“Upside-Down” Cr 2 O 3 /Pd 0 /TiO 2 Catalysts for Ambient Hydrogenation of Nitriles to Primary Amines: Insights into the Activity and Selectivity

Primary amines are of high importance in biology, medicine, and industry as precursors for a wide range of valuable products. Today, their major synthesis via the catalytic hydrogenation of nitriles suffers from insufficient selectivity. In the present work, the prepared inverse (“upside-down”) Cr2O3/Pd/TiO2 catalysts exhibit a high efficiency in facilitating the selective hydrogenation of nitriles to primary amines at room temperature and atmospheric hydrogen pressure. The catalytic performance is significantly influenced by the method used for chromium oxide deposition and the Cr3+ loading. The catalysts have been synthesized by the selective deposition of Cr2O3 species onto the surface of Pd nanoparticles via simultaneous or consecutive reduction with hydrogen of titania-supported Pd polyhydroxo-complexes and CrO42− ions. The inverse catalyst Cr2O3/1%Pd/TiO2 modified with just 0.18 wt % Cr3+ demonstrates a 30-fold increase in activity and 4−5 times gain in the selectivity compared to both 1%Pd/TiO2 and commercial 1%Pd/C samples when hydrogenating benzonitrile to benzylamine under ambient conditions. The addition of protonic acid further accelerates reaction rates and improves selectivity. A variety of nitriles were successfully converted to primary amines with a 100% selectivity and yields of 50−99% under ambient conditions, achieving turnover frequencies of up to 920 h−1. Electron paramagnetic resonance, diffuse-reflectance infrared Fourier transform spectroscopy with benzonitrile, and 1H/2H magic-angle spinning solid-state nuclear magnetic resonance were used to explore the nature of the observed effect and monitor the deposition of Cr2O3 onto a 1%Pd/TiO2 catalyst and the developing of Cr2O3/Pd interfacial interaction. The latter introduces additional sites for the adsorption and hydrogenation of nitriles, facilitates the formation of β-PdHx by a factor of 3 in comparison to unmodified 1%Pd/TiO2, and can activate nitrile toward nucleophilic hydrogenation by the polarization of the triple CN bond via acid catalysis upon nitrile adsorption on Cr2O3 of the Cr2O3/Pd interface.