Catalytic hydrogenation of dihydrolevoglucosenone to levoglucosanol with a hydrotalcite/mixed oxide copper catalyst

Levoglucosanol (LGOL) is a critical intermediate for the bio-based production of hexane-1,2,5,6-tetrol, 1,2,6-hexanetriol, and 1,6-hexanediol. Here we report on the aqueous-phase hydrogenation of cellulose-derived dihydrolevoglucosenone (Cyrene™) to LGOL using a calcined and reduced heterogeneous copper/hydrotalcite/mixed oxide catalyst, denoted as Cu8/MgAlOx-HP. The turnover frequency for LGOL conversion over this copper-containing catalyst is equal to 0.013 s-1 at 353 K as measured in a flow reactor which is half the one obtained using 0.4 wt% Pd/Al2O3. Moreover, while Cu8/MgAlOx-HP shows a stable activity, the activity of 0.4 wt% Pd/Al2O3 decreases with time-on-stream. Neither Cu-nor Al-leaching is observed (resp. <1 ppb and <1 ppm) but Mg leaching can be seen (5.5 ppm). The latter leaching relates to the acidity of the Cyrene/H2O mixture (pH 3.5-4.5 range), which is due to the occurrence of the geminal diol moiety of Cyrene, an acidic species. In contrast, additional and consecutive oxidation and reduction of the catalyst leads to a gradual decrease in activity over time. Applying still further oxidation/reduction cycles to this catalyst tends to decrease its activity with some overall stabilization being observed from the fourth run onwards. Mg-leaching is shown to change the relative meso-to-macro pore content, but leaves the total pore volume unchanged between the fresh and the spent catalyst. In spite of the high copper loading (8 wt%), small Cu-nanoparticles (2-3 nm) are present over the hydrotalcite/mixed oxide surface of the Cu8/MgAlOx-HP material, and these particles do not aggregate during the hydrogenation reaction.