Selective Cellulose Hydrogenolysis to 2,5-Hexanedione and 1-Hydroxy-2-hexanone Using Ni@NC Combined with H3PO4

The production of ketones from cellulose is critical but challenging due to the easy hydrogenation of ketone groups. Herein, Ni particles encapsulated in N-doped carbon layers (Ni@NC) were synthesized and used as efficient catalysts for direct cellulose hydrogenolysis to 2,5-hexanedione (HD) and 1-hydroxy-2-hexanone (HHO) in H3PO4 aqueous solution. HD and the first reported HHO in this work could be simultaneously produced with the yields of 34.1% and 24.5%, respectively. It was found that the hydrolyzed glucose was the key intermediate in the formation of these two target products. HD originated from the hydrolysis of 2,5-dimethylfuran (2,5-DMF) that was produced via glucose (from cellulose hydrolysis catalyzed by H3PO4) isomerization to fructose, followed by fructose dehydration to 5-hydroxymethylfurfural (5-HMF) and 5-HMF hydrodeoxgenation. In parallel, HHO was obtained from the selective hydrodeoxygenation of hexoses. The N species of Ni@NC catalysts acted as the basic sites for promoting glucose isomerization to fructose. The production of ketones could be attributed to the tailored hydrogenation ability of Ni@NC, which facilitated the selective preservation of C═O bonds. The synergy between H3PO4 (cellulose hydrolysis and C–O bond splitting by hydrogenolysis), base (aldehyde isomerization to ketone), and metallic Ni (hydrogenation) played an essential role in the formation of ketone-containing products with high yields.