INFLUENCE OF MANGANESE ON FORMATION OF ACTIVE PHASE OF COBALT CATALYST OF HYDROCARBON SYNTHESIS
Abstract
The active phase and catalytic properties of cobalt catalysts of Fischer-Tropsch synthesis promoted with manganese and prepared by impregnation were studied. KSKG silica gel having a monodisperse structure with an average pore diameter of 12–16 nm was used as a support. The cobalt content in the obtained samples was 20.8-21.8 wt.%, manganese content was 0.5–2 wt.%. The structural characteristics of the catalysts were investigated by X-ray diffraction, transmission electron microscopy, temperature-programmed reduction and temperature-programmed hydrogen desorption. The catalytic activity and selectivity of the samples in the process of hydrocarbon synthesis were researched in the flow mode at a pressure of 0.1 MPa, at a Gas Hourly Space Velocity (GHSV) of 100 h–1, at the temperatures of 150–220 °C and a ratio of CO to hydrogen in synthesis gas of 2:1. Preliminary reduction of catalysts was carried out with hydrogen at a temperature of 400 °C. In the composition of the studied samples with manganese the formation of solid solutions of metals such as xCo3O4–(1–x)Mn3O4 was confirmed. It is shown that the reduction process of Co3O4 is determined by the composition and structure of solid solutions of components that are hard-to-recover compounds. The features of the promoting action of manganese depending on the concentration are determined. The introduction of 0.5–2 wt.% manganese allows to regulate the composition and microstructure of cobalt oxide compounds, the properties of metallic Co, increases the catalytic characteristics: activity, selectivity for C5+ and reduces the yield of methane. It is shown how, depending on the content of the metal additive, the manifestations of the structural (at a concentration of 0–1 wt.%) or electronic (at a concentration of 2 wt.%) promoting action of manganese increase. The optimum concentration of manganese is
1 wt.%. The catalyst is characterized by the minimum average size of crystallites, the highest value of the specific surface of cobalt metal and the maximum catalytic indicators.
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