Methyl esters and amides are valuable compounds in fine chemicals synthesis, with methyl esters used in flavourings and perfumes, while amides are used in many industries from polymers to pharmaceuticals. Alcohols are desirable starting materials as they can be sustainably sourced, however both methyl ester synthesis from alcohols and methanol, and amide synthesis from alcohols and amines conventionally involve wasteful processes using stoichiometric quantities of metal oxidants and other toxic reagents. In this project a greener synthesis route was developed and studied for each reaction, employing plug flow reactors with a heterogeneous catalyst in the form of Au/TiO2 and green oxidants and solvents.
A continuous flow methyl ester synthesis route from benzyl alcohol and methanol with hydrogen peroxide over Au/TiO2 was studied, Scheme 1. Using experimental observations, a reaction model was constructed in Berkeley Madonna (a differential equation solver), to draw conclusions about the reaction pathway, including the role of H2O2 and factors determining product selectivity. This model was then fitted to experimental data to determine reaction parameters such as rate constants and activation energies.
Initial attempts at amide synthesis over Au/TiO2 in a single plug flow reactor were unsuccessful, therefore a system with two reactors in series was built and characterised. In the first reactor of the tandem system, the alcohol to aldehyde transformation took place over Au/TiO2 with oxygen gas, while the second reactor was for amide synthesis from the aldehyde and amine using H2O2, Scheme 2. After initial screenings of process conditions, solvents and catalysts to maximise product yields for each step, both stages of the reaction were studied separately for the reaction of benzyl alcohol and morpholine. As with the methyl ester synthesis above, a reaction model was developed in Berkeley Madonna and fitted to experimental data. Finally amide synthesis from decanol and morpholine was carried out and compared to the benzyl alcohol process.