Model Ziegler−Natta Polymerization Catalysts Fabricated by Reactions of Mg Metal and TiCl₄:  Film Structure, Composition, and Deposition Kinetics

Abstract

The film structure, composition, and deposition kinetics of model Ziegler-Natta polymerization catalysts produced by chemical vapor deposition have been studied using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). Redox reactions of metallic Mg and TiCl 4 deposited from the vapor phase on a Au substrate produce the model catalysts composed of titanium chloride and magnesium chloride (TiCl x /MgCl 2 ). The low solubility of TiCl x in MgCl 2 leads to the formation of a TiCl x monolayer on top of MgCl 2 multilayers characteristic of the supported catalysts. The TiCl 4 /Mg flux ratio during the deposition controls the oxidation state distribution and surface coverage of TiCl x species of the deposited film. The conversion of Mg to MgCl 2 is completed in the codeposition process while it is incomplete in the sequential deposition process due to a kinetic barrier for chlorine diffusion. Upon heating the sequentially deposited film, the surface TiCl x (Ti 4+ ) species react with metallic Mg atoms in underlayers to complete the redox reaction. In the case of the codeposited film, the surface TiCl x species desorb as TiCl 4 at temperatures higher than 430 K. After exposure to triethylaluminum (AlEt 3 ) cocatalyst, the model catalyst films are active for propylene polymerization.

Role of titanium oxidation states in polymerization activity of Ziegler–Natta catalyst: A density functional study

“…These catalysts have been studied by ESCA and related techniques and Ti 4+ , Ti 3+ , and Ti 2+ ions have been identified on the surface [12][13][14]. Computational studies on probable active sites in ZN catalytic systems and their single site analogues have also been reported [15][16][17] TiCl 4 and TiCl 3 , coordination on the (1 1 0) MgCl 2 face has been found to be favored relative to coordination on the (1 0 0) face [15].…”

Propylene polymerization catalyzed by novel supported titanium catalysts MgCl2/NaCl/DNBP/TiCl4 with different NaCl content

“…As a series of important properties of the polymer are determined by its structure distribution, controlling ACD of the catalyst has become a great challenge and will significantly influence polyolefin production by the traditional Z-N catalysts [5]. In spite of many difficulties, the surface structures of MgCl 2 and ACD of the MgCl 2 supported catalyst have be studied by several approaches, including kinetic studies [6][7][8][9], XPS and temperature programmed desorption (TPD) studies [10][11][12] and deconvolution of polymer MWD by multiple Flory functions [3,4,[13][14][15].…”

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

“…17 In this case, the Mg atoms in the underlayer were not converted to MgCl 2 due to a kinetic barrier for chlorine diffusion. Upon heating this sequentially deposited film, the surface TiCl x (Ti 4C ) species reacted with metallic Mg atoms in underlayers to complete the redox reaction, instead of desorption into the vacuum as TiCl 4 .…”