MgAPO molecular sieves of CHA and AFI structure — Acidity and Mg ordering

Magnesium containing molecular sieves, viz. MgAPO-44, MgAPO-34 (CHA) and MgAPO-5 (AFI), were prepared with cyclohexylamine (structures 44 and 5) and tetraethylammonium hydroxide (structure 34) as templating agents. The crystallization was performed at 473 K by means of a microwave power system. The samples were characterized by chemical analysis, thermal analysis, adsorption measurements, calorimetric measurement of the differential molar heat of NH₃ sorption, temperature-programmed NH₃ desorption, infrared spectroscopy and MAS NMR spectroscopy. For the as-synthesized samples, up to 7.7 wt% of MgO is incorporated into lattice positions of the CHA structure and substitutes aluminum.

The relative intensities of the deconvoluted ³¹P NMR signals of the as-synthesized chabazite-like samples (MgAPO-44 and MgAPO-34) depend on the type of templating molecules. The calcined samples (723 K), however, showed nearly identical results which could be interpreted in terms of different ³¹P(4 n Al, n Mg) environments. A random distribution of the Mg atoms is deduced from the NMR results.

The incorporation of Mg creates isolated bridging hydroxyls. The temperature of the structure collapse decreases with increasing Mg content.