Highly water-soluble dimeric and trimeric lanthanide carbonates with ethylenediaminetetraacetates as precursors of catalysts for the oxidative coupling reaction of methane

Abstract

Reactions of carbon dioxide with lanthanide ethylenediaminetetraacetates M[Ln(EDTA)(H₂O)₃]·5H₂O (M = NH₄, Na or K; Ln = La or Ce) result in the formation of highly water-soluble lanthanide carbonates with ethylenediaminetetraacetates (NH₄)₅[La₃(CO₃)(EDTA)₃(H₂O)₃]·12H₂O (1), Na₈[Ln₂(CO₃)₃(EDTA)₂]·17.5H₂O (Ln = La, 2; Ce, 3), and K₅[Ln₃(CO₃)(EDTA)₃(H₂O)₃]·13.5H₂O (Ln = La, 4; Ce, 5; H₄EDTA = ethylenediaminetetraacetic acid) in dimethylformamide–water solution, which have been fully characterized. The main structures of these five complexes are all based on the [Ln(EDTA)]⁻ unit, where carbonates participate due to coordination in μ₁:η²(O,O) and μ₂:η²(O,O′):η²(O,O′) fashion. The reactions of lanthanum ethylenediaminetetraacetate with CO₂ have been monitored via¹³C NMR measurements. Furthermore, the calcined products of 1 and 2 are used for the oxidative coupling reaction of methane (OCM). The results demonstrate the good catalytic performances of La₂O₃ or La₂O₂CO₃ from 1 and a mixture of La₂O₂CO₃ and Na₂CO₃ from 2. The coordination modes of the lanthanide carbonates in precursors 1 and 2 may provide a model for the interaction of lanthanide carbonates, which is useful for the OCM reaction.