Process for the preparation of 1,2-propanediol

Abstract

In a process for the preparation of 1,2-propanediol, comprising the step a) of 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 having a maximum apparent pH of 6 and an organic phase; the step b) of separating the reaction mixture into an aqueous phase (P a ) comprising 1,2-propanediol and formic acid and an organic phase (P o ); the step c) of recycling at least a part of the separated organic phase (P o ) to step a); the step d) of contacting at least a part of the separated aqueous phase (P a ) with a palladium catalyst; and the step e) of recovering 1,2-propanediol from the aqueous phase provided in step d); step d) reduces the content of formic acid.

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for the preparation of 1,2-propanediol by reacting propene with hydrogen peroxide. BACKGROUND

[0002] In the established process used in the industry, 1,2-propanediol is prepared by reacting propene oxide with water. Propene oxide can be prepared on an industrial basis using the HPPO process, which comprises the reaction of propene with hydrogen peroxide in the presence of a titanium zeolite catalyst and an organic solvent. It is then isolated and purified before the step of reacting propene oxide with water to prepare 1,2-propanediol.

[0003] WO 2017 / 089075 discloses a process for the production of 1,2-propanediol from propene and hydrogen peroxide, which comprises: a) reacting propene 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 biphasic mixture from step a) into an aqueous phase and an organic phase containing propene oxide; c) returning the propene oxide contained in the separated organic phase to the reaction of step a); and d) isolating 1,2-propanediol from the aqueous phase separated in step b).

[0004] In the process described in WO 2017 / 089075, formic acid is formed in a side reaction in reaction step a) by cleavage of 1,2-propanediol into formaldehyde and acetaldehyde and subsequent oxidation of formaldehyde into formic acid. Formic acid will be in the aqueous phase and can cause corrosion problems in subsequent high temperature work-up steps, such as in distillation steps for the recovery of 1,2-propanediol (MPG) and other valuable products such as dipropylene glycol (DPG) and tripropylene glycol (TPG). It is therefore an object of the present invention to provide a process for the preparation of 1,2-propanediol, wherein corrosion problems in subsequent distillation steps for the recovery of valuable products are mitigated. SUMMARY

[0005] The inventors of the present invention have now found that formic acid formed as a by-product can be decomposed by contacting the aqueous phase (P a ) separated from the reaction mixture with a palladium catalyst, and that carrying out such a contacting step can reduce corrosion problems in subsequent steps for the recovery of 1,2-propanediol.

[0006] The subject matter of the present invention is therefore a process for the preparation of 1,2-propanediol, said process comprising:

[0007] a) reacting propene with hydrogen peroxide in the presence of a catalyst mixture comprising a phase transfer catalyst, phosphoric acid and a heteropolytungstate in a liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase, the term "apparent pH" herein refers to a value determined by measurement with a glass electrode using a commercial pH meter calibrated with a pH known aqueous buffer solution for measuring dilute aqueous solutions;

[0008] b) separating the reaction mixture into an aqueous phase (P a ) comprising 1,2-propanediol and formic acid and an organic phase (P o );

[0009] c) recycling at least a portion of the separated organic phase (P o ) to the reaction step a);

[0010] d) contacting at least a portion of the aqueous phase (P a ) separated in step b) with a palladium catalyst to provide a treated aqueous phase, wherein no hydrogen gas is added in the contacting step d); and

[0011] e) recovering 1,2-propanediol from the treated aqueous phase provided in step d).

[0012] In another aspect, the present application relates to the use of the contacting step d) for decomposing formic acid in a process for the preparation of 1,2-propanediol, said process comprising:

[0013] a) reacting propene with hydrogen peroxide in the presence of a catalyst mixture comprising a phase transfer catalyst, phosphoric acid and a heteropolytungstate in a liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase, wherein the term "apparent pH" herein refers to a value determined by measurement with a glass electrode using a commercial pH meter calibrated with a pH known aqueous buffer solution for measuring dilute aqueous solutions;

[0014] b) separating the reaction mixture into an aqueous phase (P a ) comprising 1,2-propanediol and formic acid and an organic phase (P o );

[0015] c) recycling at least a portion of the separated organic phase (P o ) to the reaction step a);

[0016] d) contacting at least a portion of the aqueous phase (P a ) separated in step b) with a palladium catalyst to provide a treated aqueous phase; and

[0017] e) recovering 1,2-propanediol from the treated aqueous phase provided in step d). DETAILED DESCRIPTION

[0018] 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 having a maximum apparent pH of 6 and an organic phase.

[0019] Propylene can be used in pure form or in 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 propene to hydrogen peroxide of 1.1:1 to 10:1.

[0020] The 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. The crude hydrogen peroxide obtained in the extraction stage of the anthraquinone process for producing hydrogen peroxide can also be used.

[0021] 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 a polytungstophosphate). Heteropolytungstates are well known to those skilled in the art. The molar ratio of phosphorus to tungsten of 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 peroxytungstate and peroxytungstophosphate 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 includes a phase-transfer catalyst. The phase-transfer catalyst includes a cation or a compound that forms a cation in the aqueous phase, whereby the cation can form 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 includes a single-charge cation or a compound that forms a single-charge cation in the aqueous phase. Suitable phase-transfer catalysts are tertiary amines, tertiary ammonium salts, quaternary ammonium salts, and quaternary phosphonium salts. Counterions suitable for tertiary ammonium salts and quaternary ammonium salts are the following anions: chloride, bromide, nitrate, sulfate, hydrogen phosphate, dihydrogen phosphate, methanesulfonate, methylsulfate, and ethylsulfate. The amount of the phase-transfer catalyst preferably provides a molar ratio of the phase-transfer catalyst to tungsten in the liquid mixture within the range of 0.2:1 to 3:1, particularly preferably within the range of 0.4:1 to 1:1, wherein the molar ratio refers to the amount of tungsten in the phase-transfer catalyst used or the compound that forms the cation to the amount of tungsten used.

[0023] In a preferred embodiment, the phase transfer catalyst is a tertiary amine or a tertiary ammonium salt or a quaternary ammonium salt containing 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 salt, hexadecyltrimethylammonium salt, octadecyltrimethylammonium salt, methyltributylammonium salt and methyltrioctylammonium salt. More preferably, the phase transfer catalyst comprises a tertiary amine having structure R 1 R 2 R 3 NR 4+ tertiary ammonium ion or quaternary ammonium ion, wherein 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 is hydrogen or methyl.Most preferably, the phase transfer catalyst comprises methyltri(octyl / decyl)ammonium methylsulfate (CAS No. 2387913-24-6).

[0024] In another preferred embodiment, the phase transfer catalyst comprises at least one compound having the structure R 1 R 2 R 3 R 4 N + A salt of a tertiary ammonium ion or a quaternary ammonium ion, wherein R 1 is YO(C=O)R 5 A group wherein 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 Each independently is R 1 , an alkyl group having 1 to 4 carbon atoms or Y-OH. Preferred is a quaternary ammonium salt with methyl sulfate as the counter ion, wherein R 2 is a methyl group, and R 5 is a straight chain alkyl or alkenyl. 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 - , where R 5 In each case it is a straight-chain alkyl or alkenyl radical having 11 to 21 carbon atoms. Most preferred is the salt (CH3)2N + (CH2CH(CH3)O(C=O)R 5 )2CH3OSO3 - , where R5 It is an alkyl or alkenyl group with 11-17 carbon atoms. The phase-transfer catalyst of this embodiment can be prepared by esterifying ethanolamine, isopropanolamine, diethanolamine, diisopropanolamine, triethanolamine or triisopropanolamine with fatty acid, and then carrying out quaternization with dimethyl sulfate. These phase-transfer catalysts have the following advantages: they are easily biodegradable (being different from tetraalkylammonium salts), and can be introduced into biological treatment equipment without the need for further pretreatment. Compared to tetraalkylammonium halides, the corrosiveness of salts with methylsulfate as anion is also less.

[0025] The reaction in 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 the value determined by measurement with a glass electrode using a commercial pH meter calibrated with a buffered aqueous solution of known pH for measuring dilute aqueous solutions. This apparent pH differs from the theoretical 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) differs from its standard potential in pure water. The apparent pH of the aqueous phase is preferably maintained within a range of 1.0 to 3.5, particularly preferably within a 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 aqueous sodium hydroxide. Adjusting the apparent pH within this preferred range provides 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.

[0026] 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 is preferably 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 in the range of 1.1:1 to 10:1, more preferably 1.2:1 to 4:1.

[0027] 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 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.

[0028] The reaction in 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 exceeding 100°C, preferably exceeding 120°C, and having a solubility in water of less than 250 mg / kg at 20°C. Suitable solvents include 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 organic solvent is selected to provide a proportion of organic solvent in the organic phase during the reaction in the range of 10-90% by weight.

[0029] In a preferred embodiment, the organic solvent comprises epoxidized fatty acid methyl esters. Epoxidized fatty acid methyl esters can be formed in situ in the reaction mixture of step a) by using fatty acid methyl esters having unsaturated fatty acid groups, which are reacted with hydrogen peroxide to form the epoxidized fatty acid methyl esters. Particularly preferred are epoxidized fatty acid methyl esters containing fatty acid groups derived from vegetable oils, particularly soybean oil. Epoxidized fatty acid methyl esters have the advantage that they have low solubility in aqueous phases.

[0030] In another preferred embodiment, the solvent comprises an alkylated aromatic hydrocarbon with 8-12 carbon atoms. For example, suitable alkylated aromatic hydrocarbons are 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, a hydrocarbon mixture comprising an alkylated aromatic hydrocarbon with 8-12 carbon atoms exceeding 50% by weight, particularly preferably exceeding 80% by weight, is used as a solvent. The use of these solvents makes it possible to extract most of the peroxytungstate into the organic phase of the reaction mixture and recycle it, which allows the method to be operated without recovering heteropolytungstate in the aqueous phase of the reaction mixture of step a). Then, the phase transfer catalyst, the molar ratio of the phase transfer catalyst to the heteropolytungstate, the molar ratio of the heteroatoms of the heteropolytungstate to tungsten, the molar ratio of propylene to hydrogen peroxide, and the amount of solvent are preferably selected to transfer as much of the tungsten present in the liquid reaction mixture as possible to the organic phase.

[0031] Phase transfer catalyst, heteropolytungstate and optionally used solvent can be added in 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, in step a), a solvent is used, and the phase transfer catalyst and heteropolytungstate are added and dissolved in the organic phase comprising the solvent.

[0032] The reaction of step a) can be carried out in the presence of phosphoric acid. 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. The aqueous phase of the reaction mixture (P a ) is preferably 0.2-0.8 wt % (relative to the mass of the aqueous phase according to PO4 3- Phosphoric acid can also be obtained from the aqueous phase of the reaction mixture (P a ) exists due to the in-situ formation of polytungstophosphate.

[0033] The reaction in 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 within 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 occurs. The residence time of the reaction mixture is preferably adjusted so that the hydrogen peroxide conversion remains within the range of 80-99%.

[0034] During the reaction, preferably the mixed liquid mixture is to produce a large phase interface between aqueous phase and organic phase. For this reason, the reaction is preferably carried out continuously in a loop reactor, and the loop reactor has a fixed internal (internal) in a tubular section, and the liquid mixture passes through the loop reactor with a flow velocity that produces turbulence at the internal. For this reason, baffles, static mixing elements, structured packings or random packings can be used as internals. With these internals in combination or as an alternative, heat exchangers (such as plate heat exchangers or tube bundle heat exchangers) can be used, wherein turbulence is for example produced between the plates of the plate heat exchanger or in the pipe of the tube bundle heat exchanger.

[0035] 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.

[0036] 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) comprising 1,2-propylene glycol and formic acid. a ) and the 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 to achieve a more complete separation, the coalescer element comprising a structured or random packing whose surface is wetted by the dispersed phase of the biphasic mixture.

[0037] Water phase (P a ) typically contains 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 by-products (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 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. o ) usually also contains one or more salts formed by heteropolytungstate and cations of phase transfer catalyst. o It will also include propane (if the propylene starting material contains propane) and an organic solvent (if an organic solvent with low water solubility is used as described further above).

[0038] In step c) of the process of the present invention, the separated organic phase (P o ) is recycled to 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. Preferably, the propylene oxide present in the organic phase (P o ) is recycled to step a), and particularly preferably substantially the entire catalyst mixture present in the organic phase is recycled to step a).

[0039] The organic phase (P) separated from the liquid reaction mixture provided in step a) may be o ) is recycled to step a) without further treatment. If the propylene fed to step a) contains propane, then preferably a stream of unreacted propylene is separated from the organic phase in step c) before the organic phase is recycled to step a), wherein the stream of separated unreacted propylene contains as much propane as the impure propylene fed to step a). In this way, for a continuous reaction, accumulation of propane in the organic phase of the reaction mixture in step a) can be avoided. The stream of separated unreacted propylene can be passed to a C3 splitter to separate propylene and propane, and the recovered propylene can be recycled to step a).

[0040] The aqueous phase (P) obtained in step b) a ) is preferably further processed without directly or indirectly recycling any part thereof to step a).

[0041] In step d) of the process of the present invention, the aqueous phase (P a ) is contacted with a palladium catalyst to provide a treated aqueous phase. Preferably, the aqueous phase (P a ) is contacted with a palladium catalyst at a temperature between 0° C. and 200° C., preferably in the range of 100° C. to 180° C. The pressure in the contacting step d) is preferably in the range of 1-100 bar and is preferably selected so as to maintain a liquid aqueous phase during the contacting step d). Preferably, the contacting step d) is carried out without adding hydrogen. It is preferred to keep the aqueous phase (P a ) is contacted with the palladium catalyst for a time sufficient to decompose greater than 50 mole percent of the formic acid, and the amount of catalyst is preferably selected to provide this level of formic acid decomposition within a time span of 2 to 90 minutes.

[0042] The palladium catalyst used in step d) can be finely dispersed palladium metal (such as palladium black) or preferably a supported palladium catalyst, preferably containing palladium metal on a support material selected from activated carbon, SiO , TiO , ZrO , Al O and aluminum silicate. When using a supported palladium catalyst, the catalyst preferably comprises 1-5% by weight of palladium. Contact with the supported palladium catalyst can be carried out using a suspended catalyst or a catalyst fixed bed, preferably in a liquid flooded fixed bed.

[0043] In a preferred embodiment, between step b) and step d) or between step d) and step e), the aqueous phase (P a) 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, Pt, Ag, Ir, Fe, Cu, Ni, and Co on a support, with activated carbon, SiO2, TiO2, ZrO2, Al2O3, and aluminum silicate being preferred as support materials. Preferred are supported hydrogenation catalysts comprising 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 may be used in the form of a suspension or a fixed bed, with trickle bed hydrogenation using a fixed bed catalyst being preferred. Hydrogenation can prevent problems caused by the decomposition of unreacted hydrogen peroxide from step a) in step e) for 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, thereby increasing the yield of 1,2-propylene glycol. Preferably, step f) catalytic hydrogenation is performed between step b) and step d) to prevent the decomposition of hydrogen peroxide or hydroperoxide by-products by the palladium catalyst used in step d).

[0044] In step e) of the process of the invention, from the thermally treated and optionally hydrogenated aqueous phase (P a) is recovered (preferably by distillation) from the distilled water. 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 which is passed to the next distillation step and optionally further distillation steps, and a distillation step providing an overhead product comprising 1,2-propylene glycol and a residual bottom product which is 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 edition, entry "Propylene Glycol," page 4, DOI 10.1002 / 14356007.a22_163.pub2, is used, wherein a water-containing overhead product is separated from a bottom product containing 1,2-propylene glycol and higher propylene glycols in a series of 2-4 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. In a preferred embodiment, the heated aqueous phase obtained in step d) is fed directly to a distillation step, in which the water-containing overhead product is separated, so that the heat supplied in step d) can be used to recover 1,2-propylene glycol in step e).

[0045] Before recovering 1,2-propylene glycol, all or part of the separated aqueous phase (P a ) contacting the aqueous phase with a palladium catalyst reduces the content of formic acid in the aqueous phase, which prevents corrosion caused by formic acid in step e) of recovering 1,2-propylene glycol.

[0046] The present invention will now be explained in more detail with reference to Examples.

[0047] Example

[0048] Example 1

[0049] Preparation of initial epoxidation catalyst solution

[0050] A mixture of 33 g of 70 wt% hydrogen peroxide, 155 g of demineralized water, 88 g of 85 wt% phosphoric acid, and 56 g of sodium tungstate dihydrate was stirred at room temperature for 2 hours. A solution of 150 g of methyltri(octyl / decyl)ammonium methylsulfate (CAS No. 2387913-24-6) in 1020 g of Hydrosol A200ND (a mixture of C10 alkylbenzenes) was then added, and the mixture was stirred at room temperature for a further 2 hours. The aqueous and organic phases were then separated to provide 1147 g of the organic phase as the initial epoxidation catalyst solution.

[0051] Reaction of propylene with hydrogen peroxide

[0052] The reaction of propene with hydrogen peroxide was carried out at a temperature of 80° C. and a pressure of 30 bar in a loop reactor having a loop volume of 0.45 l, a circulation pump and a heat exchanger for regulating the reaction temperature. The loop reactor was heated at a rate of 130 kg h -1 The reactor was operated at a circulation rate of 130 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 loaded into the catalyst feed reservoir. The loop initially contained the reaction mixture from the previous experiment. The circulation was started and maintained at 130 kg / h. -1 , and the circulating mixture was heated to 80°C. Then, 80 g h -1 Propylene, 50g h -1 Propane, 210g h -1 15 wt% aqueous hydrogen peroxide solution containing 0.3 wt% phosphoric acid, and 320 g h -1 The organic catalyst solution from the catalyst feed reservoir is introduced into the loop reactor, and the circulating mixture is cooled to maintain a reaction temperature of 80°C. The biphasic oxidation reaction mixture is removed from the loop reactor in an amount corresponding to the amount added. The phases are separated, and 0.2% by weight of sodium sulfate is added to the decompressed and cooled aqueous phase to coagulate the emulsified organic phase. After coagulation of the emulsified organic phase, a second phase separation is performed. The combined organic phase is transferred to the catalyst feed reservoir after decompression and cooling to 25°C. After approximately 11 hours of operation, the feed of reactants and the circulation in the loop reactor are stopped. The next day, the circulation in the loop reactor is restarted, and the dosing of reactants is resumed after the reaction temperature has been established in the loop reactor, and the reaction is allowed to continue for another 11 hours. After reaching a stable operating state, the aqueous phase for further hydrogenation and contact with the palladium catalyst is collected.

[0053] Hydrogenation of the separated aqueous phase

[0054] 800 g of the aqueous phase separated from the reaction mixture of propylene and hydrogen peroxide was charged into a 1 liter spinning basket autoclave containing 75 g of a 2 wt% ruthenium / activated carbon supported catalyst in the basket. The autoclave was flushed with nitrogen and then with hydrogen, and hydrogenation was carried out at 90° C. and a hydrogen pressure of 1.6 MPa for 6 hours while the basket was rotating.

[0055] The hydrogenated aqueous phase is contacted with a palladium catalyst

[0056] 50 g of the hydrogenated aqueous phase was added to a 300 ml rotating basket autoclave containing 0.5 g of a 2 wt% palladium / alumina supported catalyst in the basket. After flushing with nitrogen, the autoclave contents were heated to 160° C. while the basket rotated and maintained at this temperature for 60 minutes under a nitrogen pressure of 15 bar. The autoclave was then cooled to ambient temperature and depressurized.

[0057] The aqueous phase was analyzed for formate before and after treatment with the palladium catalyst by ion chromatography using a conductivity detector (Metrohm ASupp 5-250 column (polyvinyl alcohol with quaternary ammonium groups), an aqueous eluent containing 1 mmol / l NaHCO and 3.2 mmol / l NaCO at 0.5 ml / min, and an aqueous suppressor regeneration agent containing 100 mmol / l sulfuric acid and 20 mmol / l oxalic acid). The ion chromatogram showed that contact with the palladium catalyst reduced the area of ​​the peak associated with formate to 49.5% of the area before contact, indicating decomposition of approximately half of the formic acid.

[0058] Example 2

[0059] Example 1 was repeated, except that the temperature of contact with the palladium catalyst was lowered from 160° C. to room temperature (about 23° C.). The ion chromatogram showed that contact with the palladium catalyst reduced the peak area associated with formate to 70.6% of the area before contact, indicating that formic acid can also be decomposed at low temperatures.

Claims

1. A method for preparing 1,2-propylene glycol, comprising: a) reacting propylene with hydrogen peroxide in the presence of a catalyst mixture comprising a phase transfer catalyst, phosphoric acid and a heteropolytungstate in a liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase, wherein the term "apparent pH" herein refers to a value determined by measurement with a glass electrode using a commercial pH meter calibrated with a buffered aqueous solution of known pH for measuring dilute aqueous solutions; b) separating the reaction mixture into an aqueous phase P comprising 1,2-propylene glycol and formic acid a and organic phase P o ; c) the separated organic phase P o at least a portion of which is recycled to reaction step a); d) making the aqueous phase P separated in step b) a contacting at least a portion of the aqueous phase with a supported palladium catalyst to provide a treated aqueous phase, wherein no hydrogen is added in the contacting step d); as well as e) recovering 1,2-propylene glycol from the treated aqueous phase provided in step d); wherein said contacting in step d) is carried out at a pressure sufficient to maintain a liquid aqueous phase; and wherein the temperature in the contacting step d) is in the range of 100°C to 180°C.

2. The process of claim 1 , wherein the supported palladium catalyst comprises palladium metal on a support material selected from the group consisting of activated carbon, SiO 2 , TiO 2 , ZrO 2 , Al 2 O 3 and aluminum silicate.

3. The method of claim 1, wherein the pressure in the contacting step d) is 1-100 bar.

4. The process 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 in the range of 80° C. to 140° C.

5. The process of claim 4, 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 in the range of 90°C to 120°C.

6. The process according to claim 1 , wherein step e) comprises 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.

7. The process as claimed in claim 6, wherein the residual bottom product is subjected to at least one further distillation step.

8. The method according to claim 1 , wherein phosphoric acid is present in step a), and the heteropolytungstate is a polytungstate phosphate.

9. The process according to any one of claims 1 to 3, wherein the organic phase in step a) comprises an organic solvent having a boiling point exceeding 100° C. at atmospheric pressure and a solubility in water at 20° C. of less than 250 mg / kg, the organic solvent being selected from alkylated aromatic hydrocarbons having 8 to 12 carbon atoms.

10. The method of any one of claims 1 to 3, wherein the phase transfer catalyst is a tertiary amine, or a tertiary ammonium salt or a quaternary ammonium salt, and contains 12 to 60 carbon atoms in total.

11. The method of claim 10, wherein the phase transfer catalyst comprises a catalyst having structure R 1 R 2 R 3 NR 4+ tertiary ammonium ion or quaternary ammonium ion, wherein 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 is hydrogen or methyl.

12. Use of the contacting step d) for decomposing formic acid in a process for preparing 1,2-propylene glycol, the process comprising: a) reacting propylene with hydrogen peroxide in the presence of a catalyst mixture comprising a phase transfer catalyst, phosphoric acid and a heteropolytungstate in a liquid reaction mixture comprising an aqueous phase having a maximum apparent pH of 6 and an organic phase, wherein the term "apparent pH" herein refers to a value determined by measurement with a glass electrode using a commercial pH meter calibrated with a buffered aqueous solution of known pH for measuring dilute aqueous solutions; b) separating the reaction mixture into an aqueous phase P comprising 1,2-propylene glycol, formic acid and the phosphoric acid ester of 1,2-propylene glycol a and organic phase P o ; c) the separated organic phase P o at least a portion of which is recycled to reaction step a); d) making the aqueous phase P separated in step b) a contacting at least a portion of the aqueous phase with a supported palladium catalyst to provide a treated aqueous phase; as well as e) recovering 1,2-propylene glycol from the treated aqueous phase provided in step d).

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