Method for producing 2-methoxyacetic acid

Abstract

A method for producing 2-methoxyacetic acid by oxidizing 2-methoxyethanol in a reaction device using oxygen at a temperature of 20 to 100° C. and an oxygen partial pressure of 0.01 to 2 MPa in the presence of water and a heterogeneous catalyst containing platinum, in which the method is carried out semi-continuously or continuously, and 2-methoxyethanol is added to the reaction device in a temporally and/or spatially distributed manner such that temporally and spatially, the mass ratio of 2-methoxyethanol to 2-methoxyethanol plus water per volume element in the reaction device is constantly ≤0.80 of the mass ratio of the added 2-methoxyethanol to the added 2-methoxyethanol plus water.

Claims

1. A method for producing 2-methoxyacetic acid comprising oxidizing 2-methoxyethanol in a reaction device using oxygen at a temperature of 20 to 100° C. and an oxygen partial pressure of 0.01 to 2 MPa in the presence of water and a heterogeneous catalyst containing platinum, wherein the method is carried out semi-continuously or continuously and in that the addition of 2-methoxyethanol to the reaction device is temporally and spatially selected such that temporally and spatially, in the liquid phase containing 2-methoxyethanol and 2-methoxyacetic acid in the reaction device, the quotient of CR/CA is constantly ≤0.80, wherein CR is defined as $CR=\frac{C\left(2-\mathrm{methoxyethanol}\mathrm{reactor}\right)}{C\left(2-\mathrm{methoxyethanol}\mathrm{reactor}\right)+C\left(\mathrm{water}\mathrm{reactor}\right)},$ and wherein C(2-methoxyethanol reactor) denotes the mass of 2-methoxyethanol per volume element of the liquid phase containing 2-methoxyethanol and 2-methoxyacetic acid, and C(water reactor) denotes the mass of water per volume element of the liquid phase containing 2-methoxyethanol and 2-methoxyacetic acid, in the semi-continuous method, CA is defined as $CA=\frac{MT\left(2-\mathrm{methoxyethanol}\mathrm{total}\mathrm{mass}\right)}{MT\left(2-\mathrm{methoxyethanol}\mathrm{total}\mathrm{mass}\right)+\mathrm{MT}\left(\mathrm{water}\mathrm{total}\mathrm{mass}\right)},$ wherein MT(2-methoxyethanol total mass) denotes the total mass of 2-methoxyethanol used in the semi-continuous method, and MT(water total mass) denotes the total mass of water used in the semi-continuous method, and in the semi-continuous method, CA is defined as $CA=\frac{MF\left(2-\mathrm{methoxyethanol}\mathrm{mass}\mathrm{flow}\right)}{MF\left(2-\mathrm{methoxyethanol}\mathrm{mass}\mathrm{flow}\right)+\mathrm{MF}\left(\mathrm{water}\mathrm{mass}\mathrm{flow}\right)},$ wherein MF(2-methoxyethanol mass flow) denotes the mass flow of 2-methoxyethanol supplied to the reaction device, and MF(water mass flow) denotes the mass flow of water supplied to the reaction device.

2. The method as claimed in claim 1, wherein the addition of 2-methoxyethanol to the reaction device is temporally and spatially selected such that temporally and spatially, in the liquid phase containing 2-methoxyethanol and 2-methoxyacetic acid in the reaction device, the quotient of CR/CA is constantly ≤0.70.

3. The method as claimed in claim 1, wherein a ratio by weight of water to 2-methoxyethanol of 1 to 5 is used, wherein this is based in the semi-continuous method on the total mass of water and 2-methoxyethanol used and in the continuous method on the mass flows of water and 2-methoxyethanol supplied to the reaction device.

4. The method as claimed in claim 1, wherein a heterogeneous catalyst containing 0.1 to 10 wt % of platinum on carbon is used.

5. The method as claimed claim 1, wherein the method is carried out semi-continuously, and 2-methoxyethanol is supplied to the reaction device over a period of 1 to 10 h.

6. The method as claimed in claim 1, wherein the method is carried out continuously and 2-methoxyethanol and water are supplied to the reaction device such that the mass flow of 2-methoxyethanol and water based on the total reactor volume in the reaction device is 0.05 to 0.5 per h.

7. The method as claimed in claim 1, wherein 80 to 99% of the 2-methoxyethanol used is reacted.

8. The method as claimed in claim 1, wherein the method is carried out semi-continuously and the reaction comprises a reactor from the group of a stirred vessel, a trickle-bed reactor, and a bubble column reactor.

9. The method as claimed in claim 1, wherein the method is carried out continuously and the reaction comprises a reactor selected from the group consisting of a stirred vessel cascade, a trickle-bed reactor cascade, a cascaded bubble column reactor, and a cascaded jet loop reactor.

10. The method as claimed in claim 1, wherein the low boilers water and 2-methoxyethanol are removed by evaporation from the reaction mixture obtained.

Description

EXAMPLES

Examples 1 to 5

(1) 375 g of water and 25.6 g of a Pt/C catalyst (source: Sigma-Aldrich, 5 wt % of Pt based on the carbon carrier, 1446 m.sup.2/g BET surface area, Pt particles in the range of 1-5 nm) were first placed in a 1.6-l reaction calorimeter with a hollow shaft gassing stirrer, the mixture was heated to 50° C. under stirring at 1000 rpm, and a total pressure of 0.3 MPa was set by adding oxygen. After this, 125 g of 2-methoxyethanol was either placed in the calorimeter (example 1, formally corresponding to addition within 0 h) or supplied at a constant rate over a period of 1.5 h (example 2) to 8 h (example 5). In the case of example 1 (0 h), the 125 g of 2-methoxyethanol was added immediately at the beginning of time measurement. In each case, by means of pressure-controlled addition of oxygen, the total pressure in the reaction calorimeter was maintained at 0.3 MPa abs throughout the entire reaction time. By means of the pressure-controlled supply, oxygen consumption was simultaneously detected, and the conversion rate of 2-methoxyethanol over the course of the reaction was thus indirectly determined. Parallel to this, the amount of heat currently produced was detected in each case. After completion of 2-methoxyethanol addition, the reaction calorimeter was allowed to stand under the set conditions until a 2-methoxyethanol conversion rate of 95% was reached in each case. After this, the reaction calorimeter was cooled to room temperature and expanded to atmospheric pressure, and the removed reaction mixture was freed of the catalyst by filtration.

(2) The filtered reaction mixture was then purified in order to remove water and unreacted 2-methoxyethanol in a continuously-operated Sambay evaporator with a surface area of 0.046 m.sup.2 at 50° C., 25 hPa abs and at an addition rate of 1 mL of reaction mixture per minute. The purified bottom product was removed and analyzed without further distillative purification by quantitative gas chromatography using 1,4-dioxane as an internal standard, and the color index according to APHA was determined. In each of the examples, the product contained a maximum of 0.3 wt % of water and a maximum of 0.3 wt % of 2-methoxyethanol. All further results obtained are shown in Table 1.

(3) The value CA is calculated from the total mass of 2-methoxyethanol and the total mass of water used. The value CA is therefore the same for all five examples.

(4) The maximum CR was determined from the course over time of the conversion of 2-methoxyethanol, wherein the stoichiometry of the reaction was of course taken into consideration.

(5) The examples show that formation of the undesirable methoxyacetic acid-2-methoxyethylester decreases as the maximum CR drops. The color index according to APHA also decreases as the maximum CR drops. Both the value CA and thus the masses of the total 2-methoxyethanol and water added, as well as the maximum required reactor volume, were the same for all five examples. With the same reaction batch size and identical reactor volume, it was possible by addition according to the invention of 2-methoxyethanol to obtain a reaction mixture of significantly greater purity. In comparative example 1 according to the prior art, the reaction mixture contained 2.6 wt % of undesirable methoxyacetic acid-2-methoxyethylester, in example 3 according to the invention, it contained only 0.9 wt %, and in example 5 according to the invention, the remaining content was as low as 0.5 wt %.

(6) With the decrease in maximum CR, the maximum amount of thermal energy produced also dropped significantly. This value was extremely high at 457 W/kg in comparison example 1, in example 3 according to the invention for example only 81 W/kg, and in example 5 according to the invention, it was as low as only 30 W/kg. In accordance with this significant decrease in the amount of heat produced, in the method according to the invention, even sharply lower cooling performance is sufficient, which makes it possible to use a smaller cooler having a significantly lower peak cooling performance.

Example 6

(7) Example 3 was repeated, using in example 6 as a reactor a 1.5 liter stirred vessel (CSTR) with a hollow shaft gassing stirrer, with the bottom outlet thereof being equipped with a sintered glass filter so that the catalyst would be left in the reactor each time it was emptied. On completion of the experiment, i.e. after a 2-methoxyethanol conversion rate of 95% was reached, the reaction mixture was removed via the sintered glass filter and analyzed as in example 3 by gas chromatography, and its color index according to APHA was determined. The catalyst remaining in the reactor was reused in the next batch under the conditions of example 3. A total of 20 such batches were processed with the same catalyst load. Activity and selectivity remained constant within the range of measurement accuracy.

(8) Example 6 shows that in the method according to the invention, the catalyst can be reused over many semi-continuous cycles, and even after 20 cycles, no loss of activity or selectivity can be detected.

(9) TABLE-US-00001 TABLE 1 Example Unit 1 (comparison) 2 (comparison) 3 (invention) 4 (invention) 5 (invention) Duration of addition of 2ME.sup.#1 [h] 0 1.5 3 5 8 Reaction time until reaching 95% 2ME [h] 6.7 8 10 11.7 13.2 conversion rate.sup.#1 CA [g/g] 0.25 0.25 0.25 0.25 0.25 Maximum concentration of 2ME.sup.#1 in the [wt %] 25.0 20.7 17.2 13.9 11.7 reaction mixture Maximum CR [g/g] 0.250 0.215 0.188 0.158 0.136 Maximum (CR .Math. 100)/CA [%] 100 86.0 75.2 63.2 54.4 Maximum heat output produced during [W/kg] 457 252 81 34 30 reaction Content of ME2MEE.sup.#3 based on content of [wt %] 2.6 1.3 0.9 0.7 0.5 2MAA.sup.#2 in purified reaction mixture Color index (APHA) of purified reaction >60 >60 <50 <50 <50 mixture .sup.#12ME = 2-methoxyethanol .sup.#22MAA = 2-methoxyacetic acid .sup.#3ME2MEE = methoxyacetic acid-2-methoxyethylester