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.