Formation of paramagnetic defect centers in aluminophosphate molecular sieves

Abstract

The formation of paramagnetic defect centers in aluminophosphate (AlPO4) molecular sieves with different structures is investigated by electron spin resonance (ESR) spectroscopy. Distinct differences in the: position and number of the ESR signals generated by dehydroxylation of AlPO4 molecular sieves under a vacuum of 10(-5) Torr at 773 K clearly show that the nature of the paramagnetic defect centers formed is significantly different in the structure type of the molecular sieves. In addition, it is found that the spin concentration of these defects depends strongly on the pore size of the AlPO4 molecular sieves as well as their hydration level before dehydroxylation. The larger ring size the molecular sieve has, the higher spin concentration of paramagnetic defects it shows, Thus, the extra-large-pore VPI-5 shows the highest spin concentration value among the molecular sieves studied here, which corresponds to 6.4 defects per 1000 unit cells. The ESR results obtained from AlPO4-based molecular sieves containing heteroatoms other than aluminum and phosphorus (such as silicon or cobalt) reveal that the formation of paramagnetic defect centers is severely restricted when the amount of heteroatoms in their framework increases. On the basis of the overall ESR results of this study, a reliable mechanism of formation of paramagnetic defect centers in AIPO(4) molecular sieves is proposed.