The effect of the PbO loading in the oxidative coupling of methane over PbO/SiO2 catalysts.

The effects of the PbO loading and the CH4/O-2 ratio on the oxidative coupling of methane (OCM) over PbO/SiO2 catalysts were studied. Special emphasis was made in the interpretation of the product distribution in the gas phase and its relationship with the nature of the catalyst surface. At CH4/O-2 greater than or equal to 2 ratio, the catalytic behavior of the 2 and 6 % PbO/SiO2 catalysts was very similar. The activity for the deep oxidation of CH4 to CO2 and the activity for the formation of C-2 hydrocarbons were almost the same in both catalysts. At CH4/O-2 = 1 ratio, the behavior of the 2 and 6 % PbO/SiO2 catalysts differed. There were a lower activity for the oxidation of CH4 to CO2 and a higher activity for the formation of C-2 hydrocarbons in the 2 % PbO/SiO2 catalyst than in the 6 % PbO/SiO2 catalyst. At any CH4/O-2 ratio, the 10 % PbO/SiO2 catalyst was very different from the 2 and 6 % PbO/SiO2 catalysts. It had a very low activity for the oxidation of CH4 to CO2 and was selective for the generation of C-2 hydrocarbons. By doubling the amount of catalyst, the amount of reacted methane doubled, while the selectivity remained almost constant. Temperature programmed reduction experiments showed almost the same behavior in all the catalysts. However, the 10 % PbO/SiO2 catalyst showed clearly reducible species at about 670 K, which were practically absent in the 2 and 6 % PbO/SiO2 catalysts. Probably, these species were responsible for the low activity for the oxidation of CH4 to CO2 in the 10 % PbO/SiO2 catalyst.