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 (Its), 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-ethy1-3-methylimidazolium chloride [EMIM]Cl, 120 degrees C, 6 h), simple trialkyl and trialkoxy aluminum species such as AlEt3 and Al((OPr)-Pr-i)3, which are much cheaper than CrCl2 (by a factor of 5 for AlEt3 or 180 for Al((OPr)-Pr-i)(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. (c) 2012 Elsevier B.V. All rights reserved.