Isomorphously substituted B-MFI hollow fibre membranes for p-xylene separation from C8 aromatic mixtures

In this study, Al-MFI and B-MFI (Si/B = 100 and Si/B = 50) hollow fibres with nanocomposite architecture have been prepared on α-alumina by pore-plugging hydrothermal synthesis at 443 K for 89 h using a precursor clear solution with molar composition 1 SiO2: 0.45 TPAOH: 27.8 H2O: xH3BO3 (x = 0–0.02). The synthesized materials were characterized by XRD, SEM, ICP-AES and 29Si, 27Al- and 11B-MAS-NMR, revealing the genesis of well-intergrown materials with isomorphously substituted boron in the MFI unit cell. The pure ethylbenzene permeance within these membranes decreased in the order Al-MFI > B-MFI (Si/B = 50) > B-MFI (Si/B = 100). All the MFI materials were selective to p-xylene in the vapour permeation of ternary p-/m-/o-xylene mixtures and quaternary p-/m-/o-xylene/ethylbenzene model mixtures in the temperature range 400–650 K and for total feed vapour pressures lower than 4.5 kPa. The p-/m-xylene separation factor increased in the sense Al-MFI < B-MFI (Si/B = 100) ≈ B-MFI (Si/B = 50). The p-/m-xylene and p-/o-xylene separation factors, as well as p-xylene mixture fluxes, decreased with the ethylbenzene feed concentration, probably due to configurational entropy effects in the mixture adsorption patterns. The membranes showed however slightly preferential p-xylene permeation than ethylbenzene at low total aromatic vapour pressures despite the similar kinetic diameter of both molecules (5.8 vs. 6.0 Å), the B-MFI (Si/B = 50) material achieving a p-xylene/ethylbenzene separation factor as high as 5.

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► We have prepared isomorphously substituted B-MFI (Si/B = 50–100) hollow fibres.
► The materials display a nanocomposite architecture instead of forming a thin film.
► The p-/m-xylene separation factor evolves: Al-MFI < B-MFI(Si/B = 100) ≈ B-MFI(Si/B = 50).
► B-MFI (Si/B = 50) membranes show p-xylene/ethylbenzene separation factors as high as 5.