Reusability studies with Lewis and Brønsted acid catalysts for dehydration of the primary alcohol 1-hexanol under energy-saving conditions†

In general, currently there is an urgent need to switch from fossil-based materials to renewable resources and this is also the case for production of alkenes with a chain length of up to C₆ due to their broad application range as bulk chemicals. For instance, such C₆ alkenes (hexenes) are accessible from 1-hexanol, for which recently a sustainable technical approach based on CO₂ and water as renewable raw materials was reported by Siemens and Evonik Industries, making use of artificial photosynthesis in combination with microbial syngas fermentation. In continuation of our very recent comprehensive study on dehydration of 1-hexanol and other alcohols with a focus on Lewis acids and initial reusability studies over a few reaction cycles, the current study presents a simplified strategy for the reuse of the Lewis acid catalysts Cu(OTf)₂ and Hf(OTf)₄ and the Brønsted acid TfOH as well as a proof-of-concept for efficient recycling over 20 cycles. Over the course of the study, the catalysts demonstrated an average alkene yield of 71–77%, with no loss of activity. The production costs were calculated on the basis of industry-appropriate prices, with the lowest being 0.34 $ per kg. A successful initial lab scale-up with a 100-fold increase in reaction volume indicates a TRL4 for the developed process and enabled a product formation of 1.3 kg. Thus, these studies underline the technical feasibility of the developed dehydration process using 1-hexanol, for which catalyst recycling represents a key criterion.