The composition of Ni-Mo phases obtained by NiMoOx-SiO2 reduction and their catalytic properties in anisole hydrogenation

Abstract

The specific features of the formation of nickel- and molybdenum-containing phases during the reduction of the oxide precursor of the NiMoOx-SiO2 catalyst with hydrogen at three different temperatures (470, 570, and 750°C) were studied. The reduction behavior was investigated and the reduction temperature of oxide forms was determined with the use of the methods of temperature-programmed reduction, X-ray diffraction analysis, and X-ray photoelectron spectroscopy. It was shown that, at a reduction temperature from 470 to 570°C, one can observe the formation of NiMoOx particles with the NiO-type structure, Ni-Mo alloys of different composition, and MoO2, which pass into the metallic phases Mo, Ni3Mo, and NixMo1-x at 750°C. Nickel located at the surface is completely reduced to the metallic state in the temperature range of 300–750°C, the Mo° content increases with the growing of treatment temperature and reaches 100% at 750°C. Based on the data obtained, a reduction scheme for the catalytic system is proposed. The catalytic properties of the systems obtained are studied in the anisole hydrogenation reaction at a temperature of 300°C and a hydrogen pressure of 6MPa. The results of catalytic experiments showed that the 750-NiMo-SiO2 catalyst possesses the highest specific activity in the anisole hydrogenation, which is probably due to the complete reduction of nickel oxide forms and molybdenum to metallic forms, which are highly active in the hydrogenation of CO bonds and aromatic rings. The highest selectivity in the formation of oxygen-free products can be attributed to the catalysts reduced at 470–570°C and whose active component contains coordinatively unsaturated molybdenum atoms. The most stable during the thermal treatment in acetic acid is the 750-NiMo-SiO2 catalyst, which can be explained by the fact that it contains Ni-Mo alloys highly stable in the acid medium.