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Benzyl Ethanoate Synthesis Essay

Abstract


Synthetic methods of benzyl acetate using different catalysts such as strong acid cation exchange resins, inorganic salts (the weight ratio of FeCl3 to carbon (46 %)  and Fe2(SO4)3), inorganic acids (H2SO4)  heteropoly compounds ((NH4)6[MnMo9O32]∙8H2O), heteropolyacids (phosphotungstic acid), solid superacids (S2O82- - Fe2O3 - ZnO, S2O82- - Fe2O3 - CoO, S2O82- - ZrO2 - Al2O3, SO42- - Al2O3, SO42- - ZrO2 - Nd2O3 and SO42-- MoO3 - TiO2), organic salts (tris(trimethylsilylmethylene)tin chloride) and ionic liquids (N-methylpyrrolidone hydrosulfate and 1-methyl-3-(3-sulfopropyl)imidazolium tungstophosphate) have been reviewed. The above mentioned catalysts improved the yield of benzyl acetate. These methods have the advantage of simple process and low investment costs.((NH4)6[MnMo9O32]∙8H2O), heteropolyacids (PW12/C), solid superacids (S2O82- - Fe2O3 - ZnO, S2O82- - Fe2O3 - CoO, S2O82- - ZrO2 - Al2O3, SO42- - Al2O3, SO42- - ZrO2 - Nd2O3 and SO42-- MoO3 - TiO2), organic salts ((Ce3SiCH2)3SnCl) and ionic liquids ([Hmnp]HSO4 and [MMPS]3PW12O40) have been reviewed in the present paper. Yields of benzyl acetate are improved by the addition of above catalysts. These methods are having the advantages of simple process and low investment costs.


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DOI: http://dx.doi.org/10.17628/ecb.2013.2.272-274

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*********************** European Chemical Bulletin *******************

1. Field of the Invention

The present invention relates to a process for producing benzyl acetate and benzyl alcohol.

Benzyl acetate itself is useful as a solvent and a perfume. Benzyl alcohol which is derived from the benzyl acetate by hydrolysis or by transesterification with methanol is an important compound useful as a powerful solvent, a nontoxic medical additive, and an intermediate compound for agricultural chemicals and medicines.

2. Description of the Related Art

For industrial production of benzyl acetate, a process is known in which benzyl chloride produced by chlorination of toluene is hydrolyzed by alkali, and the resulting benzyl alcohol is esterified with acetic acid. This process comprises multistage reactions, and includes many steps of separation and purification after the respective reactions. Therefore, the process is complex and is not advantageous economically. Moreover, in the hydrolysis reaction of the second step, an alkali such as sodium hydroxide is required in an equivalent amount or more, and a large amount of a salt containing organic compounds is formed as the by-product, which involves problems in after-treatment thereof.

In a process not industrially conducted, benzyl acetate is produced by reaction of toluene, acetic acid, and oxygen in the presence of a catalyst for oxyacetoxylation. This process produces benzyl acetate in one step reaction without formation of a by-product salt, so that it is advantageous economically and can be of low environmental load.

Many techniques of benzyl acetate production have been disclosed, for example, in JP-B-42-13081, JP-A-52-151135, JP-A-52-151136, JP-B-50-28947, JP-B-52-16101, and JP-A-63-174950 (The term "JP-A" herein means an "unexamined published Japanese patent application", and the term "JP-B" herein means an "examined Japanese patent publication"). However, in these techniques, detailed studies are not made on industrial production process including separation and purification of benzyl acetate. Therefore, known techniques are not satisfactory for production of high-purity benzyl acetate.

On the other hand, benzyl alcohol can be produced by the known processes below. Of these, the processes (1) and (3) are practiced industrially:

(1) Hydrolysis of benzyl chloride by sodium hydroxide,

(2) Hydrolysis of benzyl acetate in the presence of a catalyst, and

(3) Reduction of benzaldehyde by hydrogen in the presence of a catalyst.

The above processes (1) and (2) both produces benzyl alcohol by hydrolysis. The process (1) consumes an equivalent amount or more of sodium hydroxide for stoichiometric reaction of benzyl chloride, involving a problem of after-treatment of a large amount of an aqueous solution of organic compound-containing sodium chloride formed as a by-product. The process (3) employs relatively expensive benzaldehyde as the starting material, and is disadvantageous economically.

The process (2) of hydrolysis of benzyl acetate to produce benzyl alcohol forms useful acetic acid as the by-product without discharging waste water, thus being economical and of low environmental load.

Regarding this hydrolysis process, a method is disclosed in which a mixture of water and benzyl acetate in a volume ratio of water/benzyl acetate of 25 is hydrolyzed at a temperature of 20°-30° C. in the presence of Amberlite IR-100, a sulfonic acid type cation-exchange resin (J. Chem. Soc., No.5, 1952, 1607). However, in this process, the catalyst activity is low, and an extremely large amount of water is required, so that the starting material concentration is lowered. Therefore, this process is not industrially advantageous in consideration of the energy for removal of unreacted water from the liquid reaction mixture. Further, the above disclosure does not specifically disclose the method for isolation and purification of the resulting benzyl alcohol.

In another process (Russian Patent: SU1077875), benzyl alcohol of high purity (98%) is obtained at an improved yield by hydrolysis of benzyl acetate in a flow system at a temperature of 90°-98° C. at water/benzyl acetate ratio of 3 (by weight) through a porous sulfonic acid type cation-exchange resin containing 2.2-4.0 m-equivalent/g of nitro group by replacing acetic acid with water during the reaction. However, this process, which requires use of a special nitro group-containing resin as the catalyst, is not practical as an industrial process in view of the catalyst cost.

As discussed above, the benzyl alcohol production process has not yet been investigated sufficiently for industrialization including the separation and purification steps, and no process is satisfactory as the process for producing high-purity benzyl alcohol.

An object of the present invention is to provide a process for economical production of benzyl acetate of high purity.

Another object of the present invention is to provide a process for economical production of benzyl alcohol of high purity.

As the result of comprehensive investigations to solve the above-described problems, the inventor of the present invention found that benzyl acetate of high purity can be produced economically in one step reaction of oxyacetoxylation by constructing a distillation purification process in consideration of the boiling points, the solubilities, and the azeotropy of the components including benzyl acetate as an oxyacetoxylation product, unreacted toluene and acetic acid, and recycling the process distillation fractions to the specified process, by use of a specific catalyst composed of an alloy of palladium and bismuth supported on silica exhibiting high selectivity and has a long life of the catalyst.

The inventors of the present invention found also that benzyl alcohol, which cannot readily be isolated and purified from the hydrolysis product of benzyl acetate can be produced economically by constructing an isolation-purification process in consideration of the azeotropy of the components including benzyl alcohol and acetic acid as hydrolysis products, unreacted benzyl acetate and water, and recycling the process fractions to the specified process, and by using, as the catalyst, a sulfonated styrene-divinylbenzene copolymer having divinylbenzene units in a specified content range, and by reacting the starting materials in a specific ratio.

The inventors of the present invention further found that benzyl alcohol can be produced economically by transesterification of benzyl acetate with methanol in the presence of a basic catalyst, and combining process fractions reasonably in consideration of the later isolation-purification process.

The present invention has been accomplished based on the above findings.

FIG. 1 is a flow sheet showing specifically an example of a process of production of benzyl acetate of the present invention.

FIG. 2 is a flow sheet showing specifically an example of a process of hydrolysis in the production process of benzyl alcohol of the present invention.

FIG. 3 is a flow sheet showing specifically an example of a process of hydrolysis utilizing liquid--liquid separation in the production process of benzyl alcohol of the present invention.

FIG. 4 is a flow sheet showing specifically an example of a process of azeotropic distillation employing water in the production process of benzyl alcohol of the present invention.

FIG. 5 is a flow sheet showing specifically an example of a process of extractive distillation employing glycerin in the production process of benzyl alcohol of the present invention.

FIG. 6 is a diagram showing compositions of a ternary component system composed of benzyl acetate, acetic acid, and water in the process of production of benzyl alcohol of the present invention.

FIG. 7 is a flow sheet showing specifically an example of a process of transesterification in the production process of benzyl alcohol of the present invention.

FIG. 8 is an XRD chart of the catalyst prepared in Example 2.

FIG. 9 is an XRD chart of the catalyst prepared in Comparative Example 1.

FIG. 10 is an XRD chart of the catalyst prepared in Comparative Example 2.

FIG. 11 is an XRD chart of the catalyst prepared in Comparative Example 3.

FIG. 12 is an XRD chart of the catalyst prepared in Comparative Example 4.

Firstly, the process for production of benzyl acetate of the present invention is described.

Toluene and acetic acid are employed as the starting materials in the process of the present invention.

The toluene and the acetic acid employed as the starting materials are not limited in the production processes thereof. For example, the toluene may be a product separated from petroleum distillate, or from cracked oil derived by cracking of petroleum distillate. The acetic acid may be a product obtained by oxidation of acetaldehyde, by oxidation of a hydrocarbon, or by synthesis from methanol and carbon monoxide.

In the process of the present invention, benzyl acetate is produced by oxyacetoxylation by use of toluene, acetic acid, and oxygen in a oxyacetoxylation reactor containing an oxyacetoxylation catalyst.

The oxyacetoxylation catalyst is not specially limited provided that it is capable of causing the intended oxyacetoxylation. For example, the catalysts are preferred which contain palladium having an oxyacetoxylation activity as the main component. Specifically, JP-B-42-13081 discloses a catalyst system comprising palladium supported by alumina and an alkali metal acetate. JP-A-52-151135 and JP-A-151136 disclose catalysts composed of a combination of one of bismuth, molybdenum, manganese, vanadium, and tungsten with palladium supported on silica. JP-B-50-28947 discloses a catalyst system comprising a catalyst composed of a combination of one of bismuth, cobalt, and iron with palladium supported on silica, and potassium acetate. JP-B-52-16101 discloses a catalyst system comprising a catalyst composed of palladium, bismuth, and chromium supported on silica, and an alkali metal acetate. JP-A-63-174950 discloses a catalyst system composed of palladium, and bismuth or lead supported on silica, and a bismuth compound or a lead compound soluble in the reaction system. In the present invention, any of the above catalysts can be used without difficulty. As the catalyst containing oxyacetoxylation-active palladium as a main component, another catalyst described later is useful which comprises an alloy composed of palladium and bismuth supported on silica at a palladium/bismuth ratio ranging from 2.5 to 3.5 (atomic ratio).

Such a catalyst is used as a fixed bed, a suspension catalyst bed and so on, in a reactor, a fixed bed is preferable. The amount of the catalyst depends on its activity, and is not generally specified. However, it corresponds preferably to the total feed of toluene and acetic acid per unit catalyst volume per unit time (LHSV) in the range of from 0.1 to 30 h-1 in consideration of production cost. The material of the reactor is not specially limited provided that it is sufficiently corrosion-resistant. For example, when stainless steel is used, SUS316 of JIS or higher corrosion-resistant grade of steel is preferred in consideration of the production cost.

For the oxyacetoxylation, a continuous fixed-bed flow reaction system is preferred in which the liquid starting materials of toluene and acetic acid and a prescribed concentration of oxygen are continuously fed in the reactor, and brought into contact with the fixed catalyst bed. The oxyacetoxylation reactor may be of a single-tubular type or a multi-tubular type, but is not limited thereto. The reaction proceeds by oxidation with generation of heat. The heat control method is not specially limited, and the reaction may be conducted with an adiabatic reaction system, with a multi-tubular reactor system with removal of the reaction heat, or with fractional material feed system.

The composition of the starting material liquid is generally in the range of from 0.1 to 10 moles of acetic acid per mole of toluene. However, when the catalyst employed comprises an alloy composed of palladium and bismuth supported on silica at a palladium/bismuth ratio ranging from 2.5 to 3.5 (atom ratio) as described later, the ratio of acetic acid is generally in the range of from 0.1 to 100, preferably from 0.2 to 40 moles per mole of toluene as mentioned later.

The oxidant employed in the present invention is molecular oxygen. The oxygen may be diluted with an inert gas such as nitrogen. In the practical process, air is preferably used.

The optimum feed rate of oxygen depends on the reaction conditions, the amount of the catalyst, and so forth, and is not generally limited. However, the feed should be controlled to give the total concentration of toluene, acetic acid and benzyl acetate outside the explosion range at least at the outlet portion of the oxyacetoxylation reactor. The feed rate of oxygen per unit catalyst volume per unit time (GHSV) is preferably not more than 5000 h-1 in terms of the volume at 0° C. at 1 atmosphere.

The reaction according to the present invention is conducted usually under heated and pressurized conditions. The reaction temperature is preferably in the range of from 80° to 230° C., more preferably from 120° to 200° C. At a higher temperature, side reactions are promoted without advantage, whereas at a lower temperature, the reaction rate is low disadvantageously. The pressure is applied to maintain a liquid phase on the surface of the catalyst, and is generally in the range of from 3 to 100 kg/cm2 G, preferably from 5 to 50 kg/cm2 G. The reaction time is preferably in the range of from 0.03 to 10 hours as the liquid residence time in the reactor.

The effluent from the oxyacetoxylation reactor is introduced to a gas-liquid separator to separate the liquid phase and the gas phase.

By selecting the method of discharging the effluent from the outlet of the oxyacetoxylation reactor, the gas and the liquid can be separated without an extra independent gas-liquid separator. This reactor is regarded as an integrated reactor-separator, and may be employed in the present invention.

The liquid phase from the gas-liquid separator is a liquid mixture mainly composed of toluene, acetic acid, and benzyl acetate, and is introduced to a starting-material recovery column. In the starting-material recovery column, the liquid is subjected to distillation to recover a liquid mixture distillate composed mainly of unreacted toluene and unreacted acetic acid from the column top, and to obtain a liquid mixture mainly composed of benzyl acetate, the objective product, from the column bottom. The column top distillate, which is recovered from the starting-material recovery column and composed mainly of toluene and acetic acid, is recycled to the oxyacetoxylation reactor.

The bottom liquid recovered from the starting-material recovery column, with or without adjustment of the temperature and pressure, is fed to a low-boiler removal column. In the low-boiler removal column, distillation is conducted to separate a column top liquid mainly composed of benzaldehyde and a bottom liquid mainly composed of benzyl acetate. The distillation conditions for the low-boiler removal column is not specially limited, provided that components such as benzaldehyde having a lower boiling point than benzyl acetate can be removed from the column top.

The bottom liquid mainly composed of benzyl acetate recovered from the low-boiler removal column is, with or without adjustment of the temperature and pressure, introduced to a high-boiler removal column. In the high-boiler removal column, distillation is conducted to obtain benzyl acetate of acceptable product purity from the top of the column, and components such as benzoic acid and benzyl benzoate having a higher boiling point than benzyl acetate is removed from the bottom of the high-boiler removal column. The distillation conditions for the high-boiler removal column is not specially limited, and is selected depending on the required purity of the produced benzyl acetate. For higher quality of benzyl acetate, an additional distillation column may be employed for further distillation and purification.

The embodiments of the present invention are described more specifically by reference to drawings. The present invention includes various types of embodiments, and is not limited to the embodiment shown by the drawings.

In FIG. 1, an oxyacetoxylation reactor 1 is fed with toluene, acetic acid, and air as the starting materials, a recycled liquid phase portion mainly composed of toluene and acetic acid introduced from a starting-material recovery column 3 and a water removal column 7, and a recycled gas phase portion mainly composed of oxygen and nitrogen introduced at least from a condenser 6.

The manner of feeding the starting materials is not specially limited provided that the starting material liquid covers the surface of the catalyst to conduct the reaction in a liquid phase. The starting materials may be fed either in a gas-liquid concurrent system or in a gas-liquid countercurrent system of whether up stream or down stream. The starting liquid materials and/or air may be fed fractionally to the oxyacetoxylation reactor 1.

The gas-liquid mixed phase effluent from the oxyacetoxylation reactor 1 is fed to a gas-liquid separator 2.

The gas phase portion discharged from the gas-liquid separator 2 is introduced to a condenser 6 to separate the liquid phase mainly composed of toluene, acetic acid, and benzyl acetate from the gas phase mainly composed of oxygen and nitrogen.

At least a part of the liquid phase portion separated by the condenser 6 is mixed with the liquid portion from the gas-liquid separator 2, and the mixture is introduced to the starting-material recovery column 3, and the remainder of the liquid phase portion is fed to the water removal column 7. At least a part of the gas phase portion separated by the condenser 6 is recycled to the oxyacetoxylation reactor 1, and the remainder of the gas phase is purged to the outside of the system. The amount of the recycling part of the gas phase portion introduced to the oxyacetoxylation reactor 1 is controlled to give a desired oxygen concentration in the mixture with the fresh air introduced to the reactor 1.

Various condensing apparatuses and operation methods therefor are known, and are applicable to the process of the present invention. The condensation may be conducted in plural times. For example, a condensate containing a larger amount of high-boiling benzaldehyde and benzyl acetate is separated by a first condensation; the separated condensate is fed to the starting-material recovery column 3; the remaining gas phase portion is subjected to a second condensation to remove a condensate composed mainly of toluene, acetic acid, and water; and the condensate is fed to the water removal column 7.

By this method, the load to the starting-material recovery column 3 is reduced; the recycling amount of benzyl acetate to the oxyacetoxylation reactor 1 is reduced to improve the reaction yield of the intended benzyl acetate; and further, the recycling amount of by-product benzaldehyde to the oxyacetoxylation reactor 1 is reduced, advantageously.

Most of the liquid phase portion introduced from the condenser 6 to the water removal column 7 is composed mainly of toluene, acetic acid, and water, and is subjected to distillation in the water removal column 7 to obtain a column top distillate mainly composed of toluene and water as the main distillate fraction. This column top distillate mainly composed of toluene and water is separated into two liquid phases in a settling vessel 8, and the upper phase mainly composed of toluene is recycled without treatment to the oxyacetoxylation reactor 1, and the lower phase mainly composed of water is eliminated. From the bottom of the water removal column 7, acetic acid is obtained, which is recycled without treatment to the oxyacetoxylation reactor 1.

The liquid phase portion from the gas-liquid separator 2 is mixed with a part or the whole of the liquid phase separated by the condenser 6, and the mixture is introduced to the starting-material recovery column 3. This liquid mixture contains a non-condensable gas component such as oxygen and nitrogen dissolved therein, so that the dissolved non-condensable gas is preferably eliminated prior to introduction to the starting-material recovery column 3 by a known apparatus and operation such as a flash evaporator and a vacuum deaerator. This method enables miniaturization of the starting-material recovery column 3, and makes unnecessary an extra device and operation for reducing the loss of useful component by discharge of the non-condensable gas.

The column top distillate of the starting-material recovery column 3, which is mainly composed of toluene and acetic acid is recycled without treatment to the oxyacetoxylation reactor 1.

In the process of the present invention, the recycled liquid phase portion mainly composed of toluene and acetic acid introduced to the oxyacetoxylation reactor 1 contains a small amount of water. A high concentration of water therein can adversely affect the activity and selectivity of the catalyst. Therefore, in order to prevent increase of the water concentration,the gas-liquid separation conditions in the gas-liquid separator 2, the condensation conditions in the condenser 6, and the distillation conditions in the starting-material recovery column 3 and the water removal column 7 are preferably controlled such that the water concentration is kept to be not more than 5% by weight in the entire liquid mixture of the toluene and the acetic acid supplied to the oxyacetoxylation reactor 1 and the recycled liquid phase portion mainly composed of toluene and acetic acid.

As described above, according to the present invention, benzyl acetate of high purity can be produced at a low cost through one step of oxyacetoxylation reaction of toluene, acetic acid, and oxygen and separation from the resulting mixture of the reaction products including benzyl acetate and water and unreacted materials including toluene and acetic acid.

The efficient combination of the gas-liquid separation operation and the condensation operation facilitates recovery and purification of the unreacted materials contained in the gas-liquid mixture effluent from the reactor, reduces the load to the starting-material recovery column, reduces the equipment cost, and saves energy, which is greatly advantageous economically and industrially.

The specific catalyst employed in the present invention is described below.

In the present invention, benzyl acetate is produced by reaction of toluene, acetic acid, and molecular oxygen in a liquid phase in the presence of a catalyst constituted of an alloy supported on silica, the alloy comprising palladium and bismuth in a palladium/bismuth ratio in the range of from 2.5 to 3.5 (atomic ratio).

The silica for the catalyst in the present invention is not specially limited, and may be any silica regardless of the raw material and production process therefor. However, the silica has preferably properties of the BET specific surface area of not less than 10 m2 /g, and the pore volume of not less than 0.2 cc/g.

The shape of the catalyst is not limited specially, and may be in a form of a powder, tablet or sphere shape. For a suspension catalyst bed, the catalyst is preferably powdery or granular, and for a fixed catalyst bed, the catalyst is preferably in a form of a molded article such as tablets, sphere shape and extrusion molded articles of a column shape.

The raw materials for palladium and bismuth for preparation of the catalyst employed in the present invention is not specially limited provided that the palladium and the bismuth can finally be alloyed.

Specifically, the raw material for the palladium includes metallic palladium, ammonium hexachloropalladate, potassium hexachloropalladate, ammonium tetrachloropalladate, potassium tetrachloropalladate, sodium tetrachloropalladate, potassium tetrabromopalladate, palladium oxide, palladium chloride, palladium bromide, palladium iodide, palladium nitrate, palladium sulfate, palladium acetate, potassium dinitrosulfitepalladate, chlorocarbonylpalladium, dinitrodiamminepalladium, tetraamminepalladium chloride, tetraamminepalladium nitrate, cis-dichlorodiamminepalladium, trans-dichlorodiarnminepalladium, dichloro(ethylenediamine)palladium, potassium tetracyanopalladate, and the like. The raw material for the bismuth includes metallic bismuth, bismuth chloride, bismuth nitrate, bismuth oxychloride, bismuth acetate, bismuth oxyacetate, bismuth oxide, and the like.

The amount of the supported metal of the catalyst is usually in the range of from 0.1 to 10% by weight based on the total catalyst weight including the supporting silica as an alloy consisting of palladium and bismuth in a palladium/bismuth ratio of from 2.5 to 3.5 (atomic ratio).

The palladium supported by silica should substantially be in a state of an alloy, although excess bismuth may exist in the catalyst without forming the alloy in the present invention.

The palladium substantially in a state of a palladium-bismuth alloy can be confirmed by measuring adsorption of carbon monoxide onto the catalyst, because the palladium-bismuth alloy does not adsorb carbon monoxide. Thus, palladium in a non-alloy state adsorbs carbon monoxide, and can be readily distinguished from the alloy.

The palladium-bismuth alloy of a palladium/bismuth ratio ranging from 2.5 to 3.5 (atomic ratio) shows a characteristic X ray diffraction pattern at 2 θ ranging from 30° to 80° as shown in Table 1.

This alloy will not change its X-ray diffraction pattern even when it is heated to 400° C. in a nitrogen atmosphere, or to 200° C. in the air. If the particle size of the alloy is too small for peak assignment in the X-ray diffraction pattern, the applicability of the catalyst to the process of the present invention can be confirmed by analysis by combination of high-resolution electron microscopy with electron beam diffraction or X-ray fluorescence.

In preparation of the catalyst constituted of the alloy supported on silica, the palladium-bismuth alloy of palladium/bismuth ratio of 2.5-3.5 (atomic ratio) of the present invention, the starting material for the palladium and the bismuth may be deposited by any known method in which palladium and bismuth form the alloy. Specifically the method includes precipitation, ion-exchange, immersion, deposition, blending, and so forth.

In catalyst preparation by immersion, the method of impregnation is not specially limited provided that the palladium and the bismuth finally form an alloy. A palladium material and a bismuth material may be impregnated simultaneously, or successively. For forming a more uniform alloy, the simultaneous impregnation is preferred. In a specific example of the simplest preparation method of the catalyst of the present invention, materials for the palladium and the bismuth are dissolved in a suitable solvent (the starting material salts for palladium and bismuth being preferably mixed in a palladium/bismuth ratio ranging from 2.5 to 3.5 (atomic ratio)); the solution is mixed with silica; the mixture is kept standing for a prescribed time as necessary, and is dried; and the resulting catalyst precursor is reduced in a hydrogen atmosphere or a hydrogen-containing inert gas atmosphere. Before the reduction treatment with hydrogen, the catalyst precursor may be calcined in an oxygen atmosphere.

The reduction treatment is conducted at a temperature ranging usually from 100° to 700° C., preferably from 200° to 500° C. The reducing agent for the reduction treatment includes gases such as hydrogen, carbon monoxide, and ethylene, alcohols, and hydrazine hydrate. The reduction treatment may be conducted in a gas phase or in a liquid phase.

The calcination before the reduction treatment, if necessary, is conducted in an oxygen atmosphere, in an atmosphere of oxygen diluted with nitrogen, helium, argon or the like, or in the air at a temperature ranging usually from 200° to 700° C.

The toluene and acetic acid used as the starting materials in the present invention are not limited in their production process. For example, the toluene may be a product separated from petroleum distillate, or from cracked oil derived by cracking of petroleum distillate. The acetic acid may be a product produced by oxidation of acetaldehyde, produced by oxidation of a hydrocarbon, separated from a by-product in peracetic acid, or synthesized from methanol and carbon monoxide. The mixing ratio of toluene to acetic acid is selected as desired in the range of from 0.1 to 100 moles, preferably from 0.2 to 40 moles per mole of toluene.

The reaction may be conducted in the present invention by dissolving a soluble bismuth compound in toluene and/or acetic acid. In this case, the amount of the soluble bismuth compound dissolved in toluene and/or acetic acid is preferably in the range shown the equation below: ##EQU1##

The catalyst employed in the present invention is stable in the reaction system. Therefore, the co-existence of the soluble bismuth compound is not essential. The amount of the co-existing soluble bismuth compound is sufficient in the range shown above. The soluble bismuth compound in an amount much larger than that shown above, may deposit in the process of purification of the produced benzyl acetate to clog pipe lines.

The soluble bismuth compound includes bismuth nitrate, bismuth oxide, bismuth oxyacetate, bismuth hydroxide, bismuth chloride, bismuth oxychioride, basic bismuth carbonate, bismuth acetate, bismuth oxalate, and trimethylbismuth, but is not limited thereto.

The react ion is conducted in a liquid phase in the present invention. The method of the reaction is not specially limited provided that the surface of the catalyst is covered with the starting material liquid. The reaction may be conducted by a batch process, a semi-batch process, a continuous process, or may be conducted with a reactor of fixed-bed flow or suspension system. The catalyst of the present invention may be applied to the aforementioned oxyacetoxylation process.

The amount of the catalyst depends on the method of the reaction, and is not defined generally. However, in consideration of the production cost, the amount of the catalyst in a fixed bed process is such that the total feed of toluene and acetic acid per unit volume of the catalyst per unit time (LHSV) in a fixed bed is in the range of preferably from 0.1 to 50 h-1, more preferably from 0.1 to 30 h-1, and in a suspension bed, the concentration of the catalyst is preferably in the range of from 0.1 to 30% by weight as the starting materials.

The oxygen partial pressure in the gas phase in the reactor is preferably controlled in the range of from 0.1 to 2 kg/cm2 for maintaining the catalyst life by securing activity and selectivity of the catalyst in the present invention from industrial standpoint. At the oxygen partial pressure of lower than 0.1 kg/cm2, the activity is not sufficient industrially, whereas at the partial pressure of higher than 2 kg/cm2, elution of palladium may be promoted to deteriorate remarkably the catalyst activity. The feed rate of oxygen is preferably in the range of from 0.5 to 4.5 moles per hour per one liter of catalyst. Incidentally, the reactor in the present invention means a vessel, a column, or a tube for synthesis of benzyl acetate by reaction of toluene, acetic acid, and oxygen as the starting materials, including specifically reaction vessels having a suspension bed, a catalyst-packed column or multi-tubular reactor having a fixed bed, etc.

The reaction according to the present invention is conducted usually in a heated and pressurized state. The reaction temperature is in the range of usually from 80° to 230° C., preferably from 120° to 200° C. At a higher reaction temperature than that range, side reactions are promoted without advantage, whereas at a lower temperature, reaction rate is lower disadvantageously. The reaction pressure is not limited specially provided that the liquid phase is maintained on the catalyst surface at the reaction temperature, and is in the range usually of from 3 to 100 kg/cm2 G, preferably from 4 to 50 kg/cm2 G. The reaction pressure higher than that is not necessary, since the intended reaction proceeds satisfactorily in the above pressure range.

The oxidizing agent in the process of the present invention is oxygen. The oxygen may be diluted with an inert gas such as nitrogen, or the oxidizing agent may be air. The optimum feed rate of the oxygen depends on the reaction temperature, the amount of the catalyst, and so forth, and the gas composition is controlled to be outside the explosion range at the outlet portion of the reacter. The feed rate of oxygen per unit catalyst amount per unit time (GHSV) is preferably not higher than 5000 h-1 in terms of the rate at 0° C., and 1 atmosphere.

The reaction time depends on the reaction temperature, the reaction pressure, the catalyst amount, and other conditions, and cannot be defined generally. In a batch or semi-batch system with a suspension catalyst bed, the reaction time is not shorter than 0.5 hour, and preferably in the range of from 1 to 10 hours. In a continuous system with a suspension catalyst bed, or in a flow system with a fixed catalyst bed, the residence time is generally in the range of from 0.03 to 10 hours.

As described above, according to the present invention, industrially useful benzyl acetate can be produced by liquid phase reaction of toluene, acetic acid, and molecular oxygen, by employing a catalyst comprising a specific composition of palladium and bismuth supported on silica, with a high catalyst activity, a high selectivity, and long life of the catalyst.

Furthermore, the benzyl acetate synthesis can be continued for a long term with retention of industrially satisfactory activity and selectivity by controlling the oxygen partial pressure in the gas phase to be within the specified range, and by controlling the feed rate of oxygen to the catalyst to be within the specified range.

Next, the process for producing benzyl alcohol by hydrolysis of benzyl acetate according to the present invention is described below. The process of the present invention includes various modifications, and is not limited to the examples of the embodiment shown by the drawings.

FIG. 2 shows an example a hydrolysis reaction process according to the present invention.

In this process of the present invention, benzyl alcohol is produced by hydrolysis of benzyl acetate by use of a reactor 9 packed with a solid-acid catalyst.

The solid-acid catalyst is not specially limited, and includes inorganic solid-acid catalysts such as silica-alumina, alumina, zirconia, titania, and silica; and organic solid acid-catalyst such as strong acidic cation exchange resin. Of these, preferred are sulfonated styrene-divinylbenzene copolymers as the strongly acidic cation exchange resin.

The sulfonated styrene-divinylbenzene copolymer as the solid-acid catalyst is not specially limited in the divinylbenzene unit content, and usually the content is not higher than 20% by weight. In consideration of the catalyst life and the reaction activity, the divinylbenzene unit content is preferably lower than 8% by weight.

The sulfonated styrene-divinylbenzene copolymer is not specially limited provided that the vinylbenzene unit content is in the aforementioned range, and commercial copolymers are useful. The structure of the copolymer may be either of a gel type or of an MR type (macroreticular type). The gel type copolymer includes simple gel type copolymers and macroporous (MP) copolymers, both being useful. The MR type copolymer is a porous copolymer, and is not limited in the surface area, the porosity, and the average pore diameter. The amount of the acid of the copolymer is not specially limited, but the copolymer has preferably a total ion exchange capacity in the range of from 3.0 to 6.5 m-equivalent/g based on the dry resin.

Benzyl acetate as the starting material is not specially limited, and may be the one produced by oxyacetoxylation reaction from toluene, acetic acid, and oxygen by use of a palladium type catalyst.

As the composition of the starting material, a higher content of benzyl acetate is undesirable since by-products such as dibenzyl ether is increased and the equilibrium conversion and the reaction rate are lower in the higher content of benzyl acetate. On the other hand, at a higher content of water in the liquid mixture of the starting materials, remarkably larger amount of water remains in the reaction mixture after the reaction, which requires a larger amount of energy (latent evaporation heat) for separation and recovery of benzyl acetate from the reaction mixture. Furthermore, at the higher content of water, since the strongly ion-exchange resin is extremely affinitive to water, the fed starting materials tends to form a heterogeneous phase to prevent the adsorption of benzyl acetate onto the catalyst, which lowers the reaction rate and is disadvantageous in industrial production. Therefore, the ratio of water/benzyl acetate is selected usually in the range of from 1/9 to 3/2, preferably from 1/9 to 2/3.

At a higher content of acetic acid in the starting material liquid mixture, the acetic acid content becomes higher to decrease the equilibrium conversion, and the decreased benzyl acetate content reduces the reaction rate, since the hydrolysis reaction of benzyl acetate is an equilibrium reaction and the acetic acid is one of the reaction products. Therefore, the content of acetic acid in the starting material liquid mixture is selected to be not higher than 30% by weight, preferably not higher than 20% by weight.

As mentioned above, the composition of the starting material mixture is important in the present invention. It is particularly preferred to conduct the reaction in a specified three-component system of benzyl acetate/acetic acid/water as shown later.

A useful type of the reactor is a fixed bed continuous reaction system in which the starting materials including benzyl acetate and water, and acetic acid as necessary, are continuously fed. However, the type of the reactor is not limited thereto, and any solid-liquid contact systems may be employed irrespectively of continuous systems or batch systems.

The hydrolysis of benzyl acetate is an equilibrium reaction shown by the reaction equation (1) below.

C.sub.6 H.sub.5 CH.sub.2 OCOCH.sub.3 +H.sub.2 O⃡C.sub.6 H.sub.5 CH.sub.2 OH+CH.sub.3 COOH (1)

Therefore, the conversion in a single step reaction is limited thermodynamically even if the reaction is conducted in the aforementioned composition range.

More specifically, the equilibrium constant K shown by the equation (2) is 0.40 at 80° C. according to the investigation by the inventors of the present invention. Therefore, with an equimolar material feed composition of benzyl acetate and water (for example, Benzyl acetate/Water/Acetic acid=89.3/10.7/0% by weight), the equilibrium conversion is as low as 38.7%. The conversion of benzyl acetate can be raised by removing at least part of the produced acetic acid in a multi-stage reaction system or reaction distillation system during the reaction.

K= BzOH! AcOH!/ BzOAc! H.sub.2 O! (2)

where BzOH! is the concentration of benzyl alcohol, AcOH! is the concentration of acetic acid, BzoAc! is the concentration of benzyl acetate, and H2 O! is the concentration of water.

The reaction temperature is selected in the range usually of from 40° to 150° C., preferably from 60° to 120° C. At the reaction temperature of lower than 40° C., the reaction rate is low, whereas at the reaction temperature of higher than 150° C., formation of benzyl ether as the by-product increases to lower the reaction selectivity, and may decompose or deteriorate the catalyst cation-exchange resin.

The reaction pressure is not specifically limited. The pressure may be applied to prevent boiling of the reaction liquid or to prevent significant evolution of bubbles caused by dissolved gas. Usually the pressure is selected in the range of from an atmospheric pressure to 10 kg/cm2 G.

The liquid space velocity (LHSV) is selected in the range of usually from 0.1 to 30 h-1, preferably from 0.2 to 6 h-1.

To the hydrolysis reactor 9, are introduced mixedly the benzyl acetate fed freshly as the starting material, recycled benzyl acetate from an alcohol concentration column 11, and a recycled aqueous acetic acid solution from an acetic acid-water separation column 12. The method of mixing is not specially limited. For example, the mixing is conducted in a mixing vessel provided before the reactor for sufficient agitation and mixing, or by an inactive packed bed provided at the upstream side to the catalyst layer in the reactor.

At the outlet of the hydrolysis reactor 9, the composition of the liquid corresponds preferably at least to 70% equilibrium conversion. At the composition of lower than 70% equilibrium conversion, a larger amount of energy is required for recovery of the unreacted benzyl acetate, which can be disadvantageous in industrial production.

The reaction mixture discharged from the hydrolysis reactor 9, which is composed of benzyl acetate, benzyl alcohol, water, and acetic acid, is introduced to an acetic acid-water recovery column 10. From the top of the acetic acid-water recovery column 10, acetic acid and water is recovered, and is introduced to an acetic acid-water separation column 12. Acetic acid is recovered from the bottom of the acetic acid-water separation column 12, and the fraction mainly composed of water is removed from the top thereof, and is recycled to the hydrolysis reactor 9.

In operation of the acetic acid-water recovery column 10, the concentration of acetic acid is very important since the acetic acid as an extracting agent destroys the azeotrope composed of water and benzyl alcohol to bring benzyl acetate and benzyl alcohol to the column bottom. The concentration of the acetic acid should be controlled to be not less than 10 mol % based on the water. At the acetic acid concentration of lower than 10 mol % relative to water, azeotropes composed of water and benzyl acetate, and of water and benzyl alcohol are formed in the acetic acid-water recovery column 10, thereby benzyl acetate and benzyl alcohol being distilled out from the column top, which may make the intended separation impossible. With the aforementioned material composition, no problem is caused in the operation of the column. However, when the reaction is conducted at a higher concentration of water or at a lower hydrolysis conversion ratio, the acetic acid concentration at the inlet of the acetic acid-water recovery column becomes lower, so that acetic acid should be replenished to increase the acetic acid concentration up to 10 mol % relative to water.

The benzyl alcohol and benzyl acetate discharged from the bottom of the acetic acid-water recovery column 10 is introduced to an alcohol concentration column 11 to obtain an azeotropic mixture composed of benzyl alcohol and benzyl acetate from the column top, and benzyl acetate from the column bottom. The benzyl acetate obtained from the column bottom is recycled to the hydrolysis reactor 9. Since the bottom liquid contains a small amount of high-boiling impurities including dibenzyl ether, a part of the bottom liquid is generally introduced to a heavy separation column 13 to remove high-boiling components mainly composed of dibenzyl ether. Thereby the concentration of impurities is decreased in the recycled benzyl acetate recycled to the hydrolysis reactor 9.

FIG. 3 shows an example of the hydrolysis reaction process utilizing liquid--liquid separation.

In this process, water is removed from the reaction liquid mixture by utilizing the low solubilities of benzyl alcohol and benzyl acetate in water. The reaction mixture discharged from the hydrolysis reactor 9 is introduced to a liquid--liquid separator 14 to separate it into an aqueous phase and an organic phase. The separated aqueous phase is recycled to the hydrolysis reactor 9, and the separated organic phase is introduced to the acetic acid-water recovery column 10. By this simple operation, water and a part of the acetic acid are removed, and the large energy consumption by the acetic acid-water recovery column is saved, whereby the production cost can be reduced.

The liquid--liquid separation is conducted in principle at a temperature where the liquid--liquid separation occurs. In consideration of partition coefficients of the respective components, separation time, and separation cost, the temperature is preferably in the range of from 10° to 60° C. In the case where the reaction liquid mixture contains an extremely large amount of water or acetic acid, and the liquid--liquid separation does not occur or takes a long time, the process of FIG. 2 in which liquid--liquid separation is not conducted is preferably employed for the production of benzyl alcohol.

In the hydrolysis process shown in FIG. 2 or FIG. 3, benzyl alcohol having a purity of 99% or higher can be obtained from the top of the alcohol concentration column 11 by controlling the operation conditions of the alcohol concentration column 11. Benzyl alcohol of a higher purity containing less benzyl acetate can be obtained by further conducting azeotropic distillation or extractive distillation.

FIG. 4 shows an example of the azeotropic distillation employing water.

The column top distillate of the alcohol concentration column 11 is introduced to a first alcohol purification column 15, in which benzyl acetate is separated from benzyl alcohol by utilizing water as the azetrope. The distillate from the column top is introduced to a liquid--liquid separator 17 to separate the two liquid phases. A part of the separated aqueous phase is returned to the first alcohol purification column 15 as the recycle. The organic phase which contains benzyl acetate, benzyl alcohol, and a small amount of water is recycled to the acetic acid-water recovery column 10 or the hydrolysis reactor 9.

The bottom liquid, which contains a small amount of water in addition to benzyl alcohol, of the first alcohol purification column 15 is introduced to a second alcohol purification column 16. In the second alcohol purification column 16, the water in the benzyl alcohol is completely removed from the column top by utilizing azeotrope of benzyl alcohol and water to obtain high purity benzyl alcohol from the column bottom. The column top distillate is recycled to the first alcohol purification column 15. Benzyl alcohol of higher quality can be obtained by distillation and purification by use of another distillation column.

In the above process, benzyl alcohol is purified by means of two purification columns: the first column for separation only of benzyl acetate, and the second column for separation of water. This process is advantageous in comparison with the purification by means of one purification column in points below.

(1) The operation temperature can be lowered, and the first column can be operated at an atmospheric pressure, which reduces the equipment cost.

(2) The lower operation temperature prevents deterioration of the product quality.

(3) A less expensive heating source (low pressure steam) can be used to reduce the energy cost.

(4) The operation of the distillation columns is stabilized more.

FIG. 5 shows an example of extractive distillation process employing glycerin.

The column top distillate of the alcohol concentration column 11 is introduced to an extractive distillation column 18. In the extractive distillation column 18, glycerin is used as the extracting agent, and the introduced distillation fraction is further separated into a column top distillate composed mainly of benzyl alcohol and benzyl acetate and a column bottom liquid composed of benzyl alcohol and glycerin.

To the distillate from the column top, water is added, and then the distillate is introduced to a liquid--liquid separator 20 to separate into two liquid phases. Of the two phases, the aqueous layer composed of water and glycerin is recycles to the extractive distillation column 18, and the organic phase is recycled to the hydrolysis reactor 9. Otherwise, the aqueous phase in the liquid--liquid separator 20 may be recycled, after removal of water, to the extractive distillation column 18. The organic phase in the liquid--liquid separator 20, which contains a small amount of glycerin, is recycled to the hydrolysis reactor 9, preferably after removal of glycerin.

The column bottom liquid containing benzyl alcohol and glycerin of the extractive distillation column 18 is introduced to a product column 19, and benzyl alcohol of high purity can be obtained from the top of the column. The column bottom liquid composed mainly of glycerin is recycled to the extractive distillation column 18. Benzyl alcohol of higher quality can be obtained by distillation and purification by use of another distillation column.

In the present invention, the aforementioned columns of the acetic acid-water recovery column 10, the alcohol concentration column 11, the acetic acid-water separation column 12, the heavy separation column 13, the first alcohol purification column 15, the second alcohol purification column 16, the extractive distillation column 18, and the product columns 19 are usual continuous multi-stage distillation columns such as packed columns and plate columns. The liquid--liquid separators 14, 17, 20 are usual settling vessels.

As described above, benzyl acetate and water, and benzyl alcohol and water which are not readily separable owing to the azeotropy can be separated readily by utilizing the acetic acid in the reaction liquid of the hydrolysis reaction as the extracting agent. Thereby, water and acetic acid can be completely separated from the reaction liquid mixture to obtain high-purity benzyl alcohol at remarkably reduced production cost.

The water separated by the acetic acid-water separation column is recycled to the hydrolysis reactor, and the acetic acid is useful as an industrial chemical. Therefore, the process does not discharge waste water and causes less environmental load.

A still higher purity of benzyl alcohol can be produced efficiently by concentrating the benzyl alcohol by the alcohol concentration column, and subsequently subjecting the concentrated benzyl alcohol to azeotropic distillation with water or extractive distillation with glycerin.

The preferred hydrolysis catalyst of the present invention is explained below.

In the hydrolysis reaction of benzyl acetate of the present invention, the preferred catalyst is a sulfonated styrene-vinylbenzene copolymer which is a strongly acidic cation exchange resin.

The sulfonated styrene-vinylbenzene copolymer should contain the divinylbenzene units at a content of less than 8% by weight. The copolymer of the divinylbenzene content of 8% by weight or higher is less reactive and does not achieve the object of the present invention. The content of the divinylbenzene units is preferably not lower than 1% by weight in consideration of the catalyst strength, and is preferably not higher than 7% by weight in consideration of the catalyst life.

The sulfonated styrene-divinylbenzene copolymer employed in the present invention is not specially limited provided that divinylbenzene unit content is in the aforementioned range, and may be a commercial product. The structure may be either of a gel type or a macroreticular type (MR type). The gel type copolymer includes simple gel type copolymers and macroporous (MP) copolymers, both being useful. The MR type copolymer is a porous copolymer, and is not limited in the surface area, the porosity, and the average pore diameter. The total ion exchange capacity is preferably in the range of from 3 to 6.5 m-equivalent/g based on the dry resin.

In the present invention, the hydrolysis reaction is conducted by a continuous fixed bed reaction system in which benzyl acetate and water as the starting materials are fed continuously to a reactor packed with a catalyst composed of the aforementioned sulfonated styrene-vinylbenzene copolymer. The starting materials may further contain, as desired, acetic acid in addition to the benzyl acetate and water to improve mixing of the starting materials. The water and the benzyl acetate are used in a water/benzyl acetate mole ratio ranging usually from 0.5 to 30, preferably from 1 to 15. At the mole ratio of less than 0.5, the conversion of benzyl acetate may be lower, whereas at the mole ratio of higher than 30, the reaction rate may be lower and be disadvantageous in industrial production. The concentration of acetic acid in the entire starting material is controlled to be in the range of usually from 0 to 30% by weight, preferably from 3 to 20% by weight. At the acetic acid concentration of higher than 30% by weight, the amount of acetic acid which is a reaction product is larger and the equilibrium deviates to the starting material side disadvantageously because the hydrolysis reaction of benzyl acetate is an equilibrium reaction.

The reaction temperature is controlled in the range of usually from 40° to 150° C., preferably from 60° to 120° C. At the reaction temperature of lower than 40° C., the reaction rate is lower, whereas at the reaction temperature of 150° C., the by-product dibenzyl ether increases to lower the selectivity, and further the catalyst cation exchange resin may be decomposed or deteriorated. The reaction pressure is not specially limited, and the reaction may be conducted under pressure, as desired. The liquid space velocity (LHSV) is controlled to be in the range of usually from 0.1 to 30 h-1, preferably from 0.2 to 6 h-1. The reaction mixture discharged from the reactor is subjected to after-treatment according to a conventional process or the aforementioned process to obtain benzyl alcohol.

As described above, benzyl alcohol can be produced efficiently and economically by use of the sulfonated styrene-divinylbenzene copolymer having divinylbenzene unit content in the specified range according to the present invention.

The starting material composition specified for the hydrolysis reaction in the present invention is explained below.

The composition of the starting material liquid is explained by reference to the composition diagram (FIG. 6) for the starting materials of ternary system of benzyl acetate/acetic acid/water. Table 2 shows the starting material compositions at the indicated points in FIG. 6.

The composition of the starting material in the present invention should be in the closed region surrounded by the line segments derived by bonding the points A, B, C, and D in FIG. 6. Specifically, when the material contains y% by weight of water, x% by weight of benzyl acetate, and z% by weight of acetic acid (where x+y+z=100), the compositions of the starting materials are in the ranges:

1/9≦y/x≦3/2, and 0≦z≦30

At the starting material composition having the acetic acid content higher than the line A-D in FIG. 6 (z>30), the produced acetic acid increases, thereby lowering the equilibrium conversion and lowering the benzyl acetate content to decrease the reaction rate.

At the starting material composition having the benzyl acetate content higher than the line A-B in FIG. 6 (y/x<1/9), by-products such as dibenzyl ether increase, and the equilibrium conversion and the reaction rate become lower undesirably.

At the starting material composition having the water content higher than the line C-D in FIG. 6 (3/2<y/x), a larger amount of water remains after the reaction, and a large amount of energy (latent evaporation heat) is required for separation and recovery of benzyl acetate from the reaction liquid mixture. Moreover, the strongly acidic ion exchange resin generally is extremely affinitive to water, and in the above region, where the supplied starting material becomes heterogeneous, the adsorption of benzyl acetate to the catalyst is prevented to lower the reaction rate disadvantageously in industrial production.

As described above, the starting material composition in the present invention is in the closed region surrounded by the lines bonding the points A, B, C, and D successively. For obtaining a higher purity of benzyl alcohol, the composition is preferably in the closed region surrounded by the lines bonding the points E, B, F, and G successively, namely in the range of 1/9≦y/x≦2/3, and 0≦z≦20.

A reactor useful in the present invention is a fixed-bed continuous reaction type reactor which is fed continuously with benzyl acetate and water as the starting materials, and acetic acid as necessary, and reacts them. However, the type of the reactor is not limited thereto, but may be any usual method for solid-liquid contact. For improving the conversion of benzyl acetate, a multi-stage reaction type, a reaction distillation type, may be employed in which at least a part of the formed acetic acid is removed during the reaction.

The reaction temperature in the present invention is controlled in the range of usually from 40° to 150° C., preferably from 60° to 120° C. At the reaction temperature of lower than 40° C., the reaction rate is lower, whereas at the reaction temperature of higher than 150° C., the by-product dibenzyl ether produced more to lower the selectivity, and may decompose or deteriorate the catalyst cation exchange resin. The reaction pressure is not specially limited, but is controlled to be at such a pressure that prevents boiling of the reaction liquid or remarkable bubble formation caused by a dissolved gas. The pressure is usually in the range of from an atmospheric pressure to 10 kg/cm2 G. The liquid space velocity (LHSV) is controlled to be in the range of usually from 0.1 to 30 h-1, preferably from 0.2 to 6 h-1.

The reaction mixture discharged form the reactor is subjected to after-treatment by a known process or the aforementioned process to obtain benzyl alcohol.

As described above, according to the present invention, benzyl alcohol can be produced efficiently and economically by hydrolysis reaction of benzyl acetate in the specified range of the starting material composition.

Next, the process for producing benzyl alcohol by transesterification of benzyl acetate with methanol according to the present invention is explained below.

In the method of the present invention, benzyl acetate is fed to the upper portion of a reaction-distillation column, and methanol is fed to the lower portion of the reaction-distillation column, and the benzyl acetate and the methanol are brought into contact countercurrently in the presence of a basic catalyst to cause transesterification to produce benzyl alcohol.

In the reaction, an impurity in the starting material may lower the reaction rate or cause clogging. For example, when sodium methylate is used as the basic catalyst, an acidic substance such as benzoic acid and acetic acid, or water in the starting materials may form sodium benzoate, or sodium acetate as shown by the reaction formulas below.

C.sub.6 H.sub.5 COOH+CH.sub.3 ONa→C.sub.6 H.sub.5 COONa+CH.sub.3 OH(3)CH.sub.3 COOCH.sub.3 +H.sub.2 O→CH.sub.3 COOH+CH.sub.3 OH(4)C.sub.6 H.sub.5 CH.sub.2 OCOCH.sub.3 +H.sub.2 O→CH.sub.3 COOH+C.sub.6 H.sub.5 CH.sub.2 OH (5)CH.sub.3 COOH+CH.sub.3 ONa→CH.sub.3 COONa+CH.sub.3 OH(6)

Such a sodium salt is nearly insoluble in the benzyl acetate as the starting material and methyl acetate as a by-product of transesterification, may deposit and scale around the benzyl acetate feed plate where the concentrations of benzyl acetate and methyl acetate are high to cause flooding trouble. Furthermore, since these reactions involve the catalyst (sodium methylate), the catalyst itself may be inactivated or consumed to decelerate the reaction. When sodium hydroxide is used as the catalyst, similar reactions occur as those with sodium methylate.

Therefore, the benzyl acetate and methanol as the starting material contain desirably less concentrations of acidic substances such as benzoic acid and acetic acid, and a less concentration of water. The allowable concentrations of such impurities depend on the mole ratio of methanol to benzyl acetate, catalyst concentration, and concentrations of the respective impurities, and is not limited specially. For example, the benzoic acid concentration in benzyl acetate is usually not higher than 0.05% by weight, preferably not higher than 0.03% by weight, still more preferably not higher than 0.02% by weight. The acetic acid concentration in benzyl acetate is usually not higher then 0.1% by weight, preferably 0.05% by weight, still more preferably not higher than 0.03% by weight. The water concentration in benzyl acetate is usually not higher then 0.05% by weight, preferably 0.03% by weight, still more preferably not higher than 0.02% by weight. The water concentration in methanol is usually not higher than 0.2% by weight, preferably 0.1% by weight, still more preferably not higher than 0.05% by weight.

In a preferred embodiment of the present invention, the benzyl acetate as the starting material is produced by the oxyacetoxylation described before. The methanol is recovered by hydrolysis of methyl acetate formed in the transesterification as the by-product, and distillation by a methanol recovery column. Methanol is also recovered from the bottom of the reaction distillation column by distillation through a methanol separation column. The both kinds of methanol is recycled to the reactor. Therefore, replenishment of methanol is only a small amount corresponding to the operation loss.

The basic catalyst in the present invention includes alkali metal hydroxides such as sodium hydroxide, and potassium hydroxide; and alcoholates such as sodium alcoholate, and potassium alcoholate. These basic catalysts are solid. Therefore the catalyst is fed in a state of a solution in alcohol.

The alcohol used in the transesterification reaction is not limited to methanol. However, methanol is the most suitable in consideration of the solubility of the catalyst, and the separation of the alcohol from water after the hydrolysis of acetate ester formed in the transesterification. Accordingly, as the alcohol for dissolving the catalyst, methanol is the most suitable. Otherwise, the catalyst may be used as a solution in benzyl alcohol.

The required amount of the catalyst depends on the kind of the catalyst, and cannot generally be specified. Although a larger amount of the catalyst is advantageous to raise the reaction rate, it may cause deposition of the salt as described before. The amount of the catalyst should be decided in consideration of the above matters. For example, sodium hydroxide as the catalyst is used in an amount of from 0.01 to 1% by weight, preferably from 0.02 to 0.2% by weight to the starting benzyl acetate. Sodium methylate as the catalyst is used in an amount corresponding to that of sodium hydroxide multiplied by the molecular weight ratio (54/40).

The reaction distillation column may be of a conventional type, such as a packed column and a plate column, and is not specially limited.

The catalyst is fed to the reaction distillation column preferably at the benzyl acetate feed position (feed plate) or a higher position (upper plate).

In the reaction distillation column, preferably, the zone of the column between the methanol feed position and the benzyl acetate feed position is employed as a reaction section; the zone above the benzyl acetate feed position is employed as an enriching section; and the zone below the methanol feed position is employed as a distillation-separation section called a stripping section. The enriching section serves to keep benzyl acetate, a starting material, and benzyl alcohol, the product below the column top. The stripping section serves to keep the produced methyl acetate above the column bottom. Benzyl acetate and benzyl alcohol, which have respectively a boiling point higher sufficiently than the distilling components, namely methyl acetate and methanol, can readily be separated from the distilling components. Therefore, not so many plates are required for the stripping section. The size of the reaction section is decided in consideration of the residence time of benzyl acetate as the starting material, and the theoretical plate number of distillation. The required residence time depends on the gas-liquid contact efficiency in the column, the catalyst concentration, the mole ratio of methanol to benzyl acetate, and the reaction temperature, and is not specially limited. A longer residence time, and a larger theoretical plate number do not retard the reaction, but are not economical. Accordingly, the residence time is preferably in the range of from 1 to 60 minutes, and the total theoretical plate number of the reaction-separation column is preferably in the range of from 3 to 50 plates.

Benzyl alcohol is relatively stable to heat. However, the benzyl alcohol is preferably not brought to high temperature in the presence of the catalyst or the sodium salt. Thus the column bottom temperature of the reaction distillation column is kept preferably at a temperature lower than the normal boiling point of benzyl alcohol by discharging a part of methanol from the bottom.

The operation pressure of the reaction-distillation column is not specially limited. However, in consideration of the vapor pressures of methyl acetate and methanol which are main components at the column top, operation at an atmospheric pressure is most reasonable and economical. The temperature of the respective portions of the column is a function of the operation pressure, the feed mole ratio of benzyl acetate and methanol, the catalyst concentration, the temperature or the composition of the column bottom, the plate numbers of the respective sections, and the reflux ratio.

The feed amount of methanol is equimolar to the starting benzyl acetate or larger. However, with excessively larger amount of methanol, a larger amount of methanol is delivered together with produced methyl acetate to the hydrolysis process, and the increase of the recycled methanol increases energy consumption. Therefore, the feed mole ratio of methanol to the starting benzyl acetate is in the range of usually from 1.1 to 10, preferably from 1.5 to 5.

In the present invention, the mixture obtained from the bottom of the reaction-distillation column and mainly composed of benzyl alcohol and methanol is introduced to a methanol separation column, and is subjected to distillation. The column top distillate mainly composed of methanol is recycled to the reaction-distillation column. In this distillation, when methanol contains a larger amount of water, the distillation is controlled preferably to bring the water into the benzyl alcohol fraction at the column bottom.

The methanol separation column may be of any conventional type.

The operation pressure of the methanol separation column is not specially limited. However, the column is operated preferably under a reduced pressure to keep the benzyl alcohol at or below the normal boiling point as mentioned above.

Benzyl alcohol and methanol can be separated with less number of the plates at lower reflux ratio because of the remarkably high relative volatility thereof. Therefore, as an effective methanol separation process, methanol may be distilled out without consideration of the water concentration of the distilled methanol; the water-containing methanol is dehumidified through a dehumidification column packed with a water adsorbent such as silica gel, zeolite, and active carbon; and then the dehumidified methanol is recycled to the reaction-distillation column.

The column bottom liquid mainly composed of benzyl alcohol of the methanol separation column is introduced to an alcohol purification column. Before introduction to the alcohol purification column, the column bottom liquid of the methanol separation column is preferably concentrated by a solid separator to remove, from the liquid by deposition, the solid matter such as sodium benzoate, sodium acetate, and catalyst dissolved in the liquid, and the evaporate is introduced to the alcohol purification column. In the alcohol purification column, low-boiling impurities including methanol, water, benzaldehyde, and the like are removed from the column top, the product benzyl alcohol is taken out from the middle portion of the column, and the column bottom liquid is recycled to the solid separator.

The solid separator and the alcohol purification column are operated under a reduced pressure in consideration of the thermal stability of benzyl alcohol. The type of the solid separator is not specially limited, and exemplified by an evaporator equipped with an inside scraper, and a thin film evaporator.

The alcohol separation column may be divided into two columns: an impurity separation column, and an alcohol purification column. In such a two-column alcohol separation system, impurities including methanol, water, and benzaldehyde are removed from the top of the impurity separation column, and the bottom liquid thereof is introduced to the alcohol purification column, where benzyl alcohol as the product is taken out from the top of alcohol purification column, and the bottom liquid is recycled to the solid separator.

The mixture coming out from the top of the reaction-distillation column is mainly composed of methyl acetate and methanol, and is introduced to the middle portion of an extractive distillation column. In the extractive distillation column, distillation is conducted with water fed to the upper portion of the column, and from the top of the column, a mixture composed mainly of methyl acetate and water is taken out. This column top distillate is preferably introduced together with the distillate of the acetic acid recovery column and water to a hydrolysis reactor for hydrolysis treatment. The bottom liquid composed mainly of water and methanol of the extractive distillation column is distilled by a methanol recovery column. The column top distillate of the methanol recovery column is recycled to the reaction-distillation column, and the water recovered from the column bottom is recycled to the extractive distillation column.

The hydrolyzed mixture leaving the hydrolysis reactor is introduced to a methyl acetate recovery column, and is distilled. The mixture composed mainly of unreacted methyl acetate and methanol coming off from the top of the methyl acetate recovery column is recycled to the extractive distillation column, and the mixture composed mainly of water and acetic acid obtained from the bottom of the methyl acetate recovery column is introduced to an acetic acid recovery column. The distillate mainly composed of water or aqueous acetic acid coming off from the column top of the acetic acid recovery column is recycled to the hydrolysis reactor.

The extractive distillation column, the methanol recovery column, the methyl acetate recovery column, and the acetic acid recovery column respectively are of any conventional type.

The ratio of the amount of water to be fed to the extractive distillation column relative to the amount of the mixture composed mainly of methyl acetate and methanol introduced form the reaction-distillation column is in the range of usually from 0.05 to 5 by weight, preferably from 0.1 to 1 by weight. With a less amount of water, the methanol concentration is higher in the distillate, and the methyl acetate concentration is higher in the column bottom liquid. With a larger amount of water, energy consumption increases uneconomically.

The water content of the methanol recovered from the top of the methanol recovery column is preferably as low as possible, and is usually not higher than 0.2% by weight, preferably not higher than 0.1% by weight, more preferably not higher than 0.05% by weight. The methanol concentration of the bottom liquid composed mainly of water of the methanol recovery column is not specially limited, but is desirably lower in order not to increase the amount of water introduced in the extractive distillation column and to decrease the recycling methanol.

The amount of water to be fed to the hydrolysis reactor depends on the amounts of the methyl acetate and methanol, and the hydrolysis ratio. The molar percentage of water to the methyl acetate is in the range of usually from 1 to 10, preferably from 1 to 5. The hydrolysis reaction catalyst include liquid acids such as sulfuric acid, phosphoric acid, and alkylsulfonic acid, and solid acids such as acidic cation exchange resin, silica, silica-alumina, and acid clay. Of these, acidic cation exchange resin is suitable in view of ease of handling after the reaction, and the apparatus construction materials.

The hydrolysis reactor may be any of stirring types, fixed bed types, and column types. For use of an acidic cation exchange resin as the catalyst, a fixed bed type reactor is suitable in which the catalyst is not damaged.

In another preferred embodiment, the acidic cation exchange resin molded into a shape causing less pressure drop or held in a container is incorporated in the distillation column to conduct the reaction and the distillation in one and the same column by reaction-distillation system, thereby the methyl acetate recovery column being omitted advantageously. The size of the apparatus depends on the required contact time. For a usual fixed bed type reactor, the contact time is in the range of usually from 0.03 to 10 hours, preferably from 0.1 to 5 hours. The reaction temperature is preferably in the range of from 50° to 120° C. in view of the reaction rate and heat resistance of the acidic cation exchange resin. The operation pressure of the reactor is not specially limited, but is usually in the range of from 1 to 5 atmosphere in terms of absolute pressure.

In the methyl acetate recovery column, the methyl acetate and methanol can be separated readily since the boiling point difference is sufficiently large between the distillate constituted mainly of methyl acetate and methanol and the column bottom liquid composed mainly of acetic acid and water. Therefore, the plate number and the reflux ratio are selected from the standpoint of economy. According to the present invention, the water distilled from the acetic acid recovery column is recycled to the hydrolysis reactor. This recycled water may contain acetic acid in a small amount in consideration of the process economy.

In the production of benzyl alcohol according to the present invention, the benzyl acetate is not limited in the production process thereof. For example, the benzyl acetate can be produced by oxyacetoxylation of toluene with acetic acid and oxygen continuously and efficiently by the process described before to produce high-purity benzyl acetate, and high-purity benzyl alcohol. In the process employing oxyacetoxylation, the acetic acid recovered from the bottom the acetic acid recovery column is recycled to the oxyacetoxylation reactor.

The embodiment of the transesterification process of the present invention is described below by reference to drawings. The embodiment includes various modifications, and the invention is not limited to the embodiment shown by the drawing.

FIG. 7 shows an example of the transesterification process of the present invention.

Benzyl acetate is fed to an upper portion of a reaction-distillation column 21, and methanol is fed to a lower portion of the same column 21, whereby the benzyl acetate and the methanol is brought into contact countercurrently in the presence of a basic catalyst to cause transesterification reaction.

Most portion of the methanol as the starting material is furnished by recycle of the methanol recovered from the column top of a methanol-separation column 22 of the later step and the methanol recovered from the column top of a methanol-recovery column 26 of the methyl acetate hydrolysis process, and small amount of methanol is replenished to make up an operation loss. The water concentration of the methanol is preferably controlled as described above.

The basic catalyst is fed to the same position as the benzyl acetate feed or a higher position than that. An enriching section comprising several plates is preferably provided above the benzyl acetate feed plate. By feeding the catalyst above the benzyl acetate feed plate, the reaction is caused also in the enriching section, which enables decrease of the enriching section plate number and of the reflux ratio corresponding to the reaction contribution in the enriching section.

The intended benzyl alcohol produced by the transesterification is discharged together with a part of excess methanol from the column of the reaction-distillation column 21, and is introduced to methanol separation column 22. The methanol separated by the distillation is discharged from the column top, and is recycled to the reaction-distillation column 21. When water is present in the methanol, the distillation conditions are preferably controlled to keep the water concentration in the methanol below the prescribed level.

The column bottom liquid of the methanol separation column 22 is introduced to a solid separator 23, where the liquid is concentrated to separate a solid. The solid separator 23 is exemplified by a thin film evaporator having a solid-scraper.

The vapor evaporated by the solid separator 23 is a mixture composed mainly of benzyl alcohol, and does not contain a solid component unless entrainment occurs. The vapor is introduced to an alcohol purification column 24. From the top of the purification column 24, low-boiling impurities are removed. High-purity benzyl alcohol is taken out from the middle portion of the alcohol purification column 24, and the bottom liquid of the column 24 is recycled to the solid separator 23. Naturally, the alcohol purification column 24 may be divided into two columns: a first distillation column for separating low-boiling separation and a second distillation column for taking out the product from the column top.

The mixture derived from the top of the reaction-distillation column 21 and composed mainly of methyl acetate and a part of excess methanol is introduced to the middle portion of an extractive distillation column 25 of the methyl acetate hydrolysis process. To the top of the extractive distillation column 25, the bottom liquid of a methanol recovery column 26 composed mainly of water is introduced to conduct extractive distillation of methanol with water. The mixture discharged from the bottom of the extractive distillation column 25 and composed mainly of water and methanol is introduced to the methanol recovery column 26. The mixture discharged from the bottom of the methanol recovery column 26 and composed mainly of water is recycled to the upper portion of the extractive distillation column 25. The distillate from the top of the methanol recovery column 26 and composed mainly of methanol is fed to the lower portion of the reaction-distillation column 21 together with the methanol recycled from methanol separation column 22 and the replenished methanol.

The column top liquid of the extractive distillation column 25 composed mainly of methyl acetate and water is fed together with the column top liquid of the acetic acid recovery column 29 mainly composed of water or water and acetic acid, and replenished water to the hydrolysis reactor 27. There, the methyl acetate is hydrolyzed. The effluent from the hydrolysis reactor 27 is introduced to a methyl acetate recovery column 28. By distillation in the methyl acetate recovery column 28, the mixture composed mainly of unhydrolyzed methyl acetate and methanol is taken out from the column top, and is recycled to the extractive distillation column 25. The bottom liquid of the methyl acetate recovery column 28 is introduced to an acetic acid recovery column 29.

As described above according to the present invention, high-purity benzyl alcohol can be produced stably and economically by transesterification of benzyl acetate with methanol.