Effect of isomorphous metal substitution in zeolite framework on pervaporation xylene-separation performance of MFI-type zeolite membranes

MFI-type zeolite membranes have shown good selectivity for separation of p-xylene from its isomers. The major problem with the MFI-type zeolite membrane is that the MFI-type zeolite framework loses its size/ shape selectivity under high loadings of p-xylene because of the significant framework distortion experienced by the pore structure and as a result observed high selectivities are not stable over time. This paper proposes changing the interaction of the xylene isomers with MFI-type framework to address this problem. MFI-type zeolite membranes with aluminum and boron isomorphously substituted into the framework were synthesized and subjected to multicomponent xylene separation via pervaporation. It is found that by performing this substitution, slight changes to both surface chemistry and framework flexibility can be introduced. Essentially, the interaction of the xylene molecules with the MFI structure is modified to limit p-xylene loading, as well as diffusion pathway access to o-xylene. As a result improvement in xylene separation performance over silicalite was observed. The boron-substituted membranes demonstrated the highest selectivities for p-xylene under a wide range of feed compositions; the highest selectivity observed was ̃55 (feed, 5% p-xylene; 95% o-xylene). This is higher than any previously reported xylene separation selectivity for pervaporation through MFI-type zeolite membranes. However, the performance stability of substituted membranes over time was also investigated, and it was found that, over a period of 96 h, a reduction in selectivity of about an order of magnitude was observed.