Process for preparing 1,2-propylene glycol

Abstract

The method for preparing 1,2-propylene glycol by the following steps allows a continuous reaction operation with a small amount of tungsten and phase transfer catalyst loss: (a) continuously reacting propylene with hydrogen peroxide in the presence of a catalyst mixture in a liquid reaction mixture, the catalyst mixture comprising a quaternary ammonium salt and a polytungsten phosphate, the liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase comprising an alkyl aromatic hydrocarbon solvent; (b) discharging the liquid reaction mixture from step a) and adding a water-soluble sulfate or sulfuric acid to provide a mixture comprising 500-10000 mg / kg sulfate ions in the aqueous phase; (c) separating the mixture obtained in step b) into an aqueous phase (P) comprising 1,2-propylene glycol; a ) and the organic phase (P o ); (d) the organic phase (P o ) is recycled to the reaction step a); and (e) from the aqueous phase (P a ) to recover 1,2-propylene glycol.

Description

Technical Field

[0001] The present invention relates to a process for preparing 1,2-propylene glycol by reacting propylene with hydrogen peroxide. Background Art

[0002] In a long-established process used in the industry, 1,2-propylene glycol is produced by reacting propylene oxide with water. Propylene oxide can be produced on an industrial basis 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. Propylene oxide is then isolated and purified prior to the step of reacting propylene oxide with water to produce 1,2-propylene glycol.

[0003] WO 2017 / 089075 discloses a method for producing 1,2-propylene glycol from propylene and hydrogen peroxide, the method comprising: 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 with a maximum pH of 6 and an organic phase; b) separating the two-phase 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 in step a); and d) separating 1,2-propylene glycol from the aqueous phase separated in step b). The phase transfer catalyst may be a quaternary ammonium salt, a tertiary amine or a quaternary phosphonium salt.

[0004] The preferred phase transfer catalyst of WO 2017 / 089075 comprises a structure of R 1 R 2 R 3 R 4 N + A tertiary ammonium ion or a quaternary ammonium ion, wherein R 1 is YO(C=O)R 5 A group in which Y is CH2CH2, CH(CH3)CH2 or CH2CH(CH3), and R 5 is 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 Each independently is R 1 , an alkyl group having 1 to 4 carbon atoms, or Y-OH. These quaternary ammonium ester salts are preferred for extracting most of the tungstate into the organic phase of the reaction mixture.

[0005] Example 8 of WO 2017 / 089075 discloses a continuous process using trioctylamine as a phase transfer catalyst in an alkyl aromatic hydrocarbon solvent.

[0006] The inventors of the present invention have found that the preferred quaternary ammonium salts of WO 2017 / 089075 have YO(C=O)R 5 The disadvantage is that the ester groups hydrolyze under the reaction conditions for propylene oxidation, which leads to an increased water solubility of the quaternary ammonium ions. Extended continuous operation of the process using this phase transfer catalyst therefore requires permanent replenishment of fresh phase transfer catalyst.

[0007] The inventors of the present invention have also found that when tertiary amines are used as phase transfer catalysts, oxidation of the tertiary amine to the corresponding amine oxide occurs as a side reaction. Therefore, continued continuous operation of the process with tertiary amines as phase transfer catalysts also requires permanent replenishment of fresh phase transfer catalyst.

[0008] The inventors of the present invention have also found that if the quaternary ammonium ester salt or tertiary amine of WO 2017 / 089075 is replaced by a quaternary ammonium salt, fresh phase transfer catalyst will still need to be supplemented, because the phase separation in step b) of the process then leads to a turbid aqueous phase with emulsified droplets of organic phase, and a large amount of phase transfer catalyst and tungstate will be lost from the process with the aqueous phase.

[0009] Therefore, there is a need to reduce the loss of phase transfer catalyst and tungstate from the process of WO 2017 / 089075 during continued continuous operation of the process. Summary of the invention

[0010] The inventors of the present invention have now discovered that, by using an alkyl aromatic hydrocarbon solvent and carrying out step a) of the process with a quaternary ammonium salt as a phase transfer catalyst, the process of WO 2017 / 089075 can be operated continuously for a long time with a small loss of phase transfer catalyst and tungstate from the process, when combined with the addition of sodium sulfate to the aqueous phase of the resulting reaction mixture (which improves phase separation and reduces the loss of phase transfer catalyst and tungstate with the separated aqueous phase).

[0011] The subject of the present invention is therefore a process for preparing 1,2-propylene glycol, comprising the following steps:

[0012] a) reacting propylene with hydrogen peroxide continuously in the presence of a catalyst mixture comprising a quaternary ammonium salt and a polytungstophosphate in a liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase comprising an alkyl aromatic hydrocarbon solvent;

[0013] b) discharging the liquid reaction mixture from step a) and adding a water-soluble sulfate salt or sulfuric acid to the discharged reaction mixture to provide a mixture comprising 500-10000 mg / kg sulfate ions in the aqueous phase;

[0014] c) separating the mixture obtained in step b) into an aqueous phase (P) containing 1,2-propylene glycol a ) and an organic phase (P o );

[0015] d) The organic phase (P o ) is recycled to reaction step a); and

[0016] e) From the aqueous phase (P a ) to recover 1,2-propylene glycol.

[0017] Preferably, sodium sulfate is added in step b). Specific implementation plan

[0018] In the process of the present invention, propylene and hydrogen peroxide are reacted in step a) in the presence of a catalyst mixture comprising a quaternary ammonium salt and a polytungstophosphate. The reaction is carried out continuously in a liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase comprising an alkyl aromatic hydrocarbon solvent.

[0019] Propylene can be used in pure form or in a mixture with propane, wherein 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, the molar ratio of the propylene introduced into step a) to the hydrogen peroxide introduced into step a) is from 1.2:1 to 5:1, more preferably from 1.5:1 to 3:1.

[0020] Hydrogen peroxide is preferably used in the form of an aqueous solution, preferably having a hydrogen peroxide content of 10 to 80% by weight, particularly preferably 30 to 70% by weight. Any commercially available grade of aqueous hydrogen peroxide solution can be used. It is also possible to use the crude hydrogen peroxide obtained in the extraction stage of the anthraquinone process for the production of hydrogen peroxide.

[0021] The catalyst mixture used in step a) comprises a polytungsten phosphate. Polytungsten phosphates are well known to those skilled in the art. The preferred polytungsten phosphate has a molar ratio of phosphorus to tungsten in the range of 1:2 to 1:12. The polytungsten phosphate 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 polytungsten phosphate 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 peroxytungstate and peroxytungsten phosphate such as PO4[WO(O2)2]4 3- and HPO4[WO(O2)2]2 2-and its partially protonated form, which is presumably the catalytically active species for propylene oxidation.

[0022] The catalyst mixture used in step a) also comprises a quaternary ammonium salt, which acts as a phase transfer catalyst by forming a salt with peroxytungstate or heteropolyperoxytungstate, and the salt is soluble in the organic phase of the liquid reaction mixture. The phase transfer catalyst preferably comprises a single-charge quaternary ammonium salt. The counterions suitable for quaternary ammonium ions are the following anions: chloride, bromide, nitrate, sulfate, hydrogen phosphate, dihydrogen phosphate, methanesulfonate, methylsulfate and ethylsulfate. The amount of the quaternary ammonium salt preferably makes the molar ratio of the quaternary ammonium salt to tungsten in the liquid mixture in the range of 0.2:1 to 3:1, particularly preferably 0.4:1 to 1:1, wherein the molar ratio refers to the amount of cations in the quaternary ammonium salt used to the amount of tungsten used.

[0023] In a preferred embodiment, the quaternary ammonium salt contains at least 12 carbon atoms in total, preferably 12-60 carbon atoms. Preferred are tetraalkylammonium salts. For example, suitable quaternary ammonium salts are dodecyltrimethylammonium salt, hexadecyltrimethylammonium salt, octadecyltrimethylammonium salt, methyltributylammonium salt and methyltrioctylammonium salt. More preferably, the quaternary ammonium salt contains a group having the structure R 1 R 2 R 3 NR 4+ The quaternary ammonium ion, R 1 , R 2 and R 3 are the same or different and are each selected from an alkyl group having 8 to 10 carbon atoms, and R 4 Most preferably, the phase transfer catalyst comprises methyl tri(octyl / decyl)ammonium methylsulfate (CAS No. 2387913-24-6).

[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" herein refers to a value determined by measuring with a glass electrode using a commercial pH meter calibrated with a buffered aqueous solution of known pH for measuring dilute aqueous solutions. The apparent pH differs from the theoretical pH (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 the standard potential 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 in this range by adding an acid, preferably sulfuric acid or phosphoric acid, or by adding a base, preferably an aqueous sodium hydroxide solution. Adjusting the apparent pH within this preferred range provides a high selectivity for 1,2-propylene glycol and prevents enrichment of propylene oxide in the aqueous phase, which simplifies the subsequent separation of propylene glycol from the aqueous phase.

[0025] In reaction step a), the weight ratio of hydrogen peroxide to water fed to step a) is preferably 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, most preferably 0.15 to 0.45. The molar ratio of propylene to hydrogen peroxide fed to step a) is preferably 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., particularly preferably 70 to 90° C. The reaction pressure is preferably above the vapor pressure of propylene at the reaction temperature to ensure that most of the propylene and preferably all of the propylene is present in the liquid organic phase of the liquid reaction mixture.

[0027] The reaction of step a) can be carried out with an organic phase comprising an alkyl aromatic hydrocarbon solvent. The reaction is preferably carried out in the presence of at least one organic solvent. The alkyl aromatic hydrocarbon solvent preferably has a boiling point of more than 120° C. and preferably has a solubility in water of less than 250 mg / kg at 20° C. Suitable solvents are alcohols, ethers, esters, ketones and alkylated aromatic hydrocarbons having one or more hydroxyl groups. Preferably, the amount of the alkyl aromatic hydrocarbon solvent is selected to provide a proportion of the alkyl aromatic hydrocarbon solvent in the organic phase during the reaction in the range of 10-90% by weight.

[0028] Preferably there is an alkyl aromatic hydrocarbon solvent with 7-12 carbon atoms. For example, the aromatic hydrocarbons suitable for alkylation are toluene, 1,2-dimethylbenzene (o-xylene), 1,3-dimethylbenzene (m-xylene), 1,4-dimethylbenzene (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, an alkyl aromatic hydrocarbon solvent containing an alkylated aromatic hydrocarbon with 7-12 carbon atoms exceeding 50% by weight, particularly preferably exceeding 80% by weight, is used. The use of these solvents enables most of the peroxytungstate to be extracted into the organic phase of the reaction mixture and recycled, which allows the method to be operated without recovering polytungstate phosphate from the aqueous phase of the reaction mixture of step a). Then, the quaternary ammonium salt, the molar ratio of the quaternary ammonium salt to the polytungsten phosphate, the molar ratio of the phosphorus atoms of the polytungsten phosphate to tungsten, the molar ratio of propylene to hydrogen peroxide, and the amount of the alkyl aromatic hydrocarbon solvent are preferably selected to transfer as much tungsten present in the liquid reaction mixture as possible to the organic phase.

[0029] Quaternary ammonium salt, polytungstophosphate and alkyl aromatic hydrocarbon solvent can be added in the step a) of the method of the present invention individually or in the form of a mixture containing two or all three of these components. Preferably, quaternary ammonium salt and polytungstophosphate are added and dissolved in the organic phase comprising the alkyl aromatic hydrocarbon solvent.

[0030] The reaction of step a) can be carried out in the presence of phosphoric acid. a ) is preferably 500-30000 mg / kg (calculated as orthophosphate). Phosphoric acid can be used to provide an aqueous phase (P of the reaction mixture) of 1.0 to 3.5, preferably 2.0 to 3.0. a ) apparent pH. Phosphoric acid can also be due to the aqueous phase (P a ) exists due to the in-situ formation of polytungstophosphate.

[0031] The reaction of step a) can be carried out batchwise or continuously, wherein continuous reaction is preferred. The concentration of hydrogen peroxide in the aqueous phase is preferably maintained in the range of 0.1-5 wt %, particularly preferably 0.5-3 wt %. 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 takes place. The residence time of the reaction mixture is preferably adjusted so that the hydrogen peroxide conversion rate is maintained in the range of 80-99%.

[0032] During the reaction, preferably the liquid mixture is mixed to produce a large phase interface between the aqueous phase and the organic phase. For this reason, the reaction is preferably carried out continuously in a loop reactor, and the loop reactor has a fixed internal in a tubular section, and the liquid mixture passes through the loop reactor with a flow rate that produces turbulence at the internal. For this reason, baffles, static mixing elements, structured packings or random packings can be used as internals. In combination with these internals or as an alternative, a heat exchanger (such as a plate heat exchanger or a tube bundle heat exchanger) can be used, wherein turbulence is produced, for example, between the plates of a plate heat exchanger or in the tube of a tube bundle heat exchanger.

[0033] Preferably, all or part of the heat of reaction generated in step a) is removed while the reaction is carried out, preferably by cooling the reaction mixture in a heat exchanger. More preferably, the reaction is carried out continuously in a loop reactor comprising a heat exchanger within the reactor loop to cool the reaction mixture.

[0034] In step b) of the method of the present invention, the liquid reaction mixture is discharged from step a), and a water-soluble sulfate or sulfuric acid is added to the discharged reaction mixture to provide a mixture containing 500-10000 mg / kg sulfate ions in the aqueous phase. Preferably, sodium sulfate is added in this step, preferably in the form of an aqueous solution. Before adding the sulfate or sulfuric acid, the discharged reaction mixture is preferably cooled, preferably cooled to 15-60°C. After the sulfate or sulfuric acid has been added, the resulting mixture is preferably passed through a mixer, preferably a static mixer, to provide a uniform sulfate concentration in the aqueous phase of the mixture. Adding sulfate or sulfuric acid in an amount to provide the claimed sulfate ion concentration accelerates the coagulation of dispersed droplets of the organic phase and breaks the emulsion of the organic phase in the aqueous phase (which can be formed when the concentration of phosphoric acid and phosphate in the aqueous phase of the reaction mixture is low).

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

[0036] Water phase (P a) typically comprises water, unreacted hydrogen peroxide and the reaction product 1,2-propylene glycol. The aqueous phase also typically contains dipropylene glycol and tripropylene glycol and reaction byproducts (such as 1-hydroperoxy-2-propanol and 2-hydroperoxy-1-propanol formed by the reaction of propylene oxide with hydrogen peroxide, and formic acid, acetic acid and hydroxyacetone formed by further oxidation of 1,2-propylene glycol). The aqueous phase may also contain phosphoric acid, and if a polytungstophosphate generated in situ by combining phosphoric acid with sodium tungstate is used in step a), it may also contain the sodium salt of phosphoric acid. The organic phase (P o ) contains an alkyl aromatic hydrocarbon solvent as well as unreacted propylene and propylene oxide which is formed as an intermediate when propylene reacts with hydrogen peroxide and has not yet been hydrolyzed to 1,2-propylene glycol. The organic phase (P o ) also contains one or more salts formed from cations of polytungstophosphate and quaternary ammonium salt. If the propylene raw material contains propane, the organic phase P o Propane will also be included.

[0037] In step d) of the process of the present invention, the separated organic phase (P o ) is recycled to the reaction step a). Thus, the organic phase (P o ) is recycled to step a) to achieve complete conversion of propylene to 1,2-propylene glycol, dipropylene glycol and tripropylene glycol. o ) is also recycled to step a), and it is particularly preferred to recycle substantially the entire catalyst mixture present in the organic phase to step a).

[0038] The aqueous phase (P) obtained in step c) a ) is preferably further processed without any part thereof being recycled directly or indirectly to step a).

[0039] Steps a) to d) of the process of the invention are preferably carried out continuously.

[0040] In step e) of the process of the invention, the aqueous phase (P a), preferably by distillation. Preferably, 1,2-propylene glycol and higher propylene glycols (such as dipropylene glycol and tripropylene glycol) are recovered by a series of distillation steps, such as a multi-step distillation, comprising a first distillation step providing an overhead product comprising water and a bottom product passed to the next distillation step and optionally a further distillation step, and a distillation step providing an overhead product comprising 1,2-propylene glycol and a residual bottom product preferably subjected to at least one further distillation step. Most preferably, a series of distillation steps as described in Ullmann's Encyclopedia of Industrial Chemistry, online version, entry "Propylene glycol", page 4, DOI 10.1002 / 14356007.a22_163.pub2 is used, wherein an overhead product comprising water is separated from a bottom product comprising 1,2-propylene glycol and higher propylene glycols in a series of 2 to 4 heat-integrated distillation steps, followed by a vacuum distillation step, which provides 1,2-propylene glycol as an 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 overhead products in a further vacuum distillation step.

[0041] In a preferred embodiment, step e) comprises subjecting the aqueous phase (P a ) is subjected to hydrogenation catalysis, preferably to catalytic hydrogenation at a temperature of 80°C to 140°C, more 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, Pt, Ag, Ir, Fe, Cu, Ni and Co on a carrier, wherein activated carbon, SiO2, TiO2, ZrO2, Al2O3 and aluminum silicate are preferably used as carrier materials. Preferred is a supported hydrogenation catalyst comprising ruthenium as an 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. Hydrogenation can prevent problems caused by the decomposition of hydrogen peroxide that has not yet reacted in step a) in step e) of recovering 1,2-propylene glycol and dipropylene glycol. The hydrogenation also converts the by-products 1-hydroperoxy-2-propanol, 2-hydroperoxy-1-propanol and hydroxyacetone formed in step a) into 1,2-propylene glycol and thereby increases the yield of 1,2-propylene glycol.

[0042] The process of the present invention preferably comprises an additional catalyst reactivation step f): the organic phase (P o) is reacted with hydrogen peroxide and phosphoric acid at a temperature of 5-40° C. in a liquid mixture comprising an aqueous phase and an organic phase, and the organic phase (P oc ) is recycled to step a). Preferably, the aqueous phase (P) produced in step f) is also recycled to step a). ac ) is passed to reaction step a).

[0043] In step e), phosphoric acid is preferably used in a molar ratio of phosphorus to tungsten in the range of 1:2 to 10:1, preferably 4:1 to 8:1. The aqueous phase of the liquid mixture of step f) preferably comprises 10-40 wt. %, more preferably 15-38 wt. %, most preferably 18-35 wt. % phosphoric acid. The amount of hydrogen peroxide used preferably provides at least 2 mol of hydrogen peroxide per mol of tungsten, preferably 2-10 mol of hydrogen peroxide per mol of tungsten. It is believed that the preferred molar ratios of phosphorus to tungsten and hydrogen peroxide to tungsten and the preferred concentration of phosphoric acid in the aqueous phase convert most of the tungstate to the formula PO4[WO(O2)2]4 3- and HPO4[WO(O2)2]2 2- The peroxytungstophosphate and its partially protonated form are presumably the most catalytically active species for propylene oxidation. The temperature of 5-40° C. used in step f) prevents these species from decomposing and forming molecular oxygen before being transferred to reaction step a). Preferably, step f) is carried out at a temperature of 10-35° C., more preferably 15-30° C.

[0044] The reaction time in step f) is generally 1-200 minutes, preferably 1-20 minutes, more preferably 2-10 minutes, wherein if the liquid mixture of step f) contains propylene, the reaction time is preferably less than 20 minutes at the upper limit of the temperature range of 5-40° C. Then, only a small part of the propylene present in the liquid mixture of step f) will be oxidized, step f) can be carried out in the absence of cooling, and substantially all of the peroxytungstophosphate produced in step f) can be transferred to step a).

[0045] Step f) can be carried out in any type of reactor known to be suitable for mixing the two liquid phases present in step f). Suitable reactors for carrying out step f) are stirred vessels and the reactor types described further above for providing a large phase interface between the aqueous phase and the organic phase in step a). For short reaction times in step f), tubular reactors with fixed internals for generating turbulence can also be used.

[0046] Preferably, steps a) to d) and f) are performed continuously, and 5-50%, preferably 10-25%, of the organic phase (P o) is transferred to step f), wherein the remainder is directly recycled to step a). In this way, the peroxytungstophosphate species with the highest catalytic activity can be continuously regenerated, and step a) can be continuously operated for a long time without loss of catalytic activity (i.e., with a constant hydrogen peroxide conversion rate), even when the concentration of phosphoric acid in the aqueous phase of reaction step a) is low.

[0047] Preferably, in step f), the aqueous phase (P a ) of tungsten loss by adding tungstate.

[0048] The present invention will now be explained in more detail with reference to Examples.

[0049] Example

[0050] Preparation of initial epoxidation catalyst solution

[0051] A mixture of 88 g of 70 wt % hydrogen peroxide, 100 g of demineralized water, 211 g of 85 wt % phosphoric acid and 132 g of sodium tungstate dihydrate was stirred at room temperature for 2 hours. Then, a solution of 198 g of methyl tri(octyl / decyl) ammonium methyl sulfate (CAS No. 2387913-24-6) in 816 g of Hydrosol A200ND (a mixture of C10 alkylbenzenes) was added, and the mixture was stirred for another 2 hours at room temperature. Then, the aqueous phase and the organic phase were separated to provide 1099 g of the organic phase as the initial epoxidation catalyst solution.

[0052] Reaction of propylene with hydrogen peroxide

[0053] The reaction of propene with hydrogen peroxide was carried out at a temperature of 70° C. and a pressure of 30 bar in a loop reactor having a loop volume of 0.25 l, a circulation pump and a heat exchanger for adjusting the reaction temperature. -1 The reactor was operated at a circulation rate of 50 kg h. The reactor was equipped with a catalyst feed reservoir and a feed pump for feeding liquid propylene, liquid propane, aqueous hydrogen peroxide and liquid from the catalyst feed reservoir. The initial epoxidation catalyst solution was charged to the catalyst feed reservoir. The loop initially contained the reaction mixture from the previous experiment. The circulation was started and maintained at 50 kg h. -1 , and the circulating mixture was heated to 70°C. Then, 43 g h -1 Propylene, 27g h -1 Propane, 112 g h -1 15 wt % aqueous hydrogen peroxide solution containing 0.5 wt % phosphoric acid, and 171 g h -1The organic catalyst solution from the catalyst feed reservoir was introduced into the loop reactor, and the circulating mixture was cooled to maintain a reaction temperature of 70° C. The biphasic oxidation reaction mixture was taken out from the loop reactor in an amount corresponding to the added amount, and 18 g h -1 4.8 wt% disodium sulfate aqueous solution was added to accelerate phase separation. The organic phase and aqueous phase of the resulting mixture were separated using a coalescer, and the organic phase was transferred to a catalyst feed reservoir after decompression and cooling to 25°C. After about 24 hours of operation, the feed of the reactants and the circulation in the loop reactor were stopped.

[0054] The aqueous phase of the last 60 minutes of the run was collected and analyzed by redox titration for hydrogen peroxide, by capillary GC (25 m CP-WAX-52CB column from Agilent, He carrier gas, temperature program from 50° C. with a slope of 20 K / min to 90° C., with a slope of 10 K / min to 220° C. and with a slope of 5 K / min to 235° C., FID detector), and by ICP-OES for tungsten. The analysis showed a tungsten content of 17 mg / kg, a hydrogen peroxide conversion of 98%, and concentrations of 1,2-propylene glycol, dipropylene glycol, tripropylene glycol and hydroxyacetone corresponding to yields of 74% 1,2-propylene glycol, 9% dipropylene glycol, 1% tripropylene glycol and 1% hydroxyacetone (based on the hydrogen peroxide fed to the loop reactor). For comparison, a sample of the two-phase reaction mixture was withdrawn directly from the loop of the loop reactor and allowed to stand for 0.5 hours to allow phase separation. Analysis of the resulting aqueous phase showed a tungsten content of 6800 mg / kg.

[0055] The examples demonstrate that the addition of sodium sulfate reduces the tungstate by about 400 times as the separated aqueous phase is lost until it is significantly lower than the tungsten content in the aqueous phase achieved in the prior art WO 2017 / 089075 (66 mg / kg in Example 7 and 21000 mg / kg in Example 11).

Claims

1. A method for preparing 1,2-propylene glycol, the method comprising the following steps: a) reacting propylene with hydrogen peroxide continuously in the presence of a catalyst mixture comprising a quaternary ammonium salt and a polytungstophosphate in a liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase comprising an alkyl aromatic hydrocarbon solvent; b) discharging the liquid reaction mixture from step a) and adding a water-soluble sulfate salt to the discharged reaction mixture to provide a mixture comprising 500-10000 mg / kg sulfate ions in the aqueous phase; c) separating the mixture obtained in step b) into an aqueous phase P containing 1,2-propylene glycol a and organic phase P o ; d) the organic phase P o at least a portion of which is recycled to reaction step a); and e) from the aqueous phase P a Recover 1,2-propylene glycol from The term "apparent pH" herein refers to a value determined by measurement with a glass electrode using a commercial pH meter calibrated with an aqueous buffer solution of known pH for measuring dilute aqueous solutions.

2. The process as claimed in claim 1, wherein sodium sulfate is added in step b).

3. The method of claim 1 or 2, wherein step c) comprises making the aqueous phase P a The coalescer element comprises a surface covered with the organic phase P o Wetted structured or random packing.

4. The method according to claim 1 or 2, wherein: In step a), the aqueous phase contains phosphoric acid and / or phosphates in a total amount of 500-30000 mg / kg, calculated as orthophosphate.

5. The method according to claim 1 or 2, wherein: In step a), the concentration of hydrogen peroxide in the aqueous phase is 0.1-5% by weight.

6. The process of claim 1 or 2, wherein the molar ratio of propylene introduced into step a) to hydrogen peroxide introduced into step a) is from 1.2:1 to 5:

1.

7. The process of claim 6, wherein the molar ratio of propylene introduced into step a) to hydrogen peroxide introduced into step a) is from 1.5:1 to 3:

1.

8. The process of claim 1 or 2, wherein the alkyl aromatic hydrocarbon solvent comprises more than 50 wt% of an alkylated aromatic hydrocarbon having 7 to 12 carbon atoms.

9. The process of claim 8, wherein the alkyl aromatic hydrocarbon solvent comprises more than 80% by weight of alkylated aromatic hydrocarbons having 7 to 12 carbon atoms.

10. A process as claimed in claim 1 or 2, wherein step a) is carried out continuously in a loop reactor comprising fixed internals in a tubular section and the liquid reaction mixture passes through the reactor at a flow rate sufficient to provide turbulence at the internals.

11. The method of claim 1 or 2, wherein the quaternary ammonium salt comprises a tetraalkylammonium salt.

12. The method of claim 11, wherein the tetraalkylammonium salt has the structure R 1 R 2 R 3 NR 4+ , where R 1 , R 2 and R 3 are the same or different and are each selected from an alkyl group having 8 to 10 carbon atoms, and R 4 It's methyl.

13. The method according to claim 1 or 2, further comprising a catalyst reactivation step f): the organic phase P separated in step c) is o A portion of the ester is reacted with hydrogen peroxide and phosphoric acid at a temperature of 5-40° C. in a liquid mixture comprising an aqueous phase and an organic phase.

14. The method according to claim 13, comprising treating the organic phase P provided by step f) oc Recycle to step a).

15. The method of claim 13, wherein tungstate is added in step f) to compensate for the tungsten in the aqueous phase P separated in step c). a loss.

16. The method according to claim 1 or 2, wherein: In step a), propylene is reacted with hydrogen peroxide at a temperature of 50-110°C.

17. The method of claim 1 or 2, wherein steps a) to d) are performed continuously.