Layered Double Hydroxides-Based Materials as Oxidation Catalysts

Introduction

Layered double hydroxides (LDHs) belong to the anionic clays family having the general formula with 0.2 ≤ x ≤ 0.4 corresponding to a M^II/M^III molar ratio between 1.5 and 4 (Tichit & Coq, 2003). M^II and M^III are bivalent and trivalent cations, respectively, with ionic radii not too different from that of Mg²⁺ (Cavani et al., 1991). They are hexa-coordinated to hydroxyl groups forming brucite-like sheets which stack to create a layered structure. Counter-anions A^n– are intercalated in the inter-layer space to compensate the positive charge introduced by the M^III cations partially replacing M^II cations in the layers. Two or more cations can enter simultaneously the brucite-like sheets where they are homogeneously distributed and intimately mixed together. Notably, the LDHs can be easily prepared by coprecipitation at a pH higher than or equal to the one at which the more soluble hydroxide precipitates (Cavani et al., 1991). At the same time, a large variety of inorganic and organic anions can be intercalated between the layers, mainly by a simple anionic exchange procedure. Due to their structure, together with their compositional flexibility, the LDHs possess versatile physico-chemical properties which make them good candidates as multifunctional nanostructured catalysts and catalyst precursors, several review papers being focused so far on this subject (Vaccari, 1999; Sels et al., 2001; Tichit & Coq, 2003; Zhang et al., 2008; Othman et al., 2009; Xu et al., 2011; Fan et al., 2014).

Various transition metals cations can be introduced into the layers of the LDH structure, but also into the inter-layer space as heteropolyanions and organometallic complexes where they are responsible for the redox properties of these materials and, therefore, for their catalytic properties in oxidation reactions. They can be used as such, particularly for low temperature liquid-phase oxidation, or as mixed oxides obtained by their controlled thermal decomposition for high-temperature gas-phase selective and total oxidation.

In this chapter we summarize the development of transition-metal-containing LDH-derived catalysts for oxidation reactions, including liquid-phase oxidation and epoxidation, gas-phase selective oxidation, oxidative dehydrogenation and total oxidation.