Role of N-heterocyclic carbenes in glucose conversion into HMF by Cr catalysts in ionic liquids

Authors: Eric F. Dunn, Dajiang (D.J.) Liu, Eugene Y.-X. Chen

Abstract:
As N-heterocyclic carbenes (NHCs) can be readily formed from deprotonation of their precursors— imidazolium salts (i.e., ionic liquids, ILs) that are typically used as solvent in homogenous glucose-to5-hydroxymethylfurfural (HMF) conversion systems with CrCl_x (x = 2 or 3) as catalyst—the hypothesis that the (NHC)–CrCl_x complex is the true catalyst for such conversion exists in the literature, but it has not been tested. Reported herein are three sets of mechanistic probes to address the role of the NHC ligand in the glucose-to-HMF conversion by CrCl_x in ILs. Specifically, controls with all additives (reagents) involved in the conversion system, performances of in situ generated NHC ligands or Cr complexes as well as discrete NHC ligands and Cr complexes that were proposed to be the true catalyst, and quantitative NHC titration (poisoning) experiments conclusively show that the NHC ligand actually serves as a poison to the chromium catalyst system and that a superstoichiometric amount (2 or 3 equiv.) of the NHC ligand can completely shut down the catalysis.

Keywords:
Glucose
Ionic liquids
HMF
N-heterocyclic carbene
Chromium catalyst

Published in: Applied catalysis A: General  (Volumes 460–461, Pages 1-132, 10 June 2013)

Publisher: Elsevier

ISSN Information: 0926-860X

Role of N-heterocyclic carbenes in glucose conversion into HMF by Cr catalysts in ionic liquids

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