Reactivity of Metal Catalysts in Glucose–Fructose Conversion

Abstract

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.

Experimental and Modeling Studies on the Conversion of Inulin to 5-Hydroxymethylfurfural Using Metal Salts in Water

“…At this stage, it is not clear whether the D-glucose converts to HMF directly or is first isomerized to D-fructose by the action of a Lewis acid like Cu 2+ (tandem isomerization-dehydration) or directly to HMF. Although CuCl2 is reported to be a mediocre catalyst for the isomerization of D-glucose to D-fructose in water compared to, for instance, AlCl3 [34], our findings indicate that CuCl2 is a good active catalyst to promote HMF formation when D-fructose instead of D-glucose is used as the feedstock. It suggests that Cu-salts are more active catalysts for the dehydration of D-fructose to HMF rather than for the D-glucose-fructose isomerisation reaction.…”

“…In contrast, water-sensitive metal salts based on Al, Sn, Fe, Ga, and In are hydrolysed in water to various mono-and oligomeric species, and the formation of solutions with a pH < 7. As such, these water-sensitive metal salts in water may act both as a Brönsted and a Lewis acid [34].…”

The Role of Salts and Brønsted Acids in Lewis Acid‐Catalyzed Aqueous‐Phase Glucose Dehydration to 5‐Hydroxymethylfurfural

“…Recently, the aqua complex has been reported to be more effective in the glucose-to-fructose isomerization than complexes possessing strong s and p donor ligands, such as [Cr(H 2 O) 5 OH] 2 + and [Cr(H 2 O) 5 Cl] 2 + , and dimeric chromium complexes. [22] Also, it is well-established that at low pH (< 2), coordination of alcohols to chromium is strongly restrained. [23] Consequently, based on the evidence above, we can attribute the reduced catalytic activity of CrCl 3 ·6 H 2 O with increasing acidity to the hampered formation of glucose-chromium chelate complex, which facilitates the necessary hydride transfer to form fructose.…”

Tandem Lewis/Brønsted homogeneous acid catalysis: conversion of glucose to 5-hydoxymethylfurfural in an aqueous chromium(iii) chloride and hydrochloric acid solution

“…[14,15] The Davis group obtained HMF yields in excess of 50% in a one-pot reactor containing Lewis acidic Sn-Beta and HCl. [15] Following Davis' group pioneering work, many other tandem Lewis/Brønsted acid catalysts were investigated, [16] including aluminosilicate zeolites, [17] activated carbons, [18] and homogeneous metal chlorides, [19] such as CrCl3 [14,[20][21][22][23] and AlCl3. [24] For the homogeneous Lewis acid CrCl3/Brønsted acid HCl single pot catalysis, we revealed rather complex and unexpected interactions that render understanding experimentally challenging.…”