Hydroformylation is an important industrial process for synthesis of aldehydes from olefins and synthesis gas.
1 It is performed homogeneously over Rh or Co complexes, a process that is cost-intensive due to recycling and loss of catalyst through leaching.
1 An approach to mitigate these drawbacks and maximize metal efficiency is to utilize atomically dispersed Rh atoms on supported oxides.
2 Recent work has demonstrated that singly dispersed Rh atoms supported on ReO
x-modified γ-Al
2O
3 can enhance selectivity for ethylene hydroformylation to propanal, whereas Rh atoms on pristine γ-Al
2O
3 selectively catalyze hydrogenation to ethane.
3 The enhanced selectivity and activity of the ReO
x-modified γ-Al
2O
3 surface has been attributed to weaker binding of CO to the Rh atoms, manifesting itself in blue-shifted CO stretching vibrations relative to those of Rh-CO complexes on unmodified γ-Al
2O
3. It has been hypothesized that Rh atoms vicinal to ReO
x particles are more cationic.
Here we first characterize the electronic properties of Rh atoms on γ-Al2O3 and ReOx-modified γ-Al2O3 surfaces. We explore numerous binding sites, compute IR spectra and compare to the experimentally observed blue-shift in the CO spectra. By analyzing the electron density and the density of states, we conclude that there is indeed less Rh to CO Ï-back-donation in the presence of ReOx. Furthermore, we perform mechanistic studies and microkinetic analysis of an extensive reaction network to gain insights into the enhanced selectivity of Rh atoms on ReOx-modified γ-Al2O3 surfaces for ethylene hydroformylation. We reproduce experimental selectivity trends and kinetic parameters and reveal that selectivity to hydroformylation is due to an interplay between coordination geometry and electronic structure.
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