Method for the continuous production of aldehydes

A process for the continuous preparation of an aldehyde having from 5 to 21 carbon atoms by isomerizing hydroformylation in the homogeneous phase of an olefin composition having from 4 to 20 carbon atoms and comprising an α-olefin or an olefin having internal double bonds by means of a synthesis gas in the presence of a homogeneous rhodium catalyst complexed with an oxygen- and/or nitrogen-containing organophosphorus ligand and free ligand at elevated temperature and elevated pressure in a multistage reaction system comprising at least two reaction zones, wherein the olefin composition is firstly reacted with the synthesis gas having a CO/H2 molar ratio of from 4:1 to 1:2 at a total pressure of from 10 to 40 bar in a group of one or more first reaction zones to a conversion of the α-olefin of from 40 to 95% and the hydroformylation mixture from this group of one or more first reaction zones is reacted with the synthesis gas having a CO/H2 molar ratio of from 1:4 to 1:1000 at a total pressure of from 5 to 30 bar in a group of one or more downstream reaction zones, where the total pressure in the one or more downstream reaction zones is in each case from 1 to (T1 −Tf) bar lower than in the preceding reaction zone, where T1 is the total pressure in the preceding reaction zone and Tf is the total pressure in the reaction zone downstream of the one or more first reaction zones, with the proviso that the difference T1 −Tf is greater than I bar, and the CO partial pressure in the one or more downstream reaction zones is in each case lower than in the reaction zone preceding this reaction zone wherein the catalyst used is a complex of rhodium with a phosphoramidite ligand of the formula Ia

• where
• R15, R16, R17 and R18 are each, independently of one another, hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl, hetaryl, W′COORk, W′COO−M+, W′(SO3)Rk, W′(SO3)−M+, W′PO3(Rk)(Rl), W′(PO3)2−(M+)2, W′NE4E5, W′(NE4E5E6)+X−, W′ORk, W′SRk, (CHR1CH2O)yRk, (CH2NE4)yRk, (CH2CH2NE4)yRk, halogen, trifluoromethyl, nitro, acyl or cyano,
• where
• W′ is a single bond, a heteroatom or a divalent bridging group having from 1 to 20 bridge atoms,
• Rk, E4, E5, E6 are identical or different radicals selected from among hydrogen, alkyl, cycloalkyl and aryl,
• R1 is hydrogen, methyl or ethyl,
• M+ is a cation equivalent,
• X− is an anion equivalent and
• y is an integer from 1 to 240,
• where two adjacent radicals R15, R16, R17 and R18 together with the carbon atoms of the pyrrole ring to which they are bound may also form a fused ring system having 1, 2 or 3 further rings,
• with the proviso that at least one of the radicals R15, R16, R17 and R18 is not hydrogen and R19 and R20 are not linked to one another,
• R19 and R20 are each, independently of one another, cycloalkyl, heterocycloalkyl, aryl or hetaryl,
• a and b are each, independently of one another, 0 or 1,
• P is a phosphorus atom,
• Q is a bridging group of the formula
• where
• A1 and A2 are each, independently of one another, O, S, SiRaRb, NRc or CRdRe, where
• Ra, Rb and Rc are each, independently of one another, hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl or hetaryl,
• Rd and Re are each, independently of one another, hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl or hetaryl or together with the carbon atom to which they are bound form a cycloalkylidene group having from 4 to 12 carbon atoms or the group Rd together with a further group Rd or the group Re together with a further group Re forms an intramolecular bridging group D,
• D is a divalent bridging group selected from among the groups
• where
• R9 and R10 are each, independently of one another, hydrogen, alkyl, cycloalkyl, aryl, halogen, trifluoromethyl, carboxyl, carboxylate or cyano or are joined to one another to form a C3- to C4-alkylene bridge,
• R11, R12, R13 and R14 are each, independently of one another, hydrogen, alkyl, cycloalkyl, aryl, halogen, trifluoromethyl, COOH, carboxylate, cyano, alkoxy, SO3H, sulfonate, NE1E2, alkylene-NE1E2E3+X−, acyl or nitro,
• c is 0 or 1,
• R5, R6, R7 and R8 are each, independently of one another, hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl, hetaryl, COORf, COO−M+, SO3Rf, SO−3M+, NE1E2, NE1E2E3+X−, alkylene-NE1E2E3+X−, ORf, SRf, (CHRgCH2O)xRf, (CH2N(E1))xRf, (CH2CH2N(E1))xRf, halogen, trifluoromethyl, nitro, acyl or cyano,
• where
• Rf, E1, E2 and E3 are identical or different radicals selected from among hydrogen, alkyl, cycloalkyl and aryl,
• Rg is hydrogen, methyl or ethyl,
• M+is a cation,
• X−is an anion and
• x is an integer from 1 to 120,
• or
• R5 and/or R7 together with two adjacent carbon atoms of the benzene ring to which they are bound form a fused ring system having 1, 2 or 3 further rings.