A gas-expanded liquid solvent combines a component that is a liquid at the operating temperature and pressure with a second component that is a gas under these same conditions but which is held within the liquid phase by the pressure used in the reaction and its solubility in the liquid component.
Amines as Gas-Expander for other Solvent Liquids
There are polar substances that are easily fluidized gases: ammonia, methylamine, ethylamine, trimethylamine, sulfur dioxide, and dinitrogen tetroxide being examples. Some polar solutes may dissolve in rather apolar solvents, with the assistance of a minor quantity of such a polar fluidized gas.
There are polar substances that are easily fluidized gases: ammonia, methylamine, ethylamine, trimethylamine, sulfur dioxide, and dinitrogen tetroxide being examples. Some polar solutes may dissolve in rather apolar solvents, with the assistance of a minor quantity of such a polar fluidized gas.
For such a co-solvent mixture to retain a fixed composition, however, the dissolution needs to be done well below the boiling point of the fluidized gas. Then, heating the gas-solvent combination or placing it under a vacuum or a combination of these will remove the fluidized gas leaving the polar substrate in an essentially apolar solvent from which it is likely to crystallize readily.
Fluorinated compounds can be made to dissolve in hydrocarbon liquids in the presence of liquid carbon dioxide under pressure from which they crystallize out when the pressure is released and the carbon dioxide vaporizes away.
If the fluidized gas can be effectively recovered in usable purity and if by distillation the apolar co-solvent can be purified, then both solvent components can be recycled for a green process.
If the fluidized gas can be effectively recovered in usable purity and if by distillation the apolar co-solvent can be purified, then both solvent components can be recycled for a green process.
The most common co-solvent pair in the chemical literature is ethanol-water. Since neither ethanol nor water is a gas at ambient temperatures ethanol/water is not a gas expanded liquid. Methylamine/ethanol or ammonia/ethanol would be gas expanded liquids and could be useful at low temperatures. In the first instance, the methylamine might be recoverable by recondensation causing the crystallization to be from ethanol alone. Recovering ammonia on the other hand would be too expensive to be justified.
Ammonia is quite likely to be a satisfactory replacement for water in mixtures with other organic solvents such as ethyl acetate, isopropyl acetate, isopropanol, t-butyl alcohol, MTBE, acetonitrile, dimethoxymethane, THF, diethoxymethane, dioxane, nitromethane, nitroethane, isoamyl alcohol, ethylene glycol or DMSO and would constitute gas expanded solvents. Ammonia may also work with solvents immiscible with water that require a polar additive to dissolve a substrate, such as toluene, hexane, heptane, cyclohexane, dibutyl ether, trifluoromethyl benzene, MTBE, and ethylene carbonate.
Sulfur dioxide, bp. -10 C, is readily condensed and has intriguing solvent properties. It is a Lewis acid, meaning it can accept an electron pair. It forms a stable complex with p-dioxane for example. It can be scrubbed by ethanolamine. Because it is a liquid denser than water, it likely can be used to increase the density of other solvents with which it is miscible perhaps even to the point where they may become the lower layer in a mixture with other organic solvents.
Ammonia is quite likely to be a satisfactory replacement for water in mixtures with other organic solvents such as ethyl acetate, isopropyl acetate, isopropanol, t-butyl alcohol, MTBE, acetonitrile, dimethoxymethane, THF, diethoxymethane, dioxane, nitromethane, nitroethane, isoamyl alcohol, ethylene glycol or DMSO and would constitute gas expanded solvents. Ammonia may also work with solvents immiscible with water that require a polar additive to dissolve a substrate, such as toluene, hexane, heptane, cyclohexane, dibutyl ether, trifluoromethyl benzene, MTBE, and ethylene carbonate.
Sulfur Dioxide as Gas-Expanding Agent for Solvents
Sulfur dioxide, bp. -10 C, is readily condensed and has intriguing solvent properties. It is a Lewis acid, meaning it can accept an electron pair. It forms a stable complex with p-dioxane for example. It can be scrubbed by ethanolamine. Because it is a liquid denser than water, it likely can be used to increase the density of other solvents with which it is miscible perhaps even to the point where they may become the lower layer in a mixture with other organic solvents.
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