Claude Forano

Synthesis of Al-rich hydrotalcite-like compounds by using the urea hydrolysis reaction-control of size and morphology

Mono-dispersed sub-micron sized hydrotalcite-like compounds (HTlcs), [Mg1−xAlx(OH)2](CO3)y·2H2O, were synthesized by the urea hydrolysis method. The as-prepared compounds display platelet-like primary particles with a hexagonal shape. The breadth to thickness ratio can be varied by changing the synthesis conditions such as the precipitation rate or the medium nature. The factors influencing the secondary particle morphology (or aggregates) are also discussed. The results are compared to the constant pH coprecipitation method, commonly used for HTlcs.

Delamination and restacking of layered doublehydroxides

Layered double hydroxides recovered after exfoliation have been characterized by solid state chemistry techniques. The nature of the recovered materials is highly dependant on the drying process; when gently dried, a well-ordered phase is obtained, either by freeze-drying or reconstruction, but the material becomes amorphous after evaporation of the solvent. Delamination was used to prepare interstratified LDHs, making this technique a new route to the formation of a wide range of tunable materials.

Adsorption of MCPA pesticide by MgAl-layered double hydroxides

Abstract In the present study, the adsorption characteristics of the herbicide MCPA (4-chloro-2-methylphenoxyacetic acid) on layered double hydroxides (LDHs) were evaluated under laboratory conditions with particular attention to the effect of layer charge, original interlayer anion and morphology. The final objective is the use of LDHs and modified LDH materials as recyclable adsorbents and heterogeneous catalysts for the treatment of contaminated waste waters. The anionic clays tested were [Mg1−xAlx(OH)2]x+[Xx/mm−·nH2O] materials. The MCPA adsorption capacity was determined from adsorption isotherms and a kinetic study. We looked at the influence of the pH, the Mg2+/Al3+ ratio, i.e. the anion exchange capacity, the nature of the intercalated anion X (CO32−, NO3−, Cl−) and the morphology of the adsorbent on the extent of adsorption. The adsorption isotherms, described by Freundlich model, are of S-type with tendency to L-type for high MCPA equilibrium concentration. Furthermore, the adsorption capacity increases with the layer charge density. Hence, MCPA adsorption on LDHs occurs by anion exchange in two steps, an external exchange followed by an interlayer exchange, which explain these changes of type within the same isotherm. Besides, the adsorption capacity depends on the nature of the starting anions, following the affinity order (NO3−

Anionic Clays: Trends in Pillaring Chemistry

Minerals of the anionic clay family are reported by mineralogists since the beginning of this century: (Kurnakov and Chernykh 1926 ; Aminoff and Broome 1930 ; Read and Dixon 1933; Frondel 1941). A great variety of names is used in relation to the composition and the nature of the polytypes of these minerals (hydrotalcite, manasseite, pyroaurite, sjogrenite, stichtite, takovite, honessite, meixnerite ... ) ; Drits et al. 1987 proposed a systematic nomenclature. Anionic clay minerals are relatively rare and are often associated with serpentinites in metamorphic formations . These minerals also occur in saline deposits which shows that high temperature and pressure conditions are not absolutely necessary for their genesis.

Hybrid derivatives of layered double hydroxides

The intercalation of a series of organic anions into ZnAl- and CuCr- hydrotalcite-like compounds and Cu 2 (OH) 3 (CH 3 COO) H 2 O has been performed. The organic anions differ by the anionic function (carboxylate, sulfonate and phosphonate) or by the substituted groups on the aromatic ring (OH, Cl, NH 2 , NO 2 ). The various hybrid phases were thermally treated in order to graft the anions onto the hydroxylated layers of the host structure. The CuCr- Layered Double Hydroxide (LDH) displays a reactivity higher than the ZnAl- compound with regard to the sulfonate and phosphonate groups. The grafting of aromatic carboxylates with OH groups substituted in para or meta positions is also evidenced. The characterization of all the compounds were realized using X-ray powder diffraction, infrared spectroscopy and thermogravimetry analyses.

Anion-exchanging clay-modified electrodes: synthetic layered double hydroxides intercalated with electroactive organic anions

Abstract The layered double hydroxide (LDH) Zn 2 Cr(OH) 6 Cl · 2H 2 O, denoted [Zn-Cr-Cl], which is an anion-exchanging clay, is rapidly exchanged with electroactive organic sulphonate anions such as m -nitrobenzene sulphonate and anthraquinone mono- and disulphonates. High degrees of reaction are attained, and pillar-like layered structures are formed. Clay-modified electrodes have been produced using these materials and the precursor LDH [Zn-Cr-Cl]. The electrochemical behaviour of the substrates listed above, either intercalated or simply adsorbed on the LDH particles, has been studied using cyclic voltammetry.

Delamination of layered double hydroxides by use of surfactants

Delamination of the layered double hydroxide structure [Zn2Al(OH)6][C12H25SO4 ·nH2O] was realized by dispersion in butanol; translucent colloidal solutions are stable for at least 8 months with oriented LDH materials with extended crystallite size being obtained; the results presented here suggest that total delamination occurs in the colloidal solution.

Use of organic media to modify the surface and porosity properties of hydrotalcite-like compounds

Abstract In this paper we have focused on the surface and porosity properties of [Mg-Al-CO3] and [Mg-Al-Cl] obtained by the coprecipitation method. Different synthetic routes are used as well as post-synthesis treatments in order to measure how far textural modifications can be induced in these materials. For example, by performing the direct synthesis of [Mg-Al-CO3] in mixtures composed of water and an organic solvent, or by regenerating calcined samples in similar mixtures, the surface areas, pore volumes and pore sizes are shown to be greatly affected by some solvents: synthesis in ethylene glycol causes an increase of 81% in the surface area, while in glycerol it results in the induction of microporosity (30%). On the other hand, we have also investigated the influence of these organic solvents on the textural properties during anion exchange reactions. Through analysis of their BET nitrogen adsorption/desorption isotherms, unpillared [Mg-Al-CO3] proved to be mostly mesoporous, while [Mg-Al] hydrotalcite pillared with hexacyanoferrate (III) exhibited high microporosity, this characteristic being greatly influenced when the exchange reactions were carried out in organic media. SEM images provided evidence of the microtextural modifications undergone by the products throughout these different treatments.

Chapter 13.1 Layered Double Hydroxides

Publisher Summary This chapter describes layered double hydroxides. Among the group of minerals referred to as nonsilicate oxides and hydroxides, the layered double hydroxides (LDH) have many physical and chemical properties that are surprisingly similar to those of clay minerals. Their layered structure, wide chemical compositions (because of variable isomorphous substitution of metallic cations), variable layer charge density, ion-exchange properties, reactive interlayer space, swelling in water, and rheological and colloidal properties make LDH clay-like. However, because of their anion-exchange properties, LDH were referred to as “anionic clays.” Most metals in the first transition series can be incorporated into the hydroxyl sheet of the hydrotalcite-like structure. Thus, the formation of mixed metal-Al secondary precipitates may be a general reaction mechanism for transition metal adsorption to clay minerals.

Alkoxylation reaction catalysed by layered double hydroxides

Abstract Layered double hydroxides (LDHs) are extensively studied as precursors for catalysts, following a calcination at high temperatures to yield mixed oxides. However, these materials are less used as layered materials, i.e., without undergoing thermal activation. We have focused in this work on the use of a series of as-synthesised LDHs for the catalytic preparation of glycol ethers, which is a reaction of primary commercial importance. Two main systems are considered, namely the [Cu–Cr] and [Mg–Al] LDHs. The [Cu–Cr–Cl] LDH is obtained by the coprecipitation method, then through the appropriate chimie douc e exchange reactions the original chloride anions are replaced by a variety of oxo- and polyoxometallates, (CrO 4 ) 2− , (Cr 2 O 7 ) 2− , (V 2 O 7 ) 2− , (V 10 O 28 ) 6− and (Mo 7 O 24 ) 6− . On the other hand, the [Mg–Al] hydrotalcites, intercalated by (V 2 O 7 ) 2− , (V 10 O 28 ) 6− and [Fe III (CN) 6 ] 3− anions, are obtained by structure regeneration. This was done by rehydration of a commercial calcined material (Kyowa) in aqueous solutions containing the desired anion. The different materials have been fully characterised by conventional analytical techniques to evidence their lamellar properties and chemical nature. They were then tested in the catalytic reaction involving butan-1-ol and one or more units of ethylene oxide to make butyl-monoglycol ether (BMGE), di-glycol ether (BDGE), tri-glycol ether (BTGE), etc. The reactions were carried out between 80 and 120°C, temperature range in which no collapse of the lamellar structure is normally observed. In this paper it is shown that decavanadate exchanged LDHs proved to be very selective catalysts for the preparation of the monoglycol adduct, some samples achieving up to 100% selectivity in the screening tests.