Oct-2024

Pursue sustainability and improve catalyst performance with magnesium ethoxide when producing polyolefins (TiA)

Polyolefins consisting of polyethylene (PE) and polypropylene (PP) belong to the top five groups of plastic materials produced globally.

Minghui Zhang
Evonik Catalysts

Article Summary

Two hundred and twenty million tons of polyolefin plastics are produced annually. This versatile thermoplastic boasts a wide range of end-use applications, spanning pipes, containers, automotive parts, medical devices, and more.

First commercialised in the 1930s, the growth of the polyolefin market share over the decades is due to the discovery of and subsequent improvements to catalysts. Ziegler-Natta-type catalysts, introduced in the 1950s, are still commonly used today for the production of PE and PP. With PE, Ziegler-Natta catalysts account for almost 45% of all polymerisation processes; for PP, it is 95%.

These popular catalysts, however, need a support to achieve their best performance, which is where magnesium ethoxide excels. Marrying performance and cost-effectiveness, this material can assist companies with their sustainability goals.

Evolution of Ziegler-Natta catalysts
The second generation of catalysts for polyolefin production was introduced in the 1970s when scientists discovered the incorporation of magnesium species into the Ti-Al (titanium-organoaluminium) Ziegler-Natta catalyst. Magnesium significantly increased catalyst activity and increased the catalysts’ capability to incorporate a second monomer, a high α-olefin with four or six carbon atoms, into PE chains.

Ti-Mg-Al (titanium-magnesium-organoaluminium) Ziegler- Natta catalysts revolutionised the polyolefin industry. Current polymerisation processes have been designed based on this version of catalyst.

Polyolefin production can now take place with monomer and comonomer in the presence of Ziegler-Natta catalysts without the need for diluent, significantly reducing the environmental impact.

Additionally, the higher catalyst activity in the range (20,000-60,000 kg polyolefin)/(kg cat) leaves a minimal amount of metal residue in the resin, thereby eliminating the need for residue removal steps.

For PP production, a promoter known as the internal donor was incorporated into the aforementioned Ti-Mg-Al catalyst to selectively produce the useful grades of PP, isotactic PP (iPP), in combination with a second promoter known as the external donor.  

Continued benefits of magnesium ethoxide
The magnesium used in Ziegler-Natta catalyst production takes two forms: magnesium chloride (MgCl₂) and magnesium alkoxide. The latter is mainly in the form of magnesium ethoxide (MgE, for short). In the finished Ziegler-Natta catalyst, most magnesium species exist in the form of MgCl₂ through a series of chemical transformations.

Looking at propylene polymerisation, MgCl₂ serves as the catalyst support and is still part of the catalyst’s active components. The main composition in the finished catalysts, it utilises its unique crystal structure to disperse titanium and the internal donor, to make a highly active and highly selective catalyst for the production of desired iPP. Evonik provides proprietary olefin polymerisation catalyst components for the manufacture of polyolefin catalysts, comprising both supports and donors.

Catylen S
A precursor to a variety of ethylene and propylene polymerisation catalysts, granular MgE – Catylen S 100 series is a high-purity material. It can be utilised to conduct particle form technology to obtain the desired catalyst activity, particle size, and particle size distribution for reactor operability.

A Mg-Ti precursor solution, Catylen S 200, allows for the making of morphology in only one step, significantly reducing catalyst production cycle time and debottlenecking plant capacity without capital investment.

Catylen S 300 is a family of morphology-defined spherical MgE particles. These have particle sizes between 10 µm and 35 µm that can be commercially produced. The spherical material can undergo titanation to become a PE catalyst or, when combined with an internal donor, can be used to produce a PP catalyst.

To enhance sustainability efforts, one of the starting materials in MgE production, ethanol, is being migrated from petroleum-derived ethanol to bio-based ethanol. Currently, half of the ethanol Evonik uses for this production comes from a bio-based source, with further expansion underway.

Catylen D
Internal donors, essential components of the Ziegler-Natta catalyst system, provide stereoselectivity, hydrogen response, molecular weight, and comonomer incorporation capability while influencing the catalyst activity in the polymerisation of propylene. The recently developed Catylen D 2100 is a phthalate-free and non-aromatic internal donor that enables polyolefin producers to exceed the highest toxicity standards.

Conclusion
Where would Ziegler-Natta catalysts be without magnesium ethoxide? The support precursor provides a host of performance benefits, rendering the catalyst with high activity, a smooth polymerisation kinetic profile, longer catalyst life, and better comonomer incorporation. This combination allows polyolefin manufacturers to produce value-added specialty polymers, including multimodal polyethylene and polypropylene impact copolymer.

* Catylen S and Catylen D are marks of Evonik.

This short case study originally appeared in PTQ's Technology In Action Feature - Q4 2024 Issue

For more information: Markus.hartung@evonik.com