S. Kale et al. / Applied Catalysis A: General 490 (2015) 10–16
11
O
(A15, dry, Aldrich) were used. A70 was rested for 24 h at room
temperature in the fume hood to remove moisture. Glycerol of
high purity (>99%, Alfa Aesar), acetic acid (Aldrich) and toluene
(Applichem) were used as reactants and entrainer, respectively.
Furthermore, trimethylchlorosilane and hexamethyl disilazane
(Aldrich), dodecane (TCI Europe), hexadecane (Aldrich) and pyri-
dine (ACROS) were used for analytical purposes. Table 1 presents
a short overview on the physical properties of A70 and A15.
OH
OH
O
CH3
OH
H3C
H3C
O
O
OH
HO
+
Monoacetin
O
O
O
O
CH3
O
H3C
OH
Catalyst
HO
CH3
+
O
CH3
+
OH
O
Toluene
-H2O
O
O
Diacetin
HO
OH
O
O
CH3
O
2.2. Catalyst test procedure
H3C
O
CH3
Glycerol acetylation was performed in a three-necked 250 ml
glass flask at atmospheric pressure. The flask was equipped with
a fractionating Vigreux column, a modified Dean-Stark apparatus
equipped with thermometer to measure the temperature of the
boiling azeotropic mixture, a reflux condenser, a magnetic stirrer
(1200 rpm) and a glass tube with thermocouple inside the flask
to monitor the temperature of the reaction mixture (Fig. 2). The
flask was heated using an oil bath (150 ◦C, TI1), and the tempera-
ture fluctuation measured inside the flask was less then 1 K (TI2).
In a typical experiment, 10 g of glycerol (0.108 mol) and 39.16 g
of acetic acid (0.652 mol) representing an acetic acid to glycerol
ratio of 6: 1, 60 g of toluene (0.652 mol) and 1 g of dodecane (inter-
nal standard to evaluate liquid volume change) were introduced
into the reactor, followed by heating till the reaction mixture had
the desired temperature. Then 500 mg of catalyst was added to the
reaction mixture. Samples of 100 l volume were taken periodi-
cally. In addition, a blank test in absence of a catalyst was carried
out under similar reaction conditions. Moreover, a stop-experiment
was conducted by removal of catalyst from a regular experiment
after a defined reaction time, addition of fresh glycerol-acetic acid
feed and continuation of the run with the same reaction conditions
as before.
For proper GC analysis we had to reduce the polarity of the com-
pounds to be analyzed and therefore, the OH groups of unconverted
glycerol, MAG and DAG were silylated in a common procedure
before gas chromatographic analysis by using hexamethyl dis-
ilazane and trimethylchlorosilane with pyridine as solvent. In
addition, hexadecane was always used as an internal standard
for the GC analysis (this means that two internal standards were
used to cover the complete analysis procedure). Thereby prepared
product samples were kept at 70 ◦C in a drying oven for 45 min
and then analyzed using a gas chromatograph (HP 5890 series II)
equipped with a CP-Sil 13 CB column (25 m × 0.32 mm). The tem-
perature program was as follows: 50 ◦C for 1 min hold, heating
at 20 K/min to 310 ◦C, 2 min hold at 310 ◦C. The educts glycerol
and acetic acid as well as the silylated products MAG, DAG and
TAG were clearly eluted. For DAG, two isomers were detected in
the chromatograms, but separation was incomplete and individ-
ual quantification was impossible. Quantification of all reactants
was carried out using specific response factors determined from
GC analysis of authentic compounds in calibration mixtures of
known composition. Diglycerol tetraacetate (DGTA) was identified
by using an authentic sample prepared from linear diglycerol and
acetic anhydride; product identification was carried out using a GC-
MS combination (HP G1800 C, GCD Series II). The relative error of
O
O
Triacetin
Fig. 1. Reaction scheme of glycerol acetylation.
60 ◦C over 2 h, the selectivity for triacetin rises to 100%. But acetic
anhydride is much more expensive than acetic acid as well as haz-
ardous to health, hence it is not economically and environmentally
acceptable [25,26]. Amberlyst-15 was also used, giving 100% selec-
tivity for TAG with 41% yield at an extremely high molar ratio
of acetic acid to glycerol (24:1) and very high pressure (200 bar)
[27]. However, such extreme conditions are not attractive for an
application in industry. Hasabnis and Mahajani carried out a con-
tinuous reactive distillation process for glycerol esterification over
Amberlyst-15 with acetic acid using ethylene dichloride as an
entrainer. In this process, selectivity to TAG was 39.8% [28] with
complete conversion of glycerol.
In the available literature, the glycerol acetylation suffers from
limited selectivity to triacetin, as the reaction remains in equilib-
rium in the presence of water. In order to increase the selectivity
to the most desired product triacetin, it seems that a promising
product side, i.e. triacetin. However, the only way to remove water
via azeotropic distillation would be the addition of an external com-
ponent, referred to as an entrainer. Some results are already known
but the selectivity to triacetin is still low [cf. 28].
In the present work, the catalytic performance of acidic ion-
exchange resins Amberlyst-15 and Amberlyst-70 for glycerol
acetylation was evaluated with the main target to reach highest
TAG yield. In order to shift the chemical equilibrium to the side of
the products, favorably to TAG, toluene was used as an entrainer
to permanently remove the stoichiometric by-product water by
azeotropic distillation. The effects of reaction time, acetic acid: glyc-
erol molar ratio and catalyst amount were investigated. Fresh and
spent materials were analyzed by means of CHS analysis.
2. Experimental
2.1. Materials
Commercial ion-exchange resin Amberlyst-70 (A70, moisture
holding capacity 51–59%, Dow Chemicals) and Amberlyst-15
Table 1
Properties of the used Amberlyst catalysts.a
Catalyst
Acidity (eq./kg)
Cross-linkage (%)
Surface area (m2/g)
Average pore diameter (nm)
Thermal stability (◦C)
Amberlyst-15
Amberlyst-70
4.7
20
n.a.
0.297–0.841
0.500
53
36
30
22
120
190
2.55
2.80b
aData taken from supplier and from data reported in [35].
bDetermined by titration with KOH.