Separative Gas Transport in Ultra-Thin Membranes

Membrane-based processes are regarded as sustainable replacement to traditionally used energy-intensive gas separation methods. The preferred membrane material should be able to operate at harsh separation condition, demonstrate high permeance and selectivity to minimize compression energy and separation stages. Continuous research has been focused on developing more permeable, selective and durable membranes. It is a common belief that membranes facilitate better process efficiency with higher permeance, which can be possibly achieved at lower thicknesses. Developments in the nanotechnology research area enabled researchers to fabricate sub -1000 nm thick membranes. However, it is yet unclear, how gas permeance and selectivity vary in the ultra-thin materials in comparison to their bulk value, especially in the sub-100 nm regime of thickness. To date, fabrication of defective films obstructs drawing solid conclusion regarding transport phenomena in ultra-thin regime of thickness. Here, gas transport and separation are investigated through two types of ultra-thin membrane materials: 1. Dense polymers down to 20 nm thickness. 2. Porous layers as thin as single-atom-thick graphene. Gas transport is studied in semi-crystalline and fully amorphous, liquid-like polymers. In a first-time demonstration, semi-crystalline polyamide/imide nanofilms are synthesized at the vapor-liquid interface over CNT-buckypaper and also porous graphene scaffolds in a lab scale. Amorphous oligomeric layers are synthesized via stage-wise physical vapor deposition on a porous support. Gas permeance in the semi-crystalline layers is defined by Fickian diffusion and shows a maximum at the intermediate thickness of 30 nm. Reducing the thickness decreases the diffusion length, but at the same time would decrease the diameter of the diffusion path leading to formation of the maximum. In thin, amorphous, liquid-like polymer layers, the entrance event to the matrix imposes impedance as significant as Fickian diffusion through the matrix. At a certain critical length, entrance event would define completely the transport rate. Thus, the permeance might not necessary increase by reducing the thickness of the dense polymers at the ultra-thin regime. In addition to thickness, pore diameter is another influencing factor in gas transport through microporous material. Investigation of gas transport through zero-thick porous graphene revealed the pore size dependency of permeance and selectivity in the transition regime between free molecular and collective flow, explained by intermolecular collision between gases at the entrance of the pores. In a further investigation, controlled vapor deposition of amorphous polymer on the porous support enabled capturing two distinguished transitions from collective transport toward solution-diffusion mechanism. The fabricated polymer-graphene nanofilms demonstrate two orders of magnitude higher permeance than the-state-of-the-art membranes. The polymer-CNT nanofilms benefit from antiplasticization property due to the confinement of the polymer chains and interaction between the polymer and scaffold, which makes these films attractive for harsh separation condition of hydrocarbons separation. Application of the fabricated membranes is extended beyond the typical gas separation focus. The fabricated nanofilms are used to generate power through the first-time lab scale demonstration of gas osmosis physic. Fast transport through the fabricated nanofilms resulted in capturing gas phase concentration polarization phenomenon adjacent to the membrane surface, which can reduce the membrane process selectivity. This phenomenon rationalizes developing mixing strategies and membrane module design to minimize concentration polarization and gain maximum performance out of highly permeable membranes. The present thesis demonstrates novel methods to synthesize ultra-thin membranes as platforms to study gas transport and separation in the ultra-thin regime of thickness, as well as membrane films for high performance gas separation and power generation applications