Method for separating formic acid from a reaction mixture by means of extraction

Abstract

The invention relates to a method for separating formic acid from a reaction mixture by means of extraction, wherein, in addition to the formic acid, the reaction mixture comprises a polyoxometalate ion of general formula [PMo.sub.xV.sub.yO.sub.40].sup.n as a catalyst and a solvent that dissolves the catalyst, wherein 6x11, 16, x+y=12 and 3<n<10, wherein n, x, and y are each a whole number, wherein the separation occurs via extraction by means of a polar organic extraction agent which extracts the formic acid and the catalyst and which is N-(n-hexadecyl)formamide, N-di-n-acetamide or an N,N-dialkylcarboxamide, wherein the N,N-dialkylcarboxamide forms a phase boundary between the solvent and the extraction agent during mixing with the solvent.

Claims

1. A method for isolating formic acid from a reaction mixture by extraction, wherein the reaction mixture besides the formic acid comprises a polyoxometalate ion of the general formula [PMo.sub.xV.sub.yO.sub.40].sup.n as catalyst and a solvent which dissolves the catalyst, where 6x11, 1y6, x+y=12, and 3<n<10, where n, x and y are each an integer, where the isolating is accomplished by extraction by means of a polar organic extractant which extracts the formic acid and the catalyst and which is N(N-hexadecyl)formamide, N-di-n-acetamide or an N,N-dialkylcarboxamide, where the N,N-dialkylcarboxamide on mixing with the solvent forms a phase boundary between the solvent and the extractant, where the extractant is one which, for extraction of the catalyst present at a concentration of 1.5 wt % in water, has a partition coefficient for the catalyst at 40 C. that is greater by a factor of at least 7 than a partition coefficient for extraction at 40 C. of the formic acid present at a concentration of 5 wt % in water, where the extractant before the extraction is saturated with the catalyst or where the catalyst extracted together with the formic acid is isolated from the extractant after the extraction by means of precipitation as a salt or by means of a further extraction with a polar further extractant and with a temperature change of the extractant and/or with an increase in the pH of the extractant, and is returned to the reaction mixture.

2. The method as claimed in claim 1, wherein the extraction is carried out in two stages, by extracting the reaction mixture in a first extraction step with a first quantity of the extractant in order to extract the catalyst and extracting the reaction mixture in a second extraction step with a second quantity of the extractant in order to extract the formic acid, where the catalyst extracted in the first extraction step is returned to the reaction mixture, where the catalyst is isolated from the extractant used in the first extraction step, after the first extraction step, by means of precipitation as a salt or by means of a further extraction with a polar further extractant and with a temperature change of the extractant used in the first extraction step and/or with an increase in the pH of the extractant used in the first extraction step.

3. The method as claimed in claim 2, wherein the first quantity is smaller than the second quantity and/or the extraction with the first quantity of the extractant takes place for a first time and the extraction with the second quantity of the extractant takes place for a second time and the second time is longer than the first time.

4. The method as claimed in claim 1, wherein the catalyst by means of the precipitation as a salt is isolated simultaneously with precipitation of the extracted formic acid as a formate, or wherein the polar further extractant is the solvent and/or the increase in the pH takes place by addition of a carbonate and/or a hydroxide.

5. The method as claimed claim 1, wherein the precipitation as a salt takes place by means of a hydroxide or of another base.

6. The method as claimed in claim 1, wherein the precipitated salt is supplied to the reaction mixture and dissolved beforehand in the solvent and adjusted to a pH of the reaction mixture, and/or the further extractant after the further extraction is supplied to the reaction mixture and adjusted beforehand to a pH of the reaction mixture.

7. The method as claimed in claim 1, wherein the extractant before the extraction or before the further extraction, or at least the extractant used in the first extraction step, before the first extraction step or before the further extraction, is admixed with an additive and the catalyst is isolated from the extractant after the extraction, or from the extractant used in the first extraction step after the first extraction step, by means of the further extraction with the polar further extractant and with the temperature change and/or the increase in the pH.

8. The method as claimed in claim 7, wherein the additive is petroleum, a fraction of petroleum, n-hexane, n-octane, n-decane, oleyl alcohol, toluene, dibutyl ether or tri-n-butyl phosphate.

9. The method as claimed in claim 1, wherein the solvent is a protic and/or polar solvent.

10. The method as claimed in claim 1, wherein the N,N-dialkylcarboxamide is dipentylformamide, N,N-di-n-butylformamide, N-methyl-N-heptylformamide, N-n-butyl-N-2-ethylhexylformamide or N-n-butyl-N-cyclohexylformamide.

11. The method as claimed in claim 1, wherein the extraction is carried out in each case only with a portion of the reaction mixture, which after the extraction is returned to the rest of the reaction mixture.

12. The method as claimed in claim 1, wherein the amount of formic acid extracted from the reaction mixture in the second extraction step is only such that the pH of the reaction mixture does not rise above 3.

13. The method as claimed in claim 1, wherein the method comprises catalytic generation of formic acid by means of the catalyst and regeneration of the catalyst reduced in the process, where the catalyst is contacted at a temperature between 70 C. and 160 C. with an alpha-hydroxyaldehyde, an alpha-hydroxycarboxylic acid, a carbohydrate, a glycoside or a polymer containing a carbon chain and having at least two OH groups bonded as substituents to the carbon chain and/or having an O, N or S atom present repeatedly in the carbon chain, as substrate in the reaction mixture, where the catalyst reduced in the process is returned to its original state by oxidation, where the reaction mixture for this purpose is contacted with a gas comprising a volume fraction of at least 18% of oxygen at a gas pressure in a range of 2 bar to 33 bar, by means of a mixing apparatus or via a liquid-permeable, gas-impermeable membrane, where CO and/or CO.sub.2 formed in the reaction and entering the gas are taken off in a quantity such that the volume fraction of CO and CO.sub.2 together in the gas does not exceed 80%.

14. The method of claim 5, wherein the hydroxide is KOH or NaOH.

15. The method of claim 6, wherein the adjusting of the solvent with the precipitated salt dissolved therein and/or of the further extractant to the pH of the reaction mixture is by means of formic acid.

16. The method of claim 7, wherein the admixed additive is a polar additive, the polar further extractant is the solvent, and/or the increase in the pH is by addition of a carbonate and/or a hydroxide.

17. The method of claim 9, wherein the protic and/or polar solvent is water or a substrate which can be reacted by means of the catalyst.

18. The method of claim 12, wherein the amount of formic acid extracted from the reaction mixture in the second extraction step is only such that the pH of the reaction mixture does not rise above 2.5.

Description

(1) The invention is elucidated in more detail below with an exemplary embodiment. In the drawings:

(2) FIG. 1 shows a schematic representation of a method of the invention,

(3) FIG. 2 shows a calibration line for determining the concentration of the catalyst HPA-5 in an aqueous solution of the catalyst at a wavelength of 435 nm, and

(4) In the case of the method according to FIG. 1, the reaction vessel R accommodates a reaction mixture for generating formic acid FA, this mixture comprising glucose and [PMo.sub.7V.sub.5O.sub.40].sup.8, for example. Instead of glucose, another alpha-hydroxyaldehyde or another of the substrates identified in claim 15 may also be used. From the reaction vessel R, a portion of the reaction mixture is passed into the first extraction vessel E1, where it is extracted by means of the extractant EM, such as N,N-di-n-butylformamide (DBFA), for example, as organic phase, for about one minute at a temperature of about 30 C. A single-stage extraction is sufficient for this. For this purpose, the first extraction vessel E1 may be constructed, for example, as a mixer-settler apparatus. In order to achieve an extraction of about one minute, the rate at which the extractant EM and the reaction mixture flow through the mixer-settler apparatus must be selected so that the relevant contact time, which takes place in particular in the mixer region of the apparatus, is about one minute. Because of the much higher partition coefficient of the extractant EM for an extraction of the catalyst than for an extraction of the formic acid FA, a short extraction time of this kind is sufficient for extraction of the catalyst. During the extraction, the catalyst in particular, but also a small amount of the formic acid FA, cross into the organic phase.

(5) After the extraction, the extractant EM is passed from the first extraction vessel E1 into a neutralizing vessel N. The extractant EM comprises an apolar additive, such as n-hexane, for example. In the neutralizing vessel N, the extractant is extracted at about 90 C. with an aqueous sodium carbonate solution Na.sub.2CO.sub.3/H.sub.2O. During this extraction, sodium formate NaCOOH is formed in the aqueous solution from the formic acid present. CO.sub.2 likewise formed escapes in the form of gas. At the same time, owing to the temperature increase and the addition of sodium carbonate, the catalyst crosses into the aqueous phase. The aqueous phase containing the catalyst is passed back into the reaction vessel R, and so the catalyst is available for the further reaction of the substrate.

(6) After the first extraction step has been carried out, the aqueous phase, containing formic acid FA, is passed from the first extraction vessel E1 into a second extraction vessel E2, where it is extracted with the extractant EM for 30 minutes in a multistage extraction, by means of a countercurrent extraction apparatus, for example. The extractant EM may be the extractant remaining after the extraction with the aqueous sodium carbonate solution in the neutralizing vessel N, or fresh extractant, or a mixture of these. In this second extraction step, the formic acid crosses from the aqueous phase into the organic phase. In addition, a small amount of water H.sub.2O may dissolve in the extractant, if the extractant is not already saturated with water. After the second extraction step, the extractant EM, with formic acid and optionally water H.sub.2O dissolved therein, is passed into a first distillation apparatus D1, in which formic acid FA and primarily water H.sub.2O are isolated. This formic acid-water mixture may be used, for example, to acidify the aqueous catalyst solution originating from the neutralizing vessel before this solution is added to the reaction mixture. Acidifying the catalyst solution is advantageous in order to prevent the pH of the reaction mixture in the reaction vessel rising to more than 3, more than 2.5 or more than 2. Any such increase would lead to a reduction in the reaction rate for the generation of the formic acid.

(7) The distillation may be a relatively straightforward flash distillation. The residue from the first distillation in the first distillation apparatus D1 may be passed into a second distillation apparatus D2, where the formic acid is isolated from the extractant EM in a second distillation, which is a second flash distillation, for example. The remaining extractant EM can be passed for extraction into the first extraction vessel E1. There it can be used exclusively or together with fresh extractant EM for implementing the first step of the extraction.

(8) The partition coefficient, for the catalyst HPA-5, for example, may be determined in accordance with the following example:

(9) In a separating funnel, 40 g of N,N-di-n-butylformamide were extracted by shaking with 40 g of an aqueous phase containing 10 wt % formic acid, and with different concentrations of the catalyst HPA-5 in each case, and then left to stand at 40 C. for a week. By this means, complete phase separation was achieved. The phases were then each separated and weighed and the respective concentration of HPA-5 in the aqueous phase was measured by means of a photometer at 435 nm. The partition coefficient K was calculated according to the following formula:

(10) $K_{\mathrm{HPA}\text{-}5}=\frac{\mathrm{concentration}\mathrm{of}\mathrm{HPA}\text{-}5\mathrm{in}N,N\text{-}\mathrm{di}\text{-}n\text{-}\mathrm{butylformamide}}{\mathrm{concentration}\mathrm{of}\mathrm{HPA}\text{-}5\mathrm{in}\mathrm{the}\mathrm{aqueous}\mathrm{phase}}$

(11) The resulting partition coefficients at 40 C. are evident from the following table:

(12) TABLE-US-00001 c(cat, c(FA, c(cat, start) aqueous) c(cat, a) org.) Extractant [wt %] [wt %] [mol/m] [mol/l] K.sub.HPA-5 1 DBFA 1.5 0 2.25*10.sup.6 7.39*10.sup.3 3284 2 DBFA 1.5 1 2.72*10.sup.6 7.42*10.sup.3 2727 3 DBFA 1.5 10 3.18*10.sup.6 7.05*10.sup.3 2216

(13) The abbreviations here have the following meanings:

(14) DBFA: N,N-di-n-butylformamide; c(cat,start): concentration of the catalyst HPA-5 at the start of the experiment; c(FA, aqueous): concentration of formic acid in the aqueous phase; c(cat,a): concentration of the catalyst in the aqueous phase; c(cat,org): concentration of the catalyst in the organic phase; K.sub.HPA-5: partition coefficient K in accordance with the formula above.

(15) The corresponding calibration line for determining the concentration of the catalyst in the aqueous phase after the extraction with DBFA is shown in FIG. 2. In the figure, c(cat, aqueous) is the concentration of the catalyst in the aqueous phase after the phase separation.

(16) Further experiments showed that the phase separation with an aqueous phase comprising formic acid as well as the catalyst is accelerated relative to a phase separation with a purely aqueous solution of the catalyst.

LIST OF REFERENCE SYMBOLS

(17) R reaction vessel

(18) FA formic acid

(19) EM extractant

(20) E1 first extraction vessel

(21) E2 second extraction vessel

(22) D1 first distillation apparatus

(23) D2 second distillation apparatus

(24) N neutralizing vessel

(25) H.sub.2O water

(26) Na.sub.2CO.sub.3 sodium carbonate

(27) NaCOOH sodium formate