Separation of C6 isomers by vapor permeation and pervaporation through ZSM-5 membranes

An alkali-free synthesis method (Si/Al=600 in the gel) was used to prepare H-ZSM-5 zeolite membranes supported on porous stainless steel and alumina tubes. These membranes separated mixtures of C6 isomers with high selectivities by both vapor permeation and pervaporation. The single gas and mixture permeances were similar for vapor permeation through the stainless steel-supported membrane, indicating that separation was due to shape selectivity; the linear molecule diffused faster. The n-hexane/2,2 dimethylbutane separation selectivity went through a maximum greater than 1000 as the temperature increased, and the membrane separated the isomer mixture to at least 575 K. The alumina-supported membrane had higher permeances and similar mixture selectivities, but both preferential adsorption and shape selectivity contributed to the high selectivity for vapor permeation. The selectivities for pervaporation were lower than for vapor permeation in the stainless steel supported-membrane, the selectivity dependence on temperature was different, and the fluxes were higher and increased significantly with temperature. Membranes evaluated over 9 months showed reproducible fluxes and selectivities.