(307c) Bi-Functional MFI Zeolite Membranes for Meta-Xylene Isomerization

  Xylene isomerization is an important step in the industrial production of para-xylene (PX), which is a main chemical stock for synthesis of polyester resins and fibers. Recent studies showed that MFI-type zeolite membrane could construct a membrane reactor for xylene isomerization with improved PX yield by selective separation of PX through the membrane. A highly-efficient membrane reactor for xylene isomerization requires immediate and effective removal of PX from reaction system. It is reported that α-alumina substrates supported silicalite membranes could result in aluminum incorporated into zeolite framework during membrane synthesis. The aluminum-incorporated MFI zeolite layer could show catalytic activity for xylene isomerization after H⁺ ion exchange. Thus, the ion-exchanged membranes would have two functions with combination of separation and catalysis. In this work, we will demonstrate that the bi-functional MFI zeolite membranes exhibited very high PX selectivity for xylene isomerization.

   MFI (silicalite-1) zeolite membranes were synthesized on α-alumina substrates by in-situ hydrothermal crystallization. The separation of p-/o-xylene (PX/OX) mixture was used to evaluate the quality of MFI zeolite membranes, which showed PX permeance of 2.58×10^-8 mol/m²·s·Pa and PX/OX separation factor of 19.3 at 300 ^oC. The MFI zeolite membranes were ion exchanged with 1 M NH₄Cl aqueous solution at 80 ^oC for 24 h. Afterwards, the as-made membranes were calcainted at 500 ^oC for 5 h. EDS analysis of the original membrane showed that aluminum element coming from substrates was incorporated into zeolite structure during membrane synthesis. The effects of operation temperature, feed flow rate and sweep flow rate for m-xylene (MX) isomerization were investigated. High PX selectivity of 90.8 % with MX conversion of 7.67 % for MX isomerization was achieved at 300 ^oC on the self-catalytic membranes. It was observed that the membrane contribution to xylene isomerization increased with the increasing sweep flow rate. We also found that the PX yield were much higher when MX was fed from the substrate side, which was because of the more catalytically active sites at the intermediate layer between zeolite layer and substrate.