The preparation of aldehydes and alcohols by the addition of carbon monoxide and hydrogen across olefinic double bonds (hydroformylation) is known. The reaction is catalyzed by metals of Group VIII of the Periodic Table (IUPAC version) or compounds thereof which, under the reaction conditions, form carbonyls or hydridocarbonyls. Whereas cobalt and cobalt compounds were formerly used almost exclusively as catalysts, at present, the use of rhodium, even though it costs much more than cobalt, has increased. For this purpose, rhodium is used alone or in combination with complexing agents, for example organic phosphines. Whereas the reaction pressures required for the oxo synthesis with rhodium as the catalyst are from 25 to 30 MPa, pressures of from 1 to 5 Mpa suffice when rhodium complex compounds are used.
Rhodium catalysts have distinct advantages. They have higher activity and selectivity and, moreover, make it possible to operate the plant without problems in many respects, especially as to the carrying out of the synthesis and the removal of the products from the reactor. Finally, the classical oxo process based on cobalt catalysts can, in many cases, be converted to rhodium catalysts using the same apparatus, thereby minimizing capital costs.
However, there are considerable difficulties in the loss-free--or at least substantially loss-free--removal and recovery of the rhodium, whether it is employed as catalyst with or without an additional complexing agent. After completion of the reaction, the rhodium is present in the hydroformylation product in the form of a solution of its carbonyl compound which may contain other ligands.
For workup, the pressure of the crude oxo product is normally reduced in several stages by initially reducing the synthesis pressure, which is about 1 to 30 MPa depending on the nature of the rhodium catalyst employed, to about 0.5 to 2.5 MPa. This releases the dissolved synthesis gas. Thereafter, it is possible to reduce the pressure to atmospheric. The rhodium is removed either immediately from the crude product or from the residue from the distillation of the crude product. The first route is followed when rhodium has been employed as the catalyst without additional complexing agent in the preceding hydroformylation stage; the second variant is applied when the rhodium catalyst contains other ligands in addition to carbon monoxide, for example phosphines or phosphites in complex linkages. It can also be used when, although the hydroformylation has been carried out with rhodium alone, a complexing agent has been added to the crude product after reducing the pressure to stabilize the rhodium. It is always necessary to take into account the fact that the noble metal is present in the crude product in a concentration of only a few ppm, and thus removal thereof must be carried out very carefully. Additional difficulties may arise due to the fact that, during the reduction in pressure, the rhodium, especially when it has been employed without a ligand, undergoes partial conversion into metallic form or forms polynuclear carbonyls. The result is the formation of a heterogeneous system which is composed of a liquid organic phase and a solid phase containing rhodium or rhodium compounds.
The recovery of rhodium from the products of the oxo synthesis including the residues of crude oxo products has been investigated many times. Such studies have led to the development of numerous processes, of which a few have also been used on an industrial scale.
U.S. Pat. No. 4,400,547 relates to the hydroformylation of olefins with 2 to 20 carbon atoms in the presence of unmodified rhodium as the catalyst. After completion of the reaction, a complex-forming compound such as triphenylphosphine is added to the crude oxo product, and the aldehyde is removed by distillation. The distillation residue is subsequently treated with oxygen in order to eliminate the ligand from the complex compound again and to recover the rhodium in active form. Separation of the rhodium from the distillation residue is not possible in this procedure.
The removal of noble metals such as rhodium from high-boiling hydroformylation residues is also described in U.S. Pat. No. 3,547,964. To do this, the residues are treated with hydrogen peroxide in the presence of acids such as formic acid, nitric acid, or sulfuric acid. However, there are limits to the industrial application of the process because of the high cost of hydrogen peroxide and the difficulties of handling it.
According to DE 24 48 005 C2, a rhodium-containing distillation residue is initially treated with acids and peroxides. Excess peroxides are subsequently decomposed by heating, and the aqueous solution containing the catalyst metal is reacted with hydrohalic acid or alkali metal halides and with tertiary phosphines and carbon monoxide, or compounds releasing carbon monoxide, in the presence of a water-soluble organic solvent. This procedure also requires the use of peroxides with the disadvantages described above, as well as the use of halogen-resistant materials.
Finally, U.S. Pat. No. 4,390,473 describes a process for the recovery of rhodium and cobalt from a solution which has been employed as catalyst in a low-pressure oxo process. To remove the complex-bound metals, aqueous formic acid is added to the solution, and an oxygen-containing gas is passed through. This results in two phases, an organic phase, and an aqueous phase which contains the metals dissolved as formates. After the phases have been separated, it is possible to obtain cobalt and rhodium from the aqueous solution. In practice, however, the reducing action of formic acid has proven very bothersome. This property resulted in the rhodium being partially deposited in metallic form, and no longer amenable to recovery.