Solid-State NMR Investigations of MgCl₂ Catalyst Support

Abstract

MgCl2 is a vital component of Ziegler-Natta catalysts for olefin polymerization. Here we:synthesized anhydrous MgCl2 using different drying protocols and exploited H-1 NMR to quantify the proton content. We report on our study of neat and ball-milled MgCl2 samples by means of Mg-25 and Cl-35 solid-state NMR. DFT calculations of the quadrupole tensor aid in analysis of the spectra. The results show that, due to the morphology of the neat particles, a preferred Orientation is induced which manifests itself in unusual powder line shapes. Ball Milling reduces particle size, which subsequently leads to a small distribution of quadrupole parameters for the bulk. Surface sites, highly relevant for catalysis, are not directly observed, due to their broad lines of low intensity

Structural Characterization of Electron Donors in Ziegler–Natta Catalysts

“…In a previous publication, we showed that even thoroughly dried neat MgCl 2 samples are not completely water-free. 40 In fact, a full surface coverage of water or otherwise protonated species (−OH surface groups) was found. The binary adducts presented in this study have significantly larger surfaces areas ( Table 1 ) and might potentially take up large quantities of water.…”

“… a ⟨ L c ⟩ and ⟨ L a ⟩ are the average particle dimension perpendicular and parallel to the basal ( ab ) plane, respectively. b See ref ( 40 ). c The average size could not be determined because the XRD diffraction peaks were too broad.…”

“…The preparation of anhydrous MgCl 2 is described elsewhere. 40 Dried MgCl 2 and the respective donors were ball-milled using a Retsch PM-100 planetary ball mill. Operating inside a glovebox, the samples (2–12 g) were loaded inside an air-tight and chemically inert ceramic jar (Y-stabilized ZrO 2 ), along with 87 g of grinding ceramic balls (3 mm diameter).…”

MgCl2‐supported Ziegler‐Natta catalysts: A DFT‐D ‘flexible‐cluster’ approach. TiCl4 and probe donor adducts

“…Notably, for CLU‐24‐104/Me 2 O adducts this turned out to be complicated by the presence of multiple local minima with very similar energy values and low separation barriers, differing primarily in the local structure at corners with terminal Mg‐bonds; reversible or irreversible reconstructions from one such minimum to another were observed upon structure re‐optimization. The problem was overcome by adsorbing H 2 O molecules at the aforementioned corners, thus saturating the Mg coordination sphere; as is well‐known, H 2 O is an ubiquitous donor with high affinity for MgCl 2 (vide infra), and a common contaminant of real MgCl 2 adducts . ‘Corner‐stabilized’ CLU‐24‐104/Me 2 O adducts did not feature any propensity to reconstruct (for details, see the SI).…”

“…In all cases, notably, based on the calculated values of adsorption energy (Table ), one should expect surface saturation at room temperature to occur at a donor partial pressure as low as 1 ppb. For the specific case of H 2 O, this should be kept in mind when handling model samples of ‘activated’ MgCl 2 for fundamental (e.g., spectroscopic) studies; whereas preventing surface saturation might be feasible in a UHV cell, it certainly is not in a glove‐box …”