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 catalyst mixture containing 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. The process then involves separating the reaction mixture into an aqueous phase (P a ) containing 1,2-propanediol and an organic phase (P o ); recycling at least part of the separated organic phase (P o ) to the reaction; and recovering 1,2 propanediol from the separated aqueous phase (P a ). In the recovery, 1,2-propanediol is separated from the aqueous phase (P a ) by distillation and the apparent pH of the separated aqueous phase (P a ) is adjusted to at least 9 prior to the distillation to reduce the content of formic acid and acetic acid in the recovered 1,2-propanediol.
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;
b) separating the liquid reaction mixture into a separated aqueous phase (P a ) comprising 1,2-propanediol and a separated organic phase (P o );
c) recycling at least a part of the separated organic phase (P o ) to a); and
d) recovering the 1,2-propanediol from the separated aqueous phase (P a );
wherein in d), the 1,2-propanediol is separated from the separated aqueous phase (P a ) by distillation and an apparent pH of the separated aqueous phase (P a ) is adjusted to a value of at least 9 prior to the distillation, and
wherein apparent pH is a value determined by measurement with a glass electrode employing a commercial pH meter calibrated with aqueous buffer solution of known pH for measuring dilute aqueous solutions.
2 . The method of claim 1 , wherein the apparent pH of the separated aqueous phase (P a ) is adjusted to be in a range of 10 to 14, prior to the distillation.
3 . The method of claim 1 , wherein the apparent pH of the separated aqueous phase (P a ) is adjusted by adding an alkali metal hydroxide or an alkali metal carbonate, to the separated aqueous phase (P a ).
4 . The method of claim 1 , wherein the distillation is a multi-step distillation,
wherein in a first step, water is separated as a first overhead product and a first bottoms product is subjected to a second distillation step, wherein the first bottoms product is separated into a second overhead product comprising 1,2-propanediol and a second bottoms product.
5 . The method of claim 4 , wherein the second bottoms product of the second distillation step is subjected to at least one further distillation step.
6 . The method of claim 1 , wherein in d), prior to adjusting the apparent pH of the separated aqueous phase (P a ), the separated aqueous phase (P a ) is subjected to catalytic hydrogenation.
7 . The method of claim 1 , wherein a) is conducted continuously, and
in a), a concentration of hydrogen peroxide in the aqueous phase is from 0.1 to 5 wt. %.
8 . The method of claim 1 , wherein a) is conducted continuously in a loop reactor comprising fixed internals in a tubular section, and wherein 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 a) is conducted in the presence of phosphoric acid.
10 . The method of claim 1 , wherein the heteropolytungstate is a polytungstophosphate.
11 . The method of claim 1 , wherein the organic phase in a) comprises an organic solvent having a boiling point of more than 100° C. at atmospheric pressure and a solubility in water at 20° C. of less than 250 mg/kg.
12 . 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 quatemary ammonium salt comprises in total at least 12 carbon atoms.
13 . The method of claim 12 , 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 carbon atoms; and
R 4 is hydrogen or methyl.
14 . The method of claim 2 , wherein the apparent pH of the separated aqueous phase (P a ) is adjusted to be in a range of 12 to 13.5, prior to the distillation.
15 . The method of claim 3 , wherein the alkali metal hydroxide is sodium hydroxide.
16 . The method of claim 10 , wherein the polytungstophosphate is formed in situ from phosphoric acid and sodium tungstate.
17 . The method of claim 11 , wherein the organic solvent is an alkylated aromatic hydrocarbon having 8 to 12 carbon atoms.