Glycerol acetins: Fuel additive synthesis by acetylation and esterification of glycerol using cesium phosphotungstate catalyst

Article abstract of DOI:10.1039/c5ra17623a

Glycerol acetylation and esterification reactions with acetic anhydride and acetic acid respectively give acetins, in which di and tri acetins are commercially important products used as fuel additives. Acetylation and esterification of glycerol were studied over various solid acid catalysts namely, cesium phosphotungstate, amberlyst-15, H-beta, sulfated zirconia and montmorillonite K-10 under mild reaction conditions. The catalysts were characterized by XRD, FTIR, SEM and acidity measurements. Among all the catalysts evaluated in this study, cesium phosphotungstate showed highest activity with >98% conversion for both the reactions, whereas di and triacetins selectivity was 99.1% for acetylation and 75% for esterification reaction. The catalyst with high Br?nsted acidity gave high activity for both the reactions, whereas selectivity for di and tri acetins depends on nature of active sites.

Full text of DOI:10.1039/c5ra17623a

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Page 1 of 33
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Glycerol acetins: Fuel additive synthesis by acetylation and esterification
of glycerol using cesium phosphotungstate catalyst
Swetha Sandesh, Pandian Manjunathan, Anand B. Halgeri and Ganapati V. Shanbhag*
Materials Science Division,
Poornaprajna Institute of Scientific Research (PPISR),
Bidalur Post, Devanahalli, Bengaluru-562164, Karnataka State, India
(* corresponding author: shanbhag@poornaprajna.org, Phone: +91-80-27408552)
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Abstract
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Glycerol acetylation and esterification reactions with acetic anhydride and acetic acid
respectively give acetins, in which di and tri acetins are commercially important products
used as fuel additives. Acetylation and esterification of glycerol were studied over various
solid acid catalysts namely, cesium phosphotungstate, amberlystꢀ15, Hꢀbeta, sulfated zirconia
and montmorillonite Kꢀ10 under mild reaction conditions. The catalysts were characterized
by XRD, FTIR, SEM and acidity measurements. Among all the catalysts evaluated in this
study, cesium phosphotungstate showed highest activity with > 98% conversion for both the
reactions, whereas di and triacetins selectivity was 99.1% for acetylation and 75% for
esterification reaction. The catalyst with high Brönsted acidity gave high activity for both the
reactions, whereas selectivity for di and tri acetins depends on nature of active sites.
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1. Introduction
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The growing scarcity of fossil hydrocarbons made the researchers to find alternative
source of energy. Biomass is considered to be a potential raw material for making renewable
fuels. Transesterification of vegetable oils or animal fats through catalytic route produces
biodiesel as the main product (90 wt%) and glycerol as a byproduct (10 wt%).1 Increase in
the production of biodiesel causes increase in glycerol availability in tons. Glycerol is a
propanetriol containing three hydroxyl groups which can be functionalized as well as
removed through catalytic processes. Transformation of available glycerol to valueꢀadded
products like oxygenated fuel additives, chemicals, solvents etc. can add value to it and make
the process economically viable and valuable.2ꢀ8
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Glycerol undergoes acetylation with acetic acid or acetic anhydride in presence of
acid catalyst to yield acetins namely monoacetin, diacetin and triacetin. Di and triacetins can
be used as fuel additive which have been introduced in biodiesel formulation to improve its
viscosity property as cold flow improver and it has also been used as an antiknock additive
for gasoline. Triacetin is also used in cosmetics, whereas monoacetin and diacetin are used as
plasticizer in cigarette filters and as raw materials for the production of biodegradable
polyesters.9
Homogeneous acids such as H3PW12O40 showed higher catalytic activities towards
acetylation reaction10,11 but faced several practical difficulties in separation of catalyst,
recyclability and handling. To overcome these practical difficulties, variety of Brönsted solid
acid catalysts have been reported for the reaction of glycerol with acetic acid or acetic
anhydride. Solid acid catalysts such as amberlystꢀ15, montmorillonite Kꢀ10, beta zeolite and
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HꢀUSY
were applied as catalysts for this reaction which showed 100% glycerol
14ꢀ20
conversion using higher catalyst concentrations. Supported sulfonic acid catalysts
and
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mixed oxides like MoO3/TiO2–ZrO2, Y/SBAꢀ3 22 have been reported to be active catalysts
for acetylation of glycerol. Even supported heteropoly acid catalysts like PW on silica, Csꢀ
containing zirconia, carbon, niobic acid with high thermal stability and high surface area have
been used but showed less efficiency for this reaction.23ꢀ27 Silver ion exchanged
phosphotungstic acid catalyst is also used for the esterification of glycerol with acetic acid.28
Recently, cesium exchanged heteropoly acid has received greater attention as a
catalyst in many reactions due to its heterogeneous property, high thermal stability and higher
surface area compared to parent heteropoly acid. The catalytic properties of metal ion
exchanged heteropoly acid can be tuned by choosing appropriate metal salt and by varying
the extent of ion exchange. The studies in the literature show that the cesium
phosphotungstate is more active than parent PWA due to its high surface protonic acidity
with high acid strength of the proton associated to the polyanion.29ꢀ32
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The aim of this work is to explore a catalyst for the synthesis of glycerol acetins under
mild reaction conditions (low temperature, mole ratio and catalyst concentration) and to get
higher activity and selectivity for di and triacetins. Different well known solid acid catalysts
like cesium phosphotungstate, zeolites, resin, clay and sulfated zirconia were studied for both
acetylation and esterification reactions of glycerol. The best catalyst was taken further for
detailed studies. The physicochemical properties of the catalysts were correlated with
catalytic activity and selectivity for acetins.
2. Experimental
2.1. Chemicals and catalysts
Glycerol and acetic acid were purchased from Merck India Ltd. Cesium carbonate and
phosphotungstic acid (PWA) were procured from SD fine chemicals, India. Amberlystꢀ15
(hereafter ABꢀ15) was obtained from Alfa Aesar, USA. The montmorillonite Kꢀ10 clay
(hereafter Kꢀ10) was purchased from Sigma Aldrich, USA. Hꢀbeta (SARꢀ25) was kindly
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donated by SüdꢀChemie India Pvt Ltd. All the chemicals were of research grade and used
without any further purification.
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The acidic cesium phosphotungstate (hereafter CsPWA) was prepared following a
literature procedure.33 The final composition of the salt was found to be Cs2.5H0.5PW12O40.
Other solid acid catalyst, sulfated zirconia (SZ)was synthesized by literature method.34
2.2. Catalyst characterization
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Powder Xꢀray diffraction patterns of CsPWA and PWA were recorded with Bruker
D2 phaser Xꢀray diffractometer using CuKα radiation (λ = 1.542 Å) with high resolution
Lynxeye detector. All the samples were scanned in the 2θ range of 5 – 80°. The specific
surface areas of the catalysts were determined by nitrogen sorption measurement using
Quantachrome NOVA instrument at 77 K.
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The nature of acidic sites of catalysts was investigated by pyridine adsorption study
using PyridineꢀFTꢀIR (alphaꢀT, Bruker) and the spectra were obtained in the range of 1400–
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1600 cm− . The catalyst pellets were saturated by pyridine followed by degassing at 150 °C
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for 1 h. The FTIR spectra in absorbance mode for pyridine treated sample were subtracted
with pyridine untreated sample to obtain the peaks only due to pyridine–acid interaction.34
In addition to above method, acidity of the catalysts was determined by
potentiometric titration. About 0.05 g of sample was suspended in 5 ml of nꢀbutylamine
solution (0.05 N) in acetic acid and sonicated for 5 min to attain uniform dispersion. Then the
above solution was suspended in excess of acetic acid (90 mL) and potentiometrically titrated
against perchloric acid (0.1 N) in acetic acid. Prior to sample titration, a blank titration of
acetic acid and nꢀbutyl amine against perchloric acid was carried out to check the acidity
contribution from solutions used. ICPꢀOES was performed using a ThermoꢀiCAP 6000 series
in order to study the leaching of cesium in the reaction mixture.
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Scanning electron microscope (SEM) images of CsPWA catalyst were recorded on
Zeiss microscope to investigate the crystallite size and morphology.
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2.3 Catalytic activity studies
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Catalytic activity studies were performed in a liquid phase glass batch reactor. Prior to
the reaction, the catalysts (except Amberlystꢀ15) were activated at 120 °C to remove the
moisture.
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a) Acetylation reaction of glycerol with acetic anhydride: In a typical procedure, the reaction
was performed in a 100 ml twoꢀnecked glass reactor equipped with a magnetic stirring bar, a
Liebig condenser, and a thermometer. The glycerol and acetic anhydride were taken in the
ratio of 1: 3 in the glass reactor and 4 wt% of catalyst (with respect to total reactants) were
added into it. The reaction was performed under stirring at room temperature.
b) Esterification reaction of glycerol with acetic acid: In a typical procedure, the reaction was
carried out in a 100 ml twoꢀnecked glass reactor equipped with a magnetic stirring bar, a
Liebig condenser, and a thermometer. The required amounts of glycerol and acetic acid were
taken in the reactor and desired catalyst weight was added into it. The reaction was performed
under stirring at desired temperature.
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For both the reactions, same separation procedure was followed; the reaction mixture
was taken out and centrifuged for 10 min to separate the catalyst from liquid phase. The
obtained product was analyzed in gas chromatography (Shimadzu, GCꢀ2014) with flame
ionization detector (FID) equipped with capillary column (0.25mm I.D and 30 m length,
Stabilwax, Restek). All the products were confirmed by gas chromatography with mass
spectroscopy (Shimadzu, GCMS QP 2010).
3. Results and discussion
3.1. Catalyst characterization
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The formation of crystalline phase of CsPWA catalyst was confirmed by XRD in
comparison with phosphotungstic acid (Fig.1a). XRD pattern of PWA shows the diffraction
peaks corresponding to cubic Pn3m crystalline structure. Interestingly, the diffraction peak of
CsPWA became significantly broader with a right shift in 2θ value (25°) compared to PWA.
The shift of diffraction peak towards higher angle in CsPWA is attributed to the formation of
body centered cubic structure.29,35
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The characteristic presence of Keggin structure of CsPWA and phosphotungstic acid
was confirmed by FTIR studies (Fig. 1b). Four bands at 700ꢀ1100 cmꢀ1 region corresponding
to Keggin unit (PWA) structural vibrations are observed for PWA and CsPWA suggesting
that the framework of primary Keggin structure remained unaltered after modification of
PWA with cesium salt. The peaks corresponding to Keggin anion vibration are as follows.
The stretching frequency of P–O in the central PO4 tetrahedron is at 1084 cmꢀ1. The peak at
991 cmꢀ1 is due to the terminal W=O vibration in the WO6 octahedron and the peak at 890
and 794 cmꢀ1 were assigned to W–Ob–W and W–Oc–W bridges respectively. Weaker peak
appearing at 595 cmꢀ1 is due to bending vibrations of W–O–W bonds. 32
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The physicochemical properties of CsPWA, ABꢀ15, Kꢀ10, Hꢀbeta and sulfated
zirconia are tabulated in Table 1. The specific surface area of asꢀprepared CsPWA was found
to be 110 m2/g. Hꢀbeta and Kꢀ10 exhibited higher surface area of 450 and 250 m2/g
respectively, whereas amberlystꢀ15 and sulfated zirconia gave lower surface areas < 60 m2/g.
The interaction of pyridine nitrogen with acidic sites gave two different frequency of
bending vibrations. The bending vibrations around 1445 cmꢀ1 and 1540 cmꢀ1 are assigned as
Lewis (L) and Brönsted (B) acid sites respectively and B/L ratio were measured using the
peak intensities. PyridineꢀFTIR spectra of CsPWA catalyst showed a strong Brönsted acidity
due to the presence of protons (peak at 1540 cmꢀ1) and weak Lewis acid sites (peak at 1445
cmꢀ1) as depicted in Fig. 2. The CsPWA contained high B/L ratio of 3.86 compared to other
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solid acid catalysts used in this study except ABꢀ15 (Table 1). ABꢀ15 is a pure Brönsted acid
catalyst with sulfonic acid groups on polystyrene chain. The B/L ratio decreased in the order;
ABꢀ15 > CsPWA > Kꢀ10 > Hꢀbeta > sulfated zirconia.
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Potentiometric acidꢀbase titration revealed the total acidity of the catalysts (tabulated
in Table 1). The total acidity of CsPWA, Hꢀbeta, Kꢀ10 and SZ was found to be 1.87, 1.49,
1.10 and 1.48 mmol/g respectively. Hꢀbeta zeolite and sulfated zirconia has same amount of
acidic sites. Acidity of ABꢀ15 was 4.7 mmol/g as given by the manufacturer.
CsPWA catalyst exhibited the morphology of the spherical shaped particles with size
ranging from 70ꢀ200 nm as shown in Fig. 3.
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3.2. Catalyst screening
Acetylation and esterification of glycerol was studied over various solid acid catalysts
namely, CsPWA, ABꢀ15, Hꢀbeta, sulfated zirconia and Kꢀ10 using acetic anhydride and
acetic acid respectively. The performance of the catalyst is measured by glycerol conversion
and the selectivity to di and triacetins.
Acetylation of glycerol using acetic anhydride was carried out over different Brönsted
solid acid catalysts at room temperature (30 °C) (Fig. 4.). Prior to the catalytic reaction, a
blank run was carried out without a catalyst, which resulted in negligible glycerol conversion
(2.5%) with 100% selectivity to monoacetins. Among the solid acid catalysts screened, the
catalyst containing higher amount of acid sites viz. CsPWA (1.87 mmol/g) and ABꢀ15 (4.7
mmol/g) resulted in maximum glycerol conversion (100%) with higher glycerol diacetins and
glycerol triacetin selectivity of 99.1 and 99.9% respectively. Among the solid acid catalysts
screened, the catalyst containing higher amount of acid sites viz. CsPWA (1.87 mmol/g) and
ABꢀ15 (4.7 mmol/g) gave maximum glycerol conversion (100%) with higher selectivity
towards diacetins (17 and 23%) and triacetin (82 and 77%) respectively. The catalytic
activity of CsPWA showed higher triacetin selectivity of 82% at room temperature compared
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to all other solid acid catalysts for 2 h of reaction time. This result shows that the utilization
of acetic anhydride is maximum for CsPWA and ABꢀ15 with higher selectivity to triacetin
compared with other catalysts namely Kꢀ10, Hꢀbeta and sulfated zirconia. The glycerol
conversion reached to a maximum of 100% at the initial time period, but the triacetin
selectivity was found to increase with time for CsPWA and ABꢀ15 with a decrease in mono
and diacetins selectivity (Fig. 4). Kꢀ10 containing B/L ratio of 2.3 gave lower triacetin
selectivity of 32%, whereas Hꢀbeta catalyst (B/L ratio of 1.92) showed 80% diacetin
selectivity. Sulfated zirconia, the catalyst with higher Lewis acidic sites showed very low
glycerol conversion of 25%. These results clearly show that the catalyst with higher Brönsted
acidic sites gives higher glycerol conversion with high di and triacetins selectivity. The
glycerol conversion and triacetin selectivity increased for the catalysts in the following order;
SZ < Hꢀbeta < K10 < ABꢀ15 < Cs/PWA.
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Since CsPWA and ABꢀ15 showed complete glycerol conversion at 30 min, it was not
possible to decide the best catalyst among the two. Therefore, the catalyst concentration was
reduced to 1 wt% (w.r.t. total reactants) and as a result, the catalytic performance of ABꢀ15
showed lower glycerol conversion of 25% at 30 min. As the time increased, glycerol
converted completely with increase in triacetin selectivity. But CsPWA catalyst showed 99.8
% glycerol conversion even at less catalyst amount for 30 min with higher triacetin selectivity
compared to ABꢀ15 (Fig. S1†). The turn over frequency of all the catalysts (Table 1)
increased in the following order; SZ < ABꢀ15 < Hꢀbeta < Kꢀ10 < CsPWA. Highest TOF/h of
267 was obtained for CsPWA which clearly proves that CsPWA is highly active catalyst for
acetylation reaction of glycerol. The high selectivity towards triacetin using acetic anhydride
as acetylating agent compared to acetic acid can be explained on the basis of formation of
intermediate acylium ion (Scheme S1†).12
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In order to study the catalytic behaviour at lower reactant mole ratio, the reaction was
studied with glycerol : acetic anhydride of 1:1.5 and 1:2 (Table 2). It showed relatively lower
selectivity towards triacetin compared to higher reactant mole ratio of 1:3 suggesting that the
formation of triacetin is greater with higher amount of acetylating agent.
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Further, the esterification of glycerol was studied using acetic acid under reaction
conditions; glycerol : acetic acid of 1:8, 85 °C and 7 wt% of catalyst referred to total
reactants (Fig. 5). As observed in the acetylation reaction, a similar catalytic performance was
observed with high performance of CsPWA and ABꢀ15 compared with other catalysts. The
glycerol conversion reached to 98% using CsPWA and ABꢀ15 within 2 h with increase in
diacetins and triacetin selectivity. CsPWA exhibited higher catalytic performance with
triacetin selectivity of 27%, whereas ABꢀ15 gave 22% triacetin selectivity. Among these two
catalysts, CsPWA exhibited much higher TOF at 30.5 hꢀ1 compared to ABꢀ15 (12.3 hꢀ1)
(Table 1).
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Among lower active catalysts, large pore Hꢀbeta zeolite exhibited comparatively
greater catalytic performance than Kꢀ10 and sulfated zirconia. Glycerol conversion increased
from 28 to 80% with increase in time from 1 to 5 h using Hꢀbeta and finally reached to 37%
diacetins selectivity (5 h). Triacetin did not form with Hꢀbeta catalyst. In contrast, Kꢀ10 clay
showed lower glycerol conversion (63%) compared to Hꢀbeta zeolite but it gave triacetin
selectivity of 4% (5 h). Glycerol conversion of 70% with 20% diacetin selectivity was
observed for sulfated zirconia catalyst. It exhibited lower activity compared to other acid
catalysts which could be due to lower B/L ratio (1.46), since the esterification reactions are
predominantly catalyzed by Brönsted acid sites. Thus, the catalytic activity towards
esterification of glycerol with acetic acid gives a clear picture with respect to nature of acidic
sites (B/L ratio) of the catalyst. The turn over frequency of the screened catalysts increased in
the following order; SZ < ABꢀ15 ≈ HꢀBeta < Kꢀ10 < CsPWA.
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3.3 Influence of reaction conditions on esterification reaction of glycerol with acetic acid
The influence of reaction parameters viz. catalyst concentration, reaction temperature,
and reactants mole ratio on catalytic activity using CsPWA catalyst were studied for glycerol
esterification with acetic acid.
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Effect of catalyst concentration was studied with glycerol to acetic acid mole ratio of
1 : 8 at 85 °C for 2 h. The catalyst concentration was varied from 3 to 9 wt% (Table 3). The
glycerol conversion was found to increase from 56 to 98% with increase in the catalyst
concentration from 3 to 7 wt%. The lesser catalytic activity with catalyst concentration of 3
and 5 wt% indicates the requirement of higher active sites for the reaction. Selectivity to
diacetins (31 and 34%) was almost the same for 3 and 5 wt% catalysts, but the triacetin was
formed with 5 wt% catalyst (5% selectivity), whereas for 3 wt% catalyst, triacetin was not
observed. The catalytic activity was found to be almost the same with 7 and 9 wt% catalyst
concentrations. The glycerol conversion increased from ~ 84 to 98% as the time increased
from 30 min to 2 h. The maximum of 98% glycerol conversion was attained at 1 h using 7
wt% catalyst concentration, but the selectivity to diacetin varied from 55 to 59% after 2 h.
The maximum triacetin selectivity of 16% was obtained after 2 h. No major variation in
catalytic performance was observed with further increase in catalyst concentration to 9 wt%.
Moreover, the selectivity to all the acetins remained the same as in the case of 7 wt% catalyst
concentration. This indicates that the amount of active acidic sites in 7 wt% catalyst
concentration is sufficient to get the maximum activity of glycerol conversion and selectivity
to the desired product.
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The effect of glycerol to acetic acid mole ratio was studied from 1:4 to 1:10 at 85 °C
for 2 h (Table 4). The conversion of glycerol increased with increase in mole ratio from 1:4 to
1:8 due to increase in availability of accessible acetic acid with glycerol. The glycerol
conversion and selectivity to acetins remained almost the same with further increase in mole
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ratio of reactants from 1:8 to 1:10. The reaction condition with 1:8 mole ratio was found to be
the best compared with other mole ratios. A gradual increase in glycerol conversion from 45
to 69% with increase in reaction time was observed for 1:4. Formation of triacetin was found
to be nil at this mole ratio. This indicates that at 1:4 mole ratio, the amount of accessible
acetic acid was not sufficient for the maximum conversion of glycerol to yield higher amount
of diacetin and triacetin. For mole ratio 1:6, the glycerol conversion increased from 67 to
92% with the increase in time from 30 to 120 min. The catalytic activity with 1:8 and 1:10
mole ratio was found to be almost the same. The glycerol reached a maximum conversion of
98% with negligible changes in the acetins selectivity (diacetins and triacetin was 59 and
16% respectively). Therefore, 1:8 reactants mole ratio was found to be the best mole ratio for
further studies.
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The effect of temperature was studied at four different temperatures ranging from 65
to 95 °C using glycerol to acetic acid mole ratio of 1:8 for 2 h. From Table 5, it is observed
that the glycerol conversions were low and slowly increased with time at temperatures of 65
and 75 °C, which could be attributed to lesser formation of acylium ion from acetic acid at
lower temperatures. At higher temperatures of 85 and 95 °C, glycerol conversion reached to a
maximum of 98% and remained almost the same, indicating that the formation of acylium ion
is faster at these temperatures. It is also observed that di and triacetins increased with increase
in reaction time. The catalytic activity at 85 °C was found to be best temperature for
esterification reaction since the glycerol reached a maximum conversion of 98% with the
selectivity to di and triacetin of 59 and 16% respectively.
3.4. Plausible reaction mechanism for esterification reaction of glycerol with acetic acid
The plausible reaction mechanism for esterification of glycerol with acetic acid
proceeds by the activation of acetic acid carbonyl group by CsPWA catalyst whereby
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electrophilicity of carbonyl carbon increases (Scheme 1). Then the carbonyl carbon is
attacked by the oxygen of glycerol. The transfer of proton from the intermediate to the second
hydroxyl group of glycerol gives an activated complex with the formation of water molecule.
Later, with the loss of water molecule gives monoacetin. The aboveꢀmentioned mechanism
continues sequentially further by the reaction of monoacetin with acetic acid to form diacetin
and triacetin.
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3.5. Catalyst reusability and leaching studies
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Catalyst recyclability test was performed for CsPWA catalyst under optimized
reaction conditions for both acetylation and esterification reactions. The catalyst showed
good recyclability with similar activity after 3 recycles (Table 6). In case of acetylation, the
selectivity of triacetin was retained after initial decrease in the 1st recycle. For esterification
reaction, with each recycle, triacetin decreased marginally with the increase of monoacetin
and diacetin. XRD analysis of the fresh and 3 times recycled catalyst showed no change in
the phase purity of the catalyst (Fig. 6). FTIR analysis of the spent CsPWA catalyst was also
performed and it showed no change in the characteristic peak of Keggin structure after the
recycle (Fig. S2†).
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The leaching test was carried out for acetylation and esterification reactions by
investigating the leaching of Cs in the catalyst into the reaction media. The study was
performed under the optimized reaction conditions where the reaction was stopped at 2 and 5
h for acetylation and esterification reactions respectively and the catalyst was filtered out.
Thus obtained filtrate was subjected to ICPꢀOES analysis of Cs in order to find the leaching
of Cs. The analysis confirmed the absence of Cs in the reaction mixture under the detection
limit of 0.01ppm which suggests that the catalyst is truly heterogeneous.
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3.6. Comparison of CsPWA with the reported catalysts for the esterification of glycerol
The active CsPWA catalyst was compared with the reported catalysts for the
esterification of glycerol and the data was tabulated in Table S1†. Among the reported
catalysts, supported heteropoly acids viz. HSiW/ZrO2 and HPW/ZrO2 catalysts showed
slightly higher selectivity for di and triacetins compared to CsPWA catalyst. However, higher
reaction temperature (difference of 35 °C) and glycerol to acetic acid mole ratio were used
for these catalysts. At lower temperature and reactants mole ratio, CsPWA catalyst showed
higher activity and selectivity towards di and triacetins compared to the reported catalysts.
4. Conclusions
3
4
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Acetylation and esterification of glycerol were studied with acetic anhydride and
acetic acid respectively using different solid acid catalysts. The yields of mono, di and
triacetins were differed with the nature of acid catalysts. Among the solid acid catalysts
screened, the catalyst containing higher amount of acid sites viz. CsPWA (1.87 mmol/g) and
ABꢀ15 (4.7 mmol/g) resulted in maximum glycerol conversion (100%) with higher di and
triacetins selectivity of 99.1 and 99.9% respectively for acetylation reaction. CsPWA showed
highest triacetin selectivity of 82% at room temperature compared to all other solid acid
catalysts. The turn over frequency for acetylation of glycerol increased in the following order;
SZ < Amberlystꢀ15 < Hꢀbeta < Kꢀ10 < CsPWA with highest TOF/h of 267 for CsPWA
catalyst. CsPWA catalyst also gave highest activity and selectivity for di and triacetins for
esterification of glycerol with acetic acid. The catalytic activity towards the reaction was
correlated with B/L ratio of the catalyst. Higher catalytic activities of CsPWA and ABꢀ15 are
due to higher B/L ratio of the catalysts. Among the two catalysts, CsPWA gave highest di and
triacetins selectivity which could be due to the nature of active sites present in the catalyst.
14

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1
The catalyst exhibited good recyclability with marginal decrease in the activity after 3
recycles.
2
3
4
5
6
5. Acknowledgement
Swetha S. acknowledges CSIR, New Delhi for providing Senior Research Fellowship
and also thankful to Manipal University for permitting this research as a part of the Ph. D
programme
.
7
8
6. References
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Figures
Fig. 1. XRD patterns (a) and FTIR spectra (b) of CsPWA and PWA.
Fig. 2. Pyridine-FTIR spectra of catalysts.
Fig. 3. SEM images of CsPWA.
Fig. 4. Catalytic activity of different solid acid catalysts with glycerol and acetic anhydride.
Reaction conditions: Glycerol : Acetic anhydride = 1 : 3, Temperature = 30 °C, Catalyst
weight = 3 wt%.
Fig. 5. Catalytic activity of different solid acid catalysts with glycerol and acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1 : 8, Temperature = 85 °C, Time= 5 h, Catalyst
weight = 7 wt%.
Fig. 6. XRD patterns of fresh and recycled catalyst
Scheme 1. Plausible reaction mechanism for the esterification

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Fig. 1. XRD patterns (a) and FTIR spectra (b) of CsPWA and PWA.

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Fig. 2. Pyridine-FTIR spectra of catalysts.

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Fig. 3. SEM images of CsPWA catalyst

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Fig. 4. Catalytic activity of different solid acid catalysts with glycerol and acetic anhydride.
Reaction conditions: Glycerol : Acetic anhydride = 1 : 3, Temperature = 30 °C, Time= 120
min, Catalyst weight = 4 wt%.

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Fig. 5. Catalytic activity of different solid acid catalysts with glycerol and acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1: 8, Temperature = 85 °C, Time= 5 h, Catalyst
weight = 7 wt%.

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Fig. 6. XRD patterns of fresh and recycled CsPWA catalyst

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Scheme 1.Plausible reaction mechanism for the esterification

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Tables
Table 1. Physicochemical properties and activities of different catalysts for acetylation and
esterification of glycerol.
Table 2. Effect of reactant mole ratio on catalytic performance in acetylation of glycerol with
acetic anhydride.
Reaction conditions: Temperature = 30 °C, Time= 120 min, CsPWA catalyst = 4 wt%.
Table 3. Effect of catalyst concentration on catalytic activity in esterification of glycerol with
acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1 : 8 , Temperature = 85 °C, Catalyst =
CsPWA.
Table 4. Effect of reactant mole ratio on catalyst performance in esterification of glycerol
with acetic acid.
Reaction conditions: Temperature = 85 °C, CsPWA catalyst = 7 wt%.
Table 5. Effect of temperature on catalyst performance in esterification of glycerol with
acetic acid.
Reaction conditions: Glycerol : Acetic acid = 1: 8 , CsPWA catalyst = 7 wt%.
Table 6. Reusability test of CsPWA catalyst on acetylation and esterification of glycerol.

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Table 1: Physicochemical properties and activities of different catalysts for acetylation and
esterification of glycerol.
Catalyst
SBET
Amount Py-FTIR Acetylationa Esterificationc
(m2/g) of acidity B/L ratio
(mmol /g)
TOF
TOF
(h-1)
(h-1)
CsPWA
AB -15
110
39
1.87
4.7
3.86
-
69.7
267b
27.7
28.3b
30.5
12.3
H-beta
450
250
57
1.49
1.1
1.92
2.30
1.44
55.2
58.6
11.9
12.4
14.7
9.2
K-10
Sulfated zirconia
1.48
Turn over frequency (TOF) = Moles of glycerol converted per mole of acid site per hour.
b
c
aReaction conditions as in Fig. 4, 1wt% catalyst, Reaction conditions as in Fig. 5

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Table 2. Effect of reactant mole ratio on catalytic performance in acetylation of glycerol with
acetic anhydride.
Time Glycerol : Glycerol
Acetin Selectivity (mol%)
(h)
Acetic
anhydride
conversion
(mol%)
Mono
Di
Tri
1 h
2 h
1:1.5
1:1.5
92.6
98
19.3
13.3
40.3
41.2
40.4
45.5
1 h
2 h
1:2
1:2
98.1
98.3
12.8
12.7
33.3
31.3
54.0
56.1
1 h
2 h
1:3
1:3
100
100
1.2
0.9
21.5
17.1
77.3
82
Reaction conditions: Temperature = 30 °C, Time= 120 min, CsPWA catalyst = 4 wt%.

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Table 3. Effect of catalyst concentration on catalytic activity in esterification of glycerol with
acetic acid.
Catalyst Time
Glycerol
Acetin Selectivity (mol%)
wt%
(min) conversion
(mol%)
Mono
Di
Tri
3
3
3
3
30
60
90
30
35.4
40
95
80
72
69
5
0
0
0
0
20
28
31
120
56
5
5
5
5
30
60
90
40
52
62
70
86
75
68
61
14
25
30
34
0
0
2
5
120
7
7
7
7
30
60
90
84
97.5
98
37
35
27
25
55
56
58
59
8
9
15
16
120
98.1
9
9
9
9
30
60
90
85
95.6
97
36
30
28
26
54
58
58
58
10
12
14
16
120
98.2
Reaction conditions: Glycerol : Acetic acid = 1 : 8 , Temperature = 85 °C, Catalyst =
CsPWA.