Qingping Xin

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.

Enhanced Interfacial Interaction and CO2 Separation Performance of Mixed Matrix Membrane by Incorporating Polyethylenimine-Decorated Metal–Organic Frameworks

Polyethylenimine (PEI) was immobilized by MIL-101(Cr) (∼550 nm) via a facile vacuum-assisted method, and the obtained PEI@MIL-101(Cr) was then incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate mixed matrix membranes (MMMs). High loading and uniform dispersion of PEI in MIL-101(Cr) were achieved as demonstrated by ICP, FT-IR, XPS, and EDS-mapping. The PEI both in the pore channels and on the surface of MIL-101(Cr) improved the filler-polymer interface compatibility due to the electrostatic interaction and hydrogen bond between sulfonic acid group and PEI, and simultaneously rendered abundant amine carriers to facilitate the transport of CO2 through reversible reaction. MMMs were evaluated in terms of gas separation performance, thermal stability, and mechanical property. The as-prepared SPEEK/PEI@MIL-101(Cr) MMMs showed increased gas permeability and selectivity, and the highest ideal selectivities for CO2/CH4 and CO2/N2 were 71.8 and 80.0 (at a CO2 permeability of 2490 Barrer), respectively. Compared with the membranes doped with unfilled MIL-101(Cr), the ideal selectivities of CO2/CH4 and CO2/N2 for PEI@MIL-101(Cr)-doped membranes were increased by 128.1 and 102.4 %, respectively, at 40 wt % filler loading, surpassing the 2008 Robeson upper bound line. Moreover, the mechanical property and thermal stability of SPEEK/PEI@MIL-101(Cr) were enhanced.

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.

Enhancing the CO2 separation performance of composite membranes by the incorporation of amino acid-functionalized graphene oxide

Composite membranes are fabricated by incorporating amino acid-functionalized graphene oxide (GO-DA-Cys) nanosheets into a sulfonated poly(ether ether ketone) (SPEEK) polymer matrix. Graphene oxide (GO) nanosheets are functionalized with amino acids through a facile two-step method using dopamine (DA) and cysteine (Cys) in succession. The CO2 separation performance of the as-prepared membranes is evaluated for CO2/CH4 and CO2/N2 systems. GO nanosheets increase more tortuous paths for larger molecules, enhancing the diffusivity selectivity. Amino acids with carboxylic acid and primary amine groups simultaneously enhance the solubility selectivity and reactivity selectivity. Accordingly, CO2 molecules can transport quickly due to the enhanced selectivity. The optimum separation performance is achieved at the GO-DA-Cys content of 8 wt% with selectivities of 82 and 115 for CO2/CH4 and CO2/N2, respectively, and a CO2 permeability of 1247 Barrer, significantly surpassing the Robeson upper bound reported in 2008. Besides, the mechanical and thermal stabilities of the composite membranes are also improved compared with the pristine SPEEK membrane.

A highly permeable graphene oxide membrane with fast and selective transport nanochannels for efficient carbon capture

Ultrathin graphene oxide membranes using borate as both crosslinker of GO nanosheets and facilitated transport carrier of CO2 are designed and fabricated. The membranes exhibited high CO2 permeance up to 650 GPU and a CO2/CH4 selectivity of 75, due to the rational manipulation of the interlayer nanochannel size, sufficient facilitated transport carriers and high water content.

Perspectives on water-facilitated CO2 capture materials

Efficient separation of CO2 from other gases has become an issue of world-wide concern. Intrigued by the fascinating carbonic anhydrase-catalysed CO2 hydration and deprotonation in biological organisms, researchers have found that water can play a significant role in fast and selective CO2 transport (e.g. facilitating transport, salting-out effect, swelling, synergic sorption, etc.) and have managed to fabricate super CO2 capture materials by judiciously introducing water into solids. Considering that water usually exists in industrial CO2 resources and often acts as a negative impurity due to competitive sorption and pore blockage, exploring CO2 capture materials with the aid of water has become an important emerging strategy to provide general and excellent paradigms for practical CO2 capture technologies. In this sense, we propose a new concept, “water-facilitated CO2 capture (WFCC) materials”, which refers to solid materials (either adsorbents or membranes) with a remarkable improvement in CO2 capture performance due to entrapped water. In this way, we endeavor to answer an important question: when and how water contributes to this drastic enhancement. Strategies to avoid the negative effects of water and to enable WFCC are also tentatively proposed.

Graphene Oxide Membranes with Heterogeneous Nanodomains for Efficient CO2 Separations

Achieving high membrane performance in terms of gas permeance and carbon dioxide selectivity is an important target in carbon capture. Aiming to manipulate the channel affinity towards CO2 to implement efficient separations, gas separation membranes containing CO2 -philic and non-CO2 -philic nanodomains in the interlayer channels of graphene oxide (GO) were formed by intercalating poly(ethylene glycol) diamines (PEGDA). PEGDA reacts with epoxy groups on the GO surface, constructing CO2 -philic nanodomains and rendering a high sorption capacity, whereas unreacted GO surfaces give non-CO2 -philic nanodomains, rendering low-friction diffusion. Owing to the orderly stacking of nanochannels through cross-linking and the heterogeneous nanodomains with moderate CO2 affinity, a GO-PEGDA500 membrane exhibits a high CO2 permeance of 175.5 GPU and a CO2 /CH4 selectivity of 69.5, which is the highest performance reported for dry-state GO-stacking membranes.

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.

Protein adsorption and desorption behavior of a pH-responsive membrane based on ethylene vinyl alcohol copolymer

Protein adsorption and desorption behavior was investigated for a pH-responsive ethylene vinyl alcohol copolymer (EVAL) membrane with an interconnected porous structure. The transition of electrostatic behavior and conformation change of the poly(dimethylaminoethyl methacrylate) (poly(DMAEMA)) chain contributed to the pH-responsive protein adsorption and desorption. Protein adsorption was conducted under acidic and neutral conditions. Protein desorption was conducted under alkaline conditions. The protonated poly(DMAEMA) chain was positively charged and extended into the BSA solution below its pKa, providing a three-dimensional space for BSA adsorption. The maximum static protein adsorption capacity was obtained at pH 6.4. The dynamic adsorption capacities of membrane EVAL10 at 10% and 50% breakthrough were 45 and 99 mg BSA per g of membrane, respectively. The Q50% of membrane EVAL10 was equivalent to 22.6 mg BSA per mL of membrane, almost 95% of the static adsorption capacity. BSA was quickly desorbed from the membrane and 94% recovery of BSA was observed at pH 9.0 in the dynamic desorption process, due to a deprotonated and collapsed conformation of the poly(DMAEMA) chains. The dynamic adsorption capacity of the membrane did not change significantly after four sequential cycles.

Hemocompatible poly(lactic acid) membranes prepared by immobilizing carboxylated graphene oxide via mussel-inspired method for hemodialysis

Poly(lactic acid) (PLA) is an environmentally friendly material, but the hydrophobicity and poor hemocompatibility of PLA impede its application as hemodialysis membranes. In this study, aiming to improve the hemocompatibility of PLA membranes, dopamine-g-carboxylated graphene oxide (DA-g-GOCOOH) was synthesized and then immobilized on PLA membranes via a mussel-inspired adhesion method. The effect of carboxyl content of graphene oxide on hemocompatibility was also investigated. Attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis confirmed that DA-g-GOCOOH was successfully immobilized on the PLA membranes. The significant improvement of hydrophilicity and electronegativity of the PLA membranes effectively alleviated the surface adhesion of platelets, prolonged the recalcification time and reduced the hemolysis ratio to less than 0.3%. Moreover, the DA-g-GOCOOH modified PLA membrane showed excellent dialysis performance, especially for the clearance of middle molecule toxin, which was up to 24%. The DA-g-GOCOOH immobilized layers were relatively stable after incubating in water. The present work demonstrated a potential way to improve the hemocompatibility of PLA membranes for hemodialysis.