Separation of xylene isomer vapors with oriented MFI membranes made by seeded growth

Oriented MFI membranes made by secondary growth on the surface of alpha -Al 2O3 support disks have been evaluated for the separation of xylene isomers vapors in the temperature range 22-275 degreesC and feed partial pressures up to similar to0.7-0.9 kPa. It is found that the separation performance of these membranes is directly related to the synthesis conditions and the re sulting membrane microstructure. Thick (12-18 mum) and c-oriented membranes (type A membranes) made by 24-h growth at 175 degreesC exhibit single-comp onent p-xylene/o-xylene permselectivity as high as 150 at 100 degreesC but very modest separation factors (usually <5) are obtained with binary feed m ixtures, a result of a drastic increase in the flux of o-xylene in the pres ence of p-xylene. Thin (2-3 <mu>m) and (h0h)-oriented membranes (type B mem branes) made by 120-h growth at 90 degreesC exhibit comparable single-compo nent and binary permeation behavior but suffer from lower selectivities/sep aration factors (up to 12) as a result of cracks formed after calcination f or template removal. It is found that the addition of n-hexane in a binary p-xylenelo-xylene feed in the case of type A membranes or the dip-coating o f type B membranes with a surfactant-templated silica sol results in a cons iderable improvement of mixture separation factors up to 60 or 30-300, resp ectively. This improvement is attributed to preferrential adsorption of n-h exane in nonzeolitic micropores/grain boundaries in the case of type A memb ranes or to selective sealing of cracks by mesostructured silica in the cas e of type B membranes. Both type A and type B membranes exhibit, at 100-125 degreesC, p-xylene permeance [(2-5) x 10(-8) mol m(-2) s(-1) Pa-1] and p-x ylene/oxylene separation factors (60-300) that are comparable to or higher than the values reported for other MFI membranes in the literature.