**4. Conclusions**

An optimized 5 wt.% Ru/Al2O3 catalyst prepared with ruthenium(IV) oxide hydrate precursor was found to give superior yield of the monomeric aromatics guaiacol (34%), vanillin (13%) and vanillic acid (11%) in the aerobic oxidation of the lignin-model compound GGGE in acetonitrile at 5 bar (20% oxygen in argon) under optimized reaction conditions (160 ◦C, 20 h). In comparison, Ru/Al2O3 catalysts prepared with other ruthenium precursors were found to give lower yields of the desired products, which could be correlated to the particle sizes of the Ru-species measured by TEM. Furthermore, catalysts containing other transition metals (Ag, Fe, Mn, Co and Cu) supported on alumina, and ruthenium catalysts based on alternative supports (silica, spinel, HY and zirconia) were also significantly less active compared to the Ru/Al2O3 catalysts for the GGGE oxidation when using identical reaction conditions. Notably, the optimized Ru/Al2O3 catalyst proved robust for recycling in five consecutive reaction runs with only minor activity loss corresponding to thermal regeneration between the runs.

Improved performance of Ru/Al2O3 catalysts was obtained by the optimization of catalyst preparation and reaction conditions for the oxidation of a lignin-model compound to monomeric aromatics. However, the reactivity of lignin-model systems may not be directly transferable to a complex system with native lignin, where more stable catalysts displaying higher activity and selectivity may be required in order to extend the role of lignin applications in biorefineries.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/10/832/s1, Figure S1: High-resolution TEM images of (A) fresh 1 wt.% Ru/Al2O3 (1) catalyst, (B) used 1 wt.% Ru/Al2O3 (1) catalyst, (C) fresh 3 wt.% Ru/Al2O3 (1) catalyst, (D) used 3 wt.% Ru/Al2O3 (1) catalyst, (E) fresh 5 wt.% Ru/Al2O3 (1) catalyst and (F) used 5 wt.% Ru/Al2O3 (1) catalyst, Figure S2: Overlay of 1H-13C HSQC and 1H-13C HMBC NMR spectra of post-reaction material displayed as contour plots, showing the single and multiple-bond correlations in 2-methoxy-1,4-benzoquinone, with the inset displaying the full chemical shift assignment of the compound. Figure S3: High-resolution TEM catalysts images of (left) fresh 5 wt.% Ru/Al2O3 (3) catalyst, and (right) 5 wt.% Ru/Al2O3 (3) catalyst after five reaction runs.

**Author Contributions:** Data curation and analysis, M.M.-R., S.M. and S.S.; Writing of manuscript, M.M.-R., S.M., S.S., S.K. and A.R.; Management, A.R.

**Funding:** This research received no external funding.

**Acknowledgments:** The Danish Agency for Science, Technology and Innovation (International Network Programme, 12-132649), Haldor Topsøe A/S and the Technical University of Denmark. The 800 MHz NMR spectra were recorded using the spectrometer at the DTU NMR Center, supported by the Villum Foundation.

**Conflicts of Interest:** The authors declare no conflict of interest.
