K. Maeda, Y. Kiyozumi, and F. Mizukami
[J. Phys. Chem. B, Vol.101, pp.4402-4412, 1997]

Microporous aluminum methylphosphonates, AlMepO-alpha and -beta, prepared by different procedures were characterized mainly using ²⁷Al, ³¹P, and ¹³C MAS-NMR, TG-DTA, and IR. The MAS-NMR spectra were consistent with the crystal structures determined by the single crystal X-ray structural analysis published previously. All the ³¹P NMR signals were reasonably assigned using an assumed correlation between ³¹P chemical shift and the mean Al-O-P angle around the phosphorus sites. Nitrogen adsorption isotherms of AlMepO-beta degassed at elevated temperatures were of type I, while those on AlMepO-alpha gave two plateaus in the low relative pressure region. The stepwise adsorption was explained by a packing change of the adsorbate on adsorption.
The pore diameter calculated from the maximum nitrogen adsorption capacity was consistent with adsorption of 2, 2-dimethylpropane but was larger than the size expected from the crystal structure of both the compounds. The water vapor isotherm was type II in the low relative pressure region confirming the hydrophobic nature for both compounds. AlMepO-alpha indicated no apparent adsorption of water into the channel, but AlMepO-beta showed a sudden adsorption of water at P/P_o = 0.7. The difference in the water vapor isotherms between the compounds was explained based on relationship of the size of water clusters formed and the shape of the adsorbent channel.