WO2012076532A1

WO2012076532A1 - Process for purifying aryl group containing carbonates

Info

Publication number: WO2012076532A1
Application number: PCT/EP2011/071934
Authority: WO
Inventors: Cyrille Paul Allais; Johannes Leendert Willem Cornelis Den Boestert
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: TW201229033A

Abstract

The invention relates to a process for purifying an aryl group containing carbonate, such as a diaryl carbonate, such as diphenyl carbonate, by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used. Further, the invention relates to a process for making a polycarbonate, comprising reacting a dihydroxy aromatic compound, for example bisphenol A, with a diaryl carbonate purified in accordance with the former process.

Description

PROCESS FOR PURIFYING ARYL GROUP CONTAINING CARBONATES

The present invention relates to a process for purifying an aryl group containing carbonate, such as a diaryl carbonate, such as diphenyl carbonate.

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.

Commercial preparations of diphenyl carbonate, 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 diphenyl 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.

A first category of impurities are metal ions, which may originate from the catalyst or catalysts used in the preparation of diphenyl carbonate. Examples of such metal impurities are the cations of titanium, iron, zinc, chromium, etc. For example, US20080041712 discloses that even minor quantities of titanium in diphenyl carbonate may make the diphenyl carbonate unsuitable as a raw material for the production of a high-purity discolored polycarbonate.

A second category of impurities are organic and inorganic contaminants containing a non-metal, such as phosphorus and/or halogen (bromine, chlorine, iodine, etc.), which may also originate from the catalyst or catalysts used in the preparation of diphenyl carbonate, 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. Further, chlorides are common impurities in diphenyl carbonate that has been prepared by reaction of phenol with phosgene, (C=0)Cl2. Reference is made to the description of related art in EP0722931A1 and to paragraph 8 of EP1234845B1 wherein problems associated with the presence of such contaminants are described.

A third category of impurities covers a range of compounds identified as so-called "color bodies". This is a category of contaminants, not covering isomers or derivatives of diphenyl carbonate, but any other

compounds which may contain metallic and/or halogenide elements as discussed above and which affect the color of a product, such as polycarbonate, to be produced from the diphenyl carbonate. For example, when diphenyl carbonate is used as a monomer reactant in the preparation of polycarbonate by transesterification with bisphenol A, colors in the polycarbonate product may range from pink (iron contamination) to brown (phenyl chloroformate contamination) . 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.

EP0633241 discloses a process for removing

contaminants from crude diphenyl carbonate, such as those contaminants as referred to above. The process of

EP0633241 is a two-stage procedure starting with a water wash of molten crude diphenyl carbonate followed by distillation. Such procedure involving a separate wash step is quite cumbersome. It is desired to remove the contaminants in a simpler way.

In addition to the above three categories of

contaminants, a fourth category of contaminants comprises isomers of diphenyl carbonate. A common diphenyl

carbonate isomer is phenyl salicylate (phenyl 2- hydroxybenzoate ) . For example, US5734004 discloses that the presence of phenyl salicylate negatively affects the transesterification behavior of the diphenyl carbonate which contains such phenyl salicylate. By removing the phenyl salicylate, less catalyst may be used for starting the transesterification . Therefore, it is desired to remove phenyl salicylate as much as possible. However, to do this by means of distillation, is particularly

problematic since the boiling points of phenyl salicylate (305 °C) and diphenyl carbonate (301 °C) are virtually the same.

Another undesired contaminant related to said phenyl salicylate, that may be present in crude diphenyl

carbonate or may be formed therein, is xanthone . Generally, xanthone is formed by heating of phenyl salicylate. Such xanthone formation may thus also take place when producing polycarbonate from diphenyl

carbonate which also contains phenyl salicylate.

Further, in relation to the above-mentioned third category of impurities covering "color bodies" and the above-mentioned fourth category of contaminants

comprising isomers of diphenyl carbonate, reference is also made to US5519106 concerning the preparation of aromatic polycarbonate resins.

A final, fifth category of contaminants in crude diphenyl carbonate to be mentioned, covers impurities derived from diphenyl carbonate and/or an isomer thereof, such as phenyl salicylate, and having a molecular weight greater than that of diphenyl carbonate (MW = 214

g/mole) . Side-reactions which involve phenyl salicylate and which result in such high molecular weight byproducts may be the following:

Formula (I )

Other examples of heavy impurities are alkylated derivatives of diphenyl carbonate, that is to say

diphenyl carbonate wherein a phenyl ring is substituted by an alkyl group, for example a methyl or an ethyl group. Specific examples are 2-EPPC ( 2-ethylphenyl phenyl carbonate) and 4-EPPC ( 4-ethylphenyl phenyl carbonate) . Still other examples of heavy impurities are phenyl salicylate derivatives wherein the hydroxyl group of phenyl salicylate is replaced by an alkoxide group -OR wherein R is an alkyl group, for example a methyl or an ethyl group.

Reference is made to paragraphs 22 and 23 of

US20070270604 disclosing that certain intermediate boiling point by-products (having a boiling point between that of the alkyl aryl carbonate and that of the diaryl carbonate) and high boiling point by-products (having a boiling point higher than that of the diaryl carbonate) , such as aryloxycarbonyl- ( aryloxycarbonyl ) -arenes , cause discoloration and a deterioration in the properties of polycarbonate produced from diaryl carbonate containing such by-products.

As already mentioned above, diaryl carbonates, such as diphenyl carbonate, can be made by reacting phosgene with an aryl alcohol such as phenol. Further, such diaryl carbonates may be prepared by reaction of such aryl alcohol with a dialkyl carbonate. In a first step of the latter preparation process, transesterification of the dialkyl carbonate with the aryl alcohol takes place to yield alkyl aryl carbonate and alkyl alcohol. In a second step disproportionation of the alkyl aryl carbonate takes place to yield diaryl carbonate and dialkyl carbonate. Further transesterification of the alkyl aryl carbonate with aryl alcohol yielding diaryl carbonate and alkyl alcohol may also take place.

WO200142187 discloses a process for preparing

diphenyl carbonate from a dialkyl carbonate and phenol. In said process, use is made of 3 reactive distillation columns in series: see columns A, Bl and C in Fig. 2 of WO200142187. In said process, product stream 15 coming from column C contains essentially all of the diaryl carbonate produced together with residual catalyst, some alkyl aryl carbonate and unwanted high boiling by-products. WO200142187 discloses that this product stream 15 may be further distilled if additional purification is desired.

If such diaryl carbonate product stream is further distilled in another distillation column, the main portion of the diaryl carbonate may end up in a top stream from such other distillation column whereas the catalyst and high boiling by-products would remain, together with the remaining portion of the diaryl carbonate, in the bottom stream from such other

distillation column.

It would be desired that where said top stream comprising the diaryl carbonate, such as diphenyl carbonate, still would not meet specifications regarding the maximum amount of impurities such as those as referred to above, said impurities can be removed from such top stream in a simple and effective way, without having to distill the top stream again.

In addition, said bottom stream may still contain a substantial amount of the valuable diaryl carbonate, such as diphenyl carbonate. It would be desired to be able to recover a substantial portion of the diphenyl carbonate from such bottom stream and at the same time to separate it from contaminants such as those as referred to above, without having to distill the bottom stream again.

For any further distillation of said top and bottom streams might in fact result in the production of even more impurities. Further, it would result in yet another

(waste) bottom stream that may still contain a

substantial amount of the diaryl carbonate. Therefore, there is a need in the art for a simple and effective separation method for purifying contaminated streams containing aryl group containing carbonates, such as a diaryl carbonate.

It is an object of the present invention to provide a process for purifying an aryl group containing carbonate, such as a diaryl carbonate, such as diphenyl 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 an aryl group containing carbonate by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used.

The above-mentioned aryl group containing carbonate may be a carbonate of the following formula:

RiO(CO)OR₂

wherein Ri or R₂ is an aryl group, the other group being an alkyl group (alkyl aryl carbonate) , preferably a C₁-C5 alkyl group, or both Ri and R₂ are aryl groups (diaryl carbonate) . Preferably, both Ri and R₂ are aryl groups. That is to say, preferably, the aryl group containing carbonate is a diaryl carbonate. Further, preferably, the aryl group is a phenyl group. Examples of aryl group containing carbonates that can suitably be purified by performing the present process, are ethyl phenyl

carbonate (EPC) and diphenyl carbonate (DPC) . Most preferably, the aryl group containing carbonate is DPC.

In the present process for purifying an aryl group containing carbonate by membrane separation, a liquid feed comprising the aryl group containing carbonate and a contaminant is separated by the membrane into a liquid permeate comprising the aryl group containing 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 the aryl group containing 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 an aryl group containing carbonate.

Generally, it is known to use filters to remove relatively large solid particles from crude, molten diphenyl carbonate. For example, in the process for making diphenyl carbonate according to above-mentioned EP0633241, two separate filters are used (filters 20 and

36) . The filters have pore sizes in the order of microns, more in particular about 10 μπι, and are intended to remove solid particles from molten diphenyl carbonate. Because of such large pore size, impurities such as those referred to above in the introduction which may also be dissolved in the molten diphenyl carbonate, would not be separated by the filters of EP0633241.

Further, US5502153 discloses that in the preparation of polycarbonate from aromatic dihydroxy compound and diester carbonate, such as diphenyl carbonate, the latter reactants may first be purified by washing and

distillation or by filtration with a monofilter. In the latter case, US5502153 discloses that said reactants should be passed through a monofilter having the highest possible filtration accuracy, after which they may be sent directly to the polymerization step. Said monofilter is not further specified in US5502153.

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 stream containing an aryl group containing carbonate.

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 the aryl group containing 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 aryl group containing 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 the aryl group containing carbonate and any solvent used to dissolve said aryl group containing 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) polydimethyl- 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) F4__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 aryl group containing 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 trans-membrane 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 the aryl group containing 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 300 °C, or at most 270 °C, or at most 240 °C, or at most 210 °C, or at most 180 °C, or at most 150 °C, or at most 120 °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 aryl group containing 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 aryl group containing 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 l/h/m^. 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 the aryl group containing carbonate and a contaminant may be separated into a liquid permeate comprising the aryl group containing 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 the aryl group containing 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

WO2006040307.

It will be appreciated that in a case where no solvent is used to dissolve the aryl group containing carbonate feed, preferably the operating temperature at atmospheric pressure may be kept between the melting point and the boiling point of the aryl group containing carbonate feed in order to have a liquid inlet stream. The melting and boiling points of diphenyl carbonate are about 83 and 301 °C, respectively. Therefore, the

separation may be carried out at a temperature in the range of from 83 to 300 °C, more suitably 85 to 100 °C. However, the disadvantage of this may be that due to the relatively high operating temperature more contaminants are formed.

Preferably, the aryl group containing carbonate feed is dissolved in a solvent. The use of a solvent

advantageously allows the membrane separation to be run at a relatively low temperature which inter alia has a positive effect on membrane lifetime and rejection rate. Many suitable solvents for dissolving an aryl group containing carbonate, such as diphenyl carbonate, can be used, such as ethanol, diethyl ether, carbon

tetrachloride, acetic acid, acetone, toluene and dialkyl carbonate (for example diethyl carbonate) . Preferably, acetone or toluene, most preferably acetone, is used as the solvent for the aryl group containing carbonate feed, especially where the aryl group containing carbonate feed comprises diphenyl carbonate. The weight percentage of solvent, based on total weight of the aryl group

containing carbonate feed and solvent, may vary within wide limits. Suitably, it is of from 50 to 90 wt.%, more suitably 60 to 80 wt.%.

In a case where the solvent is acetone, the molar ratio of acetone to the aryl group containing carbonate feed, which may comprise diphenyl carbonate as the aryl group containing carbonate, is in the range of from 0.5:1 to 7:1, suitably of from 0.9:1 to 2:1. Said molar ratio of acetone to the aryl group containing carbonate feed is preferably at least 0.5:1, more preferably at least

0.6:1, again more preferably at least 0.8:1 and yet more preferably at least 0.9:1. Further, said molar ratio of acetone to the aryl group containing carbonate feed preferably is at most 5:1, more preferably at most 3.5:1, yet more preferably less than 3, again more preferably less than 2.5, and most preferably at most 2:1.

The way in which the aryl group containing 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 aryl group containing 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 aryl group

containing carbonate also may have been subjected to conventional purification and recovery techniques before it is treated in accordance with the present invention. Assuming that the aryl group containing carbonate is produced from an aryl alcohol and a dialkyl carbonate, such purification and recovery techniques typically involve the removal of unreacted aryl alcohol and dialkyl carbonate and co-product alkyl alcohol. Typically, the aryl group containing carbonate (for example ethyl phenyl carbonate and/or diphenyl carbonate) feed to be purified in the present process, has been obtained by the reaction of an aryl alcohol (for example phenol) and a dialkyl carbonate (for example dimethyl carbonate, diethyl carbonate or diisopropyl carbonate) using a metal (for example titanium) containing catalyst resulting in the aryl group containing carbonate and an alkyl alcohol (for example methanol, ethanol or isopropanol ) , and then separating unreacted aryl alcohol, unreacted dialkyl carbonate, said alkyl alcohol, and possibly some

relatively low boiling contaminants such as alkyl aryl ether (for example methyl phenyl ether, ethyl phenyl ether or isopropyl phenyl ether) , from the product mixture comprising the aryl group containing carbonate. In addition, the aryl group containing carbonate feed to be purified in the present process may originate from distilling said product mixture that is obtained by removing said compounds as mentioned hereinbefore, into a top fraction comprising the aryl group containing

carbonate and a bottom fraction comprising the aryl group containing carbonate and higher boiling contaminants, wherein said bottom fraction may be the aryl group containing carbonate feed to be purified in the present process. Still further, the aryl group containing

carbonate feed to be purified in the present process may comprise an aryl group containing carbonate feed which does not meet specifications regarding the maximum amount of impurities.

In general, the aryl group containing 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 the aryl group containing carbonate .

If the aryl group containing carbonate product to be treated is a relatively crude aryl group containing 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 aryl group containing 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 aryl group containing carbonate product to be treated is a relatively pure aryl group containing 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 aryl group containing carbonate inlet stream comprises 1 to 15 ppmw of the contaminant ( s ) .

The contaminants (impurities) in the aryl group containing carbonate to be separated in the present process may be one or more of the contaminants as

referred to above in the introduction.

Preferably, the aryl group containing carbonate to be purified in the present process originates from the reaction of an aryl alcohol with a dialkyl carbonate. As also mentioned above in the introduction, in a first step, transesterification of the dialkyl carbonate with the aryl alcohol takes place to yield alkyl aryl

carbonate and alkyl alcohol. In a second step

disproportionation of the alkyl aryl carbonate takes place to yield diaryl carbonate and dialkyl carbonate. Further transesterification of the alkyl aryl carbonate with aryl alcohol yielding diaryl carbonate and alkyl alcohol may also take place.

The present invention also relates to a process for preparing a diaryl carbonate, comprising reacting an aryl alcohol with a dialkyl carbonate resulting in a stream containing diaryl carbonate and impurities, and

recovering diaryl carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used. Preferably, in said process, the stream containing diaryl carbonate and impurities is first distilled in a distillation column into a top stream containing diaryl carbonate and a bottom stream containing diaryl carbonate and impurities, and diaryl carbonate is recovered from the latter bottom stream by said membrane separation.

Further, the present invention relates to a process for preparing a diaryl carbonate, comprising reacting an aryl alcohol with a dialkyl carbonate resulting in a stream containing alkyl aryl carbonate and impurities, and recovering alkyl aryl carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used, and reacting the recovered alkyl aryl carbonate into diaryl carbonate.

Preferably, in said processes, the dialkyl carbonate is a di (C1-C5) alkyl carbonate, such as dimethyl carbonate or diethyl carbonate.

Preferably, diaryl carbonate purified in the process of the present invention is for use as raw material in making a polycarbonate. The present invention also relates to a process for making a polycarbonate,

comprising reacting a dihydroxy aromatic compound with a diaryl carbonate purified in accordance with the process as described above or prepared in accordance with any one of the processes as described above. Further, the present invention relates to a process for making a

polycarbonate, comprising purifying a diaryl carbonate in accordance with the process as described above or

preparing a diaryl carbonate in accordance with any one of the processes as described above, and reacting a dihydroxy aromatic compound with the diaryl carbonate thus obtained. Preferably, in said processes, the

dihydroxy aromatic compound is bisphenol A which is 4,4'- (propan-2-ylidene ) diphenol . The invention is further illustrated by the following Examples .

Example 1 and Reference Example 1

Contaminants were removed from a diphenyl carbonate

(DPC) 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 dead^¬ end 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 4.5 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 20 cm².

After having applied said trans-membrane pressure, the vessel shown in Fig. 1 was filled with the feed to which a solvent had been added. 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.

In Example 1, a solvent switch was performed on the permeate, the retentate and a certain amount of the feed, by evaporating the acetone and re-dissolving the residue in the same amount of toluene in order to ensure that the concentrations would remain the same. Then the

concentrations of iron (Fe) in the retentate and the permeate that had been subjected to said solvent switch, were determined by means of inductively coupled plasma - mass spectrometry. Further, the concentrations of

chloride (CI) in the retentate, the permeate and a certain amount of the feed that had been subjected to said solvent switch, were determined by means of

coulometric titration. In Reference Example 1, the concentrations of DPC in the retentate and the permeate (no solvent switch) were determined by means of gas chromatography. Said concentrations and rejection factors for the contaminants for Example 1 and for DPC for

Reference Example 1 are mentioned in Table 1.

Table 1

n.d. = not determined

(1) = weight ratio toluene/DEC = 1:1.2 (DEC = diethyl carbonate )

(2) = value is calculated, not determined (analytically)

From Table 1 it can be seen that the Fe and CI rejection factors in Example 1 were advantageously

(almost 1) . This means that nearly all of the

contaminants were removed by only a single membrane operation. Further, Reference Example 1 demonstrates that DPC was not rejected. The DPC rejection factor was in fact below zero.

Example 2

The experiment of Example 2 was performed in

accordance with the procedure as described above for Example 1 and Reference Example 1, with the proviso that the trans-membrane pressure applied was 5.0 bar during the entire experimental period. In Table 2, further process parameters are mentioned.

A solvent switch was performed on the permeate, the retentate and a certain amount of the feed, by

evaporating the acetone and re-dissolving the residue in the same amount of toluene in order to ensure that the concentrations would remain the same. Then the

concentrations of zinc (Zn), chromium (Cr) and phosphorus (P) in the retentate, the permeate and a certain amount of the feed that had been subjected to said solvent switch, were determined by means of inductively coupled plasma - mass spectrometry. Further, the concentrations of bromide (Br) in the retentate, the permeate and a certain amount of the feed that had been subjected to said solvent switch, were determined by means of

coulometric titration. Said concentrations and rejection factors for the contaminants are mentioned in Table 2. Table 2

From Table 2 it can be seen that the rejection factors for all of the contaminants were advantageously well above zero (0.50 or higher) . This implies that the purity of the permeated DPC has substantially increased by only a single membrane operation.

Example 3

The experiment of Example 3 was performed in

accordance with the procedure as described above for Example 1 and Reference Example 1, with the proviso that instead of using a feed prepared by mixing DPC, solvent and a contaminant, the feed was prepared by dissolving a heavy carbonate fraction in acetone.

Said heavy carbonate fraction was mainly comprised of DPC. Further, the heavy carbonate fraction contained the following contaminants: phenyl salicylate, 2-EPPC (2- ethylphenyl phenyl carbonate), 4-EPPC ( 4-ethylphenyl phenyl carbonate) and other unidentified heavy

contaminants having a molecular weight greater than that of DPC (MW = 214 g/mole) up to about 650 g/mole. Still further, the heavy carbonate fraction contained titanium (Ti) and chromium (Cr) .

The titanium species as contained in said heavy carbonate fraction originated from the catalyst used in the preceding preparation of DPC from DEC (diethyl carbonate) and phenol. Said preparation involved reacting phenol and DEC in a first reactive distillation column in the presence of a titanium containing transesterification catalyst and separating by withdrawing a bottom stream containing DPC product (and its isomers), intermediate product ethyl phenyl carbonate, heavy contaminants, a portion of unreacted phenol, a portion of unreacted DEC and traces of ethanol. In a second reactive distillation column, said bottom fraction was concentrated by removing phenol, DEC, ethanol and other light contaminants over the top of the column, and further reaction into DPC took place in said second column. The DPC containing bottom fraction from said second reactive distillation column was subjected to further distillation, wherein the main portion of the DPC was overheaded via the top stream from said column and the bottom stream therefrom contained the remaining DPC and isomers of DPC (such as phenyl

salicylate) as well as heavy contaminants (including 2- EPPC and 4-EPPC as mentioned above) and titanium and chromium species as mentioned above. The heavy carbonate fraction that was used in Example 3 was taken from the latter bottom stream.

In Table 3, further process parameters are mentioned. The permeate and retentate were analyzed after evaporating the acetone. The concentrations of titanium

(Ti) and chromium (Cr) in the retentate, the permeate and a certain amount of the feed were determined by means of inductively coupled plasma - mass spectrometry. Further, the concentrations of phenyl salicylate and heavy

contaminants (HC) in the permeate and a certain amount of the feed were determined by means of gas chromatography (GC) or liquid chromatography (LC) . Said concentrations and rejection factors for the contaminants are mentioned in Table 3.

Table 3

n.d. = not determined

(1) Concentration of contaminant X = 100%* (AC_X/∑AC) , wherein AC_X is the peak area (i.e. area counts) of the peak belonging to contaminant X in the chromatogram, and ∑AC is the total peak area of DPC and all heavy

contaminants having MW > MW_DPC/ps (214 g/mole) up to about 650 g/mole in the LC chromatogram or the total peak area of PS, 2-EPPC and 4-EPPC in the GC chromatogram.

From Table 3 it can be seen that the rejection factors for all of the contaminants were advantageously well above zero (0.3 or higher) . This implies that the purity of the permeated DPC has substantially increased by only a single membrane operation.

Claims

C L A I M S

1. Process for purifying an aryl group containing

carbonate by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used.

2. Process according to claim 1, wherein the aryl group containing carbonate is an alkyl aryl carbonate or a diaryl carbonate, preferably a diaryl carbonate.

3. Process according to claim 1 or 2, wherein the aryl group containing carbonate is dissolved in a solvent.

4. Process according to any one of the preceding claims, wherein the membrane is a polymeric membrane.

5. Process according to claim 4, 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.

6. Process according to any one of the preceding claims, wherein a liquid feed comprising the aryl group

containing carbonate and a contaminant is separated by the membrane into a liquid permeate comprising the aryl group containing 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 the aryl group containing carbonate and contaminant at a concentration which is higher than the contaminant concentration in the feed.

7. Process according to any one of the preceding claims, wherein the aryl group containing carbonate originates from the reaction of an aryl alcohol with a dialkyl carbonate .

8. Process according to any one of claims 2-7, wherein the purified diaryl carbonate is for use as raw material in making a polycarbonate.

9. Process for preparing a diaryl carbonate, comprising reacting an aryl alcohol with a dialkyl carbonate resulting in a stream containing diaryl carbonate and impurities, and recovering diaryl carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used.

10. Process according to claim 9, wherein the stream containing diaryl carbonate and impurities is first distilled in a distillation column into a top stream containing diaryl carbonate and a bottom stream

containing diaryl carbonate and impurities, and diaryl carbonate is recovered from the latter bottom stream by the membrane separation.

11. Process for preparing a diaryl carbonate, comprising reacting an aryl alcohol with a dialkyl carbonate resulting in a stream containing alkyl aryl carbonate and impurities, and recovering alkyl aryl carbonate from said stream by membrane separation, wherein a membrane having an average pore size of from 0 to 10 nm is used, and reacting the recovered alkyl aryl carbonate into diaryl carbonate

12. Process according to any one of claims 9-11, wherein the dialkyl carbonate is a di (C1-C5) alkyl carbonate, such as dimethyl carbonate or diethyl carbonate.

13. Process for making a polycarbonate, comprising reacting a dihydroxy aromatic compound with a diaryl carbonate purified in accordance with the process of any one of claims 2-8 or prepared in accordance with the process of any one of claims 9-12.

14. Process for making a polycarbonate, comprising purifying a diaryl carbonate in accordance with the process of any one of claims 2-8 or preparing a diaryl carbonate in accordance with the process of any one of claims 9-12, and reacting a dihydroxy aromatic compound with the diaryl carbonate thus obtained.

15. Process according to claim 13 or 14, wherein the dihydroxy aromatic compound is bisphenol A.