EXAFS AND NMR INVESTIGATION OF ZINC, MANGANESE AND COBALT-SUBSTITUTEDALUMINOPHOSPHATES WITH THE CHABAZITE STRUCTURE

Zinc, manganese and cobalt containing aluminophosphates with the chaba zite (CHA) structure type (MeAPO-34, Me=Zn, Mn and Co) have been synth esized hydrothermally in the presence of tetraethylammonium hydroxide. All samples showed a very high crystallinity without impurity phases, as evidenced by X-ray diffraction and scanning electron microscopy. T he results of EXAFS spectroscopy on as-synthesized samples show that t he metals possess 4 oxygen atoms in the first coordination shell which is an indication for a tetrahedral coordination in the lattice. The d ecrease of the EXAFS amplitude for some samples upon calcination is in agreement with the presence of Lewis acid sites, as previously report ed. P-31 NMR confirmed the incorporation of metals in the AlPO4-34 str ucture. For Zn and Mn, the P-31 NMR spectra show various lines attribu ted to different P(nAl, 4-nMe) units and the calculation of the fracti on of metal in the samples after deconvolution of the spectra is in ve ry good agreement with elemental analysis. Comparison of the relative intensities of the lines with those obtained by a binomial theorem sho wed that metals randomly substitute Al in the AlPO4-34 framework. Inco rporation of Co in the samples was suggested by a strong decrease in b oth T-1 and T-2 relaxation times with respect to SAPO-34 or ZnAPO-34, thus confirming EXAFS data.