in the next section.
4.4.2.1 Hydrogenolysis of Glycerol
Hydrogenolysis is defined as a catalytic route that involves the selective scission of carbon–carbon or carbon–heteroatom bonds in an organic compound by reaction with molecular hydrogen. Hydrogenolysis of biomass-derived compounds comprises a promising route to several industrially important chemicals, such as hydrocarbons, by complete deoxygenation, and polyols by lysis and/or partial deoxygenation of the carbon chain [9]. Owing to the increasing availability and falling prices in the market, glycerol is now considered an important substrate and much of the focus has now been diverted towards its transformation via this route. Several chemical compounds such as 1,2-PD (1,2-propanediol), ethylene glycol (EG), 1,3-PD (1,3-propanediol), propylene, 1-propanol, etc. can be synthesized by selective glycerol hydrogenolysis using a suitable metallic catalyst. It can be considered as another possible path to enhance the productivity of biodiesel industries since the products of this route are commercially produced either from non-renewable resources or through biological routes using high-cost microorganisms.
Figure 4.6 Structure of biomass-derived CBSC.
Hydrogenolysis of glycerol is usually conducted in liquid-phase or vapor-phase. It is generally admitted in glycerol hydrogenolysis process that both 1,2-PD and 1,3-PD are formed by glycerol dehydration followed by hydrogenation of intermediates; however, the reported catalysts and operating conditions for both products are different [12, 13].
Researchers explored both homogeneous and heterogeneous catalytic routes for the glycerol hydrogenolysis to improve the selectivity and yield of desirable products 1,2-PD, 1,3-PD, and EG. In homogeneous catalysis, several homogeneous complexes of metals (Pd, Rh, Ru,) have been explored as a catalyst in the presence of a suitable solvent. However, this homogeneous catalytic route is not economically and environmentally attractive because the catalysts are irrecoverable and nonrecyclable. Moreover, the use of toxic solvents causes this process to become environmentally unfriendly [13, 14]. The heterogeneous catalytic approach of using solid catalysts can overcome these limitations of homogenous catalysis.
1,3-PD is a vital industrial chemical consumed as an intermediate or solvent in pharmaceutical, textile, and food industries. It has extensive applications in the polymer industry for the production of polyurethane and polytrimethylene terephthalate. The large-scale production of 1,3-PD occurs by glycerol fermentation using expensive genetically modified microorganisms. 1,2-PD is a chief chemical that is used extensively in the production of polymers, pharmaceuticals, plastics, and transportation fuel. It is also used as an antifreeze agent, solvent, hydraulic fluid, and used for cosmetics, and food production industries. 1,2-PD is commercially prepared from the propylene oxide through the hydration method. Propylene oxide is derived from propylene which is a product of fossil fuels. So, the generation of 1,2-PD from a renewable resource is attractive [22].
Several reports are available in the literature for glycerol hydrogenolysis into 1,3-PD using highly active, stable, and selective solid-state catalysts. These catalysts are made up of acidic and metallic sites, which are accountable for selective dehydration and hydrogenation of glycerol, respectively. The detailed mechanistic study indicates that secondary hydroxyl in the glycerol molecule is dehydrated into 3-hydroxypropionaldehyde (3-HPA) on acidic sites and further 3-HPA is hydrogenated on the surface of metallic sites into 1,3-PD (Scheme 4.1). The technological issue associated with this reaction is the selective and controlled hydrolysis of the secondary hydroxyl attached with a central carbon atom. The existence of two primary hydroxyl groups decreases the accessibility of the secondary hydroxyl to the active sites of the catalyst and thus, reduced the reactivity [23]. Therefore, the elimination of secondary hydroxyl is difficult from a thermodynamics and kinetics point of view.
Generally, the availability of Bronsted acid sites on the catalyst is required to synthesize the 1,3-PD through glycerol hydrogenolysis (Route A). This reaction must be conducted at higher hydrogen pressure and temperature (<200 °C) to prevent the subsequent dehydration of 1,3-PD. Although high temperature improves the glycerol conversion, it reduces the selectivity for 1,3-PD by producing acrolein and supports the transformation of 1,3-PD into monoalcohols (Scheme 4.1) [23]. The availability of Lewis acidic sites on the catalyst surface plays a major role in the formation of 1,2-PD (Route B). The glycerol dehydration into hydroxyacetone occurs over these acidic sites, which undergoes hydrogenation over metallic sites to 1,2-PD.
Scheme 4.1 Mechanism for the glycerol hydrogenolysis [23].
Several studies have found that a bifunctional catalyst, based on a combination of acid and metal, is a promising catalyst for the formation of 1,3-PD from glycerol. The combination of metallic sites (Pt, Ir, and Rh) and Bronsted sites (MoOx, ReOx, WOx) is required to attain the high selectivity for 1,3-PD. The acidic sites are essential for the adsorption of glycerol as well as for its conversion. Among the various metals, Pt has been recognized as the finest metal. Metallic sites activate the hydrogen molecule. Physicochemical characterization of catalysts, such as metal dispersion, surface area, crystallinity, synergetic interaction between metals, nature, and strength of the acidic sites on the support affect the glycerol conversion and selectivity towards 1,3-PD.
Several types of transition metals, such as Ru, Pt, Ir, Pd, Ni, and Cu are active towards the production of 1,2-PD. It was found that a multifunctional catalyst having both hydrogenation and dehydration capability is needed for this reaction. The glycerol dehydration into hydroxyacetone is catalyzed by acidic sites in the liquid phase and hydroxyacetone is subsequently hydrogenated to 1,2-PD over metallic sites. Mechanistic studies of 1,2-PD formation indicate that Lewis acidic sites catalyze glycerol dehydration into hydroxyacetone. It was proposed that the primary hydroxyl group is activated by a Lewis acidic site as compare to the secondary hydroxyl group [23, 24].
Maris et al. [24] have used Ru or Pt supported commercial carbon (Ru/C, Pt/C) as catalysts for glycerol hydrogenolysis in an aqueous phase at 473 K and a hydrogen pressure of 40 bar. At neutral pH, Ru/C shows the higher activity and promotes the formation of ethylene glycol over propylene glycol. Whereas, Pt/C shows less reactivity and catalyzes the formation of propylene glycol with good selectivity. The existence of a base enhances the catalytic performance of Pt/C to a bigger extend as compared to Ru/C [24].
Arcoya and coworkers have investigated the glycerol hydrogenolysis into 1,2-PD over ruthenium supported activated carbons (AC), originals AC, and HNO3 modifies AC (ACOx). The hydrogenolysis reaction was conducted in the liquid phase at T = 453 K and 8 MPa. Characterization techniques revealed that oxygenated groups were available on the surface of carbon, which increases the acidity of the catalyst. The high density of the acidic group makes it highly selective towards 1,2-PD. Ruthenium supported on activated carbon (Ru(Cl)/AC-Ox and Ru(n)/AC-Ox) shows higher selectivity towards the production of 1,2-PD. The overall catalytic activity has increased with an increase in the concentration of surface acidic groups. The Ru/AC in combination with acid catalysts is efficient for simultaneous hydrogenation and dehydration [25]. The glycerol hydrogenolysis over Ru and Pt-based catalysts are given in Table 4.2.
Table 4.2 Glycerol hydrogenolysis over Ru and Pt-based catalysts.
