4.4.1 Biological Conversion
A wide variety of microorganisms such as microalgae, fungi, and bacteria have been used for glycerol transformation by aerobic and anaerobic metabolism. The biological conversion was carried out in large bio-reactors according to the demand of microorganisms. The key product of anaerobic fermentation of glycerol using bacteria is 1,3-propanediol (PDO). In addition to PDO, co-products such as formate, lactic acid, succinic acid, butyric acid, acetic acid, butanol, 2,3-butanediol, acetone, ethanol, and H2 are also formed [19]. The fermentation of glycerol using microorganisms produces lactic acid, eicosapentaenoic acid (EPA), polyhydroxyalkanoates (PHA), citric acid, hydrogen, etc. The biological conversion is the efficient pathway for glycerol transformation. Though, some limitations such as low product yield, slow kinetics, low selectivity, and low reusability limit their uses.
4.4.2 Thermochemical Conversion
Glycerol can be transformed into valuable chemicals using catalytic techniques such as hydrogenolysis, etherification, steam reforming, esterification, oxidation, dehydration, and cyclization. Various catalytic methods adopted for the conversion of crude glycerol as feedstock are outlines in Figure 4.5. A large variety of valuable derivatives, such as fine chemicals, basic units for polymers, fuels, esters, synthesis gas, hydrogen, and fuel additives can be produced using these techniques.
Several solid acid catalysts such as metal oxide (A12O3), zeolite H-ZSM-5, metal sulfide (CdS), immobilized liquid acid (e.g. HF/AlCl3), heteropoly acid (e.g. H3PW12O40), solid superacid (SO42–/ZrO2), natural clay, etc., have been tested for different catalytic processes [20]. None of the above catalysts have shown full potential for glycerol valorization on large scale. Carbon-based materials have a large potential to be used as supports for many active metals as well as catalysts after modification owing to their large surface area, stability in both acidic and basic solutions, functional properties, and desirable acidic or basic sites. The carbon has been used as a support for various metals such as Ru, Pt, Re, Cu, etc. for the glycerol conversion into useful products [21]. In some processes, the carbon-based catalyst with acidic sites has received tremendous interest compared to homogeneous catalysts. This is attributed to its stability, efficiency, viability, and sustainability. Furthermore, carbon catalysts can be recycled numerous times without losing their activity. In particular, carbon-based sulfonated catalysts (CBSCs) are a rapidly growing field for glycerol valorization due to their easy recovery, recyclability, long-term activity, and stability.
Figure 4.5 Roadmap of selected glycerol valorization reactions.
Biomass-derived CBSC is an excellent catalyst for various applications owing to its low cost and abundance. The CBSCs contain a stable and insoluble carbon skeleton with –SO3H functionalized groups. It is amorphous having –SO3H and –COOH groups. In addition, the structure is aromatic in nature with the presence of high density –OH groups. The probable structure of the biomass-derived CBSC is given in Figure 4.6. The –SO3H groups attached to CBSC are the main acidic sites for catalysis whereas the –OH and –COOH group behave as hydrophilic reactant which favor the catalytic performance by providing access to –SO3H sites.
The basic principles, mechanisms and role of different carbon-based catalysts for different catalytic routes have been explained 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