A new solvent system: Hydrothermal molten salt

Abstract

This work proposes a new solvent system composed of a molten salt in pressurized water, so-called hydrothermal molten salt (HyMoS). This system changes the paradigm of the solubility of inorganics in supercritical water. Using as an example NaOH, a low melting temperature salt, we show the possibility to precipitate it at a temperature above its melting one, leading to the instantaneous formation of the HyMoS. The molten salt is then capable of dissolving a large amount of inorganic salt, as exemplified with Na2SO4. This solvent system opens innovative ways with a potential to impact applications in many fields including materials synthesis, biomass conversion, recycling, green chemistry, catalysis, sustainable manufacturing and others. Beyond the impact on the hydrothermal community, this work also offers previously unexplored opportunities for the molten salt field with access to flow chemistry and insights regarding salt precipitation mechanism.

The impact of hydrogen valence on its bonding and transport in molten fluoride salts

“…2, which shows the interatomic distance between a single 3 H 3 H + atom and different uorine atoms in the mixture. 3 H + becomes coordinated to uorine with an average interatomic distance of 1Å, and jumps between different uorine atoms during the simulation. In Table 1 First peak radius and coordination number of 3 H 0 / 3 H + and F À pairs in Flibe and Flinak at 973 K (design temperature for typical reactor).…”

“…Interest in high-temperature molten salts has greatly increased in the last decade due to their application in a variety of emerging clean energy technologies including thermal storage, hydrogen production, batteries, fuel cells, and nuclear power. [1][2][3][4][5] In many of these salt systems, corrosion remains a central challenge. 6 This is particularly true for halide salts where corrosion impurities are soluble in the salt, so a passivation layer between the structural materials and working uid cannot be established.…”

“…16,17 One of the greatest known challenges however, is that radioactive tritium ( 3 H) is generated by the neutronic reaction 6 LiF + n / 4He + 3 HF. 18 The presence of 3 HF drives up the redox potential in the system, 7 as hydrogen is less electropositive than lithium, which causes corrosion of metallic species such as chromium via the equilibrium reaction: 2 3 HF (g) + Cr (s) 4 CrF 2(soln) + 3 H 2(g) , forming diatomic tritium gas. The relative concentration of the M n+ /M (M ¼ Cr, Fe, Ni or other structural metal elements) and HF/H 2 couples depend on the temperature and redox potential, which is established by the Nernst equation.…”

Unveiling the complexity of salt(s) in water under transcritical conditions

“…Indeed, in this case, when the saturation limit is reached, the precipitation of the salt occurs and is instantaneously followed by the fusion of the salt. A molten salt phase is then created, as observed for systems containing sodium nitrate and nitrite, lithium and potassium nitrate [73], potassium hydroxide or potassium phosphate [74] as well as sodium hydroxide [75] leading to a supplementary phase in the water-salt phase diagram.…”

“…Nevertheless, the presence of molten salts in supercritical water was mainly seen as a problematic phenomenon up to recently mainly because of potential corrosion they can cause. The super-solvent potential of these phases in supercritical water systems was evidenced with the example of NaOH and Na2SO4 [75]. The experiment, carried out by Voisin et al, consists in the deposition of solid Na2SO4 crystals on the walls of a continuous supercritical water tubular reactor followed by the injection of a NaOH/water solution.…”