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

Abstract

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/MgAlO_x-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/MgAlO_x-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/MgAlO_x-HP material, and these particles do not aggregate during the hydrogenation reaction.