Chapter 4. Recent High Performance Polymer Membranes for CO2 Separation

Abstract

Since A. Fick had observed different transport of gas molecules through nitro-cellulose, membrane-based gas separation has been extensively studied for membrane materials as well as scale-up and application to industrial gas separation field. Numerous scientists have studied the transport mechanism, measurement of gas permeation, membrane materials, and high flux membrane by asymmetric fabrication.Acknowledged to the contribution for quantative measurement of gas permeabilities by H. A. Daynes and R. M. Barrer, a number of materials have been tested and applied for gas separation membranes: polydimethylsiloxane (PDMS), cellulose acetate (CA), polysulfone (PSf), polyethersulfone (PES), and polyimide (PI), etc. PDMS, a kind of highly permeable polymer, has been used in VOCs recovery due to its superior sorption capacitance. CA and fluorinated polyimides have been employed to natural gas calorie-up process and petrochemical industries while PES and PSf have been mainly used for nitrogen enrichment from air.In a dense polymer film, gas permeation through the membrane depends upon both sorption and diffusion known as ‘solution-diffusion’ mechanism proposed by Sir Thomas Graham at 1866. Sorption, a thermodynamic factor, is occurred when a gas molecule in bulk motion adsorb physically on a membrane surface with respect to the concentration difference, and diffusion, a kinetic factor, is related to the extent how fast a gas molecule go through the membrane. Therefore, two factors are simultaneously crucial for efficient permeation and separation.While the conventional polymer membranes exhibited somewhat low permeation and selective performances, along with the recent developments in materials science, recently reported membrane materials showed excellent permeability and selectivity for various gas pairs as well as enhanced thermal, chemical and mechanical stabilities. The recent polymer membranes can be classified largely into two classes; highly diffusive membranes due to their high free volume elements and sorption-enhanced membranes based on high sorption capacity on carbon dioxide and hydrocarbons.Glassy polymers possessing rigid structure and strong intermolecular forces have been generally known to show reduced gas permeabilities although low diffusion through the membrane enabled to separate gas molecules efficiently resulting in high diffusion selectivity. However, superglassy polymers containing distorted structure and the resulting increased free volume elements (FVE) were developed since T. Matsuda et al. reported extraordinarily permeable glassy polymer, poly[1-(trimethylsilyl)-1-propyne](PTMSP). Since PTMSP was developed in 1983, acetylene-based polymers such as poly(4-methyl-2-pentene)(PMP), poly(1-phenyl-1-propyne)(PPP) have been followed by the introduction of various substitution groups. Extraordinarily high permeabilities, for example, 9,700 Barrer of oxygen permeability for PTMSP, were obtained from these acetylene-based polymers, while the separation performances were not successful because of their relatively large FVE size for separation. Fluorinated polymers, famous as the commercial names like AF 1600, AF 2400, Hyflon AD and Cytop, are candidate polymers for gas separation as well. Recent development of polymers with intrinsic microporosities (PIMs) showed competitive performances resulted from the intrinsic FVE due to the bulky and distorted structures. Thermally rearranged (TR) polymer membranes also presented optimized performances for gas separation. Chain rearrangement at the solid state improved the FVE in the polymer matrix followed by superior permselectivities as well as high permeabilities.Sorption-enhanced polymer membranes are beneficial to obtain superior permeability as well as advanced selectivity especially for carbon dioxide, hydrocarbons and VOCs because condensable gas molecules are easier to be adsorbed on the polymer surface. Recent researches are focused on the usage of ethylene oxide group, which has unique interaction with quadrupole monetum of carbon dioxide. From poly(dimethyl siloxane)(PDMS), the original sorption-based polymer, poly(ethylene oxide)(PEO), crosslinked PEO, PEO-based block copolymers such as poly(ethylene oxide-b-amide)(Pebax), poly(ethylene oxide-b-ethylene terephthalate)(PEO-b-PET) will be discussed from the viewpoint of permeabilities, solubilities and also their temperature dependences.In this book chapter, the chemical structure, physical properties, gas permeabilities and selectivities will be surveyed on the focus of up-to-date high performance and higly permeable membranes for gas separation.