Metal Dispersion Influence on The Catalytic Performance of Mn-Na-W/SiO2 Catalyst in The Oxidative Coupling of Methane

Naseem Hayek, Chemical Engineering, Technion - Israel Institute of Technology, Haifa, Israel
Oz M.Gazit, Chemical Engineering, Technion - Israel Institute Of Technology, Haifa, Israel







Oxidative coupling of methane (OCM) is an attractive route to produce ethylene from natural gas. Despite the vast amount of studies, no practical application has been put into practice. This is due to the lack of active, selective and in particular, stable catalyst at the reaction temperature (~800C) that can overcome total oxidation of methane to CO and CO2. Mn-Na-W/SiO2 is a promising catalyst, giving a C2 yield of 25% with a remarkable stability. The phase transition of the amorphous silica to cirstobalite during the preparation procedure is shown to be crucial for high catalytic activity. This is though to be due to the higher dispersion of the catalytic metals with the formation of suitable active sites.
In this work, we study the dispersion effect of Mn before the phase transition, on the catalytic activity of the catalyst after the phase transition. For this purpose, we use the framewoek of beta-zeolite. In the first step, we extract aluminum atoms from the framewok of beta zeolite to produce defects sites. Then, we introduce manganese into these defects by impregnation. The effect of Mn precursor is evaluated using Mn nitrate (MnN) and Mn acetate (MnA). Following the Mn incorporation, Na and W are added and the catalysts is tested under reaction conditions in a tubular fixed-bed reactor. As a control, fumed silica and SBA-15 were used as silica precursors.
The results show that when using MnA, the Mn is incorporated into the beta zeolite framework besides the formation of highly dispersed extra-framework species. Conversly, using MnN forms bulk MnO2 on the outer surface of the zeolite with a poor dispersion. The catalytic tests show superior catalytic activity of the catalyst obtained from the MnA-beta zeolite.