glycerol conversion into profitable chemicals and fuels (syngas/hydrogen) will facilitate the substitution of petroleum-based products as well as promote the utilization of clean energy resources [8]. However, glycerol must be purified before any attempt to transform it into value-added products. Many different reaction pathways (biological, thermocatalytic, etc.) have been investigated to derive the various chemicals starting from glycerol. Of the various methodologies, the use of catalysis represents an efficient approach for this purpose.
Recently, several novel catalytic transformation processes have been reported in the literature. Glycerol is mainly transformed into useful chemicals using steam reforming, esterification, hydrogenolysis, dehydration, oxidation, carboxylation, etherification, and acetalization [9, 12, 13]. A large variety of valuable derivatives, such as fine chemicals, polymers, fuels, and fuel additives have been achieved. A wide spectrum of catalysts such as metal, metal salt, metal oxides, resins, zeolite, heteropolyacids, sulfonic acid, metal sulfides, and carbon catalysts have been utilized for the efficient conversion of glycerol into value-added products [12, 13]. Carbon-based catalysts have added benefits compared to metal-based catalysts due to their high durability, better textural properties, easy availability, and low cost. Carbon-based catalysts have been used as a catalyst as well as support for different transformation processes. This chapter explains the fundamentals, mechanism, and latest development in the transformation of glycerol into valuable products over carbon-based catalysts. The earlier reported works demonstrated the possibility of the establishment of several new catalytic approaches and value-added products from glycerol. Interestingly, there are still a lot of opportunities available for researchers to design novel carbon-based catalysts and develop new methodologies for glycerol conversion.
4.2 Production of Biodiesel and Crude Glycerol
Worldwide biodiesel production has been rising extensively in the last few years. In 2018, the top five biodiesel producing nations were the United States (US), Argentina, Brazil, Germany, and China [10]. Both the US and Brazil have accounted for a combined share of nearly 87% of the total production in 2018. It is predicted that in the near future, the annual production of biodiesel will rise by approximately 4.5% [10]. Biodiesel is mainly produced from a broad diversity of materials that contains triglycerides as a key component. This can be produced by vegetable oils from jatropha, sunflower, babassu, soybean, macauba, palm, oilseed radish, canola, crambe, castor bean, peanut, and lupine [4]. Alternatively, it can also be produced from photosynthetic algae, animal fat, and waste cooking oil. The transesterification or esterification of triglycerides with alcohol (methanol) produces methyl esters of fatty acids (biodiesel) and glycerol as depicted in Figure 4.1. The catalysts used for transesterification are mainly alkalis, acids, or enzymes.
The transesterification reaction takes place in three steps as shown in Figure 4.2 in which triglycerides react with methanol. First, the reaction involves the conversion of triglycerides into diglycerides which then get converted into monoglycerides. Finally, the monoglycerides transformed into glycerol. The overall reaction is reversible, so, plenty of alcohol is required to favor the reaction in the forward direction. The theoretical stoichiometric ratio for the conversion is 1:3 (triglycerides: alcohol), but for practical applications surplus amount of alcohol is required to shift the equilibrium in the direction of the product. Overall, for the formation of 9 kg of biodiesel, approximately 1 kg of glycerol is generated [5].
The flow chart for the formation of biodiesel and byproduct glycerol is shown in Figure 4.3.
The commonly used catalyst during biodiesel production is NaOH which leads to the generation of faulty smell, dark-colored glycerol with high pH. The crude glycerol produced during the production of biodiesel contains impurities including catalyst salts, unreacted glycerides, and water depending upon the nature of oil and technology employed. The amount of glycerol in crude glycerol may vary from 45 to 90% depending upon the reaction conditions [5, 6]. Upgrading and refining crude glycerol are necessary to minimize waste production. The industrial-grade glycerol can be obtained by filtration, extraction, and distillation. The glycerol produced from different bio-refineries has a different composition, so it is complicated to outline the properties of crude glycerol. The properties of pure glycerol are outlined in Table 4.1. There are many commercial grades of glycerol; it is named glycerin if the concentration of glycerol is above 95%. The structure of glycerol is shown in Figure 4.4. It is miscible in water due to the presence of hydrophilic hydroxyl groups. It is a colorless, unscented thick fluid having a boiling and melting point of 290 and 17.9 °C, respectively.
Figure 4.1 Transesterification reaction for biodiesel production.
Figure 4.2 Steps involved in the transesterification process.
Figure 4.3 Transesterification reaction for biodiesel production.
Table 4.1 Properties of pure glycerol.
| Molecular formula | C3H8O3 |
| Molar mass | 92.09 g/mol |
| Melting point | 18 °C |
| Boiling point | 290 °C |
| Relative density | 1,260 kg/m3 |
| Viscosity | 1.41 Pa s |
| Flash point | 160 °C |
| Specific heat | 2.43 kJ/kg K |
| Heat of vaporization | 82.12 kJ/kmol |
| Heat of formation | 667.8 kJ/mol |
| Surface tension | 63.4 mN/m |
| Self-ignition | 393 °C |
Figure 4.4 Chemical structure of pure glycerol.
4.3 Refining Process for Crude Glycerol
