Small-pore
molecular sieve SAPO-34 is well-known as the most promising
and effective catalyst for the conversion of methanol to olefins (MTO).
In this paper, we have investigated the formation of SAPO-34 under
different dry gel conversion (DGC) conditions using two types of synthesis
gels with one containing hydrofluoric acid (HF) and the other not.
Particular attention was paid to the Si incorporation and distribution
in final SAPO-34 products. The results indicate that under DGC (with
HF) conditions SAPO-34 is formed from the transformation of a highly
crystalline, layered prephase that is held by covalent bonds. The
final SAPO-34 product is a mixture of triclinic and trigonal phases
containing six framework Si species. The Si distribution in triclinic
and trigonal SAPO-34 is drastically different. The majority of Si
species are located in the aluminosilicate domains in trigonal phase.
Conversely, under DGC (without HF) conditions, the crystallization
of SAPO-34 involves three transformational stages. Initially, a layered,
crystalline intermediate forms and then transforms to a semicrystalline
phase. The structures of both intermediates are held by weak noncovalent
bonding interactions. The final SAPO-34 product is in pure trigonal
form and contains two major Si species. Despite the difference in
Si distribution, the SAPO-34 products prepared by using different
gels have similar bulk Si contents. MTO reaction tests show that before
50 min of time-on-stream the DGC (with HF) product gives lower ethylene
and propylene selectivity but higher propane selectivity than the
DGC (without HF) product. After 50 min on stream, similar light olefin
and propane selectivities are observed over the two SAPO-34 samples.