Synthesis, characterization and structure of SAPO-56, a member of the ABC double-six-ring family of materials with stacking sequence AABBCCBB

Abstract

SAPO-56, a silicon-containing aluminum phosphate molecular sieve, is characterized by electron microscopy, magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and Rietveld refinement of synchrotron X-ray powder diffraction data. The trigonal space group is P 3 ̄ 1c with a= b=13.7571(3) Å, c=19.9362(7) Å, α= β=90° and γ=120°. This member of the ABC six-ring-structure family contains double six-rings arranged to give gmelinite (gme) cages and another large cage (aft), which has only been observed before in AlPO-52. The stacking sequence of six-rings in SAPO-56 is AABBCCBB. This unique topology has been assigned structure-type code AFX. Analysis of X-ray peak widths reveals broadening due to slight faulting and silicon disorder. 29 Si -NMR results indicate that the silicon is distributed between isolated framework T d sites, silicon islands and silica debris.