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

MAS Al-27, P-31, and Al-27 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 P-31 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(3)cm space group. The preparation of partially hydrated VPI-5 can be monitored by H-2 NMR and by Al-27 and P-31 MAS NMR. Both partially and fully hydrated VPI-5 undergo a symmetry change from P6(3) space group at 294 K to P6(3)cm above 353 K, which indicates that the breakdown of hydrogen-bonded water structure inside the pores is not a factor in the process.