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The Effect of Ionic Liquids Solvents on the Dehydration of Alcohols

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Thesis(3.63 MB)

Date

2020

Authors

Supervisor

Blackman, Allan
Weber, Cameron

Item type

Thesis

Degree name

Master of Philosophy

Journal Title

Journal ISSN

Volume Title

Publisher

Auckland University of Technology

Abstract

Ionic liquids have a unique combination of intermolecular interactions that give rise to unusual structure features including the formation of amphiphilic nanostructures with well-defined polar and non-polar domains. The effect of these nanostructures remains poorly understood. The aim of this thesis was to examine these effects of the structure of ionic liquid solvents using the dehydration of alcohols as a model reaction. Several ionic liquids containing 1-alkyl-3-methylimidazolium cations and different anions were synthesized to explore these effects. Secondary alcohols cyclohexanol and octan-2-ol were found to undergo dehydration reactions in these imidazolium ionic liquids at elevated temperatures in hydrothermal reactors. Alkenes were formed as the major product, with traces of the corresponding ketones and ethers. The ionic liquid anion influenced the reactivity of these alcohol dehydration reactions. Hydrophobic, weakly interacting anions such as [NTf2]− lead to more effective dehydration reactions compared to more strongly interacting, hydrophilic anions such as [OTf]− and [Me2PO4]− which gave trace or no products with alcohols. Increasing the mole fraction of reactants was found to decrease the formation of products, the opposite to the trend that would be expected for an innocent solvent which highlights that the ionic liquid plays an active role in the reactivity. Increasing the alkyl chain length of the cation had anion-dependent effects on reactivity. This increased the reactivity of the alcohol when the [OTf]− anion was used while decreasing reactivity for imidazolium ionic liquids with the [NTf2]− anion. The catalytic activity of ionic liquids reveals the structural and nanostructural effects of ionic liquids in alcohol dehydration reactions. The amphiphilic nanostructure contains a polar domain consisting of the catalytically active ionic network and inactive apolar alkyl chain domain. Hence the [OTf]− anion increases the proportion of alcohol in the polar domain with its effective concentration increased for long imidazolium ionic liquid alkyl chain lengths. The [NTf2]− anion does not encourage the same partitioning into the polar region and the exclusion of the alcohol increases with increased alkyl chain length, accounting for the change in the reactivity trend. These results indicate that the amphiphilic nanostructure of ionic liquids can influence the outcome of alcohol dehydration reactions. Ionic liquids have a unique combination of intermolecular interactions that give rise to unusual structure features including the formation of amphiphilic nanostructures with well-defined polar and non-polar domains. The effect of these nanostructures remains poorly understood. The aim of this thesis was to examine these effects of the structure of ionic liquid solvents using the dehydration of alcohols as a model reaction. Several ionic liquids containing 1-alkyl-3-methylimidazolium cations and different anions were synthesized to explore these effects. Secondary alcohols cyclohexanol and octan-2-ol were found to undergo dehydration reactions in these imidazolium ionic liquids at elevated temperatures in hydrothermal reactors. Alkenes were formed as the major product, with traces of the corresponding ketones and ethers. The ionic liquid anion influenced the reactivity of these alcohol dehydration reactions. Hydrophobic, weakly interacting anions such as [NTf2]− lead to more effective dehydration reactions compared to more strongly interacting, hydrophilic anions such as [OTf]− and [Me2PO4]− which gave trace or no products with alcohols. Increasing the mole fraction of reactants was found to decrease the formation of products, the opposite to the trend that would be expected for an innocent solvent which highlights that the ionic liquid plays an active role in the reactivity. Increasing the alkyl chain length of the cation had anion-dependent effects on reactivity. This increased the reactivity of the alcohol when the [OTf]− anion was used while decreasing reactivity for imidazolium ionic liquids with the [NTf2]− anion. The catalytic activity of ionic liquids reveals the structural and nanostructural effects of ionic liquids in alcohol dehydration reactions. The amphiphilic nanostructure contains a polar domain consisting of the catalytically active ionic network and inactive apolar alkyl chain domain. Hence the [OTf]− anion increases the proportion of alcohol in the polar domain with its effective concentration increased for long imidazolium ionic liquid alkyl chain lengths. The [NTf2]− anion does not encourage the same partitioning into the polar region and the exclusion of the alcohol increases with increased alkyl chain length, accounting for the change in the reactivity trend. These results indicate that the amphiphilic nanostructure of ionic liquids can influence the outcome of alcohol dehydration reactions.

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Keywords

Ionic Liquids, On, Alcohol, Dehydration

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https://hdl.handle.net/10292/13543

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