Glycerol valorisation by clay-supported ruthenium catalysts

The valorisation of glycerol has gained significant recent attention as it is considered as an industrial waste product from biodiesel production. It is well-documented that the worldwide production of biodiesel (25 bn L/yr.) entails an enormous generation of glycerol, and Colombia especially, with its immense biofuel industry (110000 barrels per day), contributes a significant amount. Thus, the development of efficient and sustainable chemical processes for the conversion of glycerol into valuable chemical products is of great interest.  

The proposed project will focus on the synthesis of catalysts for dehydrogenation reactions to convert glycerol into dihydroxyacetone, which is an important raw material for cosmetic products. Furthermore, this process generates hydrogen gas, which can either be used as fuel or be recycled in other chemical processes (e.g. hydrogenation reactions).  

Our preliminary studies show that ruthenium-based catalysts exhibit high activity and exceptional selectivity (compared to e.g. iron compounds) for the controlled dehydrogonation of glycerol. Due to their unique stereo-electronical properties, N-Heterocyclic Carbene (NHC) ligands are versatile ancillary molecules, which form robust NHC–metal bonds that may enhance the catalytic activity of the ruthenium metal centre. Surprisingly, such NHC-ruthenium systems are under-explored as catalysts for glycerol valorisation processes.   

In collaboration with the research group of Dr Baquero (National University of Colombia) the new NHC-Ru complexes will be extensively tested as catalysts in high pressure autoclave reactors for homogeneous dehydrogenation reactions of glycerol. Depending on the solubility and the stability of the catalyst (tuned by functional groups in the NHC ligand framework), these reactions will be carried out in organic solvents or, to develop more sustainable processes, in water.  

Solid supports may provide high stability and enhanced reactivity to molecular catalysts upon chemical immobilisation. Colombian clays are earth-abundant, non-toxic, and low-cost natural minerals, which are mainly composed of SiO2, Al2O3 and Fe2O3. Due to their layered structure, these materials have a large adsorption capacity and are thus ideal heterogeneous supports for molecular catalysts. The student will explore chemical deposition of the synthesised NHC–Ru complexes onto Colombian clays, to achieve heterogeneous-supported Ru-based catalysts. These composites with expected enhanced catalytic activity will be systematically characterised and then tested for heterogeneously catalysed dehydrogenation reactions of glycerol.   

Objectives:  

1. Synthesis and characterisation of NHC–Ru(II) complexes.  
2. Deposition of NHC-Ru compounds onto Colombian clays.  
3. Catalytic dehydrogenation of glycerol with Ru complexes and Ru-clay composites.  

Synthesis of NHC ligands  

Adapting literature procedures, the student will prepare NHC ligand systems with sterically hindered moieties and appropriate substituents to achieve lipophilicity (aromatic groups) or hydrophilicity (sulfonate groups), respectively. Importantly, these ligand frameworks will contain a hydroxyl moiety, to facilitate chemical deposition onto the clay support. The new NHC ligands will be characterized by conventional analytical techniques (NMR, EA, MS, etc.).     

Synthesis of NHC–Ru(II) complexes  

The NHC-Ru complexes will be synthesised following the in situ free-carbene approach: The NHC precursors (imidazolium salts) are reacted with a base and converted to the free NHC ligands, which are subsequently reacted with a Ru precursor. The new complexes will be purified and characterised by conventional analytical techniques (NMR, EA, MS, XRD).  

Deposition of NHC–Ru(II) complexes onto Colombian clays   

To achieve the chemical deposition of NHC-Ru complexes on a substrate, the clay will be activated following an established delamination process. This superficial exfoliation of the mineral results in an increased surface area. Subsequently, condensation reactions (between Lewis Acid sites on the clay and hydroxyl groups in the Ru complex backbone) allow the immobilisation of the catalysts on the clay surface. These Ru-clay composites will be characterized by SEM, EDX, N2 adsorption-desorption isotherms analysis, ICP, powder XRD, TGA. Furthermore, the project will benefit enormously from KU’s new solid-state NMR facilities to characterize the catalyst material in the solid state.  

Catalytic tests: Dehydrogenation of glycerol (NUC)  

Both, Ru complexes and Ru-clay materials will finally be tested for the catalytic dehydrogenation of glycerol. For this part of the project, we will collaborate intensely with Dr Baquero and his team, as they have profound experience in the catalytic transformation/valorisation of natural products. All reactions will be monitored by GC-MS and NMR spectroscopy to detect the formation of dihydroxyacetone and molecular hydrogen. Recyclability of the materials will be assessed to study catalyst stability and activity.