**1. Introduction**

Lignocellulosic biomass, predominantly comprised of cellulose, hemicellulose and lignin, is an abundant and renewable carbon-based alternative to fossil resources [1]. Several applications with nanocomposites of cellulose and hemicelluloses have been reported, for example water-purification [2], (bio)sensing [3] and anti-microbial treatment [4]. However, lately, the transformation of materials, as well as their monomeric C6 and C5 carbohydrates, to value-added chemicals and fuels have been studied extensively [5–15]. In comparison, lignin has received much less attention as feedstock, possibly due to its complex polymeric structure and lower reactivity, even though it is a major part of lignocellulosic biomass, typically 30% by weight and 40% by energy content. However, recent developments have demonstrated lignin to be a potentially important feedstock for producing chemicals, especially aromatic compounds [16–23]. This progress is important for the future of biorefineries as valorization of the entire biomass substrate improves economic viability.

The direct transformation of lignin usually requires mechanical pretreatment and harsh reaction conditions, due to its poor solubility and complex heterogeneous structure. Thus, in order to understand the reactivity of lignin, in general, various lignin model compounds containing di fferent structural linkages, such as α-O-4 and β-O-4, have been widely used as substrates. Among the di fferent linkages, the most abundant structural unit in lignin is the β-O-4 (Figure 1), representing approximately 60% of hardwood and 45–50% of softwood [24].

**Figure 1.** Schematic representation of typical lignin fragments and the corresponding β-O-4 lignin model compound guaiacyl glycerol-β-guaiacyl ether (GGGE).

Several studies have converted lignin and simple aromatic model compounds by catalytic oxidation under typically harsh reaction conditions and afforded low yield and/or selectivity to the targeted products, whereas dimeric lignin model compounds containing β-O-4 linkages have only been scarcely studied [25,26]. In this context, the exploration of bulky β-O-4 lignin model compounds may provide valuable insight that can be transferred to the reactivity of the complex lignin molecule, especially on cleavage of β-O-4 linkages and further reactivity of the formed monomers [18].

Aerobic oxidation of the bulky lignin model compound guaiacyl glycerol-β-guaiacyl ether (GGGE) has primarily been examined with vanadium-based homogeneous catalyst systems. Hence, Son et al. reported a vanadium-based catalyst for the non-oxidative C-O bond cleavage of dimeric lignin model compounds with a conversion of 80% [27]. Furthermore, vanadium complexes showed promising catalytic activity for oxidative C–C bond cleavage of GGGE compounds, and promoted multistep reactions affording C–C and C–O cleavage products from alternative dimeric β-O-4 lignin model compounds [28–30]. Alternatively, Rahimi et al. introduced a two-step, metal-free organocatalytic method using first 4-acetamido-TEMPO as the catalyst for chemoselective aerobic oxidation of the secondary benzylic alcohols in GGGE followed by C–C cleavage using H2O2 [23]. In addition, Leitner et al. more recently introduced a highly active and selective ruthenium-complex catalyst system for C–C bond cleavage of β-O-4 lignin linkages involving a dehydrogenation-initiated retro-aldol reaction [31]. Despite these promising homogeneous catalyst systems for selective cleavage of C–C and C–O bonds, separation and recyclability of the catalysts remains cumbersome for such catalytic systems [30,32]. In contrast, solid catalysts with supported metals/metal oxides can easily be recovered from liquid reaction mixtures, and can often be recycled multiple times with preservation of the catalytic performance.

Supported ruthenium catalysts such as Ru/alumina, are an effective and reusable heterogeneous catalyst system for aerobic oxidation of both activated and non-activated alcohols in the presence of sulfur, nitrogen and carbon-carbon double bonds [33], and they are therefore interesting in the context of oxidative lignin valorization. We previously examined such catalysts for the aerobic oxidation of the lignin model compound veratryl alcohol to veratraldehyde in water and methanol with good results [34]. In the present study, analogous ruthenium supported catalysts with the different supports

γ-alumina (Ru/Al2O3), silica (Ru/SiO2), zirconia (Ru/ZrO2), spinel (Ru/MgAl2O4) and USY zeolite (Ru/HY), which were prepared, characterized and applied for aerobic oxidative cleavage of GGGE in acetonitrile to produce guaiacol, vanillin and vanillic acid under mild reaction conditions (Scheme 1). Acetonitrile was preferred as the reaction solvent to ensure dissolution of GGGE and the products, and reaction parameters such as temperature and time were optimized for the promising catalyst Ru/Al2O3 to increase the selectivity for the desirable products, and the recyclability of the catalyst examined by performing consecutive reaction runs. For comparison, other alumina-supported metal catalysts M/Al2O3 (M = Mn, Ag, Cu and Fe) were prepared and evaluated.

**Scheme 1.** Catalytic aerobic oxidation of guaiacyl glycerol-β-guaiacyl ether (GGGE) to guaiacol, vanillin and vanillic acid with supported metal catalysts.
