Generalized Dehydration Trends: Connecting Brønsted- with Lewis- acid catalysis.
Pavlo Kostestkyy1, Nicholas Zervoudis1, Jyoti Prakash Maheswari2, and Giannis Mpourmpakis1
1Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA
2Department of Chemical Engineering, National Institute of Technology, Warangal - 506004, T.S, India
Acid-catalyzed dehydration reactions play a key role in the conversion of (oxygenate) biomass derivatives to chemicals, such as alcohols to olefins. Both Lewis and Brønsted acid-catalyzed dehydration reactions of biomass-derived alcohols involve transition states with carbenium ion (CI) characteristics 1-5. The formation of CI intermediates occurs by means of either protonation of alcohols by Brønsted acids 3 or C-O bond elongation and subsequent Hβ transfer via a concerted E2 elimination mechanism on Lewis acids 2, 4-5. In this work, we use highly accurate quantum chemical calculations to investigate the alcohol dehydration mechanisms on a variety of Brønsted and Lewis acid catalysts, including metal oxides and homogeneous acids. We identified the proton affinity (PA) and the carbenium ion stability (CIS) of the alcohols to be excellent dehydration descriptors for various alcohols of different size and substitution. We unraveled general dehydration relationships and a strong correlation between the CIS and PA properties of the alcohols. As a result, we suggest that both PA and CIS can be used as descriptors to develop dehydration models 2that rationalize activity and selectivity trends in dehydration chemistries.
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