(255c) Novel Aromatic Polyimides for Aromatic/Aliphatic Separation by Pervaporation

The separation of aromatic and aliphatic hydrocarbons is relevant in different processes of refinery and petrochemical industries, including naphtha reforming, production of cyclohexane, and removal of sulfur from gasoline. Many of these mixtures are difficult to separate because they contain close boiling compounds that generally have similar physical and chemical properties with respect to each other. The conventional technologies to perform these separations, such as extractive distillation, azeotropic distillation and liquid-liquid extraction, are energy intensive and expensive.

In view of the potential energy savings and, more recently, the reduction in carbon dioxide emissions, pervaporation has been widely investigated for the separation of aromatic/aliphatic mixtures. Aromatic polyimides, in particular, have attracted attention for this application due to their high thermal and chemical stability [1]. From the gas separation literature, it is known that an enhancement in permeability is obtained by the inclusion of hexafluoropropane moieties, fluorene moieties, and substituents in the ortho position of the diamine relative to the amino moiety [2, 3]. However, the combination of these strategies in the development of pervaporation membranes for the proposed application remains unexplored.

In this contribution, we report a structure/property study of a series of aromatic polyimides for the separation of toluene/n-heptane and benzene/n-heptane mixtures by pervaporation. Homo- and copolyimides were synthesized by the two-step polycondensation of different aromatic dianhydrides and diamines, including 4,4'-(hexafluoroisopropylidene)-diphthalic anydride (6FDA); 4,4'-oxydiphthalic anhydride (ODPA); 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA); 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB); 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD); 9,9'-bis(4-aminophenyl) fluorene (FDA) and 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF). All polymers were characterized by FTIR, ¹H NMR, DSC, TGA, and inherent viscosity measurements. Pervaporation experiments were performed at 80^oC with feed streams containing 40 wt% aromatics. Pure-liquid sorption experiments at 25^oC were also conducted to investigate the solubility selectivity of each polymer. The long-term chemical stability of the polymers was assessed in exposure tests to benzene in a Soxhlet extractor for at least a week.

Contrary to most studies on pervaporation conducted so far, the individual contributions of the membrane material and the driving force to the permeate flux were properly evaluated. It was observed that changes in the chemical structure of the polyimide led to an increase of up to 3 orders of magnitude in the permeability of the hydrocarbons.

References

[1] B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna. Separation of organic-organic mixtures by pervaporation: a review. Journal of Membrane Science, 241, 1 ? 21, 2004.

[2] K. Tanaka, K.-I. Okamoto. Structure and transport properties of polyimides as materials for gas and vapor membrane separation. In: Y. Yampolskii, I. Pinnau, B. D. Freeman (Eds.). Materials Science of Membranes for Gas and Vapor Separation, John Wiley & Sons, 2006.

[3] M. Langsam, W. F. Burgoyne. Effects of diamine monomer structure on the gas permeability of polyimides. I- Bridged diamines. Journal of Polymer Science Part A Polymer Chemistry, 31, 909-921, 1993.