Solid-state NMR studies of the aluminophosphate molecular sieve VPI-5

Abstract

MAS 27Al, 31P, and 27Al nutation NMR spectra show that at 294 K fully hydrated VPI-5 contains three Al and P crystallographic sites in an 1:1:1 occupancy ratio and one-third of Al is 6-coordinate. The results are consistent with the P6 3 space group. Above 353 K two 31P MAS resonances in a 2:1 intensity ratio are found, and quadrupole nutation spectrum gives only one line from 4-coordinate Al, indicating that fully hydrated VPI-5 undergoes a structural transformation to a higher framework symmetry P6 3cm space group. The preparation of partially hydrated VPI-5 can be monitored by 2H NMR and by 27Al and 31P MAS NMR. Both partially and fully hydrated VPI-5 undergo a symmetry change from P6 3 space group at 294 K to P6 3cm above 353 K, which indicates that the breakdown of hydrogen-bonded water structure inside the pores is not a factor in the process.