We have studied by volumetric adsorption isotherm measurements the sorption properties of different AlPO4-5 or SAPO-5 samples, for sorbate species characterized by a large range of molecular size: D2, 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 addition, 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 clearly shows that, as a consequence of the one-dimensional character of 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 accepted value of the internal diameter of the AlPO4-5 micropore diameter, ⌀M = 7.3 Å. Moreover, an interesting phase transition phenomenon has been observed during the sorption of krypton and methane, a result that can be the consequence of the good parametric agreement between such sorbate species and the AlPO4-5 inner surface.

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