Solution‐processable exfoliated zeolite nanosheets purified by density gradient centrifugation

Abstract

Highly crystalline exfoliated MFI‐nanosheets can pave the way for large‐scale deployment of sub‐500‐nm zeolite membranes due to their processing and packing advantages. Exfoliated MFI‐nanosheets prepared by melt compounding contain a large amount of polymer and unexfoliated particles which are detrimental to the fabrication of ultrathin zeolite membranes. Complete removal of polystyrene from the nanosheet suspension in toluene is demonstrated by centrifugation of the suspension across chlorobenzene as confirmed by thermogravimetric analysis (TGA) data and transmission electron microscopy (TEM) images. Rate‐zonal centrifugation in a nonlinear density gradient fractionated exfoliated MFI‐nanosheets from unexfoliated particles. The purified nanosheets were highly crystalline as indicated by high‐resolution TEM (HRTEM) and electron diffraction (ED). Coating of purified MFI‐nanosheets on a smooth α‐alumina support, fabricated by filtration of α‐alumina suspension, led to a compact, b‐oriented, 80‐nm‐thick film. A mild hydrothermal treatment of the film led to a 200‐nm‐thick membrane, which demonstrated molecular sieving properties. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3458–3467, 2013

2D Zeolite Coatings: Langmuir–Schaefer Deposition of 3 nm Thick MFI Zeolite Nanosheets

“…Before acid treatment, the nanosheets contained about 29 wt % OSDA, the majority of which is expected to reside inside their straight pore channels. [14] After acid treatment, the amount of OSDA reduced to less than 8 wt %. The partial removal of OSDA was also indicated by a color change of the solid nanosheet material from yellow to white.…”

Zeolite membranes – a review and comparison with MOFs

“…9 Recent reports of 100-200 nm zeolite membranes are promising advances for their commercial viability. 10,11 There have been some recent industrial projects that use zeolite membranes, such as for ethanol dehydration, but so far these projects have been limited. A major constraint has been the low flux and high cost, which make the required membrane area to be economically unviable.…”

“…The permeances are consistent with the previous report on 200 nm-thick MFI membranes made from nanosheet seeds, but grown using conventional solution-based growth. 10 However, the earlier separation factor of 25-45 for p-xylene/ o-xylene is greatly improved (to 4100) by the use of gel-free secondary growth. Other efforts for ultrathin MFI membranes are discussed below.…”

Preparation of Al2O3 Supports for Thin Membrane Fabrication

“…A smoother surface with less amount of roughness can be obtained by vacuum filtration method in which colloidal processing is used (15). High throughput thin membranes necessitate high flux membrane support with homogeneous surface properties, uniform pore size and macro-defect free surface.…”

“…Dry pressing, tape casting, and extrusion are the generally used methods for the preparation of ceramic membrane supports, however colloidal processing (slip casting/vacuum-assisted filtration) of statically or sterically stable suspensions allow the fabrication of macroporous supports with appropriate surface properties (14,15). A smoother surface with less amount of roughness can be obtained by vacuum filtration method in which colloidal processing is used (15).…”