Dimeric 1,3-propanediaminetetraacetato lanthanides as the precursors of catalysts for the oxidative coupling of methane.
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 (13)C{(1)H} 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.
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. zxcao@xmu.edu.cn zhzhou@xmu.edu.cn.
Mao-Long
ML
Chen
Yu-Hui
YH
Hou
Wen-Sheng
WS
Xia
Wei-Zheng
WZ
Weng
Ze-Xing
ZX
Cao
Zhao-Hui
ZH
Zhou
Hui-Lin
HL
Wan