Separation of n-butane/iso-butane by self-supporting MFI membranes with various SiO2/Al2O3

The relation between the SiO2/Al2O3 of the MFI membrane and the separation property of n-butane/iso-butane was studied using the self-supporting MFI membranes synthesized with the parent sol of SiO2/Al2O3 ranged from 48 to ∞. The separation property was much affected by the SiO2/Al2O3 of membrane. Notably, the separation factor was critically increased below a certain value of SiO2/Al2O3: separation factor of n-butane/iso-butane was around 75 at 473 K with the membrane having the layer of SiO2/Al2O3 measured by electron dispersive spectroscopy (EDS) <80–125, whereas that was about 15 with the membrane having the layer of SiO2/Al2O3 > 80–125.Separation factor of p-xylene/o-xylene from ternary mixture of xylene isomer vapors was more than 218, indicating that the membranes were quite less defective. There was no remarkable difference in the amounts of adsorbed n- and iso-butanes among the membranes with different SiO2/Al2O3. However, the membrane with the high separation property required 4 times longer time for reaching the adsorption equilibrium of iso-butane than that with the low separation property at 473 K. On the other hand, the time for reaching the adsorption equilibrium of n-butane in the both membranes was quite faster compared to that of iso-butane. Thus, the diffusion of iso-butane seemed more restricted by the Na cation existing in the MFI pore than that of smaller molecule, n-butane. It was concluded that MFI membrane with lower SiO2/Al2O3 had a potential with high separation property for butane isomers.