GB2098211A - Production of high-boiling esters by catalytic esterification - Google Patents

Abstract

A process for producing an ester boiling above 100 DEG C comprises reacting at elevated temperature an alcohol with a carboxylic acid or anhydride in the presence of a heterogeneous esterification catalyst comprising a metal substrate coated with a stannous salt of a di- or polycarboxylic acid containing up to 20 C atoms, eg stannous oxalate, and a homogeneous esterification catalyst comprising stannous formate.

Description

SPECIFICATION Process for the production of high-boiling esters The present invention relates generally to a process for the production of high-boiling esters and in particular to a process for the production of high-boiling esters employing a combination of a heterogeneous esterification catalyst comprising a metal substrate coated with a stannous salt of a di- or polycarboxylic acid containing up to 20 carbon atoms and a homogeneous esterification catalyst comprising stannous formate.

Over the years a variety of catalysts have been employed in the production of high-boiling esters, sometimes referred to as "residue esters", which include for example phthalates and stearates. Thus strong mineral acids, such as sulphuric acid and phosphoric acid, and organic acids such as para-toluene sulphonic acid have been used extensively. Not withstanding their extensive use it has long been recognised that problems are associated therewith. Thus in order to achieve commercially viable production rates at temperatures low enough to produce commercially acceptable products it is necessary to employ such high acid concentrations which lead to dehydration of the alcohol reactant giving rise to unwanted olefins and esters and colouration of the ester products.The colour may be improved in a subsequent process, either physically, by vacuum distillation of the ester product or chemically by an oxidative treatment to convert the colour-forming impurities to substances which may be removed by extraction with an aqueous solution of a strong base.

The problems raised by strong acid catalysts are well known to be alleviated to a great extent by the use of amphoretic catalysts. These may be described as compounds of elements capable of functioning as cations or anions as added or subsequently by formation of a complex with the reagents used in the esterification reaction. These include titanium compounds as described for example in British Patent Specifications Nos: 852110, 886750, 1058242, 1061173, 1070914 and 1246346. Typical of such titanium compounds are the titanium esters, e.g.

tetraisopropyl titanate. Other amphoteric esterification catalysts are described in British Patent Specifications Nos: 879,799 [Sb2O3]; 733870 [Al(OH)3 Na OH]; 747260[AI2(SO4)3/NaOH]; 1076702[ZnO]; 111 8363[MoS2/C]; 1 372854[Zn(R)2] and USP 345729[Ph3Bi].

Another amphoteric esterification catalyst said to be useful in the production of esters having a lower colour intensity than the products obtained by the use of other amphoteric catalysts is stannous oxalate as described in British Patent Specification No: 990297.

A problem associated with the use of some of the amphoteric catalysts is their lack of solubility in the reaction mixture, it being recognised in the art that only that portion of the catalyst soluble in the reaction mixture is catalytically effective. This kinetic constraint arising from low catalyst solubility and steric hindrance of the catalyst/reagents complex leads to the use of high temperatures, generally around 220"C or above, in order to achieve a satisfactory reaction rate. Quite apart form the economic penalty the use of such high temperatures can lead to product discolouration.

Our copending UK application No; 8007063 (BP Case No: 4888) describes a heterogeneous catalyst suitable for use in the production of esters which comprises one or more metals of atomic number a12 in filament or particulate form coated with a tin-containing compound.

Furthermore our copending UK application Nos: 8007064/8012530 (BP Case Nos: 4889/4955) describe a heterogeneous catalyst suitable for use in the production of esters which catalyst comprises one or more metals of atomic number 312 in a physical form having a surface area greater than 0.0001 square feet/gram coated with a compound of a metal of Groups I to VIII of the Periodic Table other than tin and, optionally, a tin-containing compound.

The coatings are formed by contacting the metal substrate at elevated temperature with a solution and/or a suspension of the metal or metals-containing coating compound in an organic solvent such as an ester, a ketone or an alcohol. A particularly advantageous method of coating the substrate is to carry out an esterification reaction in the presence of the substrate and as an esterification catalyst an excess of the compound of the metal or metals to be incorporated into the coating. It has also been found that the coatings form slowly, several batch reaction stages being required to achieve complete coverage, and that the rate of formation of coatings can be accelerated by the use of additives which are known to facilitate the formation of smooth, compact and uniform platings in the tin electroplating industry.This forms the subject of our copending UK application No. 8108077 (BP Case No. 5127). The coatings so-produced are active as heterogeneous esterification catalysts particularly when used in combination with a homogeneous amphoteric esterification catalyst.

It has now been found that a combination of a heterogeneous esterification catalyst comprising a metal substrate coated with a stannous salt of a di- or polycarboxylic acid containing up to 20 carbon atoms and a homogeneous esterification catalyst comprising stannous formate is a particularly active catalyst system for the production of high-boiling esters.

The use of the catalyst system facilitates either the achievement of shorter batch reaction times, thus potentially expanding the production capacity of the reaction system, or the use of a smaller amount of catalyst addition to the batch esterification charge thus giving an economic advantage.

Accordingly, the present invention provides a process for the production of an ester boiling above 100"C which process comprises reacting at elevated temperature an alcohol with a carboxylic acid or a carboxylic acid anhydride in the presence of a combination of a heterogeneous esterification catalyst comprising a metal substrate coated with a stannous salt of a di- or polycarboxylic acid containing up to 20 carbon atoms and a homogeneous esterification catalyst comprising stannous formate.

The process of the invention is generally applicable to esters which boil above 1 00'C at atmospheric pressure. It is particularly applicable to esters derived from the following acids: monobasic acidFcontaining up to 20 carbon atoms, e.g. alkanoic acids such as myristic, palmitic and stearic acid, alkenoic acids such as oleic acid or derivatives of such alkanoic and alkenoic acids. e.g. ricinoleic acid; aliphatic dibasic acids especially those containing up to 20 carbon atoms, preferably up to 10 carbon atoms such as adipic, azelaic or sebacic acid; tribasic aliphatic acids such as citric acid; menobasic aromatic acidFsuitably those containing up to 20 carbon atoms, such as benzoic acid; dibasic aromatic acids and their anhydrides such as the three phthalic acids especially phthalic acid itself (ortho-phthalic acid) or phthalic anhydride, and their hydrogenation products, e.g.

hexahydrophthalic anhydride. and tribasic aromatic acids and the anhydndessuch as hemimellitic, trimellitic or trimesic acids and their anhydrides.

The preferred carboxylic aids are ortho-phthalic acid or its anhydride, adipic acid, azelaic acid or sebacic acid.

The process of the invention is particularly applicable to esters derived from the following alcohols: monohydric alcohols--containing up to 20 carbon atoms, particularly alkanols containing from 4 to 14 carbon atoms, e.g. butanol, isoheptanol, iso-octanol, 2-ethylhexanol, nonanol, decanol, tridecanol or mixtures of alcohols containing, for example, 7 to 9 carbon atoms such as are obtained form olefinic mixtures by the OXO process; dihydric alcohols--containing up to 20 carbon atoms, e.g. monoethylene glycol, diethylene glycol, triethylene glycol, mono-, di- or tripropylene glycol, the butylene glycols or 2,2,4trimethyl-pentane diol; trihydric alcohols such as glycerol; aliphatic cyclic alcoholscontaining up to 1 2 carbon atoms such as cyclohexanol; derivatives particularly other derivatives of the dihydric and trihydric alcohols, e.g. lower alkyl ether derivatives as 2-butoxy ethanol.

The preferred alcohols are the monohydric alcohols containing from 4 to 1 4 carbon atoms.

With regard to the heterogeneous esterification catalyst the metal substrate may suitably be any metallic element of atomic number 12 such as iron, titanium, aluminium, copper or nickel.

Alloys incorporating one or more of the aforesaid metals may also be employed. Iron is preferably used as an alloy, e.g. stainless steel, which contains iron as the major component and chromium and nickel as minor components. The metal may be in a high surface area from, i.e. a surface area greater than 0.0001 square feet/gram, preferably in the range 0.0001 to 1.0 square feet/gram, or a low surface area form, i.e. less than 0.0001 square feet/gram. Suitable high surface area forms of the metal are the filament form, preferably the multifilament form, and the particulate form. Suitable multifilament forms of the metals copper, titanium, aluminium and stainless steel are commercially available under the trade name "KNITMESH". Such materials are supplied as blanket-type sheets or rolls having a surface area of about 600 square feet/cubic foot.A preferred particulate form of the metal is microspheroidal, e.g. ball bearings.

Alternatively the metal substrate may be in a low surface area form. In a preferred embodiment of the invention the metal substrate is the internal metal surface of an esterification reactor fabricated in one of the aforesaid metals, for example stainless steel. The stannous salt may suitably be a salt of an alliphatic, alicyclic or aromatic di- or polycarboxylic acid containing up to 20 carbon atoms. Suitable stannous salts which may be used include stannous oxalate, stannous phthalate, stannous citrate and stannous tartrate. The preferred stannous salt is stannous oxalate.

The heterogeneous catalyst may suitably be prepared by the steps of contacting at elevated temperature the metal substrate with a solution and or a suspension of a stannous salt of a di-or polycarboxylic acid containing up to 20 carbon atoms in an organic solvent capable of dissolving from 0.001 to 1 gram of stannous salt per 100g of solvent at the elevated temperature, recovering the catalyst so-formed and optionally repeating the contact and recovery steps one or more times.

The organic solvent is preferably one capable of dissolving from 0.001 to 0.1g of stannous salt per 100g solvent at the elevated temperature employed. Preferably the organic solvent has a boiling point > 1 00'C, even more preferably within the range 1 60 to 260"C. Suitable solvents include alcohols, carboxylic acids or anhydrides, ketones and esters. The amount of stannous salt employed may suitably be greater than 19 of tin metal ion per m2 of metal surface, preferably from 3 to 1 00g per m2 metal surface.

Preferably the heterogeneous catalyst is prepared by contacting the metal substrate in a first step at elevated temperature with a reaction mixture comprising an alcohol and a carboxylic acid or a carboxylic acid anhydride in the presence of an excess of the stannous salt so as to produce an ester boiling above 100"C, separating the reaction mixture from the metal substrate in a second step and, optionally repeating the first and second steps one or more times. The result of this is to transfer stannous salt from solution on to the metal substrate. Suitably the amount of stannous salt employed may be > 0.01% w/w of the reaction mixture and preferably in the range 0.1 to 5.0% w/w.

Preferably there is also added during preparation of the catalyst one or more of the compounds glue, gelatin, peptone, beta-naphthol, resorcinol, 4,4-dihydroxydiphenylmethane, 4,4-dihydroxydiphenylsulphone, phenol sulphonic acid, cresylic acid or stannous sulphate. The amount of the, or each, additive employed may suitably be in the range from 0.1 to 10 times, preferably from 0.5 to 5 times, the weight of stannous salt added.

The elevated temperature may suitably be greater than 1 OO'C and is preferably in the range from 1 50 to 300"C. The contact and recovery steps may be repeated once or more times.

With regard to the homogeneous esterification catalyst the amount of stannous formate may suitably be greater than 0.005% w/w and is preferably in the range from 0.01 to 1.0% w/w of the reaction mixture.

With regard to the esterification process forming the subject of the present invention the esterification reaction temperature is suitably greater than 160"C and preferably is in the range from 180 to 225"C. The temperature may suitably be controlled by the addition of an inert diluent of appropriate boiling point and/or by the use of sub-atmospheric pressure. Suitably inert diluents include hydrocarbons such as toluene or ortho-xylene, of which ortho-xylene is preferred. The amount of iner diluent added may suitably be greater than 5% w/w, preferably from 1 5 to 30% w/w, based on the weight of the reaction mixture.

Preferably there is added to the reaction mixture an antioxidant. Suitable antioxidants are 2,6di-tert-butyl-4-methyl-phenol (BHT) or its dimer, hydroquinone methyl ether or pyrogallol. The amount of antioxidant added may suitably be > 0.005% w/w, preferably from 0.01 to 1.0% w/w of the reaction mixture. The addition of an antioxidant substantially overcomes loss in catalytic activity which can be caused by air ingress into the esterification reactor. In order to take the esterification reaction to completion the amount of alcohol added should be in excess of the stoichiometric amount required to react completely with the acid. The water of reaction may be removed as an azeotrope with excess alcohol, which may be separated and returned to the reaction. Up to a 50% stoichiometric excess, preferably from 10 to 30% stoichionetric excess may be employed.Alternatively or in addition the water of reaction may be removed overhead by the addition of an entrainer which may suitably be the hydrocarbon diluent.

Upon completion of the esterification reaction any unconverted alcohol present in the ester product may be removed by steam-stripping. Alcohol collected in this manner is preferably recycled to the reaction.

It is preferred to remove any residual acidity present in the ester product and to remove any esterification catalyst remaining therein. This may suitably be achieved by the addition of a strong inorganic base, such as sodium carbonate or lime, which may be added either in the form of a solid or as an aqueous solution. The tin catalyst present in the ester product may be precipitated by the base and may be removed by filtration. Tin catalyst removal may also be achieved by contacting with a suitable adsorbent prior to filtration. Procedures and conditions for carrying out this step are well-known in the art and typically involve the use of elevated temperature.

The steps of alcohol stripping and neutralisation/catalyst removal are preferably effected outside the reactor in which the esterification process is carried out because the conditions and/or reagents employed have a detrimental effect on the coated metal catalyst.

The invention will now be illustrated by reference to the following Examples. In the Examples the heterogeneous esterification catalysts used were prepared as follows.

Preparation of heterogeneous esterification catalyst A A mixture of phthalic anhydride (PA) (370g), 2-ehtylhexanol (780g), stannous oxalate (0.74g), butylated hydroxy toluene (0.37g) and beta-naphthol (0.37g) was added to a 1.51 cylindrical 845-Ti stainless steel reactor. The charge was heated to reflux at 190"C under vacuum with stirring and the reaction water produced was removed overhead from the reactor into a Dean and Stark adaptor. The progress of the reaction was monitored by determination of the changes in acidity of the reaction mixture and completion of the reaction was denoted by attainment of a 99.95% conversion of PA to ester, which was achieved after a reaction time of 9 hours Three further identical batch reactions were carried out during which the reaction time decreased from 9 hours to 6.5 hours.Thus the coating of the reactor surface was complete after a catalyst consumption of ca. 5g of tin metal ion equivalent per m2 of reactor surface.

Preparation of heterogeneous esterification catalyst B A mixture of phthalic anhydride (PA) (3709), 2-ethyl hexanol (780g), stannous oxalate (30g), butylated hydroxy toluene (0.37g) and beta-naphthol (0.379) was added to a 1.51 cylindrical 845-Ti stainless steel reactor lined with 316L stainless steel "KNITMESH" (two, 1 Ocm x 5cm diameter rolls of filament) in order to increase the area of the catalyst coating. The procedure used was identical to that used in the preparation of Catalyst A. A 99.95% conversion of PA to dioctyl phthalate was achieved in 6.25 hours. The reaction mixture was then heated at 1 90'C under stirring for a further 42 hours and at the end of this period the concentration of tin metal ion in the reaction mixture was reduced to 0.036% w/w.

The procedure used in the preparation of catalyst A was then repeated using the partially coated reactor lined with Knitmesh and a batch charge of 0.74g stannous oxalate. This gave a batch reaction time of 8.5 hours and a concentration of 0.014% w/w of tin metal ion in the reaction product which confirmed that the catalyst coating of the reaction system was incomplete. Six further batch esterifications were carried out using batch charges of 10 and 0.74g of stannous oxalate alternatively which consistently gave batch times of 5 hours and 7 hours respectively.

Preparation of heterogeneous esterification catalyst C The preparation of catalyst A was repeated using stannous formate in an amount sufficient to provide 390 ppm tin based on the crude dioctyl phthalate (DOP) product in place of stannous oxalate. The values obtained for the esterification batch time, the stannous formate remaining in the crude DOP product and the relative coating propensity of the stannous formate based on the initial and final concentrations are given in Table 1.

Preparation of heterogeneous esterification Catalyst D The preparation of catalyst C was repeated except that the stannous formate was replaced by stannous acetate.

Preparation of heterogeneous esterification Catalyst E The preparation of catalyst C was repeated except that the stannous formate was replaced by stannous phthalate.

Preparation of heterogeneous esterification Catalyst F The preparation of catalyst C was repeated except that the stannous formate was replace by stannous citrate.

Preparation of heterogeneous esterification catalyst G The preparation of catalyst C was repeated except that the stannous formate was replaced by stannous tartrate.

Preparation of heterogeneous esterification Catalyst H The preparation of catalyst C was repeated except that the stannous formate was replaced by sodium stannate.

TABLE 1 Catalyst CATALYST ESTERIFI- CATALYST Salt ADDITION CATION REMAINING IN COATING Used (ppm Sn on BATCH CRUDE DOP PROPENSITY OF crude rude DOP TIME PRODUCT CATALYST Catalyst Cation Anion product) (h) (ppm Sn) SALT A Sn" Oxylate 390 9 215 GOOD C Sn" Formate 390 6.5 351 POOR D Sn" Acetate 390 7.5 251 POOR E Sn" Phthalate 390 6.5 234 FAIR F Sn" Citrate 390 > 13 11 8 GOOD G Sn" Tartrate 390 8 203 GOOD H Na Stannate 390 > 13 8 VERY GOOD With reference to Table 1 the almost complete transfer of the stannous salt of a di- or polycarboxylic acid from solution in the esterification reactor contents on to the reactor walls during the coating experiment is indicative of a high catalyst coating propensity.Similarly the relatively high concentration of stannous formate remaining in the final DOP product indicated the poor coating propensity of this salt.

Example 1 The coated reactor, i.e. heterogeneous esterification catalyst A, was charged with a mixture of phthalic anhydride (PA) (370g), 2-ethylhexanol (7809), stannous formate (0.74g) as homogeneous esterification catalyst, butylated hydroxy toluene (0.37g) and beta-naphthol (0.37g). The charge was heated to reflux at 190"C under vacuum, with stirring and the reaction water produced was removed overhead from the reactor into a Dean and Stark adaptor. The progress of the reaction was monitored by determination of the change in acidity of the reaction mixture and completion of the reaction was denoted by attainment of a 99.95% conversion of PA to ester, which was achieved after a batch reaction time of 5 hours.

Example 2 Example 1 was repeated except that the batch charge of stannous formate was reduced from 0.749 to 0.449 (i.e. an approx. 40% reduction). The batch reaction time was 6 hours.

Comparison Test 1 Example 1 was repeated except that the batch charge of stannous formate (0.749) was replaced by stannous oxalate (0.74g). The batch time was 6.5 hours.

Comparison Test 2 Example 1 was repeated except that an uncoated reactor was used, i.e. no heterogeneous esterification catalyst was present. The batch time was 6 hours.

Example 3 Example 1 was repeated using Catalyst B, i.e. coated reactor and Knitmesh, as heterogeneous esterification catalyst in place of Catalyst A. The batch reaction time was 4 hours.

Example 4 Example 3 was repeated except that the batch charge of stannous formate was reduced from 0.749 to 0.15g. A batch reaction time of 6 hours resulted.

Comparative Test 3 Example 3 was repeated except thatthe batch charge of stannous formate (0.749) was replaced by stannous oxalate (0.74g). The batch reaction time was 7 hours. The results demonstrate that by replacing the stannous oxalate homogeneous esterification catalyst by stannous formate in the presence of a stannous oxalate coated metal heterogeneous esterification catalyst the batch reaction time can be significantly reduced, thus giving a potential expansion in reactor capacity. Alternatively replacement of stannous oxalate by a smaller amount of stannous formate can give a significant reduction in catalyst cost without any loss of reaction capacity.The results also demonstrate that it is possible to decrease catalyst costs by increasing the area of the catalyst coating obtained by use of stannous oxalate and by incorporating an addition of stannous formate in the batch esterification charge.

Example 3 Example 1 was repeated except that heterogeneous esterification catalyst A was replaced by heterogeneous esterification catalyst G. The values obtained for the catalyst remaining in the crude DOP product and the esterification batch time are given in Table 2.

Example 6 Example 5 was repeated except that heterogeneous esterification catalyst C was replaced by heterogeneous esterification catalyst F.

Example 7 Example 5 was repeated except that heterogeneous esterification catalyst C was replaced by heterogeneous esterification catalyst H.

TABLE 2 CATALYST SALT CHARGED WITH CATALYST CATALYST SALT ESTERIFICATION REMAINING USED TO COAT REAGENTS IN CRUDE ESTERIFICATION ESTERIFICATION DOP DOP PRODUCT BATCH TIME Example REACTOR NATURE ppm Sn (ppm Sn) (h) 1 Stannous Stannous Oxalate Formate 390 411 5 5 Stannous Stannous Tartrate Formate 390 323 6 6 Stannous Stannous Citrate Formate 390 354 8 7 Sodium Stannous Stannate Formate 390 311 > 12 The results in Table 2 demonstrates the increase in catalyst potency obtainable by combining stannous formate addition to a batch esterification charge with the use of a preformed tin catalyst coating on the esterification reactor walls. The most active catalyst combination is stannous formate and a reactor with stannous oxalate.

Claims (14)

1. A process for the production of an ester boiling above 1 0O'C which process comprises reacting at elevated temperature an alcohol with a carboxylic acid or a carboxylic acid an hydride in the presence of a combination of a heterogeneous esterification catalyst comprising a metal substrate coated with a stannous salt of a di- or polycarboxylic acid containing up to 20 carbon atoms and a homogeneous esterification catalyst comprising stannous formate.

2. A process according to claim 1 wherein the carboxylic acid is either a monobasic acid containing up to 20 carbon atoms, an aliphatic dibasic acid containing up to 20 carbon atoms, a tribasic aliphatic acid, a monobasic aromatic acid containing up to 20 carbon atoms, a dibasic aromatic acid or an anhydride thereof or a tribasic aromatic acid or an anhydride thereof.

3. A process according to either claim 1 or 2 wherein the carboxylic acid is either orthophthalic acid or the anhydride thereof, adipic acid, azelaic acid or sebacic acid.

4. A process according to any one of the preceding claims wherein the alcohol is either a monohydric alcohol containing up to 20 carbon atoms, a dihydric alcohol containing up to 20 carbon atoms, a trihydric alcohol, an aliphatic cyclic alcohol, or a derivative thereof.

5. A process according to claim 4 wherein the alcohol is a monohydric alcohol containing from 4 to 14 carbon atoms.

6. A process according to any one of the preceding claims wherein the metal substrate is either iron, titanium, aluminium, copper or nickel in filament or particulate form.

7. A process according to any one of the preceding claims wherein the metal substrate is the internal metal surface of an esterification reactor.

8. A process according to any one of the preceding claims wherein the stannous salt of a dior polycarboxylic acid containing up to 20 carbon atoms used to coat the metal substrate is either stannous oxalate, stannous phthalate, stannous citrate or stannous tartrate.

9. A process according to claim 8 wherein the stannous salt is stannous oxalate.

10. A process according to either claim 8 or claim 9 wherein the amount of the stannous salt employed to coat the metal substrate is in the range from 3 to 1009 per m2 metal substrate surface.

11. A process according to any one of the preceding claims wherein the amount of stannous formate employed as the homogeneous esterification catalyst is in the range from 0.01 to 1.0% w/w.

1 2. A process according to any one of the previous claims wherein the esterification reaction temperature is in the range from 1 80 to 225"C.

1 3. A process according to any one of the preceding claims wherein there is added an inert diluent.

14. A process according to any one of the preceding claims wherein there is added an antioxidant which is either 2,6-di-tert-butyl-4-methyl-phenol or its dimer, hydroquinone methyl ether or pyrogallol.

1 5. A process according to any one of the preceding claims wherein from 10 to 30% stoichiometric excess of the alcohol is employed.

1 6. A process for the production of an ester boiling above 100"C substantially as hereinbefore described by reference to Examples 1 to 7.

1 7. AEsters boiling above 100"C whenever produced by a process as claimed in claims 1 to 16.