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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.

Catalyst	Process	Main product	Medium	Conversion (%)	Selectivity (%)	Ref.
Ru/C	Hydrogenolysis	Lactic acid	NaOH	100	34	[24]
Pt/C	Hydrogenolysis	Lactic acid	CaO	100	58	[24]
(Ru(Cl)/AC-Ox	Hydrogenolysis	Ethylene glycol	–	17	43	[25]
Ru(n)/AC-Ox)	Hydrogenolysis	Ethylene glycol	–	42	30	[25]

       4.4.2.2 Esterification and Acetylation of Glycerol

      One of the potential technologies for the valorization of glycerol from the biodiesel industry is its esterification with acetic acid using a suitable homogeneous or heterogeneous acidic catalyst. The esterification reaction of glycerol mainly produces mono-, di-, and triacetate, also recognized as monoacetin (MA), diacetin (DA), and triacetin (TA) respectively). These acetins have a broad range of applications such as raw materials for the fabrication of tanning agents, polyesters, explosives, and use as solvents, food additives, plasticizers, softening, or emulsifying agents, in cryogenics, or pharmaceutical industry. Furthermore, acetins can be used as environmentally friendly fuel bio-additives. The mixture of DA and TA are useful for improving the viscosity and cold properties of fuel [26].

      Various homogeneous catalysts, for example, hydrofluoric acid, sulfuric acid, and para-toluene sulfonic acid manifest high activity and selectivity. However, these homogeneous catalysts are corrosive, lethal, and hard to separate from the products. Heterogeneous catalysts can conquer the limitations of the homogeneous catalyst due to their high recoverability and recyclability. Additionally, these catalysts show improved selectivity towards the products as compare to homogeneous catalysts. Some heterogeneous catalysts such as acid exchange resins, K-10 montmorillonite, HZSM-5, HUSY, PMo-NaUSY, niobium–zirconium mixed oxide catalysts, heteropolyacids loaded AC, and mesoporous silica has been proposed in pieces of literature [9, 12, 13]. Among various catalysts, cation-exchange resins have shown high activity and outstanding selectivity for higher esters.

Sanchez et al. have prepared a very active and stable porous carbon catalyst having acidic sites by sulfonation of carbonized sucrose using fuming sulfuric acid. The catalyst is selective towards the esterification of glycerol to acetylglycerols. Glycerol reacts with the acetic acid and leads to the formation of monoacetylglycerol (MAG) or monoacetin, di-acetylglycerol (DAG) or diacetin, tri-acetylglycerol (TAG), or triacetin with the removal of water as depicted in Scheme 4.2. The catalyst exhibits conversion above 99% and selectivity of approximately 50% towards the formation of triacetin through glycerol esterification with acetic acid [27].

      The hydrothermally prepared sulfonated carbon (SHTC) from glucose shows good activity for glycerol esterification with various carboxylic acids, i.e., acetic, caprylic, and butyric

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