Investigation of Layered Double Hydroxides Intercalated by Oxomolybdenum Catecholate Complexes

Abstract

Oxomolybdenum(VI) complexes of 3,4-dihydroxybenzoic acid (3,4-H2dhb) have been incorporated into layered double hydroxides (LDHs) by treatment of the LDH−nitrate (Zn−Al, Mg−Al) or LDH−chloride (Li−Al) precursors with aqueous or water/ethanol solutions of the complex (NMe4)2[MoO2(3,4-dhb)2]·2H2O at 50 or 100 °C. The texture and chemical composition of the products were investigated by elemental analysis and scanning electron microscopy (SEM) with coupled energy dispersive spectroscopy (EDS). Microanalysis for N and EDS analysis for Cl showed that at least 90% of nitrate or chloride ions were replaced during the ion exchange reactions. The final Mo content in the materials varied between 6.5 and 11.6 wt %. Mo K-edge EXAFS analysis, supported by IR, Raman, UV−vis, and 13C{1H} CP/MAS NMR spectroscopic studies, showed the presence of cointercalated [MoO2(3,4-dhb)2]m− and [Mo2O5(3,4-dhb)2]m− complexes in proportions that depend on the type of LDH support and the reaction conditions. The binuclear bis(catecholate) complex, with a Mo···Mo separation of 3.16 Å, was the major species intercalated in the Zn−Al and Li−Al products prepared using only water as solvent. The X-ray powder diffraction (XRPD) patterns of all the Mo-containing LDHs showed the formation of an expanded phase with a basal spacing around 15.4 Å. High-resolution synchrotron XRPD patterns were indexed with hexagonal unit cells with a c-axis of either 30.7 (for Li−Al−Mo LDHs) or 45.9 Å (for a Zn−Al−Mo LDH). Fourier maps (Fobs) calculated from the integrated intensities extracted from Le Bail profile decompositions indicated that the binuclear guest species are positioned such that the Mo → Mo vector is parallel to the host layers, and the overall orientation of the complex is perpendicular to the same layers. The thermal behavior of selected materials was studied by variable-temperature XRPD, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC)