Permeation properties of bi-layered Al-ZSM-5/Silicalite-1 membranes

The permeation properties of bi-layered Al-ZSM-5/Silicalite-1 membranes were studied using nitrogen/n-hexane permporosimetry, hydrogen/sulphur hexafluoride single gas permeation/mixture separation and n/i-pentane and p/o-xylene mixture separation. At 25 °C and in the 200 kPa feed-t-100 kPa permeate permeation mode hydrogen/sulphur hexafluoride permselectivities increased with the Al-ZSM-5 layer and decreased with the Silicalite-1 layer thickness. This behaviour was not related to defects but rather to different molecular interactions according to the chemical compositions of the membranes. Differences in hydrogen/sulphur hexafluoride permselectivities and separation factors were not as pronounced in most of the membranes for permeation measurements at 105 °C and in the 100-to-1 kPa permeation mode. The membranes composed of Al-ZSM-5 on Silicalite-1 layers were of better quality than membranes with the reverse order of layers according to permporosimetry measurements. The best membrane of this type had a hydrogen/sulphur hexafluoride ideal permselectivity and mixture separation factor of 210 and 97, respectively, at 105 °C. It achieved separation factors of 8 and 5 for the separation of 1:1 mixtures of n/i-pentane and p/o-xylene, respectively. All membranes deteriorated in quality after calcination at temperatures ⩾ 450 °C in air in comparison to the initial calcination at 400 °C in a nitrogen atmosphere with trace amounts of air.