A joint experimental and computational study on the glucose–fructose conversion in water is reported. The reactivity of different metal catalysts (CrCl3, AlCl3, CuCl2, FeCl3, and MgCl2) was analyzed. Experimentally, CrCl3 and AlCl3 achieved the best glucose conversion rates, CuCl2 and FeCl3 were only mediocre catalysts, and MgCl2 was inactive. To explain these differences in reactivity, DFT calculations were performed for various metal complexes. The computed mechanism consists of two proton transfers and a hydrogen‐atom transfer; the latter was the rate‐determining step for all catalysts. The computational results were consistent with the experimental findings and rationalized the observed differences in the behavior of the metal catalysts. To be an efficient catalyst, a metal complex should satisfy the following criteria: moderate Brønsted and Lewis acidity (pKa=4–6), coordination with either water or weaker σ donors, energetically low‐lying unoccupied orbitals, compact transition‐state structures, and the ability for complexation of glucose. Thus, the reactivity of the metal catalysts in water is governed by many factors, not just the Lewis acidity.

We developed a new acid-free and metal-free heterogeneous catalytic system for the dehydration of fructose into 5-hydroxymethylfurfural (HMF). These catalysts are ionic polymers based on 4-vinylpyridine, crosslinked with divinylbenzene. The effect of different polymer properties, like the alkyl chain length at the pyridinium nitrogen, the type of anion, the porosity, the degree of post polymerization modification as well as the effect of temperature was investigated. Poly(N-alkylvinylpyridinium bromides) showed the best results. A maximum HMF yield of 77% after 0.5 h at 180 °C, using ethanol as the low boiling solvent, was achieved. Most of the byproducts are the acetals of HMF with the alcohol medium, which are often converted in downstream processes in the same manner as HMF. Thus, one can effectively consider the catalyst system as almost fully selective to HMF