Formation and properties of second-stage layered double hydroxide materials
Formation et propriétés de materiaux Hydroxydes Doubles Lamellaires bi-intercalés
Résumé
This thesis work has been devoted to the study of the anion exchange reaction mechanism for two LDH host compositions namely Zn2Cr and Zn2Al and for a series of inorganic namely Cl-, F-, CO3 2-, SO4 2- and Fe(CN)6 3- and organic anions (tartrate, succinate, adipate and styrene-4-sulfonate VBS anions). Monitoring the process in situ by means of energy dispersive X-ray diffraction reveals two reactions mechanisms: either anion exchange reaction occurs in a one-step direct transformation from the host to the fully-exchanged phase or the reaction proceeds via a second-stage intermediate with alternating interlayer occupied by two different anions and then conversion to the first stage. The aim of this work was first to determine the factors influencing staging. One important remark concerns the high rate of these reactions, typically with a half-time of 1-2 minutes. Therefore, in order to study these reactions in details and detect the presence of intermediate phases, it was necessary to slow down the reaction by adding the guest anion solution in a dropwise manner. A general trend is that inorganic-inorganic anion exchange reactions occur in a one-step process while inorganic-organic exchanges often proceed via a second-stage intermediate. This leads us to suggest that staging occurs partly as a result of organic-inorganic separation. Yet, one must keep in mind that staging is not observed for all organic anions (for instance, in the case of the exchange of chloride anions by VBS anions) indicating that other factors must be considered such as the anion size, the solvatation and the affinity for different anions. Concerning this last parameter, we believed in a correlation between the occurrence of a two-step process and the difficulty of exchange of the initial anion, as established by O'Hare et al. in the case of LiAl2 system.1 Indeed, microcalorimetric measurements reported elsewhere2, 3 clearly indicate that chloride anions are easier to replace by inorganic anions such as sulfate, carbonate or ferricyanide anions than by organic anions like tartrate or succinate anions. In these conditions, the exchange of chloride anions by inorganic anions occurs in one stage while staging is needed to overcome the energy barrier in the case of organic anions. On the other hand, the fact that the formation of second and first-stage products obeys either a consecutive process with the formation of second-stage intermediates and then conversion into fully exchanged phases or parallel processes with a route going through the intermediate as well as a direct transformation of the host, may be attributed also to differences in anion affinities. Indeed, a rapid and consecutive process is observed for anions relatively easy to replace while low and parallel processes take place for anions difficult to replace; this interpretation is supported by the enhancing effect of temperature on the formation of second-stage intermediates indicating higher activation energies in this latter case. However, another explanation given by O'Hare et al. might be that consecutive processes would arise from an ordered replacement of interlayer anions while a random replacement would lead to parallel processes. 1 The kinetic of the exchange reaction of chloride anions in Zn2Cr-Cl with tartrate anions was examined. A rate constant k of 1.2(4)*10-2 s-1 at 25 ℃ was obtained for a addition rate 1.0 mL* min-1 of the tartrate anion solution. This value is comparable with rate constants reported elsewhere for intercalation reactions in other layered. The anion exchange reaction in LDH system is expected to be a 2D processes with deceleratory nucleation. The Avrami exponent value n obtained from Sharp-Hancock analysis i.e. 2-3 is in favor of a phase boundary controlled process i.e. the reaction rate is limited by the expansion of the interlayer space necessary to accommodate tartrate anions. An important result of this study is the formation of intermediate phases of higher-order staging than 2 during the exchange reaction of CO3 2- anions with Zn2Cr-Tart, since only LDH second-stage intermediates have been reported so far. The gradual variation of the interlayer distance of the intermediate product clearly indicates a variation of the number of the interlayer spaces concerned with the exchange process. Of course this result must be further confirmed by means of HRTEM technique to better characterize the intermediate compositions. On the other hand, one may think that for the other anion-exchange reactions examined here, the high rate of these reactions is likely to prevent the observation of higher-staging than 2, if existing. In a second step, we went further by examining the anion exchange properties of these second-stage intermediates. Selective anion-exchange reactions were observed that were a result of the segregation between organic and inorganic anions. Indeed, the reaction of Zn2Cr-Succ/Cl and Zn2Cr-Tart/Cl second-stage compounds with either fluoride or adipate anions initially occurs with preferential replacement of the hydrophilic Cl ion with F, or of the organic succinate or tartrate anions with adipate leading to the formation of new second-stage intermediates Zn2Cr-Succ/F and Zn2Cr-Tart/F or Zn2Cr-Adip/Cl. Because of this great selectivity, new heterostructures can be formed that may not be achieved starting from a first-stage product. Indeed, attempts to prepare Zn2Cr-Tart/F second stage materials from respectively the exchange Zn2Cr-Tart with fluoride anions failed, suggesting that the present phases can be obtained starting only from Zn2Cr-Tart/Cl second-stage structure.
L'objectif de ce travail est de progresser dans la compréhension du mécanisme de la réaction d'échange pour les phases Hydroxydes Doubles Lamellaires (HDL). Ainsi, les propriétés d'échange des phases Zn2Cr et Zn2Al ont été examinées pour différents anions inorganiques (Cl-, F-, CO32-,SO42-, Fe(CN)63- et organiques (tartrate, succinate, adipate, styréne-4-sulfonate). Ces échanges ont été suivis par diffraction des rayons X en dispersion d'énergie (EDXRD : Energy Dispersive X-ray Diffraction) utilisant le rayonnement synchrotron. Cette technique compatible avec une étude structurale en solution in situ en cours de réaction, nous a permis de montrer l'existence de deux chemins d'échange : une conversion directe lors de l'échange d'anions inorganiques par des anions inorganiques et un processus en deux étapes avec la formation d'un composé intermédiaire dit "de stade 2" i.e. obtenu par l'échange d'un interfeuillet sur deux, dans le cas de l'échange d'anions inorganiques par des anions organiques. Ce phénomène d'intercalation par stade serait une particularité de l'échange inorganique-organique. Une corrélation peut également être établie avec la différence d'affinité des anions pour les phases HDL, qui semble indiquer que l'échange procède par stade lorsqu'il est défavorable
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