Formic acid crosslinked cellulose acetate defect-free asymmetric membrane for gas separation

Abstract

Cellulose acetate (CA) polymeric membrane has been used as gas separator; however, the modest selectivity and trade-off between permeability and selectivity have reduced the utilization of this polymer. Thus, the objectives of this study were to investigate the effect of polymer concentration and formic acid (FA) crosslinking agent loading on the formation of the membrane morphology and gas separation performance. The CA in tetrahydrofuran (THF) flat sheet asymmetric membrane was fabricated by dry/wet phase inversion process with two conditions of dope solution formulation: (1) varying the polymer concentration ranging from 13 to 16 wt.% and (2) manipulating FA:THF ratio between 0:100 to 10:90. The prepared membrane was analyzed by using viscometer, field emission scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis, and tensile testing machine. The membrane gas permeation performance was tested using pure gases of hydrogen (H2), oxygen (O2), nitrogen (N2), carbon dioxide (CO2) and methane (CH4) by using soap bubble flow meter. The CA membrane produced possessed three layers consist of top skin layer, transition layer and porous support structure. The increase in polymer concentration had produced denser membrane with thicker skin layer and substructure, thus, significantly improved the selectivity. The optimum CA concentration obtained in this study was 15 wt.% that exhibited the highest selectivity for all gas pairs. Upon addition of FA, the membrane skin layer formation had further improved without severely sacrificed the gas permeability since the FA promotes the formation of more porous substructure. This was probably due to the crosslinking of the –OH group between CA and FA as confirmed by the FTIR. Moreover, all the gas pairs selectivities improved significantly as the FA loadings were increased. The highest selectivities obtained for H2/N2, O2/N2 and CO2/CH4 separation were 55.87, 6.83 and 48.64, respectively.