SYNTHESIS AND PROPERTIES OF SAPO-5 MOLECULAR-SIEVES - SILICON INCORPORATION INTO THE FRAMEWORK

Silicoaluminophosphate molecular sieves of SAPO-5 type are synthesized with variable Si/(Al + P + Si) mole ratios using triethylamine as the organic additive. The crystalline products are characterized by several physico-chemical methods. Incorporation of silicon into the framework of SAPO-5 is demonstrated by the variation in the unit-cell volume with silicon mole fraction, chemical analysis and magic-angle-spinning nuclear magnetic resonance. When Si/(Al + P + Si) < 0.077, silicon substitutes predominantly for phosphorus. For compositions where Si/(Al + P + Si) > 0.077, it appears that a combination of mechanisms, substitution of one silicon for one phosphorus and substitution of two silicons for aluminium-phosphorus pairs, takes place. Surface analysis using depth profiling shows an inhomogeneous distribution of silicon at higher silicon content. Preliminary catalytic results in omicron- and m-xylene isomerization are discussed.