] Valente | Novel SOx removal catalysts for the FCC process: Manufacture method, characterization, and pilot-scale testing[END_REF][START_REF] Mathieu | Adsorption of SOx by oxide materials: A review[END_REF]

. The first experiments, focused on MgO, Al 2 O 3 and MgAl spinels derived from pure MgAl hydrotalcites, showed very low performances;

Table 4 .

 4 Synthesis conditions of the LDHs and mixed oxides cited in the part "2.1.2. NOx and SOx adsorption".

	Reference	Type of	Surface area Adsorption capacity	Synthesis method /

Table 6 .

 6 Synthesis conditions of the LDHs and mixed oxides cited in the part "2.2.Adsorption of aqueous pollutants".

	Reference	Type of material	Surface area (m².g -1 )	Synthesis method
	Lehmann et al. 1999	Commercial MgAl LDH (hydrotalcite)	34.8	Commercial hydrotalcite.
	Yang et al. 2005	Calcined MgAl LDHs	47 -198	Co-precipitation at high supersaturation, then calcination at 500°C for 4h.
	Lv et al. 2006	Calcined MgAl, ZnAl, and NiAl LDHs	39.9 -240.6	Synthesis method similar to co-precipitation with the use of urea, then calcination at various

Acknowledgements

The authors acknowledge the French Ministry of High Education and Research for the Ph.D.-student fellowship allowed to Dylan Chaillot.

Other applications

Hydrogen production

The need of alternative energies to replace fossil fuels is growing since the last decade.

Hydrogen is a clean energy vector that can be produced from several processes such as steam reforming, partial oxidation, or Sorption Enhanced Reaction Process (SERP) [START_REF] Marquevich | Hydrogen production via steam reforming of sun¯ower oil over Ni/Al catalysts from hydrotalcite materials[END_REF][START_REF] He | Co-Ni Catalysts Derived from Hydrotalcite-Like Materials for Hydrogen Production by Ethanol Steam Reforming[END_REF][START_REF] He | Approaching Sustainable H 2 Production: Sorption Enhanced Steam Reforming of Ethanol †[END_REF]Halabi et al. 2012a, b;[START_REF] Chanburanasiri | Simulation of Methane Steam Reforming Enhanced by in Situ CO 2 Sorption Using K 2 CO 3 -Promoted Hydrotalcites for H 2 Production[END_REF][START_REF] Cesar | Hydrogen production from ethylene glycol reforming catalyzed by Ni and Ni-Pt hydrotalcite-derived catalysts[END_REF][START_REF] Homsi | Steam reforming of ethanol for hydrogen production over Cu/Co-Mg-Al-based catalysts prepared by hydrotalcite route[END_REF]). Hydrogen production is carried out in presence of CO 2 adsorption processes (as reported in the section "2.1.1 CO 2 adsorption -Capture and storage"). [START_REF] Sikander | A review of hydrotalcite based catalysts for hydrogen production systems[END_REF] reported in a review article the different hydrogen production processes carried out in the presence of hydrotalcite-base catalysts. The feedstock is generally constituted of light hydrocarbons such as ethanol, methanol or methane, even if biomass is a promising environment-friendly hydrogen source being highly available worldwide, relatively cheap and renewable [START_REF] He | Co-Ni Catalysts Derived from Hydrotalcite-Like Materials for Hydrogen Production by Ethanol Steam Reforming[END_REF][START_REF] He | Approaching Sustainable H 2 Production: Sorption Enhanced Steam Reforming of Ethanol †[END_REF][START_REF] Contreras | Catalysts for H 2 production using the ethanol steam reforming (a review)[END_REF][START_REF] Cesar | Hydrogen production from ethylene glycol reforming catalyzed by Ni and Ni-Pt hydrotalcite-derived catalysts[END_REF][START_REF] Zardin | Hydrogen production by methane decomposition over Co-Al mixed oxides derived from hydrotalcites: Effect of the catalyst activation with H 2 or CH 4[END_REF][START_REF] García-Sancho | Hydrogen production by methane decomposition: A comparative study of supported and bulk ex-hydrotalcite mixed oxide catalysts with Ni, Mg and Al[END_REF][START_REF] Sikander | Tailored hydrotalcite-based Mg-Ni-Al catalyst for hydrogen production via methane decomposition: Effect of nickel concentration and spinellike structures[END_REF]

List of figures

Fig1. Environmental applications of hydrotalcites discussed in this review.

Fig2. Scheme of the co-precipitation method applied to the LDHs synthesis. Table 7. Synthesis conditions of the LDHs and mixed oxides cited in the part "2.3.1.

Hydrogen production".

Reference