Читать книгу Handbook of Biomass Valorization for Industrial Applications - Группа авторов - Страница 80

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].

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	TAG	100	100	[34]
9.	SO3H-glycerol-carbon	Glycerol	Acetylation	Acetic acid	TAG	100	100	[40]
10.	[PrSO3HN] [SO3CF3]/C	Glucose	Esterification	Acetic acid Lauric acid	TAG, MLG and DLG	–	–	[32]
11.	GBCC	Crude glycerol	Esterification	Acetic acid	DAG + TAG	99	88	[39]
12.	Sulfonated carbon	Catkins from willow	Esterification	Acetic acid	DAG	98.4	54.5	[38]
13.	CX	Glucose	Acetylation	Acetic acid	DA + TA	97	75	[26]
14.	Sulfonated Peanut Shell	Peanut shell,	Etherification	Isobutylene	DTBGs + TTBG	100	92.1	[51]
15.	SCC-S	Agroindustrial Wastes	Etherification	TBA	DTBG + TTBG	80	21.3	[54]
16.	Sulfonated carbon	Sugar	Etherification	tert-butanol	Mono and di-glyceryl	–	–	[33]
17.	BCC-SF	Coffee ground wastes	Etherification	TBA	MTBG	–	42	[53]
18.	Au/AC	Activated carbon	Oxidation	–	DIHA	72	18	[47]
19.	Au/MWCNT	Multi-walled carbon nanotubes	Oxidation	–	DIHA	93	60	[47]
20.	10%Pd/C	Activated carbon	Oxidation	–	Lactic acid	99	68	[48]
21.	5% Pt/C	Activated carbon	Oxidation	–	Lactic acid	99	74	[48]
22.	0.5%Cu–1.0% Pt/AC	Activated carbon	Oxidation	–	Lactic acid	80	69.3	[49]
23.	Pt/AC	Activated carbon	Oxidation	–	Lactic acid	100	69.3	[50]
24.	10%HSiW/AC	Activated carbon	dehydration	–	Acrolein	92.6	75.1	[56]

Scheme 4.4 Steps for the formation of TAG using SO₃H-C derived from glycerol [30].

Malaika and co-workers have prepared SO₃H-modified carbon xerogels and spheres using glucose as a carbon source. These catalysts were explored for acetylation of glycerol and the optimum catalyst shows 75% selectivity for both diacetin (DA) and triacetin (TA), and almost complete glycerol conversion (About 97%). The results are comparable with commercially available ion exchange resin (Amberlyst 15) [26].

Ellis and coworkers have investigated the use of sulfonated carbon catalysts for transesterification and esterification reactions of glycerol. The active catalyst was synthesized by pyrolysis and sulfonation of the sugar char in a tube furnace. The reaction of glycerol with tert-butanol over sulfonated carbon leads to the formation of mono-glyceryl ethers isomers and di-glyceryl ether isomers [33].

Khayoon et al. have explored the potential of sulfated activated carbon catalysts (AC-SA5) for the transformation of glycerol into oxygenated fuel additives by glycerol acetylation. The AC-SA5 was prepared by functionalization of activated carbon with sulfuric acid by hydrothermal method. The catalyst shows 92% of the glycerol conversion into mono, di, and triacetyl glyceride with 38, 28, and 34% selectivity, respectively in a batch run. The catalyst shows good stability up to four consecutive steps [34].

Figure 4.7 Selective glycerol esterification with acetic acid and lauric acid over [PrSO₃HN][SO₃CF₃]/C nanorods [32].

The glycerol transformation to monoacetin, diacetin, and triacetin over heteropolyacids modified activated carbon was investigated by Castanheiro and co-workers. The activity of the catalyst enhances with an increase in the loading of dodecatungstophosphoric acid (PW) on the surface of activated carbon. The maximum catalytic activity was observed for the catalyst with 4.9% loading. On further increasing the loading the activity decreases due to the blockage in the pores of activated carbon. The catalyst is stable up to three consecutive batch runs [35]. Wang et al. have explored the benefits of sulfonated hollow sphere carbon for acetylation of glycerol. Sulfonic groups modified hollow sphere carbon (HSC-SO₃H) has been prepared by carbonization of SiO₂ core–shell polymer which was followed by elimination of the SiO₂ core and functionalization with chlorosulfonic acid. The catalyst exhibits better activity for glycerol acetylation owing to the microporous structure which allows fast mass transfer during the reaction [36]. Rice husk-derived sulfonated carbon has been used as a potential catalyst for etherification and esterification of glycerol. The catalyst was synthesized by carbonization of rice husk followed by treatment with H₂SO₄. Glycerol esterification with acetic acid exhibits about 90% transformation to mono-, di-, and triglycerides with the selectivity of 11, 52, and 37% respectively after 5 h of reaction. The glycerol etherification with tert-butyl alcohol (TBA) shows a 53% conversion to di and tri tert-butylglycerol with 25% selectivity. The key role for promoting the activity of the catalyst was played by the Bronsted acidic sites and hydrophilicity which helps in preventing the catalyst deactivation [37]. The catalyst activity for various glycerol conversions depends upon the preparation methods. The effect of sulfonation on biomass-derived carbon catalyst was studied by Tao et al. [38]. The active catalyst was synthesized by sulfonation of carbonized catkins from the willow plant using different sulfonation conditions. It was found that the sulfonation conditions affect the density of acidic sites. The resulted catalyst exhibits almost complete transformation of glycerol in the presence of acetic acid into MAG, DAG, and TAG in 2 h at 393K. Compared to a similar kind of catalyst reported in the literature, this catalyst exhibits superior heat stability, recyclability, and water tolerance. Crude glycerol from biodiesel industries has been utilized for the conversion of glycerol into valuable products as well as for catalyst preparation. Hameed and coworkers have prepared solid acid catalyst by carbonization and sulfonation of biodiesel-derived crude glycerol. The catalyst shows almost complete conversion (99%) of glycerol with acetic acid into oxygenated fuel additives (DAG and TAG) and MAG. The catalyst is stable and can be used for up to seven cycles without appreciable loss in its performance. The catalyst has the potential for industrial conversion of crude glycerol at ambient reaction conditions [39]. Similarly, Karnjanakom and coworkers have prepared a sulfonated carbon-based catalyst by in situ carbonization and sulfonation. The ultrasound-assisted glycerol acetylation with acetic acid over this catalyst exhibits 100% selectivity towards the formation of TAG. The catalyst is highly stable and can be used effectively for ten repeated cycles. The presence of acidic sites along with ultrasound radiation contributes towards 100% selectivity for TAG [40].