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Abstract:
To find alternative feedstocks for the chemical industry, the oxidative coupling of methane (OCM) reaction was developed in the 1980s, allowing the production of ethylene from methane. Though, being the topic of many investigations in the last 40 years, the success so far is limited and a yield of 30% has yet to be surpassed. One major issue of the reaction is the activation of methane. With a dissociation energy of 423 kJ/mol, highly reactive materials or high reaction temperatures are needed. Controlling the activation of the oxidant is essential to avoid an overoxidation of the desired products and achieve high selectivity. Since many materials have been screened as a suitable catalyst for the OCM without a breakthrough, basic research is necessary to understand the limiting factors of this reaction. In physical experiments, calcium oxide was found to show interesting interactions with oxygen when doped with transition metals, strongly reducing the oxygen dissociation barrier, making these materials an interesting target to study oxidation catalysts.
In this work, first-row transition metal-doped calcium oxide materials (0.1 atom% Mn, Ni, Cr, Co, and Zn) were synthesized, characterized, and tested for the OCM reaction. First, doped carbonate precursors were prepared by a co-precipitation method. The synthesis parameters were optimized to yield materials with a pure calcite phase, which was verified by XRD. EPR measurements on the Mn-doped CaO materials indicate a successful substitution of Ca2+ with Mn2+ in the CaO lattice. The materials were tested for their performance in the OCM reaction, where a beneficial towards selectivity and activity effect could be observed for Mn-, Ni-, and Zn-doped samples, where the selectivity of Co- and Cr-doped CaO was strongly reduced. The optimum doping concentration could be identified in the range of 0.05-0.10 atom%, showing the strongest decrease in the apparent activation energy, as well as the maximum increase in selectivity. The overall improvement of the catalyst was though was only minor.
In situ Raman, IR, and TG experiments revealed the formation of carbonates during the reaction. Hydroxide species, or oxygen species were not found. To investigate the oxygen activation capabilities of the materials, SSTIKA experiments were performed and a pulsed isotopic scrambling technique was developed and implemented and numerically simulated. Though giving not much insight on the role of the dopant, the contamination of the surface by water and carbon dioxide could be identified as the limiting factor for the OCM on basic oxides. Due to the blocking of the active sites of the by- and side-products of the OCM reaction, high temperatures are needed to regenerate the catalyst surface.
