Sj. Kim et al., ADSORPTION OF OXYGEN ON THE PARAMAGNETIC DEFECT CENTERS IN ALUMINOPHOSPHATE MOLECULAR-SIEVES, Catalysis letters, 44(3-4), 1997, pp. 165-169
Temperature-programmed desorption (TPD) and electron spin resonance (E
SR) techniques have been employed to investigate the oxygen adsorbed o
n the paramagnetic defect centers in dehydroxylated AlPO4-20, AlPO4-11
, AlPO4-5, and VPI-5 molecular sieves. Two different adsorption sites
are observed for the small-pore AlPO4-20, where oxygen is entrapped wi
thin the intracrystalline voids generated during the calcination step,
as well as within the beta-cages. However, the unidimensional molecul
ar sieves AlPO4-11 AlPO4-5, and VPI-5 have only one type of adsorption
site within their inner pores. The apparent activation energies of de
sorption (12.0-21.7 kcal mol(-1)) determined from the oxygen TPD resul
ts demonstrate that, as the pore size of the AlPO4 molecular sieve is
larger, the interaction between the adsorbed oxygen and the molecular
sieve framework becomes weaker.