Surface Acidic Species-Driven Reductive Amination of Furfural with Ru/T-ZrO2.

Abstract

Catalyst development for upgrading bio-based chemicals towards primary amines has increasingly attracted owing to their applications in the pharmaceutical and polymer industries. The surface acidic sites in metal oxide-based catalysts play a key role in the reductive amination of aldehydes/ketones involving H2 and NH3; however, the crucial role of the type of surface acidic species and their strength remains unclear. Herein, this study exhibits the catalytic reductive amination of furfural (FUR) to furfurylamine (FUA) with Ru supported on tetragonal (Ru/T-ZrO2) and monoclinic (Ru/M-ZrO2) ZrO2. Ru/T-ZrO2 exhibited an 11.8-fold higher rate of reductive amination than Ru/M-ZrO2, giving a quantitative yield of FUA (99%) at 80 °C in 2.5 h and is recyclable up to four runs. Catalyst surface investigation using spectroscopic techniques, like X-ray photoelectron, electron paramagnetic resonance, and Raman, confirm higher oxygen vacancy sites (1.6 times) on the surface of Ru/T-ZrO2 compared to Ru/M-ZrO2. Moreover, in-situ NH3-diffuse reflectance infrared Fourier transform spectroscopy studies display that Ru/T-ZrO2 has more moderate Bronsted acidic sites (surface H-bonded hydroxyl groups) than Ru/M-ZrO2. Further, the controlled experiments and poisoning studies with KSCN and 2,6-lutidine suggest the crucial role of Ov sites (Lewis acidic sites) and surface hydroxyl groups (Bronsted acidic sites) for selective FUA formation.