Vu A. Tuan

Isomorphous substitution of Al, Fe, B, and Ge into MFI-zeolite membranes

Abstract ZSM-5 zeolite membranes with Al, Fe, B, or Ge substituted for Si in the framework structure were prepared by in situ crystallization onto porous stainless steel supports. Their single-gas and mixture separation properties were compared to a silicalite-1 membrane. For all membranes, single-gas permeances at 473 K showed a decreasing trend as the kinetic diameter of the diffusing molecule increased. All membranes separated n -C 4 H 10 / i -C 4 H 10 , n -C 4 H 10 /H 2 , and H 2 / i -C 4 H 10 mixtures, and the selectivities depended on temperature and the substituted cation, but no trend with acidity or hydrophobicity was observed. For most separations studied, the substituted membranes had higher selectivities than a silicalite-1 membrane, and the B-ZSM-5 membrane had the highest selectivities. The n -C 4 H 10 / i -C 4 H 10 separation was due to differences in diffusion rates and adsorption coverages, and the highest selectivity was 45. The n -C 4 H 10 /H 2 separation had a maximum value of 110 at 300 K, and separation was due to preferential adsorption of n -C 4 H 10 , which permeated faster than H 2 . For H 2 / i -C 4 H 10 mixtures, however, H 2 permeated faster, but i -C 4 H 10 adsorption reduced the H 2 permeation below the single-gas value so the mixture selectivity was below the ideal value.

Gas permeation properties of ion-exchanged ZSM-5 zeolite membranes

Hydrothermally synthesized, ZSM-5 zeolite membranes (Si/Al=25 and 600) were ion exchanged with H+, Na+, K+, Cs+, Ca2+ and Ba2+ cations, and the membranes were stable after exchange. Their gas permeation properties were measured over the temperature range of 323–523 K. Both the Si/Al ratio and the exchanged ion size affected the separation performance. For the membrane with a Si/Al ratio of 600, ion exchange only changed the single gas permeances of i-C4H10, apparently because the number of exchange sites was small. For the membrane with Si/Al=25, single gas permeances increased for the exchanged forms in the order: K+<Ba2+∼Ca2+<Cs+<Na+∼H+, which coincides with the decrease in ion size and only the Cs ion does not fit this trend.

Pervaporation of aqueous organic mixtures through Ge-ZSM-5 zeolite membranes

Separation factors for pervaporation of 5 wt% organic/water mixtures through Ge-ZSM-5 zeolite membranes are strongly correlated with the liquid-phase fugacities of the organic components in the feed. Competitive adsorption coverages probably depend on fugacity. This correlation allows qualitative prediction of zeolite membrane performance and is an important advancement since it is possible to group the separation factors by functionality of the organic component. Therefore, we now have a means to determine the approximate range of separation factors for various mixtures.

Highly selective separation of n-hexane from branched, cyclic and aromatic hydrocarbons using B-ZSM-5 membranes

Boron-modified ZSM-5 zeolite membranes were effective for separating n-alkanes from 2,2-dimethylbutane, benzene and cyclohexane in binary mixtures, and enhanced selectivity was seen when both benzene and cyclohexane were mixed with n-hexane.

Preparation and pervaporation properties of a MEL-type zeolite membrane

A boron-substituted ZSM-11 (Si/B = 100, MEL structure) membrane was prepared and shown to selectively permeate methyl ethyl ketone, propan-1-ol, and propan-2-ol from aqueous solutions.