PROCESS FOR PREPARING AN 1,2-ALKYLENE CARBONATE

The present invention relates to a process for preparing an 1,2-alkylene carbonate.

Processes for the production of 1,2-alkylene carbonates in which carbon dioxide is contacted with an 1,2-alkylene oxide in the presence of a suitable carbonation catalyst, are known. See e.g. WO-A 2005/003113. The insertion of carbon dioxide into the oxirane moiety of 1, 2™alkylene oxides is a reversible reaction. That is to say, 1,2-alkylene oxide may also be formed back from 1,2-alkylene carbonate under release of carbon dioxide.

Therefore, according to WO-A 2005/003113, in order to shift the equilibrium towards the desired 1,2-alkylene carbonate, the carbonation reaction is preferably performed under increased carbon dioxide pressure.

Besides providing for the desired surplus of the carbon dioxide reactant, operation at increased pressure also permits to conduct the reaction essentially in the liquid phase, as 1,2-alkylene oxide will largely remain liquid under the process conditions.

WO-A 2005/003113 suggests to conduct the carbonation process at a total pressure which is higher than atmospheric pressure, namely at a total pressure of from 5 to 200 x 10 ⁵ N/m ² {5 to 200 bar), the partial carbon dioxide pressure preferably being of from 5 to

70 x 10 ⁵ N/m ² , more preferably 7 to 50 x 10 ⁵ N/m ² , and most preferably 10 to 20 x 10 ⁵ N/m ² .

Before introducing carbon dioxide into the above- described high pressure carbonation process, as e.g. disclosed in WO-A 2005/003113, the carbon dioxide first

has to be compressed to the desired pressure. To achieve that, in general, a stream consisting of carbon dioxide gas is sent via pipelines to a compressor arranged upstream of the carbonation reactor. Said carbon dioxide gas may be sent under a relatively low overpressure (e.g. greater than 1 bar up to 4 bar) in order to transport the gas stream. In the compressor, the carbon dioxide gas is compressed to for example the above- mentioned partial carbon dioxide pressure of from 5 to 70 x 10 ⁵ N/m ² . The temperature at which compression takes place and the compressed gas is fed to the carbonation reactor, may be room temperature. The compressed gas may also be preheated before it is sent to the carbonation reactor. In the carbonation reactor, the carbon dioxide should be in the gaseous state.

A disadvantage of transporting gaseous carbon dioxide to a compressor situated on an 1,2-alkylene carbonate production plant, is that pipelines are needed to provide such transport. A further disadvantage of such transport of gaseous carbon dioxide, is that it is relatively inefficient because gaseous carbon dioxide takes up a lot of volume so that only a relatively small amount of carbon dioxide can be transported per unit of volume. The volume taken up by a specific amount of gaseous carbon dioxide at 20 ⁰ C is at least 4 times the volume taken up by the same amount of liquid carbon dioxide at the same temperature. Still a further disadvantage is that a lot of energy is required in order to compress the gas to the desired pressure. Finally, all of the foregoing also results in that in most cases, it is economically not feasible to set up an 1,2-alkylene carbonate production plant on a small scale.

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The objective of the present invention is to provide a process for preparing an 1,2-alkylene carbonate which does not have the above-mentioned disadvantages.

Surprisingly it was found that said objective can be achieved by using liquid carbon dioxide rather than gaseous carbon dioxide, as the source for the gaseous carbon dioxide that is needed in the carbonation reaction.

Accordingly, the present invention relates to a process for producing an 1,2-alkylene carbonate comprising

(i) transporting liquid carbon dioxide to an 1,2-alkylene carbonate production plant; (ii) feeding the liquid carbon dioxide to an evaporation vessel;

(iii) evaporating the liquid carbon dioxide such that a partial pressure of gaseous carbon dioxide of at least 5 x 10 ⁵ N/m ² is generated; (iv) feeding the gaseous carbon dioxide to a reactor; and (v) contacting the gaseous carbon dioxide with an 1,2- alkylene oxide and a carbonation catalyst to produce 1,2- alkylene carbonate.

An advantage of transporting liquid carbon dioxide to an 1,2-alkylene carbonate production plant, is that liquid carbon dioxide can also be transported in small quantities and by means of small tank-trucks. Pipelines are not needed.

A further advantage of such transport of liquid carbon dioxide, is that it is relatively efficient because liquid carbon dioxide takes up far less volume than gaseous carbon dioxide so that a relatively large amount of carbon dioxide can be transported per unit of volume .

Still a further advantage of the present invention is that no energy consuming compressor is needed in order to feed gaseous carbon dioxide under a certain desired pressure to the carbonation reactor. All that needs to be done, is to evaporate the liquid carbon dioxide such that a partial pressure of gaseous carbon dioxide of at least 5 x 10 ^s N/m ² is generated.

The use of liquid carbon dioxide in making ethylene carbonate from ethylene oxide and carbon dioxide is known. See e.g. US-A-2773811 and US-A-2667497 {granted in 1956 and 1954, respectively) . However, in the examples of said US patents, liquid carbon dioxide is fed directly to the carbonation reactor. No gaseous carbon dioxide is fed to the reactor. Such direct addition of liquid carbon dioxide leads to cooling of the reaction liquid that is contained in the reactor. This disadvantageously leads to the need of additional heating of the reactor to maintain the desired reaction temperature.

The liquid carbon dioxide to be used in the process of the present invention, is compressed or pressurized carbon dioxide. Further, the carbon dioxide for use in the present process can be either pure carbon dioxide or carbon dioxide containing further compounds.

The evaporation vessel containing the liquid carbon dioxide to be used in the present process, should be a vessel suitable for storing pressurized gas. Said vessel may have a cylindrical shape and may contain a top part and a bottom part which have a spherical shape. Alternatively, the whole of said vessel may have a spherical shape. Further, said evaporation vessel should be provided with means for reducing the pressure such that liquid carbon dioxide is evaporated such that a partial pressure of gaseous carbon dioxide of at least

5 x 10 ^s N/τn ² is generated. Said means may be a valve for reducing the pressure.

Preferably, the partial pressure of gaseous carbon dioxide to be fed to the reactor in step (iv) of the process of the present invention, is of from 5 to

70 x 10 ⁵ N/m ² , more preferably 7 to 50 x 10 ⁵ N/m ² , and most preferably 10 to 20 x 10 ⁵ N/m ² . The carbonation process in step (v) of the process of the present invention may be carried out at a total pressure which is higher than atmospheric pressure, namely at a total pressure of from to 5 to 200 x 10 ⁵ N/m ² , the partial carbon dioxide pressure preferably being of from 5 to 70 x 10 ⁵ N/m ² , more preferably 7 to 50 x 10 ^s N/m ² , and most preferably 10 to 20 x 10 ⁵ N/m ² . Preferably, the process of the present invention is carried out continuously.

The carbonation catalyst for use in step (v) of the process of the present invention generally will be a homogeneous catalyst, although a heterogeneous catalyst may also be used. Suitable homogeneous catalysts include alkali metal halides, alkaline earth metal halides and quaternary ammonium halides. A specific catalyst which is known to be suitable is a homogeneous phosphorus containing catalyst. The phosphorus will usually not be present in its elemental form in the catalyst. The carbonation catalyst may be a phosphonium compound. Such catalysts are known, e.g., from US-A 5,153,333, US-A 2,994,705, US-A 4,434,105, WO-A 99/57108, EP-A 776,890 and WO-A 2005/003113. Preferably, the catalyst is a phosphonium halide of formula (R) ₄ PHaI, in which Hal means halide and each R can be the same or different and can be selected from an alkyl, alkenyl, cyclic aliphatic or an aromatic group. Preferably, the

carbonation catalyst comprises a tetraalkylphosphonium bromide. The group R suitably contains from 1 to 12 carbon atoms. Good results are obtained with R being a Ci- ₈ alkyl group. Most preferred are groups R being selected from methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, and t-butyl groups. Preferably, the halide ion is bromide or iodide. A preferred phosphonium catalyst is tetra (n-butyl) phosphonium bromide. Another preferred phosphonium catalyst is tri (n-butyl) methyl phosphonium iodide.

Preferably, a solvent for the carbonation catalyst is used. This solvent may be an alcohol. Suitable alcohol solvents are 1,2-alkylene diols, in particular 1,2- ethanediol or 1, 2-propanediol . The use of ethanediol or propanediol has a further advantage when the l,2~alkylene carbonate is converted to 1,2-alkylene glycol (alkanediol) , and the 1,2-alkylene glycol is used as solvent for the catalyst.

The amount of catalyst in the reactor may conveniently be expressed in mole catalyst per mole 1,2- alkylene oxide. Due to a lower amount of by-products, the carbonation is suitably carried out in the presence of at least 0.0001 mole of the catalyst per mole 1,2-alkylene oxide. Preferably, the amount of catalyst present is such that it ranges from 0.0001 to 0.1 mole catalyst, more preferably from 0.001 to 0.05, and most preferably from 0.003 to 0.03 mole catalyst per mole 1,2-alkylene oxide.

The 1,2-alkylene oxide that is contacted with the gaseous carbon dioxide in step (v) of the present process, is suitably a C ₂ - _I alkylene oxide, preferably ethylene oxide and/or propylene oxide, or mixtures of such C ₂ - ₄ alkylene oxides. Where ethylene oxide is used, the produced 1,2-alkylene carbonate is ethylene

carbonate. Where propylene oxide is used, the produced 1, 2-alkylene carbonate is propylene carbonate.

The reaction temperature in step (v) of the present process can be selected from a wide range. Suitably the temperature is selected from 30 to 300 ⁰ C. The advantage of relatively high temperature is the increase in reaction rate. However, if the reaction temperature is too high, side reactions may occur, or the undesired decomposition of the catalyst may be accelerated. Therefore, the temperature is suitably selected from 900 to 220 ⁰ C, more preferably from 100 to 220 ⁰ C.

The skilled person will be able to adapt other reaction conditions as appropriate. The residence time of the 1, 2-alkylene oxide and the carbon dioxide in the reactor in step (v) of the present process can be selected without undue burden. The residence time can usually be varied between 5 min and 24 hours, preferably between 10 minutes and 10 hours. Conversion of 1,2- alkylene oxide is suitably at least 95%, more preferably at least 98%. Dependent on the temperature and pressure the residence time may be adapted. The catalyst concentration may also vary between wide ranges. Suitable concentrations include from 1 to 25 %wt, based on the total reaction mixture. Good results can be obtained with a catalyst concentration of 2 to 8 %wt, based on the total reaction mixture.

In step (v) of the present process, only one reactor may be used. However, it is also feasible to carry out the reaction of step (v) in two or more reactors. In such cases it may be advantageous to provide for the optimal amount of excess carbon dioxide in the reactors by removing or adding carbon dioxide between the reactors. In one embodiment of the invention, the reactors are

suitably conducted under plug flow conditions. It is even more preferred to have a back-mix reactor, e.g. a Continuously Stirred Tank Reactor (CSTR) , followed by a plug-flow reactor. Such a combination is known from e.g. US-A 4,314,945. In another embodiment of the invention, the reactors are one or more bubble column reactors as described in US 6,080,897.

The 1,2-alkylene carbonate may be recovered from the product mixture originating from step (v) or may be reacted further. The desired product 1,2-alkylene carbonate may be recovered from the product mixture originating from step (v) in the following way. First of all, carbon dioxide and light components may be separated from the crude reactor effluent from said step (v) in one or more gas-liquid separators to form a bottoms stream containing 1,2-alkylene carbonate and catalyst. Said light components are compounds, other than carbon dioxide, which have a boiling point which is 185 ⁰ C or lower, more specifically 180 ⁰ C or lower. Examples of such light components in the crude effluent from the carbonation reactor may be unreacted 1,2-alkylene oxide and any light contaminants formed during the carbonation reaction, such as acetone, propionaldehyde, allyl alcohol and acetaldehyde . The above-mentioned bottoms stream containing 1,2- alkylene carbonate and catalyst may be sent to a distillation column where it is distilled to form a first distillation overhead stream and a first distillation bottoms stream. The first distillation overhead stream contains 1,2-alkylene carbonate. The first distillation bottoms stream contains catalyst. The first distillation bottoms stream may be partially or completely recycled to the reactor.

In a situation where an alcohol is used as a solvent for the catalyst and such alcohol has a lower boiling point than the 1,2-alkylene carbonate, as is the case when the alcohol used is 1, 2-propanediol and the 1,2- alkylene carbonate is propylene carbonate or when the alcohol used is 1, 2-ethanediol and the 1,2-alkylene carbonate is ethylene carbonate, the first distillation overhead stream contains said alcohol in addition to 1,2- alkylene carbonate. As to the way the distillation may be performed in order to separate catalyst from 1,2-alkylene carbonate and any alcohol used as solvent for the catalyst, the skilled artisan can vary the temperature and number of trays without undue burden. The above-mentioned first distillation overhead stream may be distilled to form a second distillation overhead stream and a second distillation bottoms stream. The second distillation bottoms stream contains 1,2- alkylene carbonate, i.e. the purified end product. In a situation where an alcohol is used as a solvent for the catalyst and such alcohol has a lower boiling point than the 1,2-alkylene carbonate, the distillation of the first distillation overhead stream should be carried out such that the second distillation overhead stream contains said alcohol and the final 1,2-alkylene carbonate product contains no or substantially no alcohol .

As to the way the distillation of the first distillation overhead stream may be performed in order to separate 1,2-alkylene carbonate from any alcohol used as solvent for the catalyst, the skilled artisan can vary the temperature and number of trays without undue burden.

The 1, 2-alkylene carbonate that Is produced in the present process can suitably be used for the production of 1, 2-alkanediol . This is particularly preferred when the 1, 2-alkylene carbonate is ethylene carbonate and the 1, 2-alkanediol is monoethylene glycol. Accordingly, the process of the present invention preferably comprises the following further steps:

{vi) contacting the 1, 2-alkylene carbonate with water to obtain a reaction mixture containing a 1, 2-alkylene diol; and

(vii) recovering the 1, 2-alkylene diol.

Contacting the 1, 2-alkylene carbonate with water may be performed in the presence of a hydrolysis catalyst. Processes for preparing monoethylene glycol from ethylene oxide via ethylene carbonate are described in, for example, OS 6,080,897. Preferably the ethylene carbonate is supplied directly to the hydrolysis step without any intermediate purification of the ethylene carbonate.

The 1, 2-alkylene carbonate that is produced in the present process can alternatively be used for the production of 1, 2-alkanediol and dialkylcarbonate. Accordingly, the process of the present invention preferably comprises the following further steps: {viii) contacting the 1, 2-alkylene carbonate with an alkanol to obtain a reaction mixture containing an 1,2- alkylene diol and a dialkylcarbonate; and (ix) recovering 1, 2-alkylene diol and dialkylcarbonate.

The alkanol used in above transesterification step (viii) is suitably a Cχ_4 alcohol. Preferably, the alkanol is methanol, ethanol or isopropanol. Said step (viii) may be performed in the presence of a heterogeneous transesterification catalyst.

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The transesterification reaction in itself is known. In this context reference is made to US-A 4,691,041, disclosing a process for the manufacture of ethylene glycol and dimethyl carbonate by the transesterification reaction over a heterogeneous catalyst system, in particular an ion exchange resin with tertiary amine, quaternary ammonium, sulphonic acid and carboxylic acid functional groups, alkali and alkaline earth silicates impregnated into silica and ammonium exchanged zeolites. US-A 5,359,118 and US™A 5,231,212 disclose a continuous process for preparing dialkyl carbonates over a range of catalysts, including alkali metal compounds, in particular alkali metal hydroxides or alcoholates, such as sodium hydroxide or methanolate, thallium compounds, nitrogen-containing bases such as trialkyl amines, phosphines, stibines, arsenines, sulphur or selenium compounds and tin, titanium or zirconium salts. According to WO-A 2005/003113 the reaction of alkylene carbonate with an alkanol is conducted over heterogeneous catalysts, e.g. alumina.

The invention is further illustrated by the following Examples. Example 1

Liquid carbon dioxide (CO ₂ ) is transported by means of a truck to a propylene carbonate production plant, and is subsequently fed to an evaporation vessel.

The reactor for producing the propylene carbonate is an autoclave reactor equipped with a heating jacket and a gas inlet, and is stirred by means of a gas-dispersing propeller. Following placement of 5 g of the catalyst tetrabutyl phosphonium bromide and 5 g of monopropylene glycol into the reactor, 51.2 g of propylene oxide (PO) are added. The reactor is then sealed.

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The liquid CO ₂ in the evaporation vessel is evaporated by heating using a heat exchanger, such that a partial pressure of gaseous CO ₂ of 57 x 10 ⁵ N/m ² is generated. Gaseous CO ₂ at said pressure is fed from the evaporation vessel to the reactor generating a total reactor pressure of 20 x 10 ⁵ N/m ² . Then the reactor is heated to 180 ⁰ C under stirring.

At 180 ⁰ C, the total reactor pressure is maintained at 20 x 10 ⁵ N/m ² by feeding additional CO ₂ . After 4 hours at the above-described conditions, the reactor is cooled down rapidly and allowed to decompress. Example 2

Liquid carbon dioxide (CO ₂ ) is transported by means of a truck to a monoethylene glycol production plant, and is subsequently fed to an evaporation vessel.

Ethylene carbonate is produced by reacting ethylene oxide with carbon dioxide in two bubble column reactors. Ethylene oxide, carbon dioxide, water and a tributylmethyl phosphonium iodide catalyst are continuously fed to the first bubble column reactor. The liquid CO ₂ in the evaporation vessel is evaporated by heating using a heat exchanger, such that a partial pressure of gaseous CO ₂ of 22 x 10 ⁵ N/m ² is generated. Gaseous CO ₂ at said pressure is fed from the evaporation vessel to the first bubble column reactor via a sparger.

The gas-liquid stream from the top of the first bubble column reactor is fed to the bottom of the second bubble column reactor. The gas-liquid stream from the second bubble column reactor is fed to a gas-liquid separator. The liquid stream from the gas-liquid separator was fed to hydrolysis apparatus wherein the ethylene carbonate was hydrolysed to monoethylene glycol.