Rational catalyst design for acetaldehyde upgrading – an in-depth study on the use of a solid base and the development of a second generation supported N-heterocyclic carbene catalyst

Abstract

Herein, we report the development of a second-generation supported N-heterocyclic carbene (NHC) catalyst to significantly improve the upgrading of acetaldehyde to 3-hydroxy-2-butanone (acetoin) and suppress catalyst deactivation via leaching, aided by the use of sodium carbonate (Na₂CO₃) as a solid base. We were able to demonstrate that the activity of the catalyst benefits from the use of finer sieve fractions of solid Na₂CO₃. In addition, we found a beneficial effect of intermittent addition of Na₂CO₃ on the productivity of an earlier reported supported NHC catalyst over a total duration of 10 h time-on-stream (TOS). Despite the addition of the solid base, excellent selectivity towards acetoin, S(acetoin) > 90%, was found throughout batch and continuous flow experiments. Unfortunately, deactivation of the catalyst could not be suppressed by using excess base. Therefore, a second-generation supported NHC catalyst was developed, bearing non-nucleophilic counterions to suppress the reverse Menshutkin decomposition pathway. The developed catalyst was found to be active for more than 60 h TOS, attaining very high selectivities towards acetoin, S(acetoin) > 88%, and single-pass conversion of acetaldehyde, losing only around 20% of catalytically active species through decomposition.