Influence of Synthesis Conditions on the Structure of Nickel Nanoparticles and their Reactivity in Selective Asymmetric Hydrogenation

Unsupported and SiO2-supported Ni nanoparticles (NPs) were synthesised via hot-injection colloidal route using oleylamine (OAm) and trioctylphosphine (TOP) as reducing and protective agents, respectively. By adopting a multi-length scale structural characterization, it was found that by changing equivalents of OAm and TOP not only the size of the nanoparticles is affected but also the Ni electronic structure. The synthetized NPs were modified with (R,R)-tartaric acid (TA) and investigated in the asymmetric hydrogenation of methyl acetoacetate to chiral methyl-3-hydroxy butyrate. The comparative analysis of structure and catalytic performance for the synthetized catalysts has enabled us to identify a Ni metallic active surface, whereby the activity increases with the size of the metallic domains. Conversely, at the high conversion obtained for the unsupported NPs there was no impact of particle size on the selectivity. (R)-selectivity was very high only on catalysts containing positively charged Ni species such as over the SiO2-supported NiO NPs. This work shows that the chiral modification of metallic Ni NPs with TA is insufficient to maintain high selectivity towards the (R)-enantiomer at long reaction times and provides guidance for the engineering of long-term stable enantioselective catalysts.