JPH0247454B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for continuous production of methyl formate by liquid phase oxidation of methanol. In recent years, formic acid has assumed some importance in agriculture, particularly as a storage pasture additive and preservative. One source of formic acid was a by-product of acetic acid production by oxidation of paraffinic hydrocarbon feedstocks. With the advent of a process for the exclusive production of acetic acid by carbonylation of methanol, a need arose for a process for the production of formic acid to meet market demands. One possible route to formic acid is the hydrolysis of methyl formate. U.S. Pat. No. 4,126,748 discloses that each 1-
A first straight-chain alcohol having 4 carbon atoms or a mixture of such alcohols, or a mixture of an aldehyde and a first straight-chain alcohol, is treated with molecular oxygen at elevated temperature to produce (a) Co in the liquid phase; Mn,
a catalyst which is a solution of a compound of Cr or Fe, and
(b) A method for preparing carboxylic acid esters by catalytic oxidation in the presence of an acid with a first-order dissociation constant k ₁ greater than 10 ⁻³ is described. When the alcohol is methanol, the product is methyl formate. Suitable inorganic acids with a dissociation constant greater than 10 ⁻³ such as HCl, HNO ₃ , HClO ₄ , H ₂ SO ₄ , HBr,
including H ₃ PO ₄ , HI, etc. and methanesulfonic acid,
It is said to be an organic acid such as parachlorobenzoic acid, oxalic acid, trichloroacetic acid, etc. Metal compounds are said to be catalytically active in very small amounts, and concentrations from 0.2 to 100 ppm calculated as metal are commonly used. Catalytically active metal concentrations have been described to result from corrosion of the reaction vessel even in vessels made of rust-free steel. The inventors have found that methanol can be continuously oxidized to methyl formate in the presence of a chromium catalyst in the absence of an acid with a first-order dissociation constant greater than 10 ⁻³ . Additionally, the inventors have demonstrated that even very low concentrations of soluble chromium compounds resulting from corrosion of the vessel can give significant oxidation rates when carrying out this process in a rust-proof steel reaction vessel in the absence of such acids. I found out. The invention is therefore characterized by the use of soluble chromium compounds as oxidation catalysts in a process for the continuous production of methyl formate by continuously reacting methanol with molecular oxygen in the presence of an oxidation catalyst at high temperatures in the liquid phase. provide a method. Methanol is a readily available industrial product and is generally produced from synthesis gas, a mixture of carbon monoxide and hydrogen. Although it is desirable to use substantially anhydrous methanol, water may be present in amounts up to 20% W/W. Molecular oxygen can suitably be supplied, for example, in the form of air or a mixture of gases containing more or less molecular oxygen than air. Suitable soluble chromium compounds include salts, in which the metal is added as anions, such as sodium dichromate, or as cations, such as nitrates, sulphates and chlorides, complex compounds and other metal compounds, such as chromium trioxide. Exists in any of the following. Chromium compounds calculated as metal and based on the weight of methanol supplied 0.05
present in an amount ranging from 0.5 to 100 ppm, preferably from 0.5 to 10 ppm. Chromium compounds will form due to corrosion if the reactor vessel in which the loading or oxidation is carried out is made of rustproof steel or other chromium-containing alloys. Preferably the chromium compound is supplied in the form of an aqueous or methanolic solution. Elevated temperatures suitably range from 50° to 250°C, preferably from 100° to 200°C.
The pressure used must be sufficient or greater to maintain the reaction mixture in the liquid phase at the prevailing reaction temperatures. This method can be carried out in any type of known apparatus for achieving gas/liquid reactions. An example of a suitable form of equipment is an upright tower containing a coaxial draught tube open at both ends.
The column is continuously filled with liquid reactant to at least the height of the top of the draft tube, and the gas is introduced through a evacuator located outside the draft tube and close to the bottom. The gas and liquid mixture rises until it is level with the top of the draft tube, where the gas is released from the liquid and the liquid descends through the draft tube to the bottom of the reactor, thereby providing continuous circulation of the reaction mixture. Establish. The liquid containing the reaction products is removed from the bottom area of the column at a rate equivalent to maintaining a constant liquid level in the column. In some cases, a portion of the concentrate is recovered and classified into liquid products.
Catalyst feed and stream recirculation and provisions for gaseous overhead are also provided. The device is preferably fabricated from rust-free steel or titanium. Continuous oxidation can be initiated by various methods. To avoid prolonging the induction time before the start of oxidation, in the presence or absence of methanol, the substance(s) that is more susceptible to oxidation than methanol alone is first charged and then, after the start of oxidation, It is preferable to start oxidation by supplying methanol when there are no substances susceptible to oxidation. The oxidation can thus be started initially by charging paraffinic hydrocarbons. The paraffinic hydrocarbon is preferably a paraffin or a mixture of such paraffins having from 4 to 8, more preferably from 5 to 7 carbon atoms in the molecule.
The paraffins are preferably straight chain paraffins and are used alone or in mixtures with branched chains and cycloparaffins. Paraffin distillates, especially 100
Those having a boiling point not exceeding .degree. C. can be suitably used. Particularly preferred is the use of fractions having a boiling point range of about 15° to about 95°C. Alternatively, the oxidation can be initiated by charging the product of a previous methanol oxidation or a synthetic composition corresponding to such a product, optionally with additional methanol. Other compounds which are more susceptible to oxidation than methanol and which can be used as oxidation products of the previous methanol in the first charge or added to the methanol are, for example, ethanol and 2-butane. Preferably initiation is achieved in the presence of a soluble chromium compound. The soluble chromium compound is preferably present as an oxidation initiator at a high concentration, eg up to 200 ppm higher than that used during the subsequent continuous methanol oxidation reaction. In addition to methyl formate, various proportions of by-products will be formed, such as dimethoxymethane (an acetal produced by the reaction of formaldehyde and methanol), water, formaldehyde, and formic acid. After separating the methyl formate from the liquid product, for example by distillation, it may be desirable to recycle at least some by-products remaining with unreacted methanol and catalyst to the methanol oxidation step. Alternatively, dimethoxymethane may be recovered, for example. As mentioned above, one of the major uses of methyl formate is its conversion to formic acid by hydrolysis. Therefore, another feature of the present invention is the provision of a method for producing formic acid by hydrolysis of methyl formate, characterized in that methyl formate is produced by the method described above. Many methods are known in the art for the hydrolysis of methyl formate. Typical of such techniques are US Patent No. 3907884; UK Patent No. 1490374;
Norway Patent No. 138803; EP No. 5998; DT
No. 2774313; U.S. Patent No. 2160064; U.S. Patent No.
2286407; US Patent No. 2373583; DS No. 639064
No. 628656 and British Patent No. 628656. Methyl formate can be hydrolyzed by the methods described in any of the above patent specifications. The invention will now be described with reference to the following examples. The examples use the apparatus shown in the attached figures. Referring to the drawings, 1 shows a reaction vessel made of rust-free steel, with a height of approximately 1.5 m and an internal diameter of 10 cm; 2 is a heating oil jacket; 3 is a partial cross-section of the "draft tube"; , the purpose of which is to promote the circulation of the reactor contents; 4 is an air distributor; 5 is a level-regulating liquid discharge valve; 6 is a water-cooled condenser; 7 is an aqueous methanol 8 is the air inlet pipe; 9 is the combined methanol and liquid circulation inlet pipe; and 10 is the combined catalyst solution supply and condensate return pipe. In addition, the exhaust gas is taken out from 7 under reduced pressure and measured and analyzed. Further, the condensate is taken out from below 6, passed through a pump, and returned to the reaction vessel. Example 1 Oxidation initiation using chromium and subsequent continuous operation with chromium catalyst (5 ppm) without by-product recycling. The product from the previous methanol oxidation reaction was added to the reactor along with enough sodium dichromate to provide 25 ppm Cr by weight in the mixture. The reactor was then heated to 49 bar (absolute) with nitrogen.
and the mixture was heated to 175°C. Air was then introduced through tube 8 at a rate of approximately 3 kg/hour. Oxidation began almost immediately, as indicated by the oxygen content of the exhaust gas. The liquid feed to the reactor through tubes 9 and 10 is then started;
The reaction temperature was then adjusted to the desired value. The air feed rate was adjusted to maintain the desired oxygen content in the exhaust gas and liquid product was withdrawn through valve 5 at a rate sufficient to maintain a constant level in the reactor. Several hours after the start of the reaction, the reactor was operated continuously under the following conditions: Reaction temperature: 192°C Air feed rate: 8289 g/h Oxygen content in exhaust gas: 3.9% V/V Methanol feed rate: 6233 g/h Catalyst supply rate (0.3% W/WN ₂ Cr ₂ O ₇ solution)
: 30 ml/hr Liquid product recovery rate: 6902 g/hr Liquid product composition (%W/W) Methanol: 42.2 Methyl formate: 23.2 Dimethoxymethane: 8.4 Water: 21.2 Formic acid 5.0 Example 2 Initiation of oxidation using chromium , and subsequent continuous operation with chromium catalyst (5ppmCr) with partial recirculation of by-products. The product from the previous methanol oxidation reaction was added to the reactor along with enough sodium dichromate to provide 20 ppm Cr by weight in the mixture. The reaction was initiated in the mixture using the method set forth in Example 1. The liquid product removed through valve 5 is approximately 3
The procedure of Example 1 was followed, except that a 10 mm ceramic Kraschich ring with a height of 1.5 m and an internal diameter of 10 cm was packed and fed at the midpoint of the first distillation column, which was equipped with an electrically heated metal reboiler. What was removed from the bottom of the column was a fraction containing methanol, water, catalyst and formic acid. The head product from the first column is divided into two parts, one of which is re-charged to the head of the column as reflux and the other part is fed to the second column 1.5 m from its bottom. did. The second column was 3.7 m high and had an internal diameter of 7.6 cm. This column was also filled with 10 mm ceramic Laschig rings and was also equipped with an electrically heated metal reboiler. The product from the head of the second column contained more than 90% by weight methyl formate. This stream was divided into two parts, one of which was returned to the head of the column as reflux and the other part was continuously recovered as the crude methyl formate product. The bottom product of the second column consisted essentially of dimethoxymethane and methanol. This stream was recycled to the reactor through line 9 along with fresh methanol. Several hours after the start of the oxidation reaction, the reactor was operated continuously under the following conditions: Reaction temperature: 191°C Air feed rate: 7178 g/h Oxygen content in exhaust gas: 3.7%V/V Methanol feed rate: 6260 g/h Catalyst feeding rate (0.3% W/WNa ₂ Cr ₂ O ₇ aqueous solution)
: 30ml/hour Liquid product recovery rate: 6902g/hour Recirculation feed rate: 565g/hour Recirculation composition (%W/W) Methanol: 13.9 Methyl formate: 24.3 Dimethoxymethane: 61.7 Water: 0.1 Reactor product recovery rate :7268g/hour Reaction product composition (%W/W) Methanol: 44.0 Methyl formate: 23.3 Dimethoxymethane: 8.8 Water: 20.6 Formic acid 3.4 Example 3 Initiation of oxidation using chromium and partial regeneration of by-products Chromium catalyst (50ppm, Cr) with circulation
Continued continuous operation. The start-up procedure described in Example 1 was followed. The procedure described in the example was then repeated, except that the concentration of chromium in the reaction mixture was increased to 50 ppm. A few hours after the start of oxidation, the reactor was operated continuously under the following conditions: Reaction temperature: 191°C Air feed rate: 8285 g/h Oxygen content in exhaust gas: 4.3%V/V Methanol feed rate: 6262 g/h Catalyst feeding rate (3% W/WNa ₂ Cr ₂ O ₇ aqueous solution)
: 30ml/hour Recirculation feed rate: 535g/hour Recirculation composition (%W/W) Methanol: 16.0 Methyl formate: 14.5 Dimethoxymethane: 69.5 Reactor product recovery rate: 7716g/hour Reaction product composition (%W/W) W) Methanol: 43.8 Methyl formate: 22.1 Dimethoxymethane: 8.3 Water: 22.3 Formic acid 3.5 Example 4 Initiation of oxidation using naphtha and subsequent continuous operation in the absence of naphtha without recycling of by-products. Referring to the drawing, lower carboxylic acid was added to reactor 1,
Seven batches of product from the previous naphtha oxidation containing water, hydrocarbons, and ester/ketone oxidation intermediates were charged along with an additional three naphtha oxidations. The reactor was then pressurized with nitrogen to 49 bar (absolute) and the mixture was heated to a temperature of 175°C. Air is then introduced through tube 8 at a rate sufficient to maintain an oxygen concentration of approximately 1% V/V in the exhaust gas, while naphtha is
It was fed through tube 9 at a rate of Kg/hr. The methanol feed was then started and the methanol feed rate was gradually increased to the desired value. Since methanol is even more difficult to oxidize than naphtha, it was necessary to gradually raise the reaction temperature as the methanol feed rate relative to the naphtha feed rate increased. When the desired methanol feed rate was reached, the naphtha feed rate was tapered to zero. Several hours after stopping the naphtha feed, the reaction conditions were as follows: Reaction temperature 190°C Air feed rate 1885 g/h Oxygen content in exhaust gas 0.1% V/V Methanol feed rate 2771 g/h Liquid product Collection rate 3087g/hr Liquid product composition (%W/W) Methanol 54.3 Methyl formate 19.7 Dimethoxymethane 7.5 Formic acid 2.8 Water 15.5 Metal ion concentration in liquid product (ppm, by weight) Iron: 0.4 Nickel: 0.4 Chromium: 0.1 Example 5 Initiation of oxidation using chromium, subsequent continuous operation in the absence of chromium with partial recycling of by-products. Reactor 1 was charged with the product from the previous methanol oxidation reaction along with enough sodium dichromate to provide 25 ppm Cr by weight in the mixture. The reactor was then heated to 49 bar (absolute) with nitrogen.
and the mixture was heated to a temperature of 175°C. Air was then introduced through tube 8 at a rate of approximately 3 kg/hour. Oxidation began almost immediately as indicated by the oxygen content of the exhaust gas. Liquid feed was then started to the reactor without catalytic metal compound and the reaction temperature was adjusted to the desired value. The air feed rate was adjusted to maintain the oxygen content in the exhaust gas at the desired value, and the liquid product was withdrawn through valve 5 at a rate sufficient to maintain a constant liquid level in the reactor. In this example, the liquid product withdrawn through valve 5 is placed in the middle of the first distillation column with an electrically heated metal reboiler having a height of approximately 3 and an internal diameter of 10 cm packed with 10 cm ceramic Krassich rings. supplied to the point. A fraction containing methanol, water and formic acid was removed from the bottom of the column. dividing the head product from the first column into two parts, one of which is reintroduced to the head of the column as reflux;
Another portion was fed 1.5 m from the bottom of the second column. The second column is 3.7m high and has an internal diameter
It was 7.6cm. The column was packed with 10 mm ceramic Kraschich rings and was also equipped with an electrically heated metal reboiler. The head product of the second column contained more than 90% by weight methyl formate. This stream was divided into two parts, one of which was returned to the head of the column as reflux, and the other part was continuously recovered as the crude methyl formate product. The bottom product of the second column consisted essentially of dimethoxymethane and methanol. This stream was recycled through line 9 to the reactor along with fresh methanol. A few hours after the start of the oxidation reaction, the reactor was operated continuously under the following conditions: Reaction temperature: 188.4°C Air feed rate: 6826 g/hour Oxygen content in exhaust gas: 3.4% V/V Methanol feed rate: 6310 g /hour Recycle feed rate: 341 g/hour Recycle stream composition (% W/W) Methanol: 22.9 Dimethoxymethane: 60.6 Methyl formate: 16.2 Water: 0.3 Reactor product recovery rate: 7475 g/hour Reactor product Composition (%W/W) of methanol: 48.4 dimethoxymethane: 6.3 methyl formate: 23.9 water: 18.2 formic acid 3.2 The concentration of metal ions in the liquid product was similar to that cited in the previous example, i.e. Iron (0.4ppm), Nickel (0.4ppm) and Chromium (0.1ppm). Examples 4 and 5 demonstrate that the extremely low chromium ion concentration resulting from corrosion of the rust-resistant steel reaction vessel is sufficient to maintain continuous oxidation of methanol even in the absence of carefully added chromium. Example 6 The purpose of this example is to compare the oxidation rates achieved by using a chromium-containing catalyst with those obtained in the absence of catalyst and in the presence of additives other than chromium. Insofar as the examples relate to uncatalyzed reactions and reactions using additives other than chromium, they are not illustrative of the invention. A series of experiments with different additives were performed after the initiation of oxidation. Several hours passed after charging the additive and until the oxidation rate began. Approximately 6200 methanol to the reactor for each experiment
The reaction temperature, fed in g/hr, was maintained at 189±2° C. and air was fed at a rate sufficient to maintain an oxygen concentration of about 4% V/V in the exhaust gas. Partial recycling of by-products was performed as shown in Examples 2 and 3. The various metal compound additives were fed to the reactor as aqueous solutions at concentrations calculated to give the required metal concentrations in the reactor when introduced at a rate of 30 ml/hour. The results of these experiments are shown in the table below.

[Table] The results shown in the table are for vanadium, molybdenum,
The oxidation rates measured by the oxygen uptake rates achieved using nickel, manganese, cobalt, iron and tungsten are those obtained without careful addition of reactive additives when such oxidations are carried out in rust-free steel vessels. indicates that it is equal to or slower than. A substantial increase in methanol oxidation rate is achieved only by adding chromium.

[Brief explanation of drawings]

The drawings show equipment used in embodiments of the invention.

Claims (1)

[Claims] 1. The use of a soluble chromium compound as an oxidation catalyst in a continuous process for producing methyl formate by continuously reacting methanol with molecular oxygen in the liquid phase at high temperatures in the presence of an oxidation catalyst. A method characterized by: 2. The method according to claim 1, wherein molecular oxygen is supplied in the form of air. 3. Claims 1 or 2 in which the chromium compound is present in an amount ranging from 0.05 to 100 ppm based on the weight of the methanol supplied and calculated as metal.
The method described in any one of the paragraphs. 4. The method according to claim 3, wherein the chromium compound is present in a range of 0.5 to 10 ppm. 5. A method according to any one of claims 1 to 4, in which the chromium compound is carefully added. 6. A process according to any one of claims 1 to 4, wherein the reaction vessel is fabricated from rust-free steel or other chromium-containing alloy and the chromium catalyst results from corrosion of the reaction vessel. 7. The method according to any one of claims 1 to 6, wherein the high temperature is in the range of 100° to 200°C. 8 Initiating the oxidation by initially charging the substance(s) more susceptible to oxidation than methanol in the presence or absence of methanol, and after initiation of the oxidation in the absence of the substance more susceptible to oxidation. A method according to any one of claims 1 to 7 for supplying methanol. 9. The method according to claim 8, wherein the substance more susceptible to oxidation is a paraffinic hydrocarbon. 10. The process according to claim 9, wherein the paraffinic hydrocarbon is a distillate having a boiling point range of 15° to 95°C. 11. The method of claim 8, wherein the substance more susceptible to oxidation is a product of a previous methanol oxidation or a synthetic composition corresponding to such a product. 12. The method according to claim 8, wherein the substance more susceptible to oxidation is ethanol or 2-butanone. 13. A method according to any one of claims 8 to 12, wherein initiation is achieved in the presence of a soluble chromium compound.