3.4.1 L ocalized zeolite grow th
Direct regrowth of zeolite crystals onto the silicon substrate can provide better adhesion, which can withstand cracks or delamination under high temperature
treatment. Localized zeolite growth was developed to selectively incorporate zeolite catalyst layers within the microchannel.
Using this method, zeolite growth is confined within the microchannel. This was achieved through surface modification using chemical functionalization and seeding to locally promote the growth o f the zeolite layer (Chau et al, 2000, Wan et al, 2001). Mercapto-3-propyltrimethoxysilane was introduced in the microchannel, which was then seeded with colloidal zeolite using micropipette (figure A). The seeding procedure was repeated three times to obtain the desired seed population o f (5 X lO'^seeds/m^). The seeds were then calcined in air at 823 K for 6 hours, prior to
the hydrothermal growth o f the zeolite layer. The etched microreactor pattern was then placed on a specially designed Teflon holder such that the pattern was facing downwards (figure B). The other side facing upwards was wrapped with Teflon tape to shield from the synthesis solution. After the hydrothermal treatment, the silicon sample was rinsed with deionized distilled water and dried overnight. It was then calcined in air at 823K (2K/min ramp) for 24 hours to remove the organic template and activate the catalyst.
\ u
Br\
Confined seeding Zeolite growth Final product
I I Silicon Zeolite i i Teflon
Figure 3.3 Process diagram for zeolite incorporation within microchannel
3.4.2 Preparation of TS-1 synthesis solution
The “Mixed alkoxides” method is adopted to prepare the TS-1 synthesis solution, which is based on the original procedure developed by researchers in Enichem (Taramasso et al., 1983). The schematic diagram in figure 3.4 describes the steps for preparing the synthesis solution. The procedure starts with the hydrolysis of tetraethyl orthotitanate (TEOT) and tetraethyl orthosilicate (TEOS) mixtures. Extra care should be taken to make sure that all apparatus were completely dry as the presence of water would hydrolyse the titanium compounds to form hydrated TiO]. In step 1, TEOS was first added into a Teflon vessel, which was pre-rinsed with ethanol. Together with the sealed bottle of TEOT, they were placed in the nitrogen hood and purged for an hour to prevent carbon dioxide and water absorption (Step 2). TEOT was then added into the TEOS solution (Step 3). The TEOT/TEOS mixture should change to light yellow to obtain depolymerisation o f the titanate oligomers that may be present in TEOT. The yellow colour came from partially hydrolysed TEOT, however, full hydrolysation will result in precipitation of amorphous TiO]. The above steps should be repeated if a colourless or white mixture was obtained indicating formation of TiOi and the synthesis has failed (van der Pol and van Hoff, 1993).
Mixture of tetrapropylammonium hydroxide (TPAOH) and deionized water was then added gradually to complete the hydrolysis (Step 4). The mixture should be added dropwise so that a clear solution was formed. This was to ensure that there were enough silanol groups for the condensation reaction to occur with the monomeric hydrolysed titanium species (Perego et al, 1998). After vigorous stirring for another hour, the mixture was heated at 80°C water bath for 3 hours to aid the hydrolysis (Step 5). The volume of the synthesis mixture was maintained by adding deionized water (Step 6). The obtained clear solution was then stirred overnight for
hydrothermal treatment. Table 3.1 lists the standard compositions o f TS-1 synthesis solutions used for the formation of colloidal seeds and catalyst layer.
It should be noted that the presence of metal ions in the TS-1 synthesis solution would inhibit the incorporation of titanium ions into the zeolite framework (Perego et al, 1998). All the glassware and Teflon vessels should be cleaned with acid solution ( H N O 3 ) and dried completely before preparation.
Synthesis solution TEOS TPAOH TEOT H2O
TS-1 colloidal seeds 2 0 9 0.75 404
TS-1 regrowth solution for catalyst layer 40 1 0 1 . 6 1 0 0 0 0
Table 3.1 Composition of TS-1 synthesis solutions
N2
■=1---V L ■■■ 1
Step 1 TEOS was added into the Teflon vessel.
N-
1
1 0
0
0
Step 3 TEOT was then added and the mixture continued to be stirred for another hour. The colour should change to light yellow.
O
O
Step 5 The mixture was heated at 80°C in a water bath and stirred continuously for three hours. The water level on the Teflon bomb was marked before heating.
Step 2 The Teflon vessel with TEOS was stirred under nitrogen for at least 1 hour. A sealed bottle o f TEOT was kept in the nitrogen hood at the same time.
f
O
Step 4 Mixture o f TPAOH with H2O2
was added at Iml/min into the synthesis solution after removal from the nitrogen hood. It was then stirred for an hour to obtain a clear solution.
O
O
Step 6 Deionized water was then
added back to the mark after heating. The synthesis solution was then stirred overnight before hydrothermal treatment.
Figure 3.4 Preparation of TS-1 synthesis solution
3 . 4 . 3 H y d r o t h e r m a l t r e a t m e n t o f s y n t h e s i s m i x t u r e
The schematic diagram for zeolite hydrothermal synthesis is illustrated in figure 3.5. The silicon sample was placed in a Teflon holder with the etched pattern facing downwards (figure A) to prevent any powders and precipitates from being incorporated into the growing zeolite film. The sample along with the synthesis solution was transferred into a 125 ml Teflon vessel. The synthesis solution should be 5mm above the sample holder. The Teflon vessel was sealed in a stainless steel digestion bomb where hydrothermal treatment took place at autogenous pressure. The autoclave was placed in a preheated oven at 398K for 24 hours (figure C). After each synthesis, the Teflon vessel was cleaned using IM HNO3, which was sealed in the autoclave and heat treated for another 24 hours. This was to remove all the zeolite particles attached on the vessel surface to prevent pre-seeding in the next synthesis.
A Place silicon sample B Place sample holder C Hydrothermal treatment onto specially designed into synthesis solution within autoclave
Teflon holder