Recycle and Recovery of Aqueous Catalysts

In recent years, the evaluation of environmentally benign (so-called green processes) has been a focal point of oxo developments, according to the definitions and targets of the 

In this respect, risk minimization and life cycle assessments play a major role. 

The compound [HRh(C0)(TPPTS)3] is a precatalyst and dissociates to the 16e species [HRh(C0)(TPPTS)2]. This oxo-active complex initiates the hydroformylation cycle. Under oxo conditions (presence of CO/H2, H20, and a surplus of TPPTS) the hydroxo complex [(HO)Rh(CO)(TPPTS)2] may be formed and again reversibly converted to [HRh(C0)(TPPTS)3] (equilibrium lies almost completely towards the hydride). However, higher carbon monoxide partial pressures may cause the displacement of TPPTS by CO according to Equation 5.8. 

Catalyst deactivation includes (among other reactions) the formation of inactive Rh species, ligand decomposition, or P-C cleavage by direct oxidative insertion of the rhodium metal for formation of PDSPP (propyldi[w-sulfophenyl phosphine) acting as strong electron donor reducing the amount of active Rh catalyst. It turned out to be beneficial to control the Pni/Rh ratio and the CO partial pressure very carefully. 

After years of use catalyst solutions typically contain 20 mg-L 1 iron and 0.7 mg-L 1 of nickel, thus showing no corrosivity. The Rh content of crude aldehyde is in the ppb range this corresponds to losses of less than 10 9 g kg 1 n-butanal, totalling some kg rhodium over a twenty-year period and a production of approximately 5 million metric tons of n-butanal. 

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