* The profile is defined as; initial temperature/time
→ final temperature/time
The XRD patterns of synthesized membranes listed in Table 5 are shown in
Fig. 7. These patterns indicate high crystallinity of zeolite ZSM-5
formed on the surface of tubular alumina supports. As expected, starting
the synthesis at low temperature reinforces the nucleation reactions
against the crystal growth reactions. So, a larger number of nuclei are
produced. In the following, increasing temperature, immediately, shifts
the dominant reactions from nucleation to growth reactions. So, at the
end of synthesis, a larger number of smaller crystals compared to those
formed in isothermal synthesis, are produced. It may also reduce the
thickness of the zeolite layer, but, produce a more integrated and
uniform layer with eliminated inter-crystalline gaps. The results
confirm that for a constant total synthesis time, the longer the
exposure time to higher temperatures, the higher the crystallinity, the
thicker the zeolite layer, and less inter-crystalline gaps.