Patent Publication Number: US-2022235003-A1

Title: Process for the production of diacyl peroxides

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a U.S. National-Stage entry under 35 U.S.C. § 371 based on International Application No. PCT/EP2020/066231, filed Jun. 11, 2020 which was published under PCT Article 21(2) and which claims priority to European Application No. 19179624.2, filed Jun. 12, 2019, which are all hereby incorporated in their entirety by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a process for the preparation of diacyl peroxides. 
     BACKGROUND 
     Diacyl peroxides have the general formula 
       R—C(═O)—O—O—C(═O)—R
 
     wherein the R-groups may be the same or different and are selected from aryl, arylalkyl, and linear, branched, or cyclic alkyl groups, optionally substituted with heteroatom-containing substituents. 
     Symmetrical diacyl peroxides, that is, those in which the R-groups in the above formula are the same, have been prepared by reacting an excess of acid anhydride or acid chloride with alkaline solutions of hydrogen peroxide, as illustrated by the following equations: 
       2R—C(═O)—O—C(═O)—R+Na 2 O 2 →R—C(═O)—O—O—C(═O)—R+2NaOC(═O)R
 
       2R—C(═O)Cl+Na 2 O 2 →R—C(═O)—O—O—C(═O)—R+2NaCl
 
     In this reaction scheme, Na 2 O 2  does not refer to the discrete product Na 2 O 2 , but to an equilibrium comprising H 2 O 2  and NaOOH. 
     U.S. Pat. No. 3,580,955 discloses a process for the preparation of asymmetrical diacyl peroxides by reacting an acid chloride with an aldehyde and oxygen in the presence of an acid receptor. 
     U.S. Pat. No. 3,502,701 produces asymmetrical diacyl peroxides by reacting an acid chloride with peroxyacid. 
     Acid chlorides are relatively expensive and generate chloride-containing water layers, which lead to waste waters with high salt concentration. 
     Another process that allows for the preparation of asymmetrical diacyl peroxides has been described in GB 1,156,573, and involves the reaction between an organic anhydride, an aldehyde, and oxygen, in the presence of a catalyst that comprises the lithium or magnesium salt of an organic acid. 
     GB 444,603 discloses the preparation of acetyl benzoyl peroxide by reacting benzaldehyde and acetic anhydride with an oxygen-containing gas in the presence of dibenzoyl peroxide. 
     Anhydrides, however, 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. 
     GB 901,041 discloses a process of preparing diacyl peroxides by reacting a peracid with an anhydride or a halide of an organic acid, wherein using the chloride is said to be preferred. 
     It is an object of the present disclosure to provide a process for the production of diacyl peroxides that has a relatively low carboxylic acid (salt) content in its effluent and does not require the use of acid chlorides. 
     BRIEF SUMMARY 
     This disclosure provides a process for the production of a diacyl peroxide comprising the following steps:
         a) producing a mixture comprising a diacyl peroxide and a carboxylic acid by reacting one or more anhydrides with the formula R 1 —C(═O)—O—C(═O)—R 2  with an aldehyde of the formula R 3 —C(═O)H and oxygen, wherein R 1  and R 3  are independently selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with 1 to about 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 about 2 to about 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(═O)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(═O)OH—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═O, each R 4  being independently selected from H and CH 3  to form one or more anhydrides with the formula R 1 —C(═O)—O—C(═O)—R 2 , and   f) recycling at least part of the anhydride formed in step e) to step a).       

    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the disclosure or the application and uses of the disclosure. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the disclosure or the following detailed description. 
     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 one or more anhydrides with the formula R 1 —C(═O)—O—C(═O)—R 2  with an aldehyde of the formula R 3 —C(═O)H and oxygen, wherein R 1  and R 3  are selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with 1 to about 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 about 2 to about 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(═O)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(═O)OH—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═O, each R4 being independently selected from H and CH 3 , preferably with acetic anhydride, to form one or more anhydrides with the formula R 1 —C(═O)—O—C(═O)—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. The oxidation can be performed in the same equipment as step a), which makes it very economical and further allows for the production of diacyl peroxides starting from the corresponding aldehyde, which is relatively cheap. 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 an aldehyde with an anhydride with the formula R 1 —C(═O)—O—C(═O)—R 2 . 
     R 1  in this formula is selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with 1 to about 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 about 2 to about 11, even more preferably about 2 to about 8, and most preferably about 3 to about 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 of n-propyl, isopropyl, isobutyl, n-butyl, and 2-butyl groups. 
     R 2  in this formula is selected from linear and branched alkyl, cycloalkyl, aryl, and arylalkyl groups with about 2 to about 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 about 2 to about 11, even more preferably about 2 to about 8, and most preferably about 3 to about 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 of n-propyl, isopropyl, isobutyl, n-butyl, and 2-butyl groups. 
     The anhydride can be symmetrical, meaning R 1 ═R 2 , or asymmetrical, meaning that the R 1 ≠R 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(═O)OH. If the anhydride is asymmetrical, the carboxylic acid will be a mixture of R 2 —C(═O)OH and R 1 —C(═O)OH. 
     Suitable symmetrical anhydrides are propionic anhydride, n-butyric anhydride, isobutyric anhydride, pivalic anhydride, 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, 2-propylheptanoic anhydride, decanoic anhydride, neodecanoic anhydride, undecanoic anhydride, neoheptanoic anhydride, iso-octanoic 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 the 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 disclosure. Examples of suitable asymmetrical anhydrides are methoxyacetic isononanoic anhydride, which is preferably present as admixture with methoxyacetic anhydride and isononanoic anhydride; ethoxyacetic isononanoic anhydride, which is preferably present as admixture with ethoxyacetic anhydride and isononanoic anhydride; methoxy acetic nonanoic peroxide, which is preferably present as admixture with methoxyacetic anhydride and n-nonanoic anhydride; ethoxyacetic nonanoic anhydride, which is preferably present as admixture with ethoxyacetic anhydride and n-nonanoic anhydride; 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. 
     More preferred anhydrides are isobutyric anhydride, n-butyric anhydride, 2-methylbutyric anhydride, 3-methylbutyric anhydride, 2-methylhexanoic anhydride, 2-methylpentanoic anhydride, 2-propylheptanoic anhydride, n-nonanoic anhydride, isononanoic anhydride, cyclohexanecarboxylic anhydride, 2-ethylhexanoic anhydride, n-valeric anhydride, and isovaleric anhydride. Most preferred is isobutyric anhydride. 
     In step a), the anhydride is reacted with the reaction product of an aldehyde and oxygen. 
     The aldehyde has the formula R 3 —C(═O)H, wherein R 3  is selected from linear and branched alkyl, cycloalkyl, aryl, or arylalkyl groups with carbon atoms with 1 to about 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 1 to about 11, more preferably about 3 to about 9, and most preferably about 3 to about 6. 
     In a further preferred embodiment, R 3  is a linear or branched alkyl group. 
     The most preferred R 3  groups are n-propyl and isopropyl, which means that the most preferred aldehydes with formula R 3 —C(═O)H are isobutanal, n-pentanal, and n-butanal. 
     Suitable aldehydes include acetaldehyde, propionaldehyde, n-butanal, isobutanal, 2,2-dimethylpropanal, n-pentanal, 3-methyl butanal, 2-methyl butanal, 2-ethyl butanal, 2-propylheptanal, n-hexanal, n-octanal, 4-methylpentanal, n-heptanal, 6-methylheptanal, n-octanal, n-nonanal, isononanal, n-decanal, undecanal, tridecanal, 2-ethylhexanal, tetradecanal, octadecanal, phenylethanal, cyclohexanecarbaldehyde, 3-methyl-cyclopentanal, beta-methoxy propanal, alpha-ethoxy butanal, benzaldehyde, o-, m-, and p-methylbenzaldehyde, 2,4,6-trimethylbenzaldehyde, o-, m-, and p-chlorobenzaldehyde, o-, m-, and p-bromobenzaldehyde, o-, m-, and p-nitrobenzaldehyde, o-, m- and p-acetoxybenzaldehyde, and o-, m-, and p-methoxybenzaldehyde. 
     Preferred aldehydes are n-butanal, isobutanal, 2,2-dimethylpropanal, 3-methylbutanal, 2-methylbutanal, 2-methylpentanal, 2-ethylhexanal, n-heptanal, n-pentanal, isononanal, and 2-propylheptanal. 
     More preferred aldehydes are n-butanal, isobutanal, 3-methylbutanal, n-pentanal, n-heptanal, and isononanal. 
     The most preferred aldehydes are isobutanal, n-pentanal, and n-butanal. 
     A suitable oxygen source is air, although pure oxygen, oxygen-rich or oxygen-lean air may also be used. The oxygen source can be added to the reaction mixture by feeding it as a gas to the reactor, preferably using a sparger. 
     It is preferred to dose the oxygen source and the aldehyde to the reactor containing the anhydride in such a way that losses due to evaporation are low and the reaction rate is sufficiently high. 
     The reaction of step a) is preferably performed at a temperature in the range of from about −10 to about 60° C., more preferably in the range from about 0 to about 50° C., even more preferably in the range from about 0 to about 40° C., and most preferably in the range from about 5 to about 40° C. 
     Atmospheric pressure is preferably used. At lower pressure, less oxygen dissolves in the reaction mixture and more aldehyde may evaporate. Some overpressure may be used in order to increase the reaction rate, but high pressures are generally not desired for concentrated peroxide systems. 
     The molar ratio aldehyde to anhydride is preferably in the range from about 0.8 to about 2.5, more preferably from about 1.0 to about 2.0, and most preferably from about 1.1 to about 1.7. 
     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 with the anhydride or dosed to the reaction mixture during the reaction. Suitable solvents are alkanes, esters, ethers, amides, and ketones. Preferred solvents are (mixtures of) alkanes, such as isododecane, Spirdane®, Isopar® mineral oils; esters like ethylacetate, methylacetate, ethylene glycol dibenzoate, dibutyl maleate, di-isononyl-1,2-cyclohexanedicarboxylate (DINCH), or 2,2,4-trimethylpentanediol diisobutyrate (TXIB); and phthalates, such as dimethylphthalate or dioctyl terephthalate. 
     A basic catalyst may optionally be used. Examples of suitable catalysts are oxides, hydroxides, bicarbonates, carbonates, (hydro)phosphates, and carboxylates of magnesium, lithium, sodium, potassium, or calcium. 
     The catalyst may be added in amounts of from about 0 to about 30 mol % relative to anhydride, more preferably from about 0 to about 10 mol %, and most preferably from about 0 to about 5 mol %. 
     According to step b), the carboxylic acid is extracted or separated from the mixture resulting from step a) 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(═O)—O—C(═O)—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 MgSO 4  or Na 2 SO 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 H 2 SO 4 , HCl, NaHSO 4 , KHSO 4 , and the like. Most preferably H 2 SO 4  is used. If H 2 SO 4  is used, it is preferably added as an about 90 to about 96 wt % solution. 
     Acidification is preferably performed to a pH below about 6, more preferably below about 4.5, and most preferably below about 3. The resulting pH is preferably not lower than about 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 any peroxide residues. A thermal treatment (e.g. at about 20 to about 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 continues (plate) separator. 
     In some embodiments the separation can be facilitated by salting out the organic liquid phase with a concentrated salt solution, e.g. an about 20 to about 30 wt % NaCl, NaHSO 4 , KHSO 4 , Na 2 SO 4 , or K 2 SO 4  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 disclosure, 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 from about 40 to about 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═O—each R4 being independently selected from H and CH 3 — preferably with acetic anhydride, to form an anhydride with the formula R 1 —C(═O)—O—C(═O)—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 about 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═O is preferably performed in a counter-current adsorption device, as disclosed in U.S. Pat. No. 2,589,112. The preferred ketene has the formula CH 2 ═C═O. 
     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 from about 0.5:1 to about 5:1, more preferably from about 1.5:1 to about 2.2:1, most preferably from about 1.8:1 to about 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 from about 70 to about 200° C., preferably from about 100 to about 170° C., most preferably from about 120 to about 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 from about 1 to about 100 kPa, more preferably from about 5 to about 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(═O)—O—C(═O)—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. 
     Oxidation of the aldehyde in step d) can be performed in the same equipment as step a), which makes it very economical and further allows for the production of diacyl peroxides starting from the corresponding aldehyde(s), which is relatively cheap. 
     As oxygen source for step d), air is preferably used, although pure oxygen or 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 from about 0 to about 70° C., more preferably in the range from about 10 to about 60° C., and most preferably in the range from about 20 to about 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 from about 0 to about 20 mol % relative to aldehyde, more preferably from about 0 to about 5 mol %, and most preferably from about 0 to about 2 mol %. 
     Both symmetrical and asymmetrical diacyl peroxides can be produced by the process of the present disclosure. Symmetrical diacyl peroxides are, however, preferred. If R 1 , R 2 , and R 3  in the above formulae are equal, a symmetrical diacyl peroxide will result. Examples of symmetrical diacyl peroxides for which this process is especially suitable are di-n-butanoyl peroxide, di-n-valeroyl peroxide, di-2-methylbutanoyl peroxide, di-3-methylbutanoyl peroxide, di-2-methylpentanoyl peroxide, di-cyclohexylcarbonyl peroxide, di-n-nonanoyl peroxide, di-isononanoyl peroxide, and di-isobutanoyl peroxide. Most preferred are di-2-methylbutanoyl peroxide, di-2-methylpentanoyl peroxide, and di-isobutanoyl peroxide 
     Examples of an asymmetrical diacyl peroxide for which this process is especially suitable are isononanoyl isobutanoyl peroxide, isononanoyl butanoyl peroxide, isononanoyl 2-ethylhexanoyl peroxide, isononanoyl 2-methylbutanoyl peroxide, isononanoyl 3-methylbutanoyl peroxide, isononanoyl pivaloyl peroxide, isononanoyl cyclohexylcarbonyl, isononanoyl heptanoyl peroxide, isononanoyl 2-propylheptanoyl peroxide, 3-methylbutanoyl isobutanoyl peroxide, 3-methylbutanoyl n-butanoyl peroxide, 3-methylbutanoyl 2-ethylhexanoyl peroxide, 3-methylbutanoyl 2-methylbutanoyl peroxide, 3-methylbutanoyl pivaloyl peroxide, 3-methylbutanoyl cyclohexylcarbonyl peroxide, 3-methylbutanoyl heptanoyl peroxide, 3-methylbutanoyl isononanoyl peroxide, 3-methylbutanoyl 2-propylheptanoyl peroxide, isobutanoyl butanoyl peroxide, isobutanoyl 2-ethylhexanoyl peroxide, isobutanoyl 2-methylbutanoyl peroxide, isobutanoyl 3-methylbutanoyl peroxide, isobutanoyl cyclohexylcarbonyl peroxide, isobutanoyl heptanoyl peroxide isobutanoyl 2-propylheptanoyl peroxide, n-butanoyl isobutanoyl peroxide, n-butanoyl 2-ethylhexanoyl peroxide, n-butanoyl 2-methylbutanoyl peroxide, n-butanoyl 3-methylbutanoyl peroxide, n-butanoyl pivaloyl peroxide, n-butanoyl cyclohexylcarbonoyl peroxide, n-butanoyl heptanoyl peroxide, n-butanoyl 2-propylheptanoyl peroxide, 2-methylbutanoyl isobutanoyl peroxide, 2-methylbutanoyl butanoyl peroxide, 2-methylbutanoyl 2-ethylhexanoyl peroxide, 2-methylbutanoyl 3-methylbutanoyl peroxide, 2-methylbutanoyl cyclohexylcarbonoyl peroxide, 2-methylbutanoyl heptanoyl peroxide, 2-methylbutanoyl 2-propylheptanoyl peroxide, 2-methylpentanoyl isobutanoyl peroxide, 2-methylpentanoyl butanoyl peroxide, 2-methylpentanoyl 3-methylbutanoyl peroxide, 2-methylpentanoyl cyclohexylcarbonoyl peroxide, 2-methylpentanoyl heptanoyl peroxide, 2-propylheptanoyl heptanoyl peroxide, nonanoyl isobutanoyl peroxide, nonanoyl butanoyl peroxide, nonanoyl 2-ethylhexanoyl peroxide, nonanoyl 2-methylbutanoyl peroxide, nonanoyl 3-methylbutanoyl peroxide, nonanoyl pivaloyl peroxide, nonanoyl cyclohexylcarbonoyl peroxide, nonanoyl heptanoyl peroxide, nonanoyl 2-propylheptanoyl peroxide, methoxyacetyl isononanoyl peroxide, ethoxyacetyl isononanoyl peroxide, methoxyacetyl nonanoyl peroxide, and ethoxyacetyl nonanoyl peroxide. 
     Most preferred asymmetrical diacyl peroxides are isononanoyl isobutanoyl peroxide, nonanoyl isobutanoyl peroxide, isobutanoyl heptanoyl peroxide, valeroyl 2-ethylhexanoyl peroxide, valeroyl 2-propylheptanoyl peroxide, valeroyl cyclohexylcarbonoyl peroxide, heptanoyl 3-methylbutanoyl peroxide, nonanoyl 3-methylbutanoyl peroxide, isononanoyl 3-methylbutanoyl peroxide, pentanoyl 3-methylbutanoyl peroxide, nonanoyl heptanoyl peroxide, isononanoyl heptanoyl peroxide, nonanoyl pentanoyl peroxide, isononanoyl pentanoyl peroxide, and isononanoyl nonanoyl peroxide. 
     The process according to the present disclosure 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 at 10° C., 1.8 g isobutanal, 30.9 g isododecane, 39.9 g isobutyric anhydride, and 0.42 g NaHCO 3  were added. Air was passed through the resulting mixture under fast stirring. During 4.5 hours, a mixture of 34.2 g isobutanal and 39 g isobutyric anhydride were dosed at 8-10° C. The air dosing was maintained during 16.5 h, during which the temperature dropped 3° C. 
     After cooling the resulting mixture to 0° C., 24 g Na 2 CO 3 — dissolved in 104 g water—was slowly dosed. The layers were allowed to separate at 0° C. and 117 g of organic phase and 147.4 g of aqueous phase were obtained. 
     The di-isobutyryl peroxide content of the organic phase was 47 wt %, corresponding to a yield of 63%, based on aldehyde. FTIR analysis of the product revealed that the peroxide contained a small amount of anhydride (shoulder at 1750 cm −1 ) 
     The aqueous phase was extracted with 2.3 g isododecane in order to remove traces of peroxide and was subsequently acidified to pH 2 with a 20 wt % H 2 SO 4  solution. Phase separation resulted in an organic layer with 20.9 g wet isobutyric acid. 
     GC analysis of the organic compound in the organic layer showed a content of 97% of isobutyric acid, 1% of isododecane, and 1% of volatile components (excluding water). 
     Upon azeotropic distillation of this isobutyric acid-containing layer, a bottom stream containing &gt;98% isobutyric acid and a minor amount of water was obtained. This isobutyric acid was then mixed with isobutyric acid from another source (in this case, from Sigma Aldrich), and then mixed with acetic anhydride in a molar ratio isobutyric anhydride:acetic anhydride of 2:1.05 and heated to distill the acetic acid at &lt;400 mbar and 120° C. to obtain isobutyric anhydride as the residue. The isobutyric anhydride was then recycled to the first step in which it was reacted with isobutanal. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the various embodiments in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment as contemplated herein. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the various embodiments as set forth in the appended claims.