Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C51/00—Preparation of carboxylic acids or their salts, halides or anhydrides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
  • B01J21/10—Magnesium; Oxides or hydroxides thereof
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/48—Silver or gold
  • B01J23/52—Gold
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J31/00—Catalysts comprising hydrides, coordination complexes or organic compounds
  • B01J31/02—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides
  • B01J31/0234—Nitrogen-, phosphorus-, arsenic- or antimony-containing compounds
  • B01J31/0235—Nitrogen containing compounds
  • B01J31/0237—Amines
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
  • B01J35/31—Density
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
  • B01J35/61—Surface area
  • B01J35/613—10-100 m2/g
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/009—Preparation by separation, e.g. by filtration, decantation, screening
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/02—Impregnation, coating or precipitation
  • B01J37/03—Precipitation; Co-precipitation
  • B01J37/031—Precipitation
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/06—Washing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/16—Reducing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
  • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
  • B01J21/063—Titanium; Oxides or hydroxides thereof
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2231/00—Catalytic reactions performed with catalysts classified in B01J31/00
  • B01J2231/60—Reduction reactions, e.g. hydrogenation
  • B01J2231/62—Reductions in general of inorganic substrates, e.g. formal hydrogenation, e.g. of N2
  • B01J2231/625—Reductions in general of inorganic substrates, e.g. formal hydrogenation, e.g. of N2 of CO2
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration

Definitions

• the present invention relates to a process for preparing formic acid by hydrogenation of carbon dioxide in the presence of a tertiary amine (I) and a catalyst at a pressure of from 0.2 to 30 MPa abs and a temperature of from 20 to 200°C.
• Formic acid is an important and versatile product. It is used, for example, for acidification in the production of animal feeds, as preservative, as disinfectant, as auxiliary in the textile and leather industry, as a mixture with its salts for deicing aircraft and runways and also as synthetic build- ing block in the chemical industry.
• the commonest process at present for the preparation of formic acid seems to be the hydrolysis of methyl formate.
• the aqueous formic acid obtained by hydrolysis is subsequently concentrated, for example by use of an extracting agent such as, for example, a dialkylformamide.
• formic acid can also be obtained by thermal cleavage of compounds of formic acid and a tertiary nitrogen base.
• These compounds are in general acid ammonium formates of tertiary nitrogen bases, in which the formic acid has reacted beyond the stage of classic salt formation with the tertiary nitrogen bases to give stable addition compounds bridged via hydrogen bridge bonds.
• These compounds can be prepared in various ways, such as (i) by direct reaction of tertiary amine with formic acid, (ii) by hydrolysis of methyl formate to form formic acid in the presence of the tertiary amine or with subsequent extraction of the hydrolysis product with the tertiary amine or (iii) by catalytic hydration of carbon monoxide or hydrogenation of carbon dioxide to form formic acid in the presence of the tertiary amine.
• the latter proc- ess of catalytic hydrogenation of carbon dioxide has the particular attraction that carbon dioxide is available in large quantities and is flexible in terms of source.
• EP 0 095 321 A and EP 0 181 078 A mention ruthenium- based and EP 0 151 510 A rhodium-based complex catalysts.
• Preferred tertiary amines are Ci- Cio-trialkylamines, in particular the short-chain Ci-C4-trialkylamines, and also cyclic and/or bridged amines such as 1 ,8-diazabicyclo[5.4.0]undec-7-ene, 1 ,4-diazabicyclo[2.2.2]octane, pyridine or picolines.
• the hydrogenation is carried out at a carbon dioxide partial pressure of up to 6 MPa (60 bar), a hydrogen partial pressure of up to 25 MPa (250 bar) and a temperature from about room temperature to 200°C.
• P.G. Jessop Homogeneous Hydrogenation of Carbon Dioxide, in "The Handbook of Homogeneous Hydrogenation", Ed.: J.G. de Vries and C.J. Elsevier, Volume 1 , 2007, Wiley-VCH Verlag GmbH & Co KGaA, pages 489 to 51 1 presents an overview on the typically used catalysts for the hydrogenation of carbon dioxide.
• the focus is directed to homogeneous catalysts based on elements of group VIII (groups 8, 9, 10) of the periodic table, namely Fe, Ni, Ru, Rh, Pd and Ir, but Mo and Ti are also mentioned as suitable elements.
• WO 2010/149,507 teaches a way to solve this problem by carrying out the homogeneously catalyzed hydrogenation in the presence of a tertiary amine and a polar solvent to form two liquid phases, in which one phase is enriched with the polar solvent and the formed formic ac- id/amine adduct, and the other phase is enriched with tertiary amine and the homogeneous cat- alyst, whereby the latter one containing the homogeneous catalyst is recirculated to the hydrogenation reactor. Nevertheless, the handling of the homogeneous catalysts is a disadvantage of their use.
• Heterogeneous catalysts are known to be generally much more easier separated from the reac- tion products. Unfortunately, neither finely devided metal particles nor conventional metal-based supported catalysts with the metals known from the homogeneous carbon dioxide hydrogenation catalysts show suitable activities and selectivities in the hydrogenation of carbon dioxide.
• A. Baiker discloses in Appl. Organometal. Chem. 14, 2000, pages 751 to 762 the hy- drogenation of carbon dioxide to formic acid derivatives in the presence of immobilized homogeneous catalysts.
• These specific catalysts are sythesized by functionalizing group VIII (groups 8, 9, 10) transition metal complexes, such as [Ru(PR3)3C ], with biunctional silylether-modified phosphines, like Ph 2 P(CH 2 )2Si(OEt) 3 or (CH 3 )2P(CH 2 )2Si(OEt)3, and reacting them with Si(OEt) 4 (triethoxysilan), obtaining an immobilized transition metal-based silica hydrid gel complex cata- lyst.
• group VIII groups 8, 9, 10
• transition metal complexes such as [Ru(PR3)3C ]
• biunctional silylether-modified phosphines like Ph 2 P
• the catalyst was pre- pared by treating silica with (EtO)3Si(CH2)3CI in toluene and thioacetamide in water, reacting the resulting product with RuC ⁇ 3 H2O in ethanol, and mixing the formed catalyst precursor with PP i3 to obtain the immobilized Ru-based complex catalyst, expressed as
• the process should be able to be carried out in a simple manner or at least a simpler manner than is described in the prior art, for example by means of a different, simpler process concept, simpler process stages, a reduced number of process stages or simpler apparatuses. Losses of valuable catalyst should be reduced and also the separation and recycling of the catalyst from the product phase should be simple.
• the process should also be able to be carried out with a low consumption of energy.
• the heterogeneous catalyst comprising gold to be used in the hydrogenation of carbon dioxide can be present in various types. In general, it can be gold itself or gold supported by a support material. In case of being gold itself, preferably gold black is used, but also other types like sup- ported gold nanoparticles are possible. In addition, gold alloys, i.e. Au-M on supports can also be used, where M can be a precious metal like Pd or Pt as well as other kind of metals such as Ag or Cu. Also different metal promoters can be used in one and the same catalyst.
• the heterogeneous catalyst comprising gold is a supported catalyst.
• var- ious types of materials might be used, including but not limited to inorganic oxides, graphite, polymers or metals.
• inorganic oxides silicon dioxide, aluminium oxide, zirconium oxide, magnesium oxide and/or titanium oxide are preferred, but also other inorganic oxides are applicable.
• magnesium oxide aluminium oxide, silica oxide, gallium oxide, zirconium oxide, ceria oxide and/or titanium oxide as support.
• mixtures of different inorganic oxides can also be used.
• the heterogeneous catalyst can be used in various geometric shapes and sizes, for example from powder to shaped material.
• the heterogeneous catalyst In the case of a fixed-bed catalyst, use is made of, for example, pellets, cylinders, hollow cylinders, spheres, rods or extrudates. Their average particle diameter is generally from 1 to 10 mm. In case of metals or polymers as support, also meshs or knitted and crocheted wires or fabrics are applicable.
• the heterogeneous catalyst In case of a supported catalyst, generally comprises 0.01 to 50 wt- % (% by weight), preferably 0.1 to 20 wt.-% and particularly preferably 0.1 to 5 wt.-% gold, based on the total mass of the supported catalyst. In case of a non-supported catalyst, the amount of gold is generally from 0.01 to 100 wt.-%, based on the total weight of the catalyst. Suitable heterogeneous catalysts comprising gold are commercially available or can be obtained by treatment of the support with a solution of a gold component or co-precipitation and subsequent drying, heat treatment and/or calcination by
• the heterogeneous catalyst comprising gold is a supported or non-supported catalyst and irrespective of whether it additionally contains further metals (e.g. in the form of gold alloys), the heterogeneous catalyst comprising gold generally comprises gold containing particles with a diameter of 0.1 to 50 nm, measured by X-ray diffraction spectroscopy. Additionally, it may also contain particles with a diameter of less than 0.1 nm and/or more than 50 nm.
• the heterogeneous catalyst comprising gold generally exhibits a BET surface of > 1 m 2 /g and ⁇ 1000 m 2 /g, determined in accordance with DIN ISO 9277. It preferably exhibits a BET surface of > 10 m 2 /g and ⁇ 500 m 2 /g.
• the volume of the heterogeneous catalyst comprising gold in the hydrogenation reactor is generally between 0.1 and 95% of the reactor volume, whereby the catalyst's volume is calculated by the catalyst's mass divided by its bulk density.
• the tertiary amine (I) to be used in the hydrogenation of carbon dioxide in the process of the invention preferably comprises not more than 9 carbon atoms. It is preferably an amine of the general formula (la)
• NR 1 R 2 R 3 (la) where the radicals R 1 to R 3 are identical or different and are each, independently of one another, an unbranched or branched, acyclic or cyclic or aliphatic radical having from 1 to 7 carbon atoms, preferably from 1 to 3 carbon atoms, but in total R 1 to R 3 together having not more than 9 carbon atoms, where individual carbon atoms can also be substituted, independently of one another, by a hetero group selected from the groups consisting of -O- and >N- or two or all three radicals can also be joined to one another to form a chain comprising at least four atoms in each case.
• Trimethylamine N-ethyl-dimethylamine, N-propyl-dimethylamine, N-butyl-dimethylamine, N- pentyl-dimethylamine, N-hexyl-dimethylamine, N-heptyl-dimethylamine, N-methyl-diethyl- amine, triethylamine, N-methyl-dipropylamines (including N-methyl-di-n-propylamine, N- methyl-di-iso-propylamine and the mixed isomer N-methyl-n-propyl-iso-propylamine), N- methyl-dibutylamines (including N-methyl-di-n-butylamine, N-methyl-di-iso-butylamine, N- methyl-di-tert-butylamine and mixed isomers), tripropylamines (including tri-n-propylamine, tri-iso-propylamine and mixed isomers
• a saturated amine of the general formula (la) and more particularly preferred a saturated amine (la) in which the radicals R 1 to R 3 are selected independently from the group consisting of Ci-Cz-alkyl and Cs-Ce-cyclo- alky, but in total R 1 to R 3 together having not more than 9 carbon atoms.
• amine of the general formula (la) in which the radicals R 1 to R 3 are selected independently from the group consisting of Ci-C3-alkyl.ln particular the tertiary amine (I) is trimethylamine, triethylamine and/or a tripropylamine, whereby trime- thylamine, triethylamine and tri-n-propylamine are particularly preferred.
• the amount of the tertiary amine (I) to be used in the hydrogenation process of the invention is generally from 0.05 to 0.99 ml. tertiary amine (I) per ml. of the total reactor volume and preferably from 0.2 to 0.95 ml. tertiary amine (I) per ml. of the total reactor volume, whereby the volume of the tertiary amine (I) is based on the volume of the liquid tertiary amine (I) it would have as pure substance under reaction conditions.
• the carbon dioxide to be used in the hydrogenation of carbon dioxide can be used in solid, liquid or gaseous form. It is also possible to use industrially available gas mixtures comprising carbon dioxide.
• the hydrogen to be used in the hydrogenation of carbon dioxide is generally gaseous. Carbon dioxide and hydrogen can also comprise inert gases such as nitrogen or noble gases, but surprisingly, the gold catalysts are also tolerating carbon monoxide, which is a catalyst poison when using the standard ruthenium catalysts for this reaction. However, the content of these gases, especially carbon monoxide, should not exceed 20 mol-% based on the total amount of carbon dioxide and hydrogen in the hydrogenation reactor. Although larger amounts may likewise be tolerable, they generally require the use of higher pressure in the reactor which in turn makes further compression energy necessary.
• the hydrogenation of carbon dioxide is carried out in the liquid phase at a temperature of from 0 to 200°C and a total pressure of from 0.2 to 30 MPa abs.
• the temperature is preferably at least 20°C and particularly preferably at least 30°C. Preferably, it is not more than 100°C.
• the total pressure is preferably at least 1 MPa abs and particularly preferably at least 5 MPa and also generally not more than 25 MPa abs and preferably not more than 20 MPa abs.
• the molar ratio of hydrogen to carbon dioxide in the feed to the hydrogenation reactor is pref- erably from 0.1 to 10 and particularly preferably from 1 to 3.
• the molar ratio of carbon dioxide to tertiary amine (I) in the feed to the hydrogenation reactor is generally from 0.1 to 20 and preferably from 0.5 to 3.
• the hydrogenation can be carried out in the presence or in the absence of a further solvent.
• solvents unpolar as well as polar solvents can be added.
• polar solvents like alcohols or water, their amount is generally and advantageously less than 20 wt.-% of the amount of tertiary amine (I).
• As hydrogenation reactors it is in principle possible to use all reactors which are suitable in principle for heterogeneously catalyzed gas/liquid reactions at the given temperature and the given pressure. Suitable standard reactors for the hydrogenation are indicated, for example, in K.D. Henkel, "Reactor Types and Their industrial Applications", in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH Verlag GmbH&Co. KGaA, DOI:
• the hydrogenation of carbon dioxide in the process of the invention can be carried out batch- wise or continuously.
• the reactor is typically charged with the heterogeneous catalyst and the desired tertiary amine (I), and carbon dioxide and hydrogen subsequently introduced to the desired pressure at the desired temperature.
• the reactor is generally depressurized and the liquid reaction mixture separated from the heterogeneous catalyst.
• the tertiary amine (I), carbon dioxide and hydrogen are introduced continuously.
• a fixed-bed heterogeneous catalyst it is generally present beforehand in fixed form in the reactor.
• a suspended heterogeneous catalyst it normally might also be present in the reactor beforehand or be introduced in an amount equal to that of its removal by the continuous reactor discharge. Accordingly, the liquid reaction mixture is continuously discharged from the reactor so that the average liquid level in the reactor remains constant. Preference is given to the continuous hydrogenation of carbon dioxide.
• the liquid reaction mixture is after the hydrogenation reaction generally separated from the heterogeneous catalyst.
• a fixed-bed catalyst it normally stays in the reactor when the reaction mixture is discharged, due to its immobilization.
• a non-immobilized heterogeneous catalyst it is typically either kept back in the reactor by common precautions (e.g. by a mesh or a filter at the outlet) or separated from the reaction mixture by simple filtration, decantation or centrifugation and recycled back to the hydrogenation reactor.
• the liquid reaction mixture is practically free of gold, which means 1 wt.-ppm of gold or less in the separated reaction mixture.
• the average residence time in the reactor is generally from 10 minutes to 10 hours.
• the obtained liquid product mixture comprising formic acid and the tertiary amine (I) generally contains formic acid and the tertiary amine (I) in form of a formic acid/amine adduct. If a tertiary amine of formula (la) was used, the formic acid/amine adduct usually has the general formula (III)
• x HCOOH * NR R2R 3 where the radicals R 1 to R 2 are the radicals described for the tertiary amine (la) and x is from 0.5 to 5, preferably from 1 .2 to 2.6.
• the factor x can be determined, for example by titration with KOH solution against phenolphthalein.
• the precise composition of the formic acid/amine adduct (III) depends on many parameters, for example the prevailing concentrations of formic acid and tertiary amine (la), pressure, temperature or the presence and nature of further components, in particular of polar solvents if present.
• composition of the formic acid/amine adduct (III) can therefore also change over the individual process steps in which the formic acid/amine adduct (III) is in each case referred to in the present patent application.
• the composition of the formic acid/amine adduct (III) can easily be determined in each process step by determining the formic acid content by acid-base titration and determining the amine content by gas chromatography.
• the product mixture obtained by the hydrogenation comprising formic acid and the tertiary amine (I) is then preferably subjected to a base exchange and for this preferably reacted with a tertiary amine (II) which comprises 12 to 48 carbon atoms, the released tertiary amine (I) preferably separated whereby a product mixture comprising formic acid and tertiary amine (II) is obtained, and the formic acid preferably removed from said product mixture by distillation.
• the tertiary amine (II) to be reacted with the product mixture obtained by the hydrogenation preferably has, at a pressure of 1013 hPa abs, a boiling point which is at least 10°C higher, particularly preferably at leas 50°C higher and very particularly preferably at least 100°C higher, than that of formic acid.
• a restriction in terms of an upper limit to the boiling point is not necessary since a very low vapor pressure of the tertiary amine (II) is in principle an advantage for the process of the invention.
• the boiling point of the tertiary amine (II) at a pressure of 1013 hPa abs, if necessary at a pressure extrapolated by known methods from vacuum to 1013 hPa abs, is below 500°C.
• the tertiary amine (II) which is preferably reacted with the product mixture obtained by the hy- drogenation is preferably an amine of the general formula (I la) where the radicals R 1 to R 3 are identical or different and are each, independently of one an- other, an unbranched or branched, acyclic or cyclic, aliphatic, araliphatic or aromatic radical having from 1 to 46 carbon atoms, preferably from 1 to 18 carbon atoms, but in total R 1 to R 3 together having at least 12 carbon atoms and not more than 48 carbon atoms, where individual carbon atoms can also be substituted, independently of one another, by a hetero group selected from the groups consisting of -O- and >N- or two or all three radicals can also be joined to one another to form a chain comprising at least four atoms in each case. Preference is given to at least one of the radicals bearing two hydrogen atoms on the alpha-carbon atom. Examples of suitable terti
• Tributylamines including tri-n-butylamine, tri-iso-butylamine and mixed isomers
• tripen- tylamines including tri-n-pentylamine and all other isomers
• trihexylamines including tri-n- hexylamine and all other isomers
• triheptylamines including tri-n-heptylamine and all other isomers
• trioctylamines including tri-n-octylamine and all other isomers
• trinonylamines including tri-n-nonylamine and all other isomers
• tridecylamines including tri-n-decylamine and all other isomers
• tridodecylamine including tri-n-dodecylamine and all other isomers
• tritetradecylamines including tri-n-tetradecylamine and all other isomers
• tripentadecylamine including tri-
• Triphenylamine N-methyldiphenylamine, N-ethyldiphenylamine, N-propyldiphenylamine, N- butyladiphenylamine, N-2-ethylhexyldiphenylamine, N-dipropylphenylamine, N-dibutyl- phenylamine, N-bis(2-ethylhexyl)phenylamine, tribenzylamine, N-methyldibenzylamine, N- ethyldibenzylamine and derivates thereof which are substituted by one or more methyl, ethyl, 1 -propyl, 2-propyl, 1 -butyl, 2-butyl or 2-methy-2-propyl groups.
• the tertiary amine (II) is a tri-n-pentylamine, a trihexylamine, a triheptylamine, a trioctylamine, N- methyldicyclohexylamine, a N-dioctylmethylamine and/or a N-dimethyldecylamine, whereby tri- n-pentylamine, tri-n-hexylamine, tri-n-heptylamine, tri-n-octylamine, N-methyldi-n-cyclo- hexylamine, N-di-n-octylmethylamine and N-dimethyl-n-decylamine are particularly preferred.
• the amount of the tertiary amine (II) to be used in the reaction with the product mixture obtained by the hydrogenation is generally from 5 to 95 wt.-% and preferably from 10 to 90 wt.-%, based on the amount of tertiary amine (I) in the product mixture (including free tertiary amine (I) and tertiary amine (I) bound in the adduct).
• the reaction of the tertiary amine (II) with the product mixture obtained by the hydrogenation is carried out in the liquid phase at a temperature of generally from 20 to 200°C and a total pressure of from 0.01 to 20 MPa abs.
• the temperature is preferably at least 50 °C and also preferably not more than 150°C.
• the total pressure is preferably at least 0.05 MPa abs and also generally not more than 10 MPa abs.
• reactors for the base exchange it is in principle possible to use all reactors which are suitable in principle for liquid/liquid reactions at the given temperature and the given pressure. Suitable standard reactors for the base exchange are indicated, for example, in K.D.
• the released tertiary amine (I) is then separated whereby a product mixture comprising formic acid and the tertiary amine (II) is obtained.
• the separation is generally performed by distilling off the released tertiary amine (I), but also other methods may be suitable, as for example separation by diaphragm.
• the released tertiary amine (I) is separated by distillation.
• the removal of the tertiary amine (I) can take place in the same apparatus used for the base exchange (e.g. a distillation column or a stirred tank reactor) or in a separate apparatus.
• from 10 to 100%, preferably from 50 to 100%, particu- larly preferably from 80 to 100%, very particularly preferably from 90 to 100% and in particular from 95 to 100% of the separated tertiary amine (I) is recycled to the step of the hydrogenation.
• the base exchange can be carried out batchwise or continuously.
• the obtained liquid product mixture comprising formic acid and the tertiary amine (II) generally contains formic acid and the tertiary amine (II) in form of a formic acid/amine adduct.
• the formic acid/amine adduct usually has the general formula (IV) x HCOOH * NR R2R 3 (IV) where the radicals R 1 to R 2 are the radicals described for the tertiary amine (I la) and x is from 0.5 to 5, preferably from 1 .2 to 2.6.
• the factor x can be determined, for example by titration with KOH solution against phenolphthalein.
• the precise composition of the formic acid/amine adduct (IV) depends on many parameters, for example the prevailing concentrations of formic acid and tertiary amine (Ma), pressure, temperature or the presence and nature of further components, in particular of polar solvents if present.
• the composition of the formic acid/amine adduct (IV) can therefore also change over the individual process steps in which the formic acid/amine adduct (IV) is in each case referred to in the present patent application.
• the composition of the formic acid/amine adduct (IV) can easily be determined in each process step by determining the formic acid content by acid-base titration and determining the amine content by gas chromatography.
• the obtained liquid product mixture comprising formic acid and the tertiary amine (II) is then subjected to distillation in which formic acid is released from the formic acid/amine adduct by thermal dissociation and removed.
• This step can generally be carried out under process parameter known in the prior art for the thermal dissociation of formic acid/amine adducts into free formic acid and the respective amine and, for example, described in EP 0 181 078 A or WO 2006/021 ,41 1 .
• the distillation apparatus generally comprises, in addition to the actual column body with internals, inter alia a top condenser and a bottom evaporator.
• this may optionally also comprise still further peripheral apparatuses or internals and, for example, a flash container in the feed (for example for separating gas and liquid in the feed to the column body), an interme- diate evaporator (for example for improved heat integration of the process) or internals for avoiding or reducing aerosol formation (such as, for example, thermostatable trays, demisters, coalescers or deep-bed diffusion filters).
• the column body may be equipped, for example, with structured packings, random packings or trays.
• the number of separation stages required is dependent in particular on the type of tertiary amine (II), the concentration of formic acid and tertiary amine (II) in the product mixture fed to the distillation apparatus and the desired concentration or the desired purity of the formic acid and can be determined by the person skilled in the art in the customary manner.
• the number of required separation stages is > 3, pref- erably > 6 and particularly preferably > 7.
• the product mixture can be fed to the distillation apparatus, for example, as a side stream to the column body.
• the addition can also be effected upstream of a flash evaporator, for example.
• a flash evaporator for example.
• the distillation apparatus is generally operated at a bottom temperature of from 100 to 300°C and a pressure of from 30 to 3000 hPa abs.
• the distillation apparatus is operated at a bottom temperature of > 120°C, particularly preferably of > 140°C and preferably of ⁇ 220°C and particularly preferably of ⁇ 200°C.
• the pressure is preferably > 30 hPa abs, particularly preferably > 60 hPa abs and preferably ⁇ 1500 hPa abs and particularly preferably ⁇ 500 hPa abs.
• the formic acid released by the thermal dissociation can be obtained as top product and/or side product from the distillation apparatus.
• the product mixture comprises constituents boiling lower than formic acid, it may be advantageous to separate these off by distillation as top product and the formic acid in the side take-off.
• gases may be dissolved in the product mix- ture (such as, for example, carbon monoxide or carbon dioxide), however, it is as a rule also possible to separate off the formic acid together with these as top product.
• the product mixture comprises constituents boiling higher than formic acid, formic acid is preferably separated off by distillation as top product, but optionally instead of these or in addition in the form of a second stream in the side take-off.
• the constituents boiling higher than formic acid are in this case then preferably taken off via an additional side stream.
• formic acid having a content of up to 100 wt.-% can be obtained.
• formic acid contents of from 75 to 99.995 wt.-% are achievable without problems.
• the residual content to 100 wt.-% might, for example, be water added to the hydrogenation of carbon dioxide to pro- mote the heterogeneously catalyzed reaction.-Thus, water may already be present in the product mixture fed to the distillation apparatus but may optionally also form only during the thermal separation in small amounts as a result of decomposition of formic acid itself.
• water is discharged with a part of the eliminated formic acid in a side stream.
• the formic acid content of this side stream is typically from 75 to 95 wt.-%. However, it is also possi- ble to discharge the water and the eliminated formic acid in a common top or side stream. The formic acid content of the product thus obtained is then as a rule from 85 to 95 wt.-%.
• the formic acid obtainable by the process according to the invention has a low color number and a high color number stability.
• a color number of ⁇ 20 APHA and in particular even of ⁇ 10 APHA and optionally even of ⁇ 5 APHA can be achieved without problems. Even on storage for several weeks, the color number remains virtually constant or increases only insignificantly.
• the bottom product obtained in the step of the removal of formic acid by distillation and contain- ing tertiary amine (II) is advantageously recycled to the step of the reaction of the product mixture comprising formic acid and tertiary amine (I) with tertiary amine (II) (base exchange).
• base exchange tertiary amine
• from 10 to 100%, preferably from 50 to 100%, particularly preferably from 80 to 100%, very particularly preferably from 90 to 100% and in particular from 95 to 100% of the tertiary amine (II) of the bottom product is recycled to the step of the hydrogenation.
• the bottom product taken off from the distillation unit can still comprise small residual amounts of formic acid, but the molar ratio of formic acid to tertiary amine (I) is preferably ⁇ 0.1 and particularly preferably ⁇ 0.05.
• DE 34 28 319 A has described the thermal dissociation of an adduct of formic acid and a tertiary amine having C6-Ci4-alkyl radicals in a dissociation column.
• WO 2006/021 ,41 1 also describes the thermal dissociation of an adduct of formic acid and a tertiary amine having a boiling point at atmospheric pressure of from 105 to 175°C in a dissociation column.
• EP 0 563 831 A similarly discloses the thermal dissociation of an adduct of formic acid and a tertiary amine having a boiling point higher than that of formic acid, with added formamide being said to give a particularly color-stable formic acid.
• Figure 1 shows a schematic block diagram of a possible embodiment of the process of the invention.
• the individual letters have the following meanings:
• C distillation unit
• Carbon dioxide and hydrogen are fed into the hydrogenation reactor "A”.
• the carbon dioxide and hydrogen are reacted in the presence of a heterogeneous catalyst comprising gold and a tertiary amine (I) comprising not more than 9 carbon atoms to form a product mixture comprising formic acid and tertiary amine (I).
• the obtained product mixture is fed to a base exchange unit "B” in which it is reacted with a tertiary amine (II) comprising 12 to 32 carbon atoms.
• tertiary amine (I) is released and separated from the obtained product mixture comprising formic acid and tertiary amine (II).
• the released and separated tertiary amine (I) is then preferably recycled back to the hydrogenation reactor "A".
• the obtained product mixture comprising formic acid and tertiary amine (II) is fed to the distillation unit "C” and the formic ac- id/amine adduct is thermally dissociated therein into free formic acid and tertiary amine (II).
• the free formic acid is, for example, removed as overhead product.
• the bottoms from the distillation unit "C” are preferably recycled to the base exchange unit "B".
• the process of the invention makes it possible to obtain concentrated formic acid in high yield and high purity by hydrogenation of carbon dioxide.
• it provides a particularly simple and elegant mode of operation which compared to the prio art has a simpler process concept, simpler process stages, a smaller number of process stages and simpler apparatuses.
• the heterogeneous catalyst comprising gold can be very easily completely separated from the product solution by simple operation like filtration, decantation or centrifugation or can be used as a fixed bed catalyst. Losses of catalyst and thus losses of gold are minimized by the retention of the catalyst in the reactor.
• the simpler process concept makes it possible for the production plant required for carrying out the process of the invention to be made more compact in sense of a smaller space requirement and the use of fewer apparatuses compared to the prior art. It has lower capital cost requirements and a lower energy consumption.
• CO contents lower than 0.2 mol-% in H2/CO2 mixtures were analyzed by passing a gas sample through a (i) minus 23°C cooling trap (to condense readily condensable compounds such as water, formic acid or triethylamine), (ii) a weighted soda lime cartridge (to absorb the CO2), (iii) a minus 80°C cooling trap (to condense water), and (iv) a NDIR-CO analyzer.
• a minus 23°C cooling trap to condense readily condensable compounds such as water, formic acid or triethylamine
• a weighted soda lime cartridge to absorb the CO2
• a minus 80°C cooling trap to condense water
• Au was analyzed by inductively coupled plasma (ICP).
• Example 3 Preparation of Au/MgO Moist Au(OH)3/MgO co-precipitate prepared from 101 mmol of Au as described in Example 1 was suspended in 500 mL of water and magnetically stirred to form a suspension free of lumps. To the stirred suspension 32 mL of 37 wt.-% formaldehyde (referring to 395 mmol formaldehyde) were added at room temperature. The reduction was accompanied by hydrogen evolution. The indigo-colored solid was collected on a filter, washed with water and dried under vac- uum. 75 g of 26 wt.-% Au/MgO were obtained (referring to an Au-content of 26 wt.-% of the total mass of the supported Au/MgO-catalyst).
• the off-gas contained 3.8 g (86 mmol) of CO2, 174 mg (87 mmol) of H2 and 1 1 mg (0.4 mmol) of CO.
• adduct (according to the invention) The continuous production of adduct was carried out in a magnetically driven PARR autoclave with an internal volume of 320 mL equipped with temperature and pressure sensors and with a dip tube carrying a 160 mesh steel net at its internal tip.
• the autoclave was connected with thin, wound up capillary tubes to a high pressure reservoir filled with a H2/CO2 1 :1 mixture, to a reservoir of supercritical CO2 kept at 80°C at 14 MPa abs and to a HPLC pump for the supply with NEt.3.
• the amounts of the introduced gases were determined by weighing the reservoirs and the amount of introduced NEt.3 was determined by volume.
• the autoclave was then heated at 40°C by a heater and a starting pressure of 17.8 MPa abs was attained. As shown in Fig. 3, the pressure slightly decreased with time.
• the experiment shows a significant production of the 1 .33 HCOOH/NEt.3-adduct over a time of about 240 hours.
• Gold black GB 2 afforded a yield of formic acid of approx. 71 % (1.3 mol x 1 .33 / 2.44 mol) bound in the 1.33 HCOOH/NEt.3-adduct with regard to the amounts of introduced H2 and C0 2 .
• Example 10 was repeated but with 1.5 mol-% Ar instead of 1.5 mol-% CO. After 160 hours the pressure leveled off to 4.7 MPa abs. The respective pressure/time curve is shown as curve "B" in Fig. 4. 86 mmol of CO2, 92 mmol of H2, 9.2 mmol of Ar and 0.2 mmol of CO were determined in the vented gases.
• curve "GB 2" shows the pressure/time curve of Example 8 (i.e. without addition of CO).
• the experimental set-up for example 12 was the same as in example 9, but with the difference that the autoclave additionally contained a steel net ring shaped cage for the fixation of the heterogeneous catalyst.
• the autoclave was charged with 13 g of AUROIiteTM AU/T1O2 and 140 ml_ (1 .0 mol) of NEt.3 and pressurized with 46.2 g of H2/CO2 1 :1 mixture (referring to 1004 mmol of H2 and CO2 each).
• the autoclave was then heated at 40°C by a heater and a starting pressure of 18.1 MPa abs was attained. As shown in Fig. 5, the pressure slightly decreased with time. Every time the pressure had dropped to about 13 MPa abs, approximately 18 MPa abs
• the autoclave was pressurized again to approximately 18 MPa abs with 15.6 g of H2/CO2 1 :1 mixture (referring to 339 mol of H2 and CO2 each). In total, twelve of such six-stepped runs were performed within 37 days. The corresponding pressure/time-curves are shown in Fig. 5. After the sixth pressure decrease of the twelfth cycle the procedure was stopped and the autoclave depressurized.
• the off-gas contained 20.6 g (468 mmol) of C0 2 , 0.93 g (465 mmol) of H 2 and 1.31 g (47 mmol) of CO.
• the experimental set-up for example 13 was the same as in example 12, but with the difference that a H2/CO2 1 :1 mixture 99 mol-% with 1.0 mol-% CO was used.
• the autoclave containing 13 g of AUROIiteTM AU/T1O2 and 140 ml. (1.0 mol) of NEt.3 was pressurized at 40°C with 46.2 g of H2/CO2 1 :1 mixture 99 mol-% with 1.0 mol-% CO (referring to 993 mol of H 2 and CO2 each and 17.8 mmol of CO).
• a starting pressure of 18.5 MPa abs was attained, which slightly decreased with time as shown in Fig. 6.
• the experimental set-up for example 14 was the same as in example 13.
• the autoclave containing 12.8 g of AUROIiteTM AU/T1O2 and 140 ml. (1 .0 mol) of NEt 3 was pressurized at 40°C with 46 g of H2/CO2 1 :1 mixture 99 mol-% with 1 .0 mol-% CO (referring to 989 mol of H2 and CO2 each and 17.2 mmol of CO).
• a starting pressure of 18.5 MPa abs was attained, which slightly decreased with time as shown in Fig. 7.
• HCOOH/NEt 3 -adduct (AAR 1.7) and 22 g (500 mmol) of C0 2 , 0.90 g (450 mmol) of H 2 and 3.0 g (107 mmol) of CO were removed from the autoclave.
• the adduct was withdrawn and the gases directly vented from the autoclave. This step is indicated in Fig. 7 by a second arrow above the pressure/time curve.
• 79.5 ml. (568 mmol) of NEt.3 were introduced into the autoclave with the HPLC pump while the stirring was started again.
• the autoclave was pressurized again to approximately 18 MPa abs with 30.6 g of H2/CO2 1 :1 mixture 99 mol-% with 1 .0 mol-% CO (referring to 658 mol of H 2 and C0 2 each and 1 1 mmol of CO).
• H2/CO2 1 :1 mixture was bubbled through the triphasic system at 130°C to strip off ⁇ 3.
• 95% of the NMe3 was recovered together with H2/CO2, having a (H2 C02) NMe3 molar ratio of 2, which is a suitable ratio for the new production of HCOOH/NMe3 adducts.
• the obtained raw HCOOH was finally distilled at 100°C and atmospheric pressure to give a total yield of formic acid of 92%.
• the distillation residue contained HCOOH/NMe3/NHex3 in a molar ratio of 6.3 : 1 : 1 which can be recycled.

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