Li/MgO Catalysts Doped with Alio‐valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling

Abstract

The role of Li in Li/MgO as a catalyst for oxidative coupling of methane (OCM) is to promote MgO surface morphology change rather than serve as a constituent of catalytically active sites. While Li/MgO is unstable at realistic conditions with respect to loss of Li, the resulting samples show enhanced selectivity towards C2 hydrocarbons versus CO2, although activity is low and close to pristine MgO. The way (co‐)doping with alio‐valent metal ions affects the catalytic performance of Li/MgO has now been explored. To analyze the structure and the stability of the samples, catalysts with well‐defined stoichiometry were prepared using a co‐precipitation method with freeze‐drying and subsequent annealing. Gd and Fe were used as dopants. Apart from their potential direct role in catalysis, these dopants are anticipated to stabilize Li in the catalyst under the reaction conditions, allowing further clarification of the role of Li. In the case of Gd/Li co‐doping, changes observed in EPR and 7Li‐NMR spectra indicate the formation of correlated, next‐neighbor Li−Mg⋅⋅⋅Gd+Mg pairs co‐existing with “isolated” Gd3+ ions at octahedral Mg lattice sites. For Li/Fe co‐doping, no significant change in the EPR pattern is observed in the presence of Li+ ions, indicating a larger distance between the Li+ and Fe3+ cations in the MgO lattice. Hybrid DFT calculations explain the difference between Fe and Gd co‐doping by a less efficient screening of the Coulomb repulsion between Gd3+ and neighboring cations in Gd doped samples, leading to the stabilization of LiMg near GdMg.

Magnetic Properties of Reduced and Reoxidized Mn–Na2WO4/SiO2: A Catalyst for Oxidative Coupling of Methane (OCM)

“…This method can be applied for disordered systems, as shown recently. [48][49] Figure 5 shows the transient nutation spectra of the reduced and the re- MHz. It should be noted that these ideal values only hold for the limiting case that B ZFS >> B 1 .…”

“…This method can be applied for disordered systems, as shown recently. 48,49 Figure 5 shows the TN spectra of the reduced and the reoxidized catalyst obtained under identical measurement conditions. Both the reduced and the reoxidized samples exhibit similar TN spectra with two main features along the TN axis: one maximum at a nutation frequency of ∼20 MHz and a second signal with a nutation frequency of 9.5 MHz.…”

Optimizing the Reaction Performance of La2Ce2O7‐Based Catalysts for Oxidative Coupling of Methane (OCM) at Lower Temperature by Lattice Doping with Ca Cations

“…So far, Mn/Na 2 WO 4 /SiO 2 and Li/MgO catalysts have been extensively studied and the reaction mechanisms have been researched. Although Li/MgO exhibits good initial catalytic performance, its stability is suffered from the vaporization of Li + cations, the structure modifier, at elevated temperatures . Mn/Na 2 WO 4 /SiO 2 and its promoted forms have both good catalytic performance and long‐term stability .…”

SnO2Based Catalysts with Low‐Temperature Performance for Oxidative Coupling of Methane: Insight into the Promotional Effects of Alkali‐Metal Oxides

“…As a typical example, for the nonredox type catalysts, Li/MgO catalysts have been reported to be effective for reactions in which the surface alkaline sites and oxygen species are regarded as the active sites , . However, Li/MgO catalysts still suffer from deactivation, due to the vaporization of Li + cations . As a typical example for the redox type catalysts, Mn/Na 2 WO 4 /SiO 2 catalysts have been tremendously studied over the past 30 years .…”