Interaction of dipropylamine template molecules with the framework of as-synthesized AlPO4-31

The symmetry transformation of aluminophosphate molecular sieve AlPO4-31 and the interaction of its framework with dipropylamine template molecules were studied by variable temperature XRD and multinuclear MAS, CPMAS, and MQMAS NMR. measurements. The as-synthesized material exhibited triclinic symmetry with a slightly distorted rhombohedral-like unit cell. Deviation from a rhombohedral symmetry was due to a large amount of template within cavities of a molecular sieve, which caused short contacts with the aluminophosphate framework. The template within AlPO4-31 was in the form of dipropylammonium hydroxide ionic pairs, whose arrangement did not possess a long-range order. Hydroxyl groups, not being isolated by surrounding alkyl groups, interacted with the aluminophosphate framework and gave rise to 5-coordinated aluminum sites. Dipropylammonium and hydroxyl groups were also coupled to the framework through hydrogen bonds with the framework oxygen and affected the local environments of the 4-coordinated aluminum and phosphorus sites. After one-half of the template was expelled from the sample, the interaction with the framework was reduced and the degree of crystal symmetry increased. Irreversible transformation from triclinic to rhombohedral symmetry occurred at 210 degreesC.