Computer-modelling studies on MgCl₂-supported Ziegler–Natta catalysts

Characterization of the Ziegler‐Natta olefin polymerization system: surface science studies on model catalysts

“…This plane is most likely to be the (100) plane, which is the second most stable plane of the crystal. 44,45 By assuming this structural information to be applicable to the model catalyst films, we could attribute the dominant mesitylene desorption peak at ¾200 K for TiCl x /MgCl 2 to a basal plane structure where the chlorine ions at the outermost layer are close packed and the metal ions under the chlorine layer are coordinated to six chlorine atoms. 20 In this frame, the high-temperature desorption peak at 246 K could be attributed tentatively to a non-basal plane where the surface chlorine ions are not close packed and the metal ions under the chlorine layer are under-coordinated.…”

“…41 The LEED study of a MgCl 2 film deposited by thermal evaporation in UHV showed that the film grows layer by layer and the film surface was composed of small domains of the (001) basal plane, the most stable crystallographic plane. 39,44,45 The He C ISS studies of the model Ziegler-Natta catalyst films found that the surfaces of these films are terminated with chlorine ions and no metal ions are exposed to the vacuum. 9,10,12 The SEM pictures described the TiCl 3 microcrystallites produced by a sublimation-recrystallization method as possessing a very thin, hexagonal platelet shape.…”

“…However, these surfaces have very high surface energy and undergo significant reconstruction processes. 44,45 Structural studies on the (100) and (110) planes of a MgCl 2 single crystal would provide more insight into the surface structure of the non-basal planes where isotactic PP is produced.…”

“…20 However, one could interpret the mesitylene TPD data using the crystallographic data, 41 surface structure, 39 surface compositions, 9,10 the shape of microcrystals 42,43 and thermodynamics 44,45 of the titanium chloride and magnesium chloride films or crystals obtained by other techniques. X-ray diffraction (XRD) found that these halide crystals have a layered structure, each layer consisting of two monoatomic layers of chlorine ions octahedrally coordinated to intercalating metal ions.…”

“…This could be attributed to the higher surface energies of these planes compared with the (001) basal plane. 44 If these planes were exposed at the surface, they would reconstruct or react with TiCl 4 and/or residual gas (HCl) to lower surface energy.…”