The selective hydrogenation of unsaturated aldehydes (UAL) to saturated aldehydes (SAL) and unsaturated alcohols (UOL) is an important industrial process for producing fine chemicals. More efforts were made on the selective hydrogenation of CO to UOL, because CC hydrogenation is thermodynamically favored over CO hydrogenation. A crucial step toward high selectivity is the rational design of heterogeneous catalysts. In this Review, the catalyst design for the hydrogenation of UAL to UOL are catalogued into three major strategies, namely, modifying electronic properties, forming electrophilic sites, and involving confinement/steric effect. Research works accomplished in the past decade on the catalyst design for UAL hydrogenation are systematically reviewed using the above strategies. The focus is on the selectivity-enhancing mechanism, methods to perform the chosen strategy, and the factors that influence the mechanism. Density functional theory calculations and catalyst stability are discussed to appreciate the challenges in designing catalysts. In addition, recent advances in the selective hydrogenation of CC of cinnamaldehyde to hydrocinnamaldehyde are briefly reviewed.

Highly dispersed NiCo bimetallic alloy nanoparticles have been successfully immobilized on the SiO 2 frameworks by using heteronuclear metal−organic frameworks (MOFs) as metal alloy precursors. Catalyst characterizations revealed that the average size of NiCo alloy particles was less than 1 nm, with a total metal loading of about 20 wt %. As compared to individual Ni or Co MOF-derived catalysts and the catalysts prepared by the conventional impregnation method, the ultrafine NiCo/SiO 2 -MOF catalyst showed a much better catalytic performance in the catalytic hydrogenation of furfuryl alcohol (FA) to tetrahydrofurfuryl alcohol (THFA) under mild conditions, giving 99.8% conversion of FA and 99.1% selectivity to THFA. It was found that a significant synergistic effect existed between Co and Ni within the subnanometer NiCo/SiO 2 -MOF catalyst, which was 2 and 20 times more active than Ni/SiO 2 -MOF and Co/SiO 2 -MOF, respectively.

Rh-based homogeneous catalysts with phosphine ligands are highly active in hydroformylation reactions. Using DFT calculations, we found a similar electronic effect of inorganic phosphorus in the Rh2P structure. The energy profiles demonstrated that Rh2P would significantly enhance the styrene hydroformylation activity in comparison with Rh, which was further confirmed by experiments. Triphenylphosphine (PPh3) was used as the phosphorus source, and Rh2P supported on silica was prepared by impregnation at a relatively low temperature (550 °C). The turnover frequency (TOF) of styrene hydroformylation was increased to 1496 h–1, which was comparable with some single atom catalysts (SACs). Recycling tests showed a good stability in five runs. Furthermore, HAADF-STEM, XPS, and other characterizations confirmed the synthesis of the Rh2P structure. The promotion effect of P was bifunctional. On the one hand, the doped P separated the surface Rh atoms, which eliminated the surface hollow sites and prevented excessively strong adsorption of the reactants. On the other hand, electrons transferred from Rh to P, causing the surface Rh atoms to be positively charged, which was favorable for hydroformylation reactions. The geometric effects improved the dispersion and the electronic effects changed the rate-determining step from CO insertion to phenylpropionyl hydrogenation, both leading to a higher hydroformylation activity.

No abstract

Toxic Cu–Cr catalysts or precious metal catalysts are currently used for the selective hydrogenation of biomass-derived furfural to the key intermediate furfuryl alcohol (FFA). Herein, efficient and Cr-free Ni-based nonprecious bimetallic catalysts were developed. Catalyst characterizations with XRD, TEM, and TPR and catalytic evaluation revealed that the Ni3Fe1/SiO2 catalyst showed a significant synergetic effect. Compared with the Ni/SiO2 monometallic catalyst, the activity increased by 8 times, and the selectivity to FFA increase from 50% to 91.7% over the Ni3Fe1/SiO2 bimetallic catalyst. Furthermore, the solvent effect is significant in terms of catalytic activity and product distribution. Protic solvents, especially alcohols, showed the highest reaction rate and FFA selectivity. The byproduct tetrahydrofurfuryl alcohol, which is produced by excessive hydrogenation, was almost completely inhibited due to competitive adsorption when using methanol as solvent. At optimized operating conditions, the Ni3Fe1/SiO2 catalyst showed 100% conversion and 96.5% selectivity to FFA in methanol solvent with a good recyclability, which was much better than most reported catalysts.