Haiqing Lin

High-Performance Polymers for Membrane CO2 /N2 Separation.

This Concept examines strategies to design advanced polymers with high CO2 permeability and high CO2 /N2 selectivity, which are the key to the success of membrane technology for CO2 capture from fossil fuel-fired power plants. Specifically, polymers with enhanced CO2 solubility and thus CO2 /N2 selectivity are designed by incorporating CO2 -philic groups in polymers such as poly(ethylene oxide)-containing polymers and poly(ionic liquids); polymers with enhanced CO2 diffusivity and thus CO2 permeability are designed with contorted rigid polymer chains to obtain high free volume, such as polymers with intrinsic microporosity and thermally rearranged polymers. The underlying rationales for materials design are discussed and polymers with promising CO2 /N2 separation properties for CO2 capture from flue gas are highlighted.

Membranes with Surface-Enhanced Antifouling Properties for Water Purification

Membrane technology has emerged as an attractive approach for water purification, while mitigation of fouling is key to lower membrane operating costs. This article reviews various materials with antifouling properties that can be coated or grafted onto the membrane surface to improve the antifouling properties of the membranes and thus, retain high water permeance. These materials can be separated into three categories, hydrophilic materials, such as poly(ethylene glycol), polydopamine and zwitterions, hydrophobic materials, such as fluoropolymers, and amphiphilic materials. The states of water in these materials and the mechanisms for the antifouling properties are discussed. The corresponding approaches to coat or graft these materials on the membrane surface are reviewed, and the materials with promising performance are highlighted.

Designing ultrathin film composite membranes: the impact of a gutter layer.

Industrial membranes comprised of a thin selective layer (<100 nm) requires a gutter layer (<100 nm) between the selective layer and the porous support to achieve high permeance for gas separation. The gutter layer materials must be carefully chosen to enhance overall membrane performance, i.e., high permeance and high selectivity. However, the experimental determination of the optimum gutter layer properties is very challenging. Herein we address this need using a three dimensional (3D) computational model to systematically determine the effects of the gutter layer thickness and permeability on membrane performance. A key finding is that the introduction of a gutter layer between the selective layer and porous support can enhance the overall permeance of the penetrant by up to an order of magnitude, but this gain is accompanied by an undesired decrease in selectivity. The analysis also shows for the first time that a maximum increase in permeance with negligible decrease in selectivity is realized when the thickness of the gutter layer is 1-2 times the pore radius. The modeling approach provides clear and practical guidelines for designing ultrathin multilayer composite membranes to achieve high permeance and selectivity for low-cost and energy-efficient molecular separations.

Preparation and gas transport properties of triptycene-containing polybenzoxazole (PBO)-based polymers derived from thermal rearrangement (TR) and thermal cyclodehydration (TC) processes

Polybenzoxazoles (PBOs), such as thermally rearranged (TR) polymers, have been shown to have excellent gas separation performance. Herein we report the preparation and transport properties of two new series of PBO-based polymers that were thermally derived from triptycene-containing o-hydroxy polyimide and polyamide precursors via a thermal rearrangement (TR) process and a thermal cyclodehydration (TC) process, respectively. Incorporation of triptycene units into poly(hydroxyimide) precursor structures led to a significant increase of fractional free volume and created ultrafine microporosity in the converted PBO-based TR polymers, which enabled both high gas permeabilities and high selectivities. Although the TC process of the poly(hydroxyamide) precursor led to moderate improvement in the separation performance of the resulting triptycene-containing PBO polymers as compared to the TR process, the PBO films converted via the TC process exhibited excellent mechanical properties superior to many other TR polymers previously reported in the literature as well as the triptycene-containing TR polymers in this study. In particular, the PBO film thermally rearranged at 450 °C showed a H2 pure gas permeability of 810 barrer, a CO2 permeability of 270 barrer, and CO2/CH4 and H2/CH4 selectivities of 67 and 200, respectively, at 35 °C and 11 atm, which are far beyond the upper bound limits.

Structurally Defined 3D Nanographene Assemblies via Bottom-Up Chemical Synthesis for Highly Efficient Lithium Storage

Functionalized 3D nanographenes with controlled electronic properties have been synthesized through a multistep organic synthesis method and are further used as promising anode materials for lithium-ion batteries, exhibiting a much increased capacity (up to 950 mAh g-1 ), three times higher than that of the graphite anode (372 mAh g-1 ).

Tightening polybenzimidazole (PBI) nanostructure via chemical cross-linking for membrane H2/CO2 separation

Membranes that permeate H2 and reject CO2 at temperatures above 150 °C are of great interest for low-cost H2 purification and pre-combustion CO2 capture. One of the leading polymers for this separation is poly[2,2′-(m-phenylene)-5,5′-bisbenzimidazole] (PBI), which has good thermal stability and high H2/CO2 selectivity. This study, for the first time, demonstrates that H2/CO2 selectivity can be significantly enhanced by chemical cross-linking of PBI in solid state, in distinct contrast with the literature where cross-linking PBI in solutions decreased H2/CO2 selectivity. We prepared a series of cross-linked PBIs by immersing PBI thin films in terephthaloyl chloride solutions for varying times to achieve different degrees of cross-linking, and then systematically investigated the effect of cross-linking on physical properties including gel content, thermal stability, cross-linking density, fractional free volume (FFV) and inter-chain spacing. Gas sorption and pure- and mixed-gas permeation properties were determined at temperatures ranging from 35 to 200 °C. Cross-linking decreased CO2 sorption and significantly increased H2/CO2 selectivity with a slight decrease in H2 permeability. For example, after cross-linking of PBI, the H2/CO2 selectivity increased from 15 to 23 while the H2 permeability decreased from 45 to 39 Barrers at 200 °C. The performance of this cross-linked PBI surpasses the Robesons upper bound estimated at 200 °C, indicating its promise for H2 purification and CO2 capture.

Unprecedented size-sieving ability in polybenzimidazole doped with polyprotic acids for membrane H2/CO2 separation

Polymers with efficient and tight chain-packing and thus strong size-sieving ability are of great interest for H2/CO2 separation. Herein, we demonstrate a new approach to manipulating polymer structure by acid doping, leading to superior H2/CO2 separation performance. We have doped polybenzimidazole (PBI) with polyprotic acids, specifically H3PO4 and H2SO4. These acids cross-link PBI chains and drastically decrease free volume, improving the materials H2/CO2 selectivity to far surpass the Robesons 2008 upper bound for membrane performance. For example, PBI doped with H3PO4 at a molar ratio of 1 : 1 exhibits an unprecedented H2/CO2 selectivity of 140 at 150 °C, which exceeds that of previously known polymeric materials and is superior or comparable to that of state-of-the-art 2D materials with sharp size separation, such as graphene oxide, MoS2, and metal–organic frameworks. This facile approach to enhancing polymer chain-packing efficiency opens up a new avenue for designing strong size-sieving polymers for membrane gas separations.

Synthesis of Hydrogels with Antifouling Properties As Membranes for Water Purification

Hydrogels have been widely utilized to enhance the surface hydrophilicity of membranes for water purification, increasing the antifouling properties and thus achieving stable water permeability through membranes over time. Here, we report a facile method to prepare hydrogels based on zwitterions for membrane applications. Freestanding films can be prepared from sulfobetaine methacrylate (SBMA) with a crosslinker of poly(ethylene glycol) diacrylate (PEGDA) via photopolymerization. The hydrogels can also be prepared by impregnation into hydrophobic porous supports to enhance the mechanical strength. These films can be characterized by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to determine the degree of conversion of the (meth)acrylate groups, using goniometers for hydrophilicity and differential scanning calorimetry (DSC) for polymer chain dynamics. We also report protocols to determine the water permeability in dead-end filtration systems and the effect of foulants (bovine serum albumin, BSA) on membrane performance.

Rightsizing Nanochannels in Reduced Graphene Oxide Membranes by Solvating for Dye Desalination

Membranes with high water permeance, near-zero rejection to inorganic salts (such as NaCl and Na2SO4), and almost 100% rejection to organic dyes are of great interest for the dye desalination (the separation of dyes and salts) of textile wastewater. Herein, we prepared reduced graphene oxide membranes in a solvation state (S-rGO) with nanochannel sizes rightly between the salt ions and dye molecules. The S-rGO membrane rejects >99.0% of Direct Red 80 (DR 80) and has almost zero rejection for Na2SO4. By contrast, conventional GO or rGO membranes often have channel sizes smaller than divalent ions (such as SO42-) and thus high rejection for Na2SO4. More interestingly, high salinity in typical dye solutions decreases the channel size in the S-rGO membranes and thus increases the dye rejection, while the Na2SO4 rejection decreases because of the negatively charged surface on GO and the salt screening effect. The membranes also show pure water permeance as high as 80 L m-2 h-1 bar-1, which is about 8 times that of commercial NF 90 membrane and 2 times that of a commercial ultrafiltration membrane (with a molecular weight cutoff of 2000 Da), rendering their promise for practical dye desalination.

CHAPTER 9:Doping Polymers with Ionic Liquids to Manipulate Their Morphology and Membrane Gas Separation Properties

Polymeric membranes for gas separation exhibit an intrinsic trade-off between gas permeability and selectivity, i.e., polymers with higher permeability tend to have lower selectivity. To overcome this conundrum, polymers have been doped with ionic liquids (ILs) to enhance their gas permeability and selectivity, since ILs have very low vapor pressure and high CO2 solubility and permeability. The effect of IL doping on polymer morphology (such as glass transition temperature, melting temperature and polymer crystallinity) and gas transport properties is reviewed, and quantitative models are presented. In general, IL doping depresses melting temperature and crystallinity, which improves gas permeability. Such an effect is exemplified in semi-crystalline cellulose acetate (CA) and cellulose triacetate (CTA), which have been used to prepare commercial membranes for CO2/CH4 separation. IL doping can decrease the crystallinity in CA and CTA and increase CO2/CH4 solubility selectivity, resulting in enhanced CO2/CH4 and CO2/N2 separation properties. With appropriate ILs, doping provides an effective route to overcome the intrinsic trade-off of permeability and selectivity in polymers to achieve superior separation properties.