(701e) Gold-Catalysed Ester Synthesis from Alcohols in a Continuous Flow System

Alcohols are a desirable starting point for many chemical synthesis routes as they tend to be stable and can be derived from natural products, therefore sustainably sourced. One such synthesis route is the formation of esters, which have many uses from solvents to food flavourings.

 The formation of esters from alcohols and methanol is typically achieved in two steps: First by oxidation of the alcohol to a carboxylic acid, before esterification.¹ This process often requires the use of coupling agents and organic solvents, producing large quantities of by-products with further consumption of resources in the resultant work-up process. For these reasons has been a great deal of interest in the development of an alternative methodology, using oxygen²or hydrogen peroxide with a catalyst, giving water as the only by-product.

 One promising group of catalysts for these oxidative esterification reactions are supported gold nanoparticles.^1-3A major advantage of these catalysts are that they are heterogeneous, giving better catalyst-product separation, and so greater catalyst recovery and easier product purification. They are also suitable for incorporation into a continuous flow system as a packed bed.

A method was developed for gold-catalysed esterification/oxidation of alcohols in a flow reactor, to access the benefits delivered by a larger reaction space. The catalyst chosen was Au/TiO₂, with H₂O₂as the oxidant and a methanol/water solvent. Studies of the reaction kinetics of benzyl alcohol oxidation to methyl benzoate were carried out, with measurements of activation energies and an investigation into the origins of the selectivity between the methyl ester and benzoic acid, a side product.

References

1.      F. Z. Su, J. Ni, H. Sun, Y. Cao, H. Y. He, K. N. Fan, Chem. Eur. J. 14(2008) 7131.
2.      I. S. Nielsen, E. Taarning, K. Egeblad, R. Madsen, C. H. Christensen, Catal. Lett. 116(2007) 35
3.      C. Marsden, E. Taarning, D. Hansen, L. Johansen, S. K. Klitgaard, K. Egeblad, C. H. Christensen, Green Chem. 10 (2008) 168