EXAFS and NMR investigation of zinc, manganese and cobalt substituted aluminophosphates with the chabazite structure

Abstract

Zinc, manganese and cobalt containing aluminophosphates with the chabazite (CHA) structure type (MeAPO-34, MeZn, Mn and Co) have been synthesized 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. The results of EXAFS spectroscopy on as-synthesized samples show that the metals possess 4 oxygen atoms in the first coordination shell which is an indication for a tetrahedral coordination in the lattice. The decrease of the EXAFS amplitude for some samples upon calcination is in agreement with the presence of Lewis acid sites, as previously reported. 31P NMR confirmed the incorporation of metals in the AlPO 4-34 structure. For Zn and Mn, the 31P NMR spectra show various lines attributed to different P( nAl, 4- nMe) units and the calculation of the fraction of metal in the samples after deconvolution of the spectra is in very good agreement with elemental analysis. Comparison of the relative intensities of the lines with those obtained by a binomial theorem showed that metals randomly substitute Al in the AlPO 4-34 framework. Incorporation of Co in the samples was suggested by a strong decrease in both T 1 and T 2 relaxation times with respect to SAPO-34 or ZnAPO-34, thus confirming EXAFS data.