This invention relates to a process for the preparation of diacyl peroxides.

Diacyl peroxides have the general formula

R-C(=0)-0-0-C(=0)-R wherein the R-groups are independently selected from aryl, arylalkyl, and linear, branched, and cyclic alkyl groups, optionally substituted with heteroatom- containing substituents.

Such diacyl peroxides can be prepared by reacting an excess of acid anhydride or acid chloride with alkaline solutions of hydrogen peroxide, as illustrated by the following equations:

2 R-C(=0)-0-C(=0)-R + Na ₂ 0 ₂ ® R-C(=0)-0-0-C(=0)-R + 2 NaOC(=0)R

2 R-C(=0)CI + Na ₂ 0 ₂ ® R-C(=0)-0-0-C(=0)-R + 2 NaCI

In this reaction scheme, [\la2O2 does not refer to the discrete product Na2C>2, but to an equilibrium comprising H2O2 and NaOOH.

US 3,956,396 discloses a process to prepare a diacyl peroxide comprising reacting an anhydride and hydrogen peroxide and separating the formed product from the reaction mixture by an extraction and filtration step.

WO 02/098924 discloses a process to prepare a fluorinated diacyl peroxide comprising reacting a fluorinated anhydride and hydrogen peroxide. No work up step is disclosed.

US 6,610,880 discloses a process for the preparation of a diacyl peroxide by reacting a mixed acid anhydride with a hydroperoxide, in which a peroxide and a carbonate monoester are formed. During work-up, the carbonate monoester decarboxylates to CO2 and an alcohol. Recycling of the alcohol requires phosgene. The mixed acid anhydride is prepared by contacting a carboxylic acid with a halogen formate. This route is most relevant for making peroxides where acid chlorides are expensive or not available, such as in the case of peroxides having a hydroxy group in the molecule.

Acid chlorides are relatively expensive and generate chloride-containing water layers, which lead to waste waters with high salt concentration.

The anhydrides, on the other hand, are even more expensive than acid chlorides and the waste stream of this process contains a high organic load - i.e. a high Chemical Oxygen Demand (COD) value - due to the formed carboxylic acid salt, and is therefore economically and environmentally unattractive.

It is therefore an object of the present invention to provide a process for the production of diacyl peroxides that does not require the use of acid chlorides and at the same time contains low carboxylic acid (salt) concentrations in its effluent. This object can be achieved by a process comprising the following steps:

a) producing a mixture comprising a diacyl peroxide and a carboxylic acid by reacting an anhydride with the formula R ¹ -C(=0)-0-C(=0)-R ² with H2O2, wherein R ¹ is selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with 1 -17 carbon atoms, optionally substituted with oxygen- and/or halogen-containing substituents, and R ² is selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with 2-17 carbon atoms, optionally substituted with oxygen- and/or halogen-containing substituents, b) extracting or separating the carboxylic acid from the mixture in the form of its carboxylic acid salt or adduct,

c) liberating the carboxylic acid from the salt or adduct,

d) optionally producing an additional amount of carboxylic acid by reacting an aldehyde of the formula R ² -C(=0)H with oxygen,

e) reacting the carboxylic acid obtained in step c) and optionally an additional amount of carboxylic acid of the formula R ² -C(=0)0H - said additional amount of carboxylic acid being obtained from step d) and/or obtained in another way - with an acid anhydride or a ketene of the formula C(R ⁴ )2=C=0, each R ⁴ being independently selected from H and CH3, preferably with acetic anhydride, to form an anhydride with the formula R ¹ -C(=0)-0-C(=0)- R ² , and

f) recycling at least part of the anhydride formed in step e) to step a).

This process produces a diacyl peroxide from an anhydride, which anhydride is obtained at least partly from the carboxylic acid side product. This re-use of the carboxylic acid formed in step a) makes the route economically attractive and its effluents low in COD.

Preferably, any additional amount of carboxylic acid that is required to form the amount of anhydride that is needed in step a) is obtained by oxidation of the corresponding aldehyde. It is therefore preferred to produce an additional amount of carboxylic acid in step d) and react it in step e) with acetic anhydride or a ketene.

As this process does not involve the use of corrosive or volatile reactants, it increases production safety and allows production at the location in which the diacyl peroxide is eventually used (e.g. a polymerization facility). Such on-site production allows peroxide production on demand, thereby minimizing storage capacities and the consequential safety measures.

Step a) involves the reaction of hydrogen peroxide with the anhydride with formula R ¹ -C(=0)-0-C(=0)-R ² .

R ¹ in this formula is selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with 1 -17 carbon atoms, optionally substituted with oxygen- and/or halogen-containing substituents. Examples of suitable substituents are alkoxy, chlorine, and ester substituents. The number of carbon atoms is preferably 2-1 1 , even more preferably 2-8, and most preferably 3-6 carbon atoms. In a further preferred embodiment, R ¹ is selected from linear or branched alkyl groups. Most preferably, R ¹ is selected from the group consisting of n-propyl, n- butyl, isobutyl, 2-butyl and isopropyl groups.

R ² in this formula is selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with 2-17 carbon atoms, optionally substituted with oxygen- and/or halogen-containing substituents. Examples of suitable substituents are alkoxy, chlorine, and ester substituents. The number of carbon atoms is preferably 2-1 1 , even more preferably 2-8, and most preferably 3-6 carbon atoms. In a further preferred embodiment, R ² is selected from linear or branched alkyl groups. Most preferably, R ² is selected from the group consisting of n-propyl, n- butyl, isobutyl, isobutyl, 2-butyl, and isopropyl groups.

The anhydride can be symmetrical, meaning R ¹ =R ² , or asymmetrical, meaning that the R ¹ ¹R ² .

If the anhydride is symmetrical, the carboxylic acid that is formed in step a) and extracted or separated in step b) will have the formula R ² -C(=0)0H. If the anhydride is asymmetrical, the carboxylic acid will be a mixture of R ² -C(=0)0H and R ¹ -C(=0)0H.

Suitable symmetrical anhydrides are propionic anhydride, n-butyric anhydride, isobutyric anhydride, pivalic anhydride, n-valeric anhydride, isovaleric anhydride, 2-methylbutyric anhydride, 2-methylpentanoic anhydride, 2-methylhexanoic anhydride, 2-methylheptanoic anhydride, 2-ethylbutyric anhydride, caproic anhydride, caprylic anhydride, isocaproic anhydride, n-heptanoic anhydride, nonanoic anhydride, isononanoic anhydride, 3,5,5-trimethylhexanoic anhydride, 2-propylheptanoic anhydride, decanoic anhydride, neodecanoic anhydride, undecanoic anhydride, neoheptanoic anhydride, lauric anhydride, tridecanoic anhydride, 2-ethyl hexanoic anhydride, myristic anhydride, palmitic anhydride, stearic anhydride, phenylacetic anhydride, cyclohexanecarboxylic anhydride, 3- methyl-cyclopentanecarboxylic anhydride, beta-methoxy propionic anhydride, methoxy acetic anhydride, ethoxy acetic anhydride, propoxy acetic anhydride, alpha-ethoxy butyric anhydride, benzoic anhydride, o-, m-, and p-toluic anhydride, 2,4,6-trimethylbenzoic anhydride, o-, m-, and p-chlorobenzoic anhydride, o-, m-, and p-bromobenzoic anhydride, o-, m-, and p-nitrobenzoic anhydride, o-, m-, and p-methoxybenzoic anhydride, and mixtures of two or more of the above- mentioned anhydrides.

Examples of suitable mixtures of symmetrical anhydrides are the mixture of isobutyric anhydride and 2-methylbutyric anhydride, the mixture of isobutyric anhydride and 2-methylpentanoic anhydride, the mixture of 2-methylbutyric anhydride and isovaleric anhydride, and the mixture of 2-methylbutyric anhydride and valeric anhydride.

Asymmetrical anhydrides are usually available as a mixture of asymmetrical and symmetrical anhydrides. This is because asymmetrical anhydrides are usually obtained by reacting a mixture of acids with, e.g., acetic anhydride. This leads to a mixture of anhydrides, including an asymmetrical and at least one symmetrical anhydride. Such mixtures of anhydrides can be used in the process of the present invention. Examples of suitable asymmetrical anhydrides are isobutyric-2- methylbutyric anhydride, which is preferably present as admixture with isobutyric anhydride and 2-methylbutyric anhydride; isobutyric-acetic anhydride, which is preferably present as admixture with isobutyric anhydride and acetic anhydride; propionic-isobutyric anhydride, which is preferably present as admixture with propionic anhydride and isobutyric anhydride; 2-methylbutyric-valeric anhydride, which is preferably present as admixture with 2-methylbutyric anhydride and valeric anhydride; and butyric-valeric anhydride, which is preferably present as admixture with butyric anhydride and valeric anhydride.

Preferably, the anhydride is symmetrical. More preferred symmetrical anhydrides are isobutyric anhydride, 2-methylbutyric anhydride, 2-methylhexanoic anhydride, 2-propylheptanoic anhydride, isononanoic anhydride, cyclohexanecarboxylic anhydride, 2-ethylhexanoic anhydride, caprylic anhydride, n-valeric anhydride, isovaleric anhydride, caproic anhydride, and lauric anhydride. Most preferred is isobutyric anhydride. Step a) is performed at temperatures in the range -10 to 70°C, more preferably 0 to 40°C, most preferably 5 to 30°C.

The molar ratio H2O2 : anhydride is preferably in the range 0.40:1 to 1.0:1 , more preferably 0.45:1 to 0.70:1 , and most preferably 0.50:1 to 0.60:1.

The pH of the water phase during this reaction is preferably at least 6, more preferably at least 7, and most preferably at least 9. Suitable bases that may be added to reach this pH include oxides, hydroxides, bicarbonates, carbonates, (hydro)phosphates, and carboxylates of magnesium, lithium, sodium, potassium, or calcium.

The base is preferably added in amounts of 50-200 mol%, more preferably 70- 140 mol%, and most preferably 90-120 mol%, relative to anhydride.

The reaction does not require the presence of a solvent. However, if the final product (i.e. the diacyl peroxide) requires dilution in a solvent, a solvent can be pre-charged or dosed to the reaction mixture during the reaction. Suitable solvents are alkanes, chloroalkanes, esters, ethers, amides, and ketones. Preferred solvents are (mixtures of) alkanes, such as isododecane, Spirdane®, Isopar® mineral oils; esters like ethyl acetate, methyl acetate, ethylene glycol dibenzoate, dibutyl maleate, di-isononyl-1 ,2-cyclohexanedicarboxylate (DINCH), or 2,2,4-trimethylpentanediol diisobutyrate (TXIB); and phthalates, such as dimethyl phthalate or dioctyl terephthalate.

The reaction mixture resulting from step a) is a biphasic mixture, containing an aqueous and an organic phase.

According to step b), the carboxylic acid is extracted or separated from the mixture in the form of its carboxylic acid salt or adduct. The formation of said salt or adduct requires the presence of a base. If no base was present during step a) or if the amount of base added during step a) is insufficient to transform the majority of carboxylic acid into the corresponding salt or adduct, a base or an additional amount of base may be added in step b). If the amount of base present in the mixture resulting from step a) is sufficient to transform the majority of carboxylic acid into the corresponding salt or adduct, then no additional amount of base needs to be added in step b).

Suitable bases are alkylated amines, oxides, hydroxides, bicarbonates, carbonates, and carboxylates of magnesium, lithium, sodium, potassium, or calcium. These bases will deprotonate the carboxylic acid, thereby forming a water-soluble salt that ends up in the aqueous phase. The organic phase and the aqueous phase are subsequently separated.

Other suitable bases are solid materials with basic functions that are able to capture the carboxylic acid, thereby forming an adduct. Examples of such solid materials are basic ion exchange resins such as poly(styrene-co- vinylbenzylamine-co-divinylbenzene), N-{2-[bis(2-aminoethyl)amino]ethyl} aminomethyl-polystyrene, diethylaminomethyl-polystyrene, dimethylamino methylated copolymers of styrene and divinylbenzene, polymer-bound morpholine, poly(4-vinylpyridine), zeolites or mesoporous silicas containing alkylamine groups like 3-aminopropylsilyl-functionalized SBA-15 silica, polymeric amines, and mixtures of one or more of these materials. The formed adduct can be removed from the reaction mixture by filtration.

Any residual peroxy compounds in the aqueous phase can be removed by washing the aqueous phase with a solvent and/or an anhydride, preferably the anhydride of formula R ¹ -C(=0)-0-C(=0)-R ² .

After removal of the carboxylic acid, the organic phase containing the diacyl peroxide may be purified and/or dried. Purification can be performed by washing with water, optionally containing salts, base, or acid, and/or filtration over, e.g., carbon black or diatomaceous earth. Drying can be performed by using a drying salt like MgSC>4 or Na2SC>4 or by using an air or vacuum drying step. If the diacyl peroxide is to be emulsified in water, a drying step can be dispensed with. In step c), the carboxylic acid is liberated by, for instance,

(i) acidifying the aqueous phase containing the carboxylic acid salt, (ii) splitting the adduct (e.g. by heating or acidification) and physically separating (e.g. distilling) the carboxylic acid from the solid material with basic functions, or

(iii) splitting the salt via electrochemical membrane separation, e.g., bipolar membrane electrodialysis (BPM).

Preferred acids for acidifying and protonating the carboxylic acid are acids with a pKa below 3, such as H2SO4, HCI, NaHSC>4, KHSO4, and the like. Most preferably H2SO4 is used. If H2SO4 is used, it is preferably added as a 90-96 wt% solution. Acidification is preferably performed to a pH below 6, more preferably below 4.5, and most preferably below 3. The resulting pH is preferably not lower than 1.

In addition to acid, also a small amount of a reducing agent, such as sulfite and/or iodide, may be added to the aqueous phase in order to decompose peroxide residues. A thermal treatment (e.g. at 20-80°C) can be applied in order to decompose any diacyl peroxide residues.

The organic layer containing the carboxylic acid is then separated from any aqueous, salt-containing layer. Separation can be performed by gravity, using conventional separation equipment, such as a liquid/liquid separator, a centrifuge, a (pulsed and or packed) counter current column, (a combination of) mixer settlers, or a continuous (plate) separator.

In some embodiments the separation can be facilitated by salting out the organic liquid phase with a concentrated salt solution, e.g. a 20-30 wt% NaCI, NaHSC>4, KHSO4, Na2SC>4, or K2SO4 solution. The salt reduces the solubility of the carboxylic acid in the aqueous liquid phase. This extraction can be performed in any suitable device, such as a reactor, centrifuge, or mixer-settler.

Especially for lower molecular weight acids, like butyric, isobutyric, pentanoic, and methyl- or ethyl-branched pentanoic acids, a residual amount of the acid will remain dissolved in the aqueous layer. This residual amount can be recovered by adsorption, (azeotropic) distillation, or extraction. Optionally, a salt (e.g. sodium sulfate) can be added to the aqueous layer in order to lower the solubility of the carboxylic acid. In another embodiment, liberation of the carboxylic acid is achieved by electrochemical membrane separation. Examples of electrochemical membrane separation techniques are membrane electrolysis and bipolar membrane electrodialysis (BPM). BPM is the preferred electrochemical membrane separation method.

Electrochemical membrane separation leads to splitting of the metal carboxylate in carboxylic acid and metal hydroxide (e.g. NaOH or KOH) and separation of both species. It thus leads to (i) a carboxylic acid-containing mixture and (ii) a NaOH or KOH solution, separated by a membrane.

The NaOH or KOH solution can be re-used in the process of the present invention, for instance in step a).

Depending on the temperature, the salt concentration, and the solubility of the carboxylic acid in water, the carboxylic acid-containing mixture can be a biphasic mixture of two liquid phases or a homogeneous mixture. If a homogeneous mixture is formed under the electrochemical membrane separation conditions (generally 40-50°C), cooling of the mixture to temperatures below about 30°C and/or the addition of salt will ensure that a biphasic mixture will be formed. The organic liquid layer of this biphasic carboxylic acid-containing mixture can then be separated from the aqueous layer by gravity or by using equipment like a centrifuge.

The carboxylic acid-containing organic phase is optionally purified to remove volatiles like alcohols, ketones, alkenes and water before it is used in step e). These volatiles can be removed by adsorption, distillation, or drying with salt, molecular sieves, etc. Distillation is the preferred way of purification. The distillation preferably involves two product collection stages, one to collect impurities like alcohols, and another to collect the remaining water, optionally as an azeotrope with the carboxylic acid. According to steps e) and f), the carboxylic acid is subsequently reacted with an acid anhydride or a ketene of the formula C(R ⁴ )2=C=0 - each R ⁴ being independently selected from H and CH3 - preferably with acetic anhydride, to form an anhydride with the formula R ¹ -C(=0)-0-C(=0)-R ² , which is subsequently at least partly recycled to step a) and used again to produce the diacyl peroxide.

The reaction of step e), in particular the reaction with acetic anhydride, is advantageously performed in a reactive distillation column that is fed in the middle sections with the carboxylic acid and the acetic anhydride. The product anhydride is drawn from the bottom of the column and the product acetic acid is collected from the top of the column. An alternative method is to produce the anhydride in a stirred reactor surmounted by a distillation column. This allows the acetic acid to be distilled when formed in order to shift the equilibrium. US 2005/014974 discloses a process to prepare isobutyric anhydride by reacting acetic anhydride with isobutyric acid and containing a step of distilling of the acetic acid as formed. The distillation column is preferably sufficiently efficient to get high purity acetic acid. The efficiency of the column is preferably at least 8 theoretical plates. High purity acetic acid can be sold and/or used for various purposes.

The reaction with the ketene of the formula C(R ⁴ )2=C=0 is preferably performed in a counter-current adsorption device, as disclosed in US 2,589,1 12. The preferred ketene has the formula CH2=C=0.

A catalyst may be used in step e), although it is preferred to perform the reaction in the absence of catalyst. Examples of suitable catalysts are oxides, hydroxides, bicarbonates, carbonates, and carboxylates of magnesium, lithium, sodium, potassium, or calcium.

The molar ratio of carboxylic acid to acetic anhydride is preferably in the range 0.5:1 to 5:1 , more preferably 1.5:1 -2.2:1 , most preferably 1.8:1 to 2.2:1 . A slight excess of carboxylic acid relative to acetic anhydride might be used.

The reaction is preferably performed at a temperature of 70-200°C, preferably 100-170°C, most preferably 120-160°C. The temperature can be maintained at the desired value by adjusting the pressure in the reactor. This pressure is preferably in the range 1 -100 kPa, more preferably 5-70 kPa. After completion of the reaction, any excess acetic anhydride that may be present can be distilled off in order to purify the anhydride of formula R ¹ -C(=0)-0-C(=0)- R ² .

This anhydride can then be used again in step a).

In a preferred embodiment, the carboxylic acid that is used in step e) is obtained from two or three sources. The first source is the carboxylic acid that is liberated in step c). The second source is the carboxylic acid obtained by oxidation of the corresponding aldehyde in accordance with step d), as described below. The third source is an additional amount of carboxylic acid obtained in any other way.

As oxygen source for step d), air is preferably used, although pure oxygen, oxygen-enriched, or oxygen-lean air may also be applied. The oxygen source can be added to the reaction mixture by feeding it as a gas to the reactor, preferably using a sparger.

The reaction of step d) is preferably performed at a temperature in the range of 0-70°C, more preferably in the range 10-60°C, and most preferably in the range 20-55°C.

Atmospheric pressure is preferably used; at lower pressure the aldehyde may evaporate, which is undesired.

A catalyst may optionally be used. Very good catalysts, which not only accelerate oxidation but also increase the yield of acid, are platinum black and ferric salts. Cerium, nickel, lead, copper and cobalt salts are also useful, particularly their carboxylic acid salts.

The catalyst may be added in amounts of 0 to 20 mol% relative to aldehyde, more preferably 0-5 mol%, and most preferably 0-2 mol%.

Both symmetrical and asymmetrical diacyl peroxides can be produced by the process of the present invention. If a symmetrical anhydride (R ¹ =R ² ) is used, a symmetrical diacyl peroxide will result. Examples of diacyl peroxides which can particularly suitably be produced by this process are di-2-methylbutyryl peroxide, di-iso-valeryl peroxide, di-n-valeryl peroxide, di-n-caproyl peroxide, di-2- methylhexanoyl peroxide, di-2-propylheptanoyl peroxide, di-isononanoyl peroxide, di-cyclohexylcarbonyl anhydride, di-2-ethylhexanoyl anhydride, acetyl- isobutyryl peroxide, propionyl-isobutyryl peroxide, and di-isobutyryl peroxide. The most preferred is di-isobutyryl peroxide.

The process according to the present invention and individual steps thereof can be performed batch-wise or continuously. Steps that are preferably performed in continuous mode are reactive distillation to make the anhydride in step e) and isolation and purification of the carboxylic acid in step c).

Also combinations of batch and continuous operation can be used. Examples of combinations are:

- a batch reaction to the diacyl peroxide in step a), followed by a batch separation and continuous purification of carboxylic acid and continuous reactive distillation towards the anhydride in step e),

- a continuous reaction to diacyl peroxide and separation and purification of the carboxylic acid, followed by a batch mode distillation to the anhydride in step e), or

- a batch reaction to diacyl peroxide and separation of the product, followed by a continuous mode purification of carboxylic acid and continuous reactive distillation to the anhydride in step e).

EXAMPLES

Example 1

To an empty reactor equipped with a thermometer and a turbine impellor, were charged 79 g n-butyric anhydride and 16.9 g Spirdane® D60 (ex-Total) at 10°C. While cooling the mixture at 15-17°C, 194 g 10.8 wt% NaOH solution and 12.4 g 70 wt% H2O2 solution were dosed in 17 minutes.

After a 5 minutes post reaction at 10°C, the aqueous phase was allowed to separate by gravity and was subsequently removed. The organic layer - containing 62.6 wt% di-n-butyryl peroxide - was dried with MgSC>4 -2H20 and filtered over a G3 glass filter. The aqueous phase (241 g) was stirred at 0-5°C with 18.3 g Spirdane® D60 to extract residual peroxide and the layers were allowed to separate. H2SO4 (26.8 g, 96 wt%) was subsequently added to the water layer, resulting in a temperature increase to 27°C. A 215.4 g water layer and a 51 .6 g organic layer were obtained, which layers were separated. The organic compounds in said organic layer consisted for more than 99 wt% of butyric acid.

After azeotropic removal of water in a rotary evaporator at 200 mbar, 80°C, the n-butyric acid was mixed with n-butyric acid from another source (in this case, from Sigma Aldrich) and then reacted with acetic anhydride in a molar ratio n- butyric acid:acetic anhydride of 2:1 .05 and heated to distill the acetic acid at <400 mbar and 120°C to obtain n-butyric anhydride as the residue. This anhydride was then recycled to the first step in which it was reacted with H2O2.

Example 2

To an empty reactor equipped with a thermometer and a turbine impellor were charged 45.6 g Spirdane® D60 (ex-Total) and 288 g water, at 0°C. While cooling the mixture at 5°C, 21 1 g 25 wt% NaOH solution, 32.4 g 70 wt% of H2O2 solution, and 21 1 g isobutyric anhydride were dosed in 40 minutes.

After 10 minutes post reaction at 5°C, the water layer was allowed to settle by gravity and was subsequently removed. The organic layer contained 70.9 wt% di-isobutyryl peroxide, corresponding to a yield of 94%, calculated on anhydride. The water layer contained about 24 wt% of sodium isobutyrate.

To said water layer, 10.4 g Spirdane® D60 was added and the resulting mixture was mixed at 0-5°C. After separation of the Spirdane® D60 layer, 32.5 g H2SO4- 96% was added to the water layer. The resulting 270.1 g water layer and 60.6 g of organic isobutyric acid layer were separated. The organic compounds in said organic layer consisted for more than 99.5 wt% of isobutyric acid.

After azeotropic removal of water in a rotary evaporator at 200 mbar, 80°C, the isobutyric acid was mixed with isobutyric acid from another source (in this case, from Sigma Aldrich), and then reacted with acetic anhydride in a molar ratio isobutyric acid:acetic anhydride of 2:1.05 and heated to distill the acetic acid at <400 mbar and 120°C to obtain isobutyric anhydride as the residue. This anhydride is then recycled to the first step in which it is reacted with H2O2.