Fig. 8: Effects of synthesis temperature/time on
N2/SF6 ideal selectivity of ZSM-5
zeolite membranes
Based on the above-mentioned discussion, proper intergrowth of small
crystals penetrating into the defects/gaps of the large seed layer is
the main factor for a successful different-sized seeding method. This
requirement depends on both nucleation and crystal grow reactions with
the priority of nucleation reactions [28]. Therefore, an optimum
variable temperature/time method with a short-term low-temperature
initial stage (4 h at 140˚C) facilitates the production of zeolite
nuclei which can transfer into the defects of the seed layer. The faster
growth of these nuclei in every direction during the high-temperature
final stage (8 h at 180˚C) effectively fills the inter-crystalline gaps
resulting in the formation of a high-quality zeolite layer.
It is known that the small seeds at the top layer intend to grow faster
than those penetrated into the defects at the bottom layer, wherein,
zeolite crystals with the highest vertical growth rate with respect to
the support surface have the highest probability of survival [5].
Therefore, controlling the rate of crystal growth reactions by adjusting
the synthesis temperature/time can effectively increase the likelihood
of small internal seeds growing. The slower growth rate of zeolite
crystals at the initial stage of the variable temperature method
facilitates the nutrition of nuclei/small crystals penetrating into the
defects providing them the opportunity to grow enough to fill defects
and gaps in the bottom layer [24]. The growth rate of the small
seeds on the top of the seed layer then overcome during the
high-temperature final stage accelerating crystal growth rate on top,
improving the crystal intergrowth and overgrowing by adjacent crystals
which dominate the surface of the film. It would be responsible for the
higher selectivity [24]. Contrariwise, starting the synthesis at
high temperatures, similar to the M9 membrane, favors the faster
intergrowth of small seeds on the top which eliminates the nutrition of
internal seeds and consequently increases the inter-crystalline gaps and
membrane thickness simultaneously [5, 29]. However, understanding
the effect of synthesis conditions (particularly synthesis temperature
and time) on growth competition between small crystals on the top of the
seed layer and those stuck into the defects/gaps still need to be
studied in detail.