Continuous-flow liquid-phase dehydrogenation of 1,4-cyclohexanedione in a structured multichannel reactor

A highly selective, scalable and continuous-flow process is developed for the liquid-phase dehydrogenation of 1,4-cyclohexanedione to hydroquinone in a millimetre-scale structured multichannel reactor. The square-shaped channels (3 mm × 3 mm) were filled with 10 wt% Pd/C catalyst particles and utilized for the dehydrogenation reaction in single-pass and recycle modes. For the purpose of enhancing process understanding and maximizing conversion and selectivity by process optimization, the design of experiment (DoE) methodology was utilized by studying the effect of operating parameters on the catalytic performance in the kinetic regime. The results demonstrated the strong influence of temperature and liquid feed flow on the conversion and selectivity, with liquid feed and N2 flows influencing pressure drop significantly. A multi-objective optimization methodology was used to identify the optimum process window with the aid of sweet spot plots, with design space plots developed to establish acceptable boundaries for process parameters. In single-pass mode, complete conversion per pass per channel was not achievable, whereas conversion increased from 59.8% in one channel to 78.3% for two channels in series while maintaining selectivity (>99%) with intermediate hydrogen removal. However, without the intermediate H2 removal step, selectivity decreased from >99% in one channel to 82.3% at the outlet of the second channel. In recycle mode, the dehydrogenation reaction resulted in almost complete conversion (>99%) with very high selectivity (>99%) and yield (>98%). This combination of mm-scale multichannel reactor and DoE methodology opens the way to developing highly selective and scalable dehydrogenation processes in the fine chemical and pharmaceutical industries.