The molecular basis of CO2 interaction with polymers containing fluorinated groups: computational chemistry of model compounds and molecular simulation of poly[bis(2,2,2-trifluoroethoxy)phosphazene]

Increased CO2/N2 selectivity by stepwise fluorination in isoreticular ultramicroporous metal-organic frameworks

Exploration of porous adsorbents with high CO2/N2 selectivity is of great significance for reducing CO2 content in the atmosphere. In this study, a series of isoreticular ultramicroporous fluorinated metal-organic frameworks (MOFs) were prepared to explore the benefits of fluorinated ultramicropores in improving CO2/N2 selectivity. Gas adsorption measurements revealed that the increase in the number of fluorine atoms in a ligand molecule leads to the increased CO2 uptakes and CO2/N2 selectivity. Theoretical calculations indicate that the interaction between the fluorine atoms and adsorbed CO2 molecules enhances the CO2-philicity, offering useful insight into the improvement of CO2/N2 selectivity in isoreticular frameworks.

Systematic Review of Solubility, Thickening Properties and Mechanisms of Thickener for Supercritical Carbon Dioxide

Supercritical carbon dioxide (CO2) has extremely important applications in the extraction of unconventional oil and gas, especially in fracturing and enhanced oil recovery (EOR) technologies. It can not only relieve water resource wastage and environmental pollution caused by traditional mining methods, but also effectively store CO2 and mitigate the greenhouse effect. However, the low viscosity nature of supercritical CO2 gives rise to challenges such as viscosity fingering, limited sand-carrying capacity, high filtration loss, low oil and gas recovery efficiency, and potential rock adsorption. To overcome these challenges, low-rock-adsorption thickeners are required to enhance the viscosity of supercritical CO2. Through research into the literature, this article reviews the solubility and thickening characteristics of four types of polymer thickeners, namely surfactants, hydrocarbons, fluorinated polymers, and silicone polymers in supercritical CO2. The thickening mechanisms of polymer thickeners were also analyzed, including intermolecular interactions, LA-LB interactions, hydrogen bonding, and functionalized polymers, and so on.

Analysis of the Influence of Process Parameters on the Properties of Homogeneous and Heterogeneous Membranes for Gas Separation

Heterogeneous membranes, otherwise known as Mixed Matrix Membranes (MMMs), which are used in gas separation processes, are the subject of growing interest. This is due to their potential to improve the process properties of membranes compared to those of homogeneous membranes, i.e., those made of polymer only. Using such membranes in a process involves subjecting them to varying temperatures and pressures. This paper investigates the effects of temperature and feed pressure on the process properties of homogeneous and heterogeneous membranes. Membranes made of Pebax®2533 copolymer and containing additional fillers such as SiO2, ZIF-8, and POSS-Ph were investigated. Tests were performed over a temperature range of 25-55 °C and a pressure range of 2-8 bar for N2, CH4, and CO2 gases. It was found that temperature positively influences the increase in permeability, while pressure influences permeability depending on the gas used, which is related to the effect of pressure on the solubility of the gas in the membrane.

Synthesis and gas transport properties of sulfonated poly(ether ether sulfone) membranes containing pendant sulfonic acid groups

A series of gas separation membranes based on sulfonated poly(ether ether sulfone)s (SPEESs) can be synthesized by the polycondensation and sulfonation reactions. The structural characteristics of copolymers were confirmed by 1H-NMR spectroscopy, Fourier-transform infrared spectroscopy, gel permeation chromatography, and ultraviolet–visible absorption spectroscopy. The permeability and selectivity behavior of these membranes were investigated using three single-gases (CO2, O2, and N2) at different temperatures of 25–55°C and pressures of 1.0–3.0 atm. The effect of sulfonation degree (SD), operating temperature, and pressure on gas permeability was explored and discussed. The results showed that SPEESs containing pendant sulfonic acid groups exhibited different separation performance. In particular, the SPEES-4 membrane with an SD of 67% exhibited the highest permeability (CO2 = 513.8 Barrer and O2 = 78.19 Barrer) at 55°C and 1 atm, whereas CO2/N2 and O2/N2 selectivity was 31.09 and 4.73, respectively.

Modulator-Mediated Functionalization of MOF-808 as a Platform Tool to Create High-Performance Mixed-Matrix Membranes

Modulator-mediated functionalization (MoFu) is introduced as a new and versatile platform tool to improve the separation performance of metal-organic framework (MOF)-based membranes, exemplified here by the creation of mixed-matrix membranes (MMMs) with enhanced CO₂ separation efficiency. The unique structure of MOF-808 allows incorporation of CO₂-philic modulators in the MOF framework during a one-pot synthesis procedure in water, thus creating a straightforward way to functionalize both MOF and corresponding MMM. As a proof of concept, a series of fluorinated carboxylic acids [trifluoroacetic acid (TFA), pentafluoropropionic acid (PFPA), and heptafluorobutyric acid (HFBA)] and nonfluorinated alkyl carboxylic acids (acetic acid (AA), propionic acid (PA), and butyric acid (BA)) were used as a modulator during MOF-808 synthesis. Two of the best MMMs prepared with 30 wt % MOF-TFA (100% increase in CO₂/CH₄ separation factor, 350% increase in CO₂ permeability) and 10 wt % MOF-PFPA (140% increase in CO₂/CH₄ separation factor, 100% increase in CO₂ permeability) scored very close to or even crossed the 2008 and 2018 upper bound limits for CO₂/CH₄. Because of its facile functionalization (and its subsequent excellent performance), MOF-808 is proposed as an alternative for widely used UiO-66, which is, from a functionalization point-of-view and despite its widespread use, a rather limited MOF.

Micro-scale effects of nano-SiO2 modification with silane coupling agents on the cellulose/nano-SiO2 interface

In this study, molecular dynamics simulations were used to investigate the micro-scale effects of modification of nano-SiO₂ with commonly used silane coupling agents (KH550, KH560, KH570, and KH792) on the cellulose/nano-SiO₂ interface. The relative optimum silane coupling agent and grafting density for nano-SiO₂ modification to improve the cellulose/nano-SiO₂ interface were determined. The results showed that at the same grafting density, modification of nano-SiO₂ with KH792 yielded the highest interfacial binding energy and binding energy density, the largest number of hydrogen bonds at the cellulose/nano-SiO₂ interface, the strongest binding to the cellulose chains, and the largest overlapping area at the cellulose/nano-SiO₂ interface. We found that the non-bonding interaction energy played a decisive role in the energy of the model system and the interfacial interaction force mainly consisted of van der Waals forces and the hydrogen-bonding energy. When silane coupling agents with amino groups (KH550 and KH792) were used to modify nano-SiO₂, the number of hydrogen bonds at the cellulose/nano-SiO₂ interface was larger than that for unmodified nano-SiO₂. When silane coupling agents without amino groups (KH560 and KH570) were used to modify nano-SiO₂, the number of hydrogen bonds at the cellulose/nano-SiO₂ interface was lower than the case for unmodified nano-SiO₂. Nano-SiO₂ modification with various amounts of KH792 was investigated. The results showed that the interfacial bonding energy increased with grafting density. When the grafting density was 1.57 nm^-2, the interfacial bonding energy and number of hydrogen bonds formed at the cellulose/nano-SiO₂ interface was relatively stable, which indicates that the interface had reached a relatively stable state. Modification of nano-SiO₂ with KH792 achieved the greatest improvement of the cellulose/nano-SiO₂ interface; this interface reached a relatively stable state when the grafting density of KH792 was 1.57 nm^-2.

Modeling Solvation in Supercritical CO2

In recent decades, a microscopic understanding of solute-solvent intermolecular interactions has been key to advances in technologies based on supercritical carbon dioxide. In many cases, computational work has provided the impetus for new discoveries, shedding new light on important concepts such as the local structure around the solute in the supercritical medium, the influence of the peculiar properties of the latter on the molecular behavior of dissolved substances and, importantly, CO₂ -philicity. In this Review, the theoretical work that has been relevant to these developments is surveyed and, by presenting some crucial open questions, the possible routes to achieving further progress based on the interplay between theory and experiments is discussed.

Microcosmic understanding on thickening capability of copolymers in supercritical carbon dioxide: the key role of π–π stacking

Thickening capability evaluations and microscopic understanding of St–HFDA copolymers in SC-CO₂.

Complexes of carbon dioxide with dihalogenated ethylenes: structure, stability and interaction

Interactions of ethylene and its 1,2-dihalogenated derivatives with CO₂ induce twenty four molecular complexes with stabilization energies in the range of 1.1 to 7.5 kJ mol⁻¹ as computed at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level of theory.

Assessing the non-ideality of the CO2-CS2 system at molecular level: a Raman scattering study

The dense phase of CO2-CS2 mixtures has been analysed by Raman spectroscopy as a function of the CO2 concentration (0.02-0.95 mole fractions) by varying the pressure (0.5 MPa up to 7.7 MPa) at constant temperature (313 K). The polarised and depolarised spectra of the induced (ν2, ν3) modes of CS2 and of the ν1-2ν2 Fermi resonance dyad of both CO2 and CS2 have been measured. Upon dilution with CO2, the evolution of the spectroscopic observables of all these modes displays a "plateau-like" region in the CO2 mole fraction 0.3-0.7 never previously observed in CO2-organic liquids mixtures. The bandshape and intensity of the induced modes of CS2 are similar to those of pure CS2 up to equimolar concentration, after which variations occur. The preservation of the local ordering from pure CS2 to equimolar concentration together with the non-linear evolution of the spectroscopic observables allows inferring that two solvation regimes exist with a transition occurring in the plateau domain. In the first regime, corresponding to CS2 concentrated mixtures, the liquid phase is segregated with dominant CS2 clusters, whereas, in the second one, CO2 monomers and dimers and CO2-CS2 hetero-dimers coexist dynamically on a picosecond time-scale. It is demonstrated that the subtle interplay between attractive and repulsive interactions which provides a molecular interpretation of the non-ideality of the CO2-CS2 mixture allows rationalizing the volume expansion and the existence of the plateau-like region observed in the pressure-composition diagram previously ascribed to the proximity of an upper critical solution temperature at lower temperatures.

Synthesis and characterization of low surface energy fluoropolymers as potential barrier coatings in oral care

A series of low surface energy fluorinated homopolymers and copolymers has been synthesized and characterized using thermal, optical, spectroscopic, and chromatographic techniques. Their utility as barrier technologies in oral care has been considered, and aqueous nanosuspensions of the materials have been deposited as films on model dental hard surfaces in the presence and absence of a salivary pellicle. Calcium hydroxyapatite has been used as a model for enamel, as has PMMA due to its widespread use in denture fabrication. Surface energy determinations, combined with XPS studies, have provided insights into the molecular-level organization at the surface of the film structures. Studies of solubility in supercritical carbon dioxide have identified the polymers that are suitable for processing in this medium.

Improving carbon dioxide solubility in ionic liquids

Previously we showed that CO2 could be used to extract organic molecules from ionic liquids without contamination of the ionic liquid. Consequently a number of other groups demonstrated that ionic liquid/CO2 biphasic systems could be used for homogeneously catalyzed reactions. Large differences in the solubility of various gases in ionic liquids present the possibility of using them for gas separations. More recently we and others have shown that the presence of CO2 increases the solubility of other gases that are poorly soluble in the ionic liquid phase. Therefore, a knowledge and understanding of the phase behavior of these ionic liquid/CO2 systems is important. With the aim of finding ionic liquids that improve CO2 solubility and gaining more information to help us understand how to design CO2-philic ionic liquids, we present the low- and high-pressure measurements of CO2 solubility in a range of ionic liquids possessing structures likely to increase the solubility of CO2. We examined the CO2 solubility in a number of ionic liquids with systematic increases in fluorination. We also studied nonfluorinated ionic liquids that have structural features known to improve CO2 solubility in other compounds such as polymers, for example, carbonyl groups and long alkyl chains with branching or ether linkages. Results show that ionic liquids containing increased fluoroalkyl chains on either the cation or anion do improve CO2 solubility when compared to less fluorinated ionic liquids previously studied. It was also found that it was possible to obtain similar, high levels of CO2 solubility in nonfluorous ionic liquids. In agreement with our previous results, we found that the anion frequently plays a key role in determining CO2 solubility in ionic liquids.