Effect of Ni/Co mass ratio and NiO–Co3O4 loading on catalytic performance of NiO–Co3O4/Nb2O5–TiO2 for direct synthesis of 2-propylheptanol from n-valeraldehyde†
In the direct synthesis of 2-propylheptanol (2-PH) from n-valeraldehyde, a second-metal oxide component Co3O4 was introduced into NiO/Nb2O5–TiO2 catalyst to assist in the reduction of NiO. In order to optimize the catalytic performance of NiO–Co3O4/Nb2O5–TiO2 catalyst, the effects of the Ni/Co mass ratio and NiO–Co3O4 loading were investigated. A series of NiO–Co3O4/Nb2O5–TiO2 catalysts with different Ni/Co mass ratios were prepared by the co-precipitation method and their catalytic performances were evaluated. The result showed that NiO–Co3O4/Nb2O5–TiO2 with a Ni/Co mass ratio of 8/3 demonstrated the best catalytic performance because the number of d-band holes in this catalyst was nearly equal to the number of electrons transferred in hydrogenation reaction. Subsequently, the NiO–Co3O4/Nb2O5–TiO2 catalysts with different Ni/Co mass ratios were characterized by XRD and XPS and the results indicated that both an interaction of Ni with Co and formation of a Ni–Co alloy were the main reasons for the reduction of NiO–Co3O4/Nb2O5–TiO2 catalyst in the reaction process. A higher NiO–Co3O4 loading could increase the catalytic activity but too high a loading resulted in incomplete reduction of NiO–Co3O4 in the reaction process. Thus the NiO–Co3O4/Nb2O5–TiO2 catalyst with a Ni/Co mass ratio of 8/3 and a NiO–Co3O4 loading of 14 wt% showed the best catalytic performance; a 2-PH selectivity of 80.4% was achieved with complete conversion of n-valeraldehyde. Furthermore, the NiO–Co3O4/Nb2O5–TiO2 catalyst showed good stability. This was ascribed to the interaction of Ni with Co, the formation of the Ni–Co alloy and further reservation of both in the process of reuse.