Journal of physical chemistry vol:95 issue:24 pages:10025-10031
As-synthesized VPI-5 is an aluminophosphate dihydrate phase. Upon dehydration, VPI-5 undergoes reversible phase transitions into its monohydrate and anhydrous forms. The dehydration products from VPI-5 and their interconversions are studied with Al-27 and P-31 MAS NMR and in situ XRD. At a degree of hydration intermediate between that of the monohydrate phase and the dihydrate phase, the structure is susceptible to an irreversible phase transition into the AlPO4-8 phase. AlPO4-8 formation probably starts at the outer rim of the crystals. The pore diameter in the interior of the anhydrous crystals, determined with Xe-129 NMR, amounts to ca. 1.2 nm. Dynamic argon adsorption measurements suggest that part of the pore entrances are narrowed to ca. 0.83 nm. This type of pore narrowing is observed even when the dehydration is performed under conditions were AlPO4-8 formation is not observed with XRD or P-31 MAS NMR. Pore entrance restriction may be due to deformation of some 18-membered rings in the anhydrous form of VPI-5.