Molecular transport in ionic liquid/nanomembrane hybrids

Anti-Arrhenius passage of gaseous molecules through nanoporous two-dimensional membranes

The passage of molecules through membranes is known to follow an Arrhenius-like kinetics, i.e. the flux is accelerated upon heating and vice versa. There exist though stepwise processes whose rates can decrease with temperature if, for example, adsorbed intermediates are involved. In this study, we perform temperature-variable permeation experiments in the range from -50 to +50 °C and observe anti-Arrhenius behaviour of water and ammonia permeating in two-dimensional freestanding carbon nanomembranes (CNMs). The permeation rate of water vapour is found to decrease many-fold with warming, while the passage of ammonia molecules strongly increases when the membrane is cooled down to the dew point. Liquefaction of isobutylene shows no enhancement for its transmembrane flux which is consistent with the material's pore architecture. The effects are described by the Clausius-Clapeyron relationship and highlight the key role of gas-surface interactions in two-dimensional membranes.

Ammonium-, phosphonium- and sulfonium-based 2-cyanopyrrolidine ionic liquids for carbon dioxide fixation

The development of carbon dioxide (CO₂) scavengers is an acute problem nowadays because of the global warming problem. Many groups around the globe intensively develop new greenhouse gas scavengers. Room-temperature ionic liquids (RTILs) are seen as a proper starting point to synthesize more environmentally friendly and high-performance sorbents. Aprotic heterocyclic anions (AHA) represent excellent agents for carbon capture and storage technologies. In the present work, we investigate RTILs in which both the weakly coordinating cation and AHA bind CO₂. The ammonium-, phosphonium-, and sulfonium-based 2-cyanopyrrolidines were investigated using the state-of-the-art method to describe the thermochemistry of the CO₂ fixation reactions. The infrared spectra and electronic and structural properties were simulated at the hybrid density functional level of theory to characterize the reactants and products of the chemisorption reactions. We conclude that the proposed CO₂ capturing mechanism is thermodynamically allowed and discuss the difference between different families of RTILs. Quite unusually, the intramolecular electrostatic attraction plays an essential role in stabilizing the zwitterionic products of the CO₂ chemisorption. The difference in chemisorption performance between the families of RTILs is linked to sterical hindrances and nucleophilicities of the α- and β-carbon atoms of the aprotic cations. Our results rationalize previous experimental CO₂ sorption measurements (Brennecke et al., 2021).

Thickness-Varied Carbon Nanomembranes from Polycyclic Aromatic Hydrocarbons

Despite the prospects of intrinsically porous planar nanomaterials in separation applications, their synthesis on a large scale remains challenging. In particular, preparing water-selective carbon nanomembranes (CNMs) from self-assembled monolayers (SAMs) is limited by the cost of epitaxial metal substrates and molecular precursors with specific chemical functionalities. In this work, we present a facile fabrication of CNMs from polycyclic aromatic hydrocarbons (PAHs) that are drop-cast onto arbitrary supports, including foils and metalized films. The electron-induced carbonization is shown to result in continuous membranes of variable thickness, and the material is characterized with a number of spectroscopic and microscopic techniques. Permeation measurements with freestanding membranes reveal a high degree of porosity, but the selectivity is found to strongly depend on the thickness. While the permeance of helium remains almost the same for 6.5 and 3.0 nm thick CNMs, water permeance increases by 2 orders of magnitude. We rationalize the membrane performance with the help of kinetic modeling and vapor adsorption experiments.

Surface coverage of alcohols on carbon nanomembranes under ambient conditions

Molecular adsorption on 2D membranes plays a key role in surface-mediated permeation offering selectivity benefits for chemical separation. As many vaporous compounds are demonstrated to pass 2D membranes faster than...

Triethylsulfonium-Based Ionic Liquids Enforce Lithium Salts Electrolytes

The demand for energy cheap production and efficient storage is huge nowadays. Sulfonium-based ionic liquids were shown to exhibit a useful set of physical-chemical and electrochemical properties which make them...

Vapor Adsorption Measurements with Two-Dimensional Membranes

Two-dimensional (2D) membranes display extraordinary mass transfer properties, in particular for the permeation of gaseous substances. Their ultimate thickness not only ensures the shortest diffusion pathways, but also makes the membrane surface play a significant role in accommodating and guiding the permeating molecules. As saturated vapors of water and organic solvents are often observed to pass 2D membranes faster than inert gases, condensation is believed to be responsible for surface-mediated transport. Here, we present a spectroscopic experiment to probe adsorption of condensable species on 2D membranes under realistic conditions. Polarization-modulation infrared reflection absorption spectroscopy (PM IRAS) is coupled with a reaction chamber and a vacuum system to control the vaporous environments. The measurements are demonstrated to yield quantitative information on the amount of adsorbates onto supported 2D layers. As a case study, the azeotropic mixture of water and propanol is revealed to maintain its molar composition upon interaction with carbon nanomembranes.