Recent development of layered double hydroxide-derived catalysts - Rehydration, reconstitution, and supporting, aiming at commercial application -

Layered double hydroxide (LDH) has a unique feature, i.e., reconstitution of layered structure from the calcined oxides. The application of LDH as the heterogeneous catalysts by adopting reconstitution phenomenon and supporting on appropriate carrier were reviewed aiming at their commercial uses. After a short explanation on fundamental properties of LDH, such as phase transition by calcination and rehydration of calcined LDH (reconstitution of LDH by "memory effect"), their applications in the catalyst preparations were discussed. By adopting "memory effect", the surface of LDH-derived mixed oxide was rehydrated or reconstituted as well as decorated with various metals and anions, resulting in an enhanced catalytic activity and selectivity via base sites activation and high dispersion of active metal species. Active OH- species, as Bronsted base, was produced on calcined Mg-Al LDH after the rehydration and exhibited high activity in various reactions. Catalytically active transition metals such as Ni, Fe and Mn were highly dispersed on Mg(Al)O via reconstitution, resulting in the high catalytic activity in dehydrogenation and oxidation reactions. Pt-Sn/Mg(Al)O catalysts prepared by adopting reconstitution phenomenon exhibited an excellent activity in the dehydrogenation of propane to propene. This is owing to the formation of highly dispersed and stable Pt-Sn bi-metallic species on the catalyst surface. By supporting LDH on magnetic carriers such as ferrite, the catalyst became magnetically separable and could be utilized recyclably in the fine chemicals synthesis. Paper structured catalyst prepared from LDH by utilizing paper making technology opened new type usage of LDH materials which can be applied in the reforming catalysts for fuel cells. Moreover the reconstitution phenomena donate several beneficial properties to the LDH-derived mixed oxides in their uses as the heterogeneous catalysts. Reconstitution of LDH kinetically accelerated the conversion of anions such as toxic NO3- and refractory (COO)(2-) in aqueous solution by entrapping the anions in the interlayer space of rehydrated LDH as observed on Pd-Cu/Mg(Al)O and Cu/Mg(Al)O catalysts, respectively. In the reforming of hydrocarbons for H-2] production, self-regenerative activity appeared on trace amount of Pt doped Ni/Mg(Al)O catalyst by reversible oxidation-reduction between Ni-0 and Ni2+ on/in Mg(Ni,Al)O periclase assisted by reversible change between periclase and LDH structure of the support. Both Pt-Ni/Mg(Al)O and Pt-Sn/Mg(AI)O catalysts exhibited excellent activities almost closing to the commercial uses. (C) 2016 Published by Elsevier B.V.