Fig. 10: Effect of the hybrid different-sized
seeding/varying-temperature method on gas permeation and selectivity
obtained from H2/CO2 binary mixture
It is known that the high-quality ZSM-5 zeolite membranes with few
non-zeolitic pores generally show H2/CO2selectivities lower than 1.0 at ambient temperature [30]. The
reversed selectivity for CO2 is due to the fact that
CO2 preferentially adsorbs in the ZSM-5 zeolite pores
and blocks the diffusion channels for H2. The higher the
membrane quality, the lower the H2/CO2selectivity. However, in low-quality membranes, both H2and CO2 molecules can transport through
inter-crystalline gaps under the Knudsen diffusion mechanism increasing
H2/CO2 selectivities in the reverse
trend up to Knudsen selectivity, which makes the membrane more selective
for H2, due to its lower molecular weight [31].
The results in Fig. 10 indicate that the CO2 permeances
are higher than that of H2 in M7, M9, and M14 membranes
due to the strong adsorption of CO2 in the zeolite pores
resulting in H2/CO2 selectivities lower
than Knudsen selectivity. This confirms that using the different-sized
seeding method significantly reduces the inter-crystalline gaps making
the ZSM-5 zeolite membranes selective to CO2. In
contrast, the H2/CO2 selectivity of the
M3 membrane is almost equal to Knudsen selectivity (4.7) at room
temperature showing that the single-layer seed membranes are selective
to H2. The results reveal also that both
CO2 and H2 permeances for membrane M14
are higher than those for membrane M9. Since these two membranes were
synthesized by the same seeding method, the results confirm that the
inter-crystalline gaps reduced effectively using the variable
temperature/time method due to the elimination of non-zeolitic pores
through in-situ balancing nucleation and crystal growth reaction rates.
Table 6 compares the H2/CO2 separation
performance of the ZSM-5 zeolite membranes prepared in this work with
some of the best performing membranes reported in the literature. As can
be seen, membranes synthesized in this work show the best reported
MFI-type zeolite membranes in the literature so far, if both selectivity
and permeance are taken into account. It proves that using the proposed
different-sized seeding method in combination with the variable
temperature/time profile, the membrane microstructure improves, the
density of the membrane layer increases and the effective membrane
thickness decreases. It makes these membranes a proper option for future
scale-up applications of tubular zeolite membranes.
Table 6: The H2/CO2 separation
properties of the ZSM-5 zeolite membranes