Zeolitic imidazolate frameworks blended polysulfone hollow fiber membranes for natural gas purification

Abstract

Mixed matrix membranes (MMMs) have received world-wide attention for natural gas purification due to their superior performance in terms of permeability and selectivity. In this study, zeolitic imidazole framework (ZIF) based polysulfone (PSf) hollow fiber membranes were fabricated for natural gas purification. A new micron-sized leaf-like ZIF (ZIF-L) and hexagonal nano-sized ZIF-8 were synthesized in an aqueous basic solution at room temperature with the same molar ratio of reagents (Zn+2/Hmim = 8). Furthermore, various moles of triethylamine (TEA)/total moles ratio of reactants ranging from 0–0.006 were used. Both ZIF powders were characterized by field emission scanning electron microscopy, X-ray diffraction, CO2 temperature programmed desorption, Fourier transform infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy, and surface area and pores textural properties using nitrogen adsorption-desorption analysis. ZIF- 8 particles have shown improved thermal stability, textural properties, basic sites and CO2 adsorption capacity compared to ZIF-L. The neat PSf membrane and mixed matrix hollow fiber membranes incorporated with the various loading of ZIF-8 ranging from 0–1.25% were fabricated at bore fluid rate of 1.5 and 1.8 ml/min. The prepared membranes were further investigated with respect to their structural morphology, thermal stability, functional groups, surface roughness and finally gas separation performance. The gas permeation results at room temperature showed that fabricated MMM at 1.8 ml/min of bore fluid and loaded with 0.5 wt% of ZIF-8 showed 28% higher CO2/CH4 selectivity at 6 bar (g) feed pressure compared to neat PSf membrane. High loading of ZIF-8 =0.75 wt% deteriorated the separation performances. However, CO2/CH4 selectivity decreased at elevated pressure (8 and 10 bar) due to CO2-induced plasticization. The amine modification of ZIF-8 particles with 25 ml ammonium hydroxide solution at room temperature was found to significantly improve textural properties, basic sites strength and CO2 desorption capacity. MMM prepared at 1.8 ml/min of bore fluid rate and loaded with 0.25 wt% of amine modified ZIF-8 showed 18% increase in CO2/CH4 selectivity compared to unmodified ZIF-8 based membrane. The amine modification was proven to be a membrane’s anti-plasticization agent with superior gas separation performance at elevated pressure. In comparison to the neat PSf membrane, amine modified MMM prepared at the bore fluid rate of 1.8 ml/min has shown 50, 72 and 69% higher selectivity at 6, 8 and 10 bar (g) feed pressure respectively. Also, the selectivity of A-M0.25 was 18% higher than unmodified ZIF-8 based MMM at 6 bar (g) feed pressure. The permeance of both gases decreased at an acceptable level with an increase of selectivity at elevated pressure. Hence, the promising results obtained in this study has demonstrated the potential of amine modified ZIF-8 based MMMs for natural gas purification.