Effect of Mg substitution on LaTi1−xMgxO3+δ catalysts for improving the C2 selectivity of the oxidative coupling of methane

Abstract

This study investigated the effect of the partial substitution of Ti by Mg on the La₂Ti₂O₇ perovskite-type structure towards improvements in the selective results of OCM reactions. The LaTi_1−xMg_xO_3+δ catalysts were prepared with x = 0, 0.25 and 0.50 according to a polymer complexing method. XPS analysis showed a surface reconstruction with Mg addition, increasing the formation of oxygen vacancies in the interface formed by La–O–Mg and with an Mg-enriched surface compared to bulk compositions. DRIFTS results presented spectra more restricted to mono- and bidentate carbonate bands with the Mg load increasing, which significantly influence the OCM reaction. Differences in the M–O bonding force were demonstrated by XPS, FTIR, and Raman analysis, which result in the change of the catalyst basicity with the Mg addition. The M–O bond force constants (k) were calculated and showed a linear correlation with C2 selectivity. The catalytic results showed that methane conversion and C2 selectivity increased as a function of the Mg amount. Changes in the reaction conditions confirmed that both CH₄/O₂ feed ratio and reaction temperature are important parameters for the OCM reaction. The best catalytic reaction conditions were WHSV = 30 000 mL g_cat⁻¹ h⁻¹, CH₄/O₂ = 8 and T = 800 °C for LaTi_0.5Mg_0.5O₃ (LTMO-50) catalyst, which achieved 51.4% C2 selectivity. The catalyst remained thermally stable under such conditions even after 24 h of reaction, which has proven the advantage of Mg substitution in this catalyst structure.