PREPARATION AND CHARACTERIZATION OF THIN ZEOLITE MFI MEMBRANES ON POROUS SUPPORTS

Thin (2-7 mu m) polycrystalline randomly-oriented zeolite MFI membrane s were prepared on an alpha-Al2O3 support by one or more subsequent hy drothermal treatments. Different particle sizes (275-700 nm) in the la yer were achieved by changing the synthesis temperature (371-459 K). Z eolite membranes, prepared by two subsequent hydrothermal treatments h ave an optimum quality. Membranes prepared by a single hydrothermal tr eatment have defects and zeolite membranes obtained by three or more h ydrothermal treatments become too thick and crack during the removal o f the template. The zeolite MFI membranes were characterised by permea tion measurements using single gases and mixtures of n-butane (R-C4H10 ) and isobutane (i-C4H10) Permselectivities, F-alpha, and separation f actors, alpha, for n-C4H10/i-C4H10 decrease strongly with an increasin g number of defects and hence provide a good indication of membrane qu ality. The separation, on the other hand, of a CH4/n-C4H10 mixture at high occupation of n-C4H10 is not much affected by small defects. This is ascribed to blocking of those defects by capillary condensation of the butane. Flux values decrease linearly with increasing zeolite cry stallite size in the top layer indicating that the effective membrane thickness is only one particle thick. During hydrothermal treatment ze olite particles also grow at the other side of the support but do not decrease significantly the flux through the supported zeolite membrane . (C) 1998 Elsevier Science B.V.