Homogeneous Catalyst Models

Keywords Ethylene polymerization mechanisms Heterogeneous model catalysts Homogeneous model catalysts Molecular modeling Phillips Cr/silica catalyst Polyethylene Polymerization kinetics... 

During the last decade, increasing research efforts have been performed on Phillips catalysts through various approaches including spectroscopic methods, polymerizatiOTi kinetics, heterogeneous model catalysts, homogeneous model... 

The proper treatment of the electronic subtleties at the metal center is not the only challenge for computational modeling of homogeneous catalysis. So far in this chapter we have focused exclusively in the energy variation of the catalyst/substrate complex throughout the catalytic cycle. This would be an exact model of reality if reactions were carried out in gas phase and at 0 K. Since this is conspicously not the common case, there is a whole area of improvement consisting in introducing environment and temperature effects. 

Several approaches were investigated for the synthesis of the model catalyst, and the procedure resulting in the most homogeneous sample in terms of cluster size and morphology can essentially be divided into two steps Initially, metallic molybdenum is deposited on a single-crystal Au(l 11) substrate in an atmosphere of H2S at... 

The effect of the polymer backbone on the intrinsic chemical reactivity of metal complexes has been studied in aqueous solution and in Nafion (perfluorocarbon sulfonic acid) film 44). Using a model catalyst-substrate system, the independent kinetic effects of reaction site homogeneity, substrate diffusion into the polymer film, and changes on activation parameters have been addressed. The ligand substitution reaction (6), was chosen for this purpose (Py = pyridine and its derivatives). 

Early efforts to model catalyst deactivation either utilized simplified models of the catalyst s porous structure, such as a bundle of nonintersecting parallel pores, or pseudo-homogeneous descriptions in terms of effective diffusivities and tortuosity... 

A variety of model catalysts have been employed we start with the simplest. Single-crystal surfaces of noble metals (platinum, rhodium, palladium, etc.) or oxides are structurally the best defined and the most homogeneous substrates, and the structural definition is beneficial both to experimentalists and theorists. Low-energy electron diffraction (LEED) facilitated the discovery of the relaxation and reconstruction of clean surfaces and the formation of ordered overlayers of adsorbed molecules (3,28-32). The combined application of LEED, Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), field emission microscopy (FEM), X-ray and UV-photoelectron spectroscopy (XPS, UPS), IR reflection... 

Cobalt and triphenylphosphine (PPhs) ligands are commonly encountered in homogeneous catalyzed processes. Therefore, dichlorobis(triphenylphosphine)cobalt(II) has been selected as a homogeneous model catalyst. In 1-butanol, this complex is in equilibrium with the free Co(II)Q2 and PPh3 ligands. 

Heferogeneous olefin epoxidation over solid titania-silica catalysts has been the subject of numerous publications in the open literature. The general picture that emerges is that isolated titanium (IV) species on a silica surface or in a zeolife mafrix are responsible for the high epoxidation activity [2]. This picture is supported by model catalyst work on titanium silasesquioxane complexes [3,4] that form active homogeneous epoxidation catalysts [5] and by various successful atfempfs fo prepare well-defined, site-isolated titanium complexes by grafting molecular precursors on mesoporous silica [6-9]. These site-isolated titanium complexes have been shown to possess catalytic activity in olefin epoxidation. 

A model Phillips catalyst for ethylene polymerization has been prepared by spin coating of a Cr(III) precursor (Cr(acac)3) on a flat silicon wafer (100) covered by amorphous silica. The spin coating parameters were chosen in order to obtain a homogeneous film. The model catalyst was submitted to an activation process. The surface concentration of Cr decreased from about 0.8 to 0.4 Cr atom/nm as the temperature increased from 150 to 550°C. Direct information concerning the surface molecular species and the environment of Cr was provided by ToF-SIMS and XPS. At 350°C, the catalyst precursor was decomposed Cr species were in the form oxide and surface-anchored chromates. Upon final activation at 650°C for 6 h, Cr species were below the XPS detection limit however the model catalyst was active for ethylene polymerization at 160°C and 2 bar pressure. 

An active Phillips model catalyst has been successfully prepared starting from a Cr(III) precursor. The active phase was deposited homogeneously on a silicon wafer by spin coating, and the model was submitted to the activation process usually applied to real Phillips catalysts. Using complementary surface science techniques, molecular information on the modifications of the state of the Cr during activation was provided. 

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