The recent surge in natural gas production, fueled by the exploitation of shale gas reserves, is providing an abundant supply of methane. Tapping into this supply by converting methane into higher value chemicals can significantly impact the chemical industry by adding new supplies of raw materials. However, methane activation remains a major challenge. It is difficult to break the strong and localized C-H bond. Oxidative coupling of methane offers the potential of selectively converting methane to ethane/ethylene.<br/>The work of Lunsford ignited the interest in in this field when it was shown that Li-doped MgO can partially oxidize methane into ethane/ethylene. The reaction takes place at elevated temperatures (~1,000 K) and has selectivity of &gt;50%. Most of the literature work has focused on the impact of Li-doping on MgO activity. It has been debated if Li affects the activity directly through Li participation in bond breakage or indirectly through inducing structural modifications. Recently, it was suggested that the effect of Li-doping is exposing MgO under-coordinated sites that are thought to be more active. Despite much work on the subject, there are still many open questions. There is a lack of detailed microkinetic models that covers the complete catalytic cycle. In addition, most of the literature work focuses on the first step, breaking methane C-H bond. It has been generally assumed that C-H bond activation is the rate limiting step. However, later steps, such as water formation and active site regeneration, can affect the catalyst activity. Also, few works studied the effects of alkali metals as dopants, other than Li. Other alkaline earth metal oxides, such as CaO and SrO, haven’t received much attention.<br/>In this work, we attempt to address some of these questions. To understand the impact of geometry on methane activation, we study the activity of different MgO facets that represent terrace and stepped surfaces. We also examine dopant effects by studying the impact of different alkali metal oxides on MgO activity. We develop scaling relations for adsorbates and transitions states, in both doped and pure MgO, and then use those relations as an input to a microkinetic model. Developed microkinetic model is then further extended to study CaO and SrO and compare their activity to MgO. Our results show that methane activation on MgO has strong structural dependence. It also shows potential room for improvement by using other alkaline metal oxides.

Theoretical study on oxidative coupling of methane using MgO

253rd National Meeting (2017)

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