Ubiquitous aluminum alkyls and alkoxides as effective catalysts for glucose to HMF conversion in ionic liquids

Metal halides (chlorides in particular) are employed almost exclusively as Lewis acid catalysts for the homogeneous conversion of glucose (or cellulose) to HMF (5-hydroxymethylfurfural) in ionic liquids (ILs), with CrCl2 being arguably the most effective benchmark catalyst. Reported herein is a discovery that ubiquitous aluminum alkyl or alkoxy compounds are very effective Lewis acid catalysts for the glucose-to-HMF conversion in ILs. Under the current reaction conditions (1-ethyl-3-methylimidazolium chloride [EMIM]Cl, 120 °C, 6 h), simple trialkyl and trialkoxy aluminum species such as AlEt3 and Al(OiPr)3, which are much cheaper than CrCl2 (by a factor of 5 for AlEt3 or 180 for Al(OiPr)3), are at least as effective as CrCl2 to catalyze this conversion process. The molecular structure of [EMIM]+[ClAlMe(BHT)2]−, formed upon mixing the alkylaryloxy aluminum MeAl(BHT)2 and the IL [EMIM]Cl, has been determined by X-ray diffraction; the structure simulates that of the metallate [EMIM]+[CrCl3]−, the proposed active species responsible for the effective glucose to HMF conversion by CrCl2 in [EMIM]Cl. Another significant finding is that a gradual substitution of the chloride ligand on aluminum by the alkyl ligand brings about a drastic enhancement on the HMF yield, from 1.6% by AlCl3 to 7.6% by MeAlCl2 to 17% by Et2AlCl and to 51% by AlEt3, thus showing approximately an overall 32-fold HMF yield enhancement going from AlCl3 to AlEt3.

Graphical abstractSimple, inexpensive aluminum alkyl or alkoxy compounds are effective catalysts for the glucose-to-HMF conversion in [EMIM]Cl, achieving ∼32-fold enhancement in HMF yield over the corresponding halide catalyst.Figure optionsDownload full-size imageDownload high-quality image (218 K)Download as PowerPoint slideHighlights► Glucose conversion into 5-hydroxymethylfurfural (HMF) in ionic liquids. ► Inexpensive aluminum alkyls and alkoxides are as effective as the benchmark catalyst for the conversion. ► Simple aluminum alkyl and alkoxide catalysts are far more effective than aluminum chloride catalysts. ► Catalyst structure in an ionic liquid was characterized by X-ray diffraction.