Kinetics Study of the Esterification Reaction of Cyclohexene to Cyclohexyl Acetate Catalyzed by Novel Br?nsted–Lewis Acids Bifunctionalized Heteropolyacid Based Ionic Liquids Hybrid Solid Acid Catalysts

Article abstract of DOI:10.1007/s10562-021-03626-4

A series of Br?nsted–Lewis acids bifunctionalized heteropolyacid based ionic liquids hybrid solid acid catalysts (BLA-HPA-ILs) were synthesized by combining the Br?nsted acidic ionic liquid [Bis–Bs–BDMAEE]HPW₁₂O₄₀ with metallic oxide in different composition ratios and applied in the esterification of cyclohexene to cyclohexyl acetate. Among the synthesized catalysts, the ¹/₂Cu[Bis–Bs–BDMAEE]HPW₁₂O₄₀ catalyst with Br?nsted and Lewis acidities shown the most excellent catalytic performance for the esterification of cyclohexene with acetic acid. The BLA-HPA-ILs catalysts were characterized by elemental analysis, FT-IR, Py-IR, TG, ¹H NMR, SEM and EDX. The effects of reaction temperature, catalyst dosage, and initial reactant molar ratio has been investigated in detail. A pseudohomogeneous (PH) kinetic model was used to correlate the kinetic data in the temperature range of 333.15–363.15?K, and the kinetic parameters were estimated, indicating the results calculated by the kinetic model are well coincidence with the experimental results. Moreover, as a heterogeneous reaction catalyst, ¹/₂Cu[Bis–Bs–BDMAEE]HPW₁₂O₄₀ could be easily recovered by a simple treatment and reused six times without any obvious decrease in catalytic activity, displaying good reusability. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03626-4

Catalysis Letters
https://doi.org/10.1007/s10562-021-03626-4
Kinetics Study of the Esteriꢀcation Reaction of Cyclohexene
to Cyclohexyl Acetate Catalyzed by Novel Brønsted–Lewis Acids
Bifunctionalized Heteropolyacid Based Ionic Liquids Hybrid Solid Acid
Catalysts
1
1
1
1
1
1
Binxiong Guang · Yuefeng Wu · Weihua Liu · Jianhong Wang · Yahui Xiao · Yong Liu
Received: 8 February 2021 / Accepted: 9 April 2021
©
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
A series of Brønsted–Lewis acids bifunctionalized heteropolyacid based ionic liquids hybrid solid acid catalysts (BLA-HPA-
ILs) were synthesized by combining the Brønsted acidic ionic liquid [Bis–Bs–BDMAEE]HPW O with metallic oxide in
1
2
40
diꢀerent composition ratios and applied in the esteriꢁcation of cyclohexene to cyclohexyl acetate. Among the synthesized
1
catalysts, the / Cu[Bis–Bs–BDMAEE]HPW O catalyst with Brønsted and Lewis acidities shown the most excellent
2
12 40
catalytic performance for the esteriꢁcation of cyclohexene with acetic acid. The BLA-HPA-ILs catalysts were characterized
1
by elemental analysis, FT-IR, Py-IR, TG, H NMR, SEM and EDX. The eꢀects of reaction temperature, catalyst dosage,
and initial reactant molar ratio has been investigated in detail. A pseudohomogeneous (PH) kinetic model was used to cor-
relate the kinetic data in the temperature range of 333.15–363.15 K, and the kinetic parameters were estimated, indicating
the results calculated by the kinetic model are well coincidence with the experimental results. Moreover, as a heterogeneous
1
reaction catalyst, / Cu[Bis–Bs–BDMAEE]HPW O could be easily recovered by a simple treatment and reused six times
2
12 40
without any obvious decrease in catalytic activity, displaying good reusability.
Graphic Abstract
Keywords Heterogeneous catalysis · Ionic liquids · Bifunctionality · Kinetic modeling · Cyclohexene esteriꢁcation
*
Jianhong Wang
jhwnj@163.com
Abbreviations
A
B
D
Cyclohexene
*
Yong Liu
Acetic acid
liuyong79@126.com
Cyclohexyl acetate
Extended author information available on the last page of the article
Vol.:(0
1
123456789)
3

B. Guang et al.
HPA
Heteropolyacid
between cyclohexene and water, which is only 0.02% (w/w)
at 298 K [9]. Hence, the establishment of a new technology
to replace direct hydration has become especially important.
A two-step indirect hydration method for cyclohexanol
has been developed to overcome the above shortcomings of
direct hydration method for cyclohexene [10]. For the ꢁrst
step, the cyclohexyl carboxylate is formed by the reaction
of cyclohexene with the carboxylic acid (e.g. formic acid,
acetic acid, etc.), which is an electrophilic addition esteriꢁ-
cation using acid catalyst [11]. In the second step, the ester
can be produced cyclohexanol by hydrolysis, transesteriꢁca-
tion, or hydrogenation [12]. Steyer and co-workers [13] have
conꢁrmed the feasibility of this route by the esteriꢁcation of
cyclohexene with formic acid and the subsequent hydrolysis
of the ester in a reactive distillation column. The catalytic
reaction distillation process has been used in the indirect
hydration process for the production of cyclohexanol [14,
15]. It is particularly worth mentioning that the esteriꢁcation
of cyclohexene with carboxylic acid to form cyclohexyl car-
boxylate is the decisive step in the entire indirect hydration
process [16]. In other words, the yield of the target product
cyclohexanol can be increased by increasing the esteriꢁca-
tion reaction rate of cyclohexene. Until now, various cata-
lysts such as strong cation exchange resins Amberlyst-15
have been applied in the esteriꢁcation of cyclohexene [11,
17]. However, the conversion rate of cyclohexene is rela-
tively low with the conventional solid acid catalyst in the
esteriꢁcation of cyclohexene and formic acid [18–20]. Thus,
it is particularly important to increase the activity of the
catalysts used in the esteriꢁcation.
ILs
Ionic liquids
HPA-ILs
Heteropolyacid based ionic liquids
hybrid
BLA-ILs
Brønsted–Lewis acidic ILs
Brønsted–Lewis acids bifunction-
alized heteropolyacid based ionic
liquids hybrid solid acid
BLA-HPA-ILs
[
Bis–Bs–BDMAEE] [HO S–(CH ) –BDMAEE–
3 2 4
(
CH ) –SO H]
2 4 3
PH
Pseudohomogeneous
K
Equilibrium constant
e
k+
Forward reaction rate constant
−
1
−1
(
mol min )
k−
Reverse reaction rate constant
−
1
−1
(
mol min )
k0
Pre-exponential factor
2
−2
−1
(
L mol min )
Ci
The molar concentration of compo-
−
1
nent i (mol L )
mcat
The catalyst dosage per unit vol-
−
1
ume (g L )
T
t
ΔrH0
ΔrS
x
Temperature (K)
Time (min)
−
1
The reaction enthalpy (kJ mol )
−
1
−1
Entropy (J mol K )
Conversion of cyclohexene
Calculated conversion
xcal
xexp
Ea
Experimental conversion
−
1
Activation energy (kJ mol )
SRS
Minimum sum of residual squares
In recent years, functionalized ionic liquids (ILs), as a
new type of clean catalyst and an excellent solvent, have
attracted extensive research interest of scholars from vari-
ous ꢁelds due to their distinguishing properties such as neg-
ligible vapor pressure, remarkable solubility, designability
of molecules and high chemical stability [21–24]. ILs have
been extensively applied in the esteriꢁcation, hydrolysis,
transesteriꢁcation, and gas adsorption reaction because of
their excellent catalytic activities [25–29]. However, the ILs
have several drawbacks such as a large amount of ILs needed
in reaction, and the diꢂcult recovery of ILs from the reac-
tants, which hinder their practical applications [30]. On the
other hand, heteropolyacid (HPA) with Keggin structures
have been widely applied in the many organic reactions due
to their excellent catalytic performances. Whereas, there are
many defects including the low surface area and solubility,
which have limited its practical application [31, 32]. In order
to overcome these drawbacks, immobilization of HPA could
be an eꢀective way to improve their catalytic performance
[33]. Nonetheless, immobilization of HPA onto the diꢀerent
supports suꢀers from the slow reaction rate, and leaching
of active sites [34, 35]. As a novel type of ionic liquids,
heteropolyacid based ionic liquids hybrid (HPA-ILs) were
1
Introduction
Cyclohexanol is the intermediate raw material in the produc-
tion of caprolactam, adipate and other phthalamide products,
which has been widely used in organic chemicals, coating,
and textile industries [1]. There are three main methods to
produce cyclohexanol including the oxidation of cyclohex-
ane, the hydrogenation of phenol, and the direct hydration
of cyclohexene [2–5]. Up to now, the oxidation of cyclohex-
ane to produce cyclohexanol is a main method in industry.
However, this method suꢀers from several drawbacks which
limit its development [6], including limited selectivity, high
energy consumption, and explosion hazards. In addition, the
hydrogenation of phenol develops slowly due to the high
cost of phenol and the large demand of hydrogen energy [7].
In the above method, the direct hydration of cyclohexene
to produce cyclohexanol is a promising method owing to
its advantages of atom economy, and high selectivity [8].
Unfortunately, the direct hydration reaction is still limited by
low reaction rate because of the extremely poor miscibility
1
3

Kinetics Study of the Esteriꢀcation Reaction of Cyclohexene to Cyclohexyl Acetate Catalyzed…
proposed by combining the ionic liquids with the heteropo-
lyacid hybrid, which have received widespread attention
because they not only have excellent catalytic performance,
but also easy reuse in a solid state [36, 37]. HPA-ILs have
been applied for esteriꢁcation, hydroxylation, desulfuriza-
tion, and oxidation reactions as catalysts [38–41]. In our
previous works, many HPA-ILs catalysts have been synthe-
sized and used in the esteriꢁcation [42], transesteriꢁcation
and the introduction of diꢀerent metals (Cu, Co, Ni, etc.).
These catalysts contained the advantages of ILs, HPAs and
Brønsted–Lewis acid sites, and applied in esteriꢁcation reac-
tion of cyclohexene and acetic acid. The impact of various
parameters of catalyst type, reaction temperature, initial
reactant molar ratio and catalyst dosage was investigated
in detail. A pseudohomogeneous (PH) kinetic model was
applied to correlate the experimental data, and the corre-
sponding kinetic parameters were estimated simultaneously.
[
43] and hydrolysis [44].
Acidic ILs can be classiꢁed into Brønsted [45, 46] and
Lewis types [47] in accordance with their acidic groups.
As the novel catalysts, Brønsted–Lewis acidic ILs (BLA-
ILs) which combined the exceptional performance of Lewis
and Brønsted acid sites, have been developed and applied in
many chemical reactions. Han et al. [48] assessed the cata-
lytic activity of various [HO S-pmim]Cl-xSnCl (x=0–0.8
2 Experimental
2.1 Materials
Phosphotungstic acid (H PW O ), phosphomolybdic
3
2
3
12 40
mol) by transesteriꢁcation reaction of soybean oil with meth-
anol, a satisfactory biodiesel yield of 98.6% was achieved
under optimized reaction conditions. The alkylation of isob-
utene or isobutene was investigated [49] in the presence of
Brønsted–Lewis acidic ILs [HO SC NEt ]Cl-ZnCl , exhibit-
acid (H PMo O ), silicotungstic acid (H SiW O ),
3 12 40 4 12 40
cyclohexene (≥99%) and acetic acid (≥99.5%) were sup-
plied by Sinopharm Chemical Reagent, Shanghai, China.
Bis(2-dimethylaminoethyl) ether (≥ 98%) was purchased
from J&K Scientiꢁc Ltd., Beijing, China. 1,4-Butyl sul-
tone (≥99%) was purchased by Aladdin Reagent Co., Ltd.,
Shanghai, China. All the chemicals were analytical pure and
were directly used without further treatment.
3
3
3
2
ing an outstanding catalytic performance. Yuan et al. [50]
reported a heteropolyacid hybrid organic–inorganic catalyst
3
+
Sm [TEAPS] PW O by adding Sm as the Lewis acid
0.33
2
12 40
sites on the basis of functionalized Brønsted heteropolyacid
hybrid ionic liquids [TEAPS] PW O , was used to catalyze
2.2 Preparation of BLA‑HPA‑ILs
3
12 40
the dimerization of rosin. This is because the Brønsted and
Lewis acid sites and their covalent or ionic bonding modes
are eꢀective for the synergistic catalysis of the catalyst body
in the solid–liquid system. Moreover, in our previous work,
a series of Brønsted–Lewis acids bifunctionalized ionic liq-
uids based heteropolyacid hybrid was prepared and shown
excellent in the esteriꢁcation reaction of camphene with
acetic acid to produce isobornyl acetate [51].
In this work, the novel BLA-HPA-ILs were synthesized by
combining the sulfonic acid-functionalized ionic liquids
based heteropolyacid hybrid [Bis–Bs–BDMAEE]HPW O
12
40
with CuO (Scheme 1).
The detailed synthesis procedure of [Bis–Bs–BDMAEE]
HPW O were according to our previous work [41]:
1
2
40
Bis(2-dimethylaminoethyl) ether (0.1 mol) and 1,4-
butyl sultone (0.21 mol) was added into a 100 mL ꢃask.
Then, the mixtures were placed in a water bath to 60 °C,
Based on our study on the ILs and HPA catalysts, a series
of BLA-HPA-ILs was prepared through anion exchange
Scheme 1 The preparation
1
process of / Cu[Bis–Bs–
2
BDMAEE]HPW O
12
40
1
3

B. Guang et al.
and agitated for 1 h. Afterwards, a white solid zwitterion
then rapidly put it to an ice bath to avoid its further reaction
in each experiment. These samples were analyzed by gas
chromatography (GC).
[
Bis–Bs–BDMAEE] was formed, and the unreacted com-
ponents were removed by repeated washing with ether for 3
times, then dried in a vacuum at a reduced pressure of 80 °C.
Next, a certain amount of an aqueous solution of H PW O
2.5 Analysis
3
12 40
was added drop by drop. After that, the mixtures were placed
in 80 °C water bath, reꢃuxed and agitated for 18 h. After
the reaction, the samples were ꢁltered, washed, and dried
under a vacuum to obtain the product [Bis–Bs–BDMAEE]
HPW O . And then, [Bis–Bs–BDMAEE]HPW O was
The samples were analyzed by a gas chromatograph
(Fuli, 9790) equipped with a DB-1 capillary column
(30 m×0.539 mm×1.50 μm) and a hydrogen ꢃame ioniza-
tion detector (FID). The temperatures of injector and FID
were all set at 250 °C.
1
2
40
12 40
dissolved in aqueous solution and added varied amounts of
CuO. The mixtures were constant agitation till the solid was
1
completely dissolved. The ꢁnal / Cu[Bis–Bs–BDMAEE]
2
HPW O catalysts were obtained by using the rotary evap-
3 Results and Discussions
12
40
orator to remove water. [Bis–Bs–BDMAEE]HPMo O ,
1
2
40
[
Bis–Bs–BDMAEE]H SiW O , and BLA-ILs-HPA dop-
3.1 Characterization
2
12 40
ing other metallic oxide (M=Co, Ni, Zn, Fe, Al) were also
prepared by the above description.
FT-IR spectra of catalysts were shown in Fig. 1. As can be
−
1
seen in Fig. 1c, some peaks at wave number of 1079 cm
−
1
2.3 Characterization Methods
(P–O stretching vibration), 978 cm (W=O stretching
−
1
vibration), 893 cm (W–O –W stretching vibration) and
b
−1
Fourier transform infrared spectroscopy (FT-IR) was
measured using Vertex 70 (Bruker Optics, Germany),
using KBR compression method, and the scanning range
805 cm (W–O –W stretching vibration) were observed. The
c
2
+
2−
FT-IR spectra of [Bis–Bs–BDMAEE] HPW O
and
/ Cu[Bis–Bs–BDMAEE]HPW O are similar, and also
1
2
40
1
2
12 40
−
1
was 4000–450 cm . The thermogravimetric analysis
show that they all have peaks at wave number of 1079,
−
1
(
TG) was measured within the range of 20–1000 °C using
978, 893 and 805 cm , showing they all retain the Keg-
gin structure of H PW O . As shown in Fig. 1a, b, the
DSC851E (Mettler Toledo, Switzerland). The test condi-
tions were high purity nitrogen atmosphere and the heat-
3
12 40
−
1
bands at 3450 and 3014 cm are attributed to the stretch-
ing vibration of O–H and C–H, respectively. The bands
−
1
ing rate was 10 °C min . Scanning electron microscope
−
1
(
SEM) and X-ray energy spectrum (EDX) were measured
at 2927 and 2870 cm are attributed to the stretching
−
1
by JSM-7610F (JEOL, Japan). Elemental analysis (C, S,
H, O, N) of the BLA-HPA-ILs catalysts were conducted
with an elemental analyzer (Vario EL cube, Elemental
Analysis System GmbH, Hanau, Germany). Nuclear mag-
netic resonance proꢁle (NMR) was determined using an
Avance 400 NMR instrument (Bruker AXS, Germany).
vibration of –CH −. The absorption peaks of 1479 cm
2
are attributed to bending vibration of –CH −. In addition,
2
−
1
the peak at 1163 and 1043 cm are assigned to the asym-
metrical and symmetric stretching vibration of the –S=O
1
H NMR (Bruker DPX-400) characterization results for
these catalysts were similar. Here only the representative
1
data of [Bis–Bs–BDMAEE]HPW O was given. H NMR
1
2
40
(
400 MHz, D O) δ 1.70–1.82 (m, 4H), 1.88–2.04 (m, 4H),
2
2
4
.91 (t, J= 7.6 Hz, 4H), 3.20 (s, 12H), 3.40 (t, J= 5.4 Hz,
H), 3.69 (s, 4H), 4.09 (s, 4H).
2.4 Apparatus and Procedure
A round-bottom ꢃask (50 mL) was placed in a constant
temperature water bath of 363.15 K. 0.03 mol cyclohexene
and 0.09 mol acetate acid were added to the ꢃask. After
the reactants reached the reaction temperature. The agita-
tor and time measurement were started immediately when
0
.55 g of BLA-ILs-HPA catalysts were added to the ꢃask.
The reaction was carried out for 9 h with stirring, samples
were taken out from the reactor in speciꢁc intervals, and
1
Fig. 1 FT-IR spectra for (a) / Cu[Bis–Bs–BDMAEE]HPW O , (b)
2
40
12 40
[Bis–Bs–BDMAEE]HPW O , and (c) H PW O
1
2
3
12 40
1
3

Kinetics Study of the Esteriꢀcation Reaction of Cyclohexene to Cyclohexyl Acetate Catalyzed…
to –SO H, respectively. The FT-IR spectra shows that
3
both ILs of Fig. 1a, b not only preserve the Keggin struc-
ture of H PW O , while also retaining the organic cation
3
12 40
structure.
Moreover, the Py-IR spectra illustrated in Fig. 2, which
1
indicates that / Cu[Bis–Bs–BDMAEE]HPW O exists
2
12 40
−
1
both Brønsted acid sites (1537 cm ) and Lewis acid
−
1
sites (1454 cm ), indicating that the presence of Brøn-
sted–Lewis acidity derived from sulfonic acid groups and
2
+
Cu . [Bis–Bs–BDMAEE]PW O only exists Brønsted
1
2
40
acid sites, and Cuo has no any peaks.
The TG curve of [Bis–Bs–BDMAEE]HPW O and
1
2
40
1
/
Cu[Bis–Bs–BDMAEE]HPW O is shown in Fig. 3. The
2
12 40
weight loss of catalyst is caused by evaporation of water in
the sample before 250 °C. There are three obvious weight-
lessness after 250 °C, which shows that [Bis–Bs–BDMAEE]
is gradually decomposed.
1
Fig. 2 Py-IR spectra of (a) / Cu[Bis–Bs–BDMAEE]HPW O , (b)
2
12 40
[
Bis–Bs–BDMAEE]PW O , and (c) CuO
1
2
40
1
The SEM images of / Cu[Bis–Bs–BDMAEE]HPW O
2
12 40
were shown in Fig. 4, showing that catalyst have no deꢁnite
morphology. In addition, X-ray energy spectrum (EDX) anal-
1
ysis was performed for / Cu[Bis–Bs–BDMAEE]HPW O
2
12 40
catalyst. As shown in Fig. 4b, which conꢁrmed that the pres-
ence of Cu along with other elements (C, O, N, S, and W).
1
The elemental analysis of the / Cu[Bis–Bs–BDMAEE]
2
HPW O is also carried out using an elemental analyzer.
1
2
40
The measured results are as follows: C (6.53%), H (1.80%),
O (21.68%), N (0.94%), S (2.08%), which is in agreement
with the theoretical value of C (5.80%), H (1.19%), O
(
22.70%), N (0.85%), and S (1.94%).
3.2 Catalyst Performance
The esteriꢁcation reaction of cyclohexene with acetic acid
was investigated to test their catalytic activities under the
same reaction conditions using diꢀerent BLA-HPA- ILs and
other acidic catalysts. The results were shown in Table 1.
First of all, the catalyst performance of the dual-sulfonic
acid functionalized heteropolyacid based ionic liquids
hybrid (HPA-ILs) with different heteropolyacid hybrid
1
Fig. 3 TG curve of / Cu[Bis–Bs–BDMAEE]HPW O
2
12 40
1
Fig. 4 The SEM (Α) and EDX (Β) of / Cu[Bis–Bs–BDMAEE]HPW O
2
12 40
1
3

B. Guang et al.
Table 1 The esteriꢁcation of
cyclohexene with acetic acid on
diꢀerent catalysts
a
b
c
Entry
Catalysts
Conv. (%)
Sel. (%)
Yield. (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
[Bis–Bs–BDMAEE]HPW O
40
[Bis–Bs–BDMAEE]HPMo O
40
49.62
3.08
8.71
93.65
85.69
83.97
93.75
98.61
97.41
97.10
95.32
66.93
97.79
97.62
95.09
98.70
97.47
94.02
92.85
94.98
98.12
46.47
2.64
7.31
1
2
1
2
[Bis–Bs–BDMAEE]H SiW O
2
12 40
1
1
3
/4Cu[Bis–Bs–BDMAEE]HPW O
/2Cu[Bis–Bs–BDMAEE]HPW O
/4Cu[Bis–Bs–BDMAEE]HPW O
53.28
65.92
44.94
40.16
22.72
2.30
54.15
52.19
51.10
54.87
56.08
51.32
41.91
52.81
46.49
49.95
65.00
43.78
39.00
21.66
15.39
52.95
50.95
48.59
54.16
54.66
48.25
38.91
50.16
45.62
12 40
12 40
12 40
Cu[Bis–Bs–BDMAEE]HPW O
1
2
40
5
3
1
1
1
1
1
/
4Cu[Bis–Bs–BDMAEE]HPW O
12 40
/2Cu[Bis–Bs–BDMAEE]HPW O
/3Fe[Bis–Bs–BDMAEE]HPW O
/3Co[Bis–Bs–BDMAEE]HPW O
/2Ni[Bis–Bs–BDMAEE]HPW O
/2Zn[Bis–Bs–BDMAEE]HPW O
/3Al[Bis–Bs–BDMAEE]HPW O
12 40
1
1
1
1
1
1
1
1
1
12 40
12 40
12 40
12 40
12 40
H PW O
3
12 40
H PMo O
3
12 40
H SiW O
4
12 40
H SO
2
4
a
Conversion of cyclohexene
Selectivity of cyclohexyl acetate
Yield of cyclohexyl acetate. Reaction conditions: temperature=363.15 K, catalyst dosage=7 wt.% (based
b
c
on the mass of all reactants), acetic acid/cyclohexene molar ratio=3.0, and reaction time=7.0 h
anions was investigated for determining the best heter-
opolyacid hybrid anion. As can be seen in Table 1 (entries
results indicate that Brønsted acid catalysts can generate the
more carbocation intermediate to accelerate conversion of
cyclohexene, and reduce the selectivity of cyclohexyl acetate
simultaneously. For Lewis acid, the reaction mechanism is
that the vacant orbital of Lewis acid is coordinated with
π–π electron cloud of cyclohexene, inducing cyclohexene
to form carbocation intermediate, resulting in slower rate of
carbocation intermediate generation (Scheme 2b). The over-
all rate of esteriꢁcation reaction becomes slower, and the
side reactions are also decreased at the same time. So, the
BLA-HPA-ILs with Brønsted–Lewis acid sites may increase
the conversion of the cyclohexene and the selectivity of tar-
get product cyclohexyl acetate by synergistic eꢀect between
Brønsted acid and Lewis acid.
1
–3), [Bis–Bs–BDMAEE]HPW O shows the best cata-
12 40
lytic performance compared with the other two catalysts,
and the conversion of cyclohexene and the selectivity of
cyclohexyl acetate reached 49.62% and 93.65%, respec-
tively. The BLA-HPA-ILs with Brønsted acid and Lewis
acid by adding the diꢀerent metals (M = Cu, Fe, Co, Ni,
Zn, Al) into the [Bis–Bs–BDMAEE]HPW O catalyst. As
1
2
40
1
shown in Table 1 (entries 4–14), / Cu[Bis–Bs–BDMAEE]
2
HPW O catalyst shows the most excellent catalytic per-
1
2
40
formance compared to other transition metal doped catalysts.
The conversion of cyclohexene, cyclohexyl acetate selectiv-
ity and yield reached 65.92%, 98.61% and 65.00%, respec-
tively (entry 5). This may be due to that doped metal Cu with
Lewis acidity can enhance the formation of carbon cations.
The reaction mechanism of esteriꢁcation of cyclohexene
was shown in Scheme 2. The reaction of cyclohexene with
acid ꢁrst generates carbocation intermediate, and then the
carbocation intermediate further reacts with acetic acid to
generate cyclohexyl acetate. It can be seen that the key rate
step of esteriꢁcation is the generation of the carbocation
intermediate in the ꢁrst step. For Brønsted acid, its pro-
The contents of Brønsted acid and Lewis acid have
important influence on catalytic activity of the cata-
lyst. Therefore, BLA-HPA-ILs with different content of
Brønsted acid and Lewis acid were prepared by adding
different amount of Cu, and the catalytic performance
was shown in entries 4–9. Interestingly, the conversion
of cyclohexene and the selectivity of cyclohexyl ace-
tate increases with the addition of the metal Cu. When
1
/ Cu[Bis–Bs–BDMAEE]HPW O is used as a catalyst,
2
12 40
+
ton H reactions with cyclohexene easily to generate more
the catalytic activity is optimal. However, the BLA-HPA-
ILs catalytic activity began to decline with the amount
of Cu further increase (entries 6–9), which increase the
content of Lewis acid. This is may be due to that the
carbenium ion, which makes the reaction rate faster. How-
ever, carbenium ion may also react with cyclohexene, gen-
erating cyclohexene dimer by-products (Scheme 2a). This
1
3

Kinetics Study of the Esteriꢀcation Reaction of Cyclohexene to Cyclohexyl Acetate Catalyzed…
Scheme 2 Mechanism of esteriꢁcation of cyclohexene catalyzed by (Α) Brønsted acid and (B) Lewis acid
large amount of Cu added can hinder the nucleophilic
attack of acetic acid, leading to reduce the conversion of
cyclohexene.
The conventional acidic catalyst sulfuric acid,
H PW O , H PMo O and H SiW O showed rel-
3
12 40
3
12 40
4
12 40
atively low catalytic performance (entries 15–18). In
addition, these conventional acidic catalysts have some
disadvantages such as difficult separation from the reac-
tion mixtures, environmental pollution. However, as het-
erogeneous catalysts, BLA-HPA-ILs can be easily sepa-
rated from the reaction mixtures via simple filtration. In
1
a word, / Cu[Bis–Bs–BDMAEE]HPW O was found to
2
12 40
have promising application in esterification of cyclohex-
ene with acetic acid.
Fig. 5 Eꢀect of reaction temperature on the conversion of cyclohex-
3
.3 Eꢀect of Reaction Temperature
1
ene using / Cu[Bis–Bs–BDMAEE]HPW O as catalyst. Reaction
2
12 40
conditions: catalyst dosage=7 wt.%, acetic acid/cyclohexene molar
ratio=3.0
As shown in Fig. 5, the conversion of cyclohexene at
different reaction temperatures in the range of 333.15 to
1
3
63.15 K was studied using / Cu[Bis–Bs–BDMAEE]
2
HPW O under the same reaction conditions. The con-
3.4 Eꢀect of Catalyst Dosage
1
2
40
version of cyclohexene at 7 h increased obviously from
2
8.89 to 65.92% when the reaction temperature was
The eꢀect of diꢀerent catalyst dosage on the conversion of
cyclohexene in the range of 3 to 9 wt.% was investigated
increased from 333.15 to 363.15 K, indicating that an
increase in reaction temperature is beneficial in enhanc-
ing the conversion of cyclohexene. This means that the
increase of temperature is helpful to the improvement of
the reaction rate, which is due to the temperature increase
may lead to more successful collisions between reactants.
On this account, there is sufficient energy to break the
bonds to form products, which leads to a higher conver-
sion rate of cyclohexene.
1
using / Cu[Bis–Bs–BDMAEE]HPW O as catalyst under
2
12 40
the same reaction conditions. As shown in Fig. 6, it can be
observed that the conversion of cyclohexene increased with
increasing catalyst dosage from 3 to 7 wt.%. This is due to
the increased catalyst dosage can improve the active ingre-
dient concentration, and lead to speed up the reaction rate.
The conversion of cyclohexene at 7 h increased from 38.34
to 65.92% when catalyst dosage was increased from 3 to
1
3

B. Guang et al.
Fig. 6 Eꢀect of catalyst dosage on the conversion of cyclohexene
Fig. 7 Eꢀect of initial reactant molar ratio on the conversion of
1
1
using / Cu[Bis–Bs–BDMAEE]HPW O
as catalyst. Reaction
cyclohexene using / Cu[Bis–Bs–BDMAEE]HPW O
as cata-
2
12 40
2
12 40
conditions: temperature=363.15 K, acetic acid/cyclohexene molar
lyst. Reaction conditions: temperature=363.15 K and catalyst dos-
ratio=3.0
age=7 wt.%
7
wt.%. However, the conversion of cyclohexene begins to
decrease with further increase of the catalyst dosage from
to 9 wt.%. This might be because the mass transfer resist-
acid is not conducive to the esteriꢁcation of cyclohexene,
and the optimal molar ratio is 3:1.
7
ance increases with further increase of the catalyst dosage
under the same stirring conditions. There a low reaction rate
was obtained thanks to a low mass transfer coeꢂcient with
further increase of the catalyst dosage under the same stir-
ring conditions, resulting in a lower reaction conversion rate.
Moreover, the amount of catalyst was further increased, the
catalyst easily forms a pile in the reaction system, which can
reduce contact between the active sites of catalysts and reac-
tants, leading to the lower conversion of cyclohexene. As a
consequence, excessive amounts of catalyst are disadvanta-
geous to the esteriꢁcation of cyclohexene, and the optimal
catalyst dosage is 7 wt.%.
3.6 Chemical Equilibrium
The esteriꢁcation reaction equation of cyclohexene with
acetic acid can be written as follows:
k+
A + B ⇌ D
k
(
1)
−
Here, A is cyclohexene, B is acetic acid and D is cyclohexyl
acetate. Kinetic experiments were carried out to obtain the
chemical equilibrium constants under the same conditions at
diꢀerent temperatures. Samples were taken out of the reactor
after speciꢁc time intervals for analyzed until chemical equi-
librium was reached. The chemical equilibrium constants
were calculated via Eq. (2):
3.5 Eꢀect of Initial Reactant Molar Ratio
The effect of initial reactant molar ratio on the con-
CD
version of cyclohexene was investigated using
K =
e
(2)
1
C C
B
/
Cu[Bis–Bs–BDMAEE]HPW O as catalysts under the
A
2
12 40
same reaction conditions. As can be seen in Fig. 7, the con-
version of cyclohexene increased obviously with the molar
ratio changed from 1:1 to 3:1. This is because an increase in
the concentration of acetic acid brings the reaction toward
positive reaction side for this reversible reaction. However,
when the molar ratio was further increased from 3:1 to 4:1,
the conversion of cyclohexene decreased from 65.92 to
Here K represents the equilibrium constant; C is the equi-
e
i
−
1
librium concentration of component i (mol L ).
The dependency of the equilibrium constant K on the
e
temperature can be found from a plot of ln K versus the
e
reciprocal temperature (1/T), as given in Fig. 8. It was found
that the equilibrium constant decreased with increasing tem-
perature, indicates the esteriꢁcation reaction of cyclohexene
with acetic acid is an exothermic reaction. The equilibrium
constant as shown in Eq. (3) can be expressed as follows:
6
1.63%. This may be ascribed to the fact that the concentra-
tion of cyclohexene can be diluted when the amount of acetic
acid was further increased. Therefore, an excess of acetic
1
3

Kinetics Study of the Esteriꢀcation Reaction of Cyclohexene to Cyclohexyl Acetate Catalyzed…
estimated by minimizing the sum of residual squares (SRS)
between the values for the conversion of cyclohexene cal-
culated according to the PH model and obtained by experi-
ments, as shown in Eq. (6):
ꢀ
ꢁ
ꢂ
2
SRS =
xexp − xcalc
(
6)
Samples
where SRS is the minimum sum of residual squares and x
is the conversion of cyclohexene. The subscripts exp and
cal represent experimental and calculated values, respec-
tively. According to the calculated results, it was revealed
that the conversions of cyclohexene calculated by the reac-
tion kinetic model were very consistent with those obtained
from the experiments. The comparison data between the
calculated results and the experimental results are shown in
Figs. 9, 10, and 11.
Fig. 8 Eꢀects of reaction temperature on the chemical equilibrium
constant
Therefore, the PH model provides a good description of
the kinetic behavior for the esteriꢁcation of cyclohexene
1
with acetic acid using / Cu[Bis–Bs–BDMAEE]HPW O
2
12 40
5
256.2
as catalysts.
ln K =
e
− 13.84
(3)
T
The eꢀect of the temperature on the reaction rate con-
stants in this work can be expressed by the Arrhenius
equation as follows:
0
0
The reaction enthalpy (Δ H ) and entropy (Δ S ) were
r
r
estimated via the van’t Hoꢀ equation [52, 53]:
Ea
ln k = ln k −
0
(7)
Δ H0
ΔrS0
RT
−
r
ln K =
e
+
(4)
RT
R
The activation energy (E ) and pre-exponential factor
a
0
0
(k ) were obtained by the relationship between lnk and
0
The reaction enthalpy (Δ H ) and entropy (Δ S ) were
r
r
−
1
−1
−1
1/T from Eq. (7). As shown in Fig. 12, the intercept on
found to be − 43.70 kJ mol and − 115.07 J mol
K
the ordinate (ln k ) and the slope of straight lines (− E
0
a
in this work, respectively. The negative reaction enthalpy
again indicates that this esteriꢁcation reaction of cyclohex-
ene with acetic acid is the exothermic reaction.
3.7 Reaction Kinetic Model
A pseudohomogeneous (PH) model can be used to express
many esteriꢁcation reactions systems where one of the
reactants or solvents is highly polar [54–56]. In this work,
the kinetic equation for the reversible esteriꢁcation reac-
tion of cyclohexene with acetic acid was established
according to the PH model, which can be written as Eq.
(
5):
ꢂ
ꢃ
dCA
dt
ꢀ
ꢁ
CD
Ke
r = −
= mcat k C C − k C = mcatk+ C C −
B
+
A
B
−
D
A
(5)
Here K =k /k is the equilibrium constant; m is the cata-
e
+
−
cat
lyst dosage per unit volume; C is the molar concentration of
i
component i; k and k are the rate constants of the forward
+
−
and reverse reactions, respectively.
Fig. 9 Comparison between measured and calculated at diꢀerent
1
temperatures using / Cu[Bis–Bs–BDMAEE]HPW O as catalyst.
2
12 40
The kinetic equation was integrated by a fourth-order
Runge–Kutta method. The kinetic parameters were
Reaction conditions: catalyst dosage=7 wt.%, acetic acid/cyclohex-
ene molar ratio=3.0
1
3

B. Guang et al.
Fig. 10 Comparison between
measured and calculated at
diꢀerent catalyst dosage using
1
/
Cu[Bis–Bs–BDMAEE]
2
HPW O as catalyst.
12
40
Reaction conditions: tem-
perature=363.15 K, acetic acid/
cyclohexene molar ratio=3.0
1
/
R) were obtained. The activation energy (E ) and pre-
BLA-HPA-ILs / Cu[Bis–Bs–BDMAEE]HPW O in the
2 12 40
a
exponential factor (k ) were listed in Table 2. The kinetic
esteriꢁcation reaction of cyclohexene and acetic acid were
0
data obtained has promising prospects for designing the
esterification process of cyclohexene with acetic acid
and also providing an optimal setup of various operating
parameters.
studied in detail. These reactions are liquid–solid biphasic
1
heterogeneous reactions by using / Cu[Bis–Bs–BDMAEE]
2
HPW O as catalysts, so the solid catalyst could be
1
2
40
easily recycled by simple filtration. After washing for
three times using acetone and drying at 80 °C for 12 h
under a vacuum, the catalyst can be used to the next run
directly. As shown in Fig. 13, after reusing the catalysts of
1
/
Cu[Bis–Bs–BDMAEE]HPW O six times, the selec-
12 40
2
tivity of cyclohexyl acetate is almost unchanged, and the
conversion of cyclohexene has a slight decrease, which is
may be due to the loss of the catalysts in the treatment pro-
cess. As shown in Fig. 14, the FT-IR spectra of the fresh
BLA-HPA-ILs and the repeatedly used six times catalyst.
It shows that the peaks of both catalysts are similar, which
indicates that the structure of the BLA-HPA-ILs is extremely
stable after reuse for six cycles. Therefore, the BLA-HPA-
ILs acidic active sites are not easily lost and its reusability
is excellent.
4
Conclusions
Fig. 11 Comparison between measured and calculated at diꢀerent
1
initial reactant molar ratio using / Cu[Bis–Bs–BDMAEE]HPW O
2
12 40
A series of BLA-HPA-ILs were synthesized by combin-
ing the Brønsted acidic ionic liquid [Bis–Bs–BDMAEE]
HPW O with metallic oxide in diꢀerent composition
as catalyst. Reaction conditions: temperature=363.15 K and catalyst
dosage=7 wt.%
1
2
40
ratios and applied in the esterification of cyclohexene
with acetic acid to produce cyclohexyl acetate. Compared
with BLA-HPA-ILs catalysts with doping other metallic
3.8 Reusability of Catalyst
The stability and reusability of catalysts are important
factors for the application in industry. The reusability of
oxide, HPA-ILs, H SO , H PW O , H PMo O and
2
4
3
12 40
3
12 40
1
3

Kinetics Study of the Esteriꢀcation Reaction of Cyclohexene to Cyclohexyl Acetate Catalyzed…
Fig. 12 Arrhenius plots for the esteriꢁcation of cyclohexene with ace-
1
tic acid using / Cu[Bis–Bs–BDMAEE]HPW O as catalysts
2
12 40
1
Fig. 14 FT-IR spectra of / Cu[Bis–Bs–BDMAEE]HPW O : (a) the
2
12 40
fresh catalyst and (b) reused six times catalyst
Table 2 Parameters of the PH model
k (L mol min−1
2
−2
Ea (kJ mol−1)
applied to correlate the experimental data and the kinetic
parameters were estimated. The calculated results are in
good agreement with the experimental results, indicating
the PH model gives a good representation of the kinetic
Reaction
)
0
5
Forward reaction
Reverse reaction
6.68×10
1.52×10
56.21
92.34
1
1
behavior for the esteriꢁcation of cyclohexene. In a word,
1
/
Cu[Bis–Bs–BDMAEE]HPW O catalyst possess the
12 40
2
excellent stability and reusability, showing promising poten-
tial for application in industry.
Acknowledgements This work was supported by the National Natu-
ral Science Foundations of China (No. 21676072), Henan Science
and Technology Research Project (212102210653, 202102310285),
China Postdoctoral Science Foundation (2020M672209), and Sci-
entiꢁc Research Projects for Higher Education of Henan Province
(
20A530002), the Program for Innovation Teams in Science and Tech-
nology in Universities of Henan Province (20IRTSTHN004).
Declarations
Conflict of interest The authors declare that they have no known com-
peting fnancial interests or personal relationships that could have ap-
peared to inꢃuence the work reported in this paper.
Fig. 13 Conversion rate (red column) and selectivity (green column)
References
1
of / Cu[Bis–Bs–BDMAEE]HPW O . Reaction conditions: temper-
2
12 40
ature=363.15 K, catalyst dosage=7 wt.%, acetic acid/cyclohexene
1. Xiang YZ, Li XN (2007) J Chem Ind Eng 58:3041–3045
2. Ishida H (1997) Catal Surv 1:241–246
molar ratio=3.0, and reaction time=7.0 h
3
. Fang CX, Yu Y, Wang YT, Qu YX (2012) Mod Chem Ind
3
2:16–19
1
H SiW O , / Cu[Bis–Bs–BDMAEE]HPW O were
4. Wu JM, Dai XM, Chen JL, Guo WD (2003) Chem Ind Eng Prog
22:1222–1224
4
12 40
2
12 40
found to have the most excellent catalytic activity and the
conversion of cyclohexene and cyclohexyl acetate selectiv-
ity reached 65.92% and 98.61%, respectively. In addition,
the pseudohomogeneous (PH) reaction kinetic model was
5
.
Fang DR, Lu JY, Zhang HM, Li J, Wang YY (2013) Chem Res
Chin Univ 29:743–746
6
.
Jin JJ, Li F, Yang LH, Zhang DS, Xue W, Wang YJ (2004) Acta
Petrol Sin (Pet Process Section) 30:169–174
1
3

B. Guang et al.
7
8
9
.
.
.
Guo ZW, Jin HB, Tong ZM (2006) Chem Ind Eng Prog
33. Zhou Y, Chen GJ, Long ZY, Wang J (2014) RSC Adv
4:42092–42113
2
5:852–859
Wang BY, Ge XX, Wu FL, Wu YX, Zheng HD, Qiu T (2010)
Chem Ind Eng Prog 29:861–865
34. Liu YY, Murata K, Inaba M (2006) J Mol Catal A 256:247–255
35. Leng Y, Wang J, Zhu DR, Shen L, Zhao P, Zhang M (2011) Chem
Eng J 173:620–626
Zhang H, Mahajani SM, Sharma MM, Sridhar T (2002) Chem
Eng Sci 57:315–322
36. Leng Y, Wang J, Zhu DR, Ren XQ, Ge HQ, Shen L (2009) Angew
Chem Int Ed 48:168–171
1
1
1
1
1
1
1
0. Katariya A, Freund H, Sundmacher K (2009) Ind Eng Chem Res
4
8(21):9534–9545
1. Xue W, Zhao HP, Yao J, Li F, Wang YJ (2016) Chin J Catal
7(5):769–777
2. Cao ZJ, Zhao X, He FQ, Zhou Y, Huang K, Zheng AM, Tao DJ
2018) Ind Eng Chem Res 57(19):6654–6663
3. Steyer F, Freund H, Sundmacher K (2008) Ind Eng Chem Res
37. Li KX, Chen L, Wang HL, Lin WB, Yan ZC (2011) Appl Catal A
392:233–237
3
38. Zhao PP, Zhang MJ, Wu YJ, Wang J (2012) Ind Eng Chem Res
51:6641–6647
(
39. Leng Y, Zhao PP, Zhang MJ, Wang J (2012) J Mol Catal A
358:67–72
4
7:9581–9587
40. Zhao PP, Leng Y, Wang J (2012) Chem Eng J 204:72–78
41. Huang WL, Zhu WH, Li HM, Shi H, Zhu GP, Liu H, Chen GY
(2010) Ind Eng Chem Res 49:8998–9003
4. Sang HL, Won YC, Kyung JK, Dae JC, Jae WL (2018) Chem Eng
Process 123:249–257
5. Rakesh K, Amit K, Hannsjorg F, Kai S (2011) Org Process Res
Dev 15:527–539
42. Liu Y, Wang TY, Zhai CP, Chen WP, Qiao CZ (2014) Ind Eng
Chem Res 53:14633–14640
6. Yao B, Wang Z, Xiao T, Cao F, Edwards PP (2015) Appl Petro-
chem Res 5:135–142
43. Liu Y, Liu WH, Shao XN, Wang JH, Li XY (2018) Catal Lett
148:144–153
1
1
1
2
7. Zheng GC, Li XZ (2019) Synth Commun 49(7):1–9
8. Jiang HR, Lu B, Ma LJ, Yuan X (2020) Catal Lett 150:1786–1797
9. Ma L, Xu L, Jiang H, Yuan X (2019) RSC Adv 9:5692–5700
0. Lu B, Wu ZW, Ma LJ, Yuan X (2018) J Taiwan Inst Chem Eng
44. Liu Y, Liu WH, Wang L, Su MJ, Liu FJ (2018) Ind Eng Chem Res
57:5207–5214
45. Matuszek K, Chrobok A, Coleman F, Seddonb KR, Kwaśny MS
(2014) Green Chem 16:3463–3471
8
8:1–7
46. Wang HX, Wu CM, Bu XW, Tang WL, Li L, Qiu T (2014) Chem
Eng J 246:366–372
2
2
2
2
2
2
2
2
2
3
3
3
1. Zhang QH, Zhang SG, Deng YQ (2011) Green Chem
1
3:2619–2637
47. Yang YB, He WS, Jia CS, Ma Y, Zhang XM, Feng B (2012) J Mol
Catal A 357:39–43
2. Chen FF, Huang K, Zhou Y, Tian ZQ, Zhu X, Tao DJ, Jiang DE,
Dai S (2016) Angew Chem Int Ed 55:7166–7170
3. Song ZB, Huang W, Zhou Y, Tian ZQ, Li ZM, Tao DJ (2020)
Green Chem 22:103–109
48. Han XX, Yan W, Hung CT, Liu LL, Wu PH, Ren DH, Huang SJ,
Liu SB (2016) Korean J Chem Eng 33:2063–2072
49. Liu SW, Chen CG, Yu FL, Li L, Liu ZG, Yu ST, Xie CX, Liu FS
(2015) Fuel 159:803–809
4. Hui W, Zhou Y, Dong Y, Cao ZJ, He FQ, Cai MZ, Tao DJ (2019)
Green Energy Environ 4:49–55
50. Yuan B, Xie CX, Yu FL, Yang XY, Yu ST, Zhang JL, Chen XB
(2016) Springer Plus 5:460–464
5. Yang F, Xue W, Zhang DS, Li F, Wang YJ (2016) React Kinet
Mech Catal 117:329–339
51. Liu Y, Wu YF, Su MJ, Liu WH, Li XY, Liu FJ (2020) J Ind Eng
Chem 92:200–209
6. Tao DJ, Dong Y, Cao ZJ, Chen FF, Chen XS, Huang K (2016) J
Ind Eng Chem 41:122–129
52. Tao DJ, Li ZM, Cheng Z, Hu N, Chen XS (2012) Ind Eng Chem
Res 51:16263–16269
7. Yang Z, Cui XB, Jie HM, Yu XF, Zhang Y, Feng TY, Liu H, Song
K (2015) Ind Eng Chem Res 54:1204–1251
53. JagadeeshBabu PE, Sandesh K, Saidutta MB (2011) Ind Eng
Chem Res 50:7155–7160
8. Ou YF, Wang ZZ, Zhou Y, Chen Z, Lu ZH, Yang Z, Tao DJ (2015)
Appl Catal A 492:177–183
54. Jignesh G, Surendra M, Sanjay M (2003) Ind Eng Chem Res
42:2146–2155
9. An XC, Li ZM, Zhou Y, Zhu WS, Tao DJ (2020) Chem Eng J
3
94:124859
55. Bastian S, Michael D, Julrgen G (2008) Ind Eng Chem Res
47:698–703
0. Cai XJ, Cui SH, Qu LP, Yuan DD, Lu B, Cai QH (2007) Catal
Commun 9:6
56. Tsai YT, Lee MJ (2011) Ind Eng Chem Res 50:1171–1176
1. Haꢁzi A, Ahmadpour A, Koolivand-Salooki M, Heravi MM,
Bamoharram FF (2013) J Ind Eng Chem 19:1981–1989
2. Sawant DP, Vinu A, Justus J, Srinivasu P, Halligudi SB (2007) J
Mol Catal A 276:150–157
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional aꢂliations.
Authors and Aꢀliations
1
1
1
1
1
1
Binxiong Guang · Yuefeng Wu · Weihua Liu · Jianhong Wang · Yahui Xiao · Yong Liu
1
Henan Key Laboratory of Polyoxometalate Chemistry,
College of Chemistry and Chemical Engineering, Henan
University, Kaifeng 475004, People’s Republic of China
1
3