Process for preparing diester of carbonic acid

Abstract: There is disclosed a process for preparing a diester of carboxylic acid which comprises bringing carbon monoxide into contact with a nitrite in a vapor phase in the presence of a solid catalyst having (a) a platinum group metal or a compound thereof and (b) at least one selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin carried on a carrier.

Description: This invention relates to a process for preparing a diester of carbonic acid. More particularly, it relates to a process for preparing a diester of carbonic acid which comprises bringing carbon monoxide into contact with an ester of nitrous acid selectively. A diester of carbonic acid is an extremely available compound as an organic synthesis starting material for medicine and herbicides and also as an intermediate for preparation of a polycarbonate or urethane. As a process for preparing a diester of carbonic acid, there have been practiced a method of reacting phosgene and an alcohol from extremely old times, but phosgene has extremely high toxicity and hydrochloric acid is by-produced by the reaction of phosgene and an alcohol so that great care must be taken for a material to be used as an apparatus. Thus, it has been desired to develop a method using no phosgene. Accordingly, a method for synthesizing a diester of carbonic acid from an alcohol and carbon monoxide has been researched various fields as a process of using no phosgene, and has been proposed (for example, Japanese Provisional Patent publications No. 75447/1985, No. 72650/1988 and Japanese Publication No. 38018/1988, and WO-87/7601). These are methods in which a diester of carbonic acid is synthesized by oxygen oxidation reaction of an alcohol and carbon monoxide using copper halide or palladium halide as a catalyst, but involve the problems that a selectivity of a diester of carbonic acid based on carbon monoxide is low since carbon dioxide is by-produced and purification of a diester of carbonic acid is complicated due to formation of water. Thus, as a method for improving the conventional ones, it has been proposed, in Japanese Provisional Patent Publication No. 181051/1985, a method of preparing a diester of carbonic acid which comprises reacting a nitrite and carbon monoxide are reacted in a vapor phase in the presence of a solid catalyst in which a platinum group metal or a compound thereof is carried on a carrier, and an oxidizing agent with an amount of 10 mole % or more as O 2 per carbon monoxide. However, nevertheless an oxidizing agent such as oxygen is co-existed with such a ratio to carbon monoxide in order to prevent by-production of a diester of oxalic acid, a significant amount of a diester of oxalic acid is by-produced so that a selectivity of a diester of carbonic acid is low and also the reaction rate is not sufficient. Also, a used range of a nitrite in "a mixed gas comprising a nitrite, carbon monoxide, an alcohol and oxygen" exceeds an explosion limit which is a problem in safety whereby the method is not an industrially sufficient one. Also, methods for producing dimethyl carbonate by subjecting carbon monoxide and methyl nitrite to a vapor phase contact reaction in the presence of a catalyst in which a platinum group metal or a compound thereof, and lithium, copper or iron are carried on an activated charcoal are disclosed in Chin. Sci. Bull., 34(1989), 875 and Z. f. Kat. Forsch. (China), 10(1989), 75. However, in these methods, there is a problem that catalyst activities are gradually decreased. The process for preparing a diester of carbonic acid using a nitrite conventionally known is insufficient in reaction rate between carbon monoxide and a nitrite as mentioned above, and also a selectivity of a diester of carbonic acid is low so that there is a defect that a purification processing of the diester of carbonic acid formed became complicated. Further, a used concentration range of nitrite in the reaction system exceeds an explosion limit so that there is a problem of accompanying dangerous in operations. SUMMARY OF THE INVENTION An object of the present invention is to provide an industrially suitable process for preparing a diester of carbonic acid which can produce a diester of carbonic acid under gentle reaction conditions with high selectivity and high yield. Another object of the present invention is to provide a process for preparing a diester of carbonic acid with high selectivity and high yield while maintaining an activity of the solid catalyst for a long period of time. The present inventors have intensively investigated concerning a catalyst and the like when preparing a diester of carbonic acid by the vapor phase contact reaction of carbon monoxide and a nitrite in the synthetic reaction of a diester of carbonic acid using a nitrite in order to overcome the problems in the conventional preparing process of a diester of carbonic acid as mentioned above, and consequently found that a diester of carbonic acid which is the title compound can be obtained when using a solid catalyst as mentioned below under gentle or mild reaction conditions whereby accomplished the present invention. That is, the present invention is a process for preparing a diester of carbonic acid which comprises bringing carbon monoxide into contact with a nitrite (an ester of nitrous acid) in a vapor phase in the presence of a solid catalyst having (a) a chloride of a platinum group metal and (b) at least one compound of a metal selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin carried on a carrier; characterised in that said process is carried out in the presence of a minute amount of hydrogen chloride which is supplied continuously to the reaction system, in which the minute amount of hydrogen chloride is 1 to 50 mole % per unit time (hr) based on the platinum group metal in the catalyst. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, the present invention is to be described in more detail. As a nitrite to be used in the present invention, there may be suitably mentioned a nitrite of a lower aliphatic monohydric alcohol having 1 to 4 carbon atoms such as methyl nitrite, ethyl nitrite, n- (or iso)propyl nitrite, n-(or iso)butyl nitrite and sec-butyl nitrite; a nitrite of an alicydic alcohol such as cyclohexyl nitrite; a nitrite of an aralkyl alcohol such as benzyl nitrite and phenylethyl nitrite; and the like, but preferably a nitrite of a lower aliphatic monohydric alcohol having 1 to 4 carbon atoms, particularly preferably methyl nitrite and ethyl nitrite. Also, the solid catalyst to be used in the present invention is (a) a chloride of a platinum group metal such as palladium, platinum, iridium, ruthenium and rhodium, and (b) at least one compound of a metal selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin, carried on a carrier. As the chloride of the platinum group metal, there may be specifically mentioned palladium chloride, platinum chloride, iridium chloride, ruthenium chloride and rhodium chloride, particularly preferably a chloride of palladium, ruthenium or rhodium, most preferably palladium chloride. Of course, the "chloride of the platinum group metal" in the present invention is not limited to those as mentioned above, but it may be "a platinum group metal or a compound thereof as mentioned above" which is capable of forming a complex in which the above chloride or chlorine participate the reaction under the presence of hydrogen chloride. As the above compound of a metal selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin, there may be suitably mentioned, for example, halides such as chloride, bromide, iodide and fluoride, nitrates, sulfates, phosphates and acetates of said metals, and of these, halides of said metals are particularly suitable. As a carrier to be carried the platinum group metal chloride and at least one of the compound of metal selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin, there may be suitably mentioned, for example, diatomaceous earth, activated charcoal, silicon carbide, titania and alumina, and activated charcoal is particularly preferred. A method of carrying the aforesaid platinum group metal chloride, and the compound of metals such as iron, copper, bismuth, cobalt, nickel and tin on the carrier is not specific one but it may be carried out by the conventionally practiced method such as the impregnating method (dipping adsorption method), the kneading method, the precipitation deposition method and the co-precipitation method, but in the present invention, it is desired to prepare by the impregnating method or the kneading method. A carried amount of the above platinum group metal chloride to be carried on a carrier is generally 0.1 to 10 % by weight, particularly preferably 0.5 to 2 % by weight in terms of a metal of the platinum group metal chloride. Also, a carried amount of the compound of the metal such as iron, copper, bismuth, cobalt, nickel and tin to be carried on a carrier is generally 1 to 50 gram atomic equivalents, preferably 1 to 10 gram atomic equivalents in terms of an amount of these metals. Also, in the present invention, the above catalyst can be used in the form of powder or granule, and a particle size thereof is not particularly limited, but it is preferred to use powder having 20 to 100 µm generally employed and granule with 4 to 200 mesh generally employed. Hydrogen chloride to be used in the present invention is preferably an anhydrous one. The method in which a minute amount of hydrogen chloride is added into the reaction system continuously is to be explained. An amount of hydrogen chloride is 1 to 50 mole %, more preferably 5 to 20 mole % per unit time (hr) based on the platinum metal in the catalyst. This amount corresponds to a chlorine content dissipated from the catalyst during the above reaction of carbon monoxide and a nitrite. If the amount is larger than the above range, the above reaction is inhibited due to excess adsorption of hydrogen chloride to the catalyst. If it is specifically mentioned by referring to the actual reaction example, for example, when the starting materials are reacted in a fixed bed reactor with a gas space velocity (GHSV) of 3000 hr -1 it is preferred to add 10 to 500 volume ppm, more preferably 20 to 100 volume ppm of hydrogen chloride to a feed gas and supply it to the catalyst bed continuously to supplement the chlorine content dissipated from the catalyst. Further, it is one of the characteristic features of the present invention that the contact reaction of carbon monoxide and a nitrite can be carried out under extremely gentle conditions. For example, the reaction can be carried out at a temperature of 0 to 200 °C, preferably 50 to 120 °C under normal pressure. Of course, it can be carried out under pressurized system and can be carried out at a pressure of 1 to 20 kg/cm 2 G and a temperature of 50 to 150 °C. In the present invention, the aforesaid starting material nitrite can be easily synthesized by, for example, generating a mixed gas of nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ) by nitric acid or sulfuric acid decomposition of an aqueous sodium nitrite, then making nitrogen dioxide (NO 2 ) by oxidizing part of NO in the mixed gas with molecular oxygen to obtain NO x gas with NO/NO 2 = 1/1 (volume ratio), and then contacting an alcohol thereto. When considering the synthesis of the nitrite, it is particularly preferred that the above contact reaction of carbon monoxide and the nitrite is carried out under a slightly pressured condition of 2 to 3 kg/cm 2 G or so. The above reaction system of carbon monoxide and the nitrite may be carried out either the batch system or the continuous system in the vapor phase, but the continuous system is industrially more advantageous. Also, as the existing form of the catalyst in the system, the reaction may be carried out by using either of fixed bed, moving bed or fluidized bed reactor. In the present invention, it is desired to feed carbon monoxide and a nitrite as the starting gases by diluting with an inert gas such as nitrogen gas to the aforesaid reactor, and the composition thereof is not particularly limited from the view point of the reaction. However, from the view point of safety, a concentration of the above nitrite is preferably 20 % by volume or less, more preferably 5 to 20 % by volume. Accompanying this, a concentration of carbon monoxide is preferably in the range of 5 to 20 % by volume from economical viewpoint. That is, considering an industrial production process, it is preferred to circulate gases such as carbon monoxide and a nitrite to be supplied to the reaction system, and purge part of said circulating gas out of the system. Also, from the fact that a conversion of carbon monoxide per one pass is 20 to 30 %, even when the concentration of carbon monoxide is heightened than 20 % by volume, no merit can be obtained and loss is increased, while it is made lower than 5 % by volume, productivity is lowered. However, when ignoring economic problem, the concentration of carbon monoxide can be made by 80 % by volume. That is, the nitrite may be fed by diluting with carbon monoxide instead of the above inert gas. Accordingly, a used ratio of carbon monoxide and a nitrite is preferably in the range of 0.1 to 10 mole, more preferably 0.25 to 1 mole of carbon monoxide per mole of the nitrite. In the present invention, a space velocity of a gas containing carbon monoxide and a nitrite to be fed in the above reactor is preferably in the range of 500 to 20000 hr -1 , more preferably 2000 to 5000 hr -1 . Further, in the process of the present invention, the aforesaid nitrite decomposes to produce nitrogen monoxide (NO) after participating the reaction. It is preferred that the NO is recovered from a reaction gas led out from the above reactor, reacted with oxygen and an alcohol corresponding to the above nitrite to convert into a nitrite again and circulated. Thus, a reaction gas containing, in addition to a diester of carbonic acid which is the title product, a byproduct such as a diester of oxalic acid, unreacted carbon monoxide and nitrite, nitrogen monoxide, carbon dioxide and an inert gas is led out from the reactor, and after cooling the reaction gas, uncondensed gases such as carbon monoxide, a nitrite, nitrogen monoxide, carbon dioxide and an inert gas are circulated again in the reactor while purging part thereof, and a diester of carbonic acid is separated and purified from a condensate by the conventional method such as distillation. A nitrite of the starting material can be generally prepared by reacting an alcohol and a nitrogen oxide in the presence of molecular oxygen, if necessary, as mentioned above, and unreacted alcohol and nitrogen oxide (particularly nitrogen monoxide), and a minute amount of water and oxygen depending on the cases, are contained in the prepared gas in addition to the nitrite. In the present invention, even when such a nitrite-containing gas is used as a nitrite source, good results can be obtained. In the present invention, by carrying out the treatments as mentioned above, lowering in activity of the catalyst in the conventional method can be prevented and production of a diester of carbonic acid by the reaction of carbon monoxide and a nitrite can be continued with high yield and high selectivity, and also with stable conditions for a long period of time. EXAMPLES In the following, the process of the present invention will be explained specifically by referring to Examples, Reference Examples and Comparative examples, but the present invention is not limited by these. A space time yield (STY) Y (g/ℓ·hr) in each Example and Comparative example is calculated from the following equation: Y = a/(b x ) wherein  (hr) is a contact reaction time of carbon monoxide and a nitrite, a (g) is an amount of a diester of carbonic acid and b (ℓ) is a filled amount of the catalyst to the reactor. Also, a selectivity X (%) in each Example and Comparative example is calculated by using carbon monoxide to be supplied as standard, and obtained from the following formula by making amounts of a diester of carbonic acid, a diester of oxalic acid and carbon dioxide formed by the above  (hr) as c (mole), d (mole) and e (mole), respectively. X = {c/(c + 2 x d + e)} x 100 Reference Example 1 (Preparation of a catalyst) In 100 ml of 5N hydrochloric acid were dissolved 0.35 g of palladium chloride and 0.34 g of cupric chloride (dihydrate), and after dipping 10 g of granular activated charcoal therein, the mixture was filtered and the filtrate was washed with water and dried at 100 °C to prepare a catalyst (PdCl 2 -CuCl 2 /C) carried palladium chloride and cupric chloride on the activated charcoal. (Preparation of a diester of carbonic acid) The above catalyst (7 ml) was filled in a vapor phase reactor (attached with an outer jacket) having an inner diameter of 20 mm, and then the reactor was fixed vertically and a heating medium was circulated in the Jacket of the reactor to control a temperature in a catalyst bed to 100 °C. From the upper end of the reactor, a mixed gas comprising a gas containing methyl nitrite synthesized from nitrogen monoxide, oxygen and methanol, and carbon monoxide, i.e. a mixed gas comprising a composition of 15 % by volume of methyl nitrite, 10 % by volume of carbon monoxide, 3 % by volume of nitrogen monoxide, 6 % by volume of methanol and 66 % by volume of nitrogen was supplied with a space velocity (GHSV) of 3000 hr -1 to subject the reaction under normal pressure. Then, the reaction product passed through the reactor was captured by passing through an Ice-cold methanol. The captured solution thus obtained was analyzed by gas chromatography to give the results that di methyl carbonate was formed with STY of 220 g/ℓ·hr and a selectivity of 96 %. Also, dimethyl oxalate and methyl formate were confirmed to be present as by-products. Comparative example 1 (Preparation of a catalyst) In 100 ml of 5N hydrochloric acid was dissolved 0.35 g of palladium chloride, and after dipping 10 g of granular activated charcoal therein, the mixture was filtered and the filtrate was washed with water and dried at 100 °C to prepare a catalyst (PdCl 2 /C) carried palladium chloride alone on the activated charcoal. (Preparation of a diester of carbonic acid) Dimethyl carbonate was prepared in the same manner as in Example 1 except for using the above catalyst. The captured solution thus obtained was analyzed by gas chromatography to give the results that di methyl carbonate was formed with STY of 120 g/ℓ·hr and a selectivity of 90 %. Comparative exemple 2 (Preparation of a diester of carbonic acid) Dimethyl carbonate was prepared by carrying out the reaction of carbon monoxide and methyl nitrite in the same manner as in Reference Example 1 except that the catalyst prepared in Comparative example 1 was used, and a composition of a mixed gas supplied from the upper end of the reactor was 15 % by volume of methyl nitrite, 10 % by volume of carbon monoxide, 2 % by volume of oxygen, 6 % by volume of methanol and 67 % by volume of nitrogen. The captured solution thus obtained was analyzed by gas chromatography to give the results that dimethyl carbonate was formed with STY of 90 g/ℓ·hr and a selectivity of 72 %. Examples 2 to 8 (Preparation of a catalyst) In each Example, "a catalyst having a platinum group metal compound and at least one compound of a metal selected from iron, copper and bismuth carried thereon" shown in Table 1 was prepared in accordance with the method in Example 1. Carried amounts of the platinum group metal compound in these catalysts are all 1 % by weight as a platinum group metal based on the carrier. (Preparation of a diester of carbonic acid) In each Reference Example, dimethyl carbonate in Reference Examples 2 to 7 and diethyl carbonate in Reference Example 8 were prepared in the same manner as in Reference Example 1 except that the above catalysts were used, and in Reference Example 8, a mixed gas of a gas containing ethyl nitrite synthesized from nitrogen monoxide, oxygen and ethanol, i.e. a composition of a mixed gas of 15 % by volume of ethyl nitrite, 10 % by volume of carbon monoxide, 3 % by volume of nitrogen monoxide, 6 % by volume of methanol and 66 % by volume of nitrogen was supplied from the upper end of the reactor. The results are shown in Table 1. Example 9 After filling 7 ml of a catalyst (PdCl 2 -BiCl 3 /C) in which PdCl 2 and BiCl 3 were carried on a granular activated charcoal in a vapor phase reactor (attached with an outer jacket) having an inner diameter of 20 mm, and then the reactor was fixed vertically and a heating medium was circulated in the jacket of the reactor to control a temperature in a catalyst bed to 100 °C. From the upper end of the reactor, a mixed gas comprising a gas containing methyl nitrite synthesized from nitrogen monoxide, oxygen and methanol, and carbon monoxide, i.e. a mixed gas comprising a composition of 15 % by volume of methyl nitrite, 10 % by volume of carbon monoxide, 3 % by volume of nitrogen monoxide, 6 % by volume of methanol, 100 ppm by volume of hydrogen chloride (corresponding to 32 mole % of hydrogen chloride per unit time (hr) based on the platinum group metal) and 66 % by volume of nitrogen was supplied with a space velocity (GHSV) of 3000 hr -1 to subject the reaction under normal pressure. Then, the reaction product passed through the reactor was captured by passing through an ice-cold methanol. The captured solution thus obtained was analyzed by gas chromatography. As the results, while STY of dimethyl carbonate was 500 g/ℓ·hr at an initial stage, it was lowered to 350 g/ℓ·hr at after 10 hours. However, it was not changed as 350 g/ℓ·hr by 100 hours from the initiation of the reaction, which was the completion of the reaction. Also, a selectivity of dimethyl carbonate based on carbon monoxide was 97 % at an initial stage and it was lowered to 92 % at after 10 hours from the initiation of the reaction by increase of dimethyl oxalate which is a by-product, but it was substantially the same by the completion of the reaction (100 hours after initiation of the reaction). The process of the present invention is, as described above, to provide a process which can prepare a diester of carbonic acid with high selectivity, yield and remarkably elongated lifetime under gentle conditions without accompanying any dangerous operation by reacting carbon monoxide and a nitrite in the presence of a solid catalyst having a chloride of a platinum group metal and at least one compound of a metal selected from the group consisting of iron, copper, bismuth, cobalt, nickel and tin as a second component carried on a carrier under conditions of low temperature and low pressure and subjecting to vapor phase reaction in the presence of a minute amount of hydrogen chloride supplied continuously to the reaction, said minute amount of hydrogen chloride being 1 to 50 mole % per unit time (hr) based on the platinum group metal in the catalyst, whereas the conventional process of a diester of carbonic acid by a vapor phase contact reaction of carbon monoxide and a nitrite involves the defects that the reaction rate is not sufficient, the selectivity of the diester of carbonic acid is low and separating and purifying operations of the diester of carbonic acid from the reaction product becomes complicated, and also involves the problem that a dangerous operation is involved since the concentration range of the nitrite used exceeds explosion limit. Also, as pompared with the conventionally known liquid phase process, the process of the present invention is carried out in a vapor phase so that it is not necessary to separate the catalyst from the reaction mixture and no dissolution of metal component from the catalyst whereby separation and purification of the diester of carbonic acid from the reaction mixture is easy, and thus, the process of the present invention has an effect of having high superiority in production of an industrial scale.

Claims: A process for preparing a diester of carbonic acid which comprises bringing carbon monoxide into contact with a nitrite in a vapor phase in the presence of a solid catalyst having (a) a chloride of a platinum group metal and (b) at least one compound of a metal selected,from the group consisting of iron, copper, bismuth, cobalt, nickel and tin carried on a carrier; characterised in that said process is carried out in the presence of a minute amount of hydrogen chloride which is supplied continuously to the reaction system, in which the minute amount of hydrogen chloride is 1 to 50 mole % per unit time (hr) based on the platinum group metal in the catalyst. The process according to Claim 1, wherein said nitrite is selected from the group consisting of a nitrite of a lower aliphatic monohydric alcohol having 1 to 4 carbon atoms, a nitrite of an alicyclic alcohol and a nitrite of an aralkyl alcohol. The process according to Claim 2, wherein said nitrite is selected from the group consisting of methyl nitrite, ethyl nitrite, n-propyl nitrite, isopropyl nitrite, n-butyl nitrite, isobutyl nitrite, sec-butyl nitrite, cyclohexyl nitrite, benzyl nitrite and phenylethyl nitrite. The process according to Claim 2, wherein said nitrite is a nitrite of a lower aliphatic monohydric alcohol having 1 to 4 carbon atoms. The process according to Claim 4, wherein said nitrite is methyl nitrite or ethyl nitrite. The process according to Claim 5, wherein an amount of the catalyst component (a) is 0.5 to 2 % by weight. The process according to Claim 1, wherein said chloride is selected from the group consisting of palladium chloride, platinum chloride, iridium chloride, ruthenium chloride and rhodium chloride. The process according to Claim 1, wherein the reaction is carried out at a temperature of 50 to 120 °C under normal pressure. The process according to Claim 1, wherein the reaction is carried out at a temperature of 50 to 150 °C under a pressure of 1 to 20 kg/cm 2 G. The process according to Claim 1, wherein carbon monoxide and a nitrite are fed in the vapor phase reaction system with a concentration of carbon monoxide of 5 to 20 % by volume and a concentration of the nitrite of 20 % by volume or less. The process according to Claim 10, wherein a ratio of carbon monoxide and a nitrite is 0.1 to 10 mole of carbon monoxide per mole of the nitrite. The process according to Claim 11, wherein a space velocity of a gas containing carbon monoxide and a nitrite to be fed in the reaction system is 500 to 20000 hr -1 . The process according to Claim 1, wherein a catalyst component (b) is halides, nitrates, sulfates, phosphates, or acetates of said metals. The process according to Claim 1, wherein hydrogen chloride is added in an amount from 10 to 500 ppm by volume at a gas space velocity of the starting gas of 3000 hr -1 .

Inventors: MIZUTARE KATSUHIKO, NISHIHIRA KEIGO, TANAKA SHUJI, MIZUTARE, KATSUHIKO, NISHIHIRA, KEIGO, TANAKA, SHUJI, MIZUTARE, KATSUHIKO, C/O UBE CHEMICAL FACTORY, NISHIHIRA, KEIGO, C/O UBE CHEMICAL FACTORY, TANAKA, SHUJI, C/O UBE CHEMICAL FACTORY
Applicants: UBE INDUSTRIES, UBE INDUSTRIES, LTD.
Application year: 1990
Main classification: C07C69
IPC codes: C07C68, C07C69