Catalytic Role of Solid Acid Catalysts in Glycerol Acetylation for the Production of Bio-additive: A Review

Abstract

Bio-additives obtained from the acetylation of biodiesel-derived glycerol have been extensively synthesized because of their nature as value-added products and their contribution to environmental sustainability. Glycerol acetylation with acetic acid produces commercially important fuel additives. Considering that the recovery of individual monoacetin, diacetin (DA), and triacetin (TA) is complicated, many endeavours have enhanced the selectivity and total conversion of glycerol using acetic acid during catalytic acetylation. In this work, we extensively review the catalytic activity of different heterogeneous acid catalysts and their important roles in glycerol acetylation and product selectivity. In addition, the most influential operating conditions to attain high yields of combined DA and TA are achieved by closely examining the process. This review also highlights the prospective market, research gaps, and future direction of catalytic glycerol acetylation.

Full text

Catalytic role of solid acid catalysts in glycerol
acetylation for the production of bio-additives:
a review
Pei San Kong,
ac
Mohamed Kheireddine Aroua,*
a
Wan Mohd Ashri Wan Daud,
a
Hwei Voon Lee,
b
Patrick Cognet
c
and Yolande P´
er`
es
c
Bio-additives obtained from the acetylation of biodiesel-derived glycerol have been extensively synthesized
because of their nature as value-added products and their contribution to environmental sustainability.
Glycerol acetylation with acetic acid produces commercially important fuel additives. Considering that the
recovery of individual monoacetin, diacetin (DA), and triacetin (TA) is complicated, many endeavours have
enhanced the selectivity and total conversion of glycerol using acetic acid during catalytic acetylation. In this
work, we extensively review the catalytic activity of different heterogeneous acid catalysts and their important
roles in glycerol acetylation and product selectivity. In addition, the most influential operating conditions to
attain high yields of combined DA and TA are achieved by closely examining the process. This review also
highlights the prospective market, research gaps, and future direction of catalytic glycerol acetylation.
1. Introduction
Catalytic acetylation of glycerol using acetylating agents, such
as acetic acid and acetic anhydride, has been extensively
investigated recently, which is driven by the intention to search
for new economic applications of glycerol. Acetic acid is nor-
mally used as an acetylating agent in producing biofuel addi-
tives via acetylation of glycerol; the use of acetic acid is
attributed to the its lower price (0.5 USD per kg) compared with
that of acetic anhydride (about 0.98 USD per kg).
1
S. Sandesh
et al. reported that the acetylation of glycerol with acetic anhy-
dride required a lower temperature (30 C) than glycerol acety-
lation with acetic acid (85 C).
2
Despite the fact that glycerol
acetylation with acetic anhydride can be conducted at room
temperature with low energy utilization, the high potential of
Pei San Kong is a joint-PhD
candidate of University of
Malaya (Malaysia) and INP-
ENSIACET, University of Tou-
louse (France). She holds a bach-
elor degree in Chemical
Engineering and a Master of
Engineering Science. She previ-
ously worked as an R&D engineer
in a Malaysia oleochemical
industry and mainly involved in
catalysis, process and technology
development. Her research inter-
ests include catalysis (heterogeneous/homogeneous acid catalysts),
Process engineering (reaction engineering, microwave processing),
Energy & Fuels (biodiesel, biolubricant, biofuels) and oleochemical
product developments (polyol esters, glycerol derivatives).
Dr Mohamed Kheireddine Aroua
is a senior Professor at the
Department of Chemical Engi-
neering and the Deputy Dean at
the Institute of Graduate
Studies, University of Malaya,
Malaysia. He is also heading the
Center for Separation Science
and Technology (CSST). His
research interests include CO
2
capture, membrane processes,
electrochemical processes using
activated carbon, biodiesel
production and conversion of bioglycerol to value added chem-
icals. He published more than 130 papers in ISI ranked journals
with more than 3500 citations. His h-index is 30.
a
Department of Chemical Engineering, Faculty of Engineering, University of Malaya,
50603 Kuala Lumpur, Malaysia. E-mail: mk_aroua@um.edu.my; Fax: +60
379675319; Tel: +60 379674615
b
Nanotechnology and Catalysis Research Centre (NANOCAT), University of Malaya,
Level 3, IPS Building, 50603 Kuala Lumpur, Malaysia
c
Laboratoire de G´
enie Chimique (Lab`
ege), BP84234 Campus INP-ENSIACET, 4 all´
ee
Emile Monso, 31432 Toulouse Cedex 4, France
Cite this: RSC Adv.,2016,6, 68885
Received 25th April 2016
Accepted 23rd June 2016
DOI: 10.1039/c6ra10686b
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explosion for acetic anhydride is not suitable for manufacturing
especially for large-scale production (as above 49 C explosive
vapour/air mixtures may be formed).
On the contrary, the sudden decline in crude oil prices has
signicantly reduced the prices of biodiesel during the second
half of 2014. Fig. 1(a) shows the biodiesel prices declined
strongly from 112 USD per hL (2013) to less than 80 USD per hL
(2014); the ten year forecast for biodiesel prices are expected to
recover in nominal terms close to those in 2014 level (prices vary
from 85–90 USD per hL). Fig. 1(b) indicates that the global
biodiesel production is expected to reach almost 39 billion liters
by 2024; moreover, the projected production volume of bio-
diesel is stable and is mostly policy driven.
3
Nevertheless,
conversion of biodiesel-derived glycerol into value-added
products is necessary to support long-term growth of the oleo-
chemical market. The price reported for 80% pure crude glyc-
erol is $0.24 kg
1
and that for United States Pharmacopeia
(USP)-grade glycerol is $0.9 kg
1
in mid-2014.
4
Various studies on transforming glycerol into different value-
added derivatives, such as propylene glycerol, polyglycerols,
succinic acid, gaseous hydrogen, glycerol carbonate, acrolein,
fuel additives, ethanol, glycerol esters, and lubricant additive,
were conducted.
5–12
This review aims to study the role of
heterogeneous acid catalysts in glycerol acetylation using acetic
acid given that the low selectivity of the desired products
(diacetin (DA) and triacetin (TA)) remains the greatest challenge
in catalytic acetylation. In addition, recovery of key derivatives is
a very complicated work because the mono-, di-, and tri-
substituted derivatives that constitute a mixture exhibit indis-
tinguishable boiling points.
1
This review then focuses on the
important features of solid acid catalysts and on the inuence
of operating parameters in enhancing the product selectivity of
glycerol acetylation. To our best knowledge, this work is the rst
critical review focusing on the important role of acid hetero-
geneous catalysts in producing DA and TA as bio-additives.
2. Glycerol derived bio-additives
Glycerol-based additives is suitable for efficient engine perfor-
mance and is environment friendly.
13
Table 1 shows four
Dr Wan Mohd Ashri Wan Daud
is a Professor at the department
of Chemical Engineering
University Malaya since 2008.
His research interests include
activated carbon, pyrolysis
process, second generation bio-
diesel and hydrogen production.
He has published more than 160
ISI papers that have garnered
more than 3000 citations. His h-
index is 27.
Dr Patrick Cognet received his
Chemical Engineering Diploma
from ENSIC (Ecole Nationale
Sup´
erieure des Industries Chi-
miques de Nancy) and PhD
(Electrochemical Engineering) at
Chemical Engineering Labora-
tory, Toulouse. He joins ENSIA-
CET (University of Toulouse) as
Assistant Professor in 1994 and
is a Professor since 2010. His
work is focused on Green Process
Engineering, more precisely on
reactor design, activation techniques (ultrasound, electrochem-
istry), intensication and processes involving new media. He
created the Green Process Engineering Congress (GPE) in 2007.
Dr Hwei Voon Lee is a senior
lecturer at Nanotechnology and
Catalysis Research Center
(NANOCAT), University of
Malaya, Malaysia. She received
her PhD in Catalysis (2013) and
BSc (Hons) in Industrial Chem-
istry (2008) from Universiti
Putra Malaysia. Her major
research interests are Energy &
Fuels (biodiesel, renewable
diesel, biofuels); Biomass
Conversion Technology (cata-
lytic conversion of biomass); Oleochemical Technology (methyl
ester, polyol ester), Catalysis (heterogeneous catalyst, mixed metal
oxides, acid–base catalyst) and Nano-Materials (biomass derived
nanocrystalline cellulose and application).
Dr Yolande P´
er`
es received her
PhD in coordination chemistry
in 1985 from the University Paul
Sabatier (Toulouse, France). She
joined the research group of
Professor Hoberg at the Max-
Planck-Institut f¨
ur Kohlenfor-
schung (M¨
ulheim an der Ruhr,
Germany) as a postdoctoral
fellow in 1985. Her research is
focused on the transformation of
CO
2
and olens to carboxylic
acids catalyzed by transition
metal complexes. Aer one year in industry, she obtained a posi-
tion as assistant professor at ENSIACET (University of Toulouse).
At present she carries out her research at the Chemical Engineering
Laboratory on the topics of catalytic and phytoextraction process.
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important glycerol-derived bio-additives formed by different
reactants via esterication or acetylation routes. Monoacetin
(MA), DA, and TA are produced via glycerol acetylation using
acetic acid. The specic industrial applications of MA, DA, and
TA are summarized. Given that MA is water soluble, DA and TA
are the preferred components for bio-additives. MA is proven to
be completely soluble in water. By contrast, TA is completely
soluble in ethyl acetate.
14
DA and TA are utilized as fuel bio-
additives because they effectively improve cold and viscosity
properties, they enhance octane rating, and they can reduce fuel
cloud point. Furthermore, TA and DA are alternative for tertiary
alkyl ether that causes greenhouse gas emissions.
Glycerol tertiary butyl ether (GTBE) is another potential bio-
additive and can be produced via glycerol etherication using
Fig. 1 (a) Evolution of biodiesel world prices. (b) Development of the world biodiesel market (OECD 2015 market report).
Table 1 General product applications of glycerol derived bio-additives
Bio-additives Starting materials Product applications Ref.
(i) MA Acetic acid; glycerol Excellent solvency 22 and 23
Plasticizer for cellulose acetate, nitrocellulose, ethyl
cellulose and vinylidene polymers
(ii) DA Plasticizer for cellulosic polymers and cigarette lter 13 and 24
Raw material for the production of biodegradable polyesters
(iii) TA As an antiknock additive for gasoline 25
Improve cold ow and viscosity properties of biodiesel
(i) GTBE, GDBE Isobutylene (gas phase);
glycerol or tert-butyl alcohol
(liquid phase); glycerol
Used in diesel and biodiesel reformulation 16 and 17
Oxygenated additives for diesel fuel
Decreasing cloud point of biodiesel fuel
To reduce fumes, particulate matters, carbon oxides and
carbonyl compounds in exhausts
(i) Di-, tri-GEE Ethanol; glycerol (i) Used for fuel formulation 18
(ii) Mono-GEE (ii) Important intermediate for various chemicals
Polyglycerols Glycerol Excellent lubricity and used as additive in lubricant 20 and 21
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gas-phase isobutylene or via glycerol etherication using tert-
butyl alcohol (TBA). The operation cost for gas-phase isobutylene
is higher than that for liquid TBA.
15
Isobutylene is obtained from
reneries and thus requires additional purication steps to
separate isobutylene from C4 mixtures through sulfuric acid
extraction or molecular sieve separation. On the contrary, TBA is
a biomass-derived compound and is easier to handle during
manufacture. Thus, high fraction of glycerol di-tert butyl ether
(GDBE) is a reliable oxygenated additive for diesel and biodiesel
fuels.
16,17
In addition, a fuel with 10–25 v/v% of oxygenate reduces
particulate emission.
13
Similar to DA and TA, GDBE and GTBE are
desired components over glycerol mono-tert butyl ether (GMBE)
given the low solubility of GMBE in diesel.
Exploitation of sugar-fermented bioethanol for glycerol ethyl
ether (GEE) synthesis facilitates production of a completely
biomass-derived product.
18
Bioethanol is formerly used as bio-
fuel in gasoline engines. However, di- and tri-GEE derivatives
are more suitable for diesel and biodiesel formulation because
they are water insoluble. Moreover, mono-GEE (dioxolane),
a water-soluble secondary compound, is an important inter-
mediate in producing various chemicals.
19
Oligomer diglycerols
are synthesized from single-molecule glycerol via etherication.
Polyglycerols, such as diglycerols, are also widely applied in the
pharmaceutical, microbiology, food, and automotive indus-
tries,
20
as well as used as additives in lubricants.
21
3. Mechanism acetylation of glycerol
Fig. 2 illustrates the reaction mechanism in MA, DA, and TA
production. MA is rst produced via glycerol acetylation using
acetic acid. DA is then synthesized by reacting MA with acetic
acid followed by reacting DA with acetic acid to produce TA.
Water is produced as by-product during glycerol acetylation.
3.1 Market and demand
China market analysis report revealed that the price of TA
remains rigid despite the speculated weakening of the market
of TA.
26
Fig. 3 demonstrates TA constitutes 10% of the world-
wide glycerol market among other uses, which is redrawn on the
basis of a previous study.
27,28
The current global demand for TA
is approximately 110 000 T per annum, whereas 35% of demand
comes from China. The TA production capacity of China is
approximately 55 000 T per annum, 38 500 T of which is
intended for domestic consumption and 16 500 T is exported.
The price of TA ranges from RM4273–5560 per ton (equivalent
to 1097–1428 USD per ton). In addition, the demand for TA is
recently growing by 5–10% yearly. The demand for TA is ex-
pected to remain strong.
29
Dr Kongkrapan Intarajang, Group
Chief Executive Officer of Emery Oleochemicals, mentioned in
2012 that the increase in demand for plastic has led to a steady
annual growth of 4–5% in plastic additive consumption
worldwide.
30
3.2 Conventional TA production method
The main manufacturers of TA include Croda, BASF (Cognis),
Daicel, Lanxess, ReactChem, Yixing YongJia Chemical,
YunnanHuanteng,Klkoleo,and numerous manufacturers
from China. The conventional TA manufacturing process
involves two steps.
23
Fig. 4 shows the diagram of the
conventional TA production. First, glycerol is esteried using
acetic acid in the absence of catalyst, where conversion into
MA occurs. Water is formed and is removed using an azeo-
tropic distillation system. Second, the produced MAs are
further esteried using acetic anhydride under exothermic
conditions; TA and acetic acid are formed in this step. Acetic
acid is simultaneously recycled as reactant into the rst
reactor.
31
Table 2 shows the product specication of food-
grade TA.
4. Mechanism of Brønsted and Lewis
acid-catalyzed esterification
4.1 General mechanism of glycerol acetylation
Glycerol esterication using acetic anhydride or acetic acid to
produce MA, DA, and TA can be extensively explained in the
presence of three hydroxyl groups (–OH) that are attached to the
glycerol backbone. Acetic acid will selectively attach to any –OH
of glycerol or any –OH from partially reacted glycerides; this
phenomenon is related to the steric hindrance effect. Thus, the
produced MA and DA normally present isomer forms depend-
ing on the position of acetylation in the glycerol molecule
(Fig. 2).
13
Among the obtained products, DA and TA are the most
interesting products that can be applied as fuel additive. MA
is an unfavorable product owing to its relatively high solu-
bility in water. However, direct transformation of the highly
selective DA and TA is impossible as the reaction involves
aseriesofconsecutiveesterication steps, forming various
intermediates (glycerides); moreover, each step is driven by
chemical equilibrium because of the formation of water as by-
product.
33
The selectivity of MA, DA, and TA also depends
mostly on the catalyst features (surface acidity, pore structure,
and catalyst stability)
34
and esterication parameters (glycerol
to acetic acid molar ratio, temperature, catalyst amount, and
reaction time).
35
Furthermore, the acid-catalyzed glycerol
acetylation involves two plausible reaction mechanisms
based on the types of acid catalyst used: (i) Bronsted acid-
catalyzed esterication and (ii) Lewis acid-catalyzed
esterication.
4.2 Brønsted acid-catalyzed esterication
The Brønsted acid-catalyzed esterication is also named as
Fischer esterication. Fig. 5 shows a conventional reaction
mechanism of the esterication reaction. This reaction mech-
anism involves addition of nucleophile (the glycerol) into acetic
acid followed by an elimination step, as follows:
36
(i) The acetic acid is initially protonated by the Brønsted-type
acid catalyst.
(ii) In the second step, the oxygen atom (two lone pairs) from
the –OH of glycerol acts as a nucleophile and attaches to the sp
2
carbon, leading to the loss of proton from the –OH.
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(iii) A series of fast equilibrium proton exchanges occurs in
either of the –OH of acetic acid. In this step, a new ester bond
forms between the carboxyl group carbon and the oxygen in
glycerol.
(iv) Water is then eliminated in either site.
(v) In the nal step, the excess proton leaves, regenerating
a Brønsted acid catalyst.
(vi) This process continues until all three strands of the
glycerol backbone are converted into esters.
4.3 Lewis acid-catalyzed esterication
Theoretically, Lewis acid-based esterication involves a reaction
mechanism similar to that in Brønsted acid-based reaction. In
addition, Lewis acid-based esterication involves the attack of
glycerol in a nucleophilic addition reaction. A slight difference
between these two processes is that the Brønsted-catalyzed
reaction uses a proton generated from the acid catalyst. By
contrast, the Lewis-based reaction involves a metal cation (Mn
+
)
Fig. 2 Reaction mechanism of glycerol acetylation synthesis into MA, DA and TA.
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as an electrophile to facilitate the interaction between the
carbonyl oxygen from acetic acid and the Lewis acidic site (L
+
)of
the catalyst to form carbocation. The nucleophile from glycerol
attacks the carbon cation and produces tetrahedral intermedi-
ates (Fig. 6). During esterication, the tetrahedral intermediate
eliminates water molecule to form an ester product.
37
5. Heterogeneous acid catalysts for
glycerol acetylation
Solid acid catalysts play a crucial role in esterication reaction
during ester production. In particular, solid acids have largely
replaced the traditional homogeneous acid catalyst because of
environmental, technological, and economic reasons. A variety
of solid acid catalysts have been studied for glycerol acetylation.
Their catalytic efficiency are also categorized into different
groups (Table 3).
38
The key role of solid acid catalyst in high rate
of glycerol conversion and selective formation of DA and TA
products include: (i) acidity of catalyst (especially the Brønsted
acid sites), (ii) texture, and (iii) surface morphology.
Although many studies have demonstrated the high reac-
tivity of glycerol acetylation, most catalysts exhibit low thermal
stability and unsatisfactory selectivity to DA and TA produc-
tion.
39
Furthermore, the hydrophilic character of catalyst
surface is a challenge in active site deactivation resulting from
the inevitable water formation during esterication, leading to
leaching of active components into the reaction medium. The
water-tolerant property of solid acid catalyst exhibiting
a hydrophobic-enhanced surface is thus necessary to excellently
perform glycerol acetylation. Another reason of catalyst deacti-
vation is the partial blockage of the catalyst's active sites by the
reaction medium, such as glycerol and/or partial glycerides
within the pore structure of catalysts, thereby reducing the
number of acid sites for continuous esterication until the
desirable end-products are achieved.
40
5.1 Ion exchange catalyst
Rezayat et al. synthesized MA and TA under supercritical carbon
dioxide (CO
2
) conditions by using the commercially available
Amberlyst 15 catalyst.
41
The results showed that the use of
catalyst under supercritical CO
2
, as well as the molar ratio of the
reactants, determine the yield and selectivity of the product. A
100% selectivity of TA was obtained using the parameters such
as 200 bar, 110 C, and an acetic acid to glycerol molar ratio of
24 for a 2 hour reaction time. However, the selectivity of TA
decreased at a reaction time of more than 2 h. The Lewis acidity
of CO
2
also inuences the reaction.
Dosuna recently investigated glycerol acetylation by using
ve different ion exchange resins: Amberlyst 15, Amberlyst
36, Dowex 50Wx2, Dowex 50Wx4, and Dowex 50Wx8.
42
Amberlyst 36 and Dowex 50Wx2 were both outperformed by
the other catalysts when reaction was performed at 105 C, at
a glycerol to acetic acid molar ratio of 8 : 1, under atmo-
spheric pressure, at a 6.25 g of dry catalyst/L of glycerol ratio,
Fig. 3 Distribution of TA in the worldwide glycerol market.
Fig. 4 Process flow diagram of conventional TA production.
Table 2 Product specification of TA
32
Property (unit) Specication
Appearance Clear liquid free from suspended matter
Odour Essentially odourless
Purity (%) >99.5
Colour (Hazen) <15
Acidity (%) <0.005
Moisture <0.05
Arsenic (mg kg
1
)<3
Heavy metals (mg kg
1
)<5
Viscosity (cP) 21–30
Density 25 C 1.154–1.158
Refractive index 1.429–1.431
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and at a 10 hour reaction time. Unlike the previous studies,
this work was aimed to produce high selectivity of MA. The
use of Amberlyst 36 resulted in 70.3% MA selectivity, whereas
Dowex 50Wx2 produced 80.8% selectivity of MA with excess of
glycerol. Our result showed that high selectivity toward
formation of small MA molecule was caused by cross-linkage
of the catalyst.
Zhou demonstrated that the Amberlyst 15-catalyzed glycerol
acetylation produced high selectivity of DA and TA.
35
High
amount of acid sites and sufficient pore spaces are the vital
factors in formation of large molecular DA and TA derivatives.
However, Amberlyst 15 exhibits weak acid strength, leading to
low glycerol conversion. In addition, the leaching of catalyst
Amberlyst 15, resulting from the loss of functional groups
through hydrolysis at operating temperatures above the poly-
mer thermal stability limit (>120 C) should be considered in
acetylation process.
12
As A. Ignatyev Igor et al.
43
observed the
phenomenon of protons leaching from the Amberlyst 15 in the
synthesis of glucose esters from cellulose via hydrolysis–acety-
lation steps. Therefore, Wang developed an improved swelling-
changeful polymer catalyst by using sodium 5,50-sulfonylbis(2-
chlorobenzenesulfonate) and bis(4-chlorphenyl)sulfone to
increase selectivity and conversion of glycerol acetylation.
44
The
improved polymer-based acid (PES) catalyst exhibits stronger
acid strength (two times stronger than Amberlyst 15) and better
swelling property, resulting in glycerol conversion of 98.4% with
94.9% total selectivity of DA and TA. The high acid strength of
PES catalyst is attributed to its electron-withdrawing –SO
2

group.
5.2 Zeolite-based acid catalyst
Zeolites are microporous crystalline solids that offer wide
application in oil rening industry, petrochemistry, and ne
chemical production. Zeolites are generally categorized as
aluminosilicate minerals, which are applied as catalyst support
for active species owing to their unique pore system, high
surface area, and high stability. Different zeolite systems,
Fig. 5 Brønsted-acid reaction mechanism.
Fig. 6 Lewis acid reaction mechanism.
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including ZSM-5, zeolite-beta, and USY, can be applied as
potential catalysts in glycerol esterication/acetylation.
45
The catalytic esterication reaction over zeolite-based cata-
lysts depends on their different crystal structure, Si/Al ratio, and
proton exchange level; these properties allows the catalytic
properties, such as pore size, hydrophobicity/hydrophilicity,
Brønsted/Lewis acidity, and acid strength distribution, to be
designed. For esterication, surface acidity is the most vital
characteristic in designing a zeolite-based catalyst. The acidity of
zeolite can betuned by altering their chemical composition (Si/Al
ratio) and ion-exchange abilities. Theoretically, protonic zeolite
consisting of bridging –OH groups (Al–(OH)–Si) is an active acid
site that favors Brønsted acid-catalyzed esterication reactions
(Fig. 7 (ref. 46)). Zeolites exhibiting low Al framework are the most
hydrophobic types. In addition, acidity measurements of zeolites
normally comprise both Brønsted and Lewis acid sites, acid
strength distribution, and precise location of the acid sites.
47
Gonçalves et al. investigated the production of MA, DA, and
TA during glycerol acetylation using acetic acid and catalyzed by
the zeolites HZSM-5 and HUSY. The reactivity of the zeolite
catalyst was also compared with that of the traditional acid
catalysts, such as Amberlyst-15 acid resin, clay K-10 montmo-
rillonite, and niobic acid. Although the zeolites HZSM-5 and
HUSY exhibit high surface area (374 and 566 m
2
g
1
, respec-
tively), their acidity were lower than that of Amberlyst-15
(HZSM-5 zeolite ¼1.2 mmol g
1
; HUSY zeolite ¼1.9 mmol
g
1
; and Amberlyst-15 ¼4.2 mmol g
1
). Given that the esteri-
cation rate mostly depends on the catalyst's acidity, the
decreasing order of acetylation is as follows: Amberlyst-15 > K-
10 clay > niobic acid HZSM-5 > HUSY. The poor perfor-
mance of zeolites is possibly related to difficulty of diffusion of
acetylated esters within the catalyst's cavities. This phenom-
enon resulted in low selectivity of DA and TA as both molecules
are space demanding and their formation and diffusion within
the zeolite pores are difficult.
48
Ferreira et al. attempted to improve the catalytic activity of
esterication by enhancing the acidity of zeolite-based acid
catalysts. Dodeca-molybdophosphoric acid (H
3
PMo
12
O
40
) was
encaged in USY zeolite for glycerol acetylation. H
3
PMo
12
O
40
is
known as heteropolyacid with strong Brønsted acidity and is
widely applied as acid catalyst in esterication. Nonetheless,
H
3
PMo
12
O
40
exhibits low specic surface area (1–10 m
2
g
1
) and
Table 3 Different groups of solid acid catalysts for glycerol acetylation
Solid acid catalysts Properties
Ion exchange resin Ion exchange resins are synthesized from polymers that are capable of exchanging particular ions. The drawback of the
ion exchange resin catalyst is its low temperature stability
Zeolites Crystalline solids composed of silicon and aluminum oxides arranged in a three-dimensional network of uniformly
shaped micropores (<2 nm) of tuneable topology and composition
Brønsted acid sites in zeolites are commonly generated when protons balance the negatively charged framework
induced by the presence of tetrahedrally coordinated aluminum (Al) atoms
Heteropolyacids A class of metal salts wherein the oxo-anions are balanced by a wide range of cations with varying acid strength
Metal oxides The Brønsted acid sites in metal oxides originate from highly polarized hydroxyl groups, acting as proton donors
The Lewis acid sites generated from coordinatively unsaturated cationic sites, which leave M
+
exposed to interact with
guest molecules as an acceptor of pairs of electrons
Mesoporous silica Mesoporous silica is a mesoporous form of silicate that consists of unique features: high surface area, chemical,
thermal, and mechanical stability, highly uniform pore distribution and tunable pore size, high adsorption capacity, and
an ordered porous network
This material is potentially used as solid supported catalyst due to its recyclability, enhanced catalytic reactivity, and
selectivity
Carbon Porous carbon is an attractive catalytic material as it can be prepared from various low-cost waste carbon materials
This material consist of suitable characteristics that can be used as a catalyst support, such as heat resistance, stability
in both acidic and basic media, the possibility of easy recovery of precious metals supported on it and the possibility of
tailoring both its textural and surface chemical properties
Fig. 7 Brønsted acidity of zeolite in esterification reaction.
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low thermal stability. Therefore, encapsulation of H
3
PMo
12
O
40
in acid supercages of NaUSY support increased the number of
accessible acid sites at a surface area of 713 m
2
g
1
. Moreover,
the 1.9 wt% dosing of PMo zeolite catalyst demonstrated the
highest catalytic activity, with 59% selectivity of DA and 68%
total glycerol conversion.
24
5.3 Heteropolyacids (HPAs)-based acid catalyst
HPAs, such as silicotungstic acid (HSiW), phosphotungstic acid
(HPW), and phosphomolybdic acid (HPMo), are typical
Brønsted acids containing a superacid region that displays
outstanding catalytic esterication activity both in homoge-
neous and heterogeneous phases. HPAs are complex proton
acids that incorporate the Keggin-type polyoxometalate anions
(heteropolyanions) containing metal–oxygen octahedra with
a formula XM
12
O
40x8
, where X is the central atom (Si
4+
/P
5+
), xis
its oxidation state, and M is the metal ion (Mo
6+
or W
6+
).
49
The
acid strength of crystalline HPAs generally decreases in the
following order: PW > SiW $PMo > SiMo, which is identical to
the dissociation constants presented in Table 4. Moreover,
HPAs in solution are stronger than the usual mineral acids,
such as H
2
SO
4
, HCl, and HNO
3
.
50
However, bulk HPAs exhibit
low thermal stability, low surface area (1–10 m
2
g
1
), and are
highly soluble in polar media (water, lower alcohols, ketones,
ethers, esters, etc.), which restricts their application as solid
acid catalyst in esterication reaction.
Researchers have improved the thermal stability, surface
area, and solubility of HPAs in polar media by exchanging the
protons of HPA with metal/alkali metal ions and by supporting
bulk HPA with a suitable acidic or neutral carrier (such as SiO
2
,
active carbon, acidic ion-exchange resin, or metal oxide). Zhu
utilized zirconia as a support material to develop HPA-based
catalyst.
51
HPAs display excellent catalytic performance over
a wide variety of acid-catalyzed reactions owing to their low
corrosiveness, well-built structure, as well as adjustable acidity.
Nevertheless, HSiW is deemed more active than the other
available HPAs, such as HPW and HPMo. HSiW consists of four
Keggin protons and advanced Brønsted acid sites, as well as
exhibits stronger hydrothermal stability, making HSiW
considerably superior over the two other HPAs. Therefore, the
activity of HSiW supported with ZrO
2
(H
4
SiW
12
O
40
/ZrO
2
)in
glycerol acetylation at 120 C, 0.8 wt% of catalyst, 1 : 10 molar
ratio of glycerol : acetic acid for 4 h reaction time, was investi-
gated. Three vital ndings were revealed: (i) H
4
SiW
12
O
40
/ZrO
2
is
a predominant glycerol esterication catalyst as it can be reused
up to four continuous runs without showing any deactivation;
(ii) H
4
SiW
12
O
40
/ZrO
2
can be directly used to catalyze crude
glycerol material; and (iii) a 93.6% combined selectivity of DA
and TA was achieved.
Glycerol acetylation over 12-tungstophosphoric acid (TPA)
supported on Cs-containing zirconia (TPA/Cs
x
–ZrO
2
) catalyst
was investigated to improve the selectivity of MA, DA, and TA
formation.
52
The comparative study of different Cs amount
doped on ZrO
2
support was insight studied to evaluate the
catalyst activity towards glycerol acetylation. Given that the
partial substitution of H
+
by Cs
+
has altered the total number of
available surface acid sites, the TPA–Cs catalyst demonstrated
better esterication activities than the TPA parent acid during
the reaction. In addition, this work revealed that the TPA/Cs
2
–
ZrO
2
catalyst (with Cs amount equal to 2-protons of TPA)
demonstrated the highest acidity and catalytic activity
compared to zero or excess Cs content of catalysts. By contrast,
the zero Cs-content catalyst yielded the lowest conversion, while
the catalyst with Cs amount equal to 3-protons of TPA also
showed low activity owing to the absence of residual protons.
This work concluded that the catalytic activity, acidity and
textual properties of the catalyst are varied with the amount of
Cs present on support. Where, the presence of exchangeable Cs
content has improved the surface acidic sites resulting from the
existence of residual protons. Conversion of more than 90% can
then be achieved within 2 h. When the reaction time was pro-
longed to 4 h at 120 C, 1.5 acetic acid to glycerol molar ratio,
and 0.2 wt% of catalyst concentration, the selectivity towards
DA and TA is 55% and 5%, respectively.
Further, S. Sandesh et al. reported glycerol acetylation with
acetic acid under mild reaction condition in the presence of
cesium phosphotungstate catalyst (CsPW).
2
CsPW catalyst was
prepared by precipitation of 0.75 M HPW solution with 0.47 M
aqueous cesium carbonate. This study revealed that the
conversion and selectivity of DA and TA were greatly inuenced
by the total acidity of catalyst (1.87 mmol g
1
) and Brønsted
acidic sites of CsPW catalyst. The comparative characterization
results for original HPW and Cs-precipitated CsPW catalyst
have conrmed that CsPW catalyst exhibits signicant centered
cubic assembly and unaltered primary Keggin structure. In
addition, CsPW catalyst has the highest Brønsted to Lewis acid
ratio compared to Amberlyst 15, sulfated zirconia, H-beta and K-
10 catalysts. Apparently the Cs content in CsPW catalyst has
altered the textural properties (with 110 m
2
g
1
specic surface
area), acidity as well as its catalytic activity towards acetylation.
The acetylation of glycerol with acetic acid resulted in conver-
sion of more than 98% and total 75% selectivity of DA and TA at
7 wt% of CsPW catalyst, 85 C reaction temperature, 8 : 1 molar
ratio of acetic acid to glycerol for 120 min reaction time.
Patel et al. recently investigated the use of TPA anchored on
two types of support, namely, zirconia and MCM-41, for glycerol
acetylation. The use of catalyst support successfully improved
the mechanical stability of TPA and enabled catalytic activity
modication. These results conrmed that the MCM-41-
supported TPA rendered high esterication activity (conver-
sion of 87% and 60% selectivity of DA). By contrast, the ZrO
2
-
Table 4 Dissociation constants of HPAs in acetone at 25 C (ref. 50)
Acids pK
1
pK
2
pK
3
H
3
PW
12
O
40
1.6 3.0 4.0
H
4
PW
11
VO
40
1.8 3.2 4.4
H
4
SiW
12
O
40
2.0 3.6 5.3
H
3
PMo
12
O
40
2.0 3.6 5.3
H
4
SiMo
12
O
40
2.1 3.9 5.9
H
2
SO
4
6.6
HCl 4.3
HNO
3
9.4
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supported TPA resulted in low conversion (80%) and 36%
selectivity of DA. The high-acidity prole and specic hexagonal
channels of MCM-41 also facilitated the diffusion of functional
glycerol molecule compared with those of ZrO
2
.
53
Given that the acidity and esterication activity of HPA-based
catalyst is improved by exchanging the protons of HPA with
different cations, another group researcher designed a silver-
modied HPW catalyst (HPW/Ag), which was prepared through
the ion-exchange method. The HPW/Ag catalyst consists of high
number of Brønsted acid and Lewis acid sites and exhibits
excellent water tolerance, strong stability in polar reaction envi-
ronment, and a unique Keggin structure (these properties are
responsible for the tremendous esterication reaction). The
reaction was performed at 120 C, 10 wt% of catalyst, and 4 : 1
acetic acid to glycerol molar ratio under vigorous mixing at 1200
rpm for 4 h. A total of 96.8% conversion and 90.7% selectivity
towards DA and TA was attained. The advantage of the HPW/Ag
catalyst is the existence of identical symmetry like parent HPW
together with a bonded unit cell in the form of dihydronium ions
H
2
O
2+
, where it is formed by protons exchange in the secondary
Keggin structure. Neither leaching nor deactivation was detected
in ve consecutive reaction cycles.
54
In addition, niobic acid-supported tungstophosphoric acid
catalyst (TPA/Nb
2
O
5
) for glycerol acetylation was studied.
55
TPA
Keggin ion (25 wt%) was well-dispersed in the niobic acid
support (25 wt% TPA/Nb
2
O
5
). The catalyst demonstrated high
total conversion (90%) and 76% selectivity towards DA and TA at
operating parameters of 120 C, 1 : 5 glycerol to acetic acid
molar ratio, and catalyst weight of 200 mg for a 4 hour reaction
time. The ndings of this work suggested that glycerol acetyla-
tion conversion and selectivity depend on catalyst acidity, which
is highly related to the content of niobic acid-supported TPA.
5.4 Metal oxide-based acid catalyst
The use of metal oxide-based catalysts for esterication reaction
has attracted attention owing to their strong surface acidity and
high activity at low operating temperatures. The presence of
Lewis acid (cations) and Brønsted acid (OH

group)/Brønsted
base (O
2
group) (anions) of metal oxides provided the required
catalytic sites for esterication. Fig. 8 illustrates the existence of
Lewis and Brønsted sites in the metal oxide catalyst.
38
Moreover,
functionalization of metal oxide via sulphonation (sulfated
metal oxides, such as sulfated-zirconia, -tin oxide (SnO
2
), -tita-
nium oxide, and -mixed oxides) is a convenient means of
enhancing the surface area and acidity of a catalyst.
Mallesham reported that SnO
2
is one of the potential metal
oxide solid acid catalysts prepared through the wet impregna-
tion method, where SO
42
, MoO
3
, and WO
3
were incorporated
into the SnO
2
support. Compared when metal oxide is used
alone, incorporation of promoters into SnO
2
can enhance the
thermal stability and catalytic performance of SnO
2
. The
performance of three catalysts in esterication of acetic acid
using glycerol was comparatively investigated. The results
showed that all of these catalysts consisted of both Brønsted
and Lewis acidic sites. The ascending order of the activities of
the catalysts is as follows: SnO
2
<WO
3
/SnO
2
< MoO
3
/SnO
2
<
SO
42
/SnO
2
. The sulfated SnO
2
(SO
42
/SnO
2
) showed the high-
est performance mainly because of the presence of large
number of acidic sites associated with super acidic sites.
56
Mixed metal oxide system offers interesting and enhancing
properties, especially when each component differs remark-
ably from each other. Mixed metal oxide catalysts can gener-
ally be prepared via co-precipitation, impregnation, or sol–gel
methods from its bulk oxide or metal salt as precursors.
57
The
binary metal oxide system oen establishes new acid sites or
modulates the acid properties of the bulk oxides, which are
active during esterication.
58
For instance, sulfated binary
oxide solid superacids (SO
42
/TiO
2
–SiO
2
) synthesized by Yang
et al. was utilized as catalyst in glycerol esterication using
acetic acid under toluene solvent-reaction system.
59
The TiO
2
–
SiO
2
showing the highest catalytic quality was calcined at 450
C and consists of 13.8 wt% of TiO
2
.Yangreportedthatthe
coupling of two oxides (TiO
2
–SiO
2
) can generate stronger acid
sites and higher acidity compared with bulk metal oxide owing
to the larger specic surface area of the coupling. Whereby, the
presence of strong acidity of SO
42
/TiO
2
–SiO
2
is initiated by an
excess of a negative or positive charge in a binary oxide. The
TiO
2
content, special surface area and unique structure of
modied SO
42
/TiO
2
–SiO
2
catalyst resulted in 91.4%
conversion.
Reddy et al. performed similar glycerol esterication using
acetic acid catalyzed by various types of zirconia-based cata-
lysts: ZrO
2
,TiO
2
–ZrO
2
,WO
3
/TiO
2
–ZrO
2
,andMoO
3
/TiO
2
–ZrO
2
.
MoO
3
/TiO
2
–ZrO
2
showed the highest conversion among the
investigated catalysts. A 100% conversion with 80% selectivity
towards TA was achieved at 120 C, acetic acid to glycerol
molar ratio of 6 : 1, and 5 wt% of catalyst concentration for 60
hreactiontime.
25
The results revealed that the MoO
3
promoted the number of acidic sites of TiO
2
–ZrO
2
support and
the strong interaction between the dispersed MoO
3
with
support have increased catalytic activity for glycerol acetyla-
tion. Despite its high conversion and selectivity, long opera-
tion time is required for TA production, which is not cost-
effective toward upscaling for industrial uses.
Reddy et al. further improved the activity and selectivity of
products (DA and TA) by using a modied sulfonated zirconia-
based mixed metal oxide; they used a better metal oxide support
(SO
42
/CeO
2
–ZrO
2
) for acetylation. The wet impregnated sulfate
ions on CeO
2
–ZrO
2
mixed oxides has increased the initial metal
oxide surface area resulting from the formation of porous
surface sulfate compounds between the sulfate groups and the
supports. The reaction time was successfully shortened to 1 h,
and a high glycerol conversion (100%) and 74.2% selectivity
towards DA and TA was achieved. Nevertheless, a longer reac-
tion time (40 h) was required for the steric structure of partial
glycerides to esterify into 90% highly selective TA.
60
Fig. 8 Lewis and Brønsted site of metal oxide catalyst.
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5.5 Mesoporous silica-based catalysts
Mesoporous silica materials, such as MCM-41 and SBA-15, have
attracted much attention as a catalyst support in heterogeneous
catalysis owing to their high specic surface area ($1000 m
2
g
1
), well-ordered mesoporous structure, and large pore sizes (2
nm #size #20 nm).
61
The preparation of sulfonated silica is
shown in Fig. 9, as redrawn on the basis of previous study.
62
Given that they display a high accessibility to large organic
molecules, such as fatty acids and esters, the mesoporous silica-
based catalysts were chosen as acid catalysts for esterication of
glycerol with fatty acids, especially when these catalysts are
functionalized with R–SO
3
H groups.
63
The plausible mecha-
nism of esterication of acetic acid using methanol and cata-
lyzed by acid-functionalized SBA-15 was reported by Miao.
64
The
functionalized SBA-15 obeyed dual-site mechanism (Langmuir–
Hinshelwood type), which involved adsorption of both acetic
acid and alcohol molecules. This phenomenon demonstrated
that the reaction occurred at a high rate. Fig. 10 illustrates the
dual-site esterication mechanism catalyzed by functionalized
SBA-15, which is modied on the basis of a previous study.
64
The SBA-15 silica-based catalysts are oen modied as novel
solid acid catalysts for TA and DA production. SBA-15 possesses
unidirectional channels arranged in hexagonal symmetry and
interconnected by micropores. The cross-sectional area of this
catalyst is speculated to be much larger than that of MCM-
41.
65,66
SBA-15 exhibits a large surface area (700–900 m
2
g
1
),
large pore size (5–9 nm), and thick walls (3.5–5.3 nm).
67
Gon-
çalves et al. used ve different solid acids (Amberlyst-15, K-10
montmorillonite, niobic acid, HZSM-5, and HUSY) to produce
DA and TA. Among the tested catalysts, Amberlyst-15 (acidity ¼
4.2 mmol g
1
) was the most active, showing 97% conversion
and 67% selectivity of DA and TA, followed by K-10 clay (acidity
¼0.5 mmol g
1
) with 96% conversion but a lower combined
selectivity of DA and TA (54%).
48
Despite of K-10 clay does not
possess high acidity, the predominant of medium-weak
Brønsted acid sites of K-10 clay are mainly responsible for
catalytic activity as K-10 clay is type of acetic acid-treated cata-
lyst.
68,69
As shown by the recent studies, the use of SBA-15 as
solid acid catalyst has demonstrated enormous improvement in
terms of conversion and selectivity. For instance, a hybrid SBA-
15 catalyst functionalized with molybdophosphoric acid (MPA/
SBA-15) can achieve 100% glycerol conversion with a corre-
sponding 86% combined selectivity of DA and TA. The thermal
decomposition of 15% MPA over SBA-15 support has shown
improved catalytic activity in glycerol acetylation using acetic
acid.
70
Trejda et al. also modied the SBA-15 catalyst. They modied
the mesoporous niobiosilicate (nb-SBA-15) with 3-mercapto-
propyl trimethoxysilane (MPTMS) followed by hydrogen
peroxide oxidation of the thiol species of the catalyst (denoted
as MP-nb-SBA-15 catalyst). The work revealed that, compared
with silica material, the presence of niobium in the matrix
improved the transformation of thiols into sulphonic species
via hydrogen peroxide oxidation. The optimum Si/MPTMS ratio
is 1 : 1. The elemental analysis of the catalysts revealed that the
inclusion of MPTMS into nb-SBA-15 is effective and has reduced
the surface area, pore volume, and pore diameter. Thus, the
modied catalyst used in this work does not only change the
conversion but also strongly affects the selectivity. The domi-
nant product is DA (approximately 50%), and a considerably
high selectivity of TA of up to nearly 40% was attained. Never-
theless, weak stability of catalyst was detected during recycla-
bility test.
71
Catalytic esterication of yttrium supported on silicate
framework (Y/SBA-3) during glycerol acetylation was also
studied. Graing of yttrium on silicate framework increased the
surface area of Y/SBA-3 (1568 m
2
g
1
) compared with that of the
blank silicate (SBA-3 at 1462 m
2
g
1
), conrming the inhibitory
effect of nano-sized yttrium agglomeration. The Y/SBA-3 used to
catalyze glycerol acetylation, resulting in 100% glycerol
conversion and 89% combined selectivity of DA and TA. The
Fig. 9 Preparation of sulfonated silica.
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catalyst activity was greatly inuenced by two factors: (i) acidity
of the catalyst and (ii) the combination of high surface area and
large pore size, which facilitated the diffusion of bulky glycerol
and the nal product. This work also compared the catalytic
activities of different SBA-15-based catalysts and found that the
decreasing order of the catalytic activity is as follows: 3%Y/SBA-
3 > SBAH-15 (15) (hybrid SBA-15 functionalized with molybdo-
phosphoric acid) > AC-SA5 (sulfated activated carbon).
40
Although mesoporous silica has displayed good perfor-
mance in esterication, Stawicka further improved the meso-
porous support by creating more open structures, which
improve the performance of the catalyst during esterication
using glycerol and acetic acid. Mesostructured cellular foams
(MCF) containing acidic sites derived from oxidized MPTMS
were developed. Fig. 11 shows the simplied model of the MCF
structure. MCF consists of uniform spherical cells approxi-
mately 20–40 nm in diameter and possesses surface areas up to
approximately 900 m
2
g
1
. The cells are interconnected by
uniform windows (7–20 nm in diameter), forming a continuous
3-D pore system. In addition, the walls of the MCFs are formed
from silica; the silanols present on the surface can thus be used
to modify the material. The presence of niobium (Nb/MCF) and
tantalum (Ta/MCF) doped on the MCF also improves the
Brønsted acidity of the catalyst. The acidic sites of Nb/MCF and
Ta/MCF catalysts were derived from the oxidized MPTMS and
were further enhanced by modication with Nb and Ta. Inter-
estingly, the sulphonic species on silica surfaces showed high
stability and strength aer being modied by the presence of
promoters. Both Nb/MCF and Ta/MCF showed high conversion
and selectivity of DA and TA even though the acidity of Ta/MCF
(0.57 mequiv. g
1
) was higher than that of Nb/MCF (0.32
mequiv. g
1
).
72
5.6 Mesoporous carbon-based acid catalyst
Mesoporous carbon has been actively studied generally as
a catalyst support and/or acid-functionalized carbon for esteri-
cation reaction. The presence of surface oxide group in mes-
oporous carbon enables it to provide anchoring sites for active
metals, which can tune the properties of carbon as a catalyst
support material. Furthermore, the existence of unique prop-
erties, such as high thermal–mechanical stability with low
metal leaching, as well as controllable textural and surface
chemical properties, makes carbon a suitable catalyst support.
Compared with mesoporous silica, mesoporous carbon is more
resistant to structural changes caused by hydrolytic effects in
aqueous environments. Acid-functionalized carbon, such as
sulfonated-carbon via sulfonation by concentrated H
2
SO
4
(formation of high density sulfonic acid group (–SO
3
H)), has
been extensively studied in esterication. Fig. 12 shows the
preparation of sulfonated carbon.
73,74
Sulfonated activated carbon (AC) was investigated in glycerol
esterication using acetic acid to produce DA and TA. The
porosity of the AC was measured, and the results revealed that
27% of the AC exhibits mesoporous structure. A blank AC nor-
mally exhibits large specic surface area of up to 780 m
2
g
1
.
This work found that the acid treatment has slightly reduced
the surface area of AC to 742 m
2
g
1
. The acid-functionalized AC
was prepared through hydrothermal treatment with sulfuric
acid at 85 C for 4 h. Although the sulfonated AC catalyst (AC-
SA5) demonstrated good stability during recycling (up to four
Fig. 10 Dual-site esterification mechanism (Langmuir–Hinshelwood type, L–H) through functionalized SBA-15 catalyst.
Fig. 11 Simplified model of MCF structure.
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consecutive batch runs), the conversion and selectivity results
were not encouraging, that is 91% conversion and a 62%
combined selectivity of DA and TA (slightly lower than those of
other mentioned catalysts). Nevertheless, the density of
Brønsted acid sites and mass transfer rate in the mesoporous
channels of AC are the governing factors leading to TA
synthesis.
75
A new class of sulfonated carbon catalysts termed “sugar
catalyst”was recently reported. They were prepared via incom-
plete carbonization of simple carbohydrates (starch, cellulose,
glucose, and sucrose) followed by sulphonation. S´
anchez
61
studied the effect of the porous system of a sulphonated sucrose-
derived carbon prepared via silica-template carbonization for
glycerol acetylation reaction. The presence of interconnected
micro- and mesopores (45% in 0–2 nm micropores and 51% in 2–
50 nm mesopores) in sucrose-derived carbon allowed effective
surface sulfonation (C–SO
3
H). High glycerol conversion (>99%)
with high selectivity (50%) of TA was achieved during esterica-
tion (at 9 : 1 molar ratio of acetic acid to glycerol, 5 wt% catalyst,
180 C optimum temperature, and 4 h reaction time).
Another study reported on the preparation of sulfonated
carbon catalyst, which is derived from low-cost biomass (willow
catkins), to be used in producing carbon support. The acid
density of the sulphonated carbon is greatly inuenced by sul-
phonation condition. The sulphonation temperature of 90 C
rendered the highest acidity of sulfonated carbon catalyst
compared with that obtained at 100 C, 80 C, and 70 C.
Furthermore, the study indicated that ester selectivity is mainly
correlated with catalyst acidity, where high acidity of sulfonated
carbon yields 67.2% of DA and TA products. These types of
catalysts showed good heat stability and high water tolerance at
low production cost.
76
The acid-functionalized carbon was further improved using
graphene oxide (GO) as carbon material. GO consists of oxygen-
rich functional groups, such as SO
3
H, carboxyl, hydroxyl, and
epoxide groups, which provide moderate acidic site for glycerol
esterication. The unique layered structure of graphene also
improves the accessibility of reactant for adsorption during
reaction. Moreover, the acid-functionalized GO enhanced the
acid density of the catalyst containing acidic SO
3
H groups. The
glycerol acetylation showed a good glycerol conversion of 98.5%
with high selectivity of DA (60%) and TA (24.5%). In addition,
GO has displayed high reusability without showing reduction in
catalytic activity and changes in product distribution.
77
5.7 Others
A new type of catalyst, hydroxylated magnesium uoride
(MgF
2x
(OH)
x
, where x< 0.1), which contains both Lewis and
Brønsted acid sites, was synthesized for glycerol acetylation.
The hydroxylated magnesium uoride exhibits the following
structural and chemical features: (i) high surface area with pore
diameters at the mesopore range; (ii) very low solubility in
strong polar solvents; (iii) hydrolysis resistance; (iv) medium-
strength Lewis and Brønsted acid sites; (v) possibility of easy
tuning of surface acidity; and (vi) nanoscopic particle dimen-
sion. MgF
2x
(OH)
x
showed a majority of Lewis sites that are
generated from Mg
2+
sites of Mg–F bond, while Brønsted acid
sites character from the Mg–OH group. The surface acidity of
hydroxylated magnesium uoride exhibiting high Lewis/
Brønsted ratio sites is suggested to favor DA and TA formation
during glycerol acetylation. Troncea
36
reported that the
medium-strength Lewis acid sites of catalyst (such as incom-
pletely coordinated Mg
2+
) exert a major effect on acetylation
compared with Brønsted acid sites (M–OH groups) that favor
MA formation. The increased selectivity to DA and TA with
loading of Lewis sites may be explained by considering the
double role of these sites: rst, as active catalytic sites involved
in the formation of the reactive electrophilic intermediate, and
second, as dehydrating sites coordinating the water molecules
formed during the reaction.
Recent studies have applied a magnetic solid acid catalyst
(Fe–Sn–Ti(SO
42
))-400 in glycerol acetylation and successfully
produced 99% selectivity of TA with 100% total conversion.
78
All
of the reactions were conducted at 80 C, 2.5 wt% of catalyst, 5.6
mass ratio of acetic acid to glycerol, and 0.5 h reaction time.
This magnetic-based catalyst consists of iron (Fe), tin (Sn), and
titanium (Ti), sulfated by (NH
4
)
2
SO
4
solution, and calcined at
Fig. 12 Preparation of SO
3
H–carbon.
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Table 5 Different kind of heterogeneous catalysts used for glycerol acetylation with acetic acid
Catalyst Catalyst preparation Catalyst characterization Operating parameters Performance Ref.
Ion exchange resin catalyst
Amberlyst 15 Commercial available SSA ¼37.6 m
2
g
1
T¼110 CY¼41% 41
Acidity ¼4.7 mmol g
1
P¼200 bar S¼100% TA
PD ¼30 nm t¼2h
T
stability
¼120 C Molar ratio acetic acid to
glycerol ¼24
PV ¼0.4 cm
3
g
1
Flow rate of scCO
2
¼1.2 mL
min
1
Moisture content ¼48%
Cross-linkage ¼20%
Acid strength, H
0
¼3to2.2
(i) Amberlyst 36 Commercial available Amberlyst 36: T ¼105 CAmberlyst 36: 42
SSA ¼33 m
2
g
1
t¼10 h C¼95.6%
Acidity ¼5.4 mmol g
1
Molar ratio glycerol to acetic
acid ¼8:1
S¼70.3% MA; 4.5%
DA
PD ¼24 nm
T
stability
¼140 C
PV ¼0.2 cm
3
g
1
Moisture content ¼52.4%
(ii) Dowex 50Wx2 Cross-linkage ¼12%
Dowex 50Wx2: Dowex 50Wx2:
Acidity ¼4.8 mmol g
1
C¼95.2%
T
stability
¼140 CS¼80.8% MA; 5.1%
DAMoisture content ¼76.7%
Cross-linkage ¼2%
Gel-type
Amberlyst 15 Commercial available Same as above T ¼110 CC¼97.3% 35
t¼270 min S¼47.7% DA;
44.5% TA
Molar ratio acetic
acid : glycerol ¼9:1
Improved polymer-
based solid acid catalyst
(PES)
Precipitation and ion-exchange method Acidity: 2.1 mmol g
1
T¼110 CC¼98.4% 44
The polymer with SSBCBS and BCPS molar ratio of 5 : 5 (PES-50) was
synthesized in ratio of: 0.05 mol of 4,40-biphenol (BP), 0.025 mol of
BCPS, 0.025 mol of SSBCBS and 0.058 mol of K
2
CO
3
. The polymer
PES-P was precipitated, followed by being washed thoroughly with
hot deionized water for three times. The polymer was dried in oven at
100 C for 12 h. As-synthesized PES-P polymer was exchanged in 0.5
MH
2
SO
4
at 25 C for 6 h, and then ltrated
Acid strength, H
0
¼5.6 to 3.0 t¼3h S¼45% DA; 49.5%
TA
Catalyst loading ¼0.15 g
Molar ratio acetic
acid : glycerol ¼8:1
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Table 5 (Contd. )
Catalyst Catalyst preparation Catalyst characterization Operating parameters Performance Ref.
Zeolite-based acid catalyst
(i) Zeolite HZSM-5 Zeolites HZSM-5 and HUSY were purchased from Petrobras HZSM-5: T ¼110 CHZSM-5: 48
(ii) Zeolite HUSY SSA ¼374 m
2
g
1
t¼30 min C¼30%
Acidity ¼1.2 mmol g
1
Molar ratio acetic
acid : glycerol ¼3:1
S¼10% DA & TA
T
stability
¼386 C
Si/Al ¼28
HUSY: HUSY:
SSA ¼566 m
2
g
1
C¼14%
Acidity ¼1.9 mmol g
1
S¼14% DA & TA
T
stability
¼397 C
Si/Al ¼2.6
PMo/NaUSY zeolite Preparation of NaUSY zeolite Acidity ¼0.019 g
HPA
g
cat
1
T¼not reported C¼68% 24
HUSY zeolite was neutralized by 3 repeating 2 M NaCl at 80 C. It was
washed with distilled water and then dried at 120 C
SSA ¼713 m
2
g
1
t¼3h S¼61% DA & TA
Micropore volume, mPV ¼0.17
cm
3
g
1
Catalyst ¼10 wt% of glycerol
Hydrothermal impregnation for PMo/NaUSY
The catalysts were prepared by hydrothermal impregnation method:
by encaging molybdenum(VI)oxide on NaUSY, followed by
neutralization and drying at 110 C
HPAs-based acid catalyst
H
4
SiW
12
O
40
/ZrO
2
Wetness impregnation method Acidity ¼0.69 mmol g
1
T¼120 CC¼100% 80
HSiW/ZrO
2
catalyst was prepared through incipient wetness
impregnation method. Zirconia support was impregnated with 139
mmol/HSiW solution for 8 h, followed by drying (110 C) and
calcination (250 C in static air, 4 h)
SSA ¼48.7 m
2
g
1
t¼4h S¼93.6% DA & TA
PD ¼10.7 nm Molar ratio acetic
acid : glycerol ¼10 : 1
PV ¼0.17 cm
3
g
1
Catalyst ¼0.8 wt%
TPA/Cs
2
–ZrO
2
Precipitation and impregnation method Acidity ¼not reported T¼120 CC¼90% 52
Zirconia support was prepared by precipitation method and dried at
120 C for 36 h. Zirconia support is then loaded by CsNO
3
solution,
calcined at 500 C for 2 h
t¼4h S¼55% DA; 5% TA
Later, TPA supported Cs–zirconia was prepared by impregnation of
20 wt% of TPA solution, calcined at 350 Cfor4h
Molar ratio acetic acid/glycerol
¼1.5
Catalyst ¼0.2 wt%
(i) TPA/MCM-41 Impregnation method TPA/MCM-41: T ¼100 CTPA/MCM-41: 53
Both TPA/MCM-41 and TPA/ZrO
2
catalysts were prepared by incipient
impregnation. MCM-41 was impregnated with 1% of TPA solution;
while, ZrO
2
with 10–40% of TPA, both dried at 100 C for 10 h
SSA ¼360 m
2
g
1
t¼360 min C¼87%
Acidity ¼0.855 mmol g
1
(majority of strong acid strength)
Mole ratio acetic acid : glycerol
¼6:1
S¼75% DA & TA
Catalyst ¼0.15 g
(ii) TPA/ZrO
2
TPA/ZrO
2
: TPA/ZrO
2
:
SSA ¼146 m
2
g
1
C¼80%
Acidity ¼0.840 mmol g
1
(fully
weak acid strength)
S¼40% DA & TA
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Table 5 (Contd. )
Catalyst Catalyst preparation Catalyst characterization Operating parameters Performance Ref.
HPW/Ag Ion-exchanged method Acidity ¼1.92 mequiv. g
1
T¼120 CC¼96.8% 54
HPW/Ag was prepared by ion-exchanged method. A 0.1 mol L
1
AgNO
3
solution was added drop-wise into HPW solution, aged 2 h,
evaporated, dried at 80 C (12 h) and calcined at 250 C for 4 h
t¼4h S¼90.7% DA & TA
Molar ratio acetic
acid : glycerol ¼4:1
Catalyst ¼10 wt% of glycerol
Speed ¼1200 rpm
TPA/Nb
2
O
5
Impregnation method Acidity ¼1.149 mmol g
1
T¼120 CC¼90% 55
TPA/Nb
2
O
5
was prepared by impregnation method. Nb
2
O
5
support
was impregnated with 5–30 wt% of TPA in methanol, dried at 120 C
(12 h) and calcined in air at 300 C for 2 h
SSA ¼66 m
2
g
1
t¼4h S¼76% DA & TA
PV ¼0.36 cm
3
g
1
Molar ratio acetic
acid : glycerol ¼5:1
Catalyst ¼4 wt% of glycerol
CsPWA (cesium
phosphotungstate)
catalyst
CsPWA was prepared in molar ratio Cs
2.5
H
0.5
PW
12
O
40
, by adding
dropwise aqueous cesium carbonate (0.47 M) to H
3
PW
12
O
40
(0.75 M)
at room temperature. The precipitate was aged in aqueous mixture
for 48 h at room temperature, dried in a rotary evaporator at 45 C,
then in an oven at 150 C for 1.5 h
Acidity ¼1.87 mmol g
1
T¼85 CC¼98.1% 2
SSA ¼110 m
2
g
1
t¼120 min S¼75% DA & TA
Molar ratio acetic
acid : glycerol ¼8:1
Catalyst ¼7 wt%
Metal oxide-based acid catalyst
SO
42
/SnO
2
Wet-impregnation method Acidity ¼0.186 mmol g
1
T¼70 CC¼99% 56
The catalyst was prepared by wet-impregnation method, where SnO
2
was added into 0.5 M H
2
SO
4
solution (mass ratio of 10 wt% of SO
42
),
dried at 120 C (12 h) and calcined in air at 650 C for 5 h
SSA ¼41 m
2
g
1
t¼2h
PD ¼15.79 nm Molar ratio acetic
acid : glycerol ¼1:1
PV ¼0.1623 cm
3
g
1
Catalyst ¼5 wt% of glycerol
Speed ¼800 rpm
Performed in solvent system
SO
42
/TiO
2
–SiO
2
Precipitation and impregnation method SSA ¼550 m
2
g
1
T¼120 CC¼91.4% 59
The catalyst was prepared by co-precipitation method, where Ti(OH)
4
sol, Si(OC
2
H
5
), C
2
H
5
OH and water were mixed to form Ti(OH)
4
and
Si(OH)
4
mixture sol at 80 C. It was then dried at 100 C and calcined
for 1 h at 500 C
Catalyst ¼5 wt% of glycerol
TiO
2
–SiO
2
was sulfated with 1.0 M sulfuric acid for 1 h, dried under
infrared lamp and calcined at 450 C for 3 h
Performed in solvent reaction
system
MoO
3
/TiO
2
–ZrO
2
Co-precipitation and impregnation method Acidity ¼0.61 mmol g
1
T¼120 CC¼100% 25
MoO
3
/TiO
2
–ZrO
2
was prepared by co-precipitation and impregnation
method. The solids were dried at 120 C (12 h) and calcined in air at
650 C (5 h), where the ratio of TiO
2
–ZrO
2
is 1 to 1. Meanwhile, the
ZrO
2
was prepared in advance by precipitation of ZrOCl
2
with drop-
wise of NH
4
OH solution
SSA ¼7m
2
g
1
t¼60 h S¼80% DA & TA
Molar ratio acetic
acid : glycerol ¼6:1
Catalyst ¼5 wt% of glycerol
SO
42
/CeO
2
–ZrO
2
Impregnation method SSA ¼92 m
2
g
1
T¼120 CC¼100% 60
The catalyst was prepared by wet sulfonation-impregnation method,
where 0.5 M H
2
SO
4
solution was mixed into 1 : 1 mol ratio of CeO
2
–
ZrO
2
(based on oxides), stirred for 1 h, dried at 120 C (3 h) and
calcined for 5 h at 500 C
t¼1h S¼74.2% DA & TA
Molar ratio acetic
acid : glycerol ¼6:1
Catalyst ¼5 wt%
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Table 5 (Contd. )
Catalyst Catalyst preparation Catalyst characterization Operating parameters Performance Ref.
Mesoporous silica-based acid catalyst
MPA/SBA-15 SBA-15 preparation Acidity ¼0.86 mmol g
1
T¼110 CC¼100% 70
SSA ¼754 m
2
g
1
t¼3h S¼92.3% DA & TA
Pluronic P123 was dissolved in 1.8 M HCl, followed by addition of
Brij S100. Aer 18 h, droplet of TEOS was added at 35–40 C. The
formed SBA-15 was dried at 90 C (24 h), followed by DI water
washing and re-dried at room temperature
PD ¼6.53 nm Molar ratio acetic
acid : glycerol ¼6:1
PV ¼1.24 cm
3
g
1
Catalyst ¼2 wt% of glycerol
Impregnation method
MPA/SBA-15 catalyst was impregnated by molybdophosphoric acid
solution at room temperature for 24 h. It was dried and calcined in
air for 6 h at 560 C
MP-nb-SBA-15 nb-SBA-15 silica preparation: Acidity ¼0.50 mequiv. g
1
T¼150 CC¼94% 71
Molar ratio of mixture was prepared at chemical ratio of:
1SiO
2
: 0.005Pluronic P123 : 1.45HCl : 124H
2
O. Ammonium
niobate(V)oxalate was then added into mixture and stirred for 8 h at
55 C. Thermal treating of solution was conducted at 80 C without
stirring for 16 h. The solid sample was ltered, washed and dried at
60 C (12 h) and calcined at 550 C(8h)
SSA ¼565 m
2
g
1
t¼4h S¼89% DA & TA
PD ¼6.8 nm (51%) Molar ratio acetic
acid : glycerol ¼9:1
S
TA
¼40%
PV ¼0.38 cm
3
g
1
Catalyst ¼0.6 wt%
Post synthesis of nb-SBA-15:
SBA-15 were heated (350 C, 4 h) prior to addition of MPTMS in
toluene solution. The mixtures were heated at 100 C for 20 h,
washed, dried at 100 C (4 h). Oxidation of modier was carried out
by using H
2
O
2
and H
2
SO
4
solution
Y/SBA-3 Hydrothermal method Acidity ¼1.34 mmol g
1
T¼110 CC¼100% 40
The catalyst was prepared by dissolving of CTMABr in 0.4 V% HCl
(37%) solutions. Y(NO
3
)
3
$6H
2
O was dropwise added to TEOS
solution and then aged at 12 h (room temperature). It was washed,
ltered, dried (12 h at 100 C) and then calcined in air at 560 C for 8
h (heating rate of 2 Cmin
1
)
SSA ¼1568 m
2
g
1
t¼2.5 h S¼89% DA & TA
PD ¼2.54 nm Molar ratio acetic
acid : glycerol ¼4:1
PV ¼0.81 cm
3
g
1
Catalyst ¼5 wt% of glycerol
Speed ¼350 rpm
(i) Nb/MCF Hydrothermal method Nb/MCF: T ¼150 CNb/MCF: 72
Both MCF support of the catalysts were prepared by dissolving of
P123 into 0.7 M HCl solution, added by 1,3,5-trimethylbenzene,
NH
4
F, TEOS (aer 1 h). Next, (i) ammonium niobate(V)oxalate
hydrate; (ii) tantalum(V)ethoxide, was added to form Nb/MCF and Ta/
MCF respectively. The solution was mixed (20 h), hydrothermal-
treated (100 C for 24 h), dried and calcined in air at 500 C for 8 h
Acidity ¼0.32 mequiv. g
1
t¼4h C¼89%
Ta/MCF: Molar ratio acetic
acid : glycerol ¼9:1
S¼89% DA & TA
Acidity ¼0.57 mequiv. g
1
Catalyst ¼4 wt% of glycerol Ta/MCF:
C¼91%
(ii) Ta/MCF S¼87% DA & TA
Mesoporous carbon-based acid catalyst
AC-SA5 Hydrothermal method Blank AC T ¼120 to 135 CC¼91% 75
The catalyst was prepared by hydrothermal method. Activated carbon
was treated in 5 mol L
1
of H
2
SO
4
solution at 85 C for 4 h
SSA ¼780 m
2
g
1
t¼3h S¼62% DA & TA
PV ¼0.52 cm
3
g
1
Molar ratio acetic
acid : glycerol ¼8:1
PSD ¼500–710 mm Catalyst ¼4 wt% of glycerol
AC-SA5
Acidity ¼0.89 mmol g
1
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Table 5 (Contd. )
Catalyst Catalyst preparation Catalyst characterization Operating parameters Performance Ref.
SSA ¼742 m
2
g
1
PV ¼0.47 cm
3
g
1
PSD ¼500–710 mm
Mesopore SSA ¼208 m
2
g
1
Sulfated silica template
carbonized acid catalyst
Hydrothermal method Acidity ¼1.35 mmol g
1
T¼180 CC¼99.6% 61
Na
2
SiO
3
and H
2
O were mixed at 70 C to prepare silica template.
Sucrose and HCl were then added into mixture. The solution was le
for 24 h for polymerization. Carbonized of silica was conducted at
400 CinN
2
(5 h), followed by immerse with 3 M NaOH solution (12 h
and 120 C) and hot water washing
SSA ¼556 m
2
g
1
t¼4h S¼50% TA
PD ¼2–50 nm (51%) Molar ratio acetic
acid : glycerol ¼9:1
PV ¼0.51 cm
3
g
1
Catalyst ¼5 wt%
Carbonized solids were lein contact overnight with fuming sulfuric
acid (7% of SO
3
)
Sulfonated willow
catkins-based carbon
Hydrothermal method Acidity ¼5.14 mmol g
1
T¼120 CC¼98.4% 76
The willow catkin (10 g) was carbonized in N
2
at 450 C for 5 h. The
black powder (1 g) was then heated in 5 mL of concentrated H
2
SO
4
(95–98%) for 3 h at different temperatures. It was cooled down,
ltered, dried at 80 C (5 h). The dried powder (1 g) was then treated
in 14 mL of fuming sulfuric acid (15 wt% SO
3
) at to 100, 90, 80, and 70
C, temperatures for 2 h and then cooled to room temperature
Content of –SO
3
H¼2.85 mmol
g
1
t¼2h S¼67.2% DA & TA
Molar ratio acetic
acid : glycerol ¼5:1
Catalyst ¼5 wt%
GO Hydrothermal and oxidation method Not reported T¼120 CC¼98.5% 77
t¼1h S¼94.5% DA & TA
Molar ratio acetic
acid : glycerol ¼10 : 1
S
DA
¼60%
Graphite (5 g) and NaNO
3
(2.5 g) were placed into 115 mL H
2
SO
4
in an
ice bath under vigorous stirring. Aer adding of 15 g KMnO
4
,itwas
heated to 35 C and stirred for extra 30 min. The mixture was diluted
with water and re-heated to 98 C. Subsequently, 50 mL H
2
O
2
(30
wt%) was added, followed by ltered, washed, dried (50 C). Then,
the dispersed GO was kept in water for sonication (1 h), centrifuged
and dried at ambient temperature
Catalyst ¼0.1 g
Others
MgF
2x
(OH)
x
,x< 0.1 Hydrothermal method Acidity ¼0.33 mmol g
1
T¼100 CC¼>99% 36
Metallic Mg was dissolved in methanol (50 mL) at room temperature
overnight. Aer heating under reux conditions for 3 h, HF solution
was added to the formed Mg(OCH
3
)
2
solution, then aged for 12 h and
dried under vacuum at room temperature. The solid product thus
obtained was then further dried under vacuum at 70 C for 5 h
SSA ¼424 m
2
g
1
t¼22 h S¼85% DA & TA
PV ¼0.25 cm
3
g
1
Molar ratio acetic
acid : glycerol ¼3:1
PD ¼2.2 nm
Fe–Sn–Ti(SO
42
)-400 Precipitation and hydrothermal method SSA ¼18.88 m
2
g
1
T¼80 CC¼100% 78
The magnetic matrix was made by mixture of [Fe
2
SO
4
/Fe
2
(SO
4
)
3
]at45
C. The matrix (0.1 mol L
1
) was then treated with stannic chloride
pentahydrate (17.5 g), tetrabutyl titanate (10 mL) and NH
3
$H
2
O. The
formed solid sample is dried (100 C) and further sulfated with
(NH
4
)
2
SO
4
solution for 24 h, washed, ltered, dried and calcined at
different temperatures
PV ¼0.15 cm
3
g
1
t¼30 min S¼99% TA; 1% DA
PD ¼3.8 nm Mass ratio acetic acid to
glycerol ¼5.6
Catalyst ¼2.5 wt%
SSA ¼specic surface area of catalyst; PD ¼pore diameter of a particle; PS ¼particle size of catalyst; PV ¼pore volume of catalyst; PSD ¼particle size distribution of catalyst.C¼conversion; S¼
selectivity; Y¼yield.
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400 C. The calcination temperature is a crucial factor to
develop a good acid strength of the catalyst given that a catalyst
calcined at 400 C displays weak acid sites. The catalysts calcined
at 500 C, 600 C, and 700 C promoted the formation of
a superacid structure. Furthermore, the acidity of magnetic cata-
lyst is mainly contributed by Lewis acid (from Sn and Ti metal
cations) and Brønsted acid (from proton of –OH groups). Both the
Lewis and Brønsted acid sites play key role in activating the
carbonyl group from acetic anhydride, which is further attacked
by glycerol molecule for esterication process. The (Fe–Sn–
Ti(SO
42
))-400 catalyst exhibits good stability during catalyst
recyclability test. Unfortunately, (Fe–Sn–Ti(SO
42
))-400 demon-
strated a water intolerance behavior as selectivity of TA is reduced
to 26% when small amount of water was added during the reac-
tion. This work also concluded that the catalytic activity is highly
dependent on the acid strength but not on the total acid amount
of the catalyst. Similar fact was also supported by Huang et al.,
79
where the presence of both Lewis and Brønsted acid sites are
observed from sulfated Sr–Fe oxide and sulfated Ca–Fe oxide
catalysts, respectively. The study summarized that Brønsted acid
sites are able to catalyze the esterication of fatty acids via the
protonation of the acid group (–COOH) to give oxonium ions,
while the Lewis acid sites catalyze the esterication of fatty acids
through the coordination of acid groups on the active sites.
Table 5 shows the different types of heterogeneous catalysts used
in glycerol acetylation using acetic acid.
6. Conclusions
Glycerol acetylation using acetic acid allows the cost-effective
production of MA, DA, or TA compared with the use of acetic
anhydride. Despite the spontaneous reaction and formation
of electrophilic intermediates in catalytic acetylation of glyc-
erol, the combined high conversion and selectivity of DA and
TA can now be attained under mild reaction environment by
using a well-designed heterogeneous acid catalyst. For TA, the
magnetic solid acid catalyst (Fe–Sn–Ti(SO
42
))-400 is
currently the most competent catalyst because 99% selectivity
of TA with a 100% total conversion was attained. However, TA
formation is strongly affected by the acidity of the catalyst
(more specically by weak acid strength), by pore aperture
(sufficient pore space to facilitate formation of large mole-
cule), as well as by the correct shape–structure (high cross-
linkage) at high molar ratio of acetic acid to glycerol (9 : 1).
By contrast, low-pore catalyst should be used to generate high
selectivity of small MA under excessive glycerol concentration
(1 : 8 molar ratio of acetic acid to glycerol). The vital roles of
the catalyst to increase product selectivity include controlling
the acid sites, pore diameter, hydrolysis resistance, and
hydrophobicity, whereas the molar ratio of acetic acid to
glycerol is the more inuential factor that improves the
combined selectivity of DA and TA. Developing a hydro-
phobic-enhanced magnetic solid acid catalyst to overcome
the problem on water deactivation and subsequently devel-
oping a scalable high-conversion-selectivity catalyst is
strongly recommended.
Abbreviations
MA Monoacetin
DA Diacetin
TA Triacetin
TBA tert-Butyl alcohol
GMBE Mono-tert butyl ether
GDBE Glycerol di-tert butyl ether
GTBE Glycerol tertiary butyl ether
GEE Glycerol ethyl ether
mono-GEE Glycerol mono-ethyl ether
di-GEE Glycerol di-ethyl ether
tri-GEE Glycerol tri-ethyl ether
TBA tert-Butyl alcohol
PES Polymer-based acid
H
3
PMo
12
O
40
Dodeca-molybdophosphoric acid
HPAs Heteropolyacids
HSiW Silicotungstic acid
HPW Phosphotungstic acid
HPMo Phosphomolybdic acid
TPA 12-Tungstophosphoric acid
SnO
2
Tin oxide
MPTMS 3-Mercaptopropyl trimethoxysilane
MCF Mesostructured cellular foams
AC Activated carbon
GO Graphene oxide
Acknowledgements
The authors thank University of Malaya for supporting
this research under High Impact Research grant (Grant
Number: UM.C/625/1/HIR/MOHE/ENG/59). The Laboratoir-
ede G´
enie Chimique of Campus INP-ENSIACET, SBUM
scholarship and French government scholarship are grate-
fully acknowledged.
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