Direct Observation of Kinetically Competent Surface Intermediates upon Ethylene Hydroformylation over Rh/Al2O3 under Reaction Conditions by Time-Resolved Fourier Transform Infrared Spectroscopy

Time resolved Fourier transform infrared (FTIR) spectroscopy with millisecond resolution (rapid scan technique) has been employed to detect multiple kinetically relevant surface intermediates in heterogeneous catalytic hydroformylation of ethylene over alumina-supported Rh nanoparticles at 443 K (1 atm total pressure). While corresponding single component C 2 H 4 hydrogenation over Rh/Al 2 O 3 nanoparticle catalysts yields ethane, no hydrogenation of transient ethyl intermediate is observed in the presence of CO. Hence, complete product selectivity is observed with respect to branching between hydroformylation and direct ethylene hydrogenation to ethane. Surface ethyl intermediate (C 2 H 5 Rh: 2876, 2855, 1190 cm −1 ) is converted to propionyl intermediate (C 2 H 5 C(═O)Rh: 2869, 1675 cm −1 ) by reaction with CO with a time constant of 2.7 s. The spectral assignments of propionyl were confirmed by the observation of 13 C shifts. Hydrogenation of propionyl to propionaldehyde with its characteristic C═O (1737 cm −1; 13 C═O at 1696 cm −1 ) and aldehyde C−H absorption (2715 cm −1 ) is the rate -limiting step and proceeds with a time constant of 4.4 s. This is the first time-resolved observation of consecutive elementary steps of heterogeneous catalytic hydrocarbon conversion under reaction conditions.