Study of butanol conversion to butenes over H-ZSM-5: Effect of chemical structure on activity, selectivity and reaction pathways

- Iso-butanol dehydration is a viable synthesis method for bio-based iso-butene.
- Water presence doesńt affect activity or selectivity of C4-OH dehydration reaction.
- An ab-initio based study indicates the importance of the dominant reaction pathway.
- Destabilization of the ether intermediate is key in the selectivity shift.
- Activity increase is caused by an increased surface coverage of a butanol dimer.

To evaluate the viability of the use of butanol as a green chemical key molecule, the effects of temperature and site time on the transformation of the three butanol isomers, 1-butanol, 2-butanol and iso-butanol, towards butenes over H-ZSM-5 have been studied in search of the most promising isomer. Under dehydration conditions, 2-butanol is by far the most active and intermediate ether formation is only observed for 1-butanol. On the other hand, only isobutanol allows the direct formation of isobutene, the most valuable of the butene isomers. Hence, it is especially interesting to further stimulate the biomass derived isobutanol production, e.g. via genetic modification of appropriate microorganisms, in order to allow thereafter the formation of green drop-in isobutene upon dehydration with H-ZSM-5 in existing refineries. Due to the large potential of isobutanol, a reaction path analysis via ab-initio based microkinetic simulations is conducted for this molecule. Comparing these results with simulations on 1-butanol indicates that the shift is occurring due to a shift of the dominant reaction pathway towards the direct dehydration via anti elimination. A Gibbs free energy analysis shows that the large distortion of the transition state for isobutanol etherification renders this path far less favorable, resulting in the lack of formation of the di-alkyl ether, whilst the increased degree of substitution of the alkyl chain in isobutanol is found to promote the direct dehydration path, leading to an increase of the overall activity of isobutanol as compared to 1-butanol.

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