Di(2-ethylhexyl)phosphoric acid

Di(2-ethylhexyl)phosphoric acid (DEHPA or HDEHP) is an organophosphorus compound with the formula (C₈H₁₇O)₂PO₂H. The colorless liquid is a diester of phosphoric acid and 2-ethylhexanol. It is used in the solvent extraction of uranium, vanadium and the rare-earth metals.

Di(2-ethylhexyl)phosphoric acid

Names
Preferred IUPAC name Bis(2-ethylhexyl) hydrogen phosphate
Other names Bis(2-ethylhexyl) phosphoric acid Bis(2-ethylhexyl) phosphate Bis(2-ethylhexyl) hydrophosphoric acid BEHPA BEHP BEHHPA BEHHP Di(2-ethylhexyl) phosphoric acid DEHPA D2EHPA EHPA
Identifiers
CAS Number	298-07-7 Y
3D model (JSmol)	Interactive image
ChemSpider	8918 N
ECHA InfoCard	100.005.507
PubChem CID	9275
UNII	2Z4W0L3H06 Y
CompTox Dashboard (EPA)	DTXSID1027134
InChI InChI=1S/C16H35O4P/c1-5-9-11-15(7-3)13-19-21(17,18)20-14-16(8-4)12-10-6-2/h15-16H,5-14H2,1-4H3,(H,17,18) N Key: SEGLCEQVOFDUPX-UHFFFAOYSA-N N InChI=1/C16H35O4P/c1-5-9-11-15(7-3)13-19-21(17,18)20-14-16(8-4)12-10-6-2/h15-16H,5-14H2,1-4H3,(H,17,18) Key: SEGLCEQVOFDUPX-UHFFFAOYAP
SMILES CCCCC(CC)COP(=O)(O)OCC(CC)CCCC
Properties
Chemical formula	C16H35O4P
Molar mass	322.43 g/mol
Appearance	odorless colorless liquid (impure samples are often yellow)
Density	0.9758 g/mL
Melting point	−50 °C (−58 °F; 223 K)
Boiling point	393 °C (739 °F; 666 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Preparation

DEHPA is prepared through the reaction of phosphorus pentoxide and 2-ethylhexanol:

4 C₈H₁₇OH + P₄O₁₀ → 2 [(C₈H₁₇O)PO(OH)]₂O

[(C₈H₁₇O)PO(OH)]₂O + C₈H₁₇OH → (C₈H₁₇O)₂PO(OH) + (C₈H₁₇O)PO(OH)₂

These reaction produce a mixture of mono-, di-, and trisubstituted phosphates, from which DEHPA can be isolated based on solubility.

Use in lanthanide extraction

DEHPA can be used to extract lanthanides (rare earths) from aqueous solutions, it is commonly used in the lanthanide sector as an extraction agent. In general the distribution ratio of the lanthanides increase as their atomic number increases due to the lanthanide contraction. It is possible by bringing a mixture of lanthanides in a counter current mixer settler bank into contact with a suitable concentration of nitric acid to selectively strip (back extract) some of the lanthanides while leaving the others still in the DEHPA based organic layer. In this way selective stripping of the lanthanides can be used to make a separation of a mixture of the lanthanides into mixtures containing fewer lanthanides. Under ideal conditions this can be used to obtain a single lanthanide from a mixture of many lanthanides.

It is common to use DEHPA in an aliphatic kerosene which is best considered to be a mixture of long chain alkanes and cycloalkanes. When used in an aromatic hydrocarbon diluent the lanthanide distribution ratios are lower. It has been shown that it is possible to use a second generation biodiesel which was made by the hydrotreatment of vegetable oil. It has been reported that Neste's HVO100 is a suitable diluent for DEHPA when calcium, lanthanum and neodymium are extracted from aqueous nitric acid

Use in uranium extraction

Extraction

DEHPA is used in the solvent extraction of uranium salts from solutions containing the sulfate, chloride, or perchlorate anions. This extraction is known as the “Dapex procedure” (dialkyl phosphoric extraction). Reminiscent of the behaviours of carboxylic acids, DEHPA generally exists as a hydrogen-bonded dimer in the non-polar organic solvents. For practical applications, the solvent, often called a diluent, is typically kerosene. A complex is formed from two equivalents of the conjugate base of DEHPA and one uranyl ion. Complexes of the formula (UO₂)₂[(O₂P(OR)₂]₄ also form, and at high concentrations of uranium, polymeric complexes may form.

The extractability of Fe³⁺ is similar to that of uranium, so it must be reduced to Fe²⁺ before the extraction.

Stripping

The uranium is then stripped from the DEHPA/kerosene solution with hydrochloric acid, hydrofluoric acid, or carbonate solutions. Sodium carbonate solutions effectively strip uranium from the organic layer, but the sodium salt of DEHPA is somewhat soluble in water, which can lead to loss of the extractant.

Synergistic effects

The extractive capabilities of DEHPA can be increased through synergistic effects by the addition of other organophosphorus compounds. Tributyl phosphate is often used, as well as dibutyl-, diamyl-, and dihexylphosphonates. The synergistic effects are thought to occur by the addition of the trialkylphosphate to the uranyl-DEHPA complex by hydrogen bonding. The synergistic additive may also react with the DEHPA, competing with the uranyl extraction, resulting in a decrease in extraction efficiency past a concentration specific to the compound.

Alternatives to DEHPA

Alternative organophosphorus compounds include trioctylphosphine oxide and bis(2,4,4-trimethyl pentyl)phosphinic acid. Secondary, tertiary, and quaternary amines have also been used for some uranium extractions. Compared to phosphate extractants, amines are more selective for uranium, extract the uranium faster, and are easily stripped with a wider variety of reagents. However, the phosphates are more tolerant of solids in the feed solution and show faster phase separation.