Acidic ionic liquid based UiO-67 type MOFs: A stable and efficient heterogeneous catalyst for esterification

Article abstract of DOI:10.1039/c8ra01119b

A facile strategy for the synthesis of acidic ionic liquid based UiO-67 type MOFs was developed in this study. Br?nsted acids (H₂SO₄, CF₃SO₃H and hifpOSO₃H (hexafluoroisopropyl sulfuric acid)) were introduced into UiO-67-bpy (bpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) frameworks by reacting with bipyridyl nitrogen to introduce the properties of an acidic ionic liquid into the frameworks. The prepared catalysts, denoted as UiO-67-HSO₄, UiO-67-CF₃SO₃ and UiO-67-hifpOSO₃, were characterized by XRD, SEM, FT-IR, EA, TGA and N₂ adsorption-desorption studies. The relatively high surface area was still maintained and acidic active groups were uniformly dispersed in the frameworks. The catalytic performance of UiO-67-HSO₄, UiO-67-CF₃SO₃ and UiO-67-hifpOSO₃ was evaluated by the esterification of acetic acid with isooctyl alcohol. The prepared catalysts showed good catalytic activities in the esterification, of which UiO-67-CF₃SO₃ gave the maximum isooctyl alcohol conversion of 98.6% under optimized conditions. The catalyst could be reused five times without a significant decrease in the conversion of isooctyl alcohol, and almost no active species were leached, indicating the excellent stability and reusability of the catalyst. Our study provides one effective way to synthesize heterogeneous acidic ionic liquid catalysts consisting of isolated, well defined acidic groups that will probably attract interest in acid catalyst chemistry.

Full text of DOI:10.1039/c8ra01119b

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Acidic ionic liquid based UiO-67 type MOFs:
a stable and efficient heterogeneous catalyst for
esterification†
Cite this: RSC Adv., 2018, 8, 10009
ab
a
*
Zichen Xu,
Yanqiang Zhang
Guoying Zhao, Latif Ullah,a Meng Wang,a Aoyun Wang,a
ac
a
*
and Suojiang Zhang
A facile strategy for the synthesis of acidic ionic liquid based UiO-67 type MOFs was developed in this study.
Brønsted acids (H2SO4, CF3SO3H and hifpOSO3H (hexafluoroisopropyl sulfuric acid)) were introduced into
UiO-67–bpy (bpy ¼ 2,20-bipyridine-5,50-dicarboxylic acid) frameworks by reacting with bipyridyl nitrogen
to introduce the properties of an acidic ionic liquid into the frameworks. The prepared catalysts, denoted
as UiO-67–HSO4, UiO-67–CF3SO3 and UiO-67–hifpOSO3, were characterized by XRD, SEM, FT-IR, EA,
TGA and N2 adsorption–desorption studies. The relatively high surface area was still maintained
and acidic active groups were uniformly dispersed in the frameworks. The catalytic performance of
UiO-67–HSO4, UiO-67–CF3SO3 and UiO-67–hifpOSO3 was evaluated by the esterification of acetic acid
with isooctyl alcohol. The prepared catalysts showed good catalytic activities in the esterification, of
which UiO-67–CF3SO3 gave the maximum isooctyl alcohol conversion of 98.6% under optimized
conditions. The catalyst could be reused five times without a significant decrease in the conversion of
isooctyl alcohol, and almost no active species were leached, indicating the excellent stability and
reusability of the catalyst. Our study provides one effective way to synthesize heterogeneous acidic ionic
liquid catalysts consisting of isolated, well defined acidic groups that will probably attract interest in acid
catalyst chemistry.
Received 5th February 2018
Accepted 3rd March 2018
DOI: 10.1039/c8ra01119b
rsc.li/rsc-advances
Metal–organic frameworks (MOFs) as a novel group of hybrid
materials which have generated considerable research interest
and have been applied widely in the areas of catalysis,13
1. Introduction
In the recent past, ionic liquids (ILs) have attracted much
attention due to their unique properties such as eco-friendly
nature, negligible vapour pressure, high thermal stability and
low melting points.1–4 Specially, acidic ILs have shown efficient
catalytic activities in various kinds of reactions including
esterication,5 alkylation,6 dehydration7 and Diels–Alder reac-
tions.8 However, their actual applications are greatly soured by
shortcomings such as high viscosity and being difficult to
recycle.9 In order to overcome these drawbacks, attempts have
been made to introduce ILs onto various supports, such as
mesoporous silica,10 activated carbon11 and zeolites,12 which
will supress the cost of ILs and improve the reusability of ILs.
adsorption,14 drug delivery,15 sensor,16 and catalyst carriers.17
Compared to conventional porous materials, MOFs have unique
advantages including large surface area, high porosity, tunable
topologies and low structure density which make MOFs as ideal
catalyst carriers.18–21 Moreover, MOFs are highly designable,
tuneable host–guest interactions can be created between MOFs
and guest molecules. During the past decade, the introduction
of ILs into MOFs has been reported, which are usually carried
out by post-synthetic modication (PSM) strategy. Up to now,
several approaches have been reported for the PSM of MOFs
with ILs such as impregnating MOFs into ILs solutions22 and
adopting the ship-in-bottle method for the introduction of ILs.23
By adopting PSM, ILs will be effectively introduced into MOFs
and the basic frameworks remain unchanged,24 and the
resulting materials had the properties of both ILs and MOFs
which greatly extends the applications.25 Therefore, using PSM
strategy in the areas of MOFs and ILs has become a promising
research eld.
aCAS Key Laboratory of Green Process and Engineering, State Key Laboratory of
Multiphase Complex System, Beijing Key Laboratory of Ionic Liquids Clean Process,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190,
China. E-mail: sjzhang@ipe.ac.cn; hfdong@ipe.ac.cn; Fax: +86-010-82544875; Tel:
+86-010-82544875
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cZhengzhou Institute of Emerging Industrial Technology, Zhengzhou, Henan 450003,
China
Since its discovery, zirconium-based MOFs have drawn
much attention due to its high thermo-chemical stability and
acids resistive properties.26–28 The robust nature of Zr-MOFs
endows them as excellent platforms for the applications in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra01119b
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dichloromethane (CH2Cl2) and sulfuric acid (H2SO4) were ob-
tained from Beijing Chemical Works. De-ionized water (99.5%)
was obtained from home-built distillation assembly. All chemicals
were used without further purication.
2.2 Characterization techniques
NMR data (1H, 13C and 19F) were collected on the AVANCE III
HD-600 MHz NMR spectrometer. Powder X-ray diffraction
(PXRD) patterns were performed on the Rigaku smartlab X-ray
diffractometer equipped with a Cu-Ka1 radiation source (l ¼
0.15406 nm) and the XRD patterns were collected in the steps of
0.02ꢀ between the 2q range of 5–50ꢀ. The simulated powder
pattern was carried out based on crystal structure data of UiO-
67–bpy as reported.34 The morphologies and structures were
investigated by the Hitachi SU8020 scanning electron micros-
copy (SEM) equipped with Energy Dispersive Spectrometry
(EDS) at an accelerating voltage of 5 kV and 20 kV, respectively.
All the samples were mounted on a carbon tape and coated with
gold prior to measurement. Fourier transformed infrared (FT-
IR) was conducted on the Nicolet 380 spectroscopy of Thermo
Electron Corporation, using the KBr pellet methodology, in the
range of 4000–400 cmꢁ1 with a resolution of 4 cmꢁ1. Element
Analysis (EA) was performed with the Elementar Vario Macro
Cube to determine the N and S content of the prepared cata-
lysts. The N2 adsorption–desorption isotherms were collected
using the Micromeritics ASAP 2020 HD88 surface area and
porosity analyzer at 77 K. Prior to the measurements, the
samples were activated under vacuum at 120 ꢀC for 12 h.
Thermogravimetric analysis (TGA) was performed on the Shi-
madzu TA-60WS Thermal analyzer in the range of 40–700 ꢀC at
a heating rate of 5 ꢀC minꢁ1 under air ow. Inductively coupled
plasma-optical emission spectrometry (ICP-OES) experiments
were conducted on the Shimadzu Plasma Atomic Emission
Spectrometer. Gas Chromatography Mass Spectrometry (GC-
MS) analysis was carried out on the Shimadzu GCMS-QP2020
equipped with a HP-5 capillary column (30 m ꢂ 0.25 mm,
using N2 as carrier gas) and GC analysis were performed on the
Shimadzu GC-2014 equipped with a HP-5 capillary column (30
m ꢂ 0.25 mm, using N2 as carrier gas).
Fig. 1 (a) Linker of bpydc and Zr6 cluster (gray: C; red: O; blue: N; dark
pink: Zr), (b) the octahedral cage, (c) the tetrahedral cage, (d) unit cell
of UiO-67–bpy. Hydrogen atoms are omitted for clarity.
various elds.29 Among the reported Zr-based MOFs, UiO-67–
bpy (bpy ¼ 2,20-bipyridine-5,50-dicarboxylic acid) has been
studied widely due to the rich coordination chemistry of the
2,20-bipyridine moieties. The structure of UiO-67–bpy is octa-
hedral and it crystalizes in cubic space group Fm3 with the fcu
topology as shown in Fig. 1. Studies highlight the incorporation
of noble metals/complexes or organometallic species on/in UiO-
67–bpy by coordinating with the N atoms of the 2,20-bipyridine
moieties, and the resulting materials exhibit outstanding
performance in catalysis and ratiometric thermometer.30–32
However, to the best of our knowledge, only Lei Xu et al. has
reported the introduction of ILs into UiO-67–bpy by N-
quaternization of bipyridine sites.29 In the delight of the idea
of Kavitha who prepared 2,2-bipyridinium hydrogensulfate by
treating 2,20-bipyridine with sulfuric acid,33 it is feasible to
introduce the properties of acidic ILs unto UiO-67–bpy by
reacting bipyridyl groups with Brønsted acids. Due to the acidic
nature, the resulting materials could be applied as heteroge-
neous catalysts in various chemical reactions.
Herein, we report a facile PSM strategy to synthesize acidic
ionic liquid based UiO-67 type MOF catalysts, by treating the
bipyridyl groups of UiO-67–bpy with different Brønsted acids
(H2SO4, CF3SO3H and hifpOSO3H (hexauoroisopropyl sulfuric
acid)). The newly prepared materials were characterized by
XRD, SEM, FT-IR, EA, TGA and N2 adsorption–desorption
studies. The catalytic performance of synthesized catalysts was
then evaluated by esterication of isooctyl alcohol with acetic
acid. The reaction conditions were further optimized and the
reusability of the catalysts was also carried out.
2.3 Preparation of UiO-67–bpy
The synthesis method of UiO-67–bpy was derived from Andreas
Schaate35 and Sigurd Øien36 with slight modication. Typically,
ZrCl4 (2.58 mmol, 602 mg) and DMF (100 mL, 500 eq. based on
Zr salt) were placed in a 100 mL Teon-capped glass ask.
2. Experimental
2.1 Materials
N,N-Dimethylformamide (DMF, 99.5%), isooctyl alcohol ($99%) Benzoic acid (9.46 g, 30 eq. based on Zr salt) was also added to
and anhydrous methanol were obtained from Xilong Chemical the ask which acted as a modulator. The precursors were
Industry Incorporated Co., Ltd. Benzoic acid was purchased from sonicated for 30 minutes for dissolution. Then bpydc
Sinopharm Chemical Reagents Co., Ltd. 2,20-Bipyridine-5,50- (2.58 mmol, 626 mg) and H2O (3.4 mmol, 62 mL) was added to
dicarboxylicacid (bpydc, 98%) was provided by Zhengzhou Alpha the solution, dispersed by ultrasound until no suspended
Chemical Co., Ltd. Zirconium tetrachloride (ZrCl4, 98%), hexa- particles were visible. The tightly capped ask was kept in a pre-
ꢀ
uoroisopropanol (99.5%) and triuoromethanesulfonic acid heated oven at 95 C for 4 days. Aer the ask cooled down to
(CF3SO3H, 98%) was supplied by Aladdin Chemical Co., Ltd. room temperature naturally, the white precipitate was collected
Chlorosulfonic acid (ClHSO3, 97%) was purchased from Beijing by ltration and washed 3 times in anhydrous DMF (15 mL),
Ouhe Technology Co., Ltd. Acetic acid (99.5%), anhydrous followed by washed 3 times in anhydrous methanol (15 mL).
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Finally, theꢀexchanged sample was dried under reduced pres- with anhydrous CH2Cl2 (15 mL). Finally, the powders were dried
ꢀ
sure at 120 C overnight.
under dynamic vacuum at 120 C for 12 h.
2.4 Preparation of hexauoroisopropyl sulfuric acid
(hifpOSO3H)
2.6 Catalytic reaction
The catalytic activity of the prepared catalysts was evaluated by
the esterication of acetic acid with isooctyl alcohol. The reac-
tion was carried out in the round bottom ask equipped with
a magnetic stirrer and connected to a reux condenser. In
a typical reaction, a mixture of isooctyl alcohol and acetic acid
with molar ratio 1 : 6 were charged into the ask containing
0.2 g the prepared catalysts. Then the reaction mixture was
heated to 80 ꢀC with continuously stirring for 18 h. Aer the
reaction, the reaction mixture was withdrawn, the catalyst was
separated by centrifugation and the liquid phase was analyzed
by GC-MS to dene the structure of products, and GC was
applied to analyse the conversion of isooctyl alcohol and the
yield of isooctyl acetate.
The preparation method of hifpOSO3H was following a modied
procedure reported by Witali Beichel.37 Typically, ClHSO3
(0.1 mol, 11.652 g) was placed in a three-necked ask at ꢁ78 ꢀC to
precooling. Following by slowly adding hexauoroisopropanol
(0.1 mol, 16.804 g) to the ask using a constant pressure drop-
ping funnel and then stirring at 140 ꢀC under reux until the gas
evolution had ceased. Finally, the product was distilled at
reduced pressure at 140 ꢀC gave the pure acid as a colorless
liquid. The structure was conrmed by NMR analysis and NMR
data was available in ESI.†
2.5 Preparation of acidic ionic liquid based UiO-67 type MOF
The simplied preparation processes of acidic ionic liquid
based UiO-67 type MOF (denoted as UiO-67–HSO4, UiO-67–
CF3SO3 and UiO-67–hifpOSO3) were shown in Scheme 1. Before
preparation, the as-synthesized UiO-67–bpy powders were
3. Results and discussion
3.1 Catalysts characterization
ꢀ
evacuated at 120 C for 6 h to active powders. For the prepara-
The powder XRD patterns of the simulated UiOy-67–bpy, the
pristine UiO-67–bpy, and the synthesized catalysts that treated
with different Brønsted acids were shown in Fig. 2. The
diffraction peaks for the pristine UiO-67–bpy agreed with the
simulated one which based on the single crystal data of UiO-67–
bpy as reported.34 All the main diffraction peak positions of UiO-
67–HSO4, UiO-67–CF3SO3 and UiO-67–hifpOSO3 were matched
well with the simulated pattern and the pristine one, apart from
some slight variations in the Bragg intensities. Furthermore,
from the inserted picture in Fig. 1, the three sharp peaks at
5.76ꢀ (1 1 1), 6.66ꢀ (2 0 0) and 9.46ꢀ (2 2 0) for the synthesized
catalysts indicated that they were highly crystalline. These
results indicated that the integrity of framework was preserved
tion of UiO-67–HSO4, the activated powders (0.2 g) were
immersed in H2SO4 (200 mL, 0.01 M) using anhydrous meth-
anol as solvent for 24 h, during which time the mixture was
stirred one minute every two hours. Aer treatment, the solu-
tion was decanted off and the remained powders were collected
by ltration and washed several times with anhydrous meth-
anol (15 mL). Finally, the powders were dried under dynamic
ꢀ
vacuum at 120 C for 12 h.
The preparation of UiO-67–CF3SO3 and UiO-67–hifpOSO3
were similar to the preparation of UiO-67–HSO4, except that
CF3SO3H (2 mmol, 0.3002 g) and hpOSO3H (2 mmol, 0.4962 g)
were added dropwise to anhydrous CH2Cl2 (200 mL) in an ice
bath, respectively. Then activated powders (0.2 g) were
immersed in the resulting solution for 24 h, during which time
the mixture was stirred one minute every two hours. Aer
treatment, the solution was decanted and the remaining
powders were collected by ltration and washed several times
Scheme 1 Scheme for the post-synthesis of acidic ionic liquid based Fig. 2 Powder XRD patterns of (a) the simulated UiO-67–bpy, (b) the
UiO-67 type MOF and the structures of the Brønsted acid anions. The pristine UiO-67–bpy, (c) UiO-67–HSO4, (d) UiO-67–CF3SO3, (e) UiO-
topology is shown in a simplified form as an octahedral cage.
67–hifpOSO3.
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for the prepared catalysts through the PSM processes under
acidic conditions.
To investigate the surface morphologies and crystalloid
structures of the synthesized catalysts, scanning electron
microscopy (SEM) was carried out and the results were given in
Fig. 3. The pristine UiO-67–bpy crystals had well dened octa-
hedral microcrystals with a smooth surface, and the average
crystals size was around 1.3 mm (Fig. S1(a)†). The well-dened
octahedral morphology was still maintained for the synthe-
sized catalysts and the average particles size almost remain the
same, about 1.2 mm (Fig. S1(b), (c), (d)†), aer treated by
Brønsted acids solutions of H2SO4, CF3SO3H and hifpOSO3H,
respectively. However, the surface of UiO-67–HSO4, UiO-67–
CF3SO3 and UiO-67–hifpOSO3 tended to be rough. Meanwhile,
it could be also observed that a small number of UiO-67–bpy
crystallites were disrupted or slightly changed to irregular ones
which could be attributed to the collapse of UiO-67–bpy
frameworks.
Fig. 4 FT-IR spectra for (a) the pristine UiO-67–bpy, (b) UiO-67–
HSO4, (c) UiO-67–CF3SO3, (d) UiO-67–hifpOSO3.
FT-IR studies were used to conrm whether the acidic
characteristic groups had been successfully introduced into the
frameworks. The enlarged partial FT-IR spectra of the pristine
UiO-67–bpy and the prepared catalysts were shown in Fig. 4. For
all samples, the observed vibration bands between 1300–
1750 cmꢁ1, which corresponding to carboxylate linkers38 of the
pristine UiO-67–bpy were well maintained in the all resulting
materials. For the UiO-67–HSO4, two peaks were observed at
1278 cmꢁ1 and 1180 cmꢁ1 corresponding the O]S]O
symmetric and asymmetric stretching vibrations, respectively.
In the case of UiO-67–CF3SO3, two bands at 1224 cmꢁ1 and
1164 cmꢁ1 could be ascribed to the O]S]O symmetric and
asymmetric stretching vibrations, respectively, and the peak at
1029 cmꢁ1 corresponding to the C–F stretching. Furthermore,
the bands at 638 cmꢁ1, 568 cmꢁ1, and 516 cmꢁ1 could be
attributed to the C–S stretching. The peaks at 1282 cmꢁ1 and
1166 cmꢁ1 observed over UiO-67–hifpOSO3 catalyst were the
O]S]O symmetric and asymmetric stretching vibrations,
respectively, and the peak at 1089 cmꢁ1 corresponding to the
C–F stretching. These results indicated that the acidic charac-
teristic groups of the synthesized catalysts were introduced into
the UiO-67–bpy matrix successfully.
To conrmed the N and S content in UiO-67–HSO4, UiO-67–
CF3SO3 and UiO-67–hifpOSO3, element analysis (EA) was
carried out and the results were summarized in Table 1. It could
be observed that, for the prepared catalysts, the molar ratio of N
to S was all around 2 : 1, which indicated that each linker of
UiO-67–bpy was reacted with one acids molecule. Furthermore,
as indicated by EDS elemental mapping (Fig. S2†) the acidic
characteristic groups for the synthesized catalysts were
uniformly dispersed in the frameworks.
N2 adsorption–desorption isotherms of the pristine UiO-67–
bpy UiO-67–HSO4, UiO-67–CF3SO3 and UiO-67–hifpOSO3 were
shown in Fig. 5 and the pore size distributions for the all
samples were depicted in Fig. S3.† Table 2 summarized the
textural properties of the all samples including BET surface
area, pore volume and average pore size. The pristine UiO-67–
bpy exhibited type-I isotherm, which revealed its microporous
nature. The pristine UiO-67–bpy showed a BET surface area and
pore volume of 1802.5 m2 gꢁ1 and 0.6926 cm3 gꢁ1, which were
comparable with the reported results.39 The isotherms for UiO-
67–HSO4, UiO-67–CF3SO3 and UiO-67–hifpOSO3 also exhibited
type-I isotherms which was consistent with the pristine one and
showed no sign of bulging indicating the robust nature of UiO-
67–bpy and the pores were not damaged during the preparation
conditions. Compared with the pristine UiO-67–bpy, both
Table 1 N and S content for the synthesized catalysts
Entry
Sample
N (wt%)
S (wt%)
N : Sa
1
2
3
UiO-67–HSO4
UiO-67–CF3SO3
UiO-67–hifpOSO3
6.50
5.57
4.65
7.28
6.65
5.08
2.04 : 1
1.91 : 1
2.09 : 1
Fig. 3 SEM images of (a) the pristine UiO-67–bpy, (b) UiO-67–HSO4,
(c) UiO-67–CF3SO3, (d) UiO-67–hifpOSO3.
a
Molar ratio.
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Fig. 6 TGA curves of (a) the pristine UiO-67–bpy, (b) UiO-67–HSO4,
(c) UiO-67–CF3SO3, (d) UiO-67–hifpOSO3.
Fig. 5 N2 adsorption–desorption isotherms for (a) the pristine UiO-
67–bpy, (b) UiO-67–HSO4, (c) UiO-67–CF3SO3, (d) UiO-67–
hifpOSO3.
degradation step of the synthesized catalysts were occurred
ꢀ
before 200 C which was comparable to the pristine one. The
rapid weight loss in the temperature range of 300–380 ꢀC for
UiO-67–HSO4, UiO-67–CF3SO3 and UiO-67–hifpOSO3 may be
Table 2 The textural properties of the pristine UiO-67–bpy and the
synthesized catalysts
ꢁ
ꢁ
attributed to decomposition of HSO4
,
CF3SO3
and
BET surface Pore volumea Average pore
hifpOSO3ꢁ, respectively. When the temperature increased
around 400 ꢀC, the frameworks of UiO-67–HSO4, UiO-67–
CF3SO3 and UiO-67–hifpOSO3 tended to degrade and collapse.
The results indicated the decomposition temperature for the
synthesized catalysts were slightly lower than the pristine one.
But the thermostability of the prepared catalysts was much
better than commercial catalyst Amberlyst-15 of which usually
used in esterication reactions with the thermostability of
120 ꢀC.40 Thus the synthesized UiO-67–HSO4, UiO-67–CF3SO3
and UiO-67–hifpOSO3 catalysts can be employed as heteroge-
neous catalysts for various organic reactions.
Entry Sample
area (m2 gꢁ1) (cm3 gꢁ1
)
size (nm)
1
2
3
4
Pristine UiO-67–bpy 1802.5
0.6926
1.4257
1.1795
1.0701
1.0201
UiO-67–HSO4
1232.9
1124.4
1014.7
0.4610
0.4254
0.3880
UiO-67–CF3SO3
UiO-67–hifpOSO3
a
Calculated by using Horvath–Kawazoe (HK) method.
surface area and pore volume for all of the synthesized catalysts
had been decreased from 1802.5 m2 gꢁ1 to 1014.7 m2 gꢁ1 and
from 0.6926 cm3 gꢁ1 to 0.3880 cm3 gꢁ1, respectively. The
decrease in BET surface area and pore volume could be attrib-
3.2 Catalytic assessment
uted to the introduction of acidic characteristic groups into the Esterication is one of the fundamental reactions in the eld of
cages of UiO-67–bpy and might partially occupied the channels organic synthesis. The product organic esters are considered as
of the frameworks. Furthermore, the optimized geometric high production-volume (HPV) chemicals. They are important
structures of acetic acid, isooctyl alcohol and isooctyl acetate intermediates in pharmaceutical, cosmetic and polymer
were shown in Fig. S4.† Comparing their sizes with the prepared industries for the production of fragrances, polymers, poly-
catalysts, it could be concluded that the open cavities for the esters, paints and plasticizers.41–43 Thus, we chose this reaction
prepared catalysts could still benet the diffusion of the reac- to probe the catalytic performance of prepared catalysts.
tants and products.
3.2.1 Catalytic activities of the prepared catalysts. The
Since the synthesized catalysts were achieved by treating catalytic performance of the synthesized catalysts was evaluated
UiO-67–bpy powders with strong Brønsted acids, it was neces- by choosing esterication of acetic acid and isooctyl alcohol as
sary to substantiate the thermal and structural stability. Ther- the probe reaction as shown in Scheme 2. The results under
mogravimetric analysis (TGA) was conducted and TGA curves different reaction time by using the prepared catalysts were
were depicted in Fig. 6. The two-stepped thermal degradation shown in Fig. 7. The conversion of isooctyl alcohol was
was observed for the pristine UiO-67–bpy which was consistent increased with increasing the reaction time and became steady
with the reported result.35 The initial weight loss before 150 ꢀC aer 18 h. Without catalyst, the conversion of isooctyl alcohol
was correspond to the vaporization of absorbed water and was only 66.4% and the yield of isooctyl acetate was 56.7%. With
solvent inner/outer the surfaces of the framework. The second the pristine UiO-67–bpy, the conversion of isooctyl alcohol was
ꢀ
degradation step was emerged about 450 C which was attrib- 77.5% aer 21 h with 70.5% yield of isooctyl acetate due to the
uted to the decomposition of the frameworks. The rst unsaturated site at Zr ion clusters with could provide Lewis
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Scheme 2 Esterification of acetic acid with isooctyl alcohol using the
prepared catalysts.
Fig. 8 Catalyst amount effect upon the catalytic performance of UiO-
67–CF3SO3. Reaction conditions: isooctyl alcohol to acetic acid molar
ratio 1 : 6, for 18 h, at 80 ꢀC.
Fig. 7 The catalytic activities of prepared catalysts (a) blank, (b) the
pristine UiO-67–bpy, (c) UiO-67–HSO4, (d) UiO-67–CF3SO3, (e) UiO-
67–hifpOSO3. Reaction conditions: catalyst amount 0.2 g, isooctyl
alcohol to acetic acid molar ratio 1 : 6, at 80 ꢀC.
acidity.48 It was upliing that the synthesized catalysts showed
increased catalytic performance. By employing these three
prepared catalysts, the conversion of isooctyl alcohol was
over 85.0% because of the strong acidity provided by the
introduction of the acidic active sites that promote the esteri-
cation effectively. Obviously, the catalytic performance of
UiO-67–CF3SO3 was the best amongst all the catalysts employed
and gave 98.0% isooctyl alcohol conversion which may attribute
to the stronger acidity as compared to UiO-67–HSO4 and
UiO-67–hifpOSO3. Thus, the reaction conditions were further
optimized by employing the UiO-67–CF3SO3 catalyst.
3.2.2 Optimization of reaction variables. The effect of
catalyst amount on the conversion of isooctyl alcohol was
investigated at 18 h, 80 ꢀC and molar ratio of isooctyl alcohol to
acetic acid 1 : 6, the reaction results were displayed in Fig. 8.
The conversion of isooctyl alcohol was increased with the
increasing of catalyst amount. However, the conversion of iso-
octyl alcohol was not affected by further increasing the catalyst
amount, due to the reaction reached equilibrium. Since reac-
tion temperature is the dominant factor in every chemical
reaction, the effect of temperature iꢀn our system was subse-
quently investigated from 60 to 120 C. As shown in Fig. 9, it
could be observed that the optimum temperature for the
esterication was 90 ꢀC. Further increased reaction temperature
did not show any signicant change on the conversion of iso-
octyl alcohol, however when decreased the reaction temperature
to 60 ꢀC, the reaction was greatly affected, only 67.0%
Fig. 9 Reaction temperature effect upon the catalytic performance of
UiO-67–CF3SO3. Reaction conditions: catalyst amount 0.2 g, isooctyl
alcohol to acetic acid molar ratio 1 : 6, for 18 h.
conversion and 56.3% yield obtaꢀined as compared to 98.6%
conversion and 92.9% yield at 90 C.
3.2.3 Catalytic activities of different catalysts. In order to
investigate the catalytic performance of target catalyst UiO-67–
CF3SO3, the catalytic performance of different catalysts in this
work and reported by others for the esterication of acetic acid
and isooctyl alcohol were summarized in Table 3. It should be
noted that the reaction conditions performed for the investi-
gation were catalyst amount 0.2 g, molar ratio of isooctyl alcohol
to acetic acid 1 : 6, reaction temperature 90 ꢀC and reaction
time 18 h. As shown in Table 2, in this work, under optimized
conditions, the pristine UiO-67–bpy could achieve 80.7%
conversion of isooctyl alcohol. While, the UiO-67–CF3SO3 ob-
tained much better catalytic performance and the conversion of
isooctyl alcohol could reach 98.6%. Moreover, we used
commercial H-Beta zeolite for the reaction and could reach
92.2% conversion of isooctyl alcohol. In Table 3, some reported
catalysts were listed for comparison (entries 4–7), and the
results showed that the catalytic activity of UiO-67–CF3SO3 was
comparable to or even better than the reported catalysts
10014 | RSC Adv., 2018, 8, 10009–10016
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Table 3 Catalytic performance of different catalysts for the esterification of isooctyl alcohol
Entry
Catalyst
Ioaa : Aab
Conversion (%)
Yield (%)
Reference
1
2
3
4
5
6
7
Pristine
UiO-67–CF3SO3
H-Beta
1 : 6
1 : 6
1 : 6
1 : 17.5
1 : 3
80.7
98.6
92.2
93.0
84.5
—
73.7
92.9
90.1
—
—
96.0
64.9
This work
This work
This work
44
45
46
47
H-Beta
MZrPc
(NH4)8[CeW10O36]$20H2O
[ChCl][CrCl3$6H2O]2
1 : 52
1 : 5
—
a
Ioa ¼ isooctyl alcohol. b Aa ¼ acetic acid. c Mesoporous zirconium oxophosphate.
Table 4 Results of esterification for different acids and alcohols cat-
a
alysed by UiO-67–CF3SO3
Conversion of
Entry
Acids
Alcohols
alcohols (%)
1
2
3
4
5
6
Acetic acid
Acetic acid
Acetic acid
Formic acid
Formic acid
Formic acid
Methanol
Ethanol
Cyclohexanol
Isopropanol
1-Pentanol
n-Octanol
99.2
97.8
85.1
90.7
94.6
93.5
a
Reaction conditions: catalyst amount 0.2 g, alcohols to acids molar
ratio 1 : 6, for 18 h, at 90 ꢀC.
indicating that UiO-67–CF3SO3 would be an excellent catalyst
for esterication.
Fig. 10 Catalyst reusability of UiO-67–CF3SO3. Reaction conditions:
catalyst amount 0.2 g, isooctyl alcohol to acetic acid molar ratio 1 : 6,
for 18 h, at 90 ꢀC.
In order to explore the catalytic application of UiO-67–
CF3SO3 as a catalyst for other esterication, different acids and
alcohols were chosen as the substrates for investigation. As
shown in Table 4, the UiO-67–CF3SO3 showed excellent catalytic
performance in these reactions which indicated that UiO-67–
CF3SO3 could be used as an efficient catalyst for esterication.
tested by ICP-OES, the result showed that almost no S content
was detected. According to these results, it could be conrmed
the active species were effectively introduced into UiO-67–bpy
and UiO-67–CF3SO3 possessed excellent catalytic activity.
3.3 Reusability of the catalyst
The reusability of a catalyst is prime important, consequently
the reusability of UiO-67–CF3SO3 was investigated, the esteri-
4. Conclusion
cation reaction was repeated several times under the optimized Brønsted acids (H2SO4, CF3SO3H and hifpOSO3H) were intro-
experimental conditions, the results were shown in Fig. 10. Aer duced into the frameworks of UiO-67–bpy effectively by a facile
each reaction, the catalyst was separated from the reaction PSM strategy for the synthesis of acidic ionic liquid based UiO-
mixture by using simple centrifugation (8000 rpm for 10 min), 67 type MOF. The relative high surface area for all prepared
washed with anhydrous dichloromethane and dried under catalysts were maintained and acidic active groups were
vacuum overnight at 80 ꢀC, then reused in the next run. The dispersed in the frameworks uniformly. The prepared catalysts
conversion of isooctyl alcohol decreased from 98.6% to 95.9% showed better catalytic activities in the esterication of acetic
and the yield of isooctyl acetate decreased from 92.9% to 87.1%. acid with isooctyl alcohol and UiO-67–CF3SO3 catalyst showed
These results indicated high reusability of the UiO-67–CF3SO3 best catalytic performance. Under optimized conditions, cata-
catalyst. The decrease in conversion and yield for the UiO-67– lyst amount 0.2 g, isooctyl alcohol to acetic acid molar ratio
ꢀ
CF3SO3 catalyst might be attributed to the blockage of MOFs 1 : 6, for 18 h, at 90 C, the UiO-67–CF3SO3 catalyst could give
channels and partially decomposition of catalyst which could be maximum isooctyl alcohol conversion 98.6% and isooctyl
also observed in Fig. S5(a)† of the recovered catalyst, however, acetate yield 92.9%. The UiO-67–CF3SO3 catalyst could be
the size still maintained around 1.2 mm (Fig. S5(b)†). Next, the reused ve times with no obvious reduction in catalytic
leaching of the active species of UiO-67–CF3SO3 was investi- performance and no obvious leaching was detected, which
gated. Aer the reaction, the solid catalyst was separated from indicated the excellent stability and reusability of UiO-67–
the reaction mixture, the liquid phase was diluted and then CF3SO3.
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Conflicts of interest
There are no conicts of interest to declare.
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Acknowledgements
We gratefully acknowledge the National Key Research and
Development Program of China (No. 2016YFB0601303), the
National Natural Science Foundation of China (No. 21676278),
the Instrument Developing Project of the Chinese Academy of
Sciences (No. YZ201624) and Science and Technology Open
Cooperation Project of Henan Province (No. 172106000066) for
nancial support.
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10016 | RSC Adv., 2018, 8, 10009–10016
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