GB2280672A - Process for the production of alcohol carbonates - Google Patents

Abstract

A process for the production of a carbonate of an aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic or non-aromatic heterocyclic monohydric alcohol which comprises: (a) reacting urea and a vicinal diol to form the cyclic carbonate of the diol, and (b) treating the cyclic carbonate from stage (a) with a monohydric alcohol in the presence of a catalyst to form the desired carbonate.

Description

"PROCESS FOR THE PRODUCTION OF ALCOHOL CARBONATES" This invention relates to a process for the production of carbonates of aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic and non-aromatic heterocyclic monohydric alcohols.

Carbonates of aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic and non-aromatic heterocyclic monohydric alcohols, especially dialkyl carbonates and in particular dimethyl carbonate, have many uses. They are, for example, used as high performance lubricants, solvents for cellulose derivatives and as starting materials for the preparation of diaryl carbonates, aliphatic and aromatic polycarbonates and medicaments.

They are safe alkylators and are particularly useful in the synthesis of fine and speciality chemicals.

Dimethyl carbonate (DMC) may, for example, commonly replace methyl chloride or dimethyl sulphate as a methylating agent, for example, in the methylation of amines.

DMC is also replacing phosgene in the production of isocyanates from amines. Under appropriate reaction conditions DMC and an amine will react together to form a carbamate. Upon heating the carbamate decomposes to form the desired isocyanate.

Several processes for producing carbonates of monohydric alcohols and especially DMC on a commercial scale have been proposed. One process uses phosgene and methanol as starting materials and may therefore be rather hazardous.

It is also possible to react carbon monoxide, methanol and oxygen in the presence of cuprous compounds, for example, CuCl, to obtain DMC, as discussed in Ind.

Eng. Chem. Prod. Res. Dev. 1989, 19, 396. The reaction is, however, slow and the copper catalyst is corrosive.

Large glass-lined reactors are therefore required.

A third process, described in EP-A-501507, has as a first step the reaction of an alcohol, for example, methanol, with a nitrogen dioxide/nitrogen monoxide mixture and oxygen. The product of that reaction is then treated with carbon monoxide in the presence of a catalyst which results in the production of the carbonate of the alcohol and nitrogen monoxide (which is recycled).

When methanol is used as the alcohol then the product carbonate is DMC. A similar process is described in EP-A-538676.

Carbonates of alcohols may also be made by transesterification of the cyclic carbonate of a with an appropriate alcohol in the presence of a catalyst. The reaction is well known and many different catalyst systems have been disclosed.

US 3642858 describes a process in which a cyclic carbonate and a non-tertiary hydroxy-containing compound are reacted together in the presence of a catalytic amount of an alkali metal or a derivative thereof to form the carbonate of the hydroxy-containing compound.

US 4307032 discloses a process for preparing carbonates of alcohols by contacting a cyclic glycol carbonate with an alcohol at an elevated temperature in the presence of a thallium compound.

US 4661609 discloses the cosynthesis of ethylene glycol and dimethyl carbonate by reacting methanol and ethylene carbonate in the presence of a catalyst selected from zirconium, titanium and tin or compounds or complexes of those metals.

US 4734518 discloses the cosynthesis of ethylene glycol and dimethyl carbonate by reacting methanol and ethylene carbonate in the presence of a homogeneous catalyst. The catalyst is selected from soluble and miscible tertiary phosphines, arsines and stibines, and miscible bivalent sulphur and selenium compounds.

One of the starting materials for all of these transesterification reactions is the cyclic carbonate of a diol. These may be made, for example, by a process as described in EP-A-443758, which comprises reacting a vicinal glycol and urea, optionally in the presence of a tin catalyst. The desired cyclic carbonate results together with ammonia.

The present invention provides a process for the production of a carbonate of an aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic or non-aromatic heterocyclic monohydric alcohol which comprises: (a) reacting urea and a vicinal diol, advantageously one of the formula I,

in which R1, R2, R3 and R4 may be the same or different and independently represent hydrogen, or an aliphatic, aromatic, cycloaliphatic, araliphatic or arylcycloaliphatic group, or wherein R1 and R2 and the carbon atom to which they are attached and/or R3 and R4 and the carbon atom to which they are attached each form an aliphatic or araliphatic ring or R2 and R3 and the carbon atoms to which they are attached and/or R1 and R4 and the carbon atoms to which they are attached each form an aliphatic or araliphatic ring, to form the cyclic carbonate of the diol and (b) treating the cyclic carbonate from stage (a) with an aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic or non aromatic heterocyclic monohydric alcohol in the presence of a catalyst to form the desired dialkyl carbonate and a diol.

The above process obviates the need to purify and store the cyclic carbonate.

Ammonia is formed by the reaction in stage (a) of the process and this is normally and preferably removed by, for example, venting the ammonia in gaseous form to a collection means.

The diol which is a product of the transesterification reaction in stage (b) may be recycled to stage (a).

This represents one considerable advantage of the process of the present invention over the separate processes of the prior art. A preferred embodiment of the present invention comprises two further stages in which: (c) the carbonate of the monohydric alcohol, formed in stage (b), is separated from the diol, and (d) the diol is recycled to stage (a).

The reaction in stage (a) may be illustrated as follows:

and that in the transesterification stage (b) is as follows:

R5 is determined by the structure of the desired carbonate. Although the process of the present invention is suitable for making most carbonates of aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic and non-aromatic heterocyclic monohydric alcohols, it is most suitable for those with a boiling point sufficiently different from that of the starting diol and of the other reactants to allow separation by conventional distillation techniques.

Generally the method is most suitable for making carbonates of aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic and nonaromatic heterocyclic monohydric alcohols where the alcohol contains from 1 to 16 carbon atoms.

Accordingly, the preferred aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic and non-aromatic heterocyclic monohydric alcohols for use in the process of the present invention are those containing from 1 to 16 carbon atoms.

Alcohols which are suitable for use in the present invention include methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, n-octanol, cyclohexanol, ethyl cyclohexanol, n-dodecanol and isododecanol. Alcohols such as 2-methyl2-butanol, 2-butanol, 3-buten-l-ol, benzyl alcohol, 1,2,3, 4-tetrahydro-1-naphthol, furfuryl alcohol, and 3hydroxytetrahydrofuran are also amongst those that are suitable. The process of the present invention is most suitable for making carbonates of alkyl alcohols, preferably primary alkyl alcohols. It may, however, also be used to make carbonates of cycloalkyl alcohols, secondary or tertiary alcohols, allyl alcohol, nonaromatic heterocyclic alcohols and heterocyclic- or aromatic-substituted alkyl or allyl alcohols.Aromatic alcohols, for example, phenol, are not suitable as starting materials for the present invention.

R1 R2 R3 and R4 denote groups as listed above.

It is preferred that at least one of the four groups is not hydrogen.

Advantageously at least one of R1 and R2 and at least one of R3 and R4 represent other than hydrogen.

Especially good results are achieved with such diols.

Especially good results are also achieved with diols in which any two or three of the groups or all four of the groups represent groups other than hydrogen and these are also preferred for use in the present invention.

Accordingly, the present invention also provides a process for making cyclic carbonates of diols which uses diols as defined in this paragraph as starting materials, that process being optionally followed by reaction of the resulting cyclic carbonate with a monohydric alcohol.

When any of R1 R2 R3 and R4 represent radicals other than hydrogen, they are preferably chosen from alkyl or cycloalkyl groups having from 1 to 16 carbon atoms, alkoxy groups having from 1 to 16 carbon atoms and alkenyl groups having from 2 to 16 carbon atoms. It is preferred that the diol be free from acid and ester groups. It is preferred that at least one group is an alkyl group having from 1 to 16 carbon atoms.

Preferably at least one of R1 and R2 and at least one of R3 and R4 represent an alkyl group having from 1 to 16 carbon atoms.

To produce a particular desired carbonate of a monohydric alcohol, it is necessary to choose the appropriate alcohol to be used in the reaction of stage (b). It is not, however, necessary to choose any particular vicinal diol for use in stage (a) of the process. The vicinal diol may therefore be chosen to react under the most favourable reaction conditions, to give the lowest proportion of by-products and/or to make any separation stages as simple as possible, for example, by ensuring that the boiling point of the diol used is sufficiently different from the boiling point of the desired carbonate of a monohydric alcohol to allow easy separation using conventional distillation techniques.

The vicinal diols used in the present invention are advantageously defined by Formula I given above.

In the past 1,2-diols have been used to produce cyclic carbonates. A disadvantage of the use of 1,2diols is that they lead to the formation of ammonium carbamate and 4- and 5-alkyl-oxazolidinone-2. A further disadvantage is that the cyclic carbonates form azeotropes with their starting 1,2-diols making any separation step difficult. It has been found that using an internal vicinal diol avoids the formation of such byproducts. Internal diols are those in which neither hydroxy group is attached to a primary carbon atom. The reaction has also surprisingly been found to proceed faster as well as more cleanly with an internal vicinal diol than with a 1,2-diol. The cyclic carbonates formed may be solids and may therefore be easily separated from the liquid diols.

In some cases the cis form of a cyclic carbonate and the trans form of that cyclic carbonate may not have the same state at room temperature and pressure. For example the trans-isomer of the cyclic carbonate formed from 2,3-butanediol is a solid but the cis-isomer appears to be a liquid. In such a case it is preferred that the solid trans-isomer is formed.

The reaction with urea appears to be stereospecific, at least in the case where a tin catalyst is present. To obtain the trans-isomer of the cyclic carbonate of 2,3 butanediol it is necessary to use the DL form of 2,3butanediol as the starting diol. If the meso form of 2,3-butanediol is used then the cis-isomer results.

The present invention therefore further provides the use of an internal vicinal diol, preferably a 2,3vicinal diol, in a reaction with urea to form a solid cyclic carbonate. 2,3-Butanediol and 2,3-dimethylbutanediol-2,3 (pinacol) are two diols which are especially preferred for use in the present invention.

The present invention further provides the use of a cyclic carbonate, especially a solid cyclic carbonate, of a vicinal diol, preferably one formed from an internal vicinal diol in a reaction with urea, particularly a 2,3vicinal diol, in a reaction with an aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic or non-aromatic heterocyclic monohydric alcohol to form the carbonate of the monohydric alcohol.

The process of the present invention may be a batch or a continuous process. The rate of reaction and related economics determine which type of process is preferred. Stage (b) of the process is generally run as a continuous process.

The reaction of stage (a) is preferably carried out at an elevated temperature, typically in the range of from 100 to 2000C, preferably 150 to 1800C. Suitable pressures are in the range atmospheric to 3.5 MPa in the presence of a means of eliminating ammonia but it is preferred that reactions are run under atmospheric pressure. Urea and diol may be used in molar ratios in the range 1:1 to 5:1, preferably in a 1:1 mole ratio.

The reaction may be carried out under an inert atmosphere, for example, under a nitrogen atmosphere, if desired.

A typical batch reaction time is approximately 6 hours but as the progress of the reaction is normally monitored throughout by, for example, NMR and/or GC, the precise time required for the completion of the reaction is determined by the results of the analysis rather than on a calculated time span.

The reaction of stage (b) is carried out at an elevated temperature. A temperature in the range of from 100 to 200"C is preferred, and especially a temperature in the range of from 130 to 1900C. The reaction may be carried out under vacuum, at atmospheric pressure or under an elevated pressure. The choice of pressure will depend on the type of reaction vessel and on the carbonate being formed. It may be necessary to use an elevated pressure to allow light alcohols, typically those having less than seven carbon atoms, for example, methanol to be heated to the desired reaction temperature, when, for example, that temperature is higher than the normal boiling point of the alcohol. Suitable pressures are generally within the range of from 10 KPa to 2500 KPa. Typically the reaction is carried out as a distillation.

The alcohol and cyclic carbonate are preferably used in a molar ratio of alcohol:cyclic carbonate of greater than 2:1. Molar ratios of from 2:1 up to 10:1 are preferred.

The reaction may be carried out under an inert atmosphere, for example, nitrogen, if desired.

The course of the reaction is generally followed by NMR or GC. Typical reaction times are in the range 5 to 15 hours. The reaction is advantageously run as a continuous process.

Stages (a) and (b) of the process may be carried out in solution but it is not necessary to do so. If solvents are used then different solvents may be used for stage (a) and stage (b) but it is preferred that, if a solvent is used, the same solvent is used in both stages. Suitable solvents are polar aprotic, aliphatic, cycloparaffinic or aromatic. Solvents are chosen to have boiling points sufficiently different from those of both the reagents and the products to allow separation.

When carrying out the process of the invention it is possible to pass the product mixture from stage (a) directly to the transesterification stage (b).

Alternatively, there may be a separation process between the two stages in which any by-products are removed from the mixture before it is fed into stage (b). In normal operation the reaction of stage (a) is run at an elevated temperature and so the ammonia formed is in the gaseous phase and therefore easily removed.

Stage (a) may be carried out without a catalyst.

The reaction will take place with heating but is slow and leads to the production of by-products, especially products formed by decomposition of urea.

Advantageously, therefore, a catalyst is used. This may be the same as or different from that used in stage (b).

If the catalysts are different then it may be necessary to remove the first catalyst from the product mixture of stage (a) before it is passed to stage (b). It is preferred, however, that the same catalyst is used in the two reaction stages, avoiding the need to remove the catalyst from the product mixture being passed to stage (b). Other advantages include a lower overall catalyst loss and less contamination of the final product.

Preferred catalysts are homogeneous although heterogeneous catalysts may be used.

Tin compounds are the preferred catalyst for stage (b). These compounds are also suitable as catalysts for the reaction in stage (a) and are therefore the preferred catalysts when the same catalyst is used for both stages (a) and (b). Advantageously tin dialkyl dialkoxides, for example, dibutyltin dimethoxide, dibutyltin diethoxide, dibutyltin dipropoxide, and dibutyltin dibutoxide, dialkyltin oxides, for example, dibutyltin oxide, or tin dialkyl diesters, for example, dibutyltin dilaurate, are used.

Typically the catalyst is used in an amount of up to 5 mole % based on the amount of diol, and preferably in the range of 0.5 to 2 mole % and most preferably 1 mole % in stage (a). In stage (b) the amount of catalyst used is preferably 1 to 10 mole % based on the amount of cyclic carbonate and is most preferably in the range of 1 to 5 mole %.

For stage (a) of the process quantitative conversion of vicinal diol into cyclic carbonate has been achieved with a 100% selectivity for the carbonate. In stage (b) conversion of up to 60% of the cyclic carbonate used has been achieved with a 100% selectivity for the carbonate of the monohydric alcohol.

The (optional) stage (c) of the process is a separation stage in which the desired carbonate of a monohydric alcohol is separated from the diol. Typically conventional distillation techniques are used.

Any unreacted starting materials, reaction products, by-products, catalyst and solvent in the product mixture of stage (b) may be recycled to an earlier stage of the process. It is preferred that the mixture is separated into fractions before recycling so that different parts of the mixture may be recycled to different stages of the process. The diol which is a product of the transesterification reaction in stage (b) is preferably recycled to stage (a), as described in stage (d) above.

Any unreacted alcohol is preferably recycled to stage (b). Unreacted cyclic carbonate may be recycled to stage (a) and/or stage (b). Any catalyst may be recycled back to stage (b) and/or to stage (a) if it is used in that reaction stage. In a preferred embodiment of the present invention the same catalyst is used for stages (a) and (b) and this simplifies the recycling process as there is no need to remove the catalyst before recycling the diol, the cyclic carbonate or the alcohol.

One process in accordance with the invention will now be described in greater detail by way of example only with reference to the accompanying drawing, in which the sole Figure is a flow diagram.

Urea, diol and a catalyst are fed into the reactor vessel of stage (a) where they react together. The reaction is conveniently run under atmospheric pressure and at a temperature of 1700C. At that temperature the ammonia produced in the reaction is gaseous and is vented from the reaction vessel. The product mixture, including the catalyst, is mixed with an appropriate alcohol, in this case methanol, and fed through a line 1 into the second reactor vessel (b) in which the transesterification reaction occurs. Where, as in this case, the alcohol used is methanol, the transesterification reaction is conveniently run at a temperature of 1700C and a pressure of 700 kPa.

The product mixture from (b) is then passed through a line 2 into a separation stage (c) from which the carbonate of the alcohol, in this case DMC, is recovered.

In this particular embodiment the DMC and any unreacted methanol are distilled off from the rest of the mixture in a first distillation stage (cl) and are then passed through a line 3 to a second distillation stage (c2) in which the methanol is distilled off leaving the DMC to be recovered. The methanol is recycled via a line 4 to the transesterification reaction of stage (b).

The bottoms product of the first distillation (cl) to remove the DMC and methanol contains the diol, the catalyst and may also contain unreacted cyclic carbonate.

This mixture comprising unreacted cyclic carbonate, diol and catalyst, is passed through a line 5 to the first reaction vessel (a).

The following Examples illustrate the invention: Examnle 1 (stave (a)) Reaction of urea and 2.3-butanediol to form 4 .5-di-methvl-2-oxo-1, 3-dioxolane Equimolar quantities (0.5 moles) of urea and 2,3butanediol (97 wt% DL form, 3 wt% of meso form) and 1.0 mole% of dibutyltin dimethoxide (the mole percentage being based on the amount of one starting material) were mixed together in a 50ml three-necked flask equipped with a thermometer, condenser, stirrer and nitrogen inlet.

The reaction was carried out at 1700C. The ammonia released was titrated and after 6 hours at 1700C found to be quantitative. The progress of the reaction with time was also followed by GC and NMR. After 6 hours the reaction mixture consisted of 94 wt% trans-4,5-dimethyl2-oxo-1,3-dioxolane, 3 wt% cis-4,5-dimethyl-2-oxo-1,3dioxolane and 3 wt% tin catalyst in the form of a cyclic tin/diol complex.

Quantitative conversion of the urea and the diol had been achieved and with 100% selectivity for the desired cyclic carbonate.

Example 2 (stage b)) Transesterification of 4, 5-di-methvl-2-oxo-l 3-dioxolane with methanol to dimethvl carbonate (DMC) and 2,3butanediol Cyclic carbonate resulting from stage (a) and obtainable as described in Example 1 above, containing 3 mole % of dibutyl tin dimethoxide, was dissolved in absolute methanol (1:4 ratio) and put in an autoclave. The autoclave was equipped with methanol and nitrogen sparging inlet lines, a liquid level controller, stirrer and an outlet line, through a condenser connected to an overhead product collection vessel. A backpressure of 2.4 MPa was maintained in this reactor system throughout the reaction period and the temperature of the reactor was held in the range 182 to 1850C after an initial period of approximately 30 minutes taken to reach that temperature range.The methanol flow was controlled to maintain a constant liquid level in the reactor. The overhead fraction and the reactor contents were analysed by GC as a function of time on stream. After 12 hours the reactor composition consisted of 18.7 wt% cyclic carbonate, 14.9 wtt 2,3-butanediol, 65.3 wt% methanol, 1.1 wt% DMC and tin catalyst only and the overhead fraction consisted of DMC and methanol only thereby showing that no byproducts were formed and that the reaction gave 100% selectivity for DMC. There was an overall conversion of 60 wt% based on the cyclic carbonate. The reaction had a material balance of 98.5%.

Example 3 (staae (buzz Transesterification of 4' 5-di-methvl-2-oxo-1. 3-dioxolane with isododecyl alcohol to di-isododecvl carbonate and 2. 3-butanediol A mixture of 0.5 moles of cyclic carbonate resulting from stage (a) and obtainable as described in Example 1 above, 1.05 moles of isododecyl alcohol (mixture of branched primary alcohol isomers), and 5 mole% (based on the carbonate) of dibutyltin dimethoxide was ref fluxed with slow distillation (with a reflux ratio of 5:1) through a 30cm packed column. The distillation temperature overhead was regulated at 140 to 1500C by adjusting the vacuum (20 to 13 kPa). The overhead fractions were collected and were composed of 2,3butanediol, cyclic carbonate and isododecyl alcohol.

After 6 hours 27 minutes a 57% conversion of cyclic carbonate was obtained with a total yield of 54.6 wt% of di-isododecyl carbonate and 45.4 wtt of 2,3-butanediol.

Analysis of the mixture by GC and NMR did not show any other by-products. The di-isododecyl carbonate remained as bottoms product in the distillation column, and was subsequently separated from the catalyst using conventional distillation.

Claims (25)

1. A process for the production of a carbonate of an aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic or non-aromatic heterocyclic monohydric alcohol which comprises: (a) reacting urea and a vicinal diol to form the cyclic carbonate of the diol, and (b) treating the cyclic carbonate from stage (a) with an aliphatic, cycloaliphatic, araliphatic, arylcycloaliphatic, heterocyclic aliphatic or non aromatic heterocyclic monohydric alcohol in the presence of a catalyst to form the desired carbonate and a diol.

2. A process as claimed in claim 1, wherein the vicinal diol is of the formula I,

in which R1, R2, R3 and R4 may be the same or different and independently represent hydrogen, or an aliphatic, aromatic, cycloaliphatic, araliphatic or arylcycloaliphatic group, or wherein R1 and R2 and the carbon atom to which they are attached and/or R3 and R4 and the carbon atom to which they are attached each form an aliphatic or araliphatic ring or R2 and R3 and the carbon atoms to which they are attached and/or R1 and R4 and the carbon atoms to which they are attached each form an aliphatic or araliphatic ring.

3. A process as claimed in claim 1 or claim 2, wherein the process comprises further stages in which: (c) the carbonate of the monohydric alcohol, formed in stage (b), is separated from the diol, and (d) the diol is recycled to stage (a).

4. A process as claimed in any one of claims 1 to 3, wherein the monohydric alcohol of stage (b) contains from 1 to 16 carbon atoms.

5. A process as claimed in any one of claims 1 to 4, wherein the monohydric alcohol is a primary alkyl alcohol.

6. A process as claimed in any one of claims 2 to 5, wherein at least one of R1, R2, R3 and R4 represents group other than hydrogen.

7. A process as claimed in any one of claims 2 to 6, wherein at least one of R1 and R2 and at least one of R3 and R4 represent groups other than hydrogen.

8. A process as claimed in any one of claims 2 to 7, wherein at least three of R1, R2, R3 and R4 represent groups other than hydrogen.

9. A process as claimed in any one of claims 2 to 8, wherein any of R11 R2, R3 and R4 which do not represent hydrogen, represent alkyl or cycloalkyl groups having from 1 to 16 carbon atoms, alkoxy groups having from 1 to 16 carbon atoms or alkenyl groups having from 2 to 16 carbon atoms.

10. A process as claimed in any one of claims 1 to 9, wherein the vicinal diol is substantially free from acid or ester groups.

11. A process as claimed in any one of claims 2 to 10, wherein at least one of R1, R2, R3 and R4 represents an alkyl group having from 1 to 16 carbon atoms.

12. A process as claimed in any one of claims 2 to 11, wherein at least one of R1 and R2 and at least one of R3 and R4 represent alkyl groups having from 1 to 16 carbon atoms.

13. A process as claimed in any one of claims 1 to 5, wherein the vicinal diol of stage (a) is 2,3butanediol or 2, 3-dimethyl-butanediol-2 '3.

14. A process as claimed in any one of claims 1 to 13, which is carried out as a continuous process.

15. A process as claimed in any one of claims 1 to 14, wherein a catalyst is used in the reaction of stage (a).

16. A process as claimed in claim 15, wherein the catalyst used in stages (a) and (b) is the same.

17. A process as claimed in claim 16, wherein catalyst present in the product mixture from stage (b) is recycled to stage (a).

18. A process as claimed in any one of claims 1 to 17, wherein the catalyst used in stage (b) is a tin compound.

19. A process as claimed in any one of claims 1 to 18, wherein the catalyst used in stage (b) is a tin dialkyl dialkoxide, a dialkyl tin oxide or a dialkyl tin diester.

20. A process as claimed in any one of claims 1 to 19, wherein unreacted monohydric alcohol present in the product mixture from stage (b) is recycled to stage (b) and/or unreacted cyclic carbonate present in the mixture is recycled to stage (a) and/or stage (b).

21. A process substantially as described in either Examples 1 and 2 or 1 and 3 herein.

22. A process substantially as described with reference to and as illustrated by Figure 1 herein.

23. A carbonate of a monohydric alcohol made by a process as claimed in any one of claims 1 to 22.

24. The use of an internal vicinal diol in a reaction with urea to form a solid cyclic carbonate.

25. Any new feature described herein or any new combination of herein described features.