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