**3.2 Catalysts**

The design and selection of catalyst systems have been a major research topic in organic synthesis and chemical engineering. Several factors should be considered for the adequate selection of a catalyst system including, the transition metal, supporting material, preparation methods, and solvent. For lactose hydrogenation, metals such as nickel (Ni), ruthenium (Ru), and palladium (Pd) within a range of 1–10% are commonly used due to their relatively high reactivity and selectivity toward aldehyde groups. The concentration of the metal is linearly related to its activity within a limited range of 1 to 10%. Outside the concentration range, the metal is not available for reaction. A number of metal-based catalysts have been developed for lactitol production, including nickel-based, ruthenium-based, and other metalbased catalysts.

#### *3.2.1 Nickel-based*

Raney in 1920s patented a protocol where active metal (Ni) was embedded within an inactive metal (Al) frame [7]. The activity of the Raney's catalyst results from the random distribution of nickel crystals within the inactive crystal lattices [20]. A number of metals have been added into the Raney-nickel catalysts to further increase the reactivity [21]. Chromium, molybdenum, and tungsten are examples of metals added. The use of metal promoters (Cr-, Mo-, and Fe-Ni) showed a five-fold rate enhancement over non-promoted Raney nickel in the hydrogenation of glucose [21]. Although nickel-based catalysts are an effective catalyst for lactose hydrogenation, it suffers from the deactivation problem due to the nickel leaching and catalyst sintering. This results in a loss of catalyst activity and high nickel content in the lactitol product solution.

#### *3.2.2 Ruthenium-based*

Ruthenium is another metal used as a catalyst supported in different materials, such as carbon, alumina, silica, and synthetic gel. The ruthenium catalyst was effective

**39**

*Hundred Years of Lactitol: From Hydrogenation to Food Ingredient*

for the hydrogenation of monosaccharides and disaccharides [22]. Rutheniumbased catalysts are supported in alumina (Ru/Al2O3), silica gel (Ru/gel), titanium dioxide (Ru/TiO2), crosslinked polystyrene (Ru/CP), and activated carbon (Ru/C). Ruthenium-based catalyst is more active than a nickel-based catalyst, and this leads to the higher catalyst selectivity [23]. Moreover, ruthenium-based catalyst was more stable than Ni based catalyst in the hydrogenation process; this leads to the extended

Metals such as copper (Cu) and Pd have also been studied for the hydrogenation of lactose. For instance, Cu/SiO2 was effective for the catalytic transformation of lactose to a high yield mixture (75–86%) of sorbitol and galactitol [24]. The boron nitride supported palladium (Pd/h-BN) was applied for the hydrogenation of lactose. The results indicated that the high lactose conversion ratio (up to 50%) was

Lactitol is the main product formed during the hydrogenation of lactose, followed by a considerable formation of lactulose, lactulitol, lactobionic acid, sorbitol, and galactitol [23]. All these compounds are formed through a combination of hydrogenation, isomerization, hydrolysis, and oxidation. **Figure 3** illustrates the reaction pathways occurring during the hydrogenation of lactose. A number of factors influence the occurrence and extent of a given reaction. Temperature, pressure, pH, agitation, type of catalysts, concentration, and catalyst load are examples of

Lactose is readily reduced to its corresponding alcohol, where the carbonyl group reacts with the hydrogen ion. This reaction is represented by scheme (1), and it is the main reaction occurring during the hydrogenation. Concomitantly, other reducing sugars (lactulose, galactose, and glucose) are also hydrogenated to form their respective alcohol (lactulitol, galactitol, and sorbitol). These reactions are exemplified in scheme (4), (10), and (9), respectively. Lactulose is formed through isomerization, while galactose and glucose are formed via hydrolysis of lactose.

The temperature used during hydrogenation may trigger the isomerization of lactose via enolization of the glucose molecule, scheme (4). Hypothetically, galactose, and glucose may undergo isomerization to form D-Tagatose and fructose, respectively. The yield corresponding to derives from isomerization is rather low. This is because the isomeric form of a reducing carbohydrate is prone to hydrogenation. Scheme (9) and (10) illustrate the hydrogenation of glucose and galactose,

Lactose may hydrolyze to some extent, leading to the formation of galactose and glucose, scheme (8). Lactulose and lactulitol may also hydrolyze, and their respective product can be hydrogenated. These sets of reactions are illustrated in

*DOI: http://dx.doi.org/10.5772/intechopen.93365*

lifetime of catalysts [23].

*3.2.3 Other metal catalysts*

obtained with this catalyst.

**3.3 Reaction pathways**

such factors [2].

*3.3.1 Hydrogenation*

*3.3.2 Isomerization*

respectively.

*3.3.3 Hydrolysis*

#### *Hundred Years of Lactitol: From Hydrogenation to Food Ingredient DOI: http://dx.doi.org/10.5772/intechopen.93365*

for the hydrogenation of monosaccharides and disaccharides [22]. Rutheniumbased catalysts are supported in alumina (Ru/Al2O3), silica gel (Ru/gel), titanium dioxide (Ru/TiO2), crosslinked polystyrene (Ru/CP), and activated carbon (Ru/C). Ruthenium-based catalyst is more active than a nickel-based catalyst, and this leads to the higher catalyst selectivity [23]. Moreover, ruthenium-based catalyst was more stable than Ni based catalyst in the hydrogenation process; this leads to the extended lifetime of catalysts [23].
