Synthesis and structure of as-prepared ITQ-4, a large pore pure silica zeolite: The role and location of fluoride anions and organic cations

Pure silica ITQ-4, a zeolitic material with large one-dimensional 12MR channels and an unexpectedly large microporous void volume, can be synthesized in the presence of fluoride ions and N-benzylquinuclidinium cations within a wide range of synthesis pH. Substitution of the tertiary carbon of the quinuclidine moiety by a N atom still renders the organic cation as a suitable structure-directing agent for this material. The role of F- anions and the influence of the pH of the synthesis on both the phase selectivity of the crystallization and the presence of connectivity defects in the final material are discussed. In the presence of F-, defect formation is observed to be pH-dependent, and there appears to be a threshold pH for the generation of a constant defect concentration. A plausible explanation for this observation based upon the ionization state of the condensing species is also presented. The structure of as-made ITQ-4 has been obtained and refined from high-resolution powder XRD data, and the organic and fluoride ions have been located. The sinuosity of the channel of ITQ-4 is found to coincide with the bent geometry observed for the structure-directing agent, while the fluoride ions are found to reside within a small [4(3)5(2)6] cage located around the periphery of the central pore space.