Abstract

In considering the typical experimental conditions used for studies of methane oxidation to oxygenates or hydrocarbons in the presence of catalysts, it is clear that a significant potential exists presence of catalysts, it is clear that a significant potential exists for the appearance of non-catalyzed, thermally activated gas-phase reactions of methane and oxygen. Experimentally, in a quartz reactor at 800 degrees C with a 3:1 methane-to-oxygen ratio and residence times of the order of 5–10 sec, significant conversion of methane (30%) to light hydrocarbons and COx with appreciable selectivity (25%) to C2+ components is observed due solely to these background reactions. In order to identity and describe the contribution of the gas phase processes during catalyzed reactions, a chemical kinetic model (HCT) processes during catalyzed reactions, a chemical kinetic model (HCT) developed at this Laboratory has been employed to describe these homogeneous gas phase reactions. Overall, the model predicts very well the trends and steady state results observed when tested against a series of experimental reactions comparing the effects of various reaction parameters such as residence time, temperature, and gas composition. The ability to accurately predict the magnitude of these background reactions should provide a means to begin to dissect the contributions of thermal gas phase chemistry from those of solely catalytic action.

New catalysts materials containing niobium and lanthanum have been synthesized. It is observe that niobium alone at high loading levels is an oxidation catalyst facilitating the production of carbon dioxide. This behavior is modified when lanthanum is incorporated into the catalyst material. In that case, improved selectivity to oxidative coupling products of methane is observed. These results appear to be a consequence of catalyst interaction with the existing background gas phase reactions.

Introduction

A significant economic incentive exists to discover technologies that directly convert methane to higher value fuels and chemicals. This incentive has motivated active research efforts aimed at describing catalyzed reactions that facilitate a partial oxidation or oxidative coupling reaction between methane and oxygen. Our efforts have been focussed on the synthesis, characterization, and reactions of new catalysts that perform these oxidative processes with methane. During the study of some of these new catalysts, we observed, for even chemically quite distinct materials, similar reactivity patterns. An examination of the literature shows that despite the patterns. An examination of the literature shows that despite the wide range of catalyst materials that have been studied under an equally wide variety of experimental conditions, a common reactivity pattern emerges. This pattern, characterized by the inverse pattern emerges. This pattern, characterized by the inverse relationship between methane conversion and product selectivity, has been observed for both partial oxidation and oxidative coupling reactions. In considering the typical experimental conditions used for such catalyst studies, it is clear that significant potential exists for the appearance of non-catalyzed, thermally activated gas-phase reactions of methane and oxygen. These reactions can occur in the absence of catalysts and under the appropriate experimental conditions can be quite important. Experimentally, we have observed significant conversion of methane (30%) to light hydrocarbons and COx with appreciable selectivity (25%) to C2+ components due solely to gas-phase reactions. In recognition of these thermal reactions, more experimental consideration is being given to describing the thermally induced, homogeneous gas phase (" background") reactions of methane and oxygen occurring during catalysis studies.

In order to better quantity the contribution of these background reactions and to determine whether gas-phase related reactions may in part be responsible for the common reactivity observed during catalyzed reactions, we have employed a chemical kinetic model (HCT) developed at this Laboratory to describe the overall homogeneous gas phase reaction of methane and oxygen. Here we report a description and experimental verification of the model, applied to the case of methane oxidative coupling, in a series of tests comparing the effects of various experimental parameters such as temperature, residence time, feed gas composition.

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