WO1995020560A1

WO1995020560A1 - Process for the manufacture of benzaldehyde

Info

Publication number: WO1995020560A1
Application number: PCT/NL1994/000020
Authority: WO
Inventors: Hendricus Johannes Rozie; Maria Louisa Christina Dsinter; Jihad Baka Dakka; Amikam Zoran; Yoel Sasson
Original assignee: DSM NV
Current assignee: Koninklijke DSM NV
Priority date: 1994-01-27
Filing date: 1994-01-27
Publication date: 1995-08-03
Anticipated expiration: 1996-07-27
Also published as: EE9500004A; AU5892694A

Abstract

The invention relates to a process for the oxidation of toluene in a liquid phase using an oxygen containing gas at a temperature between 120-200 °C and a pressure between 2-50 atm in the presence of a catalyst comprising cobalt or manganese as a metal ion further comprising bromide, wherein the amount of metal ion is between 0.2 and 5 wt % with respect to the reaction mixture; the molar ratio of bromide: metal ion is between 0.05 and 4; and whereby the reaction mixture further comprises an organic onium compound in a molar ratio with respect to the metal ion of 0.1 to 4, which compound is able to solubilize the metal ion-bromide complex, whereby the oxidation is carried out in such a way that at least 15 wt % of the converted toluene is benzaldehyde by limitation of the conversion of toluene, whereafter the reaction mixture is subjected to a separation step to separate toluene and benzaldehyde, the toluene being recycled to the reaction mixture.

Description

PROCESS FOR THE MANUFACTURE OF BENZALDEHYDE

The invention relates to a process for the oxidation of toluene in a liquid phase using an oxygen containing gas at a temperature between 120-200°C and a pressure between 2-50 atm in the presence of a catalyst comprising cobalt or manganese as a metal ion, further comprising bromide.

Such a process is described in JP-A-50108231. According to JP-A-50108231, 4-15 vol.% water should be present in the reaction mixture in order to optimise the selectivity towards benzaldehyde and benzylalcohol. As is apparent from the examples, the selectivity to benzaldehyde is about 20%, at 20% conversion. Further, substantial amounts of benzoic acid (about 60-75%) are formed.

It is the object of the present invention to provide an improved process for the manufacture of benzaldehyde, comprising a method by which the selectivity towards benzaldehyde can be increased at an economically acceptable conversion rate.

The aim of the present invention is achieved by performing the process for the oxidation reaction in such a way that the amount of metal ion is between 0.2 and 5 wt% with respect to the reaction mixture; the molar ratio of bromide : metal ion is between 0.05 and 4 ; and that the reaction mixture further comprises an organic onium compound in a molar ratio with respect to the metal ion of 0.1 to 4, which compound is able to solubilize the metal ion-bromide complex, whereby the oxidation is carried out in such a way that at least 15 wt% of the converted toluene is benzaldehyde by limitation of the conversion of toluene, whereafter the reaction mixture is subjected to a separation step to separate toluene and benzaldehyde, the toluene being recycled to the reaction mixture. The process according to the present invention obviates the need for an aliphatic carboxylic acid as co- solvent. It is preferred that the reaction mixture is essentially free of aliphatic carboxylic acid because this diminishes corrosion and precludes the necessity of additional equipment for recycling said acid. It should be noted that small amounts of e.g. acetic acid and formic acid are present because of side reactions during oxidation. These small amounts to not have a significant effect on the oxidation process.

According to the prior art, many processes are described that apply an aliphatic carboxylic acid - mainly acetic acid - as co-solvent. For instance, US-A-4088823 describes the use of a mixture of acetic acid and acetic anhydride, thereby obtaining a large amount of the ester of benzylalcohol and acetic acid. This ester has to be hydrolyzed to obtain benzylalcohol. CH-A-645335 describes a liquid phase oxidation of chloro-substituted toluene at a temperature between 50-100°C in acetic acid with a cobalt catalyst and potassium bromide. JP-A-56108728 describes the use of a zinc compound in addition to cobalt, bromide and an aliphatic acid. The temperature used according the examples is in general 100°C or lower. Such a low temperature has the disadvantage that reaction products like benzoic acid are not very well soluble. This may lead to difficulties in processing the resulting reaction mixture. However, according to e.g. CH-A-643335 higher temperatures cannot be applied since these lead to much lower selectivities of benzaldehyde. Furthermore, US- A-3931330 describes the use of an aliphatic acid, in order to increase the selectivity to benzaldehyde.

According to US-A-3714263 it is possible to oxydize toluene in the absence of an aliphatic acid in case a very high temperature is applied (200-300°C). The use of an aromatic acid as a solvent is described in JP-A-53005132 and EP-A-071166. According to EP-A-071166, the process is carried out under constant removal of water preferably at a temperature of below 100°C if bromine is present, whereas according to JP-A- 53005132 manganese and cobalt should be used in combination in order to improve the selectivity to benzaldehyde.

However, none of the prior art processes provide a simple and economic process for the oxidation of toluene that yields benzaldehyde in relatively large amounts. In particular it is feasible with the present invention to obtain benzaldehyde in amounts of at least 20% selectivity and even with 25% selectivity, at up to 30% conversion.

Unexpectedly, the process of the present invention gives rise to very little diphenyl-byproducts. This is a great advantage, since the diphenyl-byproducts are rather useless since these are formed in such low amounts that it is too expensive to recover these.

Another advantage of the present process is that only a low amount of organic bromide compounds is formed during the process. Thereby, the process circumvents a part of the problems of corrosion.

An essential component according to the present invention is an organic onium compound that is able to solubilize the metal ion-bromide complex. As such, it is preferred that the compound comprises 4-60 carbon atoms. It is furthermore preferred that the onium compound is an ammonium, sulfonium, phosphonium, arsonium or stibium compound. Very suitable onium compounds are trialkyl, tetraalkyl or aromatic onium compounds.

Examples of suitable onium compounds are tetraethylammonium, tetraethylphosphonium, tetrabut lammoniurn, tetrabut lphosphonium, dimethyldidecylammonium, tetraphenylphosphonium, tetrapropylammonium, tetramethylammonium, trihexylammonium (i.e. protonated trihexylamine) , pyridinium (i.e. protonated pyridine), N-methyl-pyridinium, N,N- dimethylaniline, quinoline, dibenzylbutylsulphonium, diphenylbutylsulphonium, tetraphenylarsonium, triphenylbutylstibinium, and β-triethylammonium-(β '- triethylammonium)ethylpropionate.

The onium compound can be applied either as neutral compound, as in the case of pyridine, N,N- dimethylaniline or tri-hexylamine, or as onium salt of a cation such as for instance chloride, bromide, fluoride, acetate, propionate, benzoate, hydroxide or hydrogen sulphate. Furthermore, the onium salt can be prepared in situ by reaction of an alkylating agent with an amine or phosphine compound.

The use of e.g. tetraalkylammonium salts with 17 carbon atoms or more in the oxidation of toluene is known from EP-A-300921. However, EP-A-300921 describes the oxidation of toluene towards benzoic acid and describes that with the use of the onium salt a higher conversion and selectivity towards benzoic acid is achieved. This is apparent, in particular if cobalt or manganese are used as metal component of the catalyst. It is therefore unexpected, that the use of such an onium salt gives rise to a high selectivity to benzaldehyde at low conversion rates.

The catalyst for the oxidation of toluene with an oxygen containing gas comprises cobalt or manganese as a metal ion. Cobalt, manganese, or a mixture of these should be present in the reaction mixture in at least 0.2 wt.%.

Cobalt and/or manganese may be combined with other transition metals of groups IB, IIB, VB, VIB, VIIB, VIII of the periodic system, Ce and Th. As transition metal it is preferred to use for instance one or more of the metals selected from the group consisting of Cu, Cr , V, Pb, Fe, Ni , Ce, Th, Rh and Mo. In case an additional metal is used, the amount is in general higher than 10 ppm, and lower than 5 wt.% with respect to the reaction mixture.

In general, it is not critical in which form the metal ion is provided to the reaction mixture. The metal ion can be provided as salt of inorganic acids such as for instance chloride, bromide or sulphate, or as salt of an organic acid such as the acetate, octanoate, stearate or benzoate.

A further essential component is bromide. The bromide ion can be provided as salt of the metal ion, as part of the onium compound or as a separate component. In the latter case, bromide can be supplied e.g. as NaBr, KBr or HBr .

The amounts of the three components are important in order to achieve high selectivity to benzaldehyde.

In particular, the amount of cobalt and/or manganese should be higher than 0.2 wt.% (relative to the reaction mixture). The amount - in general - will be lower than 5 wt.% because a higher amount generally does not have a positive effect. Preferably, the amount will be higher than 0.4 wt.% and most preferably, the amount is between 0.4-2 wt.%.

The required amount of bromide is related to the amount of metal ion, and the molar ratio bromide : metal ion is generally at least 0.05. Preferably, the molar ratio of bromide relative to the metal ion is higher than 0.2. In general, the molar ratio bromide : metal ion is less than 4, preferably less than 1.5. High amounts of bromide tend to increase corrosion.

The amount of onium compound on a molar basis relative to the metal ion is - in general - higher than 0.1, preferably higher than 0.4. In general the amount on molar basis relative to the metal ion is less than 5, and preferably less than 3, since higher amounts do not contribute significantly to a higher selectivity.

The amount of onium compound on a molar basis relative to bromide in general is higher than 0.2, and lower than 20. Preferably the molar ratio onium compound : bromide is in between 0.5 : 10.

The oxygen containing gas generally contains between 3-22 vol.% oxygen, although pure oxygen gas may be used. Generally, air is used as the oxygen containing gas. It might be useful to apply gas with a relatively low amount of oxygen in order to decrease the risk of explosion. The process according to the invention can be performed batchwise, or in a continuous way. Preferable, the process is performed continuously. In the course of the process water is formed as a result of the oxidation reaction. A certain amount of water may be useful in order to solubilize the onium salt. However, the presence of water is not critical.

The temperature of the reaction is between 120°C and 200°. Preferably the reaction is performed at a temperature higher than 135°C in order to increase the reaction velocity. The temperature preferably is lower than 180°C in order to diminish side reactions.

The pressure at which the process is performed is in between 2 and 50 atm (0.2-5 MPa). Preferably (because of the temperature which causes a certain pressure) the pressure is higher than 3 atm. The pressure is preferably lower than 30 atm.

Although it is preferred to carry out the reaction without an additional solvent, it is possible to perform the process according the invention in the presence of an inert solvent such as for instance benzene or naphthalene.

The process according the invention can be carried out in a reaction vessel, equipped with a gas- inlet (sparger) and with an off-gas treatment section. Because of the heat evolving from the exothermic oxidation reaction, toluene may evaporate and can be cooled and recirculated, or cooled in a reflux-condensor or handled in another well known way.

The reaction is performed in such a way that a substantial part of the toluene remains unreacted, i.e. with a limited substrate conversion. In general, at least 30% (on molar basis) of the toluene remains unreacted, preferably at least 50% and most preferred at least 60% of the toluene remains unreacted. A lower conversion generally leads to a higher selectivity towards benzaldehyde. In order to achieve an economically attractive process, it is preferred to have a conversion of toluene of at least 10%, preferably of at least 15%. The man skilled in the art can easily ascertain the conversion which he considers as optimal.

In order to arrive at a process that is economically very attractive, the reaction is performed in such a way that in a continuous process the continuously withdrawn part of the reaction mixture contains at least 2.5 wt.% benzaldhyde, preferably at least 3 wt.% benzaldehyde and very preferably at least 4 wt.% benzaldehyde. In a batch process, the amount of benzaldehyde will be slightly higher.

The resultant reaction mixture, or a continuously withdrawn part of the reaction mixture containing mainly toluene, benzaldehyde, benzoic acid, benzyl benzoate, benzylalcohol and catalyst is distilled to separate toluene. The toluene is recycled to the reaction mixture. The benzaldehyde that may be further purified if necessary is obtained by distillation as well. The resulting residue consists mainly of benzoic acid, benzylalcohol, benzylbenzoate and the components of the catalyst. Benzoic acid can be separated, if desired, by destination or crystallisation. Benzylalcohol, together with a relatively small amount of benzylformiate and/or benzylacetate, can be separated and purified, or can be recirculated to the reaction mixture for instance together with the catalyst. It is also possible, to separate the catalyst by washing the reaction mixture or a resulting residue with water, at least in these cases where the onium compound is water soluble. The components of the catalyst preferably are used again in the oxidation reaction.

While the invention will now be described in connection with certain preferred embodiments in the following Examples, it will be understood that it is not intended to limit the invention only to these particular embodiments, it being understood that the particulars described are by way of example only.

Example I

Into a laboratory autoclave provided with a stirrer, the following reagents were introduced: - 211 g of toluene (2.25 moles);

- 8.1 g of MnCl₂.4H₂0 (41 mmoles; 1 wt.% metal), and

- 12.5 g of didecylammonium bromide (31 mmole).

The autoclave was heated at 160°C and air at a pressure of 6 atmospheres was introduced at a rate of 1.8 1/min (at standard conditions) for about 90 minutes.

After cooling, the reaction mixture (in percentage on molar basis) consisted of 63% toluene, 10% benzaldehyde, 16% benzoic acid, along with small amounts of benzyl alcohol and benzyl benzoate. After cooling, the reaction mixture was separated by distillation at atmospheric pressure. The first fraction was water followed by a fraction at 109-111°C which consisted of the excess of toluene. The toluene was used in further oxidation reactions. The following fraction at 178-179°C consisted of benzaldehyde.

The yield of pure benzaldehyde obtained by distillation was 24 g. After 20% conversion of toluene the selectivity to benzaldehyde was about 40 mol%.

Example II

In the same autoclave as in Example I, the following reagents were introduced:

- 211 g of toluene (2.25 moles);

- 15.9 g of cobalt chloride hexahydrate (67 mmoles; 1.6 wt.% metal), and

- 20.1 g of didecylammonium bromide (50 mmoles).

The autoclave was heated at 160°C and air at a pressure of 20 atmospheres was introduced at a rate of 2 1/min (at standard conditions) for about 80 minutes. At the end of the reaction, a sample was taken to a gas chromatograph and the composition found (on a molar basis without the catalyst) was as follows; 58% toluene; 13% benzaldehyde, 35% benzoic acid, and 4% of a mixture of benzylic alcohol and benzyl benzoate. The toluene was again used in an oxidation reaction. The benzaldehyde was separated by distillation as described in Example I and 33 g was obtained.

Based on analysis of several samples taken from this experiment, it was concluded that a maximum selectivity of 44% was achieved after 45 minutes, while the conversion of toluene was 25%.

Examples III-VI and Comparative Experiment A

Into a laboratory autoclave provided with a stirrer and a condensor , the reagents as shown in Table 1 were introduced. A small amount (0.5 wt.%) of p-methyl- benzaldehyde was added as initiator. The autoclave was heated and brought to a pressure as shown in Table 1. Air was blown into the reaction mixture at a rate of about 5 1/min. Generally, the air was diluted with nitrogen gas, in an amount of about 1 1 nitrogen relative to 3 1 air. The reaction time was 2-4 hr , and during the reaction samples were taken and analysed. Benzoic acid was analysed with HPLC, the other reaction products were analysed with GC.

The condensor was provided with a vessel in which water was separated. Therefore, the amount of water in the reaction mixture was lower than 0.1 wt.%. Example III and experiment A were also performed with a condensor without such vessel, which resulted in an increasing amount of water in the reaction mixture of up to 2 wt.% at 40% conversion of toluene. The presence of this amount did not have significant influence on the reaction rate nor on the selectivity. T A B L E

DDAB: didecyldimethylammoniumbromide

NaBr: sodiumbromide

TBAB: tetrabutylammoniumbromide

TBPB: tetrabutylphosphoniumbromide

DDOAB: dimethyldioctadecylammoniumbromide

The conversion and selectivity was measured at several times. The results are summarized in table 2.

T A B L E

^X) conversion of toluene on molar basis

²⁾ selectivity to products on molar basis

³⁾ not determined because the reaction was stopped at 23% conversion

The amount of diphenyl byproducts was at 30% conversion in Example III less than 0.04 wt.% of the reaction mixture. In contrast, the amount found in comparison experiment A was about 0.3 wt.% at 20% conversion. Examples VII-X and Comparative Experiment B

In a manner analogous to Example III, several experiments were performed.

The starting materials and reaction conditions are summarized in Table 3.

T A B E

¹ ammoniumbromide

² pyridiniumbromide

³ didecyldimethylammoniumbromide ⁴ tetraethylammoniumbromide (20) tetramethylammoniumbromide in example VII, Co-acetate tetrahydrate was used in example VIII, Mn-octoate was used The conversion and selectivity was measured several times. The results are summarized in Table 4.

T A B L E

¹ conversion of toluene on molar basis

² selectivity to products on molar basis

³ not determined because the reaction was stopped at slightly one 20% conversion

As appears from the results of Example VIII, the use of manganese as metal ion for the oxidation reaction has the advantage that the amount of benzylbenzoate formed is very low. Examples XI-XV

In a manner analogous to example III, several experiments were performed.

The starting materials and reaction conditions are summarized in Table 5.

T A B L E

- didecyldimethylammoniumbromide

² tetramethylammoniumbromide

³ tetrabutylammoniumacetate

⁴ The molar ratio onium/bromide = 5

The conversion and selectivity was measured several times. The results are summarized in Table 6.

A B L E

- conversion of toluene on molar basis

² selectivity to products on molar basis

With respect to Example XII, a quantitative analysis appeared not possible due to the high concentration of metal. The results indicated high amounts of benzaldehyde, as in the other examples.

Claims

C L I S

1. Process for the oxidation of toluene in a liquid phase using an oxygen containing gas at a temperature between 120-200°C and a pressure between 2-50 atm in the presence of a catalyst comprising cobalt or manganese as a metal ion, further comprising bromide, characterized in that the amount of metal ion is between 0.2 and 5 wt% with respect to the reaction mixture; the molar ratio of bromide : metal ion is between 0.05 and 4; and whereby the reaction mixture further comprises an organic onium compound in a molar ratio with respect to the metal ion of 0.1 to 4, which compound is able to solubelize the metal ion-bromide complex, whereby the oxidation is carried out in such a way that at least 15 wt% of the converted toluene is benzaldehyde by limitation of the conversion of toluene, whereafter the reaction mixture is subjected to a separation step to separate toluene and benzaldehyde, the toluene being recycled to the reaction mixture.

2. Process according to claim 1, characterized in that the reaction mixture is essentially free from aliphatic carboxylic acid.

3. Process according to any of claims 1-2, characterized in that the organic onium compound comprises 4-60 carbon atoms.

4. Process according to claim 3, characterized in that the onium compound is an ammonium, sulphonium or phosphonium, arsonium or stibium compound.

5. Process according to any one of claims 3-4, characterized in that the onium compound is a trialkyl or tetraalkyl onium compound.

6. Process according to any one of claims 1-5, characterized in that the catalyst further comprises a transition metal in an amount of 10 ppm to 5 wt.% with respect to the reaction mixture.

7. Process according to any one of claims 1-6, characterized in that the molar ratio of bromide : metal is between 0.2-1.5.

8. Process according to any one of claims 1-7, characterized in that the molar ratio of onium compound : metal is between 0.4-3.

9. Process according to any one of claims 1-8, characterized in that the molar ratio of onium compound : bromide is between 0.5 : 10.

10. Process according to any one of claims 1-9, characterized in that the temperature of the reaction is between 135-180°C.

11. Process according to any one of claims 1-10, characterized in that the reaction is performed in such a way, that the conversion of toluene is between 10-50% on molar basis.

12. Process according to any one of claims 1-11, characterized in that the reaction is performed in such a way, that the resultant reaction mass contains more than 3 wt.% benzaldehyde.

13. Process according to any one of claims 1-12, characterized in that benzaldehyde is obtained by distillation.

14. Process as substantially described in the description and the examples.