Spectroscopic Evidences for TiCl4/Donor Complexes on the Surface of MgCl2-Supported Ziegler-Natta Catalysts

In this work, we have addressed the question of the location of electron donors (we chose ethylbenzoate, EB, for practical reasons) on the surface of MgCl2-supported Ziegler-Natta catalysts by synergistically coupling (a) an in situ investigation of the EB adsorption process by Fourier transform infrared spectroscopy, (b) an evaluation of the MgCl2 surfaces available for the adsorption of CO as a molecular probe at each step of the precatalyst synthesis and (c) an accurate quantum mechanical density functional theory (DFT-D) study of a few TiCl4/EB complexes. Our experimental data indicate that homogeneous-like TiCl4/EB complexes are formed, loosely bonded to the MgCl2 surfaces, as a consequence of a rather high mobility of the adsorbed EB and TiCl4. Our DFT-D computational results demonstrate that monomeric TiCl4(EB) and TiCl4(EB)2 complexes might indeed exist on different catalytically relevant MgCl2 surfaces (although steric repulsion does not allow reaching a full surface coverage), whose computed infrared spectra are highly compatible with the experimental ones. The whole set of data converge to a final scenario whereby the internal donor has the function to induce a certain mobility for TiCl4 simultaneously acting as a "surfactant", in certain cases providing a particularly exposed TiCl4 species readily available for further reaction with the aluminum alkyl activator. The relevant role of the MgCl2 support in assisting the self-organization of the catalyst components clearly emerges.