Effects Of The Modification Of The Na+/cao
EFFECTS OF THE MODIFICATION OF THE Na+/CaO
CATALYST BY CHLORIDE IONS
ON THE OXIDATIVE COUPLING OF METHANE
A. Machocki, R. Jezior, A. Denis, W. Gac
Department of Chemical Technology, Maria Curie-Skłodowska University,
Maria Curie-Skłodowska sq. 3, 20-031 Lublin, Poland
One of the most frequently used catalysts of the oxidative coupling of methane is calcium oxide promoted with sodium ions. It demonstrates moderate activity and quite good selectivity. The results of the reaction may be affected by the chloride ions present in the catalyst or introduced along with the reagents. However, literature studies imply that this influence is not unequivocal – in the cases of some catalytic materials a considerable improvement in the selectivity to ethylene formation and an increase in the yield were reported but just as frequently such positive effects of chlorides did not occur or the results of the reaction were even worse than previously.
This paper presents results of systematic studies on the influence of chloride ions introduced into the catalyst Na+/CaO on the effects of the oxidative coupling of methane under various reaction conditions – temperatures, oxygen concentrations, and various contact times of the reagents with the catalyst.
Experimental: The reaction was performed with the catalyst Na+/CaO (1.7wt% Na+) modified with chloride ions. The contents of the latter was 0,0.2, 0.5, 1.0, 2.0, 5.0 or 10.0 wt % in respect to the mass of calcium oxide. The amount of the catalyst in the reactor (W) was 0.5 g. The reaction of methane with oxygen was carried out with a single passage of the reaction mixture through the catalyst at temperatures from 700 to 850º C. Oxygen concentration in the CH4+O2 mixture was 4.76 or 9.09 % (CH4/O2 ratio = 20/1 or 10/1); the volumetric flow rate (F) was 75 or 150 cm3/min; and the contact time W/F was 0.2 or 0.4 s·g/cm3.
Results: The presence of chloride ions decreases the activity of the catalyst – even a small content of those ions was sufficient to cause a decrease the oxygen and methane conversion – and the lower was the reaction temperature the greater was the decrease. The fall in the activity of the catalysts containing
chloride ions was so great that even simultaneous increase in the reaction selectivity did not maintain the original level of the C2+ hydrocarbons yield. The selectivity of ethane formation is improved, even considerably at lower temperatures, by increasing the amounts of chloride ions to moderate levels (up to 2 wt %); larger amounts of chloride ions again decrease the selectivity of ethane formation. The effect of chloride ions on the selectivity of ethylene formation was most complex – at lower temperatures the formed amounts were lower than those formed on Na+/CaO. Above 750º C the selectivity to ethylene was higher than the level achieved on the chloride-free catalyst and this effect was greater at higher oxygen concentrations in the reaction mixture and at longer contact times. The different effects of chloride ions on the selectivities of forming ethane and ethylene, additionally differentiated by the reaction conditions, causes that ethylene content in the whole C2 fraction is higher than the level achieved on chloride-free Na+/CaO only at 850º C, at higher oxygen concentration in the reaction mixture and longer contact time. The selectivity to C3+ hydrocarbons decreases already with small amounts of the chlorides. Only at 850º C and longer contact time the selectivity to C3+ was higher than on Na+/CaO. The total selectivity of the oxidative coupling of methane improves, even significantly, by increasing the contents of chloride ions to 2 wt %; larger amounts of the chlorides again decreased total selectivity which, at lower temperatures, dropped below the level obtained on chloride-free Na+/CaO. The presence of the chloride affected the selectivity of full methane oxidation and carbon dioxide formation in a reversed way than the overall selectivity of C2+ hydrocarbons formation.
Conclusions: There exists an optimum content (about 2 wt %) of chloride ions in Na+/CaO at which the overall selectivity of the reaction and the selectivity of ethane formation reach the highest values, higher than those obtained on chloride-free Na+/CaO. The effect of chloride ions on the selectivity of ethylene formation is most complex. An improvement in the selectivity in the case of this hydrocarbon, leading to ethylene contents in the whole C2 fraction exceeding the level obtained on chloride-free catalyst Na+/CaO, is accomplished only at higher temperatures, with the reagents remaining in the reaction zone for a longer time, and with a higher oxygen concentration in the reaction mixture. These results indicate that in order to secure a high reaction selectivity, oxide ions and chloride ions on the catalyst surface should co-exist in an optimal ratio. Yet, one should also bear in mind the fact that any amount of chloride ions in the Na+/CaO catalyst decreases its activity, therefore these ions are not responsible for the activation of methane. At the same time, the effects of the reaction conditions indicates that chlorides escaping into the gas phase additionally facilitate the formation of hydrocarbons, including ethylene, and restrict their oxidation to carbon dioxide.
Related articles::