The recovered silicalite and ZSM-5 nanosheets were redispersed in DI water by alternated stirring and sonication and peptized by HNO3 at pH of 3 to 3.5 for 2 hours. The suspension was then added with HPC solution under rigorous stirring to form a stable translucent sol, which had 0.2 wt % of zeolite nanosheets and 0.1 wt % of HPC. The HPC was used as binder or DCA for preventing peeling of the multilayer nanosheet coating during drying and calcination. The zeolite membrane layer was dip coated on a homemade α-alumina disc, which was 1.2 mm thick and 2.5 cm in diameter with average pore size and porosity of ~100 nm and 33 ± 3%, respectively. The surface of the disc for coating the membrane was finely polished with #800 sandpaper. Dip coating of the zeolite film was accomplished by bringing the suspension surface to the horizontally held disc surface and contacting for 5 s. The dip-coated membrane layer was dried in an oven at 40°C for overnight and then calcined at 500°C in air for 6 hours to remove the SDA and strengthen the film consisting of multiple layers of zeolite nanosheets. The dip-coating and calcination processes were repeated once to ensure the elimination of pinholes or uncovered spots in the zeolite film. After firing at 500°C, the zeolite membrane layer was loaded with a dilute zeolite synthesis solution by dipping the zeolite-coated surface into the solution for 10 s. The precursor-loaded membrane disc was subsequently treated in the vapor of 1 M TPAOH solution at 190°C for 6 hours under autogenous pressure. The vapor phase–treated membrane was dried and calcined in air at 500°C for 6 hours. The heating and cooling rates used in the calcination were 0.5°C/min. A ZSM-5 membrane was also synthesized on the porous alumina disc by the conventional in situ crystallization method following a previously reported procedure (32). The precursor had a molar composition of 0.055 SiO2:0.0058 NaOH:0.017 TPAOH:0.92 H2O, and hydrothermal reaction was performed at 180°C for 4.5 hours under autogenous pressure. The resultant MFI zeolite membrane was washed, dried, and then calcined in air at 500°C for 8 hours for SDA removal. During the hydrothermal treatment, alumina dissolved slightly into the highly alkaline solution. Thus, both the zeolite crystals formed on the substrate and those formed in the liquid phase contained framework aluminum, which was experimentally quantified.

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