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

Abstract

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 À1 ) is converted to propionyl intermediate (C 2 H 5 C(dO)Rh: 2869, 1675 cm À1) 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 CdO (1737 cm À1 ; 13 CdO at 1696 cm À1 ) and aldehyde CÀH absorption (2715 cm À1 ) 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.

NO Oxidation Inhibition by Hydrocarbons over a Diesel Oxidation Catalyst: Reaction Between Surface Nitrates and Hydrocarbons

“…This feature will be discussed in detail later. The group of peaks at 1642, 1650, 1658, 1665 and 1672 cm -1 , on the shoulder of the 1580 cm -1 band, are associated with carboxylate groups (C=O stretch) [11,20]. The intensities of the newly formed bands increased until saturation was reached after around 130 min.…”

Alcohol Synthesis from CO2, H2, and Olefins over Alkali‐Promoted Au Catalysts—A Catalytic and In situ FTIR Spectroscopic Study

“…[38,40,41] These formates are produced by reactiono fH 2 with the adsorbed carbonate species. The intensities of these bands slightly decrease in the H 2 /C 2 H 4 /N 2 feed (Figure 8e)a nd new bands appear at 2893/2879 cm À1 , which are relatedt og aseous ethane [44,45] formed by the hydrogenationo fe thene. The intensities of these bands slightly decrease in the H 2 /C 2 H 4 /N 2 feed (Figure 8e)a nd new bands appear at 2893/2879 cm À1 , which are relatedt og aseous ethane [44,45] formed by the hydrogenationo fe thene.…”

“…[42,43] Furthermore, the activated hydrogen reduces CO 2 to CO (RWGS reaction) for which the characteristic gas-phase spectrum is also seen at 2143 cm À1 ,F igure 8c.U nder subsequent dosing of C 2 H 4 /N 2 (Figure 8d)g aseous CO is removed, but no changes of thea dsorbate bands are observed. The intensities of these bands slightly decrease in the H 2 /C 2 H 4 /N 2 feed (Figure 8e)a nd new bands appear at 2893/2879 cm À1 , which are relatedt og aseous ethane [44,45] formed by the hydrogenationo fe thene. During exposure of the catalystt ot he reaction feed CO 2 /H 2 /C 2 H 4 /N 2 no further changes could be observed (Figure 8f).…”

Nanocatalysis II: In Situ Surface Probes of Nano-Catalysts and Correlative Structure–Reactivity Studies

“…When carried out heterogeneously, homogeneous catalytic reactions can be understood in the surfacereactivity context; however, their kinetic investigation in the heterogeneous mode requires tools with time resolutions in the same regimes as residence times, usually in the millisecond time scale. In Fourier transformed IR where measurements in the vibrational time domain are exchanged for vibrational frequency domain, spectral acquisition can be gated with reactive gases pulsed in short time intervals on the millisecond scale [105,106]. Therefore, FTIR can provide the instrumental time resolution necessary to monitor the history, frozen in time, of catalytic transformations.…”