Oxidative coupling of methane over lithium doped magnesium oxide

Abstract

In this study, a monolithic structure was employed for oxidative coupling of methane reaction over lithium doped magnesium oxide catalyst and the results were compared to those over the particulate catalyst. The rationale behind the study was to see the effects of empty space within the monolith on C2 (C2H4 and C2H6) yield; and effects of monolithic structure on the stability of Li/MgO catalyst at harsh conditions of OCM reaction. 0.5 wt.% Li/MgO particulate catalyst was prepared by wet impregnation and mixed mill technique while the monolith catalyst was prepared by dip-coating technique. Furthermore, two different precursors of lithium (LiNO3 and CH3COOLi) in the particulate catalyst preparation were also investigated. All the tests were carried out in a 10 mm internal diameter quartz reactor, which had a reduced part (2 mm) exactly after the catalyst bed in order to evacuate produced gases immediately from the reaction medium. This was done to minimize gas phase reactions and send product gases to the condensers. SEM, EDX, and XRD tests were also done on the catalysts to see morphological and quantitative changes in the Li/MgO catalysts before and after catalytic tests. SEM images illustrate loss of Li during the reaction; considering this drawback, time on stream tests were done to see activity and selectivity changes by time. Experiments showed a drastic activity and selectivity decrease for particulate catalyst within first two hours of reactant gases introduction, which continued for the next hours in lesser extent. On the other hand, monolithic structure showed a poor activity and selectivity in comparison with particulate catalyst even though the stability seem to be increased by the use of monolithic structure. It is believed that poor performance of monolithic structure is related to its poor heat transfer. Reinforced monolith with metal framework, which is heated using an induction furnace is proposed as a promising technology for further investigations.