At the beginning of the twentieth century, biodiesel (BD) has emerged as an alternative fuel to combat the issues related to climate change, air pollution, energy demand and energy security. Ever increasing BD production also lead to generate enormous amount of crude glycerol (CGL) which would be approximately one tenth of BD produced. Disposal and purification of this excess CGL may pose serious threat to the environment and may cause water as well as soil pollution (Mufrodi et al 2020; Testa et al. 2013). Therefore, for the past few years, researchers are exploring various methods for efficient utilization of CGL (Okoye et al. 2017). The CGL possesses a meagre economic value because of the impurities present (methanol, catalyst, BD, trig-, di-, and mono-glycerides). Moreover, purification of CGL is quite expensive and may generate huge amount effluents, which is challenging to dispose of and may cause the environmental pollution (Chozhavendhan et al. 2020). One of the most suitable application of this CGL could be as precursor for the synthesis of fine chemicals for nonedible application. Glycerol (GL) esterification with acetic acid in presence of acidic acid leads to the formation of acetins viz., monoacetin (MA), diacetin (DA), and triacetin (TA) (Nda-Umar et al. 2020) as shown in scheme 1. TA has a variety of industrial application in pharmaceuticals, cosmetics, polymers, cryogenics, and fuels as an anti-knock additive for gasoline, and cold flow and viscosity improver additive for BD (Testa et al. 2019).
For TA synthesis most popular approach is homogeneous Brønsted acid (H2SO4, H3PO4, HClO4 etc.) catalysed GL esterification, with acetic acid (AcA), which is a low cost eco-friendly and non-hazardous chemical in comparison to acetic anhydride (AcAn) (Nda-Umar et al. 2020). Homogeneous Acid catalysts demonstrate better reactivity and product selectivity, however, they are highly corrosive, needed neutralization and removal from the reaction mixture, and are non-reusable. Thus, effluents generated during the product washing, to remove the catalyst, need to be disposed of safely to avoid any kind of environmental pollution. To simplify the issues associated with homogeneous catalysts, many researchers, in recent past, have paid additional attention in developing heterogeneous acidic catalysts. Such catalysts are easy to separate from the reaction mixture, stable and reusable (Testa et al. 2019). In case of TA synthesis, the product selectivity was found to be a function of acidic strength of the catalyst, AcA/GL molar ratio and reaction duration. The Brønsted acidic sites (e.g. PO43−, SO42− etc.) in reported solid catalysts have been frequently incorporated onto the silica surface (Maria et al. 2019; Kulkarni et al. 2020). To facilitate the magnetic separation of the catalysts, Fe3O4 magnetic nano particles (MNPs) have been embedded within the silica shell (Caon et al. 2020). Magnetic catalysts could be removed from the reaction mixture under the influence of an external magnetic field and thus overcomes the problems of frequently applied and time consuming centrifugation and filtration procedures. Additionally, due to the nano size of catalysts, large surface area facilitates the more exposure the of active sites to impart the high catalytic activity (Ajala et al. 2020). A variety of matrix (metal oxide, silica, ion exchange resins, carbon, MNPs, and zeolite) functionalized with acidic groups (PO43− or SO42− etc.) have been employed for TA synthesis via GL esterification and reactivity of a few of them are summarized in Table 1.
Table 1
Activity of the literatures reported heterogeneous acidic catalysts for the GL esterification with AcA.
Catalyst
|
wt%
(w.r.t. GL)
|
AcA/GL
(mol/mol)
|
Temp (oC)
|
Time (h)
|
MA %
|
DA %
|
TA %
|
GLC conversion %
|
Catalyst reusability
(# of cycles)
|
Reference
|
Zr(SO4)2
|
5.0
|
3:1
|
105
|
1.0
|
-
|
-
|
-
|
40.0
|
NR
|
Maria et al. 2019
|
Zr3(PO4)4
|
5.0
|
3:1
|
105
|
1.0
|
-
|
-
|
-
|
81.0
|
NR
|
Maria et al. 2019
|
Zr4(PO4)2(SO4)5
|
5.0
|
3:1
|
105
|
1.0
|
15.0
|
48.0
|
39.0
|
100
|
5
|
Maria et al. 2019
|
SiO2-H3PO4
|
5.0
|
9:1
|
100
|
4.0
|
-
|
-
|
22.84
|
100
|
NR
|
Manurung et al. 2020
|
SO42−@SiO2@Fe3O4
|
5
|
6:1
|
80
|
0.6
|
-
|
-
|
100
|
100
|
6
|
Abida et al. 2012
|
Fe4SiW12O40
|
6.0
|
3:1
|
60
|
8.0
|
24.0
|
69
|
7
|
100
|
NR
|
Silva et al 2016
|
Fe-Sn-Ti/SO42−
|
5
|
-
|
80
|
0.5
|
-
|
-
|
99
|
100
|
4
|
Sun et al. 2016
|
PO43−SiO2
|
2.0
|
6:1
|
110
|
5.0
|
20.0
|
50.0
|
30.0
|
100
|
NR
|
Ghoreishi et al. 2014
|
SO42−-SiO2
|
2.0
|
6:1
|
110
|
5.0
|
40.0
|
50.0
|
10.0
|
100
|
NR
|
Ghoreishi et al. 2014
|
MCM-41-10Ru10Cu
|
8.0
|
10:1
|
120
|
5.0
|
2.4
|
45.2
|
50.4
|
99.1
|
NR
|
Ramalingam et al.2020
|
Amberlyst-45
|
7
|
3:1
|
70
|
7
|
68.9
|
29.8
|
1.3
|
75.3
|
NR
|
Jiang et al. 2019
|
Amberlyst-36P
|
-
|
7:1
|
120
|
-
|
30
|
50
|
17
|
100
|
NR
|
Aghbashlo et al. 2019
|
S-Glycerol
|
-
|
6:1
|
110
|
2
|
21.0
|
56.0
|
23.0
|
97.0
|
6
|
Malaika et al. 2021
|
MS-Carbon
|
4.5
|
10.4:1
|
126
|
3
|
4.9
|
27.8
|
66.5
|
97.0
|
5
|
Nda-Umar et al. 2021
|
ZSM-5/MCM-41
|
1.5
|
9:1
|
125
|
24
|
55.9
|
23.4
|
20.7
|
92.6
|
NR
|
Liu at al. 2019
|
HZSM-5
|
2.65
|
9:1
|
110
|
4.5
|
-
|
25.7
|
7.7
|
85.6
|
NR
|
Zhou et al. 2013
|
w.r.t. = with respect to, GL = glycerol, AcA = acetic acid, temp = reaction temperature, MA = monoacetin, DA = diacetin, TA = triacetin, GLC = glycerol conversion, h = hour, NR = not reported, MS = mesoporous silica, S = sulfur, and P = pressure
Recently, Zr(SO4)2 and Zr3(PO4)4 catalysts have been utilized during GL esterification to obtain up to 81 % GL conversion levels on employing 3:1 AcA/GL molar ratio and 3 wt% catalyst at 105°C reaction temperature (Maria et al. 2019). The catalyst with phosphate group was found to show better activity compared to the one with sulphate group. Testa et al. anchored both sulphate and phosphate over the zirconia surface to obtain the Zr4(PO4)2(SO4)5 catalyst for the GL esterification with AcA. However, TA selectivity (39 %) was not very promising while maintaining 100 % GL conversion at 105°C within 1 h of reaction duration (Maria et al. 2019). This catalyst was reused 5 times under optimized reaction conditions; however, a decline in catalyst activity and TA/DA selectivity was observed even during the 2nd cycle. In most cases, the catalyst deactivation was associated either with the active site blockage or by the leaching of active components, such as sulphate group, from the catalyst surface (Rane et al. 2016; Yang et al. 1997). Recently, our group has prepared the magnetic catalyst by attaching the sulphate group over the silica coated magnetic (SiO2@Fe3O4) nanoparticles to prepare SO42−@SiO2@Fe3O4 catalyst. The catalyst demonstrated the excellent TA selectivity (100 %) as well GL conversion (100 %) levels, isolated under the influence of external magnetic field from the reaction mixture and reused during six consecutive reaction cycles (Abida et al. 2020).
The literature review shows that most of the heterogeneous acidic catalysts, employed in GL esterification, needed a high reaction temperature (up to 120°C) and longer reaction duration (up to 24 h) to yield high GL conversion level and higher TA selectivity. In most of the cases, the low TA selectivity and poor catalyst reusability remain issues which need to be addressed. Moreover, to the best of our knowledge, phosphate based magnetic heterogeneous catalysts have not yet been explored for the GL esterification to obtain TA. Therefore, the present study aims to develop phosphate group containing magnetic catalyst for the GL esterification to produce TA.