Partial oxidation of methane by chemical looping

Abstract

The catalytic partial oxidation of methane has recently received a great deal of attention as a process for production of synthesis gas. In this work, the catalytic partial oxidation was carried out anaerobically, in the absence of gas phase oxygen and using lattice oxygen from solid materials such as perovskites. The process was studied in two steps where methane and oxygen were periodically brought into contact with the catalyst. During re-oxidation with oxygen, the reduced form of the perovskite was oxidized whereas the catalyst was reduced during exposure to methane.

During the first part of the work, the focus was on developing a catalytic system such as perovskite-type mixed oxides which was then subsequently tested in a quartz fixed bed reactor for partial oxidation. It is important that the catalyst is selective towards syngas products with a good oxygen storage capacity, and being thermally stable, since the reactions take place at high temperatures (above 1133K). The methane conversion and the selectivity to CO and H2 in catalytic partial oxidation are favored at high temperatures. Two different reaction temperatures and flow rates were applied during the experiments.

The catalytic performance and the oxygen storage capacity of various synthesized LaFeO3 perovskites with different average crystal sizes were investigated. The superior activity and selectivity to synthesis gas and the reversibility during the redox cycle make orthorhombic LaFeO3 a promising candidate as a stable oxygen carrier in the reduction-oxidation process.

Different crystal sizes of perovskites have been obtained using various preparation methods. The reversible phase change was studied by XRD. Phase separation of the perovskite after oxygen removal by methane reduction with the formation of the La2O3, Fe2O3, Fe3O4 and graphite is indicated in the XRD spectrum. After oxygen removal, the samples lose their perovskite structure and are transferred to the mixed oxides. However, the transformation between the perovskite and the mixed oxides is reversible during the redox cycles, and the mixed oxides are transferred back to the perovskite by exposing the samples to oxygen during the re-oxidation step.

A kinetic study by varying the cluster size with different amount of O removal has been performed. The effect of the O content in the perovskite on the catalytic performance was studied by correlating the formation rates of the reaction products as a function of O removed. Lower O removal rates on smaller LaFeO3 crystals can not be explained by O diffusion in the lattice as the rate determining step. Surface reaction step is a kinetic relevant step in methane conversion and it depends on the O-Fe bond strength. Larger crystals of LaFeO3 have smaller band gaps and thus weaker O-Fe bond strength, resulting in higher activity for methane conversion and large amount of removable O. Strong O-Fe bonds lead to a higher surface O concentration that enhances the formation of CO2. Too much removal of O from the perovskites leads to carbon formation.

The nature of the active sites is highly dynamic and follows changes in O removed. These active sites can not be assigned to the specific Fe site; instead they are related to the coordination of O with Fe sites. Fe sites highly coordinated with O lead to CO2 formation. Fe sites moderately coordinated with O leads to high activity and selectivity towards CO and H2. Fe sites lower coordinated with O increase the rate of carbon formation with considerable increase in the O vacancy.

The regeneration of the catalyst during the re-oxidation step and carbon formation during the reaction with methane were also studied in this work. The reoxidation rate is faster than the reduction rate. Duration of the oxidation period depends on the degree of reduction of the sample and the formation of carbon. The influence of surface composition of perovskites on partial oxidation was also studied. Increase in the amount of Sr2+ in La1-x SrxFeO3-δ perovskites results in more oxygen available for reduction during Temperature Programmed Reduction (TPR). In addition, the oxygen release is heavily increased by the substitution of Fe3+ with Co3+. Temperature Progarmmed Surface Reaction (TPSR) indicates the existence of two different steps for oxygen removal from La1-x SrxFeO3-δ perovskites. Firstly, the reducible oxygen gives total methane combustion to CO2 and water and secondly the release of oxygen from the bulk of the material gives the selective oxidation products such as CO and H2. Increase in the Sr2+ content in the perovskites leads to low methane conversion. Cobalt-containing perovskites have also been studied and the low CO selectivity indicates that these catalysts favor the total combustion of methane.