FORMATION OF NH4+ AT THE BRONSTED SITE IN SAPO CATALYSTS

The catalytic properties of ammonia adsorption on silicoaluminophospha te (SAPO) clusters have been investigated within the framework of the ab initio self-consistent field method. Full optimization of structure s has been carried out at the DZ, DZP and TZ2P levels of theory. Two d ifferent types of ammonia adsorption on SAPO framework sites are propo sed. In one of these, the structures H3Si-OH-Al(OH)2-O-PH3...NH3 are s tabilized on the bridging OH by a single site binding with an interact ion energy of -17.49 kcal/mol. The other is a type of the structure [H 3Si-O-Al(OH)2-O-PH3][NH4+], in which the ammonium cation forms two hyd rogen bonds towards the unprotonated framework sites. Other possible s tructures like a ''bifurcated'' structure are less stable than the two H-bonded structures by about 0.48 and 0.1 kcal/mol at the DZP and TZ2 P basis set levels respectively. This indicates the free rotation of t he NH4+ on the SAPO surface site at room temperature. The interaction energies for the structures [H3Si-O-Al(OH)2-O-PH3][NH4+] are more stab le than for the structures H3Si-OH-Al(OH)2-O-PH3...NH3 by 0.5-1.36 kca l/mol depending on the basis sets. These calculated energy values are an inversion order from the zeolite/NH3 complexes. Comparison of the S APO complexes with hydrogen halides, silanol, and zeolite has demonstr ated that the hydrogen-form SAPO is at least as acidic as zeolite.