Influences of cation and anion substitutions on oxidative coupling of methane over hydroxyapatite catalysts

• Pb2+, CO32− and both substitutions in hydroxyapatite (HAP) were studied.
• Structural properties and performance of HAP catalysts in OCM reactions were revealed.
• Pb-HAP-CO3 exhibits the best stability and C2 yield under optimized conditions.
• Tuning compositions of HAP by ion substitutions tailors its OCM performances.

Lead substituted hydroxyapatite (Pb-HAP) has been an active catalyst for oxidative coupling of methane (OCM) reactions. CO32− 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 (CO32−) in HAP structure on OCM reactions were studied. The composition and physicochemical properties of HAP catalysts were changed 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 comparable C2 yield (under optimized reaction conditions) to Pb-HAP catalyst. Under different reaction temperature and/or CH4/O2 ratio in the OCM reactions, the CH4 conversion and C2 or COx (CO and CO2) selectivity showed a strong dependence on the composition of HAP-based catalysts. The present study forms a basis for understanding of the correlations between the composition, structure, and catalytic performance of HAP and other apatite structured catalysts, which are potential membrane materials for OCM reactions in membrane reactors.

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