Probing Interactions between Electron Donors and the Support in MgCl₂-Supported Ziegler–Natta Catalysts

Abstract

Olefin polymerization using Ziegler–Natta catalysts (ZNCs) is an important industrial process. Despite this, fundamental insight into the inner working mechanisms of these catalysts remains scarce. Here, we focus on the low-γ nuclei 25Mg and 35Cl for an in-depth solid-state NMR and density functional theory (DFT) study of the catalyst’s MgCl2 support in binary adducts prepared by ball-milling. Besides the bare MgCl2 support and a MgCl2–TiCl4 adduct, samples containing donors that are part of the families of 2,2-dialkyl-1,3-dimethoxypropanes and phthalates used in fourth- and fifth-generation ZNCs are studied. DFT calculations indicate that the quadrupolar coupling parameters of the chlorines differ significantly between bulk and surface sites. As a result, the NMR visibility of the chlorine sites correlates with the particle size except for the adduct with 2,2-dimethyl-1,3-dimethoxypropane donor. The DFT calculations furthermore show that the surface sites are fairly insensitive to binding of different donor molecules, making it difficult to identify specific binding motives. The surface sites with large 35Cl NMR line widths can be observed using high radio frequency field strengths. For the 2,2-dimethyl-1,3-dimethoxypropane donor, we observe additional surface sites with intermediately high quadrupolar couplings, suggesting a different surface structure for this particular adduct compared to the other systems. For 25Mg, pronounced effects of donor binding on the quadrupole interaction parameters are observed, both computationally and experimentally. Again the adduct with the 2,2-dimethyl-1,3-dimethoxypropane donor shows a different behavior of the surface sites compared to the other adducts, which display more asymmetric coordinations of the surface Mg sites. Identifying specific binding motives by comparing 25Mg NMR results to DFT calculations also proves to be difficult, however. This is attributed to the existence of many defect structures caused by the ball-milling process. The existence of such defect structures both at the surface and in the interior of the MgCl2 particles is corroborated by NMR relaxation studies. Finally, we performed heteronuclear correlation experiments, which reveal interactions between the support and Mg–OH surface groups, but do not provide indications for donor–surface interactions.

Ultrafast Crystallization of AlPO4-5 Molecular Sieve in a Deep Eutectic Solvent

“…On the contrary, the static 35 Cl spectrum of frozen DES consists of a broad pattern with a breadth comparable to that of TEACl·H 2 O. Although the line shape is ill-defined, it does have several features, suggesting that there are several distinct chlorine sites in the frozen DES and that they have largely disordered local environment . It seems that although the TEA + ions are highly mobile in the frozen DES the mobility of the Cl – ions are significantly lower.…”

“…Although the line shape is ill-defined, it does have several features, suggesting that there are several distinct chlorine sites in the frozen DES and that they have largely disordered local environment. 60 It seems that although the TEA + ions are highly mobile in the frozen DES the mobility of the Cl − ions are significantly lower. The discrepancy can be associated with the intrinsic differences in their roles in hydrogen bonding: The Cl − ions are expected to form hydrogen bonds with PE, whereas the TEA + ions are not.…”

DNP-SENS Formulation Protocols To Study Surface Sites in Ziegler–Natta Catalyst MgCl2 Supports Modified with Internal Donors

“…Step 3: The material is activated with AlEt 3 , often in the presence of an external donor (e.g., alkoxysilane), and contacted with olefins. , It was shown that the highest stereo- and regioselectivity in polymerization reactions is achieved when either aromatic esters (e.g., ethyl benzoate, dibutyl phthalate) or 1,3-diethers (e.g., 1,3-dimethoxy-2,2-dialkylpropane) are used as IDs. , Moreover, it is generally accepted that the presence of IDs increases catalytic efficiency and directs polymerization toward formation of isotactic PP . Therefore, there has been significant effort devoted to studying the materials at the various steps of the catalyst preparation and in particular at step 2 focusing on the interaction of IDs with MgCl 2 supports, with the goal in particular to clarify the location of ID on the MgCl 2 surface with the intent to understand the origins of stereoselectivity of PP-ZNC. − Therefore, in this study we focus on developing a methodology to enable the evaluation of the MgCl 2 –ID supports (Figure b), here using dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS) ( vide infra ).…”

“…Recent experimental and computational studies show that IDs are likely bound to four- and five-coodinated Mg sites on catalytically relevant (110) and (104) facets of the MgCl 2 surface. This leads to the stabilization of primary MgCl 2 particles, which upon introduction of Ti directs formation of stereoselective sites. ,,− However, little is experimentally known about the detailed interaction between the internal donor and the support even though it is likely very relevant for the understanding of the origins of stereoselectivity and further improvement of PP-ZNC.…”

Monitoring the Kinetics of Internal Donor Clean-up from Ziegler–Natta Catalytic Surfaces: An Integrated Experimental and Computational Study

“…MgCl 2 -supported Ziegler–Natta catalysts (ZNCs) dominate the ever-growing production of isotactic polypropylene, thanks to an exceedingly high stereoselectivity (isotacticity index I.I. > 98%) and a productivity in the order of several tons of polymer per gram of Ti. − These systems consist of a solid precatalyst and a soluble cocatalyst: the former is typically obtained by reacting a MgCl 2 precursor with TiCl 4 in the presence of a first Lewis base (“internal donor”, ID), whereas the latter consists of AlEt 3 and a second Lewis base (“external donor”, ED). , Surface modification by the electron donors is essential for the stereoselectivity; decades of experimental − and computational modeling − studies demonstrated that ID and ED molecules, chemisorbed in the vicinity of the active Ti sites, shape the catalytic pocket similarly to the ancillary ligands of molecular catalysts.…”