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.