Isomerization of glucose into fructose

Nowadays, there is a renewed interest for the preparation of fructose, for its food applications as a diet sugar[16] as well as for its nonfood applications as a starting material for the synthesis of furanic precursors of nonpetroleum derived polymeric materials. [17 19] 

Catalyst Glucose conversion (%) Fructose selectivity (%) Cation leaching (%) Glucose disappearance rate (x 104 s-1) 

However, no leaching effect was observed with hydrotalcites in their hydroxide or mixed carbonate-hydroxide form.[21] Modifications of their basic properties lead to an increase in the selectivity to fructose to nearly 100%, but once again at a glucose conversion lower than 20%. 

Cation-exchanged KX and CaY zeolites are also known to be used for the separation of glucose and fructose on the basis of the selective adsorption properties of that kind of material.122-251 Some experiments have then been performed in the presence of Ca- and Ba-exchanged A, X and Y zeolites. Unfortunately, the CaY zeolite claimed for the separation of glucose and fructose was not as efficient as expected for a two-stage process involving isomerization followed by separation on the same type of material. 

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We have shown that the isomerization of glucose into fructose (Scheme 7.2) was easily achieved in the presence of cation-exchanged zeolites and hydrotalcites, in water as the solvent, at higher temperatures and higher glucose concentrations than in the case of ion-exchange resins and enzymes as catalysts.[20]... 

This is the case, for instance, of glucose isomerase, which is a very relevant industrial enzyme that catalyzes the reversible reaction of isomerization of glucose into fructose with a value of the equilibrium constant of the reaction close to 1 at conditions compatible with enzyme activity (Houng et al. 1993). 

The isomerization of glucose into fructose for the production of high-fructose com symps (HFCS), a mixture of glucose and fructose, which is 1.3 times sweeter than sucrose, is the largest biocatalytical process in the world. In the next decades, the isomerization of sugars probably will... 

Consequently, to produce EMF from glucose, isomerization of glucose into fructose appears as a particularly relevant reaction. Lew et al. (2012) have reported the one-pot synthesis of EMF from glucose using a combination of Sn-BEA (for the isomerization step) and Amberlyst 131 (for the dehydration-etherification steps) with acceptable yields. 

It may be noted that simple alkaline-catalyzed isomerization of glucose to fructose is possible, but gives rise to serious lactic acid and coloured by-product formation. Alkaline catalysis, however, is still applied for the conversion of lactose to lactulose, used in treatment of constipation and PSE. The reason is that no enzyme has been found that is able to isomerize the glucose unit of lactose into a fructose moiety. As a consequence, a low conversion is applied or borate is used as a protecting group. In the latter case extra separation and recycle steps are required. 

So far, the concept has been used for the isomerization of glucose to fructose (Hashimoto et al., 1983 and Borren and Schmidt-Traub, 2004), but further applications such as thermal racemizations are conceivable. An application for reactive systems with more than two components is also possible if special catalysts or operating conditions are required. However, an even more complex design, as well as control of the process, has to be taken into account. 

Step 2 is the isomerization of glucose to fructose. This reaction involves the conversion of the aldohexose into the 2-ketohexose. Retro-aldol reaction of the aldohexose leads to a C4 and C2 sugar, whereas the ketohexose leads to the two trioses, dihydroxyacetone (DHA) and glyceraldehyde (GLY). As the pathway to LA involves the trioses, selective glucose isomerization is essential, its conversion being limited by equilibrium in the operational temperature window. The isomerization of aldo- to ketoses can proceed via an acid-catalyzed hydride shift, a base-catalyzed mechanism with a proton shift (and intermediate enol), or via a concerted 1,2-hydride shift in neutral media [96, 97]. The latter isomerization mechanism occurs at mild temperatures (100°C) in the presence of Lewis acid catalysts, first... 

In 1895, Emil Ficher proposed an enediol intermediate for this isomerization. As would be expected, the enzyme-catalyzed isomerization of glucose-6-phosphate in 2H20 is accompanied by incorporation of deuterium into the product fructose 6-phosphate at C-l. In the reverse reaction 2H-containing fructose 6-phosphate was found to react at only 45% of the rate of the 1H-containing compound. Thus, the primary deuterium isotope effect expected for a rate-limiting cleavage of the C-H bond was observed (see Chapter 12, Section B,3). 

Step 2. The Isomerization of Glucose-6-Phosphate into Fructose-6-Phosphate The reaction is... 

The next reaction in the glycolysis pathway is catalyzed by phosphoglucoi-somerase and involves the isomerization of glucose-6-phosphate into fructose-6-... 

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