Surface science of single-site heterogeneous olefin polymerization catalysts

Abstract

This article reviews the surface science of the heterogeneous olefin polymerization catalysts. The specific focus is on how to prepare and characterize stereochemically specific heterogeneous model catalysts for the Ziegler-Natta polymerization. Under clean, ultrahigh vacuum conditions, low-energy electron irradiation during the chemical vapor deposition of model Ziegler-Natta catalysts can be used to create a ''single-site'' catalyst film with a surface structure that produces only isotactic polypropylene. The polymerization activities of the ultra-high vacuum-prepared model heterogeneous catalysts compare well with those of conventional Ziegler-Natta catalysts. X-ray photoelectron spectroscopic analyses identify the oxidation states of the Ti ions at the active sites. Temperature-programmed desorption distinguishes the binding strength of a probe molecule to the active sites that produce polypropylenes having different tacticities. These findings demonstrate that a surface science approach to the preparation and characterization of model heterogeneous catalysts can improve the catalyst design and provide fundamental understanding of the single-site olefin polymerization process.Ziegler-Natta C atalysts can function by being dispersed on solid surfaces (heterogeneous catalysis) or dissolved in reaction media (homogeneous catalysis). Heterogeneous catalysts are widely used in the chemical industry because they are in general easy to handle, separate, and recycle. Homogeneous catalysts are used in synthesis of specialty chemicals, offering precise control of molecular structure and reactivity. Combining the merits of these two different catalyst systems is of great interest for production of next-generation catalysts (1). In this article, we review the surface science of the heterogeneous olefin polymerization.In the polymerization of small olefins such as ethylene and propylene, both heterogeneous and homogeneous catalytic systems are operational (Fig. 1). Heterogeneous olefin polymerization catalysts (so-called Ziegler-Natta catalysts) are used for production of more than two-thirds of the commodity polyolefins consumed in the world (2, 3). Recently, a large number of specialty polyolefins have been produced with homogeneous metallocene catalysts (4). Whereas most industrial heterogeneous catalysts producing polyethylene and polypropylene are titanium chloride-based catalysts, the homogeneous metallocene catalysts are organometallic compounds of Ti, Zn, and Hf metals in organic solvents. In both types of catalysts, the catalytic species are activated with alkyl aluminum cocatalysts to create the active sites for carbon-carbon bond formation. Triethyl aluminum (AlEt 3 ) is widely used for heterogeneous ZieglerNatta catalysts, whereas methylaluminoxane is typically used for homogeneous metallocene catalysts.The polymers produced with these catalysts can have a wide range of mechanical properties depending on how many monomers are connected (molecular weight) and how they are connected (microstructure). The mechan...

Investigation of Planar Ziegler-Natta Model Catalysts Using Attenuated Total Reflection Infrared Spectroscopy

“…Kim and Somorjai [43] reported the formation of isotactic PP with model donor-free heterogeneous catalysts prepared under ultra-high vacuum techniques. Our results are in line with reported values for PP produced with MgCl 2 /TiCl 4 /AlR 3 (I.I.…”

Evolution of the surface science of catalysis from single crystals to metal nanoparticles under pressure

“…A typical homogeneous catalyst used for olefin polymerization is a so-called single site catalyst with an active site that is a titanium ion surrounded by ligands (Figure 2b), which is 1.6 nm in size [6] . This nanosize molecule produces a million C-C bonds as it makes polypropylene [7] . The heterogeneous catalysts such as platinum nanoparticles (Figure 2c) are between 1-10 nm in size [8] .…”

Influence of Coordination Environment of Anchored Single‐Site Cobalt Catalyst on CO2 Hydrogenation

“…Atomically dispersed transition metals on a surface have been shown to form unique oxidation states that correlate directly to the coordination geometry and neighboring atoms, therefore, XPS was utilized to probe this surface sensitive phenomena. In the case of cobalt, the most stable state is the Co 2+ (Td) cation, which has a distinct 2 p 3/2 XPS peak at 780.2 eV .…”