MFI-type zeolites with their three-dimensional channel network are strong candidate materials for separation of straight chain and aromatic hydrocarbons. They have captured the main focus in the research area of zeolite membranes. High permselectivity has been observed in the separations of gases, close-boiling hydrocarbons, along with butane and xylene isomers using MFI zeolite membranes. Recently, the influence of membrane microstructure (i.e. membrane thickness, grain size, morphology and film orientation) on gas permeation properties of MFI-type membranes has been reported. It is noted that membrane chemistry also plays an important role on the transport properties of the membrane. The objectives of this study are to investigate the effects of membrane chemistry and low temperature...[ Read more ]

MFI-type zeolites with their three-dimensional channel network are strong candidate materials for separation of straight chain and aromatic hydrocarbons. They have captured the main focus in the research area of zeolite membranes. High permselectivity has been observed in the separations of gases, close-boiling hydrocarbons, along with butane and xylene isomers using MFI zeolite membranes. Recently, the influence of membrane microstructure (i.e. membrane thickness, grain size, morphology and film orientation) on gas permeation properties of MFI-type membranes has been reported. It is noted that membrane chemistry also plays an important role on the transport properties of the membrane. The objectives of this study are to investigate the effects of membrane chemistry and low temperature templates removal methods on the permeation properties of MFI zeolite membranes.

Supported zeolite membranes with engineered microstructure and tailored chemistry were prepared using a novel synthesis method. This technique involved seeding of colloidal zeolites on the substrate followed by hydrothermal synthesis to minimize inter-crystalline porosity. Applying different membrane growth sequence, the non-uniform spatial distribution of framework ion, aluminum, ZSM-5 membrane was located at different positions of the 3-layer MFI membranes could be tailored/optimized. The results were illustrated that the gas permeance was independent of the uniform and non-uniform spatial distribution of aluminum along the membrane thickness. The three-layers, non-uniform spatial distribution of aluminum MFI membranes showed a comparable gas permeance and separation behavior to the uniform single layer aluminum distributed MFI membrane. However, the permselectivity of butane isomers is sensitive to the aluminum content in the membrane. Higher permselectivity of butane isomers is observed in aluminum rich ZSM-5 membrane when compared with silicon rich Silicalite-1 membrane.

Consecutive ion exchanges with different ions were carried out successfully on the same ZSM-5 membrane. The permeation behavior of ion-exchanged ZSM-5 exhibited a strong dependency on the substituted counterions. The presence of larger counterions led to slower gas permeance. The n-butane/iso-butane ideal selectivity varied with different size of the substituted counterions in ZSM-5 membrane

The substrate used also affected the selectivity of zeolite membrane. MFI zeolite membrane grown on porous γ-alumina support selectively separated hydrogen from a commercial fuel mixture (i.e., Towngas: 49.7 % H₂, 27.7 % CH₄, 19.5 % CO₂ and 3.1% CO) whereas MFI membrane grown on porous α-alumina support preferentially separated CH₄ and CO₂. The ideal separation of butane isomers was different when using porous γ-alumina and α-alumina support membranes.

In addition to the membrane chemistry and microstructure, the removal of organic templates from zeolite pores after the membrane synthesis also affected the transport properties of zeolite membrane. In this study, stepwise oxygen plasma method and ozone treatment at low temperature were used instead of traditional air calcination at high temperature. The pemeance of permanent gases (i.e., He, H₂, N₂ and Ar) and hydrocarbons (i.e., CH₄, n-butane and iso-butane) displayed a strong dependence on the extent of removal of organic templates in zeolite pore and method used.