Acceptorless dehydrogenation of biomass-derived aromatic alcohols to aromatic ketones synergistically catalysed by Pd/C and ionic liquids

Acceptorless dehydrogenation of alcohols is a promising pathway for biomass resource utilization and hydrogen supply, but still lacking for simple and effective catalysts. Herein, we report a metal-ionic liquids (ILs) catalyst for synergistical catalytic dehydrogenation of aromatic alcohols to aromatic ketones under mild conditions without any acid/base additives. It is found that the combination of Pd/C with 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) displays high activity for the dehydrogenation of biomass-derived alcohols including 1-phenyl-1-ethanol, 1-phenyl-1-propanol and 1-(4-methoxyphenyl)ethanol. Mechanistic studies reveal that the Pd/C can be anchored and stabilized by the IL via interacting with [BMIm]+ cation, and the [BF4]- anion plays vital role in the activation of hydroxyl-H and alpha-H protons via hydrogen bonding interactions. These interactions create an effective microenvironmental for Pd/C catalyzing the cleavage of the hydroxyl-H and alpha-C-H bonds of alpha-hydroxyl aromatic alcohols to generate aromatic ketones and H2. Moreover, the Pd/C-[BMIm][BF4] catalyst could be recovered and reused for 5 runs without obvious activity loss. This strategy provides more evidences for the synergetic catalysis of metal and ionic liquids and exhibits potential application prospects for hydrogen supply from renewable biomass-derived alcohols.