Zhizhang Tian

Advances in high permeability polymer-based membrane materials for CO2 separations

Membrane processes have evolved as a competitive approach in CO2 separations compared with absorption and adsorption processes, due to their inherent attributes such as energy-saving and continuous operation. High permeability membrane materials are crucial to efficient membrane processes. Among existing membrane materials for CO2 separations, polymer-based materials have some intrinsic advantages such as good processability, low price and a readily available variety of materials. In recent years, enormous research effort has been devoted to the use of membrane technology for CO2 separations from diverse sources such as flue gas (mainly N2), natural gas (mainly CH4) and syngas (mainly H2). Polymer-based membrane materials occupy the vast majority of all the membrane materials. For large-scale CO2 separations, polymer-based membrane materials with high CO2 permeability and good CO2/gas selectivity are required. In 2012, we published a Perspective review in Energy & Environmental Science on high permeability polymeric membrane materials for CO2 separations. Since then, more rapid progress has been made and the research focus has changed significantly. This review summarises the advances since 2012 on high permeability polymer-based membrane materials for CO2 separations. The major features of this review are reflected in the following three aspects: (1) we cover polymer-based membrane materials instead of purely polymeric membrane materials, which encompass both polymeric membranes and polymer–inorganic hybrid membranes. (2) CO2 facilitated transport membrane materials are presented. (3) Biomimetism and bioinspired membrane concepts are incorporated. A number of representative examples of recent advances in high permeability polymer-based membrane materials is highlighted with some critical analysis, followed by a brief perspective on future research and development directions.

Efficient CO2 capture by humidified polymer electrolyte membranes with tunable water state

Polymer electrolyte membranes containing alkali or alkaline-earth metal salts were designed and utilized for CO2 capture. These membranes showed higher CO2 permeability than the un-doped control membrane due to the increase of water content, and CO2/gas selectivity was simultaneously enhanced due to the “salting-out” effect, which was strongly dependent on the content of bound water. More specifically, water content, water state and separation performance of polymer electrolyte membranes were strongly dependent on the salt type: (1) membranes containing alkaline-earth metal salts displayed a higher amount of bound water than those containing alkali cations, because the hydration energy of the alkaline-earth cation is relatively larger than that of the alkali cation; (2) the salts (KCl and CaCl2) that can efficiently interrupt chain packing by metal–polymer complexation facilitated the diffusion of water molecules into the polymer matrix and thus increased the total amount of absorbed water. As a consequence, CaCl2-doped membranes showed the highest CO2 permeability (2030 Barrer) and a high separation factor (108 for CO2/N2 and 31 for CO2/CH4) at 2 bar (gage pressure) and 298 K for fully humidified gas streams. The effects of annealing conditions and feed pressure were also explored to elucidate the relevant separation mechanism of the polymer electrolyte membrane.

High-performance composite membrane with enriched CO2-philic groups and improved adhesion at the interface.

A novel strategy to design a high-performance composite membrane for CO2 capture via coating a thin layer of water-swellable polymers (WSPs) onto a porous support with enriched CO2-philic groups is demonstrated in this study. First, by employing a versatile platform technique combining non-solvent-induced phase separation and surface segregation, porous support membranes with abundant CO2-philic ethylene oxide (EO) groups at the surface are successfully prepared. Second, a thin selective layer composed of Pebax MH 1657 is deposited onto the support membranes via dip coating. Because of the water-swellable characteristic of Pebax and the enriched EO groups at the interface, the composite membranes exhibit high CO2 permeance above 1000 GPU with CO2/N2 selectivity above 40 at a humidified state (25 °C and 3 bar). By tuning the content of the PEO segment at the interface, the composite membranes can show either high CO2 permeance up to 2420 GPU with moderate selectivity of 46.0 or high selectivity up to 109.6 with fairly good CO2 permeance of 1275 GPU. Moreover, enrichment of the PEO segment at the interface significantly improves interfacial adhesion, as revealed by the T-peel test and positron annihilation spectroscopy measurement. In this way, the feasibility of designing WSP-based composite membranes by enriching CO2-philic groups at the interface is validated. We hope our findings may pave a generic way to fabricate high-performance composite membranes for CO2 capture using cost-effective materials and facile methods.

Facilitated transport membranes by incorporating different divalent metal ions as CO2 carriers

Facilitated transport membranes by utilizing π complexation reactions between metal ions and penetrants have been actively explored, however, the different facilitated transport abilities of metal ions remains to be disclosed. In this study, poly(N-vinylimidazole) coated carbon nanotube particles (PVI@CNT) were prepared via precipitation polymerization of N-vinylimidazole monomers on a CNT surface. The PVI@CNT particles were then loaded with four kinds of divalent metal ions, Cu2+, Fe2+, Ca2+ and Mg2+, and incorporated into polyimide (PI) to prepare M2+–PVI@CNT hybrid membranes. The structure of M2+–PVI@CNT particles and PI–M2+–PVI@CNT membranes was analyzed by different characterization tools. Taking CO2/CH4 as the model separation system, the hybrid membranes containing Cu2+ and Fe2+ at filler content of 7 wt% showed the maximum increase of CO2 permeability of 89% and 87% compared with those of a pristine PI membrane. Meanwhile, the selectivity of these membranes shows little increase. However, membranes containing Ca2+ and Mg2+ show only little enhancement in the separation properties. Such results can be interpreted based on the π complexation mechanism, transition metal ions Cu2+ and Fe2+ possess a strong CO2 facilitated transport ability whereas main-group metal ions Ca2+ and Mg2+ possess a weak facilitated transport ability. Finally, a correlation of the electronegativity of metal ions with their CO2 facilitated transport abilities was explored.