(552d) Effect of Polymer Molecular Weight On CO2 Separation Performance of SAPO-34/6FDA-DAM/Additive Ternary Mixed Matrix Membranes

Polyimides are of interest in separation of industrial gas streams due to their outstanding thermal and chemical stability. The polyimide prepared from 4,4-hexafluoro isopropylidene diphthalic anhydride (6FDA) and 2,4,6-trimethyl-m-phenylene diamine (DAM) is attractive due to its high permeability for many gases but its selectivity for commercially important gas pairs is much lower than the industrially relevant values. We have previously reported a proof-of-concept study demonstrating that ternary (polymer-zeolite-additive) mixed-matrix membranes may be a possible route to tailor permselectivity [1].  It has been shown that by using SAPO-34 as the zeolite and 2,4,6-triaminopyrimidine (TAP) as the compatibilizing additive, separation properties of highly permeable 6FDA-DAM can be tailored to commercially attractive levels for CO₂/CH₄ application. The use of the additive served two purposes: (1) the amine functional groups of TAP formed hydrogen bonds with the OH groups of the zeolite and the carbonyl groups of the polyimide and hence providing good adhesion at the zeolite-polymer interface, (2) reducing the permeability and increasing the selectivity of polymer due to its antiplasticization effect so that permeabilities of zeolite and polymer become compatible and zeolite incorporation would favor the selectivity. The results were surprisingly good. A CO₂ permeability (35°C and 4 bar) of 8.3 Barrer and an ideal CO₂/CH₄ selectivity of  259 were obtained at 2% TAP and 20% SAPO-34 addition and this performance was much above the upper-bound. In this work, we investigate the effect of polymer molecular weight on the material and separation properties of these ternary mixed-matrix membranes. Membranes are characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), and single and binary-gas permeation measurements at 35°C for CO₂/CH₄separation. Results indicate that polymer molecular weight is a key parameter in determining the final membrane properties. The effect of the additive varies significantly when the polymer molecular weight is varied from 30,000 to 100,000. Results are interpreted in terms of additive-polymer interaction.

[1]  H. Ishakoglu, E. Erpek, S. Halitoglu, M. G. Ahunbay, S. B. Tantekin-Ersolmaz, Polymer-zeolite-additive three-component mixed matrix composite membrane for CO₂/CH₄ separation, presented at Euromembrane 2009, Montpellier, France, Sept. 06-10, 2009.