Study of the active site and activation process of Ni-based catalyst for 1,3-butadiene polymerization using X-ray absorption and crystal-field spectroscopies

Abstract

The catalytic activation process of the Ni-based Ziegler–Natta catalyst composed of Ni(naphthenate) 2, catalytic amount of 1,3-butadiene, BF 3·OEt 2 and AlEt 3, is spectroscopically observed by using synchrotron X-ray absorption and crystal field spectroscopies, and an optimum model of the nickel active site is proposed with the density functional theory (DFT) calculations. In the X-ray absorption near edge structure (XANES) spectrum of nickel naphthenate (NN), the position of the pre-edge at 8330.0 eV and both the strong edge and the weak pre-edge peaks indicate that NN possesses high O h symmetry and is in a divalent state. The similar XANES spectra of NNB (the mixture of NN and catalytic amount of 1,3-butadiene) and NNBF (the mixture of NNB and BF 3·OEt 2) indicate that 1,3-butadiene and BF 3·OEt 2 do not greatly disturb the coordination geometry around nickel. AlEt 3 functions as the alkylating agent in the presence of BF 3·OEt 2, and initiates 1,3-butadiene polymerization in NNBFA (the mixture of NNBF and AlEt 3) where the coordination geometry is altered from O h- to T d-like but nickel remains in a divalent oxidation state. In the absence of BF 3·OEt 2, AlEt 3 works as the reducing agent in NNBA (the mixture of NNB and AlEt 3). A single and well-defined peak at 1.62 Å is observed for NN due to the NiO shell in the phase-uncorrected Fourier-transformed (FT) EXAFS (extended X-ray absorption fine structure) spectrum of NN. A significant change in the EXAFS spectrum of NNBFA has occurred where the main peak shifts toward a higher distance with increased complexity. The complex nature of the peak at 1.75 Å indicates that the coordination feature around nickel differs from the NiO shell and the longer bonds, NiC and NiF, are formed. The NiF bond of NNBFA is confirmed by the NiF peak, −137 ppm, in the 19 F -NMR spectrum. The three main absorption bands, around 8830, 14660 and 24940 cm −1, are obtained with the crystal field spectra of NN, NNB and NNBF, which represent a typical octahedral structure of Ni(II) having three spin-allowed transition bands from 3 A 2 g ground state to the 3 T 2 g , 3 T 1 g (F) and 3 T 1 g (P) states, respectively. Heterogeneity of the Ziegler–Natta catalyst is evidenced by strong scattering in the UV-Vis region of NNBFA.