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Title:
A PROCESS FOR PREPARING DIMETHYL CARBONATE
Document Type and Number:
WIPO Patent Application WO/2020/127287
Kind Code:
A1
Abstract:
In a process for making dimethyl carbonate (DMC) by direct carbonylation, where dimethyl ether (DME) is used as feed, any formation of water is eliminated completely in the oxidative carbonylation step. The carbonylation of DME into DMC is catalyzed using a supported Cu or Pd-Cu catalyst.

Inventors:
JOENSEN FINN (DK)
Application Number:
PCT/EP2019/085682
Publication Date:
June 25, 2020
Filing Date:
December 17, 2019
Export Citation:
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Assignee:
HALDOR TOPSOE AS (DK)
International Classes:
C07C68/01; C07C41/09; C07C43/04; C07C69/96
Domestic Patent References:
WO2004074228A12004-09-02
WO2014144099A12014-09-18
Foreign References:
CN108727194A2018-11-02
JPH1180095A1999-03-23
US4481305A1984-11-06
US9073849B22015-07-07
US5536864A1996-07-16
US5214185A1993-05-25
CN108727194A2018-11-02
JPH1180095A1999-03-23
US20100056831A12010-03-04
US20160362355A12016-12-15
US20120078023A12012-03-29
US4318862A1982-03-09
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Claims:
Claims :

1. A process for preparing dimethyl carbonate (DMC) by direct carbonylation, where dimethyl ether (DME) is used as feed according to the equation

CH3OCH3 + CO + ½ 02 -> OC(OCH3) (1) where any formation of water is eliminated completely in the oxidative carbonylation step.

2. Process according to claim 1, wherein the carbonyl- ation of DME into DMC is catalyzed using a supported Cu or Pd-Cu catalyst at a temperature between 90 and 200°C under a pressure between 5 and 40 bar.

3. Process according to claim 2, wherein the DME feed is generated by dehydration of methanol followed by distil- lative separation of DME.

4. Process according to claim 3, wherein the dehydra tion of methanol is carried out over an acidic catalyst at a temperature between 280 and 360°C.

5. Process according to claim 4, wherein the catalyst is an alumina-based catalyst.

6. Process according to claim 1, wherein DME and CO are generated directly from synthesis gas by using a com bined methanol/DME synthesis.

7. Process according to claim 6, wherein the combined synthesis of methanol/DME from synthesis gas takes place at about 200-350°C under a pressure of 20 to 60 bar using a catalyst comprising mixtures of alumina, silica/alumina and/or zeolites and Cu/Zn/Ai 03-based catalysts.

8. Process according to claim 7, wherein a synthesis gas, in which the ¾/CO ratio is approximately 0.8, is used whereby the DME synthesis proceeds essentially according to the reaction

3H2 + 4CO -> CH3OCH3 + CO + C02 leading to a feed of approximately equimolar amounts of DME and CO for the oxidative carbonylation step.

Description:
Title: A process for preparing dimethyl carbonate

The present invention relates to a process for the prepara tion of dimethyl carbonate (DMC) , which is an organic com pound with the formula OC(OCH 3)2 - It is a colorless liquid classified as a carbonate ester.

More specifically, the invention concerns a process for the preparation of dimethyl carbonate (DMC) by direct carbonyl- ation, where dimethyl ether (DME) is used as feed according to the equation

CH 3 OCH 3 + CO + ½ 0 2 -> OC(OCH 3) (1) where any formation of water is eliminated completely in the oxidative carbonylation step.

Carbon dioxide is one of the gases contributing heavily to global warming, and its concentration in the atmosphere continues to increase. The use of fossil fuel systems ac- counts for 33-40% of the CO 2 emission, and much research has recently been focused on how to utilize the waste CO 2 .

CO 2 is a highly oxidized state of carbon, and it is there fore suitable to use it to produce molecules with high oxi- dation states. As an example of this, organic carbonates can be mentioned, wherein the chemical bonds of the car bonyl carbon are connected to oxygen. Within this group, one interesting chemical is dimethyl carbonate (DMC) . The synthesis of DMC was originally performed using the toxic reagent phosgene (COCI2) , but DMC can in fact be syn thesized through many different routes, among which direct synthesis is one that uses carbon dioxide as reactant.

In one of the routes investigated recently, DMC is produced through direct synthesis from methanol and CO2 according to the equation

2CH3OH + C0 2 <-> OC(OCH 3 ) ( DMC) + H 2 0

This direct synthesis has been considered promising due to the use of simple, less toxic reactants and the possibility to fixate carbon dioxide by converting it into valuable products. The reaction is exothermic, but under the usual conditions it is not spontaneous. Currently there are no commercial processes using this direct method to produce DMC, but much research has been made on direct synthesis of DMC and on developing effective catalysts (see e.g. US

9,073,849 B2) . However, the methanol conversion is quite limited by the thermodynamic equilibrium, and the typical conversions are only 1-2% in the absence of dehydrating agents .

According to le Chatelier' s principle, water can be removed in order to further increase the production of DMC. This removal can be performed in a separate unit with a recycle or integrated in a sorption-enhanced reactor. Because the equilibrium constant for direct synthesis of DMC is so low, water has to be removed down to small mole fractions. The currently used processes for the synthesis of DMC rely on oxidative carbonylation of methanol (the EniChem pro cess) : 2CH 3 OH + CO + ½ 0 2 -> OC(OCH 3 ) 2 (DMC) + ¾0 (2)

This process, which is described i.a. in US 5,536,864 A, is catalyzed by CuCl . It generates equimolar amounts of DMC and water, the latter resulting in a low conversion per pass, azeotrope formation and a strongly corrosive environ ment .

A competing process launched by Ube Industries (described in US 5,214,185 A) circumvents some of these drawbacks by avoiding the formation of water in the carbonylation step.

This is done by conducting the oxidation of methanol by NO and oxygen (to form methyl nitrite) in a separate step in which a CuCl/Pd catalyst is used. The current processes for the preparation of dimethyl oxa late (DMO), which is a precursor for (mono ) ethylene glycol (MEG) , and dimethyl carbonate (DMC) , both rely on oxidative carbonylation of methanol. The technology is dominated by the direct carbonylation route (the EniChem process) for DMC and the Ube process, involving methyl nitrite as an in termediate, for both DMC and DMO synthesis.

The DMC process and the DMO process proceed according to

2CH 3 OH + CO + ½ 0 2 -> OC(OCH 3 ) 2 (DMC) + ¾0 (3) 2CH 3 OH + 2 CO + ½ 0 2 -> (OCOCH 3 ) 2 (DMO) + ¾0 (4) respectively, where the latter may be hydrogenated to form MEG:

(OCOCH 3 ) + 4 H 2 -> (CH 2 OH) 2 + 2CH 3 OH (5)

Thereby, the net process (equations (4) and (5) in combina tion) becomes an all-syngas based route, in which methanol is merely an intermediate:

2CO + 4H 2 + ½ 0 2 -> (CH 2 OH) 2 + ¾0 (6)

The EniChem process only applies to DMC production, and it suffers from low conversion, azeotrope formation and a very corrosive environment. In the Ube process, both for DMC and DMO, these problems are, at least partly, circumvented by avoiding the formation of water in the carbonylation step by conducting the oxidation of methanol by NO and oxygen (to form methyl nitrite) in a separate step. Thus, the ad vantages of the Ube process are, in part, paid for by a more complex synthesis loop.

The EniChem process [(3) -(5)] involves direct oxidative carbonylation by CO in a CuCl ( copper ( I ) chloride ) slurry process to produce DMC. The process was commercialized in 1983 with the set-up of a 5500 t/a plant. The overall pro cess suffers from low conversion per pass (approx. 20%) due to water inhibition (max. 3% H 2 0) , corrosiveness and azeo trope formation. The process is one-pot (120-140°C; 20-30 bar) , applying a suspension of CuCl contained in a boiling, self-circulating U-shaped reactor. Ube Industries developed a vapor phase process [(3), (6),

(7), (8)] separating the oxidation and carbonylation steps by first oxidizing methanol by NO and oxygen to form methyl nitrite (7) and subsequently carbonylating methyl nitrite with release of NO (8) :

2CH 3 OH + 2NO + ½ 0 2 -> 2CH 3 ONO + ¾0 (non-catalytic) (7)

2CH 3 ONO + CO -> OC(OCH 3)2 + 2NO (8) applying a supported PdCl 2 /CuCl catalyst. The main ad vantage of the Ube process is that it takes place in the gas phase under anhydrous conditions, allowing for close to 100% conversion per pass and, most likely, with much less severe corrosion problems than in the EniChem process. It also simplifies separation by avoiding the formation of DMC-Me0H-H 2 0 azeotrope.

In a closely related process also developed by Ube, a very similar chemistry takes place, in which a (supported) cata- lyst consisting of Pd(0) only (i.e. comprising no CuCl) is used. This process is promoting double carbonylation at Pd and subsequent C-C coupling to produce DMO:

2CH 3 OH + 2NO + ½ 0 2 -> 2CH 3 ONO + ¾0 (non-catalytic) (7)

2CH 3 ONO + 2CO -> (OCOCH 3)2 + 2NO (9)

Other prior art documents within the field include the fol lowing : CN 108727194 A describes a method for synthesizing dimethyl carbonate by reacting dimethyl ether (DME) with carbon mon oxide and oxygen in a ratio of 2:2:1 at a temperature of 120 ° C and a pressure of 2 MPa using a Pd-Cu/Cei x Zr x C>2 cata lyst. DME is obtained from synthesis gas carrying out the reaction at a temperature of 200-280 °C and a pressure of 2- 4 MPa using a Cu-Zn-Mn/HZSM-5 catalyst.

In JP (H) 1180095 A, a method for synthesizing dimethyl carbonate is described, in which DME is reacted with CO and oxygen in the presence of a copper and a platinum catalyst according to the equation

CH 3 OCH 3 + CO + ½ o 2 -> OC(OCH 3 )

The reaction temperature is 120 °C, and the pressure is 30 kg/cm 2 .

WO 2014/144099 A1 describes a method for generating DME by dehydration of methanol over a Cu-modified alumina catalyst at a temperature between 50 and 300°C followed by distilla tion, and in US 2010/0056831 A1 a method for generating DME by dehydration of methanol over an alumina catalyst at a temperature between 300 and 480°C is described.

From US 2016/0362355 A1 , a method for producing DME from syngas is known, said method using a H2/CO molar ratio of 0.7-2.5, a reaction temperature of 200-300°C and a reaction pressure of 30-60 atm. And, finally, a method for producing a mixture of DME and methanol from syngas is known from US 2012/0078023 A1 , said method using a catalyst which

comprises alumina combined with silica-alumina at a pres sure of at least 4 MPa and a temperature of 260°C. The process according to the present invention involves a direct carbonylation, in which dimethyl ether (DME) is used instead of methanol: CH 3 OCH 3 + CO + ½ 0 2 -> OC(OCH 3) (1)

Using this process, the formation of water in the oxidative carbonylation step is eliminated completely, providing the advantages of a high conversion per pass, less or even no corrosion issues and no problems with azeotrope formation.

Studies by Amoco published in 1993, in which the EniChem catalyst was employed, concluded that CuCl 2 supported on activated carbon and B-ZSM-5 were both inactive for oxida- five carbonylation of DME, but - by changing to a bimodal carbon support, increasing the Cu loading and feeding a mixture of methanol and DME - high levels of conversion, i.e. more than 70%, could be obtained with methylal (di- methoxymethane ; CH 2( OCH 3)2) as the primary byproduct.

A process according to equation (1) warrants essentially the same advantages as the methyl nitrite based DMC process by Ube in terms of an anhydrous carbonylation step. Indeed, the Ube process is elegant in its chemistry, but it does take a rather complex process to eliminate water in the carbonylation. The simplicity in making DMC from DME in stead of methanol would provide a much less complex layout, simply by introducing a MeOH-to-DME synthesis step in front of the DMC synthesis. Thus, in one embodiment of the pro cess of the invention, shown in Fig. 1, the DME feed may be generated by dehydration of methanol, followed by distilla- tive separation of DME. Alternatively, a direct synthesis of DME from a coal gas having a H 2 /CO ratio of about 0.8, leaving essentially over one mole of CO for downstream carbonylation, would make the DMC synthesis virtually self-sustaining. Thus, in another embodiment, shown in Fig. 2, DME and CO may be generated directly from synthesis gas by making use of a combined methanol/DME synthesis. In this case it is particularly ad vantageous to apply a synthesis gas having a ¾/CO ratio of about 0.8, whereby the DME synthesis takes place essen- tially according to the equation

3H 2 + 4CO -> CH 3 OCH 3 + CO + C0 2

This way, a feed is obtained, which consists of approxi- mately equimolar amounts of DME and CO for the oxidative carbonylation step plus C0 2 , which serves as a diluent and heat sink in the exothermic DMC synthesis. As opposed to the Ube process, CO purity is not an issue in the direct carbonylation. In fact, in 1982, EniChem specifically pa- tented the use of synthesis gas in the carbonylation reac tion (US 4,318,862) . Therefore, the layout shown in Fig. 2 may entail a significant cost reduction by eliminating the need for a separate CO generation and purification. DME may be synthesized by dehydration of methanol over an acidic catalyst, typically an alumina-based acidic cata lyst, at temperatures between 280 and 360°C.

In the alternative embodiment according to the present in- vention, the combined synthesis of methanol/DME from syn thesis gas typically takes place at about 200-350°C at a pressure of 20 to 60 bar over catalysts comprising mixtures of alumina, silica/alumina and/or zeolites and Cu/Zn/A^Cd- based catalysts.

The carbonylation of DME into DMC is catalyzed using a sup- ported Cu or Pd/Cu catalyst at a temperature between 90 and 200°C under a pressure, which is preferably between 5 and 40 bar.