Polyimide/CD mixed matrix membranes for pervaporation applications

Abstract

Novel molecular-level mixed matrix membranes comprising cyclodextrins (CDs) inside polyimide (PI) matrices were fabricated and investigated as pervaporation membranes for isopropanol dehydration and n-butanol/tert-butanol (n-BuOH/t-BuOH) isomers separation, respectively. Promisingly, much improved separation performance was obtained with both types of mixed matrix membranes relative to their homologue polymeric membranes. In isopropanol dehydration, addition of ∼2 to 10 wt. % of β-CD into PI matrix has led to simultaneously enhanced flux and separation factor. In the separation of isomeric n-BuOH/t-BuOH mixture, a maximum separation factor of 1.53 with a corresponding flux of 4.4 g/m2hr was achieved at an optimal β-CD loading of 15 wt. %. The performance improvement is ascribed to the following aspects: The unique toroid structure and of CD for size, shape and constant stability selectivity, the hydrophilic exterior of CDs provide additional polymer/CD interface channels for hydrophilic water diffusion and polymer chain rigidification due to the favorable interaction between CD outer surface and polymer backbone. The effect of various types of CD (α-, β- or γ-CD) embedded in the PI membranes have also been investigated experimentally and computationally for isomeric n-BuOH/t-BuOH separation. Consistent with experimental data, the simulation results showed that the CD inclusion ability and butanol discrimination ability are dependent on both CD cavity size and butanol molecular size, and all CDs exhibited both preferential sorption and diffusion selectivity toward n-BuOH. The PI membrane incorporated with α-CD has the smallest cavity and has the highest discrimination ability for the n-BuOH/t-BuOH pair but with a low butanol flux; whereas, the mixed matrix membrane embedded with γ-CD had the lowest selectivity and the highest flux. The PI/β-CD membrane had a comparable separation performance. In addition to performance test, some interesting morphologies were also investigated and reported. Micro-phase separation displaying continuous polymer-rich and discrete CD-rich phases was observed due to the extremely hydrophilic exterior of CD with hydroxyl groups and the relatively hydrophobic property of polymer backbone with limited hydrophilic sites. This morphology presents following properties probably hindering the membrane application: the dis-orientated agglomeration of CD molecules with one-dimension channel would not permit free transfer of guest molecules easily and the observed delamination between the two phases may become non-selective defects in the long-term application.