Method for preparing 1,2-propanediol

Abstract

In a process for preparing 1,2-propanediol, comprising the steps of: a) reacting propylene with hydrogen peroxide in a liquid reaction mixture in the presence of a catalyst mixture comprising a phase transfer catalyst and a heteropolytungstate, the liquid reaction mixture comprising an aqueous phase and an organic phase having a maximum apparent pH of 6; b) separating the reaction mixture into an aqueous phase (P a ) comprising 1,2-propanediol and the phosphate ester of 1,2-propanediol and an organic phase (P o ); c) recycling at least a portion of the separated organic phase (P o ) to step a); d) heating at least a portion of the separated aqueous phase (P a ) to a temperature above 140 °C at a pressure sufficient to keep at least a portion of the aqueous phase liquid; and e) recovering 1,2-propanediol from the heated aqueous phase; the heating in step d) causes the phosphate ester of 1,2-propanediol to cleave into 1,2-propanediol and phosphoric acid.

Description

Technical Field

[0001] The present invention relates to a process for preparing 1,2 - propanediol by reacting propylene with hydrogen peroxide, in which the amount of phosphate esters of 1,2 - propanediol can be reduced. Background Art

[0002] In the well - established processes used in the industry, 1,2 - propanediol is prepared by reacting propylene oxide with water. Propylene oxide can be prepared on an industrial scale using the HPPO process, which involves the reaction of propylene with hydrogen peroxide in the presence of a titanium zeolite catalyst and an organic solvent. Then, it is separated and purified before the step of reacting propylene oxide with water to prepare 1,2 - propanediol.

[0003] WO 2017 / 089075 discloses a process for producing 1,2 - propanediol from propylene and hydrogen peroxide, which comprises: a) reacting propylene with hydrogen peroxide in the presence of a catalyst mixture comprising a phase - transfer catalyst and a heteropolytungstate, wherein the reaction is carried out in a liquid mixture comprising an aqueous phase and an organic phase having a maximum pH of 6; b) separating the biphasic mixture from step a) into an aqueous phase and an organic phase containing propylene oxide; c) returning the propylene oxide contained in the separated organic phase to the reaction of step a); and d) separating 1,2 - propanediol from the aqueous phase separated in step b). Summary of the Invention

[0004] The inventors of the present invention have now found that in the process described in WO 2017 / 089075, phosphate esters of 1,2 - propanediol are formed in a side reaction in reaction step a) by the reaction of propylene oxide present in the reaction mixture with phosphoric acid. The inventors have also found that by heating all or a part of the aqueous phase separated from the oxidation reaction mixture to a temperature above 140 °C at a pressure sufficient to keep a substantial part of the water in the liquid phase (which hydrolyzes the phosphate esters of 1,2 - propanediol to produce 1,2 - propanediol), the selectivity to 1,2 - propanediol can be increased.

[0005] Accordingly, the subject of the present invention is a process for preparing 1,2 - propanediol, which process comprises:

[0006] a) reacting propylene with hydrogen peroxide in a liquid reaction mixture in the presence of a catalyst mixture, said catalyst mixture comprising a phase - transfer catalyst, phosphoric acid and a heteropolytungstate, said liquid reaction mixture comprising an aqueous phase and an organic phase having a maximum apparent pH of 6, wherein the term "apparent pH" herein refers to the value determined by measuring with a glass electrode using a commercial pH meter calibrated with a buffer aqueous solution of known pH for measuring dilute aqueous solutions;

[0007] b) Separate the reaction mixture into an aqueous phase (P a ) containing 1,2 - propanediol and the phosphate ester of 1,2 - propanediol and an organic phase (P o );

[0008] c) Recycle at least a portion of the separated organic phase (P o ) to reaction step a);

[0009] d) Heat at least a portion of the aqueous phase (P a ) separated in step b) to a temperature above 140 °C at a pressure sufficient to keep at least 20 wt% of the water in the aqueous phase (P a ) in the liquid phase, wherein the heating in step d) is carried out without adding hydrogen; and

[0010] e) Recover 1,2 - propanediol from the heated aqueous phase of step d).

[0011] In another aspect, the present invention relates to the use of the heating in step d) for cleaving the phosphate ester of 1,2 - propanediol into 1,2 - propanediol and phosphoric acid in a process for preparing 1,2 - propanediol, the process comprising:

[0012] a) React propylene with hydrogen peroxide in the presence of a catalyst mixture in a liquid reaction mixture, the catalyst mixture comprising a phase - transfer catalyst, phosphoric acid and a heteropolytungstate, the liquid reaction mixture comprising an aqueous phase and an organic phase with a maximum apparent pH of 6;

[0013] b) Separate the reaction mixture into an aqueous phase (P a ) containing 1,2 - propanediol and the phosphate ester of 1,2 - propanediol and an organic phase (P o );

[0014] c) Recycle at least a portion of the separated organic phase (P o ) to reaction step a);

[0015] d) Heat at least a portion of the aqueous phase (P a ) separated in step b) to a temperature above 140 °C at a pressure sufficient to keep at least 20 wt% of the water in the aqueous phase (P a ) in the liquid phase; and

[0016] e) Recover 1,2 - propanediol from the heated aqueous phase of step d). Detailed Description

[0017] In the process of the present invention, in step a), propylene is reacted with hydrogen peroxide in the presence of a catalyst mixture comprising a phase transfer catalyst and a heteropolytungstate. The reaction is carried out in a liquid reaction mixture comprising an aqueous phase and an organic phase with a maximum apparent pH of 6.

[0018] Propylene can be used in pure form or in the form of a mixture with propane, where the proportion of propane can be up to 20 mol%. The proportion of propane in the propylene used is preferably less than 5 mol%. Propylene is preferably used in a molar excess relative to hydrogen peroxide, preferably in a molar ratio of propylene to hydrogen peroxide of 1.1:1 to 10:1.

[0019] Hydrogen peroxide is preferably used in the form of an aqueous solution, which preferably has a hydrogen peroxide content of 10 - 80 wt%, particularly preferably 30 - 70 wt%. Any commercially available grade of aqueous hydrogen peroxide solution can be used. It is also possible to use the crude hydrogen peroxide product obtained in the extraction stage of the anthraquinone process for the production of hydrogen peroxide.

[0020] The catalyst mixture used in step a) comprises a heteropolytungstate. The heteroatom is preferably phosphorus or arsenic, and particularly preferably phosphorus (i.e., the heteropolytungstate is particularly preferably polytungstophosphate). Heteropolytungstates are well known to those skilled in the art. The molar ratio of phosphorus to tungsten in the preferred polytungstophosphate is in the range of 1:2 to 1:12. The polytungstophosphate is preferably generated in situ by combining phosphoric acid and sodium tungstate, which can be carried out in the liquid reaction mixture itself or before adding the polytungstophosphate to the liquid reaction mixture. Phosphoric acid and sodium tungstate are preferably used in a molar ratio of phosphorus to tungsten of 1:2 to 10:1, preferably 4:1 to 8:1. The heteropolytungstate reacts with hydrogen peroxide in the liquid reaction mixture to form peroxotungstate and peroxotungstophosphate such as PO4[WO(O2)2]4 3- and HPO4[WO(O2)2]2 2- and their partially protonated forms, which are presumably the catalytically active species for the oxidation of propylene.

[0021] The catalyst mixture used in step a) further comprises a phase transfer catalyst. The phase transfer catalyst comprises a cation or a compound that forms a cation in the aqueous phase, whereby the cation can form a salt with a peroxotungstate or a heteropolyperoxotungstate, and the salt is soluble in the organic phase of the liquid reaction mixture. The phase transfer catalyst preferably comprises a monovalent cation or a compound that forms a monovalent cation in the aqueous phase. Suitable as phase transfer catalysts are tertiary amines, tertiary ammonium salts and quaternary ammonium salts, as well as quaternary phosphonium salts. Suitable counterions for the tertiary ammonium salts and quaternary ammonium salts are the following anions: chloride, bromide, nitrate, sulfate, hydrogen phosphate, dihydrogen phosphate, methanesulfonate, methyl sulfate and ethyl sulfate. The amount of the phase transfer catalyst is preferably such that the molar ratio of the phase transfer catalyst to tungsten in the liquid mixture is in the range of 0.2:1 to 3:1, particularly preferably 0.4:1 to 1:1, where the molar ratio refers to the amount of the cation or the compound forming the cation in the phase transfer catalyst used compared to the amount of tungsten used.

[0022] In a preferred embodiment, the phase transfer catalyst is a tertiary amine or a tertiary ammonium salt or a quaternary ammonium salt that in total comprises at least 12 carbon atoms, preferably 12 - 60 carbon atoms. Preferred are tetraalkylammonium salts. For example, suitable tertiary amines are dodecyldimethylamine, hexadecyldimethylamine, octadecyldimethylamine, tributylamine and trioctylamine. Suitable tertiary ammonium salts are the protonated products of these tertiary amines. For example, suitable quaternary ammonium salts are dodecyltrimethylammonium salts, hexadecyltrimethylammonium salts, octadecyltrimethylammonium salts, methyltributylammonium salts and methyltrioctylammonium salts. More preferably, the phase transfer catalyst comprises a tertiary ammonium ion or a quaternary ammonium ion having the structure R 1 R 2 R 3 NR 4+ , where R 1 , R 2 and R 3 are the same or different and each is selected from alkyl groups having 8 - 10 carbon atoms, and R 4 is hydrogen or methyl. Most preferably, the phase transfer catalyst comprises methyltri(octyl / decyl)ammonium methyl sulfate (CAS No. 2387913 - 24 - 6).

[0023] In another preferred embodiment, the phase transfer catalyst comprises at least one salt of a tertiary ammonium ion or a quaternary ammonium ion having the structure R 1 R 2 R 3 R 4 N + , where R 1 is a Y - O(C═O)R 5 group, where Y is CH2CH2, CH(CH3)CH2 or CH2CH(CH3), and R 5is an alkyl or alkenyl group having 11 to 21 carbon atoms, R 2 is hydrogen or an alkyl group having 1 to 4 carbon atoms, and R 3 and R 4 are each independently R 1 , an alkyl group having 1 to 4 carbon atoms or Y-OH. Preferred is a quaternary ammonium salt having methyl sulfate as a counter ion, wherein R 2 is methyl, and R 5 is a straight-chain alkyl or alkenyl group. Particularly preferred are the following salts: (CH3)3N + CH2CH2O(C=O)R 5 CH3OSO3 - , (CH3)2N + (CH2CH2OH)(CH2CH2O(C=O)R 5 )CH3OSO3 - , (CH3)2N + (CH2CH2O(C=O)R 5 )2CH3OSO3 - , CH3N + (CH2CH2OH)2(CH2CH2O(C=O)R 5 )CH3OSO3 - , CH3N + (CH2CH2OH)(CH2CH2O(C=O)R 5 )2CH3OSO3 - , CH3N + (CH2CH2O(C=O)R 5 )3CH3OSO3 - , (CH3)3N + CH2CH(CH3)O(C=O)R 5 CH3OSO3 - , (CH3)2N + (CH2CH(CH3)OH)(CH2CH(CH3)O(C=O)R 5 )CH3OSO3 - and (CH3)2N + (CH2CH(CH3)O(C=O)R 5 )2CH3OSO3 - , wherein R 5 is in each case a straight-chain alkyl or alkenyl group having 11 to 21 carbon atoms. Most preferred is the salt (CH3)2N + (CH2CH(CH3)O(C=O)R 5 )2CH3OSO3 - , wherein R5 is an alkyl or alkenyl group having 11 to 17 carbon atoms. The phase transfer catalyst of this embodiment can be prepared by esterifying ethanolamine, isopropanolamine, diethanolamine, diisopropanolamine, triethanolamine or triisopropanolamine with a fatty acid and then quaternizing with dimethyl sulfate. These phase transfer catalysts have the advantages that they are readily biodegradable (unlike tetraalkylammonium salts) and can be introduced into biological treatment equipment without further pretreatment. They are also less corrosive than salts with tetraalkylammonium halides as the anion, with methyl sulfate as the anion.

[0024] The reaction of step a) is carried out in a liquid reaction mixture comprising two liquid phases (an aqueous phase with a maximum apparent pH of 6 and an organic phase). The term "apparent pH" here refers to the value determined by measuring with a glass electrode using a commercial pH meter calibrated with a buffer aqueous solution of known pH for measuring dilute aqueous solutions. This apparent pH differs from the notional pH (i.e., the negative logarithm of the hydrogen ion activity) by a constant value because the standard potential of the glass electrode in the aqueous phase of the reaction mixture (containing hydrogen peroxide and diol) is different from that in pure water. The apparent pH of the aqueous phase is preferably maintained in the range of 1.0 to 3.5, particularly preferably in the range of 2.0 to 3.0. The apparent pH can be maintained within this range by adding an acid, preferably sulfuric acid or phosphoric acid, or by adding a base, preferably an aqueous solution of sodium hydroxide. Adjusting the apparent pH within this preferred range provides high selectivity for 1,2-propanediol and prevents the enrichment of propylene oxide in the aqueous phase, which simplifies the subsequent separation of propanediol from the aqueous phase.

[0025] In reaction step a), preferably the weight ratio of hydrogen peroxide to water fed to step a) is adjusted while maintaining a molar excess of propylene fed to step a) relative to hydrogen peroxide. The weight ratio of hydrogen peroxide to water preferably varies in the range of 0.05 to 1.5, more preferably 0.10 to 0.7, and most preferably 0.15 to 0.45. The molar ratio of propylene to hydrogen peroxide fed to step a) is preferably from 1.1:1 to 10:1, more preferably 1.2:1 to 4:1.

[0026] The reaction is preferably carried out at a temperature in the range of 50 to 110 °C, more preferably 60 to 100 °C, and particularly preferably 70 to 90 °C. The reaction pressure is preferably higher than the vapor pressure of propylene at the reaction temperature to ensure that most of the propylene is present in the liquid organic phase of the liquid mixture.

[0027] The reaction of step a) can be carried out with or without the addition of an organic solvent. The reaction is preferably carried out in the presence of at least one organic solvent having a boiling point above 100 °C, preferably above 120 °C, and having a solubility in water at 20 °C of less than 250 mg / kg. Suitable solvents are alcohols, ethers, esters, ketones and alkylated aromatic hydrocarbons having one or more hydroxyl groups. The addition of a solvent can improve the extraction of the salt formed from the heteropolytungstate and the phase transfer catalyst into the organic phase. Preferably, the amount of the organic solvent is selected to provide a proportion of the organic solvent in the organic phase during the reaction in the range of 10-90% by weight.

[0028] In a preferred embodiment, the organic solvent comprises an epoxidized fatty acid methyl ester. The epoxidized fatty acid methyl ester can be formed in situ in the reaction mixture of step a) by using a fatty acid methyl ester having an unsaturated fatty acid group, which reacts with hydrogen peroxide to form the epoxidized fatty acid methyl ester. Particularly preferred are epoxidized fatty acid methyl esters containing fatty acid groups derived from vegetable oils, especially soybean oil. Epoxidized fatty acid methyl esters have the advantage that they have a low solubility in the aqueous phase.

[0029] In another preferred embodiment, the solvent comprises an alkylated aromatic hydrocarbon having 8-12 carbon atoms. For example, suitable alkylated aromatic hydrocarbons are 1,2-xylene (o-xylene), 1,3-xylene (m-xylene), 1,4-xylene (p-xylene), ethylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene (mesitylene), 1-ethyl-2-toluene, 1-ethyl-3-toluene and 1-ethyl-4-toluene and n-propylbenzene. Preferably, a hydrocarbon mixture containing more than 50% by weight, particularly preferably more than 80% by weight, of an alkylated aromatic hydrocarbon having 8-12 carbon atoms is used as the solvent. The use of these solvents enables most of the peroxotungstate to be extracted into the organic phase of the reaction mixture and recycled, which allows the process to be operated without recovering the heteropolytungstate from the aqueous phase of the reaction mixture of step a). Then, preferably, the phase transfer catalyst, the molar ratio of the phase transfer catalyst to the heteropolytungstate, the molar ratio of the heteroatom of the heteropolytungstate to tungsten, the molar ratio of propylene to hydrogen peroxide, and the amount of the solvent are selected to transfer as much tungsten as possible present in the liquid reaction mixture into the organic phase.

[0030] The phase transfer catalyst, the heteropolytungstate and the optionally used solvent can be added in step a) of the process according to the invention either individually or in the form of a mixture containing two or all three of these components. Preferably, a solvent is used in step a), and the phase transfer catalyst and the heteropolytungstate are added and dissolved in the organic phase containing the solvent.

[0031] The reaction of step a) is carried out in the presence of phosphoric acid. The phosphoric acid is preferably used in an amount that provides an apparent pH of the aqueous phase (P a ) of the reaction mixture of 1.0 to 3.5, preferably 2.0 to 3.0. The concentration of phosphoric acid and phosphate in the aqueous phase (P a ) of the reaction mixture is preferably 0.2 - 0.8% by weight (calculated as PO4 3- ) relative to the mass of the aqueous phase. The phosphoric acid can be present due to the in-situ formation of polyoxotungstate phosphate in the aqueous phase (P a ) of the reaction mixture as described above.

[0032] The reaction of step a) can be carried out batchwise or continuously, with continuous reaction being preferred. The concentration of hydrogen peroxide in the aqueous phase is preferably maintained in the range of 0.1 - 5% by weight, particularly preferably 0.5 - 3% by weight. The concentration of hydrogen peroxide can be adjusted within this range by appropriately selecting the reaction temperature, the molar ratio of propylene to hydrogen peroxide, and the residence time of the liquid mixture in the reactor where the reaction occurs. The residence time of the reaction mixture is preferably adjusted so that the conversion of hydrogen peroxide is maintained in the range of 80 - 99%.

[0033] During the reaction, the liquid mixture is preferably mixed to create a large phase interface between the aqueous phase and the organic phase. For this purpose, the reaction is preferably carried out continuously in a loop reactor that has fixed internal components in the tubular section, and the liquid mixture passes through the loop reactor at a flow rate that creates turbulence at the internal components. For this purpose, baffles, static mixing elements, structured packings or random packings can be used as internal components. In combination with or as an alternative to these internal components, heat exchangers (such as plate heat exchangers or shell-and-tube heat exchangers) can be used, where turbulence is generated, for example, between the plates of a plate heat exchanger or in the tubes of a shell-and-tube heat exchanger.

[0034] Preferably, all or part of the reaction heat generated in step a) is removed while the reaction is proceeding, preferably by cooling the reaction mixture in a heat exchanger. More preferably, the reaction is carried out continuously in a loop reactor that contains a heat exchanger within the reactor loop to cool the reaction mixture.

[0035] In step b) of the process of the present invention, the liquid reaction mixture provided by step a) is separated into an aqueous phase (P a ) containing 1,2 - propanediol and the phosphate ester of 1,2 - propanediol and an organic phase (P o ). The separation of the biphasic reaction mixture provided by step a) is preferably carried out in a settler vessel. The biphasic reaction mixture is preferably passed through a coalescer element containing structured packings or random packings whose surfaces are wetted by the dispersed phase of the biphasic mixture to achieve more complete separation.

[0036] The aqueous phase (P a ) generally contains water, unreacted hydrogen peroxide, and the reaction product 1,2-propanediol. The aqueous phase generally also contains dipropylene glycol and tripropylene glycol, as well as reaction by-products (such as 1-hydroxyperoxy-2-propanol and 2-hydroxyperoxy-1-propanol formed by the reaction of propylene oxide with hydrogen peroxide, and formic acid, acetic acid, and hydroxyacetone formed by the further oxidation of 1,2-propanediol). The aqueous phase also contains phosphoric acid and phosphate esters of 1,2-propanediol, and if a polyoxotungstate formed in situ by combining phosphoric acid with sodium tungstate is used in step a), it may also contain sodium salts of phosphoric acid. The organic phase (P o ) contains unreacted propylene, as well as propylene oxide that forms as an intermediate during the reaction of propylene with hydrogen peroxide and has not yet been hydrolyzed to 1,2-propanediol. The organic phase (P o ) generally also contains one or more salts formed from the cations of heteropolyoxotungstates and phase transfer catalysts. The organic phase P o will also contain propane (if the propylene raw material contains propane) and an organic solvent (if an organic solvent with low water solubility is used as further described above).

[0037] In step c) of the process of the present invention, at least a portion of the separated organic phase (P o ) is recycled to reaction step a). Thus, the propylene oxide present in the organic phase (P o ) is recycled to step a) to achieve complete conversion of propylene to 1,2-propanediol, dipropylene glycol, and tripropylene glycol. Preferably, the heteropolyoxotungstate present in the organic phase (P o ) is recycled to step a), and particularly preferably, substantially all of the catalyst mixture present in the organic phase is recycled to step a).

[0038] The organic phase (P o ) separated from the liquid reaction mixture provided in step a) can be recycled to step a) without further treatment. If the propylene fed to step a) contains propane, it is preferred to separate a stream of unreacted propylene from the organic phase in step c) before recycling the organic phase to step a), where the separated stream of unreacted propylene contains as much propane as the impure propylene fed to step a). Thus, for a continuous reaction, the accumulation of propane in the organic phase of the reaction mixture in step a) can be avoided. The separated stream of unreacted propylene can be passed to a C3 splitter to separate propylene and propane, and the recovered propylene can be recycled to step a).

[0039] The aqueous phase (P a ) obtained in step b) is preferably further processed without recycling any part of it directly or indirectly to step a).

[0040] In step d) of the process of the present invention, at least a part and preferably all of the aqueous phase (P a ) separated in step b) is heated to a temperature above 140 °C at a pressure sufficient to keep at least 20% by weight of the water in the aqueous phase (P a ) in the liquid phase. Preferably, the aqueous phase (P a ) is heated to a temperature in the range of 142 °C to 200 °C. The pressure in heating step d) can be 4 - 200 bar, and preferably the autogenous pressure generated by the vapor pressure of water at the temperature used in step d). The heating in step d) is carried out without adding hydrogen. Preferably, the aqueous phase (P a ) is heated in step d) for a time sufficient to hydrolyze more than 30 mol%, preferably 50 - 90 mol% of the phosphate ester of 1,2 - propanediol, which generally takes 5 - 60 minutes. The heating in step d) can be carried out in any type of pressure reactor, and preferably in a plug - flow reactor such as a tubular reactor.

[0041] In a preferred embodiment, between step b) and step d) or between step d) and step e), at least a part and preferably all of the aqueous phase (P a ) obtained in step b) is subjected to catalytic hydrogenation in step f) at a temperature of 80 °C to 140 °C, preferably 90 °C to 120 °C. The hydrogenation is preferably carried out using a supported hydrogenation catalyst comprising one or more metals selected from Ru, Rh, Pd, Pt, Ag, Ir, Fe, Cu, Ni, and Co on a support, where activated carbon, SiO2, TiO2, ZrO2, Al2O3, and aluminosilicate are preferably used as the support material. Preferably, the hydrogenation catalyst contains ruthenium as the active metal. The catalytic hydrogenation is preferably carried out at a hydrogen partial pressure of 5 - 50 bar, preferably 5 - 35 bar, more preferably 7 - 30 bar, and even more preferably 8 - 25 bar. The hydrogenation catalyst can be used in the form of a suspension or a fixed bed, and trickle - bed hydrogenation using a fixed - bed catalyst is preferred. The hydrogenation can prevent problems caused by the decomposition of unreacted hydrogen peroxide in step a) during the recovery of 1,2 - propanediol and dipropylene glycol in step e). The hydrogenation also converts the by - products 1 - hydroperoxy - 2 - propanol, 2 - hydroperoxy - 1 - propanol, and hydroxyacetone formed in step a) into 1,2 - propanediol, and thus increases the yield of 1,2 - propanediol. Preferably, step f) catalytic hydrogenation is carried out between step b) and step d) to prevent the decomposition of hydrogen peroxide or hydroperoxide by - products in heating step d).

[0042] In step e) of the process of the present invention, from the heat - treated and optionally hydrogenated aqueous phase (P a) Recover 1,2 - propanediol therefrom (preferably by distillation). Preferably, 1,2 - propanediol and higher propanediols (such as dipropylene glycol and tripropylene glycol) are recovered by a series of distillation steps, such as multi - step distillation, which includes a first distillation step providing an overhead product containing water and a bottom product passed to the next distillation step and optionally additional distillation steps, and a distillation step providing an overhead product containing 1,2 - propanediol and a residual bottom product preferably subjected to at least one additional distillation step. Most preferably, a series of distillation steps as described in Ullmann’s Encyclopedia of Industrial Chemistry, online version, entry “Propylene Glycols”, page 4, DOI 10.1002 / 14356007.a22_163.pub2 are used, where the overhead product containing water is separated from the bottom product containing 1,2 - propanediol and higher propanediols in a series of 2 - 4 thermally integrated distillation steps, followed by a vacuum distillation step which provides 1,2 - propanediol as the overhead product and a bottom product containing high - boiling organic compounds and salts. From this bottom product, dipropylene glycol and tripropylene glycol can be recovered as the overhead product in an additional vacuum distillation step. In a preferred embodiment, the heated aqueous phase obtained in step d) is directly fed to a distillation step where the overhead product containing water is separated, so as to use the heat supplied in step d) for recovering 1,2 - propanediol in step e).

[0043] By converting the phosphate ester of 1,2 - propanediol (which is an unwanted by - product) into the desired product 1,2 - propanediol, heating all or part of the separated aqueous phase (P a ) increases the yield of 1,2 - propanediol.

[0044] The present invention will now be explained in more detail with reference to the examples.

[0045] Examples

[0046] Preparation of the initial epoxidation catalyst solution

[0047] A mixture of 86 g of 70 wt% hydrogen peroxide, 98 g of demineralized water, 207 g of 85 wt% phosphoric acid, and 129 g of sodium tungstate dihydrate was stirred at room temperature for 2 hours. Then, a solution of 194 g of methyl tris(octyl / decyl)methylammonium sulfate (CAS No. 2387913 - 24 - 6) in 800 g of Hydrosol A200ND (a mixture of C10 alkylbenzenes) was added, and the mixture was stirred at room temperature for another 2 hours. Then, the aqueous phase and the organic phase were separated to provide 1067 g of the organic phase as the initial epoxidation catalyst solution.

[0048] Reaction of Propylene with Hydrogen Peroxide

[0049] The reaction of propylene with hydrogen peroxide is carried out in a loop reactor at a temperature of 80 °C and a pressure of 30 bar. The loop reactor has a loop volume of 0.45 l, a circulation pump, and a heat exchanger for regulating the reaction temperature. The loop reactor is operated at a circulation rate of 130 kg h -1 . The reactor is equipped with a catalyst feed reservoir and a feed pump for feeding liquid propylene, liquid propane, aqueous hydrogen peroxide solution, and liquid feed from the catalyst feed reservoir. The initial epoxidation catalyst solution is charged to the catalyst feed reservoir. The loop initially contains the reaction mixture from a previous experiment. The circulation is started and maintained at 130 kg h -1 , and the circulating mixture is heated to 80 °C. Then, 80 g h -1 of propylene, 50 g h -1 of propane, 210 g h -1 of 15 wt% aqueous hydrogen peroxide solution containing 0.3 wt% phosphoric acid, and 320 g h -1 of the organic catalyst solution from the catalyst feed reservoir are introduced into the loop reactor, and the circulating mixture is cooled to maintain the reaction temperature of 80 °C. The biphasic oxidation reaction mixture is withdrawn from the loop reactor in an amount corresponding to the addition amount, and 18 g h -1 of 4.8 wt% aqueous sodium sulfate solution is added to the mixture at the reactor outlet to accelerate phase separation. The organic phase and the aqueous phase of the resulting mixture are separated, and the organic phase is transferred to the catalyst feed reservoir after being depressurized and cooled to 25 °C. After about 11 hours of operation, the feeding of the reactants and the circulation in the loop reactor are stopped. The next day, the circulation in the loop reactor is restarted, the feeding of the reactants is resumed after the reaction temperature has been established in the loop reactor, and the reaction is continued for another 11 hours. After reaching a steady operating state, the aqueous phase for further hydrogenation and heat treatment is collected.

[0050] Hydrogenation of the Separated Aqueous Phase

[0051] 504 g of the aqueous phase separated from the reaction mixture of propylene with hydrogen peroxide is charged to a 1 l spinning basket autoclave containing 75 g of 2 wt% ruthenium / activated carbon supported catalyst in the spinning basket. The autoclave is flushed with nitrogen and then with hydrogen, and the hydrogenation is carried out at 120 °C and a hydrogen pressure of 1.6 MPa for 5 hours while the basket is rotating.

[0052] Heat Treatment of the Hydrogenated Aqueous Phase

[0053] 50 g of hydrogenated aqueous phase with a pH of 7 was added to a 300 ml autoclave. After flushing with nitrogen, the autoclave contents were heated to 180 °C and held at this temperature for 15 minutes under a nitrogen pressure of 15 bar. The autoclave was then cooled to ambient temperature and depressurized.

[0054] Before and after the heat treatment, the 1,2-propanediol (MPG), dipropylene glycol (DPG), and tripropylene glycol (TPG) in the aqueous phase were analyzed by capillary GC (25 m CP-WAX-52CB column from Agilent, He carrier gas, temperature program starting at 50 °C with a ramp of 20 K / min to 90 °C, 10 K / min to 220 °C, and 5 K / min to 235 °C, FID detector). Table 1 shows the analysis data. Ion chromatography with a conductivity detector (Metrohm A Supp 5-250 column (polyvinyl alcohol with quaternary ammonium groups), aqueous eluent of 0.5 ml / min with 1 mmol / l NaHCO3 and 3.2 mmol / l Na2CO3, aqueous suppressor regenerant with 100 mmol / l sulfuric acid and 20 mmol / l oxalic acid) indicated that the heat treatment reduced the peak area related to propylene glycol monophosphate to 23% of the area before the heat treatment.

[0055] Table 1 Propylene glycol content before and after heat treatment (in wt%)

[0056] Before heat treatment After heat treatment MPG 22.7 23.7 DPG 3.2 3.4 TPG 0.3 0.3

Claims

1. A method for preparing 1,2 - propanediol, the method comprising: a) React propylene with hydrogen peroxide in a liquid reaction mixture in the presence of a catalyst mixture comprising a phase transfer catalyst, phosphoric acid, and a heteropolytungstate, wherein the liquid reaction mixture comprises an aqueous phase and an organic phase with a maximum apparent pH of 6, where the term "apparent pH" herein refers to the value determined by measuring with a glass electrode using a commercial pH meter calibrated with a buffer aqueous solution of known pH for dilute aqueous solutions; b) Separate the liquid reaction mixture into an aqueous phase comprising 1,2 - propanediol and the phosphate ester of 1,2 - propanediol and an organic phase; c) Recycle at least a portion of the separated organic phase to reaction step a); d) Heat at least a portion of the aqueous phase separated in step b) to a temperature above 140 °C and not higher than 200 °C at a pressure sufficient to keep at least 20 wt% of the water in the aqueous phase in the liquid phase, wherein the heating step d) is carried out without adding hydrogen; and e) Recover 1,2 - propanediol from the aqueous phase heated in step d), wherein between step b) and step d) or between step d) and step e), at least a portion of the aqueous phase is subjected to step f) catalytic hydrogenation at a temperature of 80 °C to 140 °C, wherein the phase transfer catalyst is selected from tertiary amines, tertiary ammonium salts, or quaternary ammonium salts, and the phase transfer catalyst contains 12 to 60 carbon atoms.

2. The method according to claim 1, wherein between step b) and step d) or between step d) and step e), at least a portion of the aqueous phase is subjected to step f) catalytic hydrogenation at a temperature of 90 °C to 120 °C.

3. The method according to claim 1 or 2, wherein In step d), the aqueous phase is heated to a temperature in the range of 142 °C to 200 °C.

4. The method according to claim 1 or 2, wherein the pressure in heating step d) is 4 to 200 bar.

5. The method according to claim 1 or 2, wherein step e) comprises a multi - step distillation, the multi - step distillation including a first distillation step providing a top product containing water and a bottom product passed to the next distillation step, and a distillation step providing a top product containing 1,2 - propanediol and a residual bottom product.

6. The method according to claim 5, wherein step e) comprises a multi - step distillation, the multi - step distillation including an additional distillation step providing a top product containing water and a bottom product passed to the next distillation step.

7. The method according to claim 5, wherein the residual bottom product is subjected to at least one additional distillation step.

8. The method according to claim 1 or 2, wherein the heteropolytungstate is a poly - tungstophosphate.

9. The method according to claim 1 or 2, wherein the organic phase in step a) comprises an organic solvent having a boiling point above 100 °C at atmospheric pressure and a solubility in water at 20 °C less than 250 mg / kg.

10. The method according to claim 9, wherein the organic solvent is selected from alkylated aromatic hydrocarbons having 8 to 12 carbon atoms.

11. The method according to claim 9, wherein the phase transfer catalyst comprises a tertiary ammonium ion or a 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 each is selected from alkyl groups having 8 to 10 carbon atoms, and R 4 is hydrogen or methyl.

12. Use of the cleavage of the phosphate ester of 1,2-propanediol into 1,2-propanediol and phosphoric acid in a process for preparing 1,2-propanediol, said process comprising: a) React propylene with hydrogen peroxide in a liquid reaction mixture in the presence of a catalyst mixture, the catalyst mixture comprising a phase transfer catalyst, phosphoric acid and a heteropolytungstate, the liquid reaction mixture comprising an aqueous phase and an organic phase having a maximum apparent pH of 6, where the term "apparent pH" herein refers to the value determined by measuring with a glass electrode using a commercial pH meter calibrated with a buffer aqueous solution of known pH for measuring dilute aqueous solutions; b) Separate the liquid reaction mixture into an aqueous phase comprising 1,2 - propanediol and the phosphate ester of 1,2 - propanediol and an organic phase; c) Recycle at least a portion of the separated organic phase to reaction step a); d) Heat at least a portion of the aqueous phase separated in step b) to a temperature above 140 °C and not higher than 200 °C at a pressure sufficient to keep at least 20 wt% of the water in the aqueous phase in the liquid state, where the heating step d) is carried out without adding hydrogen; and e) Recover 1,2 - propanediol from the aqueous phase heated in step d), where between step b) and step d) or between step d) and step e), at least a portion of the aqueous phase is subjected to step f) catalytic hydrogenation at a temperature of 80 °C to 140 °C, where the phase transfer catalyst is selected from tertiary amines, tertiary ammonium salts or quaternary ammonium salts, and the phase transfer catalyst contains 12 to 60 carbon atoms.

13. The use according to claim 12, wherein the method for preparing 1,2 - propanediol is carried out as defined in any one of claims 2 to 11.