Local structure analysis around the Nd center in a ternary catalyst comprising Nd(vers)3, Al(iBu)3 and Al(iBu)2Cl by XAFS

Abstract

Lanthanide-based catalysts are highly active for isoprene polymerization in hexane. In this paper, a ternary catalyst consisting of neodymium neodecanoate {Nd(vers)₃}, Al(ⁱBu)₃ and Al(ⁱBu)₂Cl was studied by using X-ray-absorption fine-structure (XAFS) technique. A sealed and moisture-proof liquid sample cell with adjustable thickness was designed for Nd L_III-edge XAFS measurements. Based on the XAFS data analysis, detailed structure changes around the Nd center were obtained. It was found that the Nd(vers)₃ molecules formed an oligomer structure in hexane solution with two Nd–O subshells (5O @ 2.39 Å and 5O @ 2.54 Å) around the Nd center. The alkylation process by adding Al(ⁱBu)₃ to the hexane solution of Nd(vers)₃ partially destroyed the aggregation degree of Nd(vers)₃ molecules in hexane solution. Al(ⁱBu)₃ ligands were bonded to the Nd center by Nd–C bonding. With the Nd : Al ratio increasing from 1 : 2.5 to 1 : 10, the O neighbors around Nd decreased from 4 to 2 but with an unchanged Nd–O bond length of 2.38 Å, and the C neighbors around Nd were kept at ca. 4 with Nd–C bond lengths in the range of 2.57–2.58 Å. The Nd–O bonds can be further replaced by Nd–C bonds during the aging process. The chlorination process by adding Al(ⁱBu)₂Cl to the mixture solution of Nd(vers)₃ and Al(ⁱBu)₃ restrained intensively the agglomeration of Nd(vers)₃ molecules in hexane solution. Al(ⁱBu)₂Cl ligands were bonded to the Nd center by Nd–Cl bonds. There were about 3–4 C neighbors at 2.58 Å, 2 Cl neighbors at 2.87 Å, and 2 Al next-neighbors at 3.14 Å around the Nd center. After allowing the ternary catalyst to stand for 5 days, the coordination numbers of Nd–C and Nd–Cl were all stabilized to 3 without bond length changes, and partial single Cl⁻ anions were also bonded to the Nd center. All these structural details and their change tendency demonstrate that the decrease of aggregation degree of Nd(vers)₃ molecules in hexane solution can improve the catalytic activity of the ternary lanthanide-based catalyst system.