We have studied by volumetric adsorption isotherm measurements the sor
ption properties of different AlPO4-5 or SAPO-5 samples, for sorbate s
pecies characterized by a large range of molecular size: D-2, Kr, Ar,
C2D2, CO2, CH4, CF4, and C(CH3)(4). The highest sorption capacities, Q
(max) greatly depend on the sorbate molecular size. A difference of 1
order of magnitude is observed between hydrogen and neopentane. In add
ition, we have found that the sorption capacities of the four analyzed
AlPO4-5 or SAPO-5 samples vary by almost a factor of 2. This result c
learly shows that, as a consequence of the one-dimensional character o
f the AFI micropore network, Q(max) is very sensitive to small amounts
of crystalline defects or impurities. For methane, Q(max) is equal to
6 mol/uc, a quantity that is not compatible with the commonly accepte
d value of the internal diameter of the AlPO4-5 micropore diameter, <e
mpty set>(M) = 7.3 Angstrom. Moreover, an interesting phase transition
phenomenon has been observed during the sorption of krypton and metha
ne, a result that can be the consequence of the good parametric agreem
ent between such sorbate species and the AlPO4-5 inner surface.