Handbook of Biomass Valorization for Industrial Applications. Группа авторов

       4.4.2.2 Esterification and Acetylation of Glycerol

      One of the potential technologies for the valorization of glycerol from the biodiesel industry is its esterification with acetic acid using a suitable homogeneous or heterogeneous acidic catalyst. The esterification reaction of glycerol mainly produces mono-, di-, and triacetate, also recognized as monoacetin (MA), diacetin (DA), and triacetin (TA) respectively). These acetins have a broad range of applications such as raw materials for the fabrication of tanning agents, polyesters, explosives, and use as solvents, food additives, plasticizers, softening, or emulsifying agents, in cryogenics, or pharmaceutical industry. Furthermore, acetins can be used as environmentally friendly fuel bio-additives. The mixture of DA and TA are useful for improving the viscosity and cold properties of fuel [26].

      Various homogeneous catalysts, for example, hydrofluoric acid, sulfuric acid, and para-toluene sulfonic acid manifest high activity and selectivity. However, these homogeneous catalysts are corrosive, lethal, and hard to separate from the products. Heterogeneous catalysts can conquer the limitations of the homogeneous catalyst due to their high recoverability and recyclability. Additionally, these catalysts show improved selectivity towards the products as compare to homogeneous catalysts. Some heterogeneous catalysts such as acid exchange resins, K-10 montmorillonite, HZSM-5, HUSY, PMo-NaUSY, niobium–zirconium mixed oxide catalysts, heteropolyacids loaded AC, and mesoporous silica has been proposed in pieces of literature [9, 12, 13]. Among various catalysts, cation-exchange resins have shown high activity and outstanding selectivity for higher esters.

Sanchez et al. have prepared a very active and stable porous carbon catalyst having acidic sites by sulfonation of carbonized sucrose using fuming sulfuric acid. The catalyst is selective towards the esterification of glycerol to acetylglycerols. Glycerol reacts with the acetic acid and leads to the formation of monoacetylglycerol (MAG) or monoacetin, di-acetylglycerol (DAG) or diacetin, tri-acetylglycerol (TAG), or triacetin with the removal of water as depicted in Scheme 4.2. The catalyst exhibits conversion above 99% and selectivity of approximately 50% towards the formation of triacetin through glycerol esterification with acetic acid [27].

      The hydrothermally prepared sulfonated carbon (SHTC) from glucose shows good activity for glycerol esterification with various carboxylic acids, i.e., acetic, caprylic, and butyric acids. The selectivity of the catalyst has been varied by changing the reaction condition. The SHTC shows one of the best selectivity for the formation of monoacetins without using excess glycerol. It was found that the catalyst gets deactivated after recycling due to the generation of sulfonated ester on the catalyst surface. The recycled catalyst was again activated by acid treatment with subsequent hydrolysis of sulfonated esters. The catalyst has excellent potential for various bio-refinery processes [28].

Alemany and coworkers have prepared carbonaceous material by pyrolysis of cellulose at 600 and 800 °C. The carbonaceous support was chemically activated with HNO₃ (3N) and impregnated with sulfonic groups through the impregnation method. The resulted catalyst named SO₃H-C is highly active and stable for acetalization reaction of glycerol with acetone in a batch and continuous flow reactor. The resulted catalyst leads to the formation of a five-member ring solketal as shown in Scheme 4.3 which is used as a fuel additive component. The catalyst shows the absolute transformation of glycerol with 100% selectivity in a continuous flow reactor. The activity of the catalyst was retained even after 300 min of continuous flow [29]. Table 4.3 summarizes the performance of different catalysts for esterification.

Scheme 4.2 Reaction scheme for the formation of actylglycerols [27].