Hydrogenation of D-fructose

Figure 8.7 Reaction schemes for hydrogenation of d-fructose, d-xylose, and d-lactose.

The study of the hydrogenation of D-fructose is of both practical importance and theoretical significance since it allows one to investigate stereochemical regularities (equation 27). 

In addition to enzymatic hydrolysis (see Section 5.8.1), inulin can be hydrolyzed chemically and subsequently dehydrated to yield hydroxymethylfurfural (van Dam et al., 1986), a key industrial chemical (Gretz et al., 1993 Fuchs, 1987 Kunz, 1993 Makkee et al., 1985 van Dam et al., 1986). Likewise, catalytic hydrogenation of D-fructose yields D-mannitol and D-sorbitol mixtures from which mannitol can be removed via crystallization (Fuchs, 1987). 

STEREOSELECTIVE HYDROGENATION OF D-FRUCTOSE TO D-MANNITOL ON SKELETAL AND SUPPORTED COPPER-CONTAINING CATALYSTS... 

D-mannitol is widely used as sweetening agent and finds also different application in the food industry and related areas [1]. D-mannitol can be directly prepared from mannose or by stereoselective hydrogenation of D-fructose. However, the hydrogenation of D-fructose in aqueous solution over different heterogeneous catalysts leads to the formation of two isomers, i.e. D-mannitol and D-sorbitol, near to a ratio of one to one [1]. 

It was observed earlier that upon hydrogenation of D-fructose high selectivities of D-mannitol could be obtained only over supported copper catalysts [2]. Further selectivity improvements were observed by addition of sodium borate to the reaction mixture [2]. 

In this work different supported and skeletal copper catalysts were investigated in the hydrogenation of D-fructose to D-mannitol. The aim of this work was to find the modes and ways to increase the selectivity of D-mannitol. 

Hydrogenation of D-fructose over Raney-copper catalysts. 

The effect of sodium borate on the rate of hydrogenation of D-fructose and the M/S ratio is shown in Figure 4. This figure shows that upon increasing the concentration of sodium borate significant decrease in the conversion value takes place. However, similarly to earlier findings [2], in the presence of sodium borate substantial increase in the M/S ratio can be achieved. 

Table 2. Hydrogenation of D-fructose in the presence of sodium haiides.

By using sodium haiides or sodium borate the hydrogenation of D-fructose could be carried out almost to the completion. These results are shown in Table 3. In these experiments upon using Raney-copper catalysts the highest selectivity of D-mannitol, 82.7 %, was obsen/ed over a cobalt containing catalysts in the presence of sodium borate. Even higher D-mannitol selectivity, i.e. 88.2, was observed over CPG supported copper catalyst in the presence of sodium borate. 

Chiral intermetallic, heterogeneous and heterogenized catalysts were put to practical use for example in the diastereoselective hydrogenation of D-fructose into mannitol and sorbitol... 

Alditols - A review on homogenous catalysts used in the hydrogenation of aldoses with emphasis on mechanistic aspects of their activity and modes of their deactivation has appeared, and the hydrogenation of D-fructose to D-mannitol has been described using ruthenium molecular sieves modified with chiral ligands as catalysts. ... 

Figures 11 and 12 show the activity data obtained in hydrogenation of D-fructose to D-mannitol. As ligands two high molecules were used. No differences between the two ligands were evidenced but the reactions performed on APO-34 revealed high convers ions. The yield to D-mannitol does not exceed 30 %.

Hydrogenation of D-fructose on these catalysis revealed conversions bwer than 50% and excess in rapport vdth D-mannitol yield of maximum 30% (Figures 11 and 12). The presence of phenyl substituent generates only a very small increase of the activity. The yield to D-mannitol is bwer than those prevbusly mentioned using chiral modifiers [20,21 ] but in this case it was obtained an enantioselective hydrogenation in presence of ligands without optical carbon and more, containing sulphur. 

Alditols are mainly produced from aldoses or ketoses by hydrogenation or reduction with amalgams, complex hydrides or by electrolytic reduction. Xyhtol is prepared by reduction of D-xylose obtained by hydrolysis of natural xylems (hemicelluloses), D-glucitol is prepared from D-glucose and D-mannitol via hydrogenation of D-fructose obtained from either starch or invert sugar. 

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