2.3. Synthesis of tubular ZSM-5 membranes
Up to three layers of ZSM-5 zeolite seeds were coated to the outer surface of the tubular support using the dip-coating technique. One layer has large seeds (1.5 μm) as the primary layer, another layer has mid-size seeds (300 nm) as the intermediate layer, and the last layer has small seeds (60 nm) as the top layer. In order to better cover the support surface, the seed particles were assembled to the support using the cationic linkage technique. In which the positive charge induces to the surface of α-alumina support using a coating layer of a proper cationic polymer. The negatively charged seed crystals are adsorbed strongly on the support surface due to the electrostatic interactions and Van der Waals forces [6]. Tubular alumina supports were rinsed first with ethanol and then five times with 0.1 mol L-1NH3 solution. The supports were then immersed in a solution of 2.5 gr PDADMAC [Poly-(diallyldimethylammonium chloride), 20 wt. % in H2O, Sigma-Aldrich] in 100 mL H2O at 60 °C for 30 min while stirring. The negative surface charge of the α-alumina supports were consequently reversed to positive charge. The positively charged supports were rinsed several times in DI water and dried at ambient temperature overnight.
The seed suspensions were prepared by mixing of 2-5 g selected ZSM-5 seed with proper particle size, 0.14 g hydroxypropyl cellulose solution (Sigma-Aldrich, 0.5 wt.% HPC, MW = 100 000 g mol−1) as a binder, 94 mL DI water and followed by reducing the pH of the solution to 3 by adding a few drops of 1 M HNO3. The positively charged supports were dip-coated with the seed suspension for 5 min and were then dried for 24 h under controlled humidity (40%) and Temperature (40°C) to prevent crack formation. The dried seeded supports were then calcined in a two-step calcination process, at 450°C for 8 h and subsequently at 650°C for 8 h with the heating/cooling rate of 20 °C h−1. The above-mentioned procedure was repeated up to three times entirely using seeds with different particle sizes according to Tables 1-3. Each seeding step was followed by a sequential drying; template-removal; calcination processes. This cycle was conducted to fix the seed particles on the bed layer during each seeding stage.
The ZSM-5 tubular zeolite membranes were synthesized on the outer surface of the alumina tubular support. First, both ends of alumina tubular substrates were sealed by homemade Teflon caps. The molar composition of 5 SiO2: 0.03 Al2O3: 2.86 NaOH: 0.2 TPAOH: 0.2 TPABr: 693 H2O was used for preparation of synthesis solution. The initial reaction mixture was prepared by dissolving aluminium sulfate octadecahydrate (Aldrich, Al2(SO4)3.18H2O, 98 wt%), sodium hydroxide, tetrapropylammonium hydroxide, and tetrapropylammonium bromide (Sigma- Aldrich, TPABr) in DI water, followed by heating under reflux apparatus up to 80˚C. Fumed silica was then gradually added into the above solution under vigorous stirring until a clear solution obtained. The mixing under reflux continued in order to perform the aging process for 6 h. The temperature was kept at 80˚C during the aging step. The aged solution was then cooled down and transferred into a Teflon-lined stainless-steel autoclave in which the calcined seeded substrates were centered horizontally, in order to obtain a membrane with uniform thickness, using a proper homemade Teflon support.
According to our experiences, in the vertical position, the thickness of the zeolite layer may gradually increase from top to bottom of the membrane especially for long-term synthesis due to the sedimentation of the reaction mixture leaving its footprint on the surface of the membrane.
The hydrothermal synthesis of the ZSM-5 membranes then was conducted at different synthesis temperature/time profiles. In the constant-temperature method (CT), the temperature and time were set to 180˚C and 10 h, respectively. Whereas, in the varying-temperature method, they were varied according to Table 5. The synthesized membranes were rinsed, dried overnight at controlled humidity and temperature (40% and 40°C, respectively), and then calcined at 450°C for 8 h with a heating and cooling rate of 20°C /h to remove the organic components.