Investigation of layered double hydroxides intercalated by oxomolybdenum catecholate complexes


Oxomolybdenum(VI) complexes of 3,4-dihydroxybenzoic acid (3,4-H(2)dhb) 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)]center dot 2H(2)O at 50 or 100 degrees 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 C-13{H-1} 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 center dot center dot center dot Mo separation of 3.16 angstrom, 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 angstrom. 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 angstrom (for a Zn-Al-Mo LDH). Fourier maps (F-obs) 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).



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Monteiro, B; Gago, S; Paz, FAA; Bilsborrow, R; Goncalves, IS; Pillinger, M

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The authors are grateful to the FCT, OE, and FEDER for financial support (Project POCI/CTM/58507/2004). The CCLRC Daresbury Laboratory Synchrotron Radiation Source (SRS) is acknowledged for providing beamtime at station 16.5 under award number 47105. We also wish to thank the European Synchrotron Radiation Facility (ESRF, Grenoble, France) for granting access to the ID31 high-resolution powder X-ray diffraction beam line and to Dr. Irene Margiolaki for technical assistance. B.M. and S.G. thank the FCT for PhD (SFRH/BD/24717/2005) and postdoctoral (SFRH/BPD/25269/2005) grants, respectively. We also wish to thank Prof. Joao Rocha for access to research facilities and Luis Mafra for assistance in the NMR experiments.

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