Fig. 5: Effect of different different-sized seeding methods on (a)
permeance and (b) ideal selectivity of tubular supported ZSM-5 zeolite
membranes
The effects of different-sized seeding method on the performance of
ZSM-5 zeolite membranes were evaluated by N2 and
SF6 pure gas permeation. The
N2/SF6 permeance and ideal selectivity
are widely used as an indicator of membrane quality due to their
significant difference in the kinetic diameter (0.36 nm and 0.55 nm for
N2 and SF6, respectively) [12]. The
permeance and ideal selectivity of the synthesized membranes are shown
in Fig. 5. The results in Tables 1-3 and Fig. 5b indicate that the
single-layer seeded membranes have low
N2/SF6 ideal selectivities due to the
poor intergrowth between zeolite crystals which results in creating more
defects and non-zeolitic pores. Membranes with dual-layer seeds (DLS
series) and triple-layer seeds (TLS series) show significantly higher
selectivities confirming the positive effect of using different-sized
seeding method in which the small seeds can effectively penetrate from
the top seed layer into the gaps of the bottom layer, grow faster
regarding both mass and diameter, and fill the defects during the
hydrothermal synthesis process [14].
The results in Fig. 5b reveals that adding the second and third seed
layers to the top of the primary seed layer significantly affects the
membrane permeance. For example, N2/SF6selectivities increase by 2100 % and 230 % by adding the second and
third seed layers, respectively, while the average of N2permeances diminishes by only 90% and 65% at the same condition (Fig.
5a). These results confirm that using the different-sized seeding method
significantly enhances the membrane performance if both permeance and
selectivity took into consideration.
A closer examining the changing trend of permeance versus seed size at
the bottom layer (Fig. 5a) discloses that increasing the seed size in
the bottom layer from 300 nm to 1.5 μm, if the size of smaller seeds at
the top layers are kept constant, significantly increases the permeance
by an average of 480%, while the selectivity decreases by only less
than 30%. It is due to the fact that using larger seeds at the bottom
layer (preferably larger than the pore size of the support),
significantly decreases the penetration of seeds into the pores
resulting in a reduction in effective membrane thickness and mass
transfer resistance. These results also reveal that the seed size at the
bottom layer mainly controls the membrane permeance. On the contrary,
results in Fig. 5a and b indicate that the
N2/SF6 ideal selectivity increases by
230 % and 160 % versus seed size change from 300 nm to 60 nm at the
top seed layer for DLS and TLS series, respectively. These results
confirm that the seed size at the top layer mainly determines the
membrane selectivity.
The effect of using a mid-size intermediate seed layer between the large
seed layer at the bottom and the small seed layer at the top can be
identified by comparison between performances of M5 and M9 membranes.
Where using a 300 nm intermediate seed layer increases selectivity up to
260 %, while decreases the N2 permeance by 40 %
showing that a proper intermediate seed layer can improve membrane
permeance, if the difference between seed sizes in the bottom and top
layers is considerable.
During the different-sized seeding process, small seeds from the top
layer penetrate into the inter-crystalline gaps/defects between large
seeds in the bottom layer. Afterward, because of their higher growth
rate compare to large seeds, small seeds effectively fill the
gaps/defects during the hydrothermal synthesis, resulting in higher
selectivities for membranes in series TLS and DLS than that of series
SLS. Consequently, the reduction in seed size in the top layer, reduces
the inter-crystalline gaps and makes the zeolite layer denser on the one
hand, and increase the membrane thickness via increasing the penetration
of seeds into the support pores, on the other hand. The former enhances
selectivity and the latter decreases permeance. So, we need to find an
optimum combination of seed sizes in the bottom and top layers to
maximize the perm-selectivity of the membranes.
Fig. 6 plots N2/SF6 ideal selectivity
versus N2 permeance for series SLS, DLS, and TLS
membranes. It can be observed in Fig. 6 that series TLS membranes locate
in high selectivity/low permeance region, while series DLS membranes
locate in moderate selectivity/permeance region. Considering the results
in Fig. 6, we selected the M9 membrane (from series TLS) for further
modification using the variable-temperature/time method. The advantages
of the new different-sized seeding method can be summarized in Table 4.