Highly Dispersed Copper Nanoparticles Supported on Activated Carbon as an Efficient Catalyst for Selective Reduction of Vanillin

Abstract

Highly dispersed copper nanoparticles (Cu NPs) supported on activated carbon (AC) are effectively synthesized by one-pot carbothermal method at temperature range of 400-700 °C. The X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller analysis reveal that Cu NPs with diameters of 20-30 nm are evenly anchored in carbon matrix. The 15 wt%-Cu/AC-600 catalyst (derived at 600 °C) exhibits best bifunctional catalysis of aqueous-phase hydrodeoxygenation (HDO) and organic-phase transfer-hydrogenation reaction (THR) to selectively transform vanillin to 2-methoxy-4-methylphenol (MMP). In HDO of vanillin, the as-prepared catalyst achieves a 99.9% vanillin conversion and 93.2% MMP selectivity under 120 °C, 2.0 MPa H₂ within 5 h. Meanwhile, near-quantitative vanillin conversion and 99.1% MMP selectivity are also obtained under 180 °C within 5 h in THR of vanillin by using 2-propanol as hydrogen donor. The transforming pathways of vanillin are also proposed: vanillin is transformed into MMP via intermediate of 4-hydroxymethyl-2-methoxyphenol in HDO case and by direct hydrogenolysis of vanillin in THR course. More importantly, the activity and the selectivity do not change after 5 cycles, indicating the catalyst has excellent stability. The Cu-based catalyst is relatively cheap and preparation method is facile, green, and easy scale-up, thus achieving a low-cost transformation of biomass to bio-oils and chemicals.

Keywords: carbothermal; copper; hydrodeoxygenation; transfer-hydrogenation; vanillin.