Method for the preparation of 1,2-propanediol
A method for preparing 1,2-propanediol involves reacting propene with hydrogen peroxide, in the presence of a phase transfer catalyst and a heteropolytungstate, in a liquid reaction mixture containing an aqueous phase with a maximum apparent pH of 6 and an organic phase containing a solvent having a solubility in water at 20° C. of less than 500 mg/kg. The method then involves separating the liquid reaction mixture into an aqueous phase containing 1,2-propanediol and an organic phase; recycling at least a part of the separated organic phase to the reaction; and extracting the separated aqueous phase with an extractant solution containing the same phase transfer catalyst and solvent as used in the reaction to provide an extracted aqueous phase and an extract phase. The method further involves recycling at least a part of the extract phase to the reaction and recovering 1,2-propanediol from the extracted aqueous phase.
1 . A method for the preparation of 1 , 2 -propanediol, comprising:
a) reacting propene with hydrogen peroxide in the presence of a catalyst mixture, comprising a phase transfer catalyst and a heteropolytungstate, in a liquid reaction mixture comprising an aqueous phase with a maximum apparent pH of 6 and an organic phase comprising a solvent(S) having a solubility in water at 20° C. of less than 500 mg/kg, wherein apparent pH refers to a value determined by measurement with a glass electrode employing a commercial pH meter calibrated with aqueous buffer solutions of known pH for measuring dilute aqueous solutions;
b) separating the liquid reaction mixture of a) into an aqueous phase (P a ) comprising 1,2-propanediol and an organic phase (P o );
c) recycling at least a part of the organic phase (P o ) to a);
d extracting the aqueous phase (P a ) with an extractant solution comprising the same phase transfer catalyst and the same solvent(S) as used in a), to provide an extracted aqueous phase (P ac ) and an extract phase (P e ) comprising a salt of the phase transfer catalyst and a heteropolytungstate or tungstate;
e) recycling at least a part of the extract phase (P e ) to a); and
f) recovering the 1,2-propanediol from the extracted aqueous phase (P ae ).
2 . The method of claim 1 , wherein c) comprises subjecting the at least a part of the organic phase (P o ) to a nanofiltration providing a retentate enriched in the heteropolytungstate and a permeate depleted in the heteropolytungstate, recycling the retentate to a), and passing the permeate to d) to provide at least part of said extractant solution.
3 . The method of claim 2 , wherein the at least a part of the organic phase (P o ) subjected to nanofiltration is combined with said extract phase (P e ) prior to said nanofiltration.
4 . The method of claim 2 , wherein phase transfer catalyst is added to said permeate before passing to d).
5 . The method of claim 1 , wherein d) is carried out in a counter current extraction column.
6 . The method of claim 1 , wherein d) is carried out in an extraction unit or a series of from 2 to 5 consecutive extraction units, each extraction unit comprising a mixer and a settler in series.
7 . The method of claim 1 , wherein a) to e) are carried out continuously.
8 . The method of claim 7 , wherein a) is conducted in a loop reactor comprising fixed internals in a tubular section, and the liquid reaction mixture is passed through the loop reactor at a flow rate sufficient to provide turbulent flow at said fixed internals.
9 . The method of claim 1 , wherein said solvent(S) comprises at least one alkylated aromatic hydrocarbon having from 7 to 12 carbon atoms.
10 . The method of claim 1 , wherein the phase transfer catalyst comprises at least one selected from the group consisting of a tertiary amine, a tertiary ammonium salt, and a quaternary ammonium salt, and
wherein the tertiary amine, the tertiary ammonium salt, and the quaternary ammonium salt each comprises in total at least 12 carbon atoms.
11 . The method of claim 10 , wherein the phase transfer catalyst comprises a tertiary or quaternary ammonium ion having the structure R 1 R 2 R 3 NR 4+ ,
wherein
R 1 , R 2 , and R 3 are the same or different and are each an alkyl group having from 8 to 10 carbon atoms, and
R 4 is hydrogen or methyl.
12 . The method of claim 1 , wherein c) comprises contacting the at least a part of the organic phase (P o ) with an aqueous reactivating solution comprising hydrogen peroxide and phosphoric acid, at a temperature of from 5 to 40° C.
13 . The method of claim 12 , wherein a further organic phase resulting from said contacting with an aqueous reactivating solution is passed to a) without any intervening steps.
14 . The method of claim 12 , wherein the at least a part of the organic phase (P o ) is combined with said extract phase (P e ) prior to said contacting with an aqueous reactivating solution.