Effect of the particle size of ZIF-8 on the separation performance of ZİF-8/PNA/PES membranes

Ayas, İlhan
Membrane based separation processes have great potential of acquiring a significant role in the gas separation processes in the coming future. In this study, the effect of the particle size of ZIF-8 on the gas separation performance of PES/pNA/ZIF-8 mixed matrix membranes (MMMs) was investigated. MMMs were prepared by solvent evaporation method, and polyethersulfone (PES) was used as the polymer, p-nitro aniline (pNA) as the low molecular weight additive and Zeolitic Imidazolate Framework-8 (ZIF-8) as filler material. Various particle sizes of ZIF-8 crystals were synthesized by using 1 hour stirring method at room temperature. ZIF-8s with particle sizes of 65, 144 and 262 nm were synthesized by using different MeOH/Zn+2 molar ratios. Recycling of the mother liquor method was used to synthesize ZIF-8s with particle sizes of 14 and 23 nm. Synthesized ZIF-8 crystals were characterized by X-ray diffractometer, nitrogen adsorption/desorption (BET), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). PES/pNA/ZIF-8 MMMs with two different ZIF-8 loadings for varying particle sizes were also investigated to understand the combined effect of particle size and loading. The single gas permeation performances of the MMMs were determined for H2, CO2 and CH4 gases at feed pressures of 3 bar. Also, the single gas permeabilities of the selected PES/pNA(4%)/ZIF-8(10%, 23 nm) MMM were measured at feed pressures of 6, 10, 12 and 15 bar in order to investigate the effect of the feed pressure on the separation performance. The binary gas separation performances of selected MMM were also investigated with CO2/CH4 mixtures for different feed gas compositions at feed pressures of 3 and 10 bar. Moreover, MMMs were characterized by scanning electron microscopy (SEM) analysis. The single gas permeabilities of all gases and ideal selectivities for all gas pairs of 10% (wt/wt) ZIF-8 loaded PES/pNA(4%)/ZIF-8 MMMs increased with decreasing particle sizes of ZIF-8 except for 14 nm ZIF-8 loaded membrane. Also, the single gas permeabilities increased significantly when the loading amount of ZIF-8 rose from 10% to 20% (wt/wt). For the 20% (wt/wt) ZIF-8 loaded PES/pNA(4%)/ZIF-8 MMMs, the single gas permeabilities had an descending trend when the particle sizes of ZIF-8s were increasing; however, the ideal selectivities increased. A significant improvement was obtained in permeabilities and ideal selectivities by using the particle size of 23 nm ZIF-8 loaded PES/pNA(4%)/ZIF-8(10%) MMM. The single gas permeations of the selected MMM were measured with increasing feed pressure from 3 bar to 15 bar. The H2 permeability was not affected by changing feed pressure and might be said as pressure independent. However, the CO2 and CH4 permeabilities decreased with increasing feed pressure. Also, the ideal selectivity of H2/CH4 pair showed the highest selectivity improvement when the feed pressure increased from 3 bar to 15 bar. It was observed that selected MMM had good gas separation performance at high pressure conditions. The binary gas separation performance of the selected MMM showed that the separation factors of CO2/CH4 pair remained nearly constant with increasing feed composition of CO2 at 3 bar, and it was similar to ideal selectivity. Also, the separation factors at 10 bar had two types of behavior according to CO2 composition of the feed. The behavior of separation factors at 10 bar had similar behavior at 3 bar until the feed was 22.5 % CO2 in the first section. However, the separation factors at 10 bar were higher than the ideal selectivity when the CO2 composition of the feed was more than 22.5% in the second section.