Ziegler-Natta catalysts are used to produce a significant fraction of Dow’s 16 billion pound annual polyethylene production. The current generation of Ziegler-Natta catalysts is based on titanium active species supported on high surface area magnesium chloride. In spite of the very successful deployment of Ziegler-Natta catalysts in industrial processes, a detailed understanding of the catalyst structure is still lacking. The local morphology and structure of salts such as MgCl2 is poorly explored because imaging the pristine state of these materials at atomic resolution is challenging for two reasons: First, their structural integrity is compromised by any exposure to oxygen or moisture. And secondly, such materials are susceptible to damage by the probing electron radiation. Elucidating the atomic-scale structure of MgCl2 materials would lead to beneficial understanding of structure-property relationships of Ziegler-Natta catalysts. Revisiting the underlying bottlenecks, we take advantage of emerging technologies and new imaging practices. The knowledge gained in these studies enriches our basic scientific knowledge about the pristine, atomic structure of previously unexplored matter. Under conventional electron imaging conditions at 300 kV, β-phase MgCl2 is observed at accumulated doses of less than 10,000 e-/Å2 at dose rates around 1500 e-/Å2/s. If the same MgCl2 material is studied with electron dose rates less than 500 e-/Å2/s, α-phase MgCl2 is detected up to and beyond accumulated doses of ~20,000 e-/Å2. By systematically increasing the dose rate, a conversion of α-MgCl2 to β-MgCl2 is observed. At significantly higher electron dose rates, chlorine loss is observed as the presence of elemental Mg. A damage retardation factor of ~ 3 has been observed for a dose rate reduction by an order of magnitude. Atomic-resolution imaging of the pristine α-MgCl2 structure has been achieved by applying low dose rates, a monochromated electron beam, and in-line holography. By limiting the electron dose rate on the sample, we are able to study dislocations in MgCl2 materials. These defect sites are particularly of interest, as they are thought to be binding sites for the active titanium species for Ziegler-Natta catalysis. Further studies of defect structure and density are currently under way.

Structure of Ziegler-Natta catalysts revealed by low-dose transmission electron microscopy

256th National Meeting (2018)

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