(650g) Highly Selective Layer-By-Layer Polymer Films for CO2/N2 Separation

The effects of global warming can be partially alleviated by the reduction of carbon dioxide emissions into the atmosphere. Flue gas from fossil fuel power generation plants is the largest contributor to CO₂ emissions. Currently, CO₂ is removed from flue gas (primarily in mixture with N₂) by technologies such as low temperature distillation, physical or chemical absorption, pressure swing adsorption and membrane separation. Cryogenic distillation, absorption technologies, and pressure swing adsorption are complex and energy intensive processes. Gas separation using membranes has garnered significant attention over the past two decades owing to the advantages of high energy efficiency, small footprint, continuous operation and lack of moving parts.

Polymeric gas separation membranes that combine low production costs with strong mechanical properties have been widely used in a variety of gas separation processes such as O₂ and N₂ separation from air and treatment of natural gas. The main obstacle encountered by polymer membranes is the inverse relationship between selectivity and permeability. A membrane with high selectivity is desired so that efficient separation occurs, but is important that the membrane is highly permeable so that a large amount of gas can be separated in a short amount of time and area.

Most polymeric membranes for CO₂/N₂ separation used the facilitated transport mechanism. In these membranes the targeted gas (e.g., CO₂) reversibly bounds to a mobile/fixed reactive carrier that transports the gas across the membrane and releases it in the downstream side (low pressure). Despite the high permeabilities for CO₂ of most facilitated-transport membranes their selectivities are not high enough to be competitive with current CO₂ separation technologies. Depending on the gas purity required, it is expected that CO₂/N₂ selectivities in excess of 200 will be competitive with conventional absorption technology.

Poly(ethylene oxide) (PEO) have been widely studied for gas separation membrane since it offers an unique mechanism for CO₂ transport due to the dipole-quadrupole interaction between the polar ether group and the CO₂. However, PEO has a strong tendency to crystallize, which is detrimental for gas permeability.

Layer-by-Layer (LbL) technique, which was developed two decades ago, has been widely studied as a simple and versatile method to create multifunctional thin films. The ease of LBL assembly technique compared to the methods employed to fabricate the current gas separation films make it an attractive alternative. Extension of the LbL assembly technique to gas purification membranes promises the ability to tune overall gas permselectivity by varying the number of deposition or variables of the deposition process (e.g., pH, deposition time, solution %wt) to suit a specific need. Kim et al. [1] reported a highly size-selective LbL light gas separation membrane. In their study, they demonstrated how gas transport properties of these assemblies can be tuned by modifying the number of layers deposited.  There has been limited number of studies on LbL assembly for gas separation, most of them have focused on the effect of film thickness but not studied in depth the effect of deposition conditions.

In the present work, a PEO/PMAA LbL assembly was investigated for CO₂/N₂ separation using a constant-volume variable-pressure apparatus. Our aim is to investigate how the permeability and selectivity is affected by number of bi-layers (film thickness) and the deposition pH. Additionally, the temperature dependence of permeability and selectivity is presented. The activation energies for CO₂ and N₂ transport are reported for 10BL and 6BL films with deposition pH 2 and for a 10 BL with a deposition pH 4. The goal is to establish a processing-property relationship that will allow to obtained thin films with high selectivity and high permeability.

[1] Kim, Daejin, et al. "Highly Size‐Selective Ionically Crosslinked Multilayer Polymer Films for Light Gas Separation." Advanced Materials 26.5 (2014): 746-751.