Abstract
This thesis focusses on the development of new catalyst systems for the DODH of diols and biomass derived polyols. Where Chapters 2 and 3 solely focus on high valent rhenium complexes, in Chapter 4 molybdenum complexes with Cp-based ligand are studied, and the use of low-valent rhenium pre-catalysts is described
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in Chapter 5.
Chapter 2 explores a Cp-based trioxo-rhenium complex, CpttReO3 (Cptt = 1,3-di-tert-butylcyclopentadienyl) as catalyst for the DODH of diols and biomass derived polyols using secondary alcohols as reductant. This study follows previous work of Klein Gebbink et. al., where a series of Cp-based trioxo-rhenium complexes has been reported and investigated for their catalytic active in the DODH of 1,2-octanediol with PPh3 as reductant. The investigations on the CpttReO3,complex include in situ NMR experiments, aiming to further investigate catalyst evolution under catalytic conditions.
Chapter 3 describes the application dioxo-rhenium complexes as catalysts for the DODH of diols and polyols. Both tetradentate N2Py2 aminopyridine ligand-supported ReO2+ complexes and monodentate pyridine-supported ReO2+ complex have been investigated. The interest in these complexes arises form the question if DODH catalysis proceeds through a ReV↔ReVII pathway or a ReV↔ReIII pathway when starting with a dioxo-rhenium species, since both pathways have been proposed for the use of trioxo-rhenium complexes in DODH catalysis.[14] The possible use of aminopyridine ligands in DODH catalysis may accelerate catalyst development through ligand design, since modifications of Cp-based ligand are rather limited. As part of this study, time-course profiles of the DODH reaction using the ReO2+ complexes are explored in order to shed light on the role of these complexes as either catalysts or pre-catalysts.
Chapter 4 focusses on the development of molybdenum-based DODH catalysts as an alternative for catalysts based on rather scarce and expensive rhenium. Previous studies have shown that the catalytic properties of Cp’ReO3 type complexes in DODH reactions are affected by modifications of the Cp ligand. Chapter 4 therefore investigates if the corresponding Cp-based Mo-complexes also show catalytic activity towards the DODH of diols and polyols. Next to catalyst identification and optimization, this chapter also addresses the mechanism of operation of one of the Mo-complexes through a series of stochiometric experiments.
Although Cp’ReO3 complexes, such as the one described in Chapter 2, are very efficient DODH catalysts, their overall synthesis suffers from rather low yields based on the starting amount of rhenium (4-49%). The low yield is mainly due to the decomposition of the trioxorhenium species under the oxidizing conditions of its synthesis from Cp’Re(CO)3. Accordingly, Chapter 5 explores the development of low valent rhenium complexes for DODH reactions and reports on the DODH activity of a series of low valent rhenium complexes supported by anionic NNO-type ligands (NNO = 2-(((2-(dimethylamino)ethyl)(methyl)amino)methyl) phenol). Both diol and biomass-based polyol substrates could be applied with these complexes under aerobic conditions, to form corresponding olefins with moderate to high yields.
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