Ion-Gated Gas Separation through Porous Graphene

Porous graphene holds great promise as a one-atom-thin, high-permeance membrane for gas separation, but to precisely control the pore size down to 3–5 Å proves challenging. Here we propose an ion-gated graphene membrane comprising a monolayer of ionic liquid-coated porous graphene to dynamically modulate the pore size to achieve selective gas separation. This approach enables the otherwise nonselective large pores on the order of 1 nm in size to be selective for gases whose diameters range from 3 to 4 Å. We show from molecular dynamics simulations that CO₂, N₂, and CH₄ all can permeate through a 6 Å nanopore in graphene without any selectivity. But when a monolayer of [emim]­[BF₄] ionic liquid (IL) is deposited on the porous graphene, CO₂ has much higher permeance than the other two gases. We find that the anion dynamically modulates the pore size by hovering above the pore and provides affinity for CO₂, while the larger cation (which cannot go through the pore) holds the anion in place via electrostatic attraction. This composite membrane is especially promising for CO₂/CH₄ separation, yielding a CO₂/CH₄ selectivity of about 42 and CO₂ permeance of ∼10⁵ GPU (gas permeation unit). We further demonstrate that selectivity and permeance can be tuned by the anion size, pore size, and IL thickness. The present work points toward a promising direction of using the atom-thin ionic liquid/porous graphene hybrid membrane for high-permeance, selective gas separation that allows a greater flexibility in substrate pore size control

FTIR investigation of the interfacial properties and mechanisms of CO2 sorption in porous ionic liquids

Porous liquids, which are liquids with permanent porosity, have received significant attention as a new class of materials with the potential for far-reaching impacts in a variety of applications including gas separation. In this work, in situ Fourier transform infrared spectroscopy measurements were conducted to investigate the mechanism of carbon dioxide absorption in a porous ionic liquid consisting of ZIF-8 combined with 8,8′-(3,6-dioxaoctane-1,8-diyl)bis(1,8-diazabicyclo[5.4.0]undec-7-en-8-ium) bis(trifluoromethanesulfonyl)imide ([DBU-PEG][(Tf2N)2]). While the vibrational modes of the pure ionic liquid remain relatively unchanged, the incorporation of carbon dioxide leads to slight structural fluctuations in the ZIF-8 framework whether it is pure solid or as integrated into the porous ionic liquid. The analysis of the vibrational modes of the porous ionic liquid suggests that the interaction of the CO2 occurs more strongly with the ring structure of the ZIF-8 framework. The splitting of the asymmetric stretch of the CO2 into multiple peaks upon sorption indicate the presence of multiple environments, which could be a combination of free and physisorbed CO2 or simply multiple binding sites within the porous ionic liquid. A better understanding of gas sorption mechanisms in this unique material could lead to new porous ionic liquids with enhanced separations properties

Benzyl-Functionalized Room Temperature Ionic Liquids for CO₂/N₂ Separation

In this work, three classes of room temperature ionic liquids (RTILs), including imidazolium, pyridinium, and pyrrolidinium ionic liquids with a benzyl group appended to the cation, were synthesized and tested for their performance in separating CO₂ and N₂. All RTILs contained the bis(trifluoromethylsulfonyl)imide anion, permitting us to distinguish the impact of the benzyl moiety attached to the cation on gas separation performance. In general, the attachment of the benzyl group increased the viscosity of the ionic liquid compared with the unfunctionalized analogs and decreased the CO₂ permeability. However, all of the benzyl-modified ionic liquids exhibited enhanced CO₂/N₂ selectivities compared with alkyl-based ionic liquids, with values ranging from 22.0 to 33.1. In addition, CO₂ solubilities in the form of Henry’s constants were also measured and compared with unfunctionalized analogs. Results of the membrane performance tests and CO₂ solubility measurements demonstrate that the benzyl-functionalized RTILs have significant potential for use in the separation of carbon dioxide from combustion products

Mechanochemistry-driven phase transformation of crystalline covalent triazine frameworks assisted by alkaline molten salts

Covalent triazine frameworks (CTFs) have shown wide applications in the fields of separation, catalysis, energy storage, and beyond. However, it is a long-term challenging subject to fabricate high-quality CTF materials via facile procedures. Herein, a mechanochemistry-driven procedure was developed to achieve phase transformation of crystalline CTFs assisted by alkaline molten salts. The transformation of CTF-1 from staggered AB to eclipsed AA stacking mode was achieved by short time (30 min) mechanochemical treatment in the presence of molten salts composed of LiOH/KOH, generating high-quality CTF-1 material with high crystallinity, high surface area (625 m2 g−1), and permanent/ordered porosity without carbonization under ambient conditions. This facile procedure could be extended to provide nanoporous three-dimensional CTF materials.This is a manuscript of an article published as Fan, Juntian, Xian Suo, Tao Wang, Zongyu Wang, Chi-Linh Do-Thanh, Shannon M. Mahurin, Takeshi Kobayashi, Zhenzhen Yang, and Sheng Dai. "Mechanochemistry-driven phase transformation of crystalline covalent triazine frameworks assisted by alkaline molten salts." Journal of Materials Chemistry A 10, no. 27 (2022): 14310-14315. DOI: 10.1039/D2TA02117J. Copyright 2022 The Royal Society of Chemistry. Posted with permission. DOE Contract Number(s): AC02-07CH11358; AC05-00OR22725

Free Energy Relationships in the Electrical Double Layer over Single-Layer Graphene

Fluid/solid interfaces containing single-layer graphene are important in the areas of chemistry, physics, biology, and materials science, yet this environment is difficult to access with experimental methods, especially under flow conditions and in a label-free manner. Herein, we demonstrate the use of second harmonic generation to quantify the interfacial free energy at the fused silica/single-layer graphene/water interface at pH 7 and under conditions of flowing aqueous electrolyte solutions ranging in NaCl concentrations from 10^–4 to 10^–1 M. Our analysis reveals that single-layer graphene reduces the interfacial free energy density of the fused silica/water interface by a factor of up to 7, which is substantial given that many interfacial processes, including those that are electrochemical in nature, are exponentially sensitive to interfacial free energy density