FORMATION OF PARAMAGNETIC DEFECT CENTERS IN ALUMINOPHOSPHATE MOLECULAR-SIEVES

The formation of paramagnetic defect centers in aluminophosphate (AlPO 4) molecular sieves with different structures is investigated by elect ron spin resonance (ESR) spectroscopy. Distinct differences in the: po sition and number of the ESR signals generated by dehydroxylation of A lPO4 molecular sieves under a vacuum of 10(-5) Torr at 773 K clearly s how that the nature of the paramagnetic defect centers formed is signi ficantly different in the structure type of the molecular sieves. In a ddition, it is found that the spin concentration of these defects depe nds strongly on the pore size of the AlPO4 molecular sieves as well as their hydration level before dehydroxylation. The larger ring size th e molecular sieve has, the higher spin concentration of paramagnetic d efects it shows, Thus, the extra-large-pore VPI-5 shows the highest sp in concentration value among the molecular sieves studied here, which corresponds to 6.4 defects per 1000 unit cells. The ESR results obtain ed from AlPO4-based molecular sieves containing heteroatoms other than aluminum and phosphorus (such as silicon or cobalt) reveal that the f ormation of paramagnetic defect centers is severely restricted when th e amount of heteroatoms in their framework increases. On the basis of the overall ESR results of this study, a reliable mechanism of formati on of paramagnetic defect centers in AIPO(4) molecular sieves is propo sed.