Gas-expanded Liquids Research Articles

Toward a CO 2-free ethylene oxide process: Homogeneous ethylene oxide in gas-expanded liquids

Among industrial chemical processes, ethylene oxide manufacture emits the largest amount of CO 2 (∼2–3 million tons/yr), as byproduct from the burning of both the ethylene (feed) and ethylene oxide. Further, the conventional silver-based catalytic process presents safety challenges due to the formation of explosive ethylene oxide/O 2 mixtures in the gas phase. By judicious choice of the catalyst (methyltrioxorhenium), oxidant (H 2O 2) and reaction medium (methanol/water), a homogeneous liquid phase catalytic system has been demonstrated that eliminates CO 2 formation while producing ethylene oxide at >90% selectivity at near-ambient temperatures. Given its high volatility, the ethylene oxide is easily recovered from the reaction phase by distillation. The vicinity of the gaseous ethylene feed to its critical temperature (9 °C) is exploited to significantly increase its solubility in the liquid reaction phase by facile compression beyond the critical pressure of ethylene (∼50 bar). Since H 2O 2 is stable at typical reaction temperatures (40 °C or less), potentially explosive ethylene oxide/O 2 mixtures are avoided in the gas phase. In addition to the potential of arresting the carbon footprint of a large-scale industrial process, the demonstrated process concept shows how gas-expanded liquids can be generally exploited in homogeneous catalysis to enhance productivity.

Probing the structure of gas expanded liquids using relative permittivity, density and polarity measurements

Gas expanded liquids (GXLs) have been shown to be useful solvents with interesting properties between those of liquids and supercritical fluids. In this work the physical properties of 15 fluids are quantified at 50 bar CO2 pressure and 25 °C and the data were used to gain an insight into the bulk and local structure upon pressurisation. It is shown that high CO2 solubilities can be obtained in all solvents except the higher alcohols. Density measurements show that upon pressurisation the free volume of most solvents increases by up to 10%. The Kamlet and Taft and ET parameters for the expanded solvents show that preferential solvation of the indicator solute occurs but that the ratio of solvent in the cybotactic and bulk regions remain roughly constant at a given pressure.

A critical look at reactions in class I and II gas-expanded liquids using CO2 and other gases

This short review aims to give a summary of the publications on reactions in class I and II gas-expanded liquids (GXLs) (those with organic or aqueous liquid components), and to draw conclusions from the trends in the current literature.

Solvation and Solvatochromism in CO2-Expanded Liquids. 3. The Dynamics of Nonspecific Preferential Solvation

Subpicosecond time-resolved fluorescence of trans-4-dimethylamino-4'-cyanostilbene (DCS) is used to measure solvation dynamics in the gas-expanded liquid (GXL) system CH(3)CN + CO(2) at 25 degrees C along the liquid-vapor coexistence curve. These measurements are supplemented by measurements of the steady-state solvatochromism of DCS and of its rotation and isomerization times. Molecular dynamics computer simulations and a semiempirical spectral model that reproduces the observed solvatochromism in this system are used to interpret the experimental results. Simulations indicate that at compositions of x(CO2) > 0.5, the cybotactic region surrounding DCS is enriched in CH(3)CN molecules, and the extent of this enrichment is greater in S(1) than that in S(0). Solvation dynamics are dominated by the CH(3)CN component. These dynamics are biphasic, consisting of a subpicosecond inertial component, followed by a slower picosecond component, related to the redistribution of CH(3)CN molecules between the cybotactic region and the bulk solvent.

Gold Nanoparticle Deposition Using CO2 Expanded Liquids: Effect of Pressure Oscillation and Surface−Particle Interactions

Postsynthesis processing of nanoparticles to obtain mesoscale hierarchal nanostructures is the key for the development of nanotechnology and smart composites/coatings from these materials. We have utilized gas-expanded liquid deposition of alkyl-coated gold nanoparticles to study the effects of variable process flowrates, variable flow oscillation and variable interaction potential of the substrate on nanoparticle array quality. Array quality is measured here as completeness of area surface coverage of approximately a monolayer of nanoparticles. Quantitative values for surface coverage are averages obtained from multiple TEM photomicrographs using Image J digital analysis. The process was modified using higher CO2 addition rate outside of the pressure range necessary for deposition, and this modified process produced an excellent film quality while reducing overall processing time by 45%. The effects of pressure oscillation during deposition appeared to anneal the film at the lower flow rates, 0.5 and 1.0 mL/min, but a reduction in area coverage was observed with pressure oscillation at 3.0 mL/min. Pressure oscillation has emerged as a useful tool for researchers to tune the film uniformity and therefore the surface roughness. Calculations based on Hamaker theories for surface-particle interactions on various substrates were performed, and better surface coverage was predicted for C-based surfaces compared to Si3N4 and SiO2 surfaces. Indeed, experimental studies verified this general ordering, indicating that if surface interactions with the particles are strong deposition directly on the surface rather than on pre-existing nanoparticle islands may govern uniform deposition.

Local Polarity in CO2-Expanded Acetonitrile: A Nucleophilic Substitution Reaction and Solvatochromic Probes

Many processes that use highly tunable gas-expanded liquids (GXLs) rely on the fact that CO2 addition can greatly affect the polarity of the solvent. We have examined several measures of bulk and local polarity in CO2-expanded acetonitrile to enable more effective exploitation of these polarity changes. The rate of the nucleophilic substitution reaction of tributylamine with methyl p-nitrobenzenesulfonate has been analyzed as a function of solvent composition by using in situ high-pressure UV/vis spectroscopy. We have also measured solvatochromic properties including the Kamlet-Taft pi* parameter and Kosower's Z-value. We correlate these local polarity-based kinetic and solvatochromic measures to develop a better understanding of these property changes as a function of bulk and local solvent composition. The data suggest that local composition enhancement in CO2-expanded acetonitrile has a significant impact on the reaction kinetics.

Hydroformylation Catalyst Recycle with Gas-Expanded Liquids

Tunable solvents such as gas-expanded liquids offer unique opportunities as benign and economic reaction media. We have improved the turnover frequency of a water-soluble catalyst system by a factor of 85 for the hydroformylation of 1-octene by using a tunable solvent system, which also increases substrate solubility. In our approach, the reaction is run homogeneously in a miscible water−organic solution, and after reaction, CO2 is used as a “miscibility switch” for efficient product and catalyst recovery. Two water-soluble ligands, TPPTS (tris(3-sulfophenyl)phosphine) and TPPMS ((3-sulfophenyl)diphenylphosphine), were compared to triphenylphosphine for hydroformylation activity of their rhodium complexes. The catalyst was recycled three times with no loss of activity.

Thermodynamic Analysis of Nanoparticle Size Selective Fractionation Using Gas-Expanded Liquids

A thermodynamic model was developed for the size-selective fractionation of ligand-stabilized nanoparticles by a CO2 gas-expanded liquid precipitation process. The tunable solvent strength of gas-expanded liquids, via CO2 pressurization, results in an effective method to fractionate nanoparticles, based on the size-dependent dispersibility of the particles. Specifically, the thermodynamic model is used to estimate the size of dodecanethiol-capped Ag nanoparticles that can be dispersed at a given CO2 pressure by equating the total interparticle interaction energy to the Boltzmann threshold stabilization energy (−3/2kBT). The ligand−solvent interaction is found to have the greatest impact on the total interaction energy. This model illustrates that the entire length of the ligand is not accessible to the solvent, and three phenomenological model variations were developed to vary the ligand−solvent interaction.

Viscosity measurements on saturated gas-expanded liquid systems—Ethanol and carbon dioxide

A recently developed method of falling-weight viscometry was employed to determine the experimental viscosities of the gas-expanded liquid system of ethanol and carbon dioxide at saturation. By adding carbon dioxide into an isochoric system containing metered amounts of ethanol, carbon dioxide concentrations within the gas-expanded liquid system were systematically varied between 0.15 and 0.80 mole fraction in 0.05 increments at constant system temperatures of 25, 30, 35 and 40 °C. Similar to the gas-expanded liquid system of methanol and carbon dioxide, an increase in carbon dioxide concentration in the liquid phase, liquid volume expansion, liquid density and system pressure resulted in a decrease in liquid phase viscosity. The significance of system temperature however, is more apparent with the gas-expanded liquid system of ethanol and carbon dioxide when compared to the gas-expanded liquid system of methanol and carbon dioxide. At equal compositions but varying system temperatures, not only is the viscosity of the ethanol and carbon dioxide system affected to a greater extent, the point at which viscosity reduction is insignificant with an increase in carbon dioxide composition occurs earlier with increasing system temperatures. The estimation of liquid phase viscosities of gas-expanded liquid systems is therefore difficult without experimental data.

Solvent Effects on the Kinetics of a Diels−Alder Reaction in Gas-Expanded Liquids

Gas-expanded liquids (GXLs) form a unique class of environmentally benign solvents that offer many of the benefits of both organic liquids and supercritical fluids. A more complete understanding of the interactions between the gas, the organic liquid, and solutes at the molecular level will enable the full exploitation of GXLs. Combining kinetic and solvatochromic studies, we have developed a comprehensive multiparameter approach to add insight into the molecular interactions related to a Diels−Alder reaction in CO2-expanded acetonitrile. We have studied the kinetics of the Diels−Alder reaction of anthracene and 4-phenyl-1,2,4-triazoline-3,5-dione as a function of solvent composition using in situ high-pressure fluorescence spectroscopy. We have also measured the values of the Kamlet−Taft solvatochromic parameters π* (dipolarity/polarizibility), α (hydrogen bonding acidity), and β (hydrogen bonding basicity) in CO2-expanded acetonitrile using in situ high-pressure UV/vis spectroscopy. A linear solvation e...

Tunable solvents for fine chemicals from the biorefinery

Making biorefineries economically and environmentally sustainable is the biggest barrier to the mass commercialization of biofuels in this country. One way to add economic value to the biofuel process is to isolate and extract fine chemicals from waste biomass such as lignin, extractives, and unreacted cellulose and hemicellulose. In this paper we demonstrate a technique for extracting high-value added chemicals such as vanillin, syringaldehyde, and syringol from lignin using a novel CO2-expanded organic solvent (gas-expanded liquid). This method incorporates many principles of green chemistry while offering several economical advantages to the biorefinery: low operating costs, easy recycling of organic solvents, use of a renewable feedstock, and a way to produce chemicals without wasteful synthesis. Furthermore, this technique demonstrated the ability to produce high-value chemicals ($5–25 lb−1) from a waste source that is presently being burned for a fuel value of 2–3 cents lb−1. We believe the process presented in this paper will spark interest in developing other sustainable techniques to extract fine chemicals from biorefinery waste.

Molecular Dynamics Simulation of the Cybotactic Region in Gas-Expanded Methanol−Carbon Dioxide and Acetone−Carbon Dioxide Mixtures

Local solvation and transport effects in gas-expanded liquids (GXLs) are reported based on molecular simulation. GXLs were found to exhibit local density enhancements similar to those seen in supercritical fluids, although less dramatic. This approach was used as an alternative to a multiphase atomistic model for these mixtures by utilizing experimental results to describe the necessary fixed conditions for a locally (quasi-) stable molecular dynamics model of the (single) GXL phase. The local anisotropic pair correlation function, orientational correlation functions, and diffusion rates are reported for two systems: CO2-expanded methanol and CO2-expanded acetone at 298 K and pressures up to 6 MPa.

Probing the Cybotactic Region in Gas‐Expanded Liquids (GXLs)

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Solvation and Solvatochromism in CO2-Expanded Liquids. 1. Simulations of the Solvent Systems CO2 + Cyclohexane, Acetonitrile, and Methanol

Molecular dynamics simulations of CO(2)-expanded cyclohexane, acetonitrile, and methanol are reported at various compositions along the experimental bubble-point curve at 298 K. Simulated properties include energies, local compositions, viscosities, diffusion coefficients, and dielectric constants and relaxation times. On the basis of the limited comparisons to experimental data currently available, the results indicate that simple intermolecular potential models previously developed for simulating the pure components provide reasonable representations of the energetics and dynamics of these gas-expanded liquids.

Supercritical CO 2 intercalation of layered silicates

In this study, we use supercritical CO 2 as the processing medium for intercalating the structure of natural clay, Cloisite Na +, and depositing CO 2-philic sugar acetate, β- d-galactose pentaacetate, between the layers of the clay. Based on our measurements of the phase behavior of the β- d-galactose pentaacetate–CO 2 system, we select two processing conditions: a dense gas solution (DGS) at 40 °C and 11.7 MPa with 2.4 wt.% sugar acetate in CO 2 and a gas-expanded liquid (GEL) at 40 °C and 7.9 MPa with 6.1 wt.% sugar acetate in CO 2. The morphology of the as-received natural clay and clay samples processed under the DGS and the GEL conditions is determined by wide-angle X-ray diffraction (XRD), transmission electron microscopy (TEM) and low-energy electron diffraction (LEED) methods. The XRD results indicate intercalation of the clay layers. While the ordered structure remains, the gallery spacing between Cloisite Na + layers is doubled, from 0.3 1 to 0.52 and 0.59 nm for DGS- and GEL-processing conditions, respectively. The LEED patterns of the processed clays show bright spots characteristic of the highly crystalline sugar acetate superimposed on the halo pattern of the clay, confirming the deposition of the sugar acetate in the intercalated structure of the clay after scCO 2 processing.

Gas-expanded liquids: Determination of the expansion coefficient by an acoustic technique

This paper presents an experimental method for liquid level detection in gas-expanded liquids by the sound speed and amplitude. Volumetric expansion of acetonitrile and 2-propanol as a function of CO 2 pressure was determined between 298–333 K and 0–6 MPa. Propane expansion of 2-propanol was done up to 393 K and up to V/ V 0 = 8. It was clearly demonstrated that this technique is useful also for multiphase detection with high accuracy and for obtaining accurate speed of sound values without calibration. The paper also presents an experimental method for gas composition sensing. The speed of sound and sound attenuation as a function of hydrogen concentration in CO 2 and in propane atmosphere at ambient conditions were measured to obtain information whether an acoustic technique could be an alternative method for the determination of H 2 solubility in liquefied gases and in gas-expanded solvents.

Dependence of Photoresist and Etch Residue Removal on CO2 Pressure in Alcohol-Based Gas-Expanded Liquids

With rapid technology development of integrated circuits, new challenges arise. Current liquid based removal chemistries are limited by continuous decrease in feature devices as well by their hazardous nature. Gas expanded liquids (GXLs) and supercritical fluids may offer a more environtmentally benign alternative. The GXL formulated by CO2 expanded ethanol (up to 75 mole % CO2) removed PHOST (polyhydroxystyrene) photoresist film indicating that the inclusion of CO2 does not inhibit photoresist removal. In situ interferometry measurements of the PHOST layers generated insight into the removal mechanism. The GXL CO2 expanded TMAHCO3/CH3OH at a temperature of 90oC and pressures above 1000 psig removed photoresist and post plasma etch residue. Cleaning efficiency depended upon CO2 pressure; at low pressures the CO2 reduced the mole fraction of the TMAHCO3/CH3OH while at higher pressures the CO2 assisted removal.

Probing the Cybotactic Region in Gas-Expanded Liquids (GXLs)

Gas-expanded liquids (GXLs) are a new and benign class of liquid solvents, which may offer many advantages for separations, reactions, and advanced materials. GXLs are intermediate in properties between normal liquids and supercritical fluids, both in solvating power and in transport properties. Other advantages include benign nature, low operating pressures, and highly tunable properties by simple pressure variations. The chemical community has only just begun to exploit the advantages of these GXLs for industrial applications. This Account focuses on the synergism of experimental techniques with theoretical modeling resulting in a powerful combination for exploring chemical structure and transport in the cybotactic region of GXLs (at the nanometer lengthscale).

Photoresist and Residue Removal Using Gas-Expanded Liquids

Rapid technology development and demands for state-of-the-art generations of integrated circuits bring new challenges to microelectronic device manufacture. Specifically, decreasing dimension sizes may limit the effectiveness of liquid-based removal of photoresist and plasma etch residues. In addition, the hazardous solvents currently in use pose environmental concerns. Gas-expanded liquids (GXLs) and supercritical fluids may offer cleaning or residue removal approaches that overcome some of the drawbacks of current surface preparation methods. Removal of PHOST (polyhydroxystyrene) photoresist films has been demonstrated with -expanded ethanol (up to 75 mol % ), indicating that the inclusion of does not inhibit photoresist removal. In situ interferometry measurements of the PHOST layers allowed insight into the removal mechanism. The GXL mixtures were also invoked to remove post-plasma etch residues using -expanded . At a temperature of 90°C and pressures above 1000 psig the GXL mixture removed the photoresist and etch residue. Cleaning efficiency depended on pressure; at lower pressures the reduced the mole fraction of the while at higher pressures the assisted removal.

Photoresist and Residue Removal Using Gas-Expanded Liquids

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