Synthesis in fluoride and hydroxide media and structure of the extra-large pore pure silica zeolite CIT-5

Abstract

The new extra-large pore molecular sieve CIT-5 has been synthesised in both hydroxide and fluoride aqueous media. It has been found, contrary to previous reports, that Li⁺ is not an essential component of the synthesis gel, although it accelerates the crystallisation. The pure silica polymorph synthesised in the F⁻ medium presents almost no connectivity defects, in contrast to that synthesised in the OH⁻ medium. IR and ²⁹Si MAS NMR spectra of the new samples synthesised in the F⁻ medium show very good resolution of bands owing to the high local order which arises from the very low concentration of connectivity defects. The structure of this phase has been reinvestigated and solved successfully by direct methods and refined using synchrotron X-ray powder diffraction data. The framework topology previously proposed for this phase is found to be correct, although the structure is more accurately described in Im2a rather than Imma symmetry. Despite the increase in the number of tetrahedra forming the minimum window of the pore, from 12 to 14, the effective pore diameter of CIT-5 appears to be almost identical to that of the unidimensional 12-ring zeolite SSZ-24.