Synergism studies on alumina-supported copper-nickel catalysts towards furfural and 5-hydroxymethylfurfural hydrogenation

The structure of alumina-supported copper-nickel catalysts prepared by impregnation method was studied by using a combination of various characterization techniques including XRD, N₂-sorption, TEM, H₂-TPR, NH₃-TPD, XANES/EXAFS and CHNS methods. A series of characterizations revealed that dispersion of copper increases with an increase in nickel loading on ɣ-Al₂O₃ due to strong interaction between nickel and copper oxide particles. This also led to the formation of mixed copper-nickel oxides in calcined catalysts at the highest loading of nickel and copper (i.e. Cu/Ni = 1). In catalytic activity, both the monometallic Cu/ɣ-Al₂O₃ and Ni/ɣ-Al₂O₃ showed lower activity and selectivity towards hydrogenation of both Furfural and 5-Hydroxymethylfurfural to 2-methylfuran (2-MF) and 2, 5-dimethylfuran (DMF), respectively. However, with increase in nickel loading, the activity and the selectivity of Cu/ɣ-Al₂O₃ drastically increased for both the cases and Cu-Ni/ɣ-Al₂O₃ (Cu/Ni = 1) showed the highest catalytic activity. Furthermore, combination of Cu/Ni ratios and temperature plays a significant role in the product distribution, as in the case of furfural hydrogenation, at a lower temperature, furfuryl alcohol (FOL) appears as the main product while at a higher temperature, 2-methylfuran (2-MF) is found to be the dominant product over Cu-Ni/ɣ-Al₂O₃ (Cu/Ni = 1) catalysts. Similarly, 2,5-bishydroxymethylfuran (BHF) is found to be the major product at a lower temperature and 2,5-dimethylfuran (DMF) is selectively produced at a higher temperature in the HMF hydrogenation. Furthermore, reaction pathways are discussed for both the reactions.