Solvent free alkene oxidation using supported nano-gold catalysts

• Main Author: Bawaked, Salem Mohammed
• Published: Cardiff University 2011
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.585214

Oxidation is an important route for the activation of chemical feedstock for the synthesis of chemical intermediates. Alkene epoxidation by the electrophilic addition of oxygen to a carbon-carbon double bond is a major challenge in oxidation catalysis. In particular it is important to use molecular oxygen as the oxidant to avoid the formation of reagent by products. Although molecular oxygen is the most environmentally benign oxidant in many cases, far more reactive forms of oxygen are required to achieve reaction, and this can lead to by-products with a heavy environmental burden with respect to their disposal. Free solvent selective epoxidation of cw-cyclooctene with air and small catalytic amount of radical initiator using supported nano-gold catalysts has been conducted in the liquid phase. Optimization of reaction conditions has been attempted to improve the selectivity by minimizing the background reactions of the support and radical initiator in the absence of the catalyst. Five different radical initiators were tested namely: di-/-butyl peroxide (DTBP), /-butyl hydroperoxide (TBHP), cumene hydroperoxide (CHP), azobisisobutyronitrile (AIBN) and dibenzoyl peroxide (DBP). TBHP was selected for more detailed study as it combine the best activity and selectivity in the presence of the catalyst and minimum background reactions in the absence of the catalyst. The influence of the metal nano-particle support was also studied using six different supports. Use of graphite as a support was found to give the best combination of selectivity and conversion. In general the selectivity to the epoxide increased with reaction temperature from 60-80 C and was highest at 80 C. Other carbon supports, e.g. activated carbon and silicon carbide were found to be less effective. Au supported on TiO2, SiO2 and AI2O3 catalysts were also selective for the epoxidation reaction and the general order of activity was: graphite > SiC > TiO2 > SiO2 AI2O3 Ac. Extensive studies concerning the reusability of the gold/graphite catalyst was conducted as catalyst reusability is a key feature of green chemistry. The catalyst is found to be inhibited by the epoxide product but we demonstrate the effect of this is negligible for reused catalysts over a long reaction time. The observation of an induction period may in part be due to the adsorption of the radical initiator blocking surface sites as well as the establishment of the reactive species. The use of bimetallic catalyst, to be exact Au-Pd supported on graphite, shows significant effect on c/s-cyclooctene reactivity. The Au-Pd ratio has a major effect on the conversion with very low activities being associated with Au:Pd weight ratios of ca. 4:1 and 1:4 which may be associated with structured alloys that can be formed at these compositions. The molar Pd surface percentage were found to have dramatic effect on the conversion as low Pd surface would produce low activity and contrariwise. The selectivity to the epoxide is not affected by the Au:Pd ratio. Catalyst preparation method was also studied and sol immobilization method was found to be the most effective for gold catalyst, however, DP method was the preferred one in the bimetallic catalysts. The Au-Pd/graphite catalyst series has been further evaluated using crotyl alcohol (trans- but-2-en-l-ol) as starting material and using the same reaction conditions that had adopted for the epoxidation of c/s-cyclooctene. With crotyl alcohol, /-butyl hydroperoxide was not required for activity. In the absence of Pd, crotonaldehyde was formed, but the introduction of Pd leads to an isomerisation pathway to 3-buten-l-ol being favored over epoxidation and crotonaldehyde was a minor product. Dimerisation of crotonaldehyde and carbon carbon bond cleavage was observed with Au catalyst. However, with Pd catalysts Wacker-like oxidation of the isomerisation pathway was the major by-product.