Method for Catalytically Producing Formic Acid and Regenerating the Catalyst Used in the Process with Little Overpressure

Abstract

The invention relates to a method for catalytically producing formic acid and regenerating the catalyst used in the process. A vanadyl ion, vandate ion, or polyoxometallate ion, which is used as the catalyst, of the general formula [PMo.sub.xV.sub.yO.sub.40].sup.n is brought into contact with an alpha hydroxyl aldehyde, an alpha hydroxy carboxylic acid, a carbohydrate, a glycoside, or a polymer, which contains a carbon chain and which comprises at least one OH group that is bound to the carbon chain as a substituent in a repeating manner and/or an O, N, or S atom contained in the carbon chain in a repeating manner, in a liquid solution (12) in a vessel (10) at a temperature above 70 C. and below 160 C., wherein 6x11, 1y6, 3<n<10, and x+y=12, where n, x, and y is each a whole number. The catalyst reduced in the process is returned to its starting state by oxidation. For this purpose, the solution (12) is brought into contact with a gas (18) which contains a volume percent of oxygen of at least 18% at a pressure of at least 2 bar and maximally 16 bar by means of a mixing device or via a liquid-non-permeable gas-permeable membrane. CO and/or CO.sub.2 resulting during the reaction and merging with the gas (18) is discharged in such a quantity that the volume percent of CO and CO.sub.2 combined does not exceed 80% in the gas (18).

Claims

1. A method for catalytically producing formic acid and regenerating the catalyst employed, where a vanadyl ion, vanadate ion or polyoxometalate ion of the general formula [PMo.sub.xV.sub.yO.sub.40].sup.n serving as catalyst is contacted at a temperature above 70 C. and below 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 one OH group bonded repeatedly as substituent to the carbon chain and/or having an O, N or S atom present repeatedly in the carbon chain, in a liquid solution (12) in a vessel (10), where 6x11 and 1y6 and 3<n<10 and x+y=12, where n, x and y are each an integer, where the catalyst reduced is returned by oxidation to its original state, characterized in that the solution (12) for this purpose is contacted with a gas (18) comprising a volume fraction of at least 18% of oxygen at a pressure of at least 2 bar and at most 16 bar, by means of a mixing apparatus or via a liquid-impermeable, gas-permeable membrane, where CO and/or CO.sub.2 formed in the reaction and passing into the gas (18) are/is taken off in a quantity such that the volume fraction of CO and CO.sub.2 together in the gas (18) does not exceed 80%.

2. The method as claimed in claim 1, characterized in that the volume fraction of CO and CO.sub.2 together in the gas (18) is at least 20% and/or in that the CO and/or CO.sub.2 formed in the reaction and passing into the gas (18) are/is taken off in a quantity such that the volume fraction of CO and CO.sub.2 together in the gas (18) does not exceed 70%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25% or 20%.

3. The method as claimed in claim 1, characterized in that the CO and/or CO.sub.2 formed in the reaction and passing into the gas (18) are/is taken off in a quantity such that the volume fraction of CO and CO.sub.2 together in the gas (18) does not exceed 80%, 70%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25% or 20% by using fresh gas (18) to replace the gas (18) contacting the solution, or at least a part of this gas (18), permanently or intermittently, no later than on attainment of the volume fraction of 80%, 70%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25% or 20%, or by separating the CO and/or the CO.sub.2 from the gas (18).

4. The method as claimed in claim 1, characterized in that the vanadyl ion, vanadate ion or polyoxometalate ion is contacted at a temperature above 80 C., more particularly above 90 C., and/or below 150 C., more particularly below 140 C., with the alpha-hydroxyaldehyde, the alpha-hydroxycarboxylic acid, the carbohydrate, the glycoside or the polymer.

5. The method as claimed in claim 1, characterized in that the pressure is at least 3 bar, at least 4 bar or at least 5 bar and/or at most 15 bar, 12 bar, 10 bar, 9 bar, 8 bar, 7 bar or 6 bar.

6. The method as claimed in claim 1, characterized in that the CO and/or CO.sub.2 formed in the reaction and passing into the gas are/is taken off in a quantity such that, with a pressure restricted to a maximum value or with pressure held constant, the oxygen partial pressure in the gas is diminished by the CO and/or CO.sub.2 by not more than 10%.

7. The method as claimed in claim 1, characterized in that for the oxidation of the catalyst, a portion of the liquid solution (12) is led out of the vessel (10), contacted with the gas (18), and subsequently supplied again to the remainder of the liquid solution (12).

8. The method as claimed in claim 1, characterized in that vaporous formic acid formed in the gas, more particularly before the replacement of the solution-contacting gas (18) by fresh gas (18), is absorbed from the gas (18) by means of an absorbent suitable for absorbing formic acid, more particularly a linear alcohol, more particularly 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol or 1-decanol, or an amide, more particularly an N,N-dialkylcarboxamide, more particularly dipentylformamide, N(N-hexadecyl)formamide, N,N-di-n-butylformamide (DBFA), N-di-n-acetamide, N-methyl-N-heptylformamide, N-n-butyl-N-2-ethylhexylformamide or N-n-butyl-N-cyclohexylformamide, and is subsequently desorbed therefrom, or is absorbed from the gas (18) by a base, more particularly an aqueous solution of NaOH or KOH, and the resulting salt solution is led off or is condensed at the vessel (10) or outside the vessel (10), where a condensate formed as a result is led back into the vessel, supplied to an extraction, more particularly by means of one of the stated absorbents, to separate the formic acid from the water contained therein, or led off for the separation of the formic acid.

9. The method as claimed in claim 1, characterized in that the mixing apparatus comprises a static mixer (34), a reactive mixing pump, a nozzle, more particularly a Venturi nozzle (36) or a spraying nozzle, and/or a gas introduction stirrer.

10. The method as claimed in claim 1, where the mixing apparatus is designed in such a way and operated in such a way, or the membrane is constructed in such a way, that the surface area of the solution is increased by a factor of at least 1000 as a result.

11. The method as claimed in claim 1, characterized in that the CO and/or the CO.sub.2 are/is separated from the gas (18) by means of a membrane which is permeable for the CO and/or the CO.sub.2 and impermeable or of only limited permeability for O.sub.2, a combination of two or more membranes with different permeabilities, or by means of a pressure swing adsorption.

12. The method as claimed in claim 1, characterized in that the gas (18) is guided in a circuit, in particular without a pressure drop of more than 2.5 bar.

13. The method as claimed in claim 1, characterized in that the method is carried out as a continuous process.

14. The method as claimed in claim 1, characterized in that the catalyst and the formic acid are separated from the solution (12) or from a portion of the solution (12) that is subsequently resupplied to the solution (12), by means of at least one polar organic extractant which extracts the formic acid and the catalyst and which, on mixing with the solution (12), forms a phase boundary between the solution (12) and the extractant, more particularly with the pH being maintained at not more than 3, more particularly not more than 2.5, in the solution (12), 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 of the formic acid present at a concentration of 5 wt % in water at 40 C., and where the extractant before the extraction is saturated with the catalyst or where the separated catalyst is separated from the extractant and resupplied to the liquid solution (12) in the vessel (10).

15. The method as claimed in claim 14, characterized in that the separation takes place in a two-stage process, by extracting the solution (12) in a first extraction step with a first quantity of the extractant for a first time, to extract the catalyst, and extracting the solution in a second extraction step with a second quantity of the extractant for a second time, to extract the formic acid, where the catalyst extracted in the first extraction step is supplied again to the liquid solution (12) in the vessel (10).

16. The method as claimed in claim 14, characterized in that the extractant is admixed with an additive, more particularly an apolar additive.

17. The method as claimed in claim 14, characterized in that the extractant is an amide, more particularly an N,N-dialkylcarboxamide, more particularly dipentylformamide, N(N-hexadecyl)formamide, N,N-di-n-butylformamide (DBFA), N-di-n-acetamide, N-methyl-N-heptylformamide, N-n-butyl-N-2-ethylhexylformamide or N-n-butyl-N-cyclohexylformamide.

18. The method as claimed in claim 14, characterized in that the catalyst is separated by means of precipitation as a salt, more particularly simultaneously with a precipitation of extracted formic acid as formate, or by means of further extraction with a polar further extractant, more particularly a solvent of the solution (12), and with a temperature change of the extractant and/or an increase in the pH of the extractant, more particularly through addition of a carbonate and/or a hydroxide.

Description

[0040] The invention is elucidated in more detail below using exemplary embodiments. In the drawings:

[0041] FIG. 1 shows a schematic representation of an apparatus for implementing the method of the invention,

[0042] FIG. 2 shows a schematic representation of an alternative apparatus for implementing the method of the invention, with a static mixer,

[0043] FIG. 3 shows a schematic representation of an alternative apparatus for implementing the method of the invention, with a Venturi nozzle, and

[0044] FIG. 4 shows a graphic representation of the yield of formic acid as a function of the volume fraction of CO and CO.sub.2 together in the gas.

[0045] FIG. 1 shows a vessel 10 which contains the solution 12 with the catalyst and with a substrate in the form of an alpha-hydroxyaldehyde, an alpha-hydroxycarboxylic acid, a carbohydrate, a glycoside or a polymer containing a carbon chain and having at least one OH group bonded repeatedly as substituent to the carbon chain and/or having an O, N or S atom present repeatedly in the carbon chain.

[0046] Opening into the vessel 10 is the supply line 14 for supplying the gas 18 comprising a volume fraction of at least 18% of oxygen. The gas 18 may be supplied from outside or via the return line 24 described below. The surface of the solution 12 forms the interface 16 with the gas 18, this gas collecting in the vessel 10 above the solution 12 and exhibiting a gas pressure of at least 5 bar and at most 33 bar. The gas is supplied to the separator 22 via the offtake line 20. In the separator 22 there is a separation of CO and/or CO.sub.2 from the gas by means of a membrane which is permeable to CO and CO.sub.2 but impermeable or of only limited permeability to O.sub.2, or by means of pressure swing adsorption. The remaining gas is fed back into the supply line 14, via the return line 24 and the compressor 26, before being blown back into the solution 12. The oxygen in the gas that is not consumed by oxidation of the catalyst is circulated accordingly, while the CO and/or CO.sub.2 formed in the solution during the reaction are/is separated.

[0047] FIG. 2 shows an alternative apparatus which allows the method of the invention to be implemented with a greater efficiency than the apparatus shown in FIG. 1. The solution 12 of the aforementioned substrate, present in the vessel 10, is led out of the vessel 10 through the liquid line 28 and is pumped by means of the pump 30 first through the heat exchanger 32 and then through the static mixer 34. The static mixer 34 is supplied via the supply line 14 with a gas comprising a volume fraction of at least 18% of oxygen and having a gas pressure of at least 5 bar and at most 33 bar, from the outside or from the feed line 23, described below, and said gas is contacted with the solution 12 in the static mixer 34 under this pressure. This is accompanied by a large increase in the surface area of the solution 12 and hence an increase in the interface 16 with the gas 18, and consequently by effective oxidation of the catalyst. By means of the heat exchanger 32, the solution 12 is kept at a constant temperature.

[0048] After passing through the static mixer 34, the solution 12 with the gas 18 dispersed therein in small bubbles is introduced through the liquid line 28 into the vessel 10 and is jetted into the solution 12. The effect of this, in addition to the intense commixing which takes place in the static mixer 34, is a further intense commixing of the solution 12 with the gas 18, and so as a result of this as well there is oxidation of the catalyst in the vessel 10. The gas 18 under pressure over the solution 12 is supplied via the offtake line 20 to the separator 22, in which CO.sub.2 and/or CO are/is separated from the gas 18 by means of a membrane or pressure swing adsorption. The gas 18 is then fed via a compressor 26 and the feed line 23 into the supply line 14, from where it is fed into the static mixer 34. The compressor 26 conveys the pressurized gas 18 within the circuit thus formed. The oxygen in the gas 18 that is not needed for the oxidation of the catalyst is circulated in this way, while the CO and/or CO.sub.2 formed by the reaction are/is separated in the separator 22. This prevents the volume fraction of CO and CO.sub.2 together in the gas exceeding 55%.

[0049] The apparatus shown in FIG. 3 differs from the apparatus shown in FIG. 2 in that, rather than the static mixer 34, there is a Venturi nozzle 36. The solution 12 supplied to the Venturi nozzle 36 via the liquid line 28 flows through the Venturi nozzle 36, entraining, as it does so, the gas 18 supplied via the supply line 14, as a result of the Venturi effect. The Venturi nozzle 36 brings about intense commixing of the gas 18 with the solution 12. Further intense commixing with the solution 12 is brought about by the jetting of the solution 12, with the gas 18 dispersed therein as small bubbles, from the Venturi nozzle 36 into the solution 12 in the vessel 10. The solution 12 may be jetted via the liquid line 28. Also possible, however, is for the Venturi nozzle 36 to be disposed directly on the vessel 10 and for the solution 12 to be jetted directly from the Venturi nozzle 36 into the solution 12.

[0050] Because the Venturi nozzle 36 conveys the gas 18 in a circuit, there is no need here for a compressor 26. The provision of the Venturi nozzle 36 simplifies the apparatus and so makes it more cost-effective. It is also possible to lead the solution 12 first through a static mixer 34 and then through a Venturi nozzle 36, or initially through a Venturi nozzle 36 and then through a static mixer 34, and to supply the gas 18, by means of a divided supply line 14, both to the static mixer 34 and to the Venturi nozzle 36. In this case as well, there may be no need for a compressor 26.

[0051] FIG. 4 shows the outcome of the determination of the yield of formic acid when using sugar as the reactant in the method of the invention, for different defined volume fractions of CO and CO.sub.2 together in the gas. The yield in this case is reported as the specific product yield (space-time yield). Here it is found that in the case of this substrate, the yield of formic acid drops significantly at and above a volume fraction of CO and CO.sub.2 together of 55%. Depending on the substrate, this drop may also take place at a lower or higher volume fraction. The highest volume fraction at which the drop takes place is 80%.

[0052] Oxidation of a Fermentation Residue

[0053] 2800 g of an aqueous solution 12 containing 21 g of H.sub.8PMo.sub.7V.sub.5O.sub.40 and 50.6 g of H.sub.2SO.sub.4 are admixed with 195 g of separated fermentation residue (dry matter) from a biogas plant. In a stirred tank autoclave, the solution 12 is contacted with oxygen under an oxygen partial pressure of 4 bar and is stirred at 130 C. For the supply of O.sub.2 and removal of resultant CO.sub.2, flushing takes place with a gas stream of 0.7 In O.sub.2 per minute. After a reaction time of just 5.5 hours, the formic acid content of the reaction solution already corresponds to a yield of approximately 24%.

[0054] Oxidation of Spent Distillery Grains

[0055] The reaction is carried out as for the oxidation of the fermentation residue. In deviation from that procedure, 98 g of toluenesulfonic acid instead of H.sub.2SO.sub.4, and 195 g of dry spent grains from a bioethanol production facility, instead of the fermentation residue, are used. In further deviation, the solution 12 is contacted with oxygen under an oxygen partial pressure of 8 bar. After a reaction time of only 2 hours, the formic acid content of the reaction solution corresponds to a yield of approximately 51%.

[0056] Oxidation of Cattle Manure

[0057] The reaction is carried out as for the oxidation of the fermentation residue. In deviation from that procedure, 195 g of cattle manure (dry matter), instead of the fermentation residue, are used. In further deviation, the temperature is raised after one hour from 110 C. to 120 C. and after a further 2 hours to 130 C. After a reaction time of 4 hours, the formic acid content of the reaction solution corresponds to a yield of approximately 23%.

[0058] Oxidation of Banana Skins

[0059] 2400 g of an aqueous solution 12 comprising 43.7 g of H.sub.8PMo.sub.7V.sub.5O.sub.40 and 82.7 g of toluenesulfonic acid are admixed with 90 g of dried banana skin. In a stirred tank autoclave, the solution 12 is contacted with oxygen under an oxygen partial pressure of 10 bar and is stirred at 90 C. To supply O.sub.2 and remove resultant CO.sub.2, flushing takes place with a gas stream of 1.0 In O.sub.2 per minute. After a reaction time of 5 hours in batch operation, the formic acid content of the solution 12 corresponds to a yield of approximately 36%.

[0060] Oxidation of Fruit Marc

[0061] The reaction is carried out as for the oxidation of the banana skins. In deviation from that procedure, 90 g of dried marc are used instead of the banana skins. After a reaction time of 5 hours in batch operation, the formic acid content of the solution 12 corresponds to a yield of approximately 25%.

[0062] Oxidation of Sucrose without Gas Exchange

[0063] The reaction is carried out like the oxidation of the fermentation residue. In deviation therefrom, 1652 g of water, 3 g of H.sub.8PMo.sub.7V.sub.5O.sub.40, 5.7 g of toluenesulfonic acid and 200 g of sucrose are used. In further deviation, the solution 12 is contacted with oxygen under an oxygen partial pressure of 10 bar, without any gas exchange during the reaction, and is stirred at 90 C. After a reaction time of 4 hours, the formic acid content of the reaction solution corresponds to a yield of approximately 30%.

[0064] Oxidation of Sucrose with Permanent O.sub.2 Stream

[0065] The reaction is carried out like the oxidation of the fermentation residue. In deviation procedure, 1652 g of water, 3 g of H.sub.8PMo.sub.7V.sub.5O.sub.40, 5.7 g of toluenesulfonic acid and 200 g of sucrose are used. In further deviation, the solution 12 is contacted with oxygen under an oxygen partial pressure of 10 bar and is stirred at 90 C. To supply O.sub.2 and remove resultant CO.sub.2, flushing takes place with a gas stream of 5 In O.sub.2 per minute. After a reaction time of 4 hours, the formic acid content of the reaction solution corresponds to a yield of approximately 40%.

LIST OF REFERENCE NUMERALS

[0066] 10 vessel [0067] 12 Solution [0068] 14 supply line [0069] 16 interface [0070] 18 gas [0071] 20 offtake line [0072] 22 separator [0073] 23 feed line [0074] 24 return line [0075] 26 compressor [0076] 28 liquid line [0077] 30 pump [0078] 32 heat exchanger [0079] 34 static mixer [0080] 36 Venturi nozzle