Density functional theory study of the selective oxidation of polynuclear aromatic hydrocarbons

• Main Author: Hickey, Niamh
• Published: Cardiff University 2014
• Subjects: 547; QD Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.620141

Density functional theory has been used to model the selective catalytic oxidation of model polynuclear aromatic hydrocarbons, in particular pyrene. Two transition metal catalysts have been employed, ruthenium tetroxide and a peroxy tungsten complex. The B3LYP method was used in all calculations. The LANL2DZ effective core potential was used for ruthenium and tungsten atoms. The 6-31G(d,p) basis set was used for all other atoms. Selectivity studies show that catalyst-substrate adducts will form at the carbon-carbon bond of the polynuclear aromatic hydrocarbon that will give the least reduction in aromaticity. The strain induced in planar polynuclear aromatic hydrocarbon structures did not have a bearing on catalyst selectivity. Interior carbon-carbon bonds of polynuclear aromatic hydrocarbons will not be attacked, meaning that oxidative fragmentation of these compounds starts at the exposed carbon-carbon bonds. For the ruthenium catalysed oxidation system, multiple catalyst attacks at similar sites on polynuclear aromatic hydrocarbons is possible but unlikely due to the catalytic amounts used. It is indicated that the presence of aliphatic functional groups at the adduct formation sites for the ruthenium catalyst will not hinder adduct formation. Water is necessary for the success of both catalysts. The pathway found for ruthenium catalysed oxidation shows a stable dione intermediate, this intermediate has been confirmed by experimental findings and the literature. For the ruthenium catalyst water is required as it is involved in the reaction mechanism, facilitating the abstraction of hydrogen atoms adjacent to the catalyst adduct formation site. Water is not as directly involved in the tungsten catalyst mechanism, taking the role of a stabilising agent for transition state structures. The reaction pathways found for both catalysts show that the final reaction intermediate before breaking the carbon-carbon bond is an organic anhydride. Each reaction type shows a different route to this structure. Both reaction mechanisms show an energetically favourable trend from reactants to products. Transition state energy barriers are not prohibitive.