Syntheses of novel halogen-free Br?nsted-Lewis acidic ionic liquid catalysts and their applications for synthesis of methyl caprylate

Article abstract of DOI:10.1039/c4gc01470g

A series of benign halogen-free ionic liquid (IL) catalysts were synthesized by combining the Br?nsted acidic ionic liquid [HSO₃-pmim]⁺HSO₄^? with ZnO in different composition ratios. The IL catalysts, which possess both Br?nsted and Lewis acidities, were employed as acidic catalysts for the esterification of n-caprylic acid to methyl caprylate. The [HSO₃-pmim]⁺(1/2Zn²⁺)SO₄^2?, prepared by cooperating equimolar amounts of Br?nsted and Lewis acid sites, was found to exhibit an optimal catalytic performance and excellent durability. This is attributed to a synergy of Br?nsted and Lewis acidities manifested by the catalyst. The response surface methodology (RSM) based on the Box-Behnken design (BBD) was utilized to explore the effects of different experimental variables (viz. catalyst amount, methanol to caprylic acid molar ratio, temperature, and reaction time) on the esterification reaction. Analysis of variance (ANOVA) was also employed to study the interactions between variables and their effects on the catalytic process. Accordingly, the deduced optimal reaction conditions led to a high methyl caprylate yield of 95.4%, in good agreement with experimental results and those predicted by the BBD model. Moreover, a kinetic study performed under optimal reaction conditions revealed an apparent reaction order of 1.70 and an active energy of 33.66 kJ mol^?1

Full text of DOI:10.1039/c4gc01470g

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DOI: 10.1039/C4GC01470G
ARTICLE TYPE
Syntheses of novel halogen-free Brønsted-Lewis acidic ionic liquid
catalysts and their applications for synthesis of methyl caprylate
*
,a
a
b
b
b
c
Xiao-Xiang Han, Huan Du, Chin-Te Hung, Li-Li Liu, Pei-Hao Wu, Da-Hai Ren, Shing-Jong
d
*,b,e
Huang and Shang-Bin Liu
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A series of benign halogen-free ionic liquid (IL) catalysts were synthesized by combining the Brønsted
+

acidic ionic liquid [HSO
3
-pmim] HSO
4
with ZnO in different composition ratios. The IL catalysts,
which possess both Brønsted and Lewis acidity, were employed as acidic catalysts for esterification of n-
+
2+
2
1
1
2
0
5
0
caprylic acid to methyl caprylate. The [HSO -pmim] (½Zn )SO , prepared by cooperating equimolar
3 4
amount of Brønsted and Lewis acid sites, was found to exhibit an optimal catalytic performance and
excellent durability. This is attributed to a synergy of Brønsted and Lewis acidity manifested by the
catalyst. The response surface methodology (RSM) based on the Box-Behnken design (BBD) was utilized
to explore the effects of different experimental variables (viz. catalyst amount, methanol to caprylic acid
molar ratio, temperature, and reaction time) on esterification reaction. Analysis of variance (ANOVA)
was also employed to study interactions between variables and their effects on the catalytic process.
Accordingly, the deduced optimal reaction conditions led to a high methyl caprylate yield of 95.4%, in
good agreement with experimental results and that predicted by the BBD model. Moreover, a kinetic
study performed under optimal reaction conditions revealed an apparent reaction order of 1.70 and active
energy of 33.66 kJ/mol.
disadvantages such as low activity, easy deactivation, formidable
1
5
separation and recovery and so on. Hence, it is essential to
develop environmental benign catalysts to replace the
aforementioned catalysts.
Ionic liquids (ILs), which have been considered as new eco-
friendly catalysts, have received considerable attention in various
chemical syntheses owing to their unique characteristics such as
Introduction
4
0
Esterification of alcohols and carboxylic acids are fundamental
and important reactions in organic synthesis. The corresponding
products, carboxylic esters, have been extensively applied in
areas such 1as cosmetics, plastics, food, medicine and
2
3
3
5
0
5
intermediates. Conventionally, syntheses of carboxylic esters 45 low melting point, high thermal stability, negligible volatility,
mostly invoke homogeneous catalysts, such as sulfuric, p-toluene
sulfonic, and phosphoric acids. However, problems caused by
these catalysts are not in conformity with the requirements of
easy solubility, adjustable properties, recyclability, and
1
6, 17
18-28
reusability.
and Lewis types
Acidic ILs may be classified into Brønsted
2
9,30
in accordance with their acidic groups. While
sustainable
development,
e.g.,
equipment
corrosion,
various Brønsted-Lewis acidic ILs (B-L AILs) have been
31,32
environmental pollution, undesirable side reactions, plaguy 50 extensively developed and applied,
the most common types,
catalyst recovery and reuse. To avoid these drawbacks,
which invoke incorporation of metal halides, are moisture-
sensitive and hence are inapplicable for aqueous systems. To
overcome this problem, efforts have been made in preparing
halogen-free B-L AILs using copper oxide as the source of Lewis
2
-5
heterogeneous catalysts such as solid super acids,
heteropolyacid acids, zeolites, resins and enzymes
6
7-9
10
11-14
have
been employed for esterification. Unfortunately, limited
applications were found for these solid catalysts due to the 55 acidity. Herein, we report a series of novel halogen-free B-L
3
3
AIL catalysts synthesized by combining Brønsted acidic IL
+

[
3 4
HSO -pmim] HSO with varied amount of ZnO as precursor for
_
_________________________________________________________
Lewis acidity. The acid catalysts so fabricated were characterized
by a variety of different analytical and spectroscopic techniques,
such as thermogravimetric analysis (TGA), Fourier transform
a
Department of Applied Chemistry, Zhejiang Gongshang University,
Hangzhou, 310035, China. Tel: +86-571-28008975; E-mail:
hxx74@126.com
Institute of Atomic and Molecular Sciences, Academic Sinica, Taipei
0617, Taiwan. Tel: +886-2-23668230; E-mail: sbliu@sinica.edu.tw
6
0
1
13
infrared (FT-IR), and H and C nuclear magnetic resonance
NMR) spectroscopy. Moreover, the acidic properties of these B-
b
(
1
31
c
L AILs were investigated by solid-state P magic-angle spinning
Department of Chemical & Biomolecular Engineering, Clarkson
University, Potsdam, New York 13699-5705, USA
(
MAS) NMR of adsorbed trimethylphosphine oxide (TMPO) as
34-36
d
6
5
the probe molecule.
The catalytic activities of these B-L AILs
Instrumentation Center, National Taiwan University, Taipei 10617,
Taiwan
during esterification of caprylic acid with methanol were
investigated and relevant reaction conditions invoked during the
synthesis of methyl caprylate were optimized using the response
e
Department of Chemistry, National Taiwan Normal University, Taipei
11677, Taiwan
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Page 3 of 10
Green Chemistry
surface methodology (RSM). The effects and correlations of
different experimental variables such as catalyst amount,
methanol to caprylic acid molar ratio, temperature, and reaction 55 evacuation at 323 K. Furthermore, the sample was s
time on the esterification reaction were addressed by the Box-
Behnken design (BBD) model, followed by a kinetic study
performed under these optimized experimental parameters.
environment. The sealed sample vessel was then connected to a
vacuum manifold, followed by removal of the CH Cl solvent by
2
2
V
u
i
b
ewje
A
cted
le Otonline
r
t
ic
DOI: 10.1039/C4GC01470G
thermal treatment at 393 K for at least 24 h to ensure a uniform
5
adsorption of adsorbate probe molecules in the substrate. Finally,
the TMPO-loaded sample was transferred into a MAS rotor in a
N glove box and then sealed by a gas-tight Kel-F cap.
2
Experimental
6
0
Esterification reaction
Catalyst preparation
n-Caprylic acid (0.1 mol), a desirable amount of methanol, and a
known amount of the AIL catalyst were placed into a three-
necked flask equipped with a mechanical stirrer, reflux condenser,
and a drop funnel containing microporous 3A molecular sieves to
65 absorb water produced from esterification. The esterification of
n-caprylic acid with methanol was first carried out at different
temperatures for a desired period of time, then, the reaction
mixture was cooled to RT. Typically, owing to the self-separation
characteristics of the reaction system, the reaction mixture tends
to spontaneously separate into two layers. The upper layer was
extracted by distillation to obtain the methyl caprylate product,
and the lower layer consisting of AIL catalyst was reused after
further treatment. Quantitative analysis of the products was
performed by gas chromatography using an Agilent 6890N GC
apparatus equipped with a FID detector and HP-5 capillary
column. Reactants and products were identified by comparison
with authentic samples using methyl laurate as an internal
standard.
The novel B-L AIL catalysts were synthesized by combining
1
1
2
0
5
0
sulfonic acid-functionalized Brønsted acidic ionic liquid [HSO -
3
+

pmim] HSO
carried out by first preparing the [HSO
known procedure outlined in the literature,
4
(denoted as ILa) with zinc oxide (ZnO). This was
+

3
-pmim] HSO
4
using a
followed by
3
7,38
adding varied amounts of ZnO into the aqueous solution. The
mixture was stirred continuously till the solid was completely
dissolved. The resultant products were evacuated after water
removal to obtain the final B-L AIL catalysts with varied L/B
7
0
+
+
2+
2
ratios, namely [HSO
3
-pmim] (½H ·¼Zn )SO
4
,
3
[HSO -
+
2+
2
+
2+
+
2+
2
pmim] (½Zn )SO
4
, [(½H ·¼Zn ) SO
3
-pmim] (½Zn )SO
4
,
2
+
+
2+
2
[(½Zn )SO
3
-pmim] (½Zn )SO
4
, denoted as ILb, ILc, ILd, and
ILe, respectively, as specified in Table 1.
7
5
Table 1 Characteristics of various acidic IL catalyst and their catalytic
performances during esterification of caprylic acid with methanol
Catalyst
Conv. Yield
The conversion of n-caprylic acid was calculated as follows:
a
b
c
Name Formula
L/B (%)
(%)
80
Conversion = (1  a
1
/ a
2
) × 100%,
(1)
+

ILa
ILb
ILc
ILd
ILe
Null
[HSO
[HSO
[HSO
3
3
3
-pmim] HSO
4
+
0:4 92.0
1:3 93.0
2:2 94.8
3:1 86.7
4:0 22.1
91.4
92.5
94.3
86.1
20.7
4.8
where a and a represents the initial and final acidity of the
1
2
+
2+
2
-pmim] (½H ·¼Zn )SO
4
reaction mixture, respectively. The acidity values were monitored
by sodium hydroxide (NaOH) titration.
+
2+
2
-pmim] (½Zn )SO
4
+
2+
+
2+
2
3 4
[(½H ·¼Zn ) SO -pmim] (½Zn )SO
+ 2+
2
+
2
[(½Zn )SO
---
3 4
-pmim] (½Zn )SO
Design of experiments and response surface methodology
---
5.3
a
39,40
2
5
Molar ratio of Lewis/Brønsted acid sites.
Reaction conditions: catalyst amount = 6 wt%, methanol/caprylic acid
molar ratio = 5.0, reaction time = 3.0 h, and temperature = 363 K.
Product yield of methyl caprylate determined by GC.
85 Response surface methodology (RSM)
was employed to
b
obtain optimal experimental conditions for esterification of
caprylic acid with methanol. A Box-Behnken design (BBD) was
applied to study the effects of three process variables, namely
amount of catalyst (x ), methanol to caprylic acid molar ratio (x ),
c
1
2
Catalyst characterization
90 and reaction time (x ), as specified in Table 2. A total of 17
3
experimental sets, which included 12 factorial points and 5
3
3
4
4
5
0
5
0
5
0
FT-IR spectra of various AIL catalysts were obtained using KBr
wafer conducted on a Bruker IFS-28 spectrometer in the region of
centring points, were adopted. The three experimental variables
were designed at three levels coded with a plus signs (+1; high
value), zero (0; central value), or a minus signs (-1; low value).
-1
4
000-400 cm . All thermogravimetric and differential
thermogravimetric (TG-DTG) curves were recorded on a TG-209
NETZSCH) thermal analyzer. Samples were heated from room
temperature (RT; 298 K) to 873 K at a heating rate of 10 K/min
9
5
The coded values of these factors were obtained by the equation:
(
ꢃ ꢅꢃ
ꢄ
ꢆ
1
13
ꢀꢁ ꢂ
(2)
under dynamic N
2
atmosphere. H and C NMR spectra were
∆ꢃꢄ
3
7,38
31
acquired on a Bruker AV-500 spectrometer.
Solid-state
P
where x , X , and X (i = 13) represents the coded, real, and
MAS NMR measurements were performed on a Bruker-Biospin
Avance-III 500 spectrometer at a Larmor frequency of 202.46
i
i
0
central value of the independent variable, respectively, and ΔX
variable at high level  variable at low level)/2, which denotes
00 the step change value.
i
=
3
1
(
MHz using a 4-mm double-resonance MAS probehead.
P
1
spectra were acquired using a single-pulse sequence under a
sample spinning rate of 12 kHz using an excitation pulse of 1.5
3
1
Table 2 List of symbols and coded levels for corresponding experimental

s (π/6 pulse) and a recycle delay of 10 s. The P chemical shifts
*
variables and ranges adopted for the esterification process
were referenced to 85% H PO aqueous solution.
3
4
Detailed acid features, namely types, relative concentration,
and strength of Brønsted vs Lewis acidity in the B-L AIL
Range and level
Variable (unit)
Symbol
3
1
34-36
-1
0
1
catalysts were studied by P MAS NMR of adsorbed TMPO.
Catalyst amount (wt%)
x
1
x
2
x
3
4.0 6.0 8.0
2.5 5.0 7.5
1.5 2.0 2.5
Prior to the adsorption of TMPO probe molecule, each catalyst
was first subjected to dehydration treatment at 393 K for 24 h
Methanol/caprylic acid ratio (mol/mol)
−5
under vacuum (< 10 Torr). Subsequently, a known amount of
TMPO dissolved in anhydrous CH Cl was introduced into a
Reaction time (h)
*
+
2+
2
2
2
Over the [HSO
-pmim] (½Zn )SO
catalyst at 363 K.
3
4
vessel containing the dehydrated sample in a glove box under N2
1
05
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 2

Green Chemistry
Page 4 of 10
A model equation based on quadratic polynomial given by
Results and discussion
RSM was used to reveal interactive effects between experimental
variables, to optimize the reaction process, and to predict the
yield of the product (i.e., methyl caprylate). The model equation
may be expressed as:
View Article Online
DOI: 10.1039/C4GC01470G
Catalyst characterization
1
13
5
55 Similar H and C NMR spectra were observed for various B-L
AIL catalysts. As an example, the NMR data obtained from
ꢇ ꢂ ꢈ ꢊ ∑ꢋ ꢈ ꢀ ꢊ ∑ ꢈ ꢀꢁ ꢊ ∑ ꢈ ꢀ ꢀ
ꢋ
ꢎ
ꢋ
+
2+
2
(3)
ꢉ
ꢁꢌꢍ
ꢁ
ꢁ
ꢁꢌꢍ ꢁꢁ
ꢁꢐꢏ ꢁꢏ
ꢁ
ꢏ
[HSO
amount of Brønsted and Lewis acid sites are depicted below: H
NMR (500 MHz, D O): δ 2.31 (m, 2H), 2.91 (t, 2H), 3.89 (s, 3H),
60 4.36 (t, 2H), 7.44 (s, 1H), 7.51 (s, 1H), 8.74 (s, 1H) ppm;
NMR (500 MHz, D O): δ 27.57, 38.20, 49.69, 50.21, 124.68,
26.26, 138.68 ppm.
Fig. 1 displays the FT-IR spectra of various IL catalysts
3
-pmim] (½Zn )SO
4
(i.e. ILc) catalyst having equimolar
1
where Y is the predicted response (i.e., product yield), x
i
and x
j
2
are the coded levels of the independent variables, β , β , β , and β
13
0
i
ii
ij
C
denote the regression coefficient representing the offset, linear,
quadratic, and interaction term, respectively. A Design-Expert
.0.5 software (Stat-Ease, USA) was used to analyse the
2
1
1
2
0
5
0
1
6
experimental data, to perform analysis of variance (ANOVA),
and to evaluate the regression equation. Accordingly, the fitted
polynomial equation may further be expressed in terms of
response surface and contour plots to facilitate visualization of
the correlations between the response and experimental variables
at various coded levels, and to infer optimized process conditions.
specified in Table 1. In spite of the marginal variations in peak
intensities and peak positions, distinct characteristic vibrational
6
7
7
8
5
0
5
0
+

bands responsible for Brønsted acidic IL [HSO
3 4
-pmim] HSO
+
were observed before (ILa) and after substitution of protons (H )
2+
-1
with Zn cations. The absorption peaks at 1638 and 1573 cm
may be attributed to the stretching vibrations of C=C and C=N
2
The coefficient of determination (R ) may be used to evaluate the
-1
bonds on the imidazole ring, respectively. The peak at 2930 cm
accuracy and applicability of the second order multiple regression
model. The significance of its regression coefficient was checked
with the F-test value.
should be associated with asymmetric stretching vibration of
-1
CH
2
and the bonds at 1460 and 748 cm may be assigned to
bending and rocking vibrations of CH
3
, respectively. Whereas
-1
the strong absorption bonds at 1080 and 1045 cm may be
attributed to the asymmetric and symmetric stretching vibrations
of S=O, respectively. Moreover, a notable decrease in peak
Kinetic study
The reaction rate (r) for esterification of n-caprylic acid to methyl
caprylate may be expressed as:
-1
intensity of the band responsible for OH vibration at 3400 cm
+
2+
with increasing substitution of H with Zn ions (i.e., increasing
L/B ratio in Table 1) was observed, revealing the progressive
incorporation of Lewis acidity onto the pristine Brønsted acidic
ꢑ ꢂ ꢒ ꢓꢖ ꢂ ꢗꢘꢙꢚꢛꢙ ꢒ ꢗꢅꢙꢔꢞꢙ
ꢓꢔꢕ
ꢝ
ꢠ
(4)
2
5
ꢜ
ꢟ
4
1,42
(
ILa) catalyst.
where C , C , C , and C represent the instant concentration of n-
A B C D
caprylic acid, methanol, methyl caprylate, and water, respectively,
while α, β, γ, and η denote their corresponding reaction order.
+ 
Whereas, k and k account for the rate constants associate with
d
3
0
the forward and inverse reaction, respectively.
ILe
ILd
ILc
Given that water had been effectively removed by
microporous 3A zeolite adsorbent during esterification reaction,
the reaction could be considered as an irreversible process. In this
context, the second term associated with inverse process in Eq. (4)
may be ignored. Moreover, since the concentration of methanol is
always much higher than that of n-caprylic acid, k
3
5
β
B
+
C
in Eq. (4)
1638 1573
1180
592
1045
could be deemed as constant, leading to a simplified rate equation:
3400
ꢓꢔꢕ
ꢑ ꢂ ꢒ ꢓꢖ ꢂ ꢗꢙꢚꢛ
ILb
ILa
(5)
1460
748
2930
3100
β
B
where k = k
+
C
refers to the modified rate constant. By taking
4
0
the natural logarithm, Eq. (5) may further be expressed as:
ln ꢑ ꢂ ln ꢗ ꢊ ꢡ ln ꢙꢢ
(6)
4
000 3500 3000 2500 2000 1500 1000 500
The values of k and α at different temperatures may therefore be
obtained easily by linear fitting of the ln r vs ln C curve using
-1
Wavenumber (cm )
A
the Origin 8.5 software.
+

4
5
It is well-known that the variation of reaction rate with 85 Fig. 1 FT-IR spectra of various IL catalysts: ILa = [HSO
temperature may be expressed by the Arrhenius equation as:
3
-pmim] HSO
[HSO
4
,
+
+
2+
2
ILb
=
[HSO
3
2
-pmim] (½H ·¼Zn )SO
4
,
ILc
=
3
-
+
2+
+
2+
+
2+
2
pmim] (½Zn )SO
4
3 4
, ILd = [(½H ·¼Zn ) SO -pmim] (½Zn )SO ,
+ 2+
ꢣꢤ
1
2+
2
and ILe = [(½Zn )SO -pmim] (½Zn )SO
3
4
.
ln ꢗ ꢂ ln ꢗ ꢒ
(7)
0
ꢥ ꢦ
9
0
Thus, the activation energy (E ) and pre-exponential factor (k )
a
0
TGA-DTG analyses were performed to investigate thermal
stability of various AIL catalysts. Since similar TGA-DTG
profiles were observed for most B-L AIL catalysts over the
temperature range from RT to 873 K, only assorted TGA-DTG
results obtained from the zwitterion 1-sulfonicacidpropyl-3-
may be derived from the slope and intercept of the Arrhenius (ln
k vs 1/T) plot.
5
0
9
5
3
methylimidazole (MIMPS), Brønsted acidic ILa ([HSO -
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

Page 5 of 10
Green Chemistry
+
), and B-L AIL with L/B = 1.0 (i.e. ILc) samples
pmim] HSO
4
the two peaks observed for ILd at 77 and 66 ppm due to TMPO
adsorbed on Brønsted and Lewis acid sites, respectively. In this
context, the broad multiple resonance around 63 ppm in ILe
are illustrated and discussed. The TGA curves observed for
MIMPS and ILa showed similar weight-loss profiles at ca.
00380 and 575-675 K. For the latter ILa ([HSO
catalyst, an initial weight loss occurred at 356 K (Fig. 2b), which 55 viscosity of these AILs, ILa, ILb, and ILc were found to exhibit a
View Article Online
DOI: 10.1039/C4GC01470G
+

3
3
-pmim] HSO
4
)
indicates that the AIL mainly manifests Lewis acidity. In terms of
5
0
5
may be attributed to desorption of physisorbed water, while the
second weight-loss peak was observed at 625 K, corresponding to
the decomposition of the organic MIMPS (Fig. 2a). By
liquid phase with increasing viscosity, but ILd appeared to be a
mixture of liquid with solid, whereas ILe was a solid powder.
These results are in line with the  P and corresponding
linewidths of the adsorbed TMPO in various catalyst substrates.
3
1
comparison,
pmim] (½Zn )SO
the
TGA-DTG
(i.e., ILc) shown in Fig. 2c revealed two
profile
of
[HSO
3
-
+
2+
2
1
4
60
major weight-loss peaks at 333 and 636 K. Thus, the acidic ILc
catalyst appeared to be thermally more stable than ILa after
incorporating Lewis acidity. It is noteworthy that these novel B-L
63
61
67
ILe
ILd
ILc
*
+
2+
2
3 4
AILs (such as [HSO -pmim] (½Zn )SO ) retain their structural
1
integrity over the range of reaction temperatures (323363 K)
examined.
66
*
77
*
*
77
100
10 100
100
0.4
0
0
0
.4
.2
.0
(c)
(
a)
(b)
0
.2
.0
81
5
8
6
4
2
0
0
0
0
0
80
80
60
40
0
0
-
-
0.2
0.4
-0.2
60
ILb
ILa
86.2
-
-
-
-
5
-
-
-
-
0.4
0.6
0.8
1.0
10
15
20
-0.6
40
20
0
-
-
-
0.8
1.0
1.2 20
150
120
90
60
30
-25
-1.4
-1.2
3
00 500 700 900
300 500 700 900
300 500 700 900
Temperature (K)
31
P chemical shift (ppm)
Temperature (K)
Temperature (K)
2
0
+

3 4
Fig. 2 TG-DTG curves of (a) MIMPS, (b) [HSO -pmim] HSO , and (c)
3
1
+
2+
2
Fig. 3 P NMR spectra of various acidic IL catalysts: ILa = [HSO
3
-
-
[
3 4
HSO -pmim] (½Zn )SO .
+

+
+
2+
2
6
5
pmim] HSO
pmim] (½Zn )SO
ILe = [(½Zn )SO
denote spinning sidebands.
4
2+
, ILb = [HSO
3
-pmim] (½H ·¼Zn )SO
4
, ILc = [HSO
3
2
+
2
+
2+
+
2+
4
, ILd = [(½H ·¼Zn ) SO
3
-pmim] (½Zn )SO
4
,
2
+
+
2+
2
3
-pmim] (½Zn )SO . The asterisks in the spectra
4
3
1
2
5
Solid-state P MAS NMR of adsorbed TMPO has been
proven to be feasible and powerful approach for
characterization of acid properties of solid acid catalysts.
a
3
4-36,40,43
The base TMPO probe molecule tends to interact with the acidic
+
70 Role of Brønsted and Lewis acidity on catalytic activity
proton to form TMPOH complex. It has been demonstrated that
3
1
3
3
4
0
5
0
a linear correlation between the observed P NMR chemical shift
To explore the effects and correlations of Lewis and Brønsted
acid sites on catalytic performance during esterification of n-
caprylic acid with methanol. Catalytic activities of various AIL
catalysts were attained, as summarized in Table 1. Accordingly, it
75 is indicative that Brønsted acid sites played the primary role than
Lewis acidity during catalytic esterification reaction. This may be
seen by the notable decreases in conversion and product yield
with increasing Lewis acidity. Nevertheless, the ILa catalyst,
which solely possesses ultra-strong Brønsted acidity, showed
3
1
(
 P) and acidic strength of Brønsted acidity may be inferred by
31 34-36
means of such P-TMPO NMR approach.

A higher observed
3
1
P value of the adsorbed TMPO therefore represents stronger
acidic strength of the catalyst, and vice versa. Fig. 3 shows
NMR spectra of TMPO adsorbed on various AILs. The
3
1
1
P
P
3
spectrum of ILa exhibited a sharp singlet at 86.2 ppm, indicating
the sole presence of Brønsted acidity whose average strength is in
3
4-36,40
the proximity of the threshold for superacidity (86 ppm).
is indicative that the parent [HSO -pmim] HSO
3 4
It
catalyst indeed
possesses ultra-strong Brønsted acidity. Upon progressing
+

8
8
9
0
5
0
inferior catalytic activity compared to ILb and ILc. This reveals
that ultra-strong acidity may be unfavorable for esterification
reaction, likely due to the occurrence of acid-catalysed reverse
reactions. Interestingly, optimal conversion (94.8%) and methyl
caprylate yield (94.3%) were obtained when equimolar Lewis and
Brønsted acid sites were incorporated in the AIL (i.e., ILc). On
the basis of these results, it is indicative that a synergy of Lewis
and Brønsted acid sites manifests the catalytic activity during
formation of methyl caprylate over these novel AIL catalysts,
particularly when an equimolar amount of both types of acidities
was implemented. A possible catalytic mechanism is proposed in
2
+
31
incorporation of Lewis metallic center (Zn ), the P resonances
become broaden due to consistent increase in viscosity. This is
also accompanied by a gradual decrease in  P. For examples, a
single broad resonance peak was observed for TMPO adsorbed
on ILb and ILc, corresponding to  P of 81 and 77 ppm,
respectively. Clearly, Brønsted acidity became weaker as more
H are being substituted by the Zn ion, creating Lewis acidity.
On the other hand, multiple resonances were observed for ILd
and Ile, among them, the peak in the vicinity of 63 ppm may be
3
1
3
1
4
5
+
2+
2
+
3
4,35,43
Scheme 1. In brief, the Lewis center (Zn ) may incorporate with
5
0
attributed to the presence of Lewis acidity.
Thus, we assign
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 4

Green Chemistry
Page 6 of 10
the oxygen atom in the carbonyl groups, which possess strong
provoke efficient esterification reaction. Nonetheless, as the ratio
electronegativity, to induce interactions between the oxygen in 80 of methanol to caprylic acid exceeded 5:1, the excessive
View Article Online
the alcohol hydroxyl group and the carbon atom in the carbonyl
group. On the other hand, couplings of Brønsted acidic H with
methanol no longer had decisive impact on the conversion rate
+
DOI: 10.1039/C4GC01470G
and product yield due to dilution of the catalyst in the reaction
5
the oxygen atom in the carbonyl group tends to lower the
negative charge, hence, is favorable for the release of Zn .
mixture.
2
+
Influence of reaction time and temperature on the
30 esterification was also investigated, as shown in Figs. 4c and 4d.
It is evident that the methyl caprylate yield reached ca. 88.9%
rapidly during the initial stage (1.0 h), then increased gradually
till reaching a maximum of 94.4% at 2.0 h. A marginal decrease
in product yield beyond reaction time of 2.0 h was observed,
35 mostly likely due to partial hydrolization of methyl caprylate. As
for effect of reaction temperature, the yield of methyl caprylate
reached a maximum as the temperature reached 363 K at 2.0 h.
However, while water removal by methanol evaporation may be
more efficient at higher temperatures, no further increase in n-
40 caprylic acid conversion and product yield was observed above
1
1
1
3
63 K, likely due to inevitable loss of methanol above its boiling
point.
1
0
Scheme 1 Possible mechanism invoked during esterification of n-caprylic
+
2+
2
3 4
acid with methanol over the B-L AIL [HSO -pmim] (½Zn )SO
(
a)96
96
92
88
84
(b) 95
80
catalyst.
95
92
88
84
80
Effects of experimental variables on esterification
65
65
50
35
+
2+
2
Since the [HSO
3
-pmim] (½Zn )SO
4
AIL catalyst having the
50
1
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
Lewis/Brønsted molar ratio of 1:1 exhibited the best catalytic
performance, the ILc catalyst was chosen for further process
optimization study. Four important experimental variables,
namely the catalyst amount, methanol/n-caprylic acid reactant
ratio, reaction time, and temperature, were investigated. Among
them, the amount of catalyst used plays a key role during
esterification due to its high effectiveness to the whole reaction
35
2
4
6
8
10
1:1 2.5:1 5:1 7.5:1 10:1
Methanol/caprylic acid molar ratio
Catalyst amount (wt%)
(c) 96
(d) 96
96
92
88
96
90
84
78
72
9
2
8
90
8
84
+
2+
2
84
84
80
76
3 4
process. Fig. 4a shows the effect of [HSO -pmim] (½Zn )SO
8
0
6
78
loading on catalytic activity during esterification of n-caprylic
acid with methanol. It is noteworthy that while performing the
experiments with varied catalyst loading, other experimental
parameters were kept constants: methanol/caprylic acid ratio =
7
72
0
.5 1.0 1.5 2.0 2.5 3.0 3.5
320
340
360
380
Time (h)
Temperature (K)
5
.0, reaction time = 3.0 h, and temperature = 363 K. Accordingly,
Fig. 4 Effects of (a) catalyst amount, (b) substrates molar ratio, (c)
10 reaction time, and (d) temperature on conversion and methyl caprylate
yield during esterification of n-caprylic acid with methanol over the
a consistent increases in both conversion and methyl caprylate
yield with increasing amount of catalyst dosage was observed,
eventually reaching a respective plateau for conversion (94.8%)
and product yield (94.3%) at a reaction time of 3 h and a catalyst
loading of 6 wt%. This is likely due to the suitable amount of
active moieties desirable for the reaction. However, as the
catalyst amount exceeded 6 wt%, the conversion of n-caprylic
acid and yield of methyl caprylate retained practically unchanged.
This may be attributed to dissolution of the AIL catalyst in the
presence of methanol, but the catalyst is practically unsolvable in
methyl caprylate. As such, at a prolonged reaction time, as the
product ester became the primary component of the reaction
mixture, leading to the limited solubility of the catalyst. Thus, in
view of the observed high yield of methyl caprylate and the
relevant cost effectiveness issue of the process, an optimal
catalyst amount of ca. 6 wt% may be inferred.
Regarding to the effect of feeds, Fig. 4b displays the variation
of methanol to n-caprylic acid molar ratio on esterification
activity. It was found that, as the reactant ratio increased, the
initial yield of ester also increased significantly and eventually
leveled off at 94.3% when the ratio of methanol to caprylic acid
reached 5:1. Since esterification is a reversible reaction, an
adequate amount of methanol is required to drive the equilibrium
towards formation of methyl caprylate. Meanwhile, the AIL
catalyst is highly soluble in methanol, thus, resulted in a
homogeneous distribution of catalyst in the reaction mixture to
1
1
+
2+
2
[HSO -pmim] (½Zn )SO
3
4
catalyst. Note that while varying each
experimental variable, other parameters were kept constants at their
predicted optimal values: catalyst amount = 6 wt%, methanol/caprylic
15 acid molar ratio = 5.0, reaction time = 23 h, and temperature = 363 K.
Process and product optimization
The factorial experimental design was applied to minimize the
waste of catalyst and to reduce the number of cumbersome
35 experiments without significant loss of information. The design
of experiments, experimental results, and predicted responses for
the esterification of n-caprylic acid with methanol over the
1
+
2+
2
[
HSO
3
-pmim] (½Zn )SO
4
catalyst are summarized in Table 3.
No significant difference between experimental results and
40 predicted data can be found. The methyl caprylate yield (Y) may
be correlated with independent experimental variables by a
quadratic model, which can be expressed as:
1
ꢎ
ꢇ ꢂ ꢊ94.37 ꢊ 2.78x ꢊ 8.80x ꢊ 3.17x ꢒ 2.97x
ꢒ
ꢍ
ꢎ
ꢋ
ꢍ
ꢋ
8
.23xꢎꢎ ꢒ 1.88xꢋꢎ ꢒ 0.67x x ꢒ 1.37x x ꢒ 1.69x x
(8)
ꢍ
ꢎ
ꢍ
ꢋ
ꢎ
1
1 2 3
10 where x , x and x are the coded values respectively representing
three experimental variables, namely catalyst amount, molar ratio
of alcohol to acid, and reaction time (cf. Table 2).
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

Page 7 of 10
Green Chemistry
Table 3 List of experimental design and response values during
esterification of n-caprylic acid with methanol over the [HSO
The fitting quality of the quadratic model (Eq. 8) was verified
3
-
by the standard analysis of variance (ANOVA) presented in Table
+
2+
2
pmim] (½Zn )SO
4
catalyst
View Article Online
4
. The model F-value (309.76) was much greater than the tabular
DOI: 10.1039/C4GC01470G
a
10 counterpart (F0.01, 9, 7 = 6.71), implied that the model was indeed
significant. In addition, the obtained P-value (< 0.0001) revealed
that the chance (0.01%) such a large ‘model F-value’ could occur
was close to noise level. The observed “Lack of Fit F-value” of
Variable and level
Methyl caprylate yield (%)
Entry
x
1
x
2
x
3
Experimental
71.01
77.18
90.51
94.00
81.92
90.93
90.85
94.40
70.82
91.24
80.68
94.33
94.97
93.93
94.23
93.82
94.89
Predicted
70.93
77.82
89.87
94.08
82.21
90.50
91.28
94.11
70.61
91.59
80.33
94.54
94.37
94.37
94.37
94.37
94.37
1
2
3
4
5
6
7
8
9
-1
1
-1
-1
1
0
0
0
2
.00 also implied that the Lack of Fit was not significant relative
2
-1
1
1
2
2
3
5
0
5
0
to pure error. A coefficient of determination (R = 0.9975 was
attained, indicating that the model was reliable in predicting the
response. The “Adeq Precision” (48.755), which measured the
signal to noise ratio, was also much greater than the desirable
value of 4, as expected. Moreover, the coefficient of variation
(C.V. = 0.73%) also demonstrated that the experiments were
carried out reliably. On the basis of these statistical values, it is
conclusive that the quadratic model was adequate to predict a
reliable methyl caprylate yield within the range of the variables
studied. This is supported by the observed F- and P-values, which
revealed that all three independent variables were highly
significant, and that the order of their significance was
methanol/caprylic acid molar ratio > reaction time > amount of
catalyst to n-caprylic acid. Moreover, mutual interactions
between each pair of these independent variables were also
highly significant to the esterification reaction.
1
0
-1
1
0
-1
-1
1
0
-1
1
0
0
1
0
-1
1
-1
-1
1
1
0
1
0
1
0
-1
1
1
1
1
1
1
1
2
3
4
5
6
7
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
a
Variables and levels specified in Table 2.
5
Table 4 Estimated regression coefficients and corresponding F- and P-values for methyl caprylate yield.
Source
Sum of squares
Degree of Freedom
Mean square
F-value
P-value
Significance
model
1142.19
61.72
619.34
80.33
37.08
284.86
14.81
1.80
9
1
126.91
61.72
619.34
80.33
37.08
284.86
14.81
1.80
309.76
150.63
1511.67
196.06
90.51
< 0.0001
< 0.0001
< 0.0001
< 0.0001
< 0.0001
< 0.0001
0.0005
**
**
**
**
**
**
**
x
1
x
2
x
3
x
1
x
2
x
3
x
1
x
1
x
2
1
1
2
2
2
1
1
695.28
36.14
1
x
2
1
4.38
0.0746
x
x
3
7.45
1
7.45
18.19
0.0037
**
**
11.46
2.87
1
11.46
0.41
27.97
0.0011
3
Residual
Lack of fit
Error
7
1.72
3
0.57
2.00
0.2559
1.15
4
0.29
Cor total
1145.06
16
*
* represents highly significant.
3
5
The three dimensional (3-D) response surface and contour plots
changed slightly with the catalyst amount. Nonetheless, the
obtained from the predicted model are shown in Fig. 5. The 55 contour plot in Fig. 5e revealed that the interaction between
correlations between methanol/n-caprylic acid molar ratio and
catalyst amount at a fixed reaction temperature and time are
shown in Figs. 5a and 5d. It is clear that the yield of methyl
caprylate rapidly increased to the optimal value with increasing
catalyst amount and reaction time played an important role in
esterification, as also indicated by the results shown in Table 4.
Figs. 5c and 5f illustrate the interaction between reaction time
and methanol/caprylic acid molar ratio with respect to the product
4
0
methanol/acid molar ratio, however, no much improvement in the 60 yield. The yield increased significantly before the methanol/acid
response for the latter over 5:1 was observed. Meanwhile, the
yield improved gradually as the amount of catalyst increased but
leveled off when the amount exceeded ca. 6 wt%. It is indicative
that the methanol/acid molar ratio is a more important variable
molar ratio reached 5:1, but remained practically unchanged
beyond this ratio. A slight reduction in ester yield was observed
at prolonged reaction time (> 2 h). Again, this is attributed to
partial hydrolysis of the methyl caprylate product when the
4
5
for esterification reaction than the catalyst amount; the notion is 65 reaction time exceeded its maximum value (2 h). Since
consistent with the ANOVA result (Table 4). Fig. 5b shows the
correlations of catalyst amount and reaction time with methyl
caprylate yield. The yield was found to increase with increasing
reaction time, reaching a maximum (94.4%) at 2.0 h, then,
esterification is a reversible reaction, the hydrolysis rate of ester
would increase with reaction time while the yield of methyl
caprylate decreased. The effect of interaction of these two
variables was also significant, revealing an ellipse mound shape
5
0
decreased slightly thereafter. On the other hand, the yield only 70 (Fig. 5e), in good agreement with the ANOVA result (Table 4).
This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 |
6

Green Chemistry
Page 8 of 10
7
8
6
5
0
5
recovered AIL catalyst could be reused for at least five
consecutive runs without significant loss in catalytic activity, as
View Article Online
summarized in Table 5. The gradual decrease in product yield
DOI: 10.1039/C4GC01470G
observed after repeated cycles is ascribed due to leaching of Zn.
Additional elemental analysis by inductively coupled plasma
(ICP) revealed that the concentration of Zn in the spent catalyst
decreased from 9.71 wt% (fresh catalyst) to 9.45 wt% after the
fifth run.
+
2+
2
Table 5 Stability of the [HSO
3
-pmim] (½Zn )SO
4
AIL catalyst during
esterification of n-caprylic acid with methanol over the [HSO
3
-
+
2+
2
pmim] (½Zn )SO
4
1 2
catalyst (Reaction conditions: x = 7 w%, x = 6:1,
x
3
= 2 h, and T = 363 K)
Recycle time
Conversion (%) Methyl caprylate yield (%)
1
2
3
4
5
95.9
95.6
95.2
93.8
93.0
95.4
95.1
94.6
93.2
92.6
5
5
Kinetic model
To establish the kinetic model for esterification of n-caprylic acid
+
2+
2
to methyl caprylate over the [HSO
AIL catalyst, additional experiments were conducted with x
wt%, x = 6:1, x
20 reaction, ca. 1 mL sample was withdrawn from the mixture for
3
-pmim] (½Zn )SO
4
B-L
1
= 7
Fig. 5 (a-c) 3D response surface and (d-f) contour plots showing
variations between a pair of experimental variables (Table 1) on predicted
values of methyl caprylate yield while keeping other variables at a
constant level of 0: (a) and (d) methanol/n-caprylic acid molar ratio vs
catalyst amount; (b) and (e) reaction time vs catalyst amount; (c) and (f)
reaction time vs methanol/n-caprylic acid molar ratio.
2
3
= 2.0 h at different temperatures. During the
1
4
4-46
analysis at different time intervals.
The variations of
) versus
5
instantaneous concentration of n-caprylic acid (C
A
reaction time at five different temperatures (323, 333, 343, 353,
and 363 K) are shown in Fig. 6. Accordingly, the instant reaction
25 rate (r) may be estimated by performing differentiation of the
decay curve using a software package (Origin 8.5) to employ in
the subsequent linear curve fitting to obtain values of k and α in
Eq. (6).
1
Optimized responses and model verification
1
1
2
0
5
0
Based on RSM results, the mathematical model predicted an
optimal methyl caprylate yield of 96.16% for esterification of n-
caprylic
pmim] (½Zn )SO
conditions: x
acid
with
methanol
AIL catalyst under the following process
= 6.59 wt% (catalyst amount), x = 6.17:1
(methanol/n-caprylic acid molar ratio), and x = 2.26 h (reaction
over
the
3
[HSO -
+
2+
2
4
2.5
1
2
323K
3
2.0
1.5
1.0
0.5
0.0
333K
343K
353K
time) at a reaction temperature of 363 K. To confirm the validity
of the model and optimal process conditions, three additional
experiments were conducted in parallel at 363 K with x = 7 wt%,
363K
1
2 3
x = 6:1, x = 2.0 h. Accordingly, methyl caprylate yields of 95.1,
95.5, and 95.7% were obtained, leading to an average
experimental yield of 95.4%, in good agreement with the
predicted value. Thus, it is conclusive that the model was reliable
and that the regression equation could truly reflect influence of
the three variables on the yield of esterification.
0
30
60
90
120
Time (min)
7
0
2
5
Catalyst recycling
A
Fig. 6 Variations of instantaneous concentration of n-caprylic acid (C )
To evaluate the reusability of such B-L AIL catalyst, cyclic
experiments were conducted under the same optimal operating
with reaction time at different temperatures.
+
2+
2
conditions obtained above over the [HSO
3
-pmim] (½Zn )SO
4
.
75
For each cycle, the lower layer of the reaction mixture was
separated from the system after the esterification reaction,
extracted with diethyl ether, and vacuum dried for 5 h at 393 K.
Taking the experiment conducted at 323 K as an example, the
plot of ln r versus ln C fitted Eq. (6) well with a correlation
coefficient (R) of 0.9841, as shown in Fig. 7a. Likewise,
3
0
A
+
2+
2
satisfactory fits were achieved for plots obtained at other
The recovered [HSO
3
-pmim] (½Zn )SO
4
catalyst was assessed
1
1
10 temperatures. Accordingly, the k and α value so deduced from
various temperatures are summarized in Table 6, from which an
average reaction order α of 1.70 was derived for the esterification
by H NMR spectroscopy to ensure no traces of reactants or
products were retained in the catalyst. It was found that the
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

Page 9 of 10
Green Chemistry
Conclusions
A series of new halogen-free Brønsted-Lewis acidic io
V
n
ie
i
w
c
A
l
r
i
t
q
ic
u
leid
O
s
nline
DOI: 10.1039/C4GC01470G
were synthesized and exploited to catalyse esterification of n-
+
2+
2
1
1
1
1
3 4
25 caprylic acid with methanol. The [HSO -pmim] (½Zn )SO
catalyst, which exhibited the highest catalytic activity, was
employed for process optimization to obtain optimal
esterification conditions at a reaction temperature of 363 K:
catalyst amount of 7 wt%, methanol/n-caprylic acid molar ratio of
30 6, reaction time of 2.0 h, leading to a maximum methyl caprylate
yield of 95.4%. These optimized experimental values are in good
agreement with those predicted by response surface methodology
based on the Box-Behnken design model. A reaction order of 1.7
and activation energy of 33.66 KJ/mol was deduced from the
35 kinetic model developed. In addition to superior catalytic activity,
the Brønsted-Lewis acidic ionic liquid catalysts report herein are
also advantaged by facile recovery and recycle use without
significant loss in catalytic activity. Our results indicate that the
+
2+
2
novel eco-friendly [HSO
3
-pmim] (½Zn )SO
4
catalyst is a
40 promising substitute for conventional catalyst and of great value
in chemical industry.
Acknowledgements
The supports of this work by National Natural Science
Foundation of China (No. 21177110), National Natural Science
7
5
Foundation of Zhejiang province, China (No. LY13B070005),
and Ministry of Science and Technology, Taiwan (NSC 101-
2
113-M-001-020-MY3) are gratefully acknowledged.
A
Fig. 7 Plots of (a) ln k vs ln C (at 323 K) and (b) ln k vs 1/T.
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1
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ꢓꢔ
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ꢋꢋ.ꢪꢪ
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ꢭꢙꢚꢍ.ꢮꢉ
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ꢑ ꢂ ꢒ
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