Sustainable biomass conversion

: conversion of carbohydrates to platform molecules in DMSO and GVL

Abstract

Replacement of depleting non-renewable fossil resources with sustainable and greener substitutes is one of the most important challenges of contemporary research and development in chemistry and chemical engineering. γ-Valerolactone (GVL) has been previously suggested as a sustainable liquid for the production of transportation fuels and carbon-based chemicals. GVL is renewable as it can be produced by four consecutive reactions from carbohydrates: the acid catalyzed dehydration of fructose, glucose, sucrose, starch, or cellulose to 5-hydroxymethylfurfural (HMF), the acid catalyzed hydration of HMF to levulinic acid (LA) and formic acid (FA), the catalytic hydrogenation of LA to 4-hydroxyvaleric acid (4-HVA), followed by the ring closure of 4-HVA by dehydration to GVL. Catalytic transfer hydrogenation of LA with FA to 4-HVA is a particularly attractive approach as the overall process of converting carbohydrates to GVL becomes independent of hydrogen gas and readily useable even in remote parts of the world. Mechanistic studies on the conversion of fructose, glucose, and sucrose to HMF and/or to LA and FA have been performed by the use of 13C- and deuterium-labeled carbohydrates in both DMSO and GVL. Several intermediates and different reaction paths were identified and confirmed in the acid catalyzed conversion reactions of fructose to HMF in DMSO. The irreversibility of reaction routes from the fructofuranosyl oxocarbenium ion to HMF and the similar pyranose path was determined by structural information combined with isotopic-labeling experiments. GVL was successfully used as a green solvent for all steps, demonstrating many advantages including full compatibility, easy separation, and enhanced productivity. Reaction conditions including initial fructose concentration, acid concentration, reaction temperature, GVL amount, and different starting carbohydrates were optimized in GVL. The best yield of LA from carbohydrates was 70%, achieved by using 2 mmol fructose, 1.5 mL 5 mol/L sulfuric acid, and 10 mL GVL at 130 °C for 2 hours. After optimization the yield of LA starting from HMF was nearly 90% at 5 mol/L H2SO4 in GVL. A similar HMF yield (80%) was reached at the same H2SO4 concentration in DMSO, but further conversion to LA was much slower with poorer yields. Several solid acid catalysts (MCM-41 anchored sulfonic acid, cesium salts of heteropoly acids, supported WO3 and MoO3, etc.) have been synthesized and tested for the HMF production from fructose in DMSO. The use of MCM-41 anchored sulfonic acid and the salt Cs3HPW12O40 resulted in the full conversion of fructose and about 80% yield of HMF. These catalysts were better than the commercially available Brønsted acidic solid catalysts such as BETA zeolite and Nafion resins. The one pot conversion of fructose to GVL in GVL as the solvent has been carried out using 13C6-fructose as the starting material and H2SO4 (5 mol/L) as the catalyst at 130 °C. The formation of 13C6-HMF by the dehydration of 13C6-fructose was followed by its hydration to 13C5-LA and 13C-FA. The Shvo-catalyst was added to the reaction mixture when the conversion to LA and FA was highest, to achieve the subsequent conversion to 13C5-GVL at 100 °C in two hours. 13C NMR and GC-MS were used to confirm the formation of 13C5-GVL in GVL.

Date of Award	15 Feb 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Istvan Tamas HORVATH (Supervisor)