Twin-free, directly synthesized MFI nanosheets with improved thickness uniformity and their use in membrane fabrication

Abstract

Zeolite nanosheets can be used for the fabrication of low-defect-density, thin, and oriented zeolite separation membranes. However, methods for manipulating their morphology are limited, hindering progress toward improved performance. We report the direct synthesis (i.e., without using exfoliation, etching, or other top-down processing) of thin, flat MFI nanosheets and demonstrate their use as high-performance membranes for xylene isomer separations. Our MFI nanosheets were synthesized using nanosheet fragments as seeds instead of the previously used MFI nanoparticles. The obtained MFI nanosheets exhibit improved thickness uniformity and are free of rotational and MEL intergrowths as shown by transmission electron microscopy (TEM) imaging. The nanosheets can form well-packed nanosheet coatings. Upon gel-free secondary growth, the obtained zeolite MFI membranes show high separation performance for xylene isomers at elevated temperature (e.g., p -xylene flux up to 1.5 × 10 −3 mol m −2 s −1 and p -/ o -xylene separation factor of ~600 at 250°C).

Twin suppression effect of dihydroxy-benzene isomers during the secondary growth of b-oriented zeolite MFI nanosheet films

“…Very recently, the preparation of thickness-improved MFI nanosheets without large nanocrystals was realized by using nanosheet fragments as seeds. 40 Future work should explore using these twin-free MFI nanosheets in combination with C 6 H 6 O 2 additives towards the complete elimination of twins.…”

MXenes and Other Two-Dimensional Materials for Membrane Gas Separation: Progress, Challenges, and Potential of MXene-Based Membranes

“…However, the fragmentation during the exfoliation process limits the in-plane dimension below 1 μm. Some progress to address this issue has recently been reported . In this study, the direct synthesis of MFI nanosheets by using zeolite flakes as the seeds advanced the uniformity of the MFI membranes and allowed for better control over the thickness of their membranes.…”

Layered Zeolite for Assembly of Two‐Dimensional Separation Membranes for Hydrogen Purification

“…The small sieving pores (6‐MR) endowed the 2D SOD membrane with high separation selectivity, whereas gas permeance was unsatisfactory because of the narrow transport pore channels. The transport channels in MFI membranes (10‐MR) were mostly used for xylene isomer separation rather than small‐sized gas separation because of their larger pore sizes (5.3 Å×5.6 Å) [27] . The blank of suitable 2D zeolite membranes for purifying H 2 can be addressed by developing 2D medium‐aperture membranes composed of 8‐MR zeolites.…”