Aldehydes and ketones and chemical compounds containing these functionalities, are of great interest as valuable intermediates in different industries including fine chemicals and pharmaceuticals’ synthesis. Oxidation of primary and secondary alcohols using oxidants such as oxygen or hydrogen peroxide, in the absence of organic solvents, is a green and promising approach toward production of such compounds. Different types of catalysts have been used for homogeneous oxidation of alcohols among which inorganic water-soluble tungsten-containing compounds (oxyanions) have shown enhanced performance. In such systems typically a phase transfer catalyst along with the catalyst and oxidant is used to facilitate the transport of the catalyst into organic phase. However, these reactions are usually carried out in batch for a long reaction time (hours) to reach complete conversion.
In this study, commercially available sodium tungstate and phosphotungstic acid were used as water-soluble homogeneous catalysts for oxidation of alcohols in continuous systems. A spiral microreactor was utilized to optimize the reaction conditions. Different phase transfer catalysts were evaluated and the best one was shown to be Tetrabutyl ammonium hydrogen sulfate (a quaternary ammonium salt). It was found that complete conversion of alcohols (benzyl alcohol, 1-phenyl ethanol, …) with high selectivity towards partially oxidized intermediate (aldehyde) can be reached in short residence times (within 10 minutes), by taking advantage of merits of continuous microreactor. Conversion and selectivity were further enhanced using homemade zinc-substituted polyoxotungstate. Quantitative conversion along with very high selectivity to aldehyde (>90%) was achieved at much higher ratios of substrate to catalyst. In addition, continuous separation of two phases and catalyst removal were achieved with the aid of membrane separators. After two separation stages, less than 3% of the catalyst stayed in the product stream.
In order to show the scalability of the process, a Low Flow Reactor (LFR) manufactured by Corning, was used. The specific design of the channels in this reactor provides efficient mixing of the organic and aqueous phases and hence maintains the high conversion while increasing the production rate.