WO2012076519A1

WO2012076519A1 - Process for purifying dialkyl carbonate

Info

Publication number: WO2012076519A1
Application number: PCT/EP2011/071902
Authority: WO
Inventors: Timothy Michael Nisbet; Garo Garbis Vaporciyan; Sanne Wijnans
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2010-12-08
Filing date: 2011-12-06
Publication date: 2012-06-14
Also published as: TW201237029A

Abstract

The invention relates to a process for purifying dialkyl carbonate by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used. The dialkyl carbonates cover both acyclic dialkyl carbonates, such as dimethyl carbonate and diethyl carbonate, and alkylene carbonates wherein the alkyl groups are linked together to form a ring, such as ethylene carbonate.

Description

PROCESS FOR PURIFYING DIALKYL CARBONATE

The present invention relates to a process for purifying dialkyl carbonate, including both acyclic dialkyl carbonates, such as diethyl carbonate, and cyclic dialkyl carbonates wherein the alkyl groups are linked together to form a ring, that is to say alkylene

carbonates, such as ethylene carbonate.

Alkylene carbonate, such as ethylene carbonate, is an important starting material in the production of acyclic dialkyl carbonate and monoalkylene glycol, by reaction with an alkanol. For example, ethylene carbonate and ethanol may be reacted to form diethyl carbonate and monoethylene glycol.

In turn, acyclic dialkyl carbonate, such as dimethyl carbonate or above-mentioned diethyl carbonate, is an important chemical product. It may for example be used as a starting material in the production of diphenyl

carbonate, by reaction with phenol. For example, dimethyl carbonate is widely described in the literature (see e.g. D. Delledonne et al . , Appl . Catal . A, 2001, 221, 241-251) for a range of applications, such as diphenyl carbonate manufacturing (as already mentioned above) , isocyanate manufacturing (via carbamate intermediate) , use as a methylating reagent, use as a solvent in various

applications and use as an oxygenate blending molecule in liquid fuels.

Above-mentioned diphenyl carbonate is an important starting material in the commercial production of

polycarbonates. Diphenyl carbonate may be polymerized with a dihydroxy aromatic compound, for example bisphenol A which is 4, 4 ' - (propan-2-ylidene) diphenol, into a polycarbonate. It is important that the diphenyl carbonate starting material is of sufficient purity before it is reacted with the dihydroxy aromatic

compound. This could be achieved by ensuring that the starting materials used in preceding steps to make diphenyl carbonate, for example above-mentioned ethylene carbonate and diethyl carbonate, are of sufficient purity.

In addition, ensuring that above-mentioned dialkyl carbonate starting materials are of sufficient purity would also result in less side-reactions, and therefore a higher yield and/or selectivity for the reactions in questio .

Preparations of dialkyl carbonate, i.e. including both acyclic dialkyl carbonates and alkylene carbonates, regardless of the process employed, inevitably contain various contaminant compounds in varying quantities.

These impurities should be removed as much as possible before any subsequent reaction step is performed using that dialkyl carbonate, in order to prevent any

interference of such further reaction by those

impurities .

For example, as disclosed in EP0633241, the presence of contaminants in diphenyl carbonate can affect the rate of polymerization with bisphenol A. The resulting polymer product may have a low intrinsic viscosity. In addition, the presence of these contaminants may also affect the color of the polycarbonate polymer.

Exemplary of impurities that can be found in such dialkyl carbonate preparations are metal ions, which may originate from the catalyst or catalysts used in the preparation of the dialkyl carbonate in question.

Examples of such metal impurities are the cations of titanium, iron, zinc, chromium, etc. Another category of impurities are organic and inorganic contaminants containing a non-metal, such as phosphorus and/or halogen (such as bromine, chlorine and iodine) , which may also originate from the catalyst or catalysts used in the preparation of the dialkyl

carbonate in question, or from one of the reactants used. Examples of organophosphorus contaminants also containing halogen, are tetraalkylphosphonium halogenides used as transesterification catalyst, such as

tetraalkylphosphonium bromides as disclosed for example in WO2005003113, that can be used in the production of alkylene carbonates, such as ethylene carbonate.

Still a further category of impurities covers a range of compounds identified as so-called "color bodies". This is a category of contaminants covering compounds which may contain metallic and/or halogenide elements as discussed above and which affect the color of a product, to be produced from the dialkyl carbonate. Further,

US20040225162, the disclosure of which is herein

incorporated by reference, mentions that by the terms

"color" and "color bodies" are meant the existence of visible color that can be quantified by the use of a spectrometer in the range of visible light, using

wavelengths of approximately 400-800 nm, and by

comparison to pure water.

For example, WO200005228 relating to removing color in organic carbonates, more specifically (cyclic)

alkylene carbonates, discloses the following.

Discoloration in organic carbonates may arise from the formation of colored impurities or by-products during synthesis of the organic carbonate. The presence or absence of color in organic carbonates may also reflect the degree of refinement or purification to which the carbonate has been subjected. Alternatively,

discoloration may arise from contaminants acquired during storage or handling of the carbonate. Discoloration in organic carbonates disadvantageously lowers product value. Reduction of color may be desirable for esthetic reasons or for other reasons, such as when a product of sufficient purity is required, as already discussed above .

As appears from the above, there is a need in the art for a simple and effective separation method for

purifying contaminated dialkyl carbonate containing streams, i.e. said dialkyl carbonates including both acyclic dialkyl carbonates and alkylene carbonates.

It is an object of the present invention to provide a process for purifying dialkyl carbonate that fulfils said need. This object is achieved by using a membrane which is a non-porous membrane (no pores) or nanofiltration membrane (pores having an average size of at most 10 nm) . Such non-porous and nanofiltration membranes are commonly referred to in the art as dense membranes.

Accordingly, the process according to the present invention is a process for purifying dialkyl carbonate by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used. Either said dialkyl carbonate is of formula RiO(CO)OR₂ wherein R_x and R₂ are alkyl groups, which alkyl groups are not linked together to form a ring, or said dialkyl carbonate is a cyclic carbonate of formula RiO(CO)OR2 wherein Ri and R2 are linked together to form a ring.

That is to say, within the present specification, the term "dialkyl carbonate" covers both (i) acyclic dialkyl carbonates (i.e. wherein the alkyl groups are not linked together to form a ring) and (ii) cyclic dialkyl carbonates wherein the alkyl groups are linked together to form a ring, that is to say alkylene carbonates.

In the present invention, for acyclic dialkyl

carbonates as mentioned under (i) above, the alkyl groups Ri and R2 may be the same or different, preferably the same. Further, preferably, the alkyl groups Ri and R2 in such acyclic dialkyl carbonate, which groups may be straight, branched and/or cyclic, are Ci-s alkyl groups, more preferably Ci-6 alkyl groups, such as isopropyl, ethyl and methyl, suitably ethyl. Preferably, the acyclic dialkyl carbonate is dimethyl carbonate or diethyl carbonate, more preferably diethyl carbonate.

In the present invention, for cyclic dialkyl

carbonates as mentioned under (ii) above, that is to say alkylene carbonates, the alkylene carbonate may be a 1,2- carbonate of a C2-8 olefin, preferably a C2-6 olefin, such as ethylene, propylene, butadiene and cyclohexene.

Preferably, the alkylene carbonate is ethylene carbonate or propylene carbonate, more preferably ethylene

carbonate.

In the present process for purifying dialkyl

carbonate by membrane separation, a liquid feed

comprising dialkyl carbonate and a contaminant is

separated by the membrane into a liquid permeate

comprising dialkyl carbonate and either no contaminant or contaminant at a concentration which is lower than the contaminant concentration in the feed, and a liquid retentate comprising dialkyl carbonate and contaminant at a concentration which is higher than the contaminant concentration in the feed. That is to say, in the present process, the phases of the feed and the permeate are both liquid. Therefore, there is no phase change. In general, when heating a liquid consisting of two or more components, the bubble point is the point where first bubble of vapor is formed. For single component mixtures, the bubble point is the same as the boiling point. In the separation technique wherein

"pervaporation" is applied using a membrane, the pressure and temperature on the permeate side of the membrane are such that the permeate is above its bubble point causing the permeate to be a vapor.

Applicants have found that when using a non-porous or nanofiltration membrane, good separation results are obtained in such a process for purifying dialkyl

carbonate .

The use of a non-porous or nanofiltration membrane in purifying hydrocarbons in general, is described in

WO2001060771 (which is in the name of Shell) . This document discloses a process for purifying a contaminated liquid hydrocarbon product, wherein the product stream is contacted with a non-porous or nanofiltration membrane and the purified product stream is recovered as the permeate. Although there is no specific limitation as to the nature of the liquid hydrocarbon product in

WO2001060771, the products specifically mentioned are typically industrially produced chemical product streams containing a polymerisable olefinic bond. The products may include one or more heteroatoms, and named examples of liquid hydrocarbon products include cyclopentadiene, dicyclopentadiene, 1 , 3-cyclohexadiene, cyclohexene, styrene, isoprene, butadiene, cis-1 , 3-pentadiene, trans-1 , 3-pentadiene, benzene, toluene, xylenes, ethene and propene . Named liquid hydrocarbon products containing heteroatoms are methyl acrylate, ethyl acrylate and methylmethacrylate . However, there is no mention in WO2001060771 of purification of a contaminated dialkyl carbonate stream.

The non-porous or nanofiltration membrane to be used in the present invention may be of the ceramic or

polymeric type. The membrane used may be hydrophobic or hydrophilic .

Examples of suitable non-porous and nanofiltration membranes are reverse osmosis type membranes. Non-porous and nanofiltration membranes should be distinguished from ultrafiltration membranes which are always porous.

Ultrafiltration membranes have an average pore size of greater than 10 nm up to about 800 nm. Where

nanofiltration membranes are used which are porous, they have an average membrane pore size which is at most 10 nm (nanoporous membranes) . Where such nanofiltration or nanoporous membrane is used in accordance with the present invention, the average membrane pore size is suitably less than 10 nm, preferably at most 8 nm, more preferably at most 7 nm, more preferably at most 6 nm, more preferably at most 5 nm, more preferably less than 5 nm, more preferably at most 4 nm, more preferably at most 3 nm, more preferably at most 2 nm, more preferably at most 1 nm, more preferably at most 0.7 nm, more

preferably at most 0.5 nm, and most preferably at most 0.3 nm .

Ultrafiltration is a pressure difference driven membrane filtration technique, wherein porous membranes are used which have an average pore size greater than 10 nm. One of the disadvantages of using ultrafiltration membranes as discussed above, is that the membranes foul during operation (membrane pores getting clogged or plugged) and have eventually to be taken out of operation for cleaning purposes or replaced in case the pore fouling is irreversible. This will severely decrease the separation efficiency in time.

One way of determining the suitability of a membrane for separating a contaminant X from dialkyl carbonate, is by calculating the rejection factor, as follows:

rejection factor = (1 - ([X]_P/[X]_f)) wherein [X]_p is the concentration of contaminant X in the permeate and [X]_f is the concentration of contaminant X in the feed. Where in the present specification reference is made to rejection of a contaminant, the rejection factor defined in the above way is meant.

In all cases where the rejection factor is greater than zero, separation of the contaminant from the

compound to be purified has taken place. That is to say, relatively more contaminant has been retained by the membrane than passed through the membrane. In such case, the purity of the passed through dialkyl carbonate has increased, as is the object of the present invention. However, it is preferred that the rejection factor is greater than 0.2, more preferably greater than 0.4, even more preferably greater than 0.6, still even more

preferably greater than 0.8, and most preferably greater than 0.9. A general discussion of rejection factors in membrane separation can be found in "Basic Principles of Membrane Technology" by Prof. Marcel Mulder (ISBN 0-7923-

4248-8) .

As mentioned above, the non-porous or nanofiltration membrane to be used in the present invention may be a ceramic membrane. The advantage of said ceramic type membranes is that they do not have to swell in order to work under optimal conditions. Examples of ceramic types are mesoporous titania, mesoporous gamma-alumina,

mesoporous zirconia and mesoporous silica. However, in a preferred embodiment of the present invention, the non-porous or nanofiltration membrane is a polymeric membrane. Such polymeric membrane is preferably cross-linked to provide the necessary network for

avoiding dissolution of the membrane once being in contact with dialkyl carbonate and any solvent used to dissolve said dialkyl carbonate. In general, cross- linking can be effected in several ways, for instance by reaction with cross-linking agents (chemical cross- linking) and/or by irradiation. Preferably, the membrane layer has a siloxane structure which has been cross- linked by means of irradiation, as is for example

described in WO199627430.

Examples of suitable, presently available cross- linked non-porous or nanofiltration membranes are cross- linked silicone rubber-based membranes, of which the cross-linked polysiloxane membranes are a particularly useful group of membranes. Such cross-linked polysiloxane membranes are known in the art, for example from

US5102551.

Typically, the polysiloxanes used contain the

repeating unit -Si-O-, wherein the silicon atoms bear hydrogen or a hydrocarbon group. Preferably the repeating units are of the formula (I)

-Si (R) (R' ) -0- (I)

wherein R and R' may be the same or different and

represent hydrogen or a hydrocarbon group selected from the group consisting of alkyl, aralkyl, cycloalkyl, aryl, and alkaryl. Preferably, at least one of the groups R and R' is an alkyl group, and most preferably both groups are alkyl groups, more especially methyl groups. The alkyl group may also be a 3, 3, 3-trifluoropropyl group. Very suitable polysiloxanes for the purpose of the present invention are (-OH or -NH2 terminated) polydirciethyl- siloxanes and polyoctylmethylsiloxanes . Thus, preferably, the polysiloxane is cross-linked. The cross-linking may be effected through a reactive terminal -OH or -NH2 group of the polysiloxane. Preferred polysiloxane membranes are cross-linked elastomeric polysiloxane membranes.

Examples of suitable cross-linked elastomeric

polysiloxane membranes are extensively described in above-mentioned US5102551. Thus, suitable membranes are composed of a polysiloxane polymer such as described supra having a molecular weight of 550 to 150,000, preferably 550 to 4200 (prior to cross-linking) , which is cross-linked with, as cross-linking agent, (i) a

polyisocyanate, or (ii) a poly ( carbonyl chloride) or (iii) R4__aSi(A)_a wherein A is -OH, -NH₂, -OR, or -OOCR, a is 2, 3, or 4, and R is hydrogen, alkyl, aryl,

cycloalkyl, alkaryl, or aralkyl. Further details

regarding suitable polysiloxane membranes can be found in US5102551.

For the purpose of the present invention the

preferred non-porous membrane is a polydimethylsiloxane or polyoctylmethylsiloxane membrane, which is preferably cross-linked. Also other rubbery non-porous membranes could be used. In general, rubbery membranes can be defined as membranes having a non-porous top layer of one polymer or a combination of polymers, of which at least one polymer has a glass transition temperature well below the operating temperature, i.e. the temperature at which the actual separation takes place. Yet another group of potentially suitable non-porous membranes are the so called superglassy polymers. An example of such a

material is poly (trimethylsilylpropyne) . The non-porous or nanofiltration membrane is

typically supported on at least one porous substrate layer to provide the necessary mechanical strength.

Suitably, this other porous substrate layer is made of a porous material of which the pores have an average size greater than 10 nm. Such other porous material may be a microporous, mesoporous or macroporous material which is normally used for microfiltration or ultrafiltration, such as poly ( acrylonitrile ) . The thickness of the base layer should be sufficient to provide the necessary mechanical strength. In addition, this substrate may in return be supported on a further porous support to provide the required mechanical strength. Typically, the thickness of the base layer is of from 10 to 250 μπι, more suitably of from 20 to 150 μπι. Where the non-porous or nanofiltration membrane is combined with such base layer, the membrane suitably has a thickness of from 0.5 to 10 μπι, preferably of from 1 to 5 μπι.

The combination of a thin top membrane layer and a thick porous support layer is often referred to as composite membranes or thin film composites. Preferably, in the present invention, such composite membrane

consists of said membrane layer and said support layer, implying that no other layers are present in the

composite membrane. The membrane is suitably so arranged that the permeate flows first through the membrane top layer and then through the base layer, so that the pressure difference over the membrane pushes the top layer onto the base layer. Suitable porous materials for the base layer having an average pore size greater than

10 nm, are poly ( acrylonitrile ) , poly ( amideimide ) + T1O₂, poly ( etherimide ) , polyvinylidenedifluoride and

poly ( tetrafluoroethylene ) . Poly ( acrylonitrile ) is especially preferred. A preferred combination according to the present invention is a poly (dimethylsiloxane ) - poly ( acrylonitrile ) combination or a

poly (octylmethylsiloxane) -poly (acrylonitrile)

combination.

The non-porous or nanofiltration membrane may also be used without a substrate layer, but it will be understood that in such a case the thickness of the membrane should be sufficient to withstand the pressures applied. A thickness greater than 10 μπι may then be required.

This is not preferred from a process economics viewpoint, as such thick membrane will significantly limit the throughput of the membrane, thereby decreasing the amount of purified product which can be recovered per unit of time and membrane area.

The dialkyl carbonate permeates through the selective membrane layer, after which it desorbs at the permeate side. The main driving force for permeation is a

hydrostatic pressure differential across the membrane barrier (in literature often referred to as transmembrane pressure) .

In the present invention, the pressure and

temperature on the permeate side of the membrane are such that the permeate is below its bubble point causing the permeate to be a liquid. That is to say, the pressure on the permeate side of the membrane should be above the bubble point pressure of the permeate at a given

temperature and a given composition. Alternatively, the temperature on the permeate side of the membrane should be below the bubble point temperature of the permeate at a given pressure and a given composition.

In the separation technique wherein "pervaporation" is applied using a membrane, the pressure and temperature on the permeate side of the membrane are such that the permeate is above its bubble point causing the permeate to be a vapor, which is therefore different from the present invention.

Preferably, in the present invention, the temperature of the liquid feed comprising dialkyl carbonate and a contaminant, which may also be referred to as the

operating temperature of the present process, is such that under the pressure on the permeate side of the membrane no vapor is formed on that side of the membrane.

For example, said feed temperature may be at most 220 °C, or at most 190 °C, or at most 160 °C, or at most 130 °C, or at most 100 °C.

An advantage of using non-porous membranes as compared to the use of nanoporous membranes is that there is no plugging effect. This means that there is no possibility of the membrane becoming blocked by larger molecules plugged in the pores. This could happen in porous membranes, as a result of which it is more

difficult to regenerate a stable flux. Therefore, it is preferred for the purpose of the present invention to use a non-porous or dense membrane.

The retentate will still comprise valuable dialkyl carbonate and for that reason the retentate may suitably be recycled to the membrane separation step and mixed with fresh feedstock. However, when recycling retentate, part of the retentate will have to be discharged such as to avoid build up of the contaminant or contaminants which is (are) to be separated from the dialkyl carbonate by means of said membrane process. Instead of recycling the retentate within the same process, it may also be subjected to a second and optionally further separation step, in which case the retentate of a first separation step is used as the feed for a second separation step.

Further, instead of recycling (part of) the retentate or further purifying it in a second and optionally further step, the retentate may also be discharged in its entirety. This is most likely advantageous where the composition of the retentate is such that it has some value as a starting material in another process, without having to further treat the retentate before such use (no further processing) . The permeate has been upgraded in the sense that its contamination level has been lowered. Consequently the permeate has obtained a higher value compared to the original product. The retentate, which contains an increased proportion of contaminants as compared to the original product, has a value depending on the contaminant concentration and the perceived end use. The retentate value may be lower than or similar to the value of the original feed.

Stage cut is defined as the weight percentage of the original feed that passes through the membrane and is recovered as permeate. By adjusting the stage cut, it is possible to vary the concentration of a contaminant in the permeate, as well as the concentration of said same contaminant in the retentate. The higher the stage cut, the higher the contaminant concentration in the

retentate .

In the present invention, the stage cut can vary within broad limits: 5 to 99% by weight, suitably 30 to 95% by weight or 50 to 90% by weight.

The desired stage cut can be set by varying, for a given permeability of the membrane, the trans-membrane pressure and/or the feed flow. The first option implies that, for a given feed flow, increasing the trans- membrane pressure results in a greater flux or flow of the permeate through the membrane, and therefore in a higher stage cut. According to the second option, such higher stage cut may also be achieved by decreasing the feed flow whilst maintaining a certain permeate flow through the membrane .

In the present invention, the volume flux through the membrane is typically in the range of from 5 to 1000, suitably 10 to 500, and more suitably 15 to 200 1/h/m². The flux through the membrane may also be expressed as mass flux. Preferably, the flux through the membrane is constant in time. Further, the inlet stream is contacted with the membrane at a trans-membrane pressure (pressure difference) which is typically in the range of from 1 to 60 bar, suitably 3 to 35 bar, and more suitably 3 to

25 bar. The permeability of the membrane is typically in the range of from 1 to 100, suitably 2 to 50, and more suitably 3 to 10 l/h/m²/bar.

In accordance with the present invention, a liquid feed comprising dialkyl carbonate and a contaminant may be separated into a liquid permeate comprising dialkyl carbonate and either no contaminant or contaminant at a concentration which is lower than the contaminant concentration in the feed, and a liquid retentate comprising dialkyl carbonate and contaminant at a concentration which is higher than the contaminant concentration in the feed.

Preferably, the contaminant concentration in said permeate is from essentially zero to at most 20,000 ppmw (parts per million by weight) , more preferably at most

1000 ppmw, more preferably at most 250 ppmw, more preferably at most 50 ppmw, and most preferably at most 1 ppmw, on the basis of total weight of the permeate. Such permeate may suitably be used as raw material in making a polycarbonate.

The membrane separation will be performed in a membrane unit, which comprises one or more membrane modules. Examples of suitable modules are typically expressed in how the membrane is positioned in such a module. Examples of these modules are the spirally wound, plate and frame (flat sheet), hollow fibres and tubular modules. Preferred module configurations are spirally wound and plate and frame. Most preferably, the non- porous or nanofiltration membrane is applied in a

membrane unit, which comprises spirally wound membrane modules. These membrane modules are well known to the skilled person as for example described in Encyclopedia of Chemical Engineering, 4^th Ed., 1995, John Wiley & Sons

Inc., Vol 16, pages 158 - 164. Examples of spirally wound modules are described in for example, US5102551,

US5093002, US5275726, US5458774, US5150118, and

O2006040307.

The dialkyl carbonate feed may be dissolved in a solvent. Many suitable solvents for dissolving dialkyl carbonate can be used, such as ethanol, diethyl ether, carbon tetrachloride, acetic acid, acetone and toluene. The weight percentage of solvent, based on total weight of dialkyl carbonate feed and solvent, may vary within wide limits. Suitably, it is of from 50 to 90 wt.%, more suitably 60 to 80 wt.%.

The way in which the dialkyl carbonate to be purified in accordance with the present invention is prepared, is immaterial to the present invention. Any known

preparation process may have been applied. The dialkyl carbonate to be treated in the process according to the present invention may be the product directly obtained from the known preparation processes. Alternatively, said directly obtained dialkyl carbonate also may have been subjected to conventional purification and recovery techniques before it is treated in accordance with the present invention.

Typically, where in the present invention the dialkyl carbonate to be purified is an acyclic dialkyl carbonate, the acyclic dialkyl carbonate (for example diethyl carbonate) feed to be purified in the present process, has been obtained by the reaction of an alkanol (for example ethanol) and an alkylene carbonate (for example ethylene carbonate) using a catalyst resulting in the acyclic dialkyl carbonate (for example diethyl carbonate) and a monoalkylene glycol (for example monoethylene glycol) .

Further, typically, where in the present invention the dialkyl carbonate to be purified is a (cyclic) alkylene carbonate, the alkylene carbonate (for example ethylene carbonate) feed to be purified in the present process, has been obtained by the reaction of carbon dioxide and an alkylene oxide (for example ethylene oxide) using a catalyst resulting in the alkylene

carbonate (for example ethylene carbonate) .

The dialkyl carbonate feed to be purified in the present process may comprise a dialkyl carbonate feed which does not meet specifications regarding the maximum amount of impurities.

In general, the dialkyl carbonate stream to be treated in the process of the present invention comprises at least 35 wt.%, more preferably at least 45 wt.%, more preferably at least 55 wt.%, more preferably at least 65 wt.%, more preferably at least 75 wt.%, more preferably at least 85 wt.%, and most preferably at least 95 wt . % of dialkyl carbonate.

If the dialkyl carbonate product to be treated is a relatively crude dialkyl carbonate stream, such product may contain 5% by weight or less of the contaminant ( s ) based on total weight of the product. However, the present method is particularly suitable when the dialkyl carbonate product to be treated contains 3% by weight or less, suitably 1% by weight or less, and more suitably 0.1% by weight or less of the contaminant ( s ) . Even at such relatively high contaminant levels, the process of the present invention is highly effective.

If the dialkyl carbonate product to be treated is a relatively pure dialkyl carbonate stream, such product preferably contains less than 500 ppmw, suitably less than 300 ppmw, more suitably less than 200 ppmw, more suitably less than 100 ppmw, more suitably less than 50 ppmw, and most suitably less than 20 ppmw of the contaminant ( s ) . Typically, such relatively pure dialkyl carbonate inlet stream comprises 1 to 15 ppmw of the contaminant ( s ) .

The contaminants (impurities) in the dialkyl

carbonate to be separated in the present process may be one or more of the contaminants as referred to above in the introduction.

The present invention also relates to a process for preparing an acyclic dialkyl carbonate, as defined above, comprising reacting an alkanol with an alkylene carbonate resulting in a stream containing acyclic dialkyl

carbonate and impurities, and recovering acyclic dialkyl carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used. Further, the present invention also relates to a process for preparing an alkylene carbonate, comprising reacting carbon dioxide with an alkylene oxide resulting in a stream containing alkylene carbonate and impurities, and recovering alkylene carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used.

The invention is further illustrated by the following Examples .

Example 1

Contaminants were removed from a dialkyl carbonate containing feed by using a dead-end membrane unit. The experimental set-up used in these examples is

schematically shown in Fig. 1, wherein the reference numerals have the following meanings:

1 nitrogen inlet

2 feed mixture

3 membrane

4 permeate outlet

5 collection vessel

The pressure difference over membrane 3 of the deadend membrane unit of Fig. 1, necessary for effecting the flow of permeate through the membrane, was applied by means of pressurisation by feeding nitrogen gas via nitrogen inlet 1. In addition, the nitrogen was used as a blanket covering feed mixture 2. The trans-membrane pressure applied was 16 bar during the entire

experimental period.

The membrane was a supported membrane wherein the top layer having a thickness of approximately 3 μπι was made of hydrophobic dense cross-linked poly (dimethylsiloxane) (PDMS) . The total membrane surface was 17 cm². After having applied said trans-membrane pressure, the vessel shown in Fig. 1 was filled with the feed.

Subsequently, stirring of this feed under a blanket of nitrogen gas was started. ( Semi- ) turbulent flow of feed mixture 2 above the membrane was ensured. The temperature during the entire experimental period was room

temperature .

At the end of the experiment, the part of the feed mixture which remained inside the vessel and which did not pass through the membrane as the permeate, was decanted and recovered as the retentate.

In Table 1, further process parameters are mentioned. The concentrations of iron (Fe) in the feed, permeate and retentate were determined by means of inductively coupled plasma - mass spectrometry. The concentrations of chloride (CI) in the feed, permeate and retentate were determined by means of coulometric titration. Further, the concentrations of the dialkyl carbonates, namely dimethyl carbonate (DMC) , diethyl carbonate (DEC) and ethylene carbonate (eC) , in the feed, permeate and retentate were determined by means of gas chromatography. Still further, color values were determined for the feed, permeate and retentate by means of a LICO300 universal colorimeter from Dr. Bruno Lange GmbH. The measured color values were expressed according to the APHA and Gardner color scales. Said concentrations, rejection factors and said color values are mentioned in Table 1.

From Table 1 it can be seen that the Fe and CI rejection factors in Example 1 were advantageously high (almost 1) . This means that nearly all of the

contaminants were removed by only a single membrane operation. Further, it appears that DMC and DEC were substantially not rejected (rejection factors of only 0.07 and 0.02, respectively) . The rejection factor for eC was 0.33, which is still advantageously small, especially since the difference between the rejection factors for the contaminants (almost 1) and the rejection factor for eC (0.33) is sufficiently large to also enable eC

purification .

Table 1

(1) DMC = dimethyl carbonate; DEC = diethyl carbonate; eC = ethylene carbonate. Example 2

The experiment of Example 2 was performed in

accordance with the procedure as described above for Example 1, with the proviso that the membrane was a supported membrane wherein the top layer having a thickness of approximately 3 μπι was made of hydrophobic dense cross-linked poly (octylmethylsiloxane ) (POMS), the total membrane surface was 20 cm² and the trans-membrane pressure applied was 4.6 bar during the entire

experimental period.

In Table 2, further process parameters are mentioned.

From Table 2 it can be seen that a clear color improvement was achieved by only a single membrane operation. The measured color value of the permeate is significantly lower than that of the feed, for both color scales. Further, it appears that DEC and eC were

substantially not rejected. For both compounds, the rejection factors were below 0.08.

Table 2

(1) DEC = diethyl carbonate; eC = ethylene carbonate. (2) value is calculated, not determined (analytically)

Claims

C L A I M S

1. Process for purifying dialkyl carbonate by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used, wherein the dialkyl carbonate is of formula RiO(CO)OR.2 wherein R_x and R₂ are alkyl groups, which alkyl groups are not linked together to form a ring, or wherein the dialkyl carbonate is a cyclic carbonate of formula RiO(CO)OR₂ wherein R_x and R₂ are linked together to form a ring.

2. Process according to claim 1, wherein the membrane is a polymeric membrane.

3. Process according to claim 2, wherein the polymeric membrane is made from a polysiloxane which contains repeating units of the formula (I)

-Si (R) (R' ) -0- (I) wherein R and R' may be the same or different and

represent hydrogen or a hydrocarbon group selected from the group consisting of alkyl, aralkyl, cycloalkyl, aryl, and alkaryl.

4. Process according to any one of the preceding claims, wherein a liquid feed comprising dialkyl carbonate and a contaminant is separated by the membrane into a liquid permeate comprising dialkyl carbonate and either no contaminant or contaminant at a concentration which is lower than the contaminant concentration in the feed, and a liquid retentate comprising dialkyl carbonate and contaminant at a concentration which is higher than the contaminant concentration in the feed.

5. Process for preparing an acyclic dialkyl carbonate, comprising reacting an alkanol with an alkylene carbonate resulting in a stream containing acyclic dialkyl

carbonate and impurities, and recovering acyclic dialkyl carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used.

6. Process for preparing an alkylene carbonate,

comprising reacting carbon dioxide with an alkylene oxide resulting in a stream containing alkylene carbonate and impurities, and recovering alkylene carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used.