Silico-alumino-phosphate

The catalyst in this case is a silico-alumino-phosphate molecular sieve. Each particle has millions of pores, each the size of 4 A or less. (A or angstrom is equal to approximately 10" centimeters and is a measure used to gauge the size of single molecules. Tiny indeed.) The catalyst is buried in those 4 A pores, so the size olefins that can get created are mostly ethylene and propylene. The catalyst is therefore highly selective in generating its products. 

Zeolitic catalysts are constituted of [TO4] tetrahedrons which are assembled by sharing oxygens to form one of several topologically distinct framework structures. In the case of alumino-silicates, T=Si with some A1 substitutions, while in silico-alumino-phosphates (SAPO), T sites are occupied by alternating A1 and P atoms with some Si substitutions. The substitutions destroy the electro-neutrality of the framework and necessitate the presence of compensating cations. While mono- or bivalent metallic cations usually occupy crys-tallographically well-characterized sites in the cavities, protons are chemically bonded to framework oxygens. 

The proton is always bonded to one of the oxygens belonging to the substituted T-site A1 in alumino-silicates or Si in silico-alumino-phosphates. Since the two vicinal tetrahedral sites are practically never substituted, the immediate neighborhood of the protonated framework oxygen is identical in both alumino-silicates and SAPOs, as indicated in the above scheme. This bridging OH group or Br0nsted acid site is at the heart of the catalytic activity of zeolites and SAPOs. 

Aluminophosphate zeolites with interesting molecular sieve properties have also been synthesized (69) but the pentavalency and trivalency of P and Al respectively do not result in a supplementary negative charge as observed in silico-aluminate zeolite, and subsequently no acidity is expected. However, alumino-silico-phosphate zeolites with variable Al and P contents may well be of interest in the future. 

---