Theoretical Insights into the Heterogeneous Hydroformylation of Ethylene on Atomically Dispersed Rh-Oxide Promoter Pairs on ?-Al2O3 Support

Atomically dispersed late-transition state metals on oxide supports have emerged as a new frontier in catalysis as they combine the advantages of both homogeneous and heterogeneous catalysts.¹ Successful heterogenization of the hydroformylation reaction, an industrially significant process for aldehyde production conventionally performed on homogeneous Rh or Co complexes,² has been reported recently for atomically dispersed Rh on oxide supports.^3-4 Interestingly, the presence of oxide promoters on the support seems to enhance the catalyst’s selectivity for hydroformylation versus hydrogenation of the olefin. Ro et al.³ have demonstrated that Rh-ReO_x pairs atomically dispersed on γ-Al₂O₃ are more selective for the hydroformylation of ethylene than Rh alone.

In this paper, we present a theoretical study of ethylene hydroformylation by Rh atomically dispersed on γ-Al₂O₃(110) in the absence and presence of two oxide promoters, Re₂O₇ and WO₃, and evaluate the developed mechanisms by comparing the predicted kinetic observables (selectivity, apparent activation energies, and reaction orders) with experimental values. We analyze how the promoters modify the electronic properties of the Rh(I) active site and elucidate their role in the catalyst’s enhanced selectivity for the hydroformylation pathway. The Re(VII) promoter seems to induce no electronic changes to Rh that are related to the latter’s catalytic function, and it only blocks the dehydrogenation pathway by posing a steric hindrance. On the other, the W(VI) promoter seems to have a more profound electronic effect as it must be reduced to the W(IV) state before it is activated.

1. Liu J. et al. ACS Catal., doi. 10.1021/acscatal.5b01816 (2017).
2. Franke R. et al. Chem. Rev., doi. 10.1021/cr3001803 (2012).
3. Ro I. et al. ACS Catal., doi. 10.1021/acscatal.9b02111 (2019).
4. Lang R. et al. Angew. Chem. Int., doi. 10.1002/anie.201607885 (2016).