Efficient conversion of biomass-derived organic compounds requires acid-catalyzed elimination of hydroxyl groups from alcohols. When this reaction happens within the pores of microporous materials, confinement effects and the distribution of solvents in the pores greatly influence the intrinsic activity of the hydronium ion and reaction mechanisms. In this study, we performed the dehydration of cyclohexanol on zeolites with varying pore diameters (0.55 to 0.74 nm) and in the presence of decalin, 1,4-dioxane, or water in order to understand how to tune the reaction pathways for dehydration. In water, the rates per hydronium ion increase with decreasing the pore diameter of the zeolite. The fast rates are attributed to a lower activation enthalpy in the tighter confines. In contrast, in decalin and 1,4-dioxane the intrinsic rates increase with increasing pore diameter. We propose that this opposing trend is due to the higher steric demands of the intermediate dimeric structures, formed prior to water elimination, in the organic solvents than the water-cyclohexanol dimer.<br/>In terms of solvent effect, the negative reaction order in cyclohexanol, when the reaction is performed in decalin, is attributed to the low reactivity of the cyclohexanol-cyclohexanol dimer. In contrast, in dioxane the positive reaction orders in cyclohexanol (that level off at low concentrations) indicate that the solvent disrupts the formation of these species producing heterodimers instead, which dehydrate at faster rates. Hence, we demonstrate that the routes for alcohol dehydration can be manipulated by tuning the combined effects of the kind of solvent and the pore size of zeolite.

Towards understanding the convoluted effects of confinement and solvent for alcohol dehydration on zeolites

256th National Meeting (2018)

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