Process for oxidative esterification of aldehydes to carboxylic acid esters

Abstract

The present invention relates to a novel process for performing a heterogeneously catalyzed reaction for oxidative esterification of aldehydes to give carboxylic esters. Against this background, it has been possible by the present process according to the invention to perform such processes for longer periods without disruption, with constant or even increased activities and selectivities. This gives rise to the possibility of performing such processes in a very simple, economically viable and environmentally benign manner.

Claims

1. A process, comprising: continuously performing a reaction for oxidative esterification of methacrolein with an alkyl alcohol and oxygen to produce an alkyl methacrylate in the presence of a gold catalyst in a reactor system comprising a reactor, the reactor comprising a gas feed and offgas outlet, wherein a molar ratio of the alkyl alcohol at a steady-state concentration to the methacrolein at a steady-state concentration in the reactor is less than 10:1, an oxygen concentration in a gas phase at the offgas outlet in the reactor is less than 4.5% by volume, a steady-state molar ratio of the alkyl alcohol to the methacrolein in the reactor to a steady-state molar ratio of the alkyl alcohol to the methacrolein in the gas feed is between 1.5 and 15, and a steady-state concentration of the methacrolein in the reactor is less than 21% by weight.

2. The process according to claim 1, wherein the molar ratio of the alkyl alcohol at a steady-state concentration to the methacrolein at a steady-state concentration in the reactor is between 4:1 and less than 10:1, the steady-state molar ratio of the alkyl alcohol to the methacrolein in the reactor to the steady-state molar ratio of the alkyl alcohol to the methacrolein in the gas feed is between 1.8 and 15, and the steady-state concentration of the methacrolein in the reactor is less than 15% by weight.

3. The process according to claim 1, wherein the molar ratio of the alkyl alcohol at a steady-state concentration to the methacrolein at a steady-state concentration in the reactor is between 5:1 and less than 9.5:1, the steady-state molar ratio of the alkyl alcohol to the methacrolein in the reactor to the steady-state molar ratio of the alkyl alcohol to the methacrolein in the gas feed is between 2 and 13, and the steady-state concentration of the methacrolein in the reactor is less than 12% by weight.

4. The process according to claim 1, wherein the catalyst is in the form of catalyst particles comprising oxygen, silicon, aluminium, at least basic element, gold and optionally one of nickel, cobalt, iron and zinc, wherein the at least one basic element is an alkali metal, an alkaline earth metal, a rare earth metal or mixtures of these metals.

5. The process according to claim 1, wherein a reaction temperature is between 60 and 100° C. and an internal reactor pressure is between 1 and 20 bar.

6. The process according to claim 5, wherein the reaction temperature is between 70 and 95° C. and the internal reactor pressure is between 2 and 10 bar.

7. The process according to claim 1, wherein a partial oxygen pressure at the offgas outlet in the reactor is between 0.01 and 0.8 bar, and a partial oxygen pressure in a gas mixture fed to the reactor is less than 10 bar.

8. The process according to claim 1, wherein the steady-state concentration of the methacrolein in the reactor is 3% or more and less than 21% by weight.

9. The process according to claim 8, wherein the steady-state concentration of the methacrolein in the reactor is between 5% and 20% by weight.

10. The process according to claim 1, wherein the alkyl alcohol is methanol and the alkyl methacrylate is methyl methacrylate.

11. The process according to claim 1, wherein a ratio of a mass of the catalyst to a liquid reaction mixing volume in the reactor is between 0.01 and 0.3 kg/l.

12. The process according to claim 1, wherein the reactor is a slurry reactor.

13. The process according to claim 1, wherein a reaction mixture is discharged continuously from the reactor and the catalyst, after being separated off, remains in the reactor.

14. The process according to claim 1, wherein a reaction mixture, after being withdrawn continuously from the reactor, is worked up in at least one distillation column, and the alkyl alcohol and methacrolein are separated off as distillate and returned to the reactor.

15. The process according to claim 1, wherein the oxygen concentration in the gas phase at the offgas outlet in the reactor is less than 3.0% by volume.

16. The process according to claim 3, wherein the steady-state molar ratio of the alkyl alcohol to the methacrolein in the reactor to the steady-state molar ratio of the alkyl alcohol to the methacrolein in the gas feed is between 1.5 and 3.5.

Description

DRAWINGS

(1) The FIGURE shows the experimental results from Table 2 further down expressed in the form of a graph. More particularly, what is shown here is the dependence of the space-time yield and of the selectivity of the reaction on the O.sub.2 concentration.

EXAMPLES

(2) The Catalyst Preparation

(3) A 250 ml beaker is initially charged with 21.36 g of Mg(NO.sub.3).sub.2*6H.sub.2O and 31.21 g of Al(NO.sub.3).sub.3*9H.sub.2O together, which are dissolved in 41.85 g of demineralized water while stirring with a magnetic stirrer. Thereafter, 1.57 g of 60% HNO.sub.3 are added while stirring. 166.67 g of silica sol (Köstrosol 1530AS from Bad Köstritz, 30% by weight of SiO.sub.2, median size of the particles: 15 nm) are weighed into a 500 ml three-neck flask and cooled to 15° C. while stirring. 2.57 g of 60% HNO.sub.3 are added gradually to the sol while stirring. At 15° C., the nitrate solution is added to the sol within 45 min while stirring. After the addition, the mixture is heated to 50° C. within 30 min and stirred at this temperature for a further 24 h. After this time, the mixture is spray-dried at exit temperature 130° C. The dried powder (spherical, median particle size 60 μm) is heated in a thin layer in a Naber oven to 300° C. within 2 h, kept at 300° C. for 3 h, heated to 600° C. within 2 h and finally kept at 600° C. for 3 h.

(4) A suspension of 10 g of the SiO.sub.2—Al.sub.2O.sub.3—MgO support from the preceding paragraph in 33.3 g of demineralized water is heated to 90° C. and stirred at this temperature for 15 min. Added to this suspension while stirring is a solution, heated to 90° C. beforehand, of HAuCl.sub.4*3H.sub.2O (205 mg) and Ni(NO.sub.3).sub.2*6H.sub.2O (567 mg, 1.95 mmol) in 8.3 g of water. After the addition, the mixture was stirred for a further 30 min, then cooled, filtered at room temperature, and subsequently washed six times with 50 ml each time of water. The material was dried at 105° C. for 10 h, finely crushed with a mortar and pestle, then heated from 18° C. up to 450° C. within 1 h and calcined at 450° C. for 5 h.

Comparative Examples 1 to 5

(5) Gold-containing catalyst (for amount see Table 1) from the preceding description was continuously tested in a stirred pressure reactor with a stirrer that draws in gas with a reaction mixing volume of 200 ml at pressure 5 bar. In the course of this, a feed composed of 41% by weight of methacrolein (MAL hereinafter) and 59% by weight of methanol (44 g/h), and also a 0.5-1.0% by weight NaOH solution in methanol (7.3 g/h), were introduced continuously into the reactor. Air was utilized as oxygen source and was metered directly into the liquid reaction mixture. The offgas was cooled at −20° C. downstream of the reactor and the oxygen content therein was measured continuously. The amount of air introduced into the reactor was adjusted such that the oxygen concentration in the offgas was 4.0% by volume. After filtration, the liquid product mixture was discharged continuously from the reactor, cooled down and analyzed by means of gas chromatography (GC).

(6) TABLE-US-00001 TABLE 1 STY Mass MAL MeOH/MAL (max) TOS to of cat. conc. in reactor C(MAL) S(MMA) [mol/kg deactivation Ex. [g] [wt %] [mol/mol] [%] [%] ct-h] >10% CE1 5 22.3 5.6 35.3 86.0 16.1 100 h CE2 10 12.9 9.8 56.4 91.1 14.2 500 h CE3 15 10.0 12.2 66.2 94.8 10.6 >1000 h CE4 20 8.2 14.0 72.5 96.1 8.8 >1000 h CE5 25 6.9 16.6 76.7 96.0 7.5 >1000 h Ex.: Example; CE: Comparative Example; conc.: concentration; C: conversion; S: selectivity; STY: space-time yield; TOS: time on stream

(7) As can be seen, the process can be conducted at the set oxygen concentration of 4% by volume with a good selectivity and long time on stream only when the molar ratio of methanol to MAL is greater than 10.

Comparative Examples 6 to 16 (O.SUB.2 .Content)

(8) Gold-containing catalyst (20 g) from the preceding description was tested as described in the comparative examples above. The only variation was the adjustment of the oxygen concentration in the offgas, which was varied between 1.9% and 6.6% by volume. Further process parameters and results are listed in Table 2.

(9) TABLE-US-00002 TABLE 2 MAL O.sub.2 in the conc. in MeOH/MAL STY offgas reactor in reactor C(MAL) S(MMA) [mol/kg Example [mol %] [wt %] [mol/mol] [%] [%] ct-h] CE6 1.9 11.07 10.32 64.2 95.1 7.6 CE7 2.2 10.44 10.94 65.9 95.1 7.9 CE8 2.4 9.62 11.89 68.2 94.1 8.2 CE9 2.8 8.59 12.97 71.2 93.8 8.4 CE10 2.9 8.36 13.29 72.4 93.4 8.4 CE11 3.0 9.01 12.52 70.1 93.2 8.5 CE12 3.6 8.19 14.20 72.5 93.5 8.6 CE13 4.0 8.29 14.04 72.2 92.0 8.6 CE14 5.9 7.54 15.51 74.0 92.2 8.6 CE15 6.1 8.13 14.49 72.2 93.4 8.6 CE16 6.6 8.55 13.84 72.2 90.4 8.5

(10) It can be seen from these experiments that, with a relatively high methanol content compared to the steady-state MAL concentration, the space-time yield increases up to an O.sub.2 concentration of about 3% by volume and beyond that is apparently no longer dependent thereon. The selectivity, by contrast, appears to decrease slightly with rising O.sub.2 concentration. This relationship is additionally depicted in the FIGURE.

(11) With an O.sub.2 concentration above 6% by volume, more particularly, relatively rapid deactivation of the catalyst and an associated reduction in STY were observed. The overall finding was that a rising O.sub.2 concentration has an adverse effect on the catalyst time on stream.

(12) Moreover, it can be clearly seen from the series of experiments that the selectivity of the reaction, S(MMA), reduces with rising O.sub.2 offgas concentration. The reason for the loss of selectivity and catalyst deactivation in the region of higher O.sub.2 concentrations is probably attributable to the formation of oligo- and polymers of methacrolein.

(13) It can be seen from the dependence of the space-time yield STY on the O.sub.2 concentration that the reaction is limited by oxygen supply at relatively low O.sub.2 contents, whereas, over and above a particular O.sub.2 concentration (in this case between 3% and 4% by volume), there is no longer any apparent dependence, and so a further increase in the O.sub.2 concentration does not result in any further increase in the STY.

(14) A preferred implementation of the oxidative esterification in the range of kinetic limitation by oxygen supply is thus advantageous; in other words, with a slight deficiency of O.sub.2, the latter is used up rapidly for the reaction without increasing by-product formation (for example by polymerization). In an oxygen excess (in this case with more than 4% by volume of O.sub.2 in the offgas), by contrast, there are higher amounts and concentrations of free oxygen in the reaction mixture, which promotes by-product formation through polymerization.

Examples 1 to 10 and Comparative Example 17 (Variation in the Methanol/MAL Concentrations)

(15) Gold-containing catalyst (20 g) from the preceding description was tested analogously to the comparative examples above, but here with an oxygen concentration of 2.0% by volume in the offgas. A constant amount of feed of MAL and methanol (45 g/h, see table below for MAL concentration) and 0.5% to 1% by weight of NaOH in methanol (7.6 g/h) was introduced continuously into the reactor. Results are shown in Table 3.

(16) TABLE-US-00003 TABLE 3 MAL MAL in MeOH conc. in MeOH/MAL MeOH/MAL Ratio STY feed in feed reactor in feed in reactor reactor/ C(MAL) S(MMA) [mol/kg Ex. [wt %] [wt %] [wt %] [mol/mol] [mol/mol] FEED [%] [%] ct-h] 10 40.06 59.94 11.07 3.27 10.32 3.15 64.2 94.3 7.62 1 41.54 58.46 11.65 3.08 9.48 3.08 63.5 94.3 7.83 2 41.24 58.76 12.39 3.12 8.84 2.84 61 94.5 7.43 3 41.85 58.15 12.64 3.04 8.70 2.86 60.7 94.4 7.53 4 42.81 57.19 13.98 2.92 7.79 2.67 57.8 94.3 7.41 5 44.2 55.80 16.34 2.76 6.33 2.29 53.7 94.6 7.05 6 44.7 55.30 16.51 2.71 6.46 2.39 53.2 94.8 7.09 7 47.76 52.24 20.1 2.39 5.09 2.13 48.1 94.4 6.91 8 47.74 52.26 19.88 2.39 5.09 2.13 48.2 92.8 6.70 9 47.85 52.15 20.48 2.38 4.98 2.09 46.8 91.4 6.36 CE17 49.7 50.30 23.76 2.21 3.96 1.79 41.8 90.0 6.01

(17) The steady-state ratio of alkyl alcohol to methacrolein in the reactor to the ratio of the substances in the feed in the steady state, in the experiments listed in Table 3, is between 1.5 and 3.5.

(18) As can be seen, the esterification reaction can surprisingly be conducted without any problem and with high selectivity even with a lower molar ratio of methanol to MAL if a relatively low oxygen concentration in the offgas (2% by volume) was set.

Examples 11 and 12 (Long-Term Test)

(19) Gold-containing catalyst (20 g) from the preceding description was continuously tested in a stirred pressure reactor with a stirrer that draws in gas with a reaction mixing volume of 200 ml at pressure 5 bar. In the course of this, a feed composed of 44.5% by weight of MAL and 55.5% by weight of methanol (44 g/h), and also a 1.0% by weight NaOH solution in methanol (7.3 g/h), were introduced continuously into the reactor. Air was utilized as oxygen source and was metered directly into the liquid reaction mixture. The offgas was cooled at −20° C. downstream of the reactor and the oxygen content therein was measured continuously. The amount of air introduced into the reactor was adjusted such that the oxygen concentration in the offgas was 2.0% by volume. After filtration, the liquid product mixture was discharged continuously from the reactor, cooled down and analyzed by means of GC.

(20) TABLE-US-00004 MAL Run MAL in conc. in MeOH/MAL STY time feed reactor in reactor C(MAL) S(MMA) [mol/kg Ex. [h] [wt %] [wt %] [mol/mol] [%] [%] ct-h] 11 500 44.5 16.5 6.5 53.2 94.8 7.09 12 2000 44.5 16.6 6.5 53.0 94.8 7.06

(21) As can be seen, the esterification reaction here too can surprisingly be conducted without any problem over a prolonged period with high selectivity even with a lower molar ratio of methanol to MAL if a relatively low oxygen concentration in the offgas was set.