Method for recovering rhodium from reaction products of oxosynthesis

#The present invention relates to an improved process for removing and recovering rhodium from oxo process reaction products.

#The preparation of aldehydes and alcohols by addition of carbon monoxide and hydrogen to olefinic double bonds (hydroformylation) is well known. The reaction is catalyzed by metals or compounds thereof of the 8th transition group of the Periodic Table which form carbonyl or hydridocarbonyls under the reaction conditions. While cobalt and cobalt compounds used to be used as catalysts, rhodium catalysts are today finding use to an increasing extent, even though rhodium is several times more expensive than cobalt. Rhodium is used alone or in combination with complexing agents, for example organic phosphines. While the oxo process requires reaction pressures of from 25 to 30 MPa using rhodium as catalyst, pressures of from 1 to 5 MPa suffice when rhodium complexes are used.

#In many cases, rhodium catalysts have distinct advantages. They have higher activity and selectivity and additionally facilitate uncomplicated operation of the production plants, in particular relating to conduct of the synthesis and the excavation of the products from the reactor. Finally, the classic oxo process based on cobalt catalysts may be converted in many cases to rhodium catalysts using the available apparatus parts with only minimal capital expenditure.

#1 000 g of a used catalyst solution having the following composition: 55% by weight of n-butyraldehyde, 2% by weight of triphenylphosphine oxide, 6.3% by weight of triphenylphosphine, 36.2% by weight of high-boilers (generally aldol condensation products of butyraldehyde) and 0.5% by weight of low-boilers (generally hydrocarbons having from 3 to 7 carbon atoms) and comprising 699 mg of rhodium (6.79 mmol) and 234 mg of cobalt (3.97 mmol) are diluted by mixing with 1 000 g of fresh n-butyraldehyde in a 4 l three-necked flask and then extracted twice with 100 g of 65% phosphoric acid for 30 minutes at 22° C. each time. The phases are separated under gentle stirring likewise within 30 minutes. The organic phase is then washed twice with 50 g of deionized water each time which had been set using 3 ml of NaOH (10% strength) to a pH of 5.8. After subsequent phase separation, the remaining organic phase is transferred to a 4 l three-necked flask equipped with a frit attachment and frit filter and aerated with 150 l/h of air at 55° C. over 12 hours at 55° C. After the oxidation had ended, 1969 g of organic phase were obtained. The organic phase is transferred together with 199.8 g of an aqueous solution of trisodium tri(m-sulfophenyl)phospine (TPPTS) (corresponds to 101.9 mmol of P III, Rh: P=1:15) into a 5 l steel autoclave and stirred in intensively for 3 hours at 5 MPa of CO/H2 pressure at 125° C. The reaction mixture is then transferred to a 4 l three-necked flask equipped with a lower outlet and lower aqueous phase (144.2 g) is removed from the upper organic phase (1979 g). The aqueous TPPTS solution contains 687.7 mg of rhodium, corresponding to 98.4% of the rhodium quantity originally present in the starting solution. The cobalt content in the aqueous TPPTS solution was below the analytical detection limit.

#The Rh/TPPTS solution (P:Rh=14:1) obtained according to example 1 is adjusted by adding fresh TPPTS to a P:Rh ratio of 100:1. The rhodium content is 260 ppm. The solution is charged into a 0.2 l stainless steel autoclave. The 0.2 l stainless steel autoclave equipped with a stirrer is charged with propylene and a CO/H2 mixture consisting of equal volume fractions in such a quantity as 10 l/h (STP) of off gas may be withdrawn from the reactor [l/h (STP) means 1 liter of off gas under atmospheric conditions (20° C. and 1 at) per hour]. At the same time, 300 ml per hour of aqueous catalyst solution are circulated through the reactor. The hydroformylation is carried out semicontinuously over 8 hours. The remaining reaction parameters and the results of the hydroformylation are given in table 1.

#TABLE 1 Hydroformylation of propylene in the presence of a worked-up catalyst solution Experiment duration [h] 8 Temperature [° C.] 122 Pressure [bar] 50 Rh content [mg/kg] 260 P (III) content [mmol/kg] 263 Ligand/Rh 100 C3 introduction rate [g/h] 40 Activity [mol of aldehyde/mol of Rh · min] 15.08 Productivity [kg of aldehyde/l cat. sol. · h] 0.213 Conversion [%] 37 n/i ratio 93/7