*3.1. General*

Guaiacyl glycerol-β-guaiacyl ether (>99%, GGGE) was prepared from acetovanillone through a multiple step synthesis route using a reported procedure [44]. Vanillin (99%), vanillic acid (99%) manganese(II) acetate tetrahydrate (>99%), copper(II) acetate monohydrate (p.a.), silver(I) nitrate (p.a.), iron(III) nitrate nonahydrate (p.a.), ruthenium(IV) oxide hydrate (>99.9%), ruthenium(III) acetylacetonate (>97%), ruthenium(III) chloride hydrate (>99%), zirconium(IV) oxide (99%), magnesium aluminate (spinel) (99%), silica gel 60 (high-purity grade) and acetonitrile (99.8%) were purchased from Sigma Aldrich. γ-Aluminium oxide (>99%) was provided by Saint Gobain. Compressed oxygen (>99.99%) and argon (>99.99%) were purchased from Air Liquide, Denmark. All chemicals and gases were used as received.
