Interaction of different poisons with MgCl2/TiCl4 based Ziegler-Natta catalysts

- The interactions between different classes of poison molecules and TiCl4/MgCl2 type Ziegler-Natta catalyst is considered.
- Poisons strongly stabilize MgCl2 crystal surfaces, mostly Ti active center relative to the unpoisoned solid.
- Poison molecules decrease catalyst activity by increasing ETS in olefin polymerization.
- Poison molecules do not have significant effect in stereospecifity of ZN catalysts in propylene polymerization.

Adsorption of different poison molecules on activated MgCl2 is investigated within DFT using a cluster model of the MgCl2 surface with (MgCl2)16 formula containing four 4-coordinated and eight 5-coordinated Mg atoms as (110) and (104) surfaces, respectively. Studied poison molecules are chosen as possible impurities in hydrocarbon solvents and monomer feeds and contain water, hydrogensulfide, carbondioxide, molecular oxygen and methanol. First, adsorption of 1-4 molecules of different poisons to the (104) and (110) lateral cuts of MgCl2, as well as their adsorption on [MgCl2]/TiCl2Et active center and AlEt3 cocatalyst is considered. Results reveal that poisons strongly stabilize both crystal surfaces, mostly Ti active center relative to the unpoisoned solid. Second, energy barrier (ETS) for ethylene insertion in the presence of different poisons located on the first and second Mg atom relative to the active Ti is calculated. While poison molecule located on the second Mg does not change ETS, coordination of it into the first Mg atom increases ETS by 0.9-1.2 kcal mol−1. In the last part of this manuscript, the stereoselective behavior of active Ti species, with and without poison molecules and external electron donor, is fully explored.