Dimeric 1,3-propanediaminetetraacetato lanthanides as the precursors of catalysts for the oxidative coupling of methane

Abstract

From neutral solutions, dimeric 1,3-propanediaminetetraacetato lanthanides (NH₄)₂[Ln₂(1,3-pdta)₂(H₂O)₄]·8H₂O [Ln = La, 1; Ce, 2] and K₂[Ln₂(1,3-pdta)₂(H₂O)₄]·11H₂O [Ln = La, 3; Ce, 4] (1,3-H₄pdta = 1,3-propanediaminetetraacetic acid, C₁₁H₁₈N₂O₈) were isolated in high yields. The reaction of excess strontium nitrate with 1 resulted in the formation of a two dimensional coordination polymer [La₂(1,3-pdta)₂(H₂O)₄]_n·[Sr₂(H₂O)₆]_n·[La₂(1,3-pdta)₂(H₂O)₂]_n·18nH₂O (5) at 70 °C. Complexes 1–4 show a similar central molecular structure. The lanthanide ions are coordinated by two nitrogen atoms, four carboxy oxygen atoms from one 1,3-pdta ligand, two from the neighboring 1,3-pdta ligand forming a four-membered ring and two water molecules. Complex 5 has two kinds of dimeric lanthanum unit and extends into a 2D coordination polymer through strontium ions and bridged oxygen atoms, and forms a fourteen membered ring linked by oxygen atoms from carboxy groups of pdta. Complexes 1–4 are soluble in water. The ¹³C{¹H} NMR experiments for complex 1 were tested in solution. Thermal products from 1 and 5 show good catalytic activities towards the oxidative coupling reaction of methane (OCM). The conversion of methane and selectivity to C₂ reached 29.7% and 51.7% at 750 °C for the product of 5. From TGA, XRD and SEM analyses, the thermal products from 1 and 5 are rod- and poly-shaped, which are assigned as lanthanum oxocarbonate and a mixture of La₂O₃, SrCO₃ and La₂O₂CO₃ for 1 and 5, respectively. The precursor method is favorable for the formation of regular shaped mixed oxides.