JPH0871429A - Esterification catalyst and production of ester - Google Patents

Abstract

PURPOSE: To obtain an esterification catalyst having high catalytic activity, capable of synthesizing ester in a high yield even when acid and alcohol as starting materials are used by nearly equimolar amts. and ensuring a high rate of reaction even at a low temp. and a very slight side reaction and to produce ester with the catalyst. CONSTITUTION: This esterification catalyst contains a compd. of a titanium family metal selected from among halides, nitrates, carboxylates, alcoholates and acetylacetone complexes as at least one of active components and ester is produced from carboxylic acid and alcohol with this catalyst.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing an ester from a carboxylic acid and an alcohol using a titanium group metal compound as a catalyst.

[0002]

2. Description of the Related Art An extremely wide variety of compounds useful in various fields belong to esters consisting of carboxylic acid and alcohol. Among the esters found in natural products, many such as fats and oils and fragrances are widely used as valuable resources. Initially, ester synthesis was used as a means for artificially producing natural products, and played a major role in the development of the chemical industry. However, in recent years, many synthetic compounds have been produced by applying ester synthesis reactions to synthesize novel substances that are not naturally present.

Although many methods for synthesizing an ester are known, the simplest, cheapest, and most versatile method is a dehydration reaction between a carboxylic acid or acid anhydride and an alcohol. It is a method for producing an ester. In this method, an acid catalyst such as sulfuric acid, p-toluenesulfonic acid or a metal oxide is generally used to accelerate the reaction. However, when a strong acid such as sulfuric acid or paratoluenesulfonic acid is used among these esterification catalysts, various side reactions occur, for example, dehydrating alcohol to monomolecularly produce olefin, and bimolecularly. Produces ether. Further, the sulfuric acid catalyst itself reacts with alcohol to form a sulfuric acid ester, which becomes an alkali salt of a sulfuric acid ester during neutralization washing in the ester purification step, which causes emulsification which makes separation of the ester layer and the aqueous layer difficult. In addition, it may cause coloration or gelation of the product, requiring a purification step of the crude ester after the reaction.

On the other hand, when a metal oxide, particularly a titanium group metal oxide, is used as a catalyst to react with a relatively weak acid, side reactions are suppressed but the catalytic activity is insufficient. Therefore, it is necessary to shift the equilibrium to the production system in order to increase the reaction rate. Therefore, it is necessary to charge either the acid or the alcohol as a raw material in an excess amount and to remove the unreacted raw material added in excess after the reaction is completed. However, it may be difficult or impossible to remove an excessive amount of unreacted matter, and in this case, a highly pure ester cannot be obtained. Even when the unreacted material can be removed, there are problems that the process requires a lot of time and energy, the yield is reduced, and the recovered unreacted material is not easily reused. . Further, it is known that compounds containing a titanium group metal have catalytic activity in various reactions. For example, titanium compounds are dehydration, hydration catalysts, oxidation or cracking catalysts, various other synthetic reaction catalysts and stereospecific compounds. It is widely used as a synthetic catalyst for organic polymers. Zirconium is also used as a catalyst for hydrocarbon decomposition, isomerization, alkylation reaction and olefin polymerization reaction.

The method of using a titanium group compound as an esterification catalyst is described in JP-A-52-22592 and JP-A-56-
27504, JP-A-61-207358,
JP-A-62-79849, JP-A-2-62849
Although many disclosures have been made, such as Japanese Patent Laid-Open No. 5-170698 and Japanese Unexamined Patent Publication No. 5-170698, there are problems in occurrence of side reactions, insufficient catalytic activity, and handling. For example, Japanese Patent Application Laid-Open No. 62-289236 discloses polytitanium polyol as a highly active catalyst, but preparation of the catalyst is difficult, and the reaction requires 25 mol% excess of alcohol and is not necessarily highly active. Japanese Patent Sho 56
No. 27504 discloses titanium alkoxides and zirconium alkoxides as esterification catalysts, but these produce colloids when the catalyst decomposes and are difficult to remove. JP-A-3-294243 discloses a zirconium clay adsorbent, but it is difficult to prepare a catalyst. JP-A-53-94296 discloses a titanium alkoxylate, but it is expensive and
It is difficult to decompose and remove. Japanese Patent Laid-Open No. 4-1156 discloses
Zirconium hydroxide is exemplified as a Group IV metal hydroxide, but it is judged that the catalyst is difficult to prepare, the acid / alcohol ratio is high, the reaction temperature is high, the time is long, and there is also coloring. It is not recognized as highly active.

[0006]

The present invention has a high catalytic activity and is capable of synthesizing an ester in a high yield even when the raw materials acid and alkali are used in approximately equimolar amounts, and also has a high reaction rate even at a low temperature. And an object of the present invention is to provide an excellent esterification catalyst that has extremely few side reactions.

[0007]

The present invention is based on the discovery that a specific compound containing a titanium group metal has an excellent activity as an esterification catalyst. That is, the present invention is an esterification catalyst containing a titanium group metal compound selected from the group consisting of titanium group metal halides, nitrates, carboxylates, alcoholates and acetylacetone type complexes as at least one of the active ingredients. Further, it is a method for producing an ester by esterifying a carboxylic acid and an alcohol using this esterification catalyst. The compound containing a titanium group metal of the present invention has significantly higher catalytic activity than various compounds including other titanium group metal-containing compounds which have hitherto been known as esterification catalysts.

Preferred examples of the halides of the titanium group metal in the present invention are MX ₄ , MX ₃ , MOX ₂ , MOX ₂ · M.
O ₂ , RMX ₃ and R ₂ MX ₂ [wherein M represents Ti, Zr or Hf, X represents a halogen atom, and R represents a hydrocarbon group]
Is. It is particularly preferable that X is chlorine. Xs in the same molecule do not necessarily have to be the same species. M is particularly preferably Zr. R is a saturated or unsaturated hydrocarbon group which may have a branch, and may have a substituent such as halide. It is preferably a hydrocarbon group having 1 to 40 carbon atoms, and more preferably 1 to 20 carbon atoms, and may be aliphatic, alicyclic, aromatic or aliaromatic. Specific examples of particularly preferable halides include ZrCl ₄ , ZrCl ₃ , TiOCl ₂ and T.
_{i (C 2 H 5) 2} Cl 2, ZrOCl 2, Zr (C 2 H 5) 2 C
L ₂ , Zr (C ₆ H ₅ ) ₂ Cl ₂ and the like. As the titanium group halides, those which are generally available as a catalyst for cationic polymerization or alkylation as a Lewis acid type catalyst may be used as they are.

The titanium group metal nitrates used in the present invention include MO (NO ₃ ) ₂ , ROM (NO ₃ ) ₃ , MR (NO ₃ ) ₃ ,
A compound represented by MR ₂ (NO ₃ ) ₂ [M and R have the same meanings as described above] may be used, and Ti or Zr, particularly Zr, is preferable.

Examples of carboxylates of titanium group metals include M
(R'COO) ₄ , O = M (R'COO) ₂ , R''OM (R'CO
O) ₃ , (R "O) ₂ M (R'COO) ₂ , R" M (R'CO
O) ₃ , O = M (R'COCHCOO) [wherein M is as defined above, R'represents a hydrogen atom or a hydrocarbon group], especially O = M (R'COO) ₂ Is preferred.
M is particularly preferably Zr. R'is a hydrogen atom or a saturated or unsaturated hydrocarbon group, which may have a substituent such as a halogen or a hydroxyl group. The carboxylic acid may also be a polycarboxylic acid such as fumaric acid, malonic acid, tartaric acid, phthalic acid, trimellitic acid and the like. Therefore, (R'COO) ₂ may be a dicarboxylic acid residue. Preferred R ′ has 1 to 40 carbon atoms, more preferably 1
~ 20 hydrocarbon groups, which are aliphatic, alicyclic, aromatic,
It may be any of aliphatic and aromatic compounds. Examples of suitable R'include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, Docosyl, tricosyl, tetracosyl, pentacosyl, isopropyl,
tert-butyl, 2-methylpropyl, 2,2'-dimethylpropyl, 3-methylbutyl, 2-ethylhexyl, vinyl, allyl, 2-butenyl, 6-methyl-4-heptenyl, cyclopentyl, cyclohexyl, methylcyclohexyl, ethylcyclohexyl , Cyclohexenyl,
Examples include 1,3-cyclohexadienyl, phenyl, tolyl, cumyl, benzyl, benzhydryl, phenethyl, trityl and naphthyl.

A method for preparing such a titanium group metal carboxylate catalyst is as follows: 1 mol of zirconium oxychloride, 2.1 mol of lauric acid, and 2 mol of pyridine are placed in a 1-liter four-necked flask. . Attach a condenser and stir at 100 ° C. for 1 hour under a stream of nitrogen. The salt formed is filtered off and dissolved in petroleum ether. Wash in a separatory funnel using methanol, sodium carbonate water, water in this order, and dry the oil layer with sodium sulfate. The desiccant is removed and the petroleum ether is removed under reduced pressure to obtain a viscous liquid with a yield of 90% by weight. For easy handling, it can be dissolved in mineral terpene to obtain a 50% by weight solution as a working solution. Other titanium group metal carboxylates can be prepared in a similar manner.

R is an alcoholate of a titanium group metal.
² M (OR ³ ) ₃ , R ² ₂ M (OR ³ ) ₂ and R ² ₃ M (OR ³ ) [wherein M is as defined above and R ² and R ³ are the same or different. And may represent a hydrogen atom or a hydrocarbon group]. The hydrocarbon group represented by R ² and R ³ is a saturated or unsaturated hydrocarbon group having 1 to 40 carbon atoms, preferably 1 to 20 carbon atoms, and has a substituent such as halogen and hydroxyl group. May be.

As the acetylacetone type complex of titanium group metal, O = M [-OC (R ⁴ ) = CHC (= O) R ⁵ ] ₂ , (O
R) ₂ M ^{[-OC (R 4) = CHC (} = O) R 5 ] _2, (R'COO)
₂ M [-OC (R ⁴ ) = CHC (= O) R ⁵ ] ₂ [wherein M is as defined above, R ⁴ and R ⁵ represent the same or different hydrocarbon groups] Can be mentioned. M is particularly preferably Zr. R ⁴ and R ⁵ are saturated or unsaturated hydrocarbon groups, which may have a substituent such as halogen and hydroxyl group. Preferred R ⁴ and R ⁵ are hydrocarbon groups having 1 to 40 carbon atoms, more preferably 1 to 20 carbon atoms, and may be any of aliphatic, alicyclic, aromatic and aliaromatic. Examples of suitable R'are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosyl. , Tricosyl, tetracosyl, pentacosyl, isopropyl, tert-butyl, 2-methylpropyl, 2,2'-dimethylpropyl, 3-methylbutyl, 2-ethylhexyl, vinyl, allyl, 2-butenyl, 6-methyl-4-heptenyl, Cyclopentyl, cyclohexyl, methylcyclohexyl, ethylcyclohexyl, cyclohexenyl, 1,3-cyclohexadienyl, phenyl, tolyl,
Cumyl, benzyl, benzhydryl, phenethyl, trityl, naphthyl.

The method for preparing such an acetylacetone type complex catalyst of titanium group metal is as follows: When sodium hydroxide and acetylacetone are mixed in water and stirring is continued, sodium acetylacetone precipitates. It is filtered off and dried. When 2 mol of the sodium acetylacetone thus obtained and 1 mol of TiOCl ₂ are stirred in toluene at 100 ° C., acetylacetone titanium and NaCl are produced. When NaCl is filtered off and toluene is removed, acetylacetone titanium is deposited.

The ester production using the catalyst of the present invention can be carried out in the same manner as in the conventional ester reaction.
For example, among the catalysts of the present invention, halides and nitrates are solid catalysts and are used by being dispersed as solid powder in the reaction system, and other catalysts such as carboxylic acid salts and complex compounds are used by being dissolved in the reaction system. To do. The amount of catalyst used is 1 ppm to 5,000 ppm, preferably 10 ppm to 1,000 ppm, based on the total weight of the carboxylic acid and alcohol, as the weight of the titanium group metal.

The reaction temperature is 50 ° C to 300 ° C, preferably 120 ° C to 220 ° C. Removal of generated water is indispensable to increase the reaction rate of esterification, but this can be effectively performed by a method such as performing the reaction under reduced pressure or using an azeotropic agent such as xylene. However, the bubbling of the nitrogen gas introduced to prevent the coloration of the system is sufficient for removal. The completion of the reaction is judged by measuring the acid value and the hydroxyl value in the reaction product. In the case of a solid catalyst, the crude ester (reaction refined product) after the reaction can be separated by a method such as filtration or centrifugation to obtain a high-purity, high-quality ester containing almost no unreacted substances and by-products. It can be obtained in yield. In the case of a homogeneous system in which the catalyst is dissolved in the produced ester, a solid adsorbent is added to the crude product to adsorb the catalyst, and then the target product can be separated by filtration or centrifugation.

As the adsorbent, activated carbon, synthetic zeolite,
Natural zeolite, silica gel, activated clay, activated alumina, bone charcoal, charcoal, bauxite, magnesia, frass earth, etc. can be used, but activated carbon is particularly preferable.

Examples of the carboxylic acid that can be used in the production of the ester of the present invention include aliphatic monocarboxylic acids such as caproic acid, capric acid, caprylic acid, lauric acid, myristic acid, oleic acid and stearic acid, maleic anhydride, Maleic acid, fumaric acid, adipic acid, sebacic acid,
Aromatic polycarboxylic acids such as aliphatic dicarboxylic acids such as azelaic acid, phthalic acid, phthalic anhydride, isophthalic acid, trimellitic acid, pyromellitic acid, trimellitic anhydride, or their anhydrides, and polyacrylic acid. Mention may be made of molecular carboxylic acids.

Examples of alcohols include butanol, hexanol, heptanol, octanol, 2
-Ethyl hexanol, decanol, dodecanol, stearyl alcohol and other aliphatic monohydric alcohols, benzyl alcohol and other aromatic monohydric alcohols, ethylene glycol, propylene glycol, neopentyl glycol, trimethylolpropane, trimethylolethane, pentaerythritol , Dipentaerythritol,
Polyhydric alcohols such as sorbitol and polyvinyl alcohol can be mentioned.

Regarding the charging ratio of the carboxylic acid and the alcohol in the ester synthesis, almost all the carboxylic acid is esterified in a short time if the alcohol is 1 equivalent or more per 1 equivalent of the carboxylic acid. The value is that the ester can be obtained in high yield even if the amount of alcohol is almost equivalent to that of the carboxylic acid. Therefore, a preferable charging ratio for the present invention is to use 1.0 to 1.1 equivalents of alcohol to 1 equivalent of carboxylic acid.

[0021]

The present invention will be described with reference to the following examples. Example 1 1 mol of capric acid and 1.03 mol of stearyl alcohol were placed in succession in a 1-liter 4-neck separable flask at room temperature, and then titanium tetrachloride (1st grade reagent manufactured by Ishizu Pharmaceutical Co., Ltd.) was added to carboxylic acid and alcohol. 2000 for the total amount of
An amount of ppm was added. In order to avoid moisture absorption of titanium tetrachloride, it is preferable to handle it in a dry box, and since hydrogen chloride gas is generated by the reaction, the reaction is preferably performed inside the draft. After the propeller-shaped stirring blade was attached through the lid, the lid was put on, and the reflux condenser, the mercury thermometer, and the nitrogen introduction tube were attached. The separable flask was immersed in an oil bath, nitrogen was bubbled through the nitrogen introduction tube, stirring was started, and the oil bath was heated to start the reaction. After the oil bath temperature reaches 190 ° C, set the temperature to 19
The reaction was continued for 3 hours while keeping the temperature constant at 0 ° C. The acid value at this point was 0.5. At this point, the separable flask was taken out of the oil bath and cooled at room temperature while continuing stirring and introducing nitrogen. (Please add about removal of reaction water). After cooling to room temperature, the solid catalyst was separated by filtering the contents of the separable flask through a glass filter. Put the filtrate in a separatory funnel, wash with dilute sodium carbonate water,
It was then washed with deionized water. The yield after removing water was 98%.

As a result of evaluating the product by subjecting the filtrate to infrared spectroscopy and NMR, it was confirmed that 99% of the product was caprylic acid stearyl ester and 1% was stearyl alcohol. The result of measuring the hue was G (Gardner) 1.

Example 2 1 mol of maleic anhydride, 2.05 mol of lauryl alcohol, and 1000 ppm of zirconium oxychloride (manufactured by Shin Nippon Metal Chemical Co., Ltd.) with respect to the total amount thereof were placed in a four-necked flask, and Example 1 was used. The reaction was performed in the same manner as in. 200 ° C
When reacted for 4 hours, the acid value was 0.3. Purification was performed by the same procedure to obtain the target ester with a yield of 99%. The product was colorless and transparent.

Example 3 1 mol of sebacic acid, 2.2 mol of octanol, and zirconium naphthenate prepared from zirconium oxychloride and naphthenic acid according to the method described above were added to 1000 ppm based on the total amount of sebacic acid and octanol. And were placed in a four-necked flask and reacted under reflux with flowing nitrogen gas at 190 ° C. for 3 hours, and the acid value was 0.5. At this point, reflux was discontinued and residual alcohol was removed under reduced pressure. Yield is almost 100% based on sebacic acid
Met. Add 50 g of activated carbon to this and add 3
The mixture was stirred for a long time, zirconium naphthenate was sufficiently adsorbed, and then filtered through a glass filter to obtain octyl sebacate ester having a purity of 99%. The product was almost colorless and transparent.

Example 4 Zirconium acetate was prepared from zirconium oxychloride and acetic acid according to the method described above. In a four-neck flask, 1 mol of 1-naphthoic acid, 1. myristyl alcohol.
Along with 02 mol, 5000 ppm of this zirconium acetate was charged with respect to the total amount of both, and the reaction was carried out at 190 ° C. The acid value reached 0.7 when the reaction proceeded for 5 hours, so the reaction was terminated. The hydroxyl value was 2.1.

Example 5 1 mol of linoleic acid, 1.02 mol of cetyl alcohol, and 2000 ppm of acetylacetotitanate (first-class reagent manufactured by Wako Pure Chemical Industries, Ltd.) with respect to the total amount of both were placed in a four-necked flask, The mixture was reacted at 190 ° C. for 3 hours while flowing nitrogen gas. The acid value after the reaction was 0.2. No color was seen in the product.

Example 6 Reaction was carried out in the same manner as in Example 1 using 1 mol of myristic acid, 1.3 mol of decanol and 1000 ppm of zirconium nitrate based on the total amount of both. 19
The acid value at 0.5 ° C. for 3 hours was 0.5. To remove the catalyst, 30 g of activated carbon was added, and the mixture was reacted at 130 ° C. for 1 hour. When activated metal was removed by filtration and the metal in the obtained ester was analyzed, zirconium was 1 ppm. The acid value was 0.3 and the hydroxyl value was 2.1.

Examples 7 to 15 In Examples 7 to 15, esters were synthesized using the catalyst of the present invention for various combinations of carboxylic acids and alcohols. Table 1 shows the amount of each drug used and the synthesis conditions. The conditions and methods other than those in Table 1 were the same as in Example 1. The acid value, hydroxyl value and residual metal concentration of the resulting ester are also listed in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

Comparative Example 1 In a four-necked flask, 1 mol of oleic acid, 1.1 mol of stearyl alcohol, and paratoluenesulfonic acid as a catalyst were used as raw materials, and the total amount of acid and alcohol was 0.5.
% And prepared. When reacted at 220 ° C. for 20 hours, the acid value was 2.5. It took 40 hours to further proceed the reaction and bring the acid value to 1.0 or less. Further, the obtained ester was colored astringent brown due to a side reaction by the catalyst.

Comparative Example 2 The reaction was carried out in the same manner as in Comparative Example 1 except that no catalyst was used. The acid value was still 2.0 after 40 hours.
The reaction was continued for another 10 hours, but the acid value no longer decreased, and the reaction stopped. In addition, the produced ester was colored brown because it was heated for a long time.

Comparative Example 3 1-naphthoic acid myristyl ester was synthesized in the same manner as in Example 4 except that cobalt naphthenate was used as the catalyst instead of zirconium acetate. As a result, the acid value after 5 hours was 3.8 and the catalytic activity was low. The reaction was continued for a further 5 hours, but the acid value dropped only to 3.4.

[0034]

As is clear from the above examples, according to the method for producing an ester using the esterification catalyst of the present invention,
The carboxylic acid and the alcohol can be used in a nearly equimolar ratio to produce the ester in a high yield in a relatively short time. That is, the effects of the present invention are: Shortening reaction time 2. The acid and alcohol charge ratios are close to equimolar, and the yield is high. Color reduction of the reaction product (ester) can be mentioned.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] March 29, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

The reaction temperature is 50 ° C to 300 ° C, preferably 120 ° C to 220 ° C. Removal of generated water is indispensable to increase the reaction rate of esterification, but this can be effectively performed by a method such as performing the reaction under reduced pressure or using an azeotropic agent such as xylene. However, the bubbling of the nitrogen gas introduced to prevent the coloration of the system is sufficient for removal. The completion of the reaction is judged by measuring the acid value and the hydroxyl value in the reaction product. In the case of a solid catalyst, the crude ester (reaction product) after the reaction can be separated by a method such as filtration or centrifugal separation to obtain a high-purity, high-quality ester containing almost no unreacted substances and by-products. It can be obtained in yield. In the case of a homogeneous system in which the catalyst is dissolved in the produced ester, a solid adsorbent is added to the crude product to adsorb the catalyst, and then the target product can be separated by filtration or centrifugation.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction content]

[0021]

The present invention will be described with reference to the following examples. Example 1 1 mol of capric acid and 1.03 mol of stearyl alcohol were placed in succession in a 1-liter 4-neck separable flask at room temperature, and then titanium tetrachloride (1st grade reagent manufactured by Ishizu Pharmaceutical Co., Ltd.) was added to carboxylic acid and alcohol. 2000 for the total amount of
An amount of ppm was added. In order to avoid moisture absorption of titanium tetrachloride, it is preferable to handle it in a dry box, and since hydrogen chloride gas is generated by the reaction, the reaction is preferably performed inside the draft. After the propeller-shaped stirring blade was attached through the lid, the lid was put on, and the reflux condenser, the mercury thermometer, and the nitrogen introduction tube were attached. The separable flask was immersed in an oil bath, nitrogen was bubbled through the nitrogen introduction tube, stirring was started, and the oil bath was heated to start the reaction. After the oil bath temperature reaches 190 ° C, set the temperature to 19
The reaction was continued for 3 hours while keeping the temperature constant at 0 ° C. The acid value at this point was 0.5. At this point, the separable flask was taken out of the oil bath and cooled at room temperature while continuing stirring and introducing nitrogen. After cooling to room temperature, the solid catalyst was separated by filtering the contents of the separable flask through a glass filter. The filtrate was placed in a separatory funnel and washed with dilute sodium carbonate water and then with deionized water. The yield after removing water was 98%.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C07C 69/44 69/533 69/587 69/602 69/76 A 9546-4H 69/80 A 9546 -4H 69/82 A 9546-4H // C07B 61/00 300

Claims (10)

[Claims] 1. A titanium group metal halide, nitrate,
An esterification catalyst containing a titanium group metal compound selected from the group consisting of carboxylates, alcoholates and acetylacetone-type complexes as at least one active ingredient. 2. A halide of a titanium group metal is MX ₄ , M.
X ₃ , MOX ₂ , MOX ₂ · MO ₂ , RMX ₃ and R ₂ MX
₂ The esterification catalyst according to claim 1, which is represented by the formula: wherein M represents Ti, Zr or Hf, X represents a halogen atom, and R represents a hydrocarbon group. 3. The titanium group metal nitrates are MO (N
O ₃ ) ₂ , ROM (NO ₃ ) ₃ , MR (NO ₃ ) ₃ , MR ₂ (NO ₃ ) ₂
The esterification catalyst according to claim 1, which is represented by the formula: wherein M and R are as defined above. 4. The carboxylic acid salt of a titanium group metal is M (R′C).
OO) ₄ , O = M (R'COO) ₂ , R''OM (R'COO) ₃ ,
(R ″ O) ₂ M (R′COO) ₂ , R ″ M (R′COO) ₃ , O =
M (R'COCHCOO) [wherein M is as defined above, R '
And R ″ may be the same or different and each represents a hydrogen atom or a hydrocarbon group], the esterification catalyst according to claim 1. 5. The titanium group metal alcoholate is R ² M.
(OR ³ ) ₃ , R ² ₂ M (OR ³ ) ₂ and R ² ₃ M (OR ³ )
The esterification catalyst according to claim 1, which is represented by the formula: wherein M is as defined above, R ² and R ³ may be the same or different and each represents a hydrogen atom or a hydrocarbon group. . 6. An acetylacetone-type complex of a titanium group metal has the formula O = M [-OC (R ⁴ ) ═CHC (═O) R ⁵ ] ₂ , (O
R) ₂ M ^{[-OC (R 4) = CHC (} = O) R 5 ] _2, (R'COO)
₂ M [-OC (R ⁴ ) = CHC (= O) R ⁵ ] ₂ [wherein M represents the same meaning as described above, R ⁴ and R ⁵ represent the same or different hydrocarbon groups] An esterification catalyst according to claim 1 represented. 7. The esterification catalyst according to claim 1, wherein the titanium group metal is selected from Ti, Zr and Hf. 8. A method for producing an ester, which comprises esterifying a carboxylic acid and an alcohol using the esterification catalyst according to claim 1. 9. The method for producing an ester according to claim 8, wherein the esterification catalyst is used in an amount of 5 × 10 ^{−5 to} 5 × 10 ⁻² mol with respect to 1 mol of the carboxylic acid. 10. The method for producing an ester according to claim 8, wherein 1.0 to 1.1 equivalents of alcohol is used with respect to 1 equivalent of carboxylic acid.