Total hydrogenation of hydroxymethylfurfural via hydrothermally stable Ni catalysts and the mechanistic study

Total hydrogenation of hydroxymethylfurfural (HMF) (including hydrogenation of both furan ring and carbonyl group) produces 2,5-bis-(hydroxymethyl) tetrahydrofuran (BHTF), which can be used as rigid polymer monomer, green solvent, or precursor of high-value chemicals and has great market potential. Although Ni is regarded as the most selective metal for the total hydrogenation of HMF, Ni catalysts often suffer sintering and leaching under hydrothermal conditions. In this regard, development of hydrothermally stable Ni catalysts is highly desirable. Here, Ni on well mixed oxides (Ni/MgaAl10-aOx) derived from layered double hydroxide (LDH) precursors was successfully synthesized via co-precipitation. The Ni/MgaAl10-aOx showed superior hydrothermal stability during the aqueous phase total hydrogenation of HMF, without noticeable activity loss, leaching and sintering after multiple reuse. In contrast, significant activity loss, leaching and sintering was observed for impregnated Ni counterparts. Catalyst characterization revealed the formation of highly dispersed Ni and well mixed porous oxides after the thermal reduction of the NiMgAl-LDH precursors as well as the stronger metal-support interactions of the NiMgAl-LDH as compared to impregnated Ni catalysts with similar Ni particle size. The markedly improved hydrothermal stability was due to enhanced metal support interactions derived from the LDH precursors, which prevents the undesired agglomeration and leaching of Ni in the hydrothermal environment. Moreover, a high BTHF yield of 99.0% was obtained using the Ni/MgaAl10-aOx under optimal reaction conditions. Density functional theory calculations suggest that the total hydrogenation of HMF over the Ni catalysts proceeds through the alkoxy pathway via “flat-lying” adsorption.