Читать книгу Handbook of Biomass Valorization for Industrial Applications - Группа авторов - Страница 83

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].