Influences of Cation and Anion Substitutions on Oxidative Coupling of Methane over Hydroxyapatite Catalysts

Lead substituted hydroxyapatite (Pb-HAP) has been an active catalyst for oxidative coupling of methane (OCM) reactions. CO₃^2- substituted HAP (HAP-CO₃) has showed enhanced oxide ion conductivity than bare HAP in high temperature solid oxide fuel cells. Substitutions for both cations and anions in HAP structure (Pb-HAP-CO₃) are promising to integrate the catalytic property of Pb-HAP and oxide ion conductive property of HAP-CO₃ into one apatite-based ceramic material that can be manufactured into membrane reactors for possessing CH₄ activation and O₂ permeation capabilities for efficient OCM reactions. In this work, the effects of substitutions for both cation (Pb²⁺) and anion (CO₃^2-) in HAP structure on OCM reactions were studied. The basicity and acidity of HAP catalysts were strengthened by the cation and anion substitutions, respectively, and as consequences, they influenced the catalytic performances of HAP structure in OCM reactions. The selectivity to C₂ (ethylene and ethane) products increased in the order of HAP-CO₃ < HAP < Pb-HAP-CO₃ < Pb-HAP, while Pb-HAP-CO₃ showed the best stability and highest C₂ yield (under optimized reaction conditions) among these four HAP-based catalysts. Upon varying the reaction temperature and/or CH₄/O₂ ratios in the reactions, the CH₄ conversion and C₂ or CO_x (CO and CO₂) formation showed a strong dependence on the composition of HAP-based catalysts.