*3.2.3 Other metal catalysts*

*Lactose and Lactose Derivatives*

molecules in the bulk phase.

fructose, xylose, and lactose [18].

rapid and therefore neglected within the rate equation.

*3.1.2 Surface reaction*

*3.1.3 Desorption*

**3.2 Catalysts**

based catalysts.

*3.2.1 Nickel-based*

content in the lactitol product solution.

*3.2.2 Ruthenium-based*

lactose and hydrogen is followed a dissociative mechanism (H2↔2H\*) due to the action of transition metals. Dissociative adsorption requires an adjacent vacant site, and the rate of attachment is proportional to the square of the vacant concentration [16]. The adsorption of reagents occurs within a very short timeframe. Once the adsorption is completed, the adsorbed molecules are in equilibrium with those

Examples of reaction mechanisms occurring at the surface include duel-site, single-site, two adsorbed species, and unabsorbed species [17]. Such mechanisms have been used for hydrogenation of a number of carbohydrates including, glucose,

The products of the surface reaction are subsequently desorbed into the reaction medium. Theoretically, the rate of desorption is exactly the opposite in sign to the rate of adsorption [19]. However, the desorption of reaction products is regarded as

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 metal-

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

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

**38**

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 obtained with this catalyst.
