The dehydration of alcohols by heterogeneous catalysis is the focus of renewed interest in the field of lignocellulosic biomass conversion into platform substances for the chemical industry. One of the difficulties encountered when it comes to optimizing processes is improving the efficiency of this catalyzed conversion, which requires a better understanding of the phenomena involved on a molecular scale. In particular, it is necessary to determine the nature of the active sites and reaction intermediates, while at the same time establishing models that provide the most accurate possible reflection of the experimental conditions used in the lab
As part of the drive to improve knowledge in this field, researchers at UPMC (Laboratoire de Réactivité de Surface) and IFP Energies nouvelles recently elucidated the reaction mechanisms related to the dehydration of isopropanol into propene and diisopropylether on catalysts based on gamma alumina (γ-Al2O3). To do so, they used a multiscale method, incorporating ab initio calculation*, kinetic modeling based on these ab initio calculations and experimental reaction kinetics measurements. The results gave rise to two recent publications in ACS Catalysis [1,2].
The combination of ab initio calculations and experimental results demonstrated that, in the conditions used (reagents and products in gas phase), the (100) surface of the gamma alumina was the only active one. In addition, it also explained why: this surface presents Lewis acid sites (aluminum ions), which, combined with oxygen atoms with basic properties, prove to have sufficient strength to activate the alcohol molecule - but not excessively - so that the water produced by the reaction does not poison the active sites (Figure 1-a). This contrasts with the (110) facets, which present sites that very strongly coordinate water and are therefore very quickly poisoned. In addition, potential mechanisms suggested involving the Brønsted acidity of the surface hydroxyls were shown to be unlikely on the basis of the ab initio calculations.
Figure 1. Multiscale study of the isopropanol dehydration mechanism on gamma alumina:
(a) ab initio study, enabling identification of active sites (ellipsis: structure of transition states) for the formation of propene and ether (gray: Al, red: O, blue: C, yellow: H);
(b) microkinetic modeling based on ab initio modeling, presenting the macrosite considered;
(c) comparison of the experimental results (symbols) and modeling results (unbroken lines).
Click on the picture to enlarge it
Ultimately, a microkinetic model was constructed and fed by the rate constants calculated on the basis of ab initio electronic analysis, for all the reactions considered. Based on the ab initio mechanistic studies, a macro-site was considered (Figure 1-b), including the alumina site required for the formation or propene or ether, and two connected sites required for the formation of propene, on the one hand, and ether, on the other. The model obtained provides a remarkable reproduction of the experimental activities and selectivities over a broad range of operating conditions (Figure 1-c). This approach reveals the major importance of the effects of water coverage, with water playing multiple roles:
This mechanistic data obtained using a multiscale method opens up a number of possibilities in terms of directing the activity and selectivity of the reaction studied.
* Calculations based on quantum chemistry.
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 Mechanistic investigation of isopropanol conversion on alumina catalysts: location of active sites for alkene / ether production
K. Larmier, C. Chizallet, N. Cadran, S. Maury, J. Abboud, A-F. Lamic-Humblot, E. Marceau, H. Lauron-Pernot
ACS Catalysis, 5, 4423−4437, 2015.
 Influence of co-adsorbed water and alcohol molecules on isopropanol dehydration on γ-alumina: Multi-scale modeling of experimental kinetic profiles
K. Larmier, A. Nicolle, C. Chizallet, N. Cadran, S. Maury, A-F. Lamic-Humblot, E. Marceau, H. Lauron-Pernot
ACS Catalysis, 6, 1905−1920, 2016.