Abstract
The selective oxidation of veratryl alcohol (VA), a model compound of lignin, with oxygen molecules to produce veratraldehyde (VAld) was studied over monometallic Au, Pd, and bimetallic Au:Pd nanoparticles supported on a Ce0.62Zr0.38O2 mixed oxide for the first time. These bimetallic Au-Pd catalysts with Au:Pd molar ratios from 0.4 to 4.3 were synthesized by the sol-immobilization method. Furthermore, all the catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), N2 physisorption, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, energy dispersive X-ray spectroscopy (EDXS), and temperature programmed reduction (TPR) techniques. A synergistic effect between gold and palladium was observed over all the bimetallic catalysts in a wide range of studied Au:Pd ratios. Remarkably, the optimum Au:Pd ratio for this reaction was 1.4 with a turnover frequency of almost six times larger than for the monometallic gold and palladium catalysts. Selectivity to veratraldehyde was higher than 99% for the monometallic Au, Pd, and all the bimetallic Au-Pd catalysts, and stayed constant during the reaction time.
Highlights
The development of strategies for the valorization of lignin to high-value chemicals is becoming increasingly important due to their potential application as sustainable supplements or replacements of fossil-based chemicals or fuels
The monometallic catalysts were labeled according to their Pd and Au loadings based on weight as determined from inductively coupled plasma-atomic emission spectroscopy (ICP-AES)
It can be observed that the actual Au and Pd loadings were lower than the expected values, indicating that the sol-immobilization method did not lead to complete deposition of Au and Pd on the Ce0.62Zr0.38O2 support
Summary
The development of strategies for the valorization of lignin to high-value chemicals is becoming increasingly important due to their potential application as sustainable supplements or replacements of fossil-based chemicals or fuels. Because of the complexity of the structure of lignin feedstocks, the majority of academic research has focused on the oxidation of model compounds, such as 2-phenoxy-1-phenylethanol [7], vanillyl alcohol [8], 2-aryloxy-1-arylethanols [8], veratryl alcohol [4,9], apocynol [10], p-sinapyl alcohol [9], and p-hydroxy benzyl alcohol [9], among others. The studies on these model compounds could be relevant in terms of the reactivity of some motifs found in lignin, as well as how specific bonds (β-O-4, α-O-4, etc.) can be broken. The oxidation of veratryl alcohol to veratraldehyde is a benzylic oxidation representing the valorization of one of the β-O-4 model compounds of lignin [4,9]
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