Intimate Ni-I metal-acid bifunctional catalysts for selective reductive etherification of carbonyl compounds
Reductive etherification of carbonyl compounds using metal–acid bifunctional catalysts is an important strategy for the synthesis of ethers. Conventional metal–acid bifunctional catalysts suffer from poor synergistic effects due to the spatially separated distribution of hydrogenation sites on metal nanoparticles and acid sites on supports. In this study, bifunctional Ni-I catalysts with intimate metal–acid bifunctional sites were prepared through the surface modification of Ni nanoparticles with organic iodide under H 2 atmosphere. Hydrogenolysis of C-I bonds over Ni nanoparticles promotes the selective deposition of I atoms on the Ni surface. Electronegative I atoms withdraw electrons from Ni nanoparticles, which enables the heterolytic dissociation of H 2 over Ni-I polar sites, leading to the in-situ generation of Ni(H - )-I(H + ) bifunctional sites on the metal surface. The reductive etherification of furfural and ethanol over Ni-I bifunctional catalysts demonstrated high efficiency with an 89.4 % yield to furfuryl ethyl ether (FEE) at mild reaction conditions. Mechanistic studies revealed that both the selective deactivation effect and the in-situ generation of Brønsted acidity contribute to this reaction pathway. While the I atoms selectively deactivate the consecutive Ni sites, suppressing the over-hydrogenation of the furan ring, the in-situ generated Brønsted acid sites catalyze the acetalization. Furthermore, temperature-programmed pyridine FTIR analysis revealed that the Brønsted acidity on the Ni-I catalyst is only evident at high temperatures (e.g., over 100°C) due to the high dissociation energy required for H 2 on non-noble metal surfaces. The developed Ni-I bifunctional catalysts, featuring intimate metal–acid sites and low-cost, holds great promise for the catalytic upgrading of biomass-derived oxygenates into fine chemicals.
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