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.