Methane activation: Oxidation goes soft

C-H bond activation in light alkanes: a theoretical perspective

Alkanes are the major constituents of natural gas and crude oil, the feedstocks for the chemical industry. The efficient and selective activation of C-H bonds can convert abundant and low-cost hydrocarbon feedstocks into value-added products. Due to the increasing global demand for light alkenes and their corresponding polymers as well as synthesis gas and hydrogen production, C-H bond activation of light alkanes has attracted widespread attention. A theoretical understanding of C-H bond activation in light hydrocarbons via density functional theory (DFT) and microkinetic modeling provides a feasible approach to gain insight into the process and guidelines for designing more efficient catalysts to promote light alkane transformation. This review describes the recent progress in computational catalysis that has addressed the C-H bond activation of light alkanes. We start with direct and oxidative C-H bond activation of methane, with emphasis placed on kinetic and mechanistic insights obtained from DFT assisted microkinetic analysis into steam and dry reforming, and the partial oxidation dependence on metal/oxide surfaces and nanoparticle size. Direct and oxidative activation of the C-H bond of ethane and propane on various metal and oxide surfaces are subsequently reviewed, including the elucidation of active sites, intriguing mechanisms, microkinetic modeling, and electronic features of the ethane and propane conversion processes with a focus on suppressing the side reaction and coke formation. The main target of this review is to give fundamental insight into C-H bond activation of light alkanes, which can provide useful guidance for the optimization of catalysts in future research.

Mg-promotion of Ni natural clay-supported catalysts for dry reforming of methane

Mg-promotion of natural clay based Ni-catalysts was considered, as a way of boosting the dry reforming of methane (DRM) activity of these materials. The results of the DRM experiments performed at temperatures from 600 °C to 850 °C evidenced much higher methane and CO₂ conversions for the Mg-promoted catalysts. Mg-promotion led of course to a significant increase of CO₂-adsorption ability (basicity). However, the increased catalytic activity of the Mg-promoted materials was rather linked to increased Ni-dispersion and Ni⁰ crystallite size. Indeed, independent of the physico-chemical properties of the support, the presence of Mg led to the formation of a MgNiO₂ mixed phase that, upon reduction, resulted in the formation of metallic Ni clusters having sizes around 7–9 nm, considerably smaller than in any of the non-promoted catalysts. Carbon formation was found to take place to a greater extent in the presence of the Mg-promoted catalysts, due to C–H bond activation leading also to favored direct methane decomposition (DMD). In spite of this, the activity of the Mg-promoted catalysts was well maintained over 5 hour DRM experiments performed at 750 °C.

Mg-promotion of natural clay based Ni-catalysts was considered, as a way of boosting the dry reforming of methane (DRM) activity of these materials.

Single-site trinuclear copper oxygen clusters in mordenite for selective conversion of methane to methanol

Copper-exchanged zeolites with mordenite structure mimic the nuclearity and reactivity of active sites in particulate methane monooxygenase, which are enzymes able to selectively oxidize methane to methanol. Here we show that the mordenite micropores provide a perfect confined environment for the highly selective stabilization of trinuclear copper-oxo clusters that exhibit a high reactivity towards activation of carbon–hydrogen bonds in methane and its subsequent transformation to methanol. The similarity with the enzymatic systems is also implied from the similarity of the reversible rearrangements of the trinuclear clusters occurring during the selective transformations of methane along the reaction path towards methanol, in both the enzyme system and copper-exchanged mordenite.

Copper-exchanged zeolites with mordenite structure can mimic the active sites in particulate methane monooxygenase. Here, the authors show that mordenite micropores can stabilize trinuclear copper-oxo clusters that exhibit a high reactivity towards activation of carbon–hydrogen bonds in methane.

The structure, carbon deposition and stability of a ZrOx/Ni–MnOx/SiO2 catalyst for the CO2 reforming of methane

A ZrO_x/Ni–MnO_x/SiO₂ catalyst with both high activity and stability was obtained. Most of the deposited carbon was CO₂-oxidizable with the release of CO and H₂.

Rethinking biological activation of methane and conversion to liquid fuels

If methane, the main component of natural gas, can be efficiently converted to liquid fuels, world reserves of methane could satisfy the demand for transportation fuels in addition to use in other sectors. However, the direct activation of strong C-H bonds in methane and conversion to desired products remains a difficult technological challenge. This perspective reveals an opportunity to rethink the logic of biological methane activation and conversion to liquid fuels. We formulate a vision for a new foundation for methane bioconversion and suggest paths to develop technologies for the production of liquid transportation fuels from methane at high carbon yield and high energy efficiency and with low CO2 emissions. These technologies could support natural gas bioconversion facilities with a low capital cost and at small scales, which in turn could monetize the use of natural gas resources that are frequently flared, vented or emitted.