(743f) Non-Oxidative Methane Coupling Using Metal Hydrides As Hydrogen Storage Materials

Methane has experienced a meteoric rise due to its vast availability and its prominence in the constitution of natural gas, which is increasingly becoming the dominant energy source in the United States. Although highly desirable, the direct conversion of methane into useful chemicals is limited due to the molecule’s high symmetry and strong C-H bonds. In this work, we seek to upgrade methane to higher hydrocarbons. Historically, the most studied method for the upgrade of methane is the oxidative coupling of methane (OCM), 2CH₄ + ½O₂ → C₂H₆ + H₂O [1]. However, this method has failed to yield an economically viable process due to poor carbon selectivities, which are a result of the over-oxidation of the reactive CH₃ intermediate into CO and CO₂[2].

To avoid the over-oxidation of methane, we employ hydride-forming metals as hydrogen storage materials (HSMs) in a chemical looping approach. In the first step, hydrogen is catalytically abstracted from methane and stored within the HSM while the methyl groups are combined to form ethane. This first step follows the reaction 2CH₄ + M → C₂H₆ + MH₂, where M represents the hydride-forming metal. In the second step, the HSM is regenerated through thermal release, forming H₂. With the regenerated HSM, the first step can be repeated, thus creating a chemical loop.

Through both density functional theory simulations and experimental work, we have identified five Group III and IV metals (scandium, titanium, yttrium, zirconium, and hafnium) that have strong hydrogen storage capabilities and also exhibit the ability to cleave C-H bonds. In particular, titanium and yttrium have shown promise due to experimentally demonstrated rapid hydride formation kinetics and a theoretical activation energy barrier of less than 0.6 eV for the initial C-H bond scission. In addition, titanium’s conversion and selectivity towards higher hydrocarbons can be tuned and improved by alloying titanium with nickel and aluminum, respectively. We have found that the use of metal hydrides and their alloys can be propitious not only for the upgrade of methane, but also for other catalytic dehydrogenation reactions.

References:

[1] Hammond, C., Conrad, S. & Hermans, ChemSusChem 5, (2012) 1668–86.

[2] Zaera, F, Catal. Letters 91, (2003) 1-10.