4 Supercritical fluids Flashcards
Define what the critical point of a compound represents.
The critical point of a compound is the end point of a phase equilibrium curve. At the critical point (Pc and Tc), phase boundaries vanish, and the substance can have behaviors of both a gas and a liquid.
Beyond 374°C and 218 atm, water properties are completely different from those of water under standard conditions. Discuss this point in terms of dielectric constant and ion product.
- Ion product: under conditions of high T and high P, its value increases considerably. At 340 atm, T~300°C, the [H+]~3.6x10^-6 mol/L, about 30x the concentration occurring at normal atmospheric T and P.
- Dielectric constant: very low dielectric constant (from 80 to about 2-30) and poor hydrogen bonding (~17%) because there is about 30% free monomeric H2O molecules, no surface tension (no interfaces exist). Therefore, it behaves as a non-polar solvent.
The synthesis of lavandulol exploits 3 properties of scH2O.
- Ability to dissolve non-polar or slightly polar organic compounds.
- Presence of higher concentrations of H+ and OH- compared to water in standard conditions, thus its capacity to protonate the molecule easily.
- Capability of carrying the synthesis very quickly without extra additives (solvents or catalysts).
Which kind of molecules can mix in scH2O in virtually all the proportions?
Organic and Apolar molecules.
Mixtures of O2 and scH2O are able to oxidize almost any type of organic molecule. For what applications is this system utilized in applied chemistry?
Since scH2O has a small dielectric constant, it behaves almost like a non-polar fluid, and the solubility of organic substance and gases into water increases greatly. This mixture of gases allows for SCWO (Supercritical Water Oxidation) technology, which can be used in the destruction of hazardous wastes such as PCBs. scH2O has a reversed solubility behavior so that chlorinated hydrocarbons become soluble, allowing single-phase oxidation of aqueous waste with dissolved O2 and precipitating salts out of the solution. The products of SCWO include CO2, H2O and salts, with NOx, SOx, and particulate concentrations at or below detection limits, without any post-treatment. This technology has been used for destroying the hydrolysis products resulting from the caustic neutralization of chemical weapons, for example.
Explain why scH2O is an excellent medium to hydrolyse condensation polymers like PET (Polyethylene Terephthalate).
ScH2O will have high density as in a liquid and high kinetic energy as in gas. Additionally, in scH2O we’ll have both OH- and H+, so the reaction will occur very quickly without any additional additives. In it, PET depolymerizes very quickly, delivering terephthalic acid and ethylene glycol.
scCO2 can be efficiently used as an ideal solvent in metal complexes-catalyzed oxidations and hydrogenations. Sketch a flow chemistry apparatus for these kind of applications using a solid-supported metal catalyst.
Combining a solid catalyst, like CpMn(CO3) and scCO2 in a flow reactor is simple and has excellent mass transfer capability.
SEE SKETCH
What are the main uses of scCO2 in analytical chemistry?
As a carrier in analytical chromatography, in extractions of hydrophobic compounds (such as that of caffeine). As a carrier in analytical chromatography and because of its properties, scCO2 can deliver the substance the fastest, as well as offer a rapid purification of grams to kgs and a unique cleaning-in-place feature, that enables the fastest production.
Define what CO2 expanded liquid phases are and provide an example.
As CO2 (or any compressible gas) dissolves into an organic liquid, the liquid expands volumetrically, forming a gas-expanded liquid. There are three classes: Class I liquids (ex. water) have insufficient ability to dissolve water and, therefore, do not significantly expand and have essentially no change in their properties (except acidity). Class II liquids (ex. methanol, hexane, THF) dissolve large amounts of CO2 and, therefore, expand greatly, and undergo significant changes in virtually every physical property. Class III liquids (ex. ILs and liquid polymers) dissolve only moderate amounts of CO2 and expand only moderately. Some properties change significantly (like viscosity) while others do not (polarity).
How can a phase splitting of water/THF mixture be exploited in the presence of CO2?
In the presence of CO2: 10 bar and 40°C, the CO2 will form 2 liquid phases from a mixture of 2 miscible liquids: THF(l)+CO2(l), H2O(l). In higher pressures, the THF will fully migrate with the CO2 to gas, leaving the H2O(l) behind.
SEE DIAGRAM
Define the nature of switchable-polarity solvents and how does polarity change in the absence or presence of CO2.
SPS are solvents that can be reversibly converted between hydrophobic and hydrophilic upon the application or removal of a trigger, such as CO2. They have low polarity until they are exposed to an atmosphere of CO2, at which point they will change into high-polarity ionic liquids, with many solutes being soluble in only one form of the solvent.
Give an example of switchable-polarity solvent, and how it can be exploited in catalyst recycling.
Secondary amines; in green the low-polarity form, in red the high-polarity one.
For catalyst recycling, the reaction could be performed, then with a switchable-polarity solvent the solvent can go to the other form to facilitate the recovery of the product, catalyst, or other chemical of interest.
SEE SKETCH
Define how a switchable-polarity solvent is formed.
By pressurizing the solvent with the gas (for example, CO2), for example via bubble CO2 and in a pressurized atmosphere, the CO2 dissolves in the water, forming hydrated carbon dioxide, which forms an equilibrium with carbonic acid (H2CO3). A hydrophobic substance can be protonated by the carbonic acid to form a bicarbonate salt; the salt will be much more soluble in water, due to its ionic/polar bonds. To reverse the polarity of the solvent, a flow of N2 will slowly eliminate all the CO2.
