Hydroformylation is one of the most important industrial processes for the production of aldehydes. In addition to alkenes, aldehydes can be available substrates for the synthesis of higher aldehydes and/or alcohols via hydroformylation. Notably, the hydroformylation of formaldehyde is considered as a pivotal intermediate reaction in the one-step synthesis of ethylene glycol from syngas, which significantly determines the target product selectivity. The conventional catalysts primarily rely on rhodium (Rh) carbonyl compounds, which catalyze formaldehyde hydroformylation via a homogeneous approach under severe conditions. Therefore, it is challenging to design efficient and stable catalysts and to facilitate the separation of precious metal catalysts. Herein, an efficient Rh/CeO2 heterogeneous catalyst was successfully constructed by the wet-chemical method to catalyze the hydroformylation of formaldehyde. High glycolaldehyde formation rate (~ 190 h-1, comparable to homogeneous catalysts) and selectivity (81.7%) were achieved at the mild reaction conditions (100 °C and 7 Mpa). The collection of characterizations, including HAADF-STEM, CO adsorption IR, XPS, and H2-TPR/TPD, demonstrates a significant effect of support and Rh particle size on the formaldehyde hydroformylation. The atomically dispersed Rh clusters formed on 0.7 wt% Rh/CeO2 are more active for the production of glycolaldehyde than isolated Rh atoms and large Rh nanoparticles. The synergistic effect of Rh+ and Rh0 species on the Rh cluster promotes the insertion of CO and the dissociation of H2, bringing a remarkable enhancement in the hydroformylation rates. While the isolated Rh atoms in the high-oxidation state are inert to hydroformylation but facilitate the formaldehyde hydrogenation to methanol. Moreover, CeO2 supported Rh clusters exhibit good stability in the recycle experiments for formaldehyde hydroformylation with a negligible decrease in the conversion and selectivity. The synthesis strategy for Rh/CeO2 in this work can provide a new approach for constructing high-efficient catalysts and the understanding of the structure-performance relationship shed light on the heterogeneous catalytic processes for the hydroformylation reaction.

Insight into the size effect of Rh species on heterogeneous hydroformylation of formaldehyde

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