Comparison of Homogeneous and Heterogeneous Catalysts for Glucose‐to‐Fructose Isomerization in Aqueous Media

Abstract

Herein, the first comparison of the mechanisms of glucose-to-fructose isomerization in aqueous media enabled by homogeneous (CrCl3 and AlCl3 ) and heterogeneous catalysts (Sn-beta) by using isotopic-labeling studies is reported. A pronounced kinetic isotope effect (KIE) was observed if the deuterium label was at the C2 position, thus suggesting that a hydrogen shift from the C2 to C1 positions was the rate-limiting step with the three catalysts. (13) C and (1) H NMR spectroscopic investigations confirmed that an intra-hydride-transfer reaction pathway was the predominant reaction channel for all three catalysts in aqueous media. Furthermore, the deuterium atom in the labeled glucose could be mapped onto hydroxymethylfurfural and formic acid through reactions that followed the isomerization step in the presence of Brønsted acids. In all three catalysts, the active site appeared to be a bifunctional Lewis-acidic/Brønsted-basic site, based on a speciation model and first-principles calculations. For the first time, a mechanistic similarities between the homogeneous and heterogeneous catalysis of aldose-to-ketose isomerization is established and it is suggested that learning from homogeneous catalysis could assist in the development of improved heterogeneous catalysts.

Mechanistic Study of Glucose-to-Fructose Isomerization in Water Catalyzed by [Al(OH)2(aq)]+

“…These results clearly supported an intramolecular hydride shift mechanism catalyzed by AlCl3. 6,18 These results also indirectly explained that the coordination of Al with acyclic glucose on the C1-O and C2-O positions facilitated the following intramolecular C2 → C1 hydride shift.…”

“…This result proved that the hydride shift from C2 to C1 positions of glucose was the rate-limiting step. 6,18 As shown in Table S3, 40 it seemed that the concentration of AlCl3 had no 21 22 23 24...…”

“…Generally accepted pathway for the isomerization of glucose-to-fructose As a cheap and low-toxic catalyst, AlCl3 behaved well in the glucose-to-fructose isomerization in aqueous media. 17,18,[22][23][24][25][26] However, the active species, normally existed as hydrolyzed forms, and their interaction with glucose under the reaction conditions are still lack of detailed information.…”

Multiscale molecular modeling can be an effective tool to aid the development of biomass conversion technology: A perspective

“…Isomerization of glucose to fructose or that of xylose to xylulose in water, catalyzed either by a heterogeneous catalyst like Sn-beta or by homogeneous catalysts like CrCl 3 /AlCl 3 , proceeds via ring opening of the glucose molecule and an intramolecular hydride shift Bermejo-Deval et al, 2012a;Choudhary et al, 2013a). Yang et al (2013) reported the mechanism of glucose isomerization into fructose on Sn-beta zeolite using DFT and the rate limiting hydride shift step was found to have a barrier of 77 kJ/mol.…”

Glucose isomerisation into fructose over magnesium-impregnated NaY zeolite catalysts

“…Basic homogeneous catalysts, however, tend to form higher amounts of by-products [17,25]. Very few works have been so far carried out using Lewisacid homogenous catalysts, and only two salts have been tested, CrCl 3 and AlCl 3 [27].…”