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 4.8 depicts that the presence of a suitable acidic catalyst is necessary for the reaction. Several reports have shown that solid acid catalysts exhibit a higher selectivity towards acrolein; however, catalysts deactivation is the main problem [55]. Deactivation study indicates that polycondensed and cyclic C6+ generated due to reaction of glycerol with acrolein deposited inside the pores and block the active acidic sites. Therefore, the growth of a highly efficient and stable heterogeneous catalyst for the long-term stable transformation of glycerol to acrolein is still required.
Figure 4.8 Glycerol dehydration in the presence of acidic catalyst [55].
Very few studies have been reported on the carbon-based catalyst for glycerol dehydration. Lili et al. have utilized activated carbon-supported silicotungstic acid catalyst for the glycerol dehydration into acrolein. Activated carbon was selected as support due to its high surface area, superior stability over a wide pH range, and strong interaction with acidic silicotungstic material. The activity of the catalyst depends upon the loading of silicotungstic acid, its dispersion, and the relative amount of acidic sites. The catalyst with 10% loading showed the highest activity and selectivity [56].
4.4.2.7 Cyclization
N-heterocyclic carbene–silica nanoparticles have been used for glycerol cyclization into cyclic acetals. The catalyst was prepared by immobilization of 1-Butylimidazole onto rice husk ash by 3-chloropropyltriethoxysilane (CPTES). The chlorine of the catalyst was replaced by the phosphate and sulfate group, and the resulting catalysts were named RHABIm-H2PO4 and RHABIm-HSO4 respectively. The RHABIm-H2PO4 catalyst show superior catalytic efficiency and selectivity as compared to RHABIm-HSO4 owing to the presence of extra free acidic proton. The cyclization reaction followed a pseudo-first-order rate low [57].
Conclusion
The utilization of glycerol to valuable products has attracted the attention of researchers because glycerol is produced in large quantities as waste during biodiesel production. The effective utilization of this waste will be an important factor that can reduce the cost of biodiesel and promote its commercialization. Since glycerol is a biomass-derived chemical feedstock, research efforts are being made globally for its transformation into marketable chemicals which can be employed as substitutes for chemicals derived from fossil resources. A catalytic route is an efficient approach for its transformation into valuable chemicals.
This chapter explained many catalytic processes for the conversion of glycerol into useful chemicals using carbon-based materials as a catalyst. Carbon-based catalysts have shown the potential to exhibit high glycerol conversions with quite a good selectivity. However, only a small number of studies have been conducted using carbon-based catalysts, and reaction conditions have not yet
