(Pt–Re/C) have been employed for the transformation of glycerol into hydrogen or synthesis gas. It was observed that a Pt–Re/C catalyst with an atomic ratio Pt/Re =1 showed high catalytic activity, selectivity to syngas, and long-term stability [45]. The carbon was used as support owing to its long-term hydrothermal stability [44].
The pyrolysis of glycerol with the support of carbonaceous catalyst is an important method for the formation of syngas with a higher H2/CO ratio. Other methods used for the formation of synthesis gas include dry and steam reforming of glycerol. Fernandez et al. [46] have compared the above three methods (steam reforming, dry reforming, and pyrolysis) for the synthesis of syngas using commercially available activated carbon as a catalyst. Carbon-base catalyst was favorable for generating syngas with H2/CO ratio not far from 1, reduce the CO2 fraction in the gaseous product. The reforming of glycerol uses CO2 (dry reforming) or H2O (steam reforming) as an oxidizing agent which encourages the higher conversion of glycerol as compared to pyrolysis. The dry reforming generates the lowest amount of hydrogen and syngas, and the highest amounts gas fraction, whereas the reverse occurs in the steam reforming process. They have also compared the microwave-assisted process with the conventional heating process. The microwave-assisted method promotes more conversion of glycerol as compared to the conventional method [46].
4.4.2.4 Oxidation of Glycerol
Due to its highly functionalized nature, glycerol is also used as a feedstock for the synthesis of useful oxygenates. Several oxygenated compounds such as mesooxalic acid, glyceric acid, dihydroxyacetone, oxalic acid, tartronic acid, hydroxyethanoic acid, etc., can be produced from the complex reaction pathway through oxidation of glycerol. These compounds, especially mesooxalic acid and tartronic acid, are important chelating agents, which have the potential for the production of valuable polymers and chemicals. Dihydroxyacetone can be used as a building block in organic synthesis and as a basic repeating unit of new degradable polymers [12, 13]. To date, these chemicals have inadequate utilization because they are either synthesized using expensive and environmentally harmful oxidation processes (e.g. K2Cr2O7, HNO3, H2CrO4) or less productive fermentation process The possible reaction pathways to oxygenated derivatives of glycerol is shown in Scheme 4.5.
The heterogeneous catalytic oxidation process can be used for the oxidation of a unique structure of glycerol using low-cost oxidizing agents such as oxygen, air, and H2O2 instead of environmentally harmful oxidants, e.g., K2Cr2O7, HNO3, H2CrO4, etc. The synthesis of a highly selective catalyst is the main challenge in this oxidation reaction. This catalyst must be selective towards either the oxidation of the primary alcohol group, to produce glyceric acid, or the oxidation of the secondary alcohol group, to synthesized hydroxypyruvic acid and dihydroxyacetone. Several studies have been reported for chemoselective glycerol oxidation over supported noble metal nanoparticles such as Pt, Pd, and Au. In general, the selective oxidation of glycerol takes place in the aqueous medium. Table 4.3 summarizes the performance of different catalysts.
The Au particles supported on multiwalled carbon nanotubes enhance the chemoselectivity for glycerol oxidation towards the formation of dihydroxyacetone. The catalyst shows 60% selectivity along with the high activity. The results were compared by using activated carbon under similar metal loading and particle size. The activated carbon encourages the synthesis of glyceric acid. The study suggests that the type of supports play significant role in chemoselectivity [47]. Selective oxidation of crude glycerol into lactic acid and glyceric acid has been carried by different groups using carbon as a support for Pt, Pd, Cu–Pt and Au–Pd etc. [48–50].
Scheme 4.5 Plausible reaction plan for the production of oxygenated derivatives from glycerol [9].
4.4.2.5 Etherification
The glycerol etherification reaction in the presence of isobutylene yields a mixture of mono-tert-butylglycerols (MTBGs), di-tert-butylglycerols (DTBGs), and tri-tert-butylglycerols (TTBGs). Sulfonated peanut shell catalysts have been used as competent and stable catalysts for glycerol etherification. This carbon-based catalyst was synthesized by partial carbonization of peanut shell in concentrated H2SO4 at 483K. The study shows that the resulted catalyst is amorphous with a porous structure having good thermal stability and catalytic efficiency owing to the presence of acidic sites. The sulfonic acid groups are covalently bonded with the carbon framework. During the etherification, the hydroxyl group of glycerol reacts with isobutylene and leads to the production of five types of glycerol ethers according to Scheme 4.6 [51]. The etherification produces two MTBGs (2-tert-butoxy-1,3-propanediol and 3-tert-butoxy-1,2-propanediol), two DTBGs (1,3-di-tertbutoxy-2-propanol and 2,3-di-tert-butoxy-1-propanol), and one TTBG (1,2,3-tri-tert-butoxy propane). This catalyst is cheap, green, and easily available.
Devi and coworkers have prepared a novel carbon-based catalyst by partial carbonization and sulfonation of glycerol pitch using concentrated H2SO4. The resulted catalyst is loaded with –OH, –SO3H, and –COOH functionalities. This carbon-based catalyst has shown tremendous potential for the conversion of glycerol to tetrahydropyranyl (THP) ethers and tetrahydropyranyl protection/deprotection of phenols and alcohols at ambient temperature. The catalyst is advantageous due to its easy synthesis, high yields, reusability, and operational simplicity [52].
Scheme 4.6 Glycerol etherification in the presence of isobutylene [51].
Goncalves et al. have utilized sulfonated carbon black for etherification of glycerol in the presence of TBA into MTBG, DTBG, and TTBG. The catalyst was obtained from the carbonization and sulfonation of coffee grounds (BCC). The oxygen and sulfur groups were effectively incorporated either with sulfuric acid labeled as BCC-S or with fuming sulfuric acid labeled as BCC-SF. The BCC-SF catalyst exhibits a higher amount of sulfur groups that are accountable for its high activity and stability as compare to BCC-S [53]. Table 4.3 summarizes the performance of different catalysts for etherification.
Carvalho and coworkers have utilized sulfonated carbon-based catalysts for glycerol etherification. The catalyst was synthesized by controlled pyrolysis of agroindustrial wastes such as sugar cane bagasse, coconut husk, and coffee grounds at 673 K under N2 flow. The pyrolyzed samples were functionalized with sulfuric acid. The catalysts were investigated for glycerol etherification with TBA in the liquid phase under the batch reactor. The glycerol conversion of about 80% with a selectivity of 21.3% was observed for the formation of DTBG and TTBG in a short reaction time of 4 h which was equivalent to commercially available resin and various catalysts reported in the literature [54].
4.4.2.6 Dehydration of Glycerol
Glycerol dehydration is a promising technique for converting glycerol into useful chemicals such as acrolein and hydroxyacetone. Acrolein is used as a reactant for producing acrylic acid, while hydroxyacetone is used for the manufacturing of propanediol. The process is carried out either in the gas phase or in the liquid phase. Figure