Molecular sieve films: New synthesis methods for improving the performance of zeolite ZSM-5 membranes
The separation performance of MFI type of zeolite membranes for gas and vapor permeation has been improved in this dissertation in several ways.
In the h0h-oriented membranes, defects were sealed by a sol-gel technique, while in the c-oriented MFI membranes, defects could be sealed in situ by introducing a third strongly adsorbing species in the feed stream. In both cases, the selectivity for separation of xylene isomers was greatly improved. However, the concomitant decrease in permeance indicated that the membrane performance was only partially improved by these methods.
A novel seeded growth method was proposed which allowed us to fabricate the first functional b-oriented ZSM-5 membranes with quality good enough to demonstrate separation of mixtures mainly based on molecular sieving effects. Membrane microstructure was optimized in this seeded growth process to be thin, less defective and b-oriented by strategies such as the use of asymmetric supports, oriented seed monolayers and appropriate structure directing agents. Although the process was proposed mainly for MFI type of membranes, it is anticipated that those idea can be extended to the preparation of other types of zeolite membranes.
The microstructurally optimized b-oriented ZSM-5 membrane was applied for the separation of several close-boiling hydrocarbon mixtures, such as xylene isomers, benzene/cyclohexane, and benzene/2,2-dimethylbutane. Significant improvement in both permeance and separation factor was achieved compared to the results reported in the literature.