Handbook of Biomass Valorization for Industrial Applications. Группа авторов

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].

Scheme 4.3 Reaction scheme for glycerol acetalization with acetone [29].

Similarly, Chandrakala et al. have used sulfonic acid-modified heterogeneous carbon for the conversion of glycerol into TAG, a biofuel additive in two-step processes. The catalyst was synthesized by controlled carbonization and sulfonation of glycerol at 220 °C. The complete reaction scheme is shown in Scheme 4.4. The first step involves the esterification of glycerol in the presence of acetic acid and sulfonic acid functionalized heterogeneous carbon catalyst which was followed by acetylation of glycerol ester mixture with acetic anhydride using the same catalyst. The catalyst is reusable for up to five cycles without appreciable loss in its activity. The complete process is economically effective because of a highly stable and reusable carbon catalyst [30]. Higher esters (di- and tri-esters) of glycerol have a high boiling point, good miscibility with traditional fuel, and high octane and cetane numbers, so they are chosen as fuel components [31].

Sun et al. have explored the potential of rod-like carbon-based sulfonic acid-modified ionic liquids for selective glycerol esterification with the help of acetic acid or lauric acid into valuable products. The formation of catalyst takes place in two steps. The first step involves the hydrothermal treatment of glucose and cyanamide at 160 °C. In the second step, the hydrothermally synthesized nitrogen-enriched carbon nanorods undergo quaternary ammonization in the presence of 1,3-propanesultone and anion substitution with HSO₃CF₃. The final catalyst is labeled as [PrSO₃HN][SO₃CF₃]/C nanorods. The performance of the catalyst was evaluated for selective glycerol esterification using acetic acid and lauric acid. Glycerol in the presence of acetic acid leads to the formation of TAG and with lauric acid, MAG, and DAG was formed as shown in Figure 4.7. The catalyst [PrSO₃HN] [SO₃CF₃]/C nanorods show superior catalytic performance as compared to propylsulfonic acid-modified SBA-15, Amberlyst-15, p-toluenesulfonic acid, and [PrSO₃HN][SO₃CF₃] functionalized carbonaceous framework [32].

Table 4.3 Performance of different carbon catalysts for different valorization processes.

S. N.	Name of catalyst	Source of catalyst	Type of process	Reactant	Glycerol derivatives	Conversion of glycerol (%)	Selectivity (%)	Ref.
1.	TC-L carbon	Rice husk	Esterification	Acetic acid	DAG + TAG	90	90	[37]
2.	TC-L carbon	Rice husk	Etherification	TBA	DTBG + TTBG	53	25	[37]
3.	TAC-673	Sucrose	Esterification	Acetic acid	TAG	<99	50	[27]
4.	AC-SA5	Activated carbon	Acetylation	Acetic acid	DAG + TAG	91	62	[34]
5.	PW2-AC	Activated Carbon	Esterification	Acetic acid	Diacetin	86	63	[35]
6.	SHTC	D-glucose	Esterification	Acetic acid	Mono acetin	70	89	[28]
7.	SO3H-C,	Cellulose	Acetalization	Acetone	Solketal	80	100	[29]
8.	SO3H-carbon	Glycerol	Esterification Acetylation	Acetic acid, Acid anhydride