Influence of the Si/Al-ratio on the permeation properties of MFI-membranes

The most frequently prepared molecular sieve membrane type is the Al-free silicalite MFI-membrane. For this membrane type high separation factors are reported for mixtures of technical importance as the xylene-isomers, hydrogen-alkanes or n/i-hydrocarbons. Other molecular sieve membranes as types LTA, ERI, FAU show small separation factors in shape selective separations, except the separation of water-solvent mixtures, which are based on hydrophilic/hydrophobic interactions rather than on molecular size exclusion. For MFI-type membranes the introduction of Al in the crystal lattice is possible in a wide range: from Si/Al ∼ 30 for the ZSM-5 type up to Si/Al ∼ ∞ for the isomorphous silicalite type. The enhancement of the Al-content in MFI-crystals of a membrane changes its hydrophilic/hydrophobic permeation properties, the stability against acids, the concentration of intercrystalline defect pores and the catalytic activity by the Brönsted centres created. In this work a systematic variation of the Si/Al-ratio was done and the membranes were characterized by single gas permeation, by permporosimetry and by mixture gas permeation. With increasing Al-content of the membrane, i.e. decreasing Si/Al-ratio, water permeance and water selectivity increase but simultaneously increasing concentrations of intercrystalline defect pores are formed. As a result of the interplay of these effects the water selectivity passes a maximum as a function of the Si/Al-ratio. The influence of Al-containing supports and of the template content of the synthesis gel on the Si/Al-ratio in both the membrane layer and in the free sediment crystals were investigated by EDX and ICP/OES. By dissolution of Al especially from γ and to a lower extent from α-alumina supports, Al-containing ZSM-5 membranes are formed from Al-free silicalite batches.