Process for preparing formic acid by reaction of carbon dioxide with hydrogen

A process for preparing formic acid, comprising the steps of

• (a) homogeneously catalyzed reaction of a reaction mixture (Rg) comprising carbon dioxide, hydrogen, at least one polar solvent selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol and water, and a tertiary amine of the general formula (A1)
• NR1R2R3 (A1)
• in which
• R1, R2, R3 are each independently an unbranched or branched, acyclic or cyclic, aliphatic, araliphatic or aromatic radical having in each case 1 to 16 carbon atoms, where individual carbon atoms may each independently also be substituted by a hetero group selected from the —O— and >N— groups, and two or all three radicals may also be joined to one another to form a chain comprising at least four atoms in each case,
• in the presence of at least one transition metal complex as a catalyst, comprising at least one element selected from groups 8, 9 and 10 of the Periodic Table and at least one phosphine ligand with at least one organic radical having at least 13 carbon atoms,
• in a hydrogenation reactor
• to obtain, optionally after addition of water, a biphasic hydrogenation mixture (H) comprising
• an upper phase (U1) comprising the catalyst and the tertiary amine (A1), and
• a lower phase (L1) comprising the at least one polar solvent, residues of the catalyst and a formic acid-amine adduct of the general formula (A2)
• NR1R2R3*xiHCOOH (A2)
• in which
• xi is in the range from 0.4 to 5 and
• R1, R2, R3 are each as defined above,
• (b) workup of the hydrogenation mixture (H) obtained in step (a) according to one of the following steps:
• (b1) phase separation of the hydrogenation mixture (H) obtained in step (a) in a first phase separation apparatus into the upper phase (U1) and the lower phase (L1),
• or
• (b2) extraction of the catalyst from the hydrogenation mixture (H) obtained in step (a) in an extraction unit with an extractant comprising a tertiary amine (A1) to obtain
• a raffinate (R1) comprising the formic acid-amine adduct (A2) and the at least one polar solvent and
• an extract (E1) comprising the tertiary amine (A1) and the catalyst
• or
• (b3) phase separation of the hydrogenation mixture (H) obtained in step (a) in a first phase separation apparatus into the upper phase (U1) and the lower phase (L1) and extraction of the residues of the catalyst from the lower phase (L1) in an extraction unit with an extractant comprising a tertiary amine (A1) to obtain
• a raffinate (R2) comprising the formic acid-amine adduct (A2) and the at least one polar solvent and
• an extract (E2) comprising the tertiary amine (A1) and the residues of the catalyst,
• (c) separation of the at least one polar solvent from the lower phase (L1), from the raffinate (R1) or from the raffinate (R2) in a first distillation apparatus to obtain
• a distillate (D1) comprising the at least one polar solvent, which is recycled into the hydrogenation reactor in step (a), and
• a biphasic bottoms mixture (B1) comprising
• an upper phase (U2) comprising the tertiary amine (A1), and a lower phase (L2) comprising the formic acid-amine adduct (A2),
• (d) optional workup of the bottoms mixture (B1) obtained in step (c) by phase separation in a second phase separation apparatus into the upper phase (U2) and the lower phase (L2),
• (e) dissociation of the formic acid-amine adduct (A2) present in the bottoms mixture (B1) and/or possibly in the lower phase (L2) in a thermal dissociation unit to obtain the corresponding tertiary amine (A1), which is recycled to the hydrogenation reactor in step (a), and formic acid, which is discharged from the thermal dissociation unit.