In this work, a new concept for the on-purpose synthesis of butadiene was studied. Starting from acetaldehyde, derived from (bio-)ethanol as C₂ building block, a reaction cascade involving 3-hydroxy-2-butanone (acetoin) and 2,3-butanediol as intermediates was explored. The proposed route includes three main steps: N-heterocyclic carbene (NHC) catalysed self-addition of acetaldehyde to form acetoin, hydrogenation of acetoin to 2,3-butanediol, and subsequent dehydration to produce 1,3-butadiene. For the NHC-catalysed acetaldehyde upgrading, thiazolium-based NHC precursors, combined with mild bases demonstrated excellent catalytic performance in both neat and solvated conditions. A key feature of the reaction was the exceptionally high selectivity towards acetoin, as well as its tolerance to ethanol as a solvent. The latter attribute is of particular significance, as it mitigates the requirements of a potential ethanol dehydrogenation reaction step with respect to acetaldehyde yield. Despite their high catalytic efficiency, homogeneous NHC precursors suffered from poor recyclability. To address this limitation, an immobilised NHC catalyst was developed and analysed. In this regard, solid-state NMR was performed to confirm the successful grafting of the catalytically active species onto a support material. Though highly selective in the acetaldehyde self-addition, the heterogeneous catalyst exhibited activity loss under continuous-flow conditions. Careful evaluation elucidated that this decay could be attributed to leaching of the grafted species. To improve the catalyst stability, an ion exchange with a non-nucleophilic counterion was conducted which significantly enhanced longevity. This modified heterogenised catalyst enabled continuous-flow operation for up to 60 hours while maintaining high acetoin selectivity (> 94%). Hydrogenation of the acetoin-containing product mixture from the continuous-flow acetaldehyde self-addition reaction was performed using a commercial 5 wt% Ru/C catalyst in batch mode, achieving near-quantitative conversion. Subsequent distillation enabled the isolation of 2,3-butanediol with approximately 95% purity, demonstrating the feasibility of separating the product from the reaction mixture, which is crucial for further downstream processing of the prepared 2,3-butanediol. Finally, the gas-phase double dehydration of 2,3-butanediol via 3-butene-2-ol as essential reaction intermediate was investigated. A hydrothermally synthesised, Sc₂O₃ catalyst (HT-800-Sc₂O₃) exhibited high 3-butene-2-ol selectivity (> 90%) and stability over 10 hours of time-on-stream. When using a zeolite (ZSM-5)-catalysed second dehydration step in the same flow-reactor, a near-quantitative 2,3-butanediol conversion and an excellent selectivity (80%) towards 1,3-butadiene were achieved. The developed double dehydration process achieved a productivity nearly four times the industrial production barrier that is discussed in literature. By employing the best performing processes and catalytic systems investigated in this work, a proof-of-concept for the proposed synthesis route was established, achieving an overall process efficiency of 17% for 1,3-butadiene production based on the mass of acetaldehyde fed into the beginning of the process.
Maurice Belleflamme (Wed,) studied this question.