Investigation of Sn(IV) catalysts in glycerol acetylation

Article abstract of DOI:10.1016/j.mcat.2020.111130

The catalysts dibutyltin dichloride (Bu₂SnCl₂), dimethyltin dichloride (Me₂SnCl₂), butyltin trichloride (BuSnCl₃), dibutyltin dilaurate (Bu₂SnLau₂), butyl stannoic acid [BuSnO(OH)], and di-n-butyl-oxo-stannane (Bu₂SnO) were investigated in the acetylation of glycerol (GLY). To the best of our knowledge, this is the first time that this family of catalysts is employed for this reaction and the catalysts bearing chlorine substituents provided the best results. The most active system (BuSnCl₃) leads to the total conversion of GLY and shows very good selectivity (25.7, 43.5 and 30.8 percent for MA, DA and TA, respectively) under mild reaction conditions (80 °C, AA:GLY molar ration of 4:1 with 3 h of reaction). The apparent rate constants (k_ap) for the GLY conversion confirm these results, since values of 29.1 × 10^?3, 67.9 × 10^?3 and 87.2 × 10^?3 h^?1 were obtained at 40, 80 and 120 °C, respectively. The activation energy for GLY conversion was determined and for the reaction performed without catalyst the estimated value was 23.9 kJ mol^?1 while using BuSnCl₃ it was 14.3 kJ mol^?1, representing a decrease of around 50 percent.

Full text of DOI:10.1016/j.mcat.2020.111130

MolecularCatalysis494(2020)111130
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Investigation of Sn(IV) catalysts in glycerol acetylation
Débora S. da Silva, Felyppe M.R.S. Altino, Janaína H. Bortoluzzi, Simoni M.P. Meneghetti*
GCAR, Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, Av. Lourival de Melo Mota, s/ nº, Maceió, AL, 57072-970, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Esterification
Acetins
Tin
Acetic acid
The catalysts dibutyltin dichloride (Bu2SnCl2), dimethyltin dichloride (Me2SnCl2), butyltin trichloride (BuSnCl3),
dibutyltin dilaurate (Bu2SnLau2), butyl stannoic acid [BuSnO(OH)], and di-n-butyl-oxo-stannane (Bu2SnO) were
investigated in the acetylation of glycerol (GLY). To the best of our knowledge, this is the first time that this
family of catalysts is employed for this reaction and the catalysts bearing chlorine substituents provided the best
results. The most active system (BuSnCl3) leads to the total conversion of GLY and shows very good selectivity
(25.7, 43.5 and 30.8 % for MA, DA and TA, respectively) under mild reaction conditions (80 °C, AA:GLY molar
ration of 4:1 with 3 h of reaction). The apparent rate constants (kap) for the GLY conversion confirm these results,
since values of 29.1 × 10−3, 67.9 × 10−3 and 87.2 × 10−3 h−1 were obtained at 40, 80 and 120 °C, respec-
tively. The activation energy for GLY conversion was determined and for the reaction performed without catalyst
the estimated value was 23.9 kJ mol−1 while using BuSnCl3 it was 14.3 kJ mol−1, representing a decrease of
around 50 %.
1. Introduction
cosmetics industry, as an emulsifier, perfume fixer and antiseptic
[21,22].
The development of chemical inputs and fuels from renewable
sources has been the subject of intense research and technological de-
velopment, mainly due to economic and environmental aspects. In this
scenario, the production of biodiesel through the transesterification of
vegetable oils has increased rapidly in recent years [1–3]. In biodiesel
production, glycerol (GLY) forms as the main co-product (10 % by
weight) and in order to add value and to make the process more sus-
tainable several reactions are studied [4–13] and GLY esterification is a
promising alternative with potential industrial applications [3,14–16].
Acetins or GLY acetals are obtained by reacting GLY with a car-
boxylic acid, usually acetic anhydride or acetic acid (AA), producing
monoacetin (1- and 2-MA), diacetin (1,3- and 1,2-DA) and triacetin
(TA) [17,18].
MA is a dense and hygroscopic liquid that is widely applied as a
gelatinizing agent, a solvent for paints and in the manufacture of ex-
plosives [18]. Similarly, DA is a hygroscopic liquid and is used as a
lubricant, softening agent and solvent [19]. In addition, MA and DA are
employed in the food and pharmaceutical industries and as building
blocks in cryogenics and biodegradable polyester [20]. TA represents
10 % of the global glycerol market and is consumed mainly as fuel
additives, such as an anti-knock agent for gasoline, diesel and biodiesel
[18]. Its use is also widespread in the production of cigarette filters, as a
cellulose plasticizer [19]. Furthermore, it can be applied in the
The acetylation of GLY is a three-step reaction (Eqs. (1)–(3), and
3 mol of AA is required to react with 1 mol of GLY, in order to ac-
complish the MA conversion to TA with the release of 3 molecules of
water as a by-product [23]. These reactions are reversible and equili-
brium-controlled, so several parameters influence the reaction rate,
including temperature, molar ratio, type of catalyst and loading (i.e.,
number of available active sites), and speed of stirring.
GLY + AA → MA + H2O
MA + AA → DA + H2O
DA + AA → TA + H2O
(1)
(2)
(3)
This type of reaction is traditionally conducted with catalysts in the
homogeneous phase, the most common being sulfuric acid, hydro-
chloric acid and para-toluenesulfonic acid [18,24]. However, the use of
these catalysts is increasingly limited due to environmental problems
related to the discharge of effluents with a high degree of toxicity and
also due to adverse effects on reaction system due to the high corrosion
power [25,26].
In order to overcome these problems, different catalysts have been
developed, with notable examples being ion exchange resins [1,23] and
heteropolyacids [20]. The esterification of glycerol has been in-
vestigated using strong acid ion exchange resins (Amberlyst 15) and
⁎ Corresponding author.
E-mail addresses: simoni.plentz@gmail.com, simoni.plentz@iqb.ufal.br (S.M.P. Meneghetti).
https://doi.org/10.1016/j.mcat.2020.111130
Received 12 May 2020; Received in revised form 27 June 2020; Accepted 18 July 2020
Availableonline03August2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

D.S. da Silva, et al.
MolecularCatalysis494(2020)111130
Fig. 1. GLY conversion and MA, DA and TA yields (%) in acetylation of GLY at 40, 80 and 120 °C, as a function of reaction time (min), using a GLY/AA molar ratio of
1:4 in the absence of catalyst and in the presence of BuSnO(OH), Bu2SnO and Bu2SnLau2 (ND = not determined; the errors determined were 0.8. 0.5. 0.7 and 0.4 %
for GLY, MA, DA and TA, respectively).
acid zeolites (HZSM-5 and HUSY) as catalysts, at 110 °C, with an acetic
acid/glycerol molar ratio of 9:1 and reaction time of 4.5 h. It was ob-
served that Amberlyst 15 showed the best results, with a high conver-
sion (above 98 %) and values for selectivity to MA, DA and TA of 33.42,
47.62 and 18.94, respectively [23]. It is important to note that the
HZSM-5 and HUSY catalysts can present diffusion problems and their
active sites can be deactivated by the formation of water in the reaction
[23]. In the studies conducted using p-toluenesulfonic acid (p-TSA),
Amberlyst-15 and Amberlyst-36 as catalysts, conversions close to 100 %
of glycerol and values for selectivity of 43, 44 and 13 % for MA, DA and
TA, respectively, were attained, at 100 °C, with an acid/glycerol molar
ratio of 7:1 and 6 h of reaction [27]. The catalytic behavior of mesos-
tructured materials functionalized with sulfonic acid was evaluated and
a conversion of up to 90 % of glycerol and more than 80 % selectivity
for MA and TA were observed, after 4 h at 125 °C and using an AA/GLY
molar ratio of 9:1. According to the authors, the excellent performance
obtained is due to the high acid strength of sulfonic acid compared to
conventional acid catalysts, such as H2SO4 [25]. The performance of the
cesium-supported heteropolyacid catalyst (CsPWA) was evaluated in
the glycerol esterification with acetic acid and the results showed that
there was a maximum conversion of glycerol (98 %) and selectivity for
MA, DA and TA (25, 58 and 15 %, respectively) at 110 °C, with an AA/
GLY molar ratio of 9:1 and 2 h of reaction [20]. Catalysts based on
mesoporous carbons were studied in glycerol acetylation and the
functionalization of these materials with SO3H groups was responsible
for the better results obtained at 110 °C, with an AA/GLY molar ratio of
9:1 and 6 h of reaction (conversion of 95 % and values for selectivity for
MA, DA and TA of 58, 20 and 22 %, respectively) [28].
There are some examples of the use of Sn(IV)-based catalytic sys-
tems in esterification and transesterification reactions, to produce
chemicals and biofuels [29–34]. Nevertheless, to the best of our
knowledge, no information is available on the performance of this class
of catalysts in GLY acetylation to produce MA, DA and TA. In this
context, a systematic study employing several Sn(IV) catalysts was
carried out and the results are presented and discussed in order to
understand the behavior of these systems in this reaction.
2. Experimental
2.1. Materials
Dibutyltin
dichloride
(Bu2SnCl2),
dimethyltin
dichloride
2

D.S. da Silva, et al.
MolecularCatalysis494(2020)111130
Fig. 2. GLY conversion and MA, DA and TA yields (%) in acetylation of GLY at 40, 80 and 120 °C, as a function of reaction time (min), using a GLY/AA molar ratio of
1:4 in the absence of catalyst and in the presence of Me2SnCl2, Bu2SnCl2 and BuSnCl3 (the errors determined were 0.8. 0.5. 0.7 and 0.4 % for GLY, MA, DA and TA,
respectively).
At the end of the reaction, the reaction mixture was placed in an
evaporator and left for 5 min to remove unreacted acetic acid. The
conversion of GLY (%) and the yields (%) of MA, DA and TA were
determined by gas chromatography with flame ionization detector (GC-
FID), using a 6 m × 0.32 mm × 0.25μm Stabilwax column. The
quantification was based on the normalization method with the addi-
tion of an internal standard (benzyl benzoate) and the errors were de-
termined as 0.8, 0.5, 0.7 and 0.4 % for GLY, MA, DA and TA, respec-
tively. The samples were analyzed at least in duplicate.
Table 1
Apparent rate constants (kap) of the GLY conversion relative to the natural
logarithm results for the conversion (%) as a function of reaction time (h).
kap x 10−3 (h−1
)
Catalyst
40 °C
80 °C
120 °C
Without
Bu2SnO
0.3
ND
0.3
0.3
11.9
8.7
29.1
2.5
5.5
7.6
5.6
25.2
30.6
67.9
29.2
29.4
28.4
23.7
68.4
68.4
87.2
BuSnO(OH)
Bu2SnLau2
Me2SnCl2
Bu2SnCl2
BuSnCl3
The 13C NMR spectra were acquired at a central frequency of
100 MHz on a Bruker Avance-400 NMR Ultra Shield (9.4 T) spectro-
meter at 295.15 K, employing a solution of 40 mg of sample in 0.6 ml of
deuterated chloroform (CDCl3, Aldrich, USA).
ND = not determined.
3. Results and discussion
(Me2SnCl2), butyltin trichloride (BuSnCl3), dibutyltin dilaurate
(Bu2SnLau2), butyl stannoic acid [BuSnO(OH)], and di-n-butyl-oxo-
stannane (Bu2SnO) were purchased from Aldrich. Benzyl benzoate were
purchased from Oakwood Chemical, AA and GLY were acquired from
Merck (analytical grade). All chemicals were used as received without
further purification.
A series of Sn(IV) complexes was investigated in the esterification of
glycerol and acetic acid, in order to investigate the effect of the reaction
conditions and the nature of the catalyst on the GLY conversion and
selectivity for MA, DA and TA (Figs. 1 and 2). It is important to high-
light that this reaction is autocatalytic, since the AA acts as a Bronsted
acid catalyst, and the results must be compared to the reaction run
without catalyst [34].
2.2. Acetylation of GLY
Additionally, the values for the GLY conversion, during kinetic
control of the reaction, were employed to calculate the apparent rate
constants (kap) for the systems evaluated (Table 1). Under isothermal
conditions, a pseudo-first-order reaction can be considered, since GLY is
considered to be a limiting reactant due to the excess of AA used [35].
Then, treating the data using the natural logarithm of the GLY con-
version as a function of reaction time provides straight lines and their
Acetylation of GLY was performed at room temperature/pressure in
a closed batch system, which consisted of 5-mL flasks immersed in an
oil bath equipped with a temperature probe and a magnetic stirrer,
operating at 3000 rpm. The reactions were carried out using an AA/
GLY/CAT molar ratio of x/1/0.166, where x = 3, 4 or 5. Several re-
action temperatures were tested (40, 80 or 120 °C) over 15–180 min.
3

D.S. da Silva, et al.
MolecularCatalysis494(2020)111130
Fig. 3. GLY conversion and MA, DA and TA yields (%) in acetylation of GLY at 80 °C, as a function of reaction time (min), using GLY/AA molar ratios of 1:3, 1:4 and
1:5 without catalyst and in the presence of BuSnCl3.
detected mainly at 120 °C in all cases, and at 80 °C for the more active
catalytic systems (Figs. 1 and 2). This finding can be attributed to this
Table 2
Chemical shifts (ppm) observed in the 13C NMR spectra of BuSnCl3 before and
after GLY acetylation and AA contact.
esterification reaction being endothermic, thus requiring
a large
amount of heat to drive the process [37]. Therefore, MA formation
prevails at 40 and 80 °C, whereas DA and TA are mainly formed at
120 °C.
AA
BuSnCl3
BuSnCl3 + AAa
BuSnCl3 + AA + GLYa
C1 carbonyl
C2 CH3
C1 butyl
C2 butyl
C3 butyl
178.0
20.8
–
–
177.7
20.8
13.3
25.6
26.9
33.7
–
176.0
20.8
13.4
25.5
27.0
On analyzing the results, it should be noted that in the reactions
conducted with the catalysts BuSnO(OH), Bu2SnO and Bu2SnLau2, at all
temperatures, there is no significant difference in comparison with the
reactions conducted without a catalyst applying the same temperature
and reaction times (Fig. 1), in terms of conversion or selectivity. After
30 min, for instance, in the absence of catalyst no conversion was ob-
served at 40 °C while 23.2 % and 58.4 % of conversion was obtained at
80 and 120 °C, respectively. For these three catalysts the reactivity
observed was almost the same, since after 30 min at 80 °C conversions
of only 20.3 %, 15.3 % and 15.4 % were observed for BuSnO(OH),
Bu2SnO and Bu2SnLau2, respectively. All of these results are corrobo-
rated by the apparent rate constants (kap) (Table 1). In terms of se-
–
–
–
–
–
13.4
25.7
26.8
32.9
–
C4 butyl
MA + DA + TA
34.0
170.0–172.0
a
80 °C, 1 h, stirring.
slopes give the values for the apparent rate constants (kap), according to
Eq. (1).
ln GLY % = kt + ln 100
(1)
In general the values of the apparent rate constants (kap) are con-
sistent in terms of magnitude order with data reported in the literature
[20,35,36]. As expected, the temperature has a notable effect in this
reaction, even in the absence of a catalyst. In all cases, as the tem-
perature is increased there is a considerable increase in the GLY con-
version and this is confirmed by the apparent rate constants (kap
(Figs. 1 and 2 and Table 1). In addition, the selectivity is strongly in-
fluenced by an increase in the temperature, since the formation of TA is
lectivity, without
a catalyst or using BuSnO(OH), Bu2SnO and
Bu2SnLau2, the formation of MA and DA was observed at 80 °C and
120 °C and, the formation of TA was only detected at 120 °C.
In contrast, Me2SnCl2, Bu2SnCl2 and BuSnCl3 showed considerable
activity even at 40 °C (Fig. 2 and Table 1), where conversions of 24.5,
17.8 and 51.0 %, respectively, were detected after 15 min. The catalysts
Me2SnCl2 and Bu2SnCl2 seemed to exhibit comparable reactivity, for
)
instance, the apparent rate constant (kap
)
at 120 °C was
Table 3
Results taken from the literature related to acetylation of GLY (mentioned in the Introduction) in comparison with the performance of BuSnCl3, investigated in this
study.
Catalytic system
T (ºC)
AA:GLY
t (h)
GLY conversion (%)
MA/DA/TA
(%)
Reference
Mesostructured functionalized materials
Amberlyst-15
SAS
Wpy-20
Sb2O5
Amberlyst-36
Cesium-supported heteropolyacid catalyst (CsPWA)
Amberlyst-36
125
110
105
110
120
100
110
110
80
9:1
9:1
3:1
6:1
6:1
7:1
9:1
8:1
4:1
4
4.5
3
10
3.5
6
2
5
3
90
15/45/35
33.42/47.62/18.94
49/51/0
10/25/70
33.2/54.2/12.6
43/44/13
25/58/15
70.3/4.5/0
25.7/43.5/30.8
[15]
[13]
[4]
[1]
[35]
[17]
[10]
[18]
80.58
100
98
96.3
100
98
95.6
100
BuSnCl3
This study
4

D.S. da Silva, et al.
MolecularCatalysis494(2020)111130
68.4 × 10−3 h−1 for both. In addition, in the case of the selectivity, no
significant differences were observed between these two catalytic sys-
tems, with the average formation of around 29.4, 43.2 and 22.0 % of
MA, DA and TA.
In the case of BuSnCl3, the most active catalyst, high GLY conver-
sions were obtained, with conversions after 15 min of 51.0 % at 40 °C,
78.1 at 80 °C and 92.4 % at 120 °C. After 60 min of reaction, at 80 and
120 °C, the system appeared to have reached equilibrium, exhibiting a
conversion of around 95.3 %.
The notable activity of BuSnCl3 was also illustrated by the apparent
rate constants (kap), which were 29.1 × 10−3 h−1, 67.9 × 10−3 h−1
and 87.2 × 10−3 h−1 at 40, 80 and 120 °C, respectively. Based on these
values, the activation energy (AE) related to the GLY conversion in the
absence and presence of BuSnCl3 can be evaluated. Here, the Arrhenius
equation derived from transition state theory was considered, in view of
the temperature dependence of the reaction rate [36]. In the case of the
reaction performed without a catalyst, the estimated AE for the GLY
conversion was 23.9 kJ mol−1 and with the use of BuSnCl3 it was
14.3 kJ mol−1. This reduction of approximately 50 % in the AE on using
this catalyst highlights the efficiency of the Sn(IV) catalyst in this
process.
performed and analyzed in NMR tubes (see experimental section). If
substituent exchange occurs on the catalyst, a displacement of the 13C
NMR signals (butyl substituent) and the appearance of new signals re-
lated do acetate coordination to metal center are expected. Table 2
summarizes the 13C NMR signals detected after GLY acetylation.
Indeed, these results (Table 2) strongly suggest that there is no
modification in the nature of the substituents for BuSnCl3 during
acetylation or in contact with AA. The detection of MA, DA and TA
signals in the region of 170.0–172.0 ppm proves that acetylation has
occurred [42].
In addition, on comparing the more active system investigated
herein with other studies reported in the literature (Table 3), it is
possible to observe that the use of BuSnCl3 leads to the total conversion
of GLY and shows promising selectivity (25.7, 43.5 and 30.8 % for MA,
DA and TA, respectively) under mild reaction conditions (80 °C,
AA:GLY molar ration of 4:1 at 3 h of reaction), showing the good po-
tential of Sn(IV) species to catalyze this type of reaction.
4. Conclusions
The catalytic systems based on Sn(IV) species investigated in this
study are active in the acetylation of GLY and the most active system,
namely BuSnCl3, leads to the total conversion of GLY and promising
selectivity (25.7, 43.5 and 30.8 % for MA, DA and TA, respectively)
under mild reaction conditions (80 °C, AA:GLY molar ration of 4:1 at 3 h
of reaction). The apparent rate constants (kap) of the GLY conversion
confirms these results and for the reaction performed without catalyst
the estimated value of the activation energy for the GLY conversion was
23.9 kJ mol−1 while the value with the use of BuSnCl3 was 14.3 kJ
mol−1, representing a decrease of around 50 %.
The effect of the AA/GLY molar ratio was also investigated, at 80 °C,
using BuSnCl3 and in the absence of catalyst, applying several different
reaction times. Fig. 3 reports the results and the values for the apparent
rate constant for the GLY conversion.
In the absence of a catalyst, the increase in the amount of AA ob-
tained for the stoichiometric condition (3:1) compared to 4:1 or 5:1
results in a slight increase in the GLY conversion. At a 4:1 M ratio, the
observed formation of MA was greater, and this is converted to DA and
TA at a 5:1 M ratio. In the presence of BuSnCl3, the increase in AA
enhances the GLY conversion and almost 100.0 % of conversion is
observed at a 5:1 M ratio. However, at a molar ratio of 4:1 a higher
amount of TA is observed, while at 5:1 this amount decreased, since it is
possible to detected a notable quantity of DA. It is important to high-
light that several studies show the dependence, mainly in terms of se-
lectivity, of the ratio of reactants in GLY acetylation [25,35,38–41], and
the results presented herein are in line with this finding.
CRediT authorship contribution statement
Débora S. da Silva: Conceptualization, Methodology, Investigation,
Writing - original draft. Felyppe M.R.S. Altino: Conceptualization,
Methodology,
Investigation.
Janaína
H.
review
Bortoluzzi:
editing,
Conceptualization, Methodology, Writing
-
&
It should be noted that the Sn(IV) catalytic systems investigated in
this study present Lewis acidity and empty orbitals that are able to
interact with electron donor substrates. In this context, there are two
mechanisms established for esterification catalyzed by organotin(IV)
complexes, namely, typical Lewis acid coordination or coordination
followed by ligand exchange [31]. It is known that organotin(IV)
chloride and its carboxylated derivatives act via the typical Lewis acid
mechanism, and oxo, alkoxy and hydroxyl derivatives follow the co-
ordination-ligand exchange mechanism [30,31].
Supervision. Simoni M.P. Meneghetti: Conceptualization, Writing -
review & editing, Supervision, Project administration, Funding acqui-
sition.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Thus, the more active Sn(IV) systems investigated in this study are
those that exhibit chloride substituents, even though the oxygen sub-
stituents are slightly more electronegative than chlorine, and the me-
chanism involved is typical Lewis acid coordination. In this case, it can
be proposed that the better catalytic performance of these systems is
due to less steric hindrance of the substituents present in the systems
investigated here, allowing better access of the reactants to the metal
center. Additionally, the fact that BuSnCl3 is more active than Me2SnCl2
and Bu2SnCl2 could be related to the number of alkyl substituents, since
as the number of alkyl ligands increases, for organotin(IV) compounds,
the Lewis acidity decreases [31]. However, it is important to highlight
that it was not possible to observe a difference between the methyl and
butyl substituents in terms of their reactivity in the GLY acetylation
reactions.
It is should be noted that in a previous study on esterification (using
caprylic or acetic acid as model FAs, and methanol-d4) in the presence
of Bu2SnCl2 and BuSnCl3, 119Sn NMR investigations and computational
studies were conducted and no modifications were observed in the
catalysts [33]. Nevertheless, in order to verify if ligand exchange pro-
cess on BuSnCl3 takes place during the reaction, GLY acetylation were
Acknowledgements
This study was supported by the National Council for Scientific and
Technological Development (CNPq), the Brazilian Federal Agency for
the Improvement of Higher Education Personnel (CAPES), the Brazilian
Innovation Agency (FINEP) and the Alagoas Research Foundation
(FAPEAL). DSS and FMA express their appreciation for fellowships
granted by CAPES. SMPM is grateful to CNPq for a research fellowship.
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