*2.4. Catalyst Reuse*

To examine catalyst viability, the 5 wt.% Ru/Al2O3 (3) catalyst was finally subjected to reuse in five consecutive oxidation reactions of GGGE under optimized reaction conditions, i.e., 160 ◦C and 20 h (Figure 6). The results revealed that the catalyst was recyclable and maintained good catalytic performance with high GGGE conversion during the five reaction runs. However, some decrease in the yields of guaiacol (34 to 30%), vanillin (13 to 8%) and vanillic acid (11 to 6%) occurred over the five recycles. This decline suggested that part of the catalytically active metal sites of the Ru/Al2O3 (3) catalyst was gradually lost during the reaction sequence. In this connection, TEM analysis of the catalyst after the five-time use confirmed that the spent catalyst contained Ru-based nanoparticles that were more uniformly shaped and larger (30–100 nm) than the fresh catalyst (7–60 nm) (Figure S3). The larger particles most likely formed during the intermediate catalyst calcinations, as also previously observed for Ru supported catalysts [41–43], and were expected to contain less catalytically active sites and therefore be (comparably) less active than smaller-sized particles.

**Figure 6.** Reuse of 5 wt.% Ru/Al2O3 (3) catalyst in five consecutive GGGE oxidation reactions. Reaction conditions: GGGE to catalyst mass ratio = 1.30, 160 ◦C, 5 bar (20% oxygen in argon).
