## Abstract

The 18-ring pore molecular sieve VPI-5 recrystallizes to the 14-ring sieve AIPO_{4}-8. This recrystallization appears to be reversible under proper conditions, as could be explained on the basis of the structural relationship between the two materials. VPI-5 has an 18-ring channel bounded by six 6-rings and six pairs of adjoining 4-rings. In the conversion to AIPO_{4}-8, two-thirds of the adjoining 4-rings are converted to 6-rings and reconnected to form a smaller, 14-ring channel bounded by eight 6-rings, two pairs of adjoining 4-rings, and two single 4-rings. The space group for AIPO_{4}-8 [Cmc2_{1}: a = 33.0894(25), b = 14.6832(13), c = 8.3630(6) A ̊ at room temperature] is a direct subgroup of that for VPI-5 [P6_{3}cm: a = 18.549(1), c = 8.404(1) A ̊ for the dehydrated form; orthohexagonal setting: a = 32.128, b = 18.549, c = 8.404 A ̊]. Interconversion of the two structures may be facilitated by extracoordination of framework aluminum atoms with water molecules. The observed small adsorption capacity of AIPO_{4}-8 can be explained by stacking faults, where, within a single channel, different pairs of adjoining 4-rings are left unbroken.

Original language | English |
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Pages (from-to) | 13-19 |

Number of pages | 7 |

Journal | Zeolites |

Volume | 12 |

Issue number | 1 |

Publication status | Published - 1 Jan 1992 |

Externally published | Yes |

## Keywords

- AIPO-8
- Aluminophosphate
- neutron diffraction
- structure
- ultrawide pore
- VPI-5
- X-ray diffraction

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