For the first time, the formation of paramagnetic defect centers in Al
PO4 molecular sieves has been monitored by electron spin resonance (ES
R) spectroscopy. Results from ESR measurements demonstrate that the st
ructural defects in hydrated AlPO4-5 and VPI-5 molecular sieves change
into paramagnetic defect centers when the molecular sieves are dehydr
oxylated under a high vacuum (similar to 10(-5) Torr) at 773 K. Adsorp
tion of O-2 or NO on dehydroxylated molecular sieves causes an immedia
te loss of the ESR signals from paramagnetic defects. This suggests th
at the defect centers formed are F-type centers, which can serve as el
ectron donors for electron-withdrawing adsorbates.