Separation of Xylene Isomer Vapors with Oriented MFI Membranes Made by Seeded Growth

Abstract

Oriented MFI membranes made by secondary growth on the surface of α-Al 2 O 3 support disks have been evaluated for the separation of xylene isomers vapors in the temperature range 22−275 °C and feed partial pressures up to ∼0.7−0.9 kPa. It is found that the separation performance of these membranes is directly related to the synthesis conditions and the resulting membrane microstructure. Thick (12−18 μm) and c -oriented membranes (type A membranes) made by 24-h growth at 175 °C exhibit single-component p -xylene/ o -xylene permselectivity as high as 150 at 100 °C but very modest separation factors (usually <5) are obtained with binary feed mixtures, a result of a drastic increase in the flux of o -xylene in the presence of p -xylene. Thin (2−3 μm) and ( h 0 h )-oriented membranes (type B membranes) made by 120-h growth at 90 °C exhibit comparable single-component and binary permeation behavior but suffer from lower selectivities/separation factors (up to 12) as a result of cracks formed after calcination for template removal. It is found that the addition of n -hexane in a binary p -xylene/ o -xylene feed in the case of type A membranes or the dip-coating of type B membranes with a surfactant-templated silica sol results in a considerable improvement of mixture separation factors up to 60 or 30−300, respectively. This improvement is attributed to preferrential adsorption of n -hexane in nonzeolitic micropores/grain boundaries in the case of type A membranes or to selective sealing of cracks by mesostructured silica in the case of type B membranes. Both type A and type B membranes exhibit, at 100−125 °C, p -xylene permeance [(2−5) × 10 -8 mol m -2 s -1 Pa -1 ] and p -xylene/ o -xylene separation factors (60−300) that are comparable to or higher than the values reported for other MFI membranes in the literature.