New Catalytic Systems with Ionic Liquids as Reaction Media for the Synthesis of Renewable Chemicals

Abstract

Producing chemicals from renewable sources is a field that has gained increased attention in recent years. In this connection, ionic liquids (ILs) have proven to be interesting reaction media for the conversion of sugars to platform chemicals. This thesis describes the synthesis of 5-(hydroxymethyl)furfural (HMF) and 2,5 furandiacrboxyl acid (FDA) from carbohydrates in ionic liquids. In chapter 2 an IL/water system for the enzymatic isomerization of glucose to fructose was demonstrated. The IL N,N -dibutylethanolammonium octanoate together with 18 wt% water proved to be a benign reaction medium for the enzyme glucose isomerase. In addition to fructose, mannose was formed via the Lobry de Bruyn-van Ekenstein transformation. Chapter 3 deals with the dehydration of glucose to HMF where new potential catalysts are investigated. Chromium(III) chloride exhibited best performance in imidazolium based ILs with halides as anions. Lanthanide salts also proved to have a catalytic effect for glucose dehydration in alkylimidazolium halides, where ytterbium triflate had the strongest effect. Increasing the alkyl chain length of the imidazolium cation of the IL increased the yield of HMF. In addition to the metal based catalytic systems, boric acid was found to enable synthesis of HMF in 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl), affording HMF in 42 and 66 % from glucose and sucrose respectively. Experimental evidence led to the conclusion that boric acid promoted the isomerization of glucose to fructose, and [EMIm]Cl subsequently mediated the dehydration of fructose to HMF. Calculations with Density Functional theory (DFT) confirmed a decrease in energy for the isomerization pathway when glucose was complexed with boric acid. Further derivatization of HMF to FDA was investigated in Chapter 4 by the aerobic oxidation in ILs with supported ruthenium hydroxide catalysts. The best yield obtained for FDA was 48 %, using Ru(OH)x/La2O3 as catalyst in 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) at elevated pressure. Leaching studies showed that the catalyst was not entirely stable under the reaction conditions. In Chapter 5 an assessment of a process in ILs was made where chemistry, as well as technological and economical parameters, was taken into account. The assessment concluded that an IL-based process for HMF production is highly dependent on the number of recycles of the IL and the price of the feedstock.