Nearly all of the aluminophosphate molecular sieves with pore sizes from 0.4 to 0.8 nm
(8-, lo-, or 12-rings) exhibit a characteristic adsorption pore volume for oxygen
(and in some cases hydrocarbons) that is only 50-40% that of the water pore volume. This
requires that their framework structures contain both large-pore voids and voids accessible through small pores (6-rings) which admit only water. In the case of the
known A1PO4-5 structure, the excess water volume is associated with small voids outlined by columns of twisted chains of 4-rings and 6-rings parallel to the c axis.