Separating nitrogen/methane on zeolite-like molecular sieves

Inverse gas chromatography has been used to determine rapidly the specific retention volumes and isosteric enthalpies of adsorption of N2 and CH4 on the silicoaluminophosphates SAPO-18 and SAPO-34, and the titanosilicate ETS-4. In the SAPOs studied, specific retention volumes and enthalpies of adsorption are found to be controlled by pore structure and framework composition, respectively. The SAPOs will separate nitrogen/methane mixtures, but their selectivity favours methane over nitrogen. The inclusion of extra-framework metal cations, as found in ETS-4 and the zeolite clinoptilolite, are likely to be important in obtaining the desired (reverse) selectivity.