Selective oxidation of methane over vycor glass, quartz glass and various silica, magnesia and alumina surfaces

Abstract

The reaction between methane and oxygen was investigated in empty Vycor glass and quartz tubes and over various silica compounds in powder form. In the temperature range of 893-993 K and with methane-to-oxygen ratios close to unity, formaldehyde, ethane and ethene were produced with a combined 53-96% selectivity over Vycor glass reactor surfaces. Quartz reactor surfaces were less active, but gave the same products. Silica compounds in powder form such as Cab-O-Sil, Ludox gels and silicic acid were investigated at temperatures lower than 893 K where the blank activity of the quartz reactor became negligible. These silica compounds followed similar trends of activity as the Vycor or quartz reactor tubes, although at much lower temperatures. High oxygen-to-methane ratios improved the rates of overall methane reaction and C2 formation. Short residence times enhanced the formaldehyde over the carbon monoxide and carbon dioxide selectivity. The reaction rates increased with pressure. However, in contrast to the results on empty reactor surfaces, no ethane was produced over the silica compounds, the ethene yields were lower, and the carbon dioxide production was higher. While the apparent activation energies for the overall methane reaction and for the production of carbon monoxide, carbon dioxide, ethene and ethane were dependent on the nature of the catalytic surface, the apparent activation energy for the formaldehyde formation appeared to be independent of the catalyst and remained constant at about 130 kJ/mol. This might imply that formaldehyde is generated in the gas phase and that carbon monoxide and carbon dioxide can be formed in secondary reactions of formaldehyde involving surface interactions. The effect of the acid/base character of the catalyst was explored by comparing the activities of the silica compounds with those of MgO and [gamma]-Al2O3. MgO and [gamma]-Al2O3 were more active than the silicas, but produced only carbon monoxide and carbon dioxide.