Influence of tetralkylammonium cations on the formation of silicoaluminophosphates CAL-2

SAPO molecular sieves were synthesized under hydrothermal conditions from gels containing AlPO-kanemite, silica and a series of four tetralkylammonium (TAA+) salts. The influence of each of the tetralkylammonium cations on the molecular sieve syntheses was studied at constant SiO2/Al2O3 and TAA+/SiO2 molar ratios of 1.0. The obtained materials were characterized by typical physico-chemical techniques for solid materials such as powder X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), thermal analysis (thermogravimetry), elemental analysis (CHN), scanning electron microscopy (SEM) and multinuclear 13C- and 29Si-solid state nuclear magnetic resonance. A theoretical evaluation of the n-butylammonium conformers, energy of interconversion and molecular sizes, was useful to understand the behavior of each system containing different SDA’s. Thus different structures were obtained depending on the TAA+ cation used: in the case of tetramethylammonium the sodalite structure was prepared, tetraethylammonium and tetrapropylammonium cations on the other hand, directed the formation of chabazite structures and for the tetrabutylammonium cation the layered structure of the reactant was not changed. The presence of silicon islands is clear and is an important part of this work. 29Si-solid state nuclear magnetic resonance, elemental analysis of C, H and N and energy dispersive spectroscopy (EDS) techniques were all applied to characterize this aspect.