(Funding from DOE Grant # DE-EE0003129)

HYDROGENATION OF AROMATICS AND HETEROAROMATICS BY SUPPORTED METAL NANOPARTICLES

Fossil fuels—coal, oil and natural gas—currently provide over 80% of the energy consumed in the US and will continue to be the predominant energy source worldwide by 2035, in spite of the aggressive development of alternative renewable energy sources. (U.S. Energy Information Administration, Washington, DC, 2010, 2011. International Energy Agency, Paris, France, 2010).

Environmental regulations impose increasingly severe restrictions on the aromatics, sulfur, and nitrogen content allowed in fuels and the low levels required are difficult to achieve with current refining technologies. Catalytic hydrogenation plays a pivotal role in lowering the proportion of aromatic hydrocarbons (HYD), and in removal of nitrogen and sulfur through hydrodenitrogenation (HDN) and hydrodesulfurization (HDS). Conventional metal sulfide hydrotreating catalysts (Co-Mo, Ni-Mo and Ni-W) saturate only a moderate proportion of aromatics due to thermodynamic equilibrium limitations under operating conditions. Supported noble metal catalysts can function at lower temperatures far from equilibrium conditions, but they are easily poisoned by N- and S-containing compounds present in refinery feeds. Therefore, there is a continuing need for catalysts capable of promoting hydrogenation of aromatics, while being resistant to poisoning by N- and S-species.

Our working hypothesis is that a surface nanostructure composed of small metallic particles immobilized on a support that is rich on basic sites can promote rare heterolytic hydrogen splitting and ionic hydrogenation mechanisms through a metal-support bifunctional effect, thereby avoiding strong substrate adsorption to active sites that leads to catalyst poisoning:

• To develop novel nanostructured materials specifically designed to promote the heterolytic splitting of molecular hydrogen and ionic hydrogenation mechanisms, by use of metallic nanoparticles immobilized on solid supports containing strongly basic functionalities

• To optimize their catalytic properties in the hydrogenation of aromatics and heteroaromatics representative of components of petroleum-derived fuels, and the dehydrogenation of N-heterocycles, of possible utility as liquid hydrogen storage materials, and to understand the principal reaction mechanisms

Pyrolysis of lignocellulosic biomass (wood, agricultural waste, grass) produces bio-oil, which displays poor combustion properties mainly due to its high oxygen content. Catalytic hydrotreating is needed to remove oxygen and produce liquid hydrocarbons. Supported metal sulfide catalysts used in petroleum hydrotreating or supported noble metals can be applied to bio-oil but they are not efficient enough for industrial applications. Therefore, new effective HDO catalysts are needed.

A key step in HDO is the cleavage of C-O bonds but due to the intrinsic complexity of the solid catalysts and surface reaction networks involved, crucial mechanistic issues remain unsolved. Our aim is to understand the factors that influence organometallic reactions leading to selective C-O bond scission of bio-fuel components, and the mechanistic pathways by which such processes take place. We study the interactions of transition-metal complexes of tripodal P- and mixed P,N-donor ligands with model substrates by experimental and theoretical methods.