Friedel-crafts acylation of p-Xylene over sulfonated zirconium terephthalates

Article abstract of DOI:10.1007/s10562-014-1242-4

In this study, a sulfonated Zr-terephthalate metal-organic framework (MOF) (UiO-66-SO₃H) was prepared using two different methods: post-synthetic grafting onto the Zr open metal sites and direct solvothermal method using monosodium 2-sulfoterephthalate. Their catalytic activities for the acid-catalyzed Friedel- Crafts acylation of p-xylene with benzoyl chloride were compared. The physicochemical properties and catalytic performance of the functionalized MOF structure was found to be strongly dependent on the synthesis scheme. UiO-66-SO₃H prepared by a direct solvothermal route exhibited excellent catalytic activity and stability compared to that prepared by post-synthetic grafting. Springer Science+Business Media New York 2014.

Full text of DOI:10.1007/s10562-014-1242-4

Catal Lett (2014) 144:817–824
DOI 10.1007/s10562-014-1242-4
Friedel–Crafts Acylation of p-Xylene over Sulfonated Zirconium
Terephthalates
•
Young-Min Chung Hee-Young Kim
•
Wha-Seung Ahn
Received: 14 January 2014 / Accepted: 13 March 2014 / Published online: 22 March 2014
Ó Springer Science+Business Media New York 2014
Abstract In this study, a sulfonated Zr-terephthalate
metal-organic framework (MOF) (UiO-66-SO3H) was
prepared using two different methods: post-synthetic
grafting onto the Zr open metal sites and direct solvo-
thermal method using monosodium 2-sulfoterephthalate.
Their catalytic activities for the acid-catalyzed Friedel–
Crafts acylation of p-xylene with benzoyl chloride were
compared. The physicochemical properties and catalytic
performance of the functionalized MOF structure was
found to be strongly dependent on the synthesis scheme.
UiO-66-SO3H prepared by a direct solvothermal route
exhibited excellent catalytic activity and stability com-
pared to that prepared by post-synthetic grafting.
problems with the use of homogeneous catalysts, such as
separation, equipment corrosion, and disposal of corrosive
waste, there have been continuing efforts to develop
effective heterogeneous catalysts in the pursuit of envi-
ronmentally-benign reaction processes [3–8].
Metal-organic frameworks (MOFs) or porous coordi-
nation polymers are highly ordered crystalline structures, in
which the nodes of metal clusters are interconnected by
multidentate organic moieties via coordination bonding in
one-, two-, or three-dimensional networks [9, 10]. The
unique organic–inorganic hybrid structures of MOFs allow
greater chemical diversity than those of zeolites or meso-
porous materials, and these structures have been under
active investigation in recent years for gas storage, sepa-
ration, and catalysis [11–16]. Apart from their high surface
area, the most promising feature of MOFs regarding cata-
lytic applications appears to be the large number of active
sites built in their structure, in which the constitutional
metal clusters offer free or exchangeable coordination sites
available for chemisorption [17]. MOFs, in this way, can
behave as a solid acid catalyst when solvent molecules
coordinated to the metal centers are removed and the
resulting coordinately unsaturated metal sites (CUS) act as
a Lewis acid. Therefore, range of MOF structures have
been reported to be efficient catalysts for Lewis acid-cat-
alyzed reactions [18–21]. Brønsted-acid-catalyzed reac-
tions over MOFs, however, have attracted less attention
[22]. Fine tuning of the catalytically active sites is possible
in MOFs either by diverse post-synthesis chemical treat-
ment on the CUS or by working directly on the organic
linkers [23].
Keywords Metal-organic frameworks ꢀ UiO-66 ꢀ
Sulfonation ꢀ Solid acid ꢀ Acylation
1 Introduction
Friedel–Crafts alkylation or acylation, both of which pro-
ceed via electrophilic aromatic substitution in the presence
of a strong Lewis or Brønsted acid, such as AlCl3, FeCl3,
BF3, H2SO4, and HF, is one of the most important means of
preparing alkyl ketones [1, 2]. To overcome the intrinsic
Y.-M. Chung (&)
Department of Nano & Chemical Engineering, Kunsan National
University, 558 Daehak-ro, Kunsan, Jeollabuk-Do 573-701,
Republic of Korea
e-mail: ymchung@kunsan.ac.kr
Cavka et al. [24] recently reported the solvothermal
synthesis of a highly porous Zr-based MOF designated as
UiO-66, which showed excellent chemical and hydrother-
mal stability, and evaluated a range of post-synthesis
H.-Y. Kim ꢀ W.-S. Ahn (&)
Department of Chemistry and Chemical Engineering, Inha
University, Incheon 402-751, Republic of Korea
e-mail: whasahn@inha.ac.kr
123

818
Y.-M. Chung et al.
modifications [25]. In the present study, a sulfonated Zr-
terephthalate MOF (UiO-66-SO3H) was prepared using
two different routes, post-synthetic grafting and direct
solvothermal method, and their catalytic activities for the
Friedel–Crafts acylation of p-xylene with benzoyl chloride
were investigated. The dependence of the textural proper-
ties/acidity and catalytic performances of the sulfonated
UiO-66 samples on the synthesis schemes employed is
reported.
washed with deionized water and ethanol. The wet material
was re-suspended in 1 wt% aqueous H2SO4 solution for a
further 4 h. Finally, the sulfonic acid grafted UiO-66
[hereafter UiO-66-SO3H (grafting)] was rinsed extensively
with deionized water, dried at 393 K, and stored in a
desiccator.
UiO-66-SO3H was also synthesized by a direct solvo-
thermal method using the mixed ligands according to the
procedure reported by Kitagawa and co-workers [30]. Ini-
tially, 1.137 mmol of ZrCl4, 0.205 mmol of monosodium
2-sulfoterephthalate, and 0.933 mmol of H2BDC were
dissolved in 50 ml of a mixed solvent (DMF:acetic
acid = 9:1 v/v). The mixture was placed in a 100 ml
Duran-bottle and heated at 393 K for 40 h in an oven. After
cooling to room temperature, the white product was sepa-
rated by centrifugation and washed three times with DMF.
The product was washed further with acetone and dried in
air. For comparison, MIL-101 [chromium(III) terephthal-
ate] was prepared using the literature procedure [31].
2 Experimental
2.1 Chemicals
Zirconium(IV) chloride (ZrCl4, [99.5 %), 1,4-benzenedi-
carboxylic acid (H2BDC, 98 %), cysteamine ([98 %),
benzoyl chloride ([99 %), p-xylene ([99 %), tetradecane
([99 %), chromium(III) nitrate nonahydrate (99 %),
hydrofluoric acid (47 %) and BasoliteÒ C300 (copper
benzene-1,3,5-tricarboxylate, Cu-BTC or HKUST-1) [26]
were purchased from Aldrich. Ethanol ([99.5 %),
dimethylformamide (DMF, 99 %), acetic acid (99.5 %),
and sulfuric acid ([96 %) were supplied by Samchun
Chemicals in Korea. Monosodium 2-sulfoterephthalate
(98 %) and HBEA (Si/Al = 12.5) were obtained from TCI
(Japan) and Zeoylst (US), respectively.
2.3 Catalyst Characterization
The specific surface area and pore volume of the catalysts
were determined by N2 adsorption using a BELSORP-Max
(BEL, Japan) at 77 K. Prior to adsorption, UiO-66 and
UiO-66-SO3H were degassed under vacuum conditions at
393 K overnight. MIL-101 and Cu-BTC were activated
overnight under vacuum at 423 and 373 K, respectively.
The surface area and pore volume were calculated using
the BET (Brunauer–Emmett–Teller) and MP (micropore
analysis) models, respectively. X-ray powder diffraction
(XRD) was performed on a PANalytical Empyrean system
using Cu-Ka radiation in continuous scan mode from 5° to
90° 2h at a scanning rate of 0.2°/s. The sulfur content of
UiO-66-SO3H was measured by elemental analysis (Flas-
hEA 1112, Thermo). Ammonia temperature-programmed
desorption profiles (NH3-TPD) of the UiO-66 and its
functionalized forms were obtained using a homemade
TPD apparatus. Before ammonia adsorption, samples were
activated in a helium flow of 100 ml/min at 373 K for 2 h.
The samples were subsequently exposed to pulses of
ammonia (1,000 ppm with H2 balance) until saturation, and
then purged with helium flow at 373 K for 1 h. The tem-
perature of the samples was increased to 673 K at a
ramping rate of 5 K/min, and the desorbed ammonia was
detected using a quadrupole mass unit (Agilent 5000A).
2.2 Catalyst Preparation
UiO-66 was synthesized based the procedure reported by
Zlotea et al. [27] with minor modification. ZrCl4
(0.227 mmol) and H2BDC (0.227 mmol) were mixed in
DMF (340 mmol) at room temperature, and heated under
static conditions in a preheated oven at 393 K for 24 h.
After the solvothermal reaction, the resulting crystallized
solid phase was filtered, washed with DMF, and dried in an
oven at 393 K for 12 h. Post-synthesis grafting of sulfonic
acid on UiO-66 was carried out by thiol group attachment
to the CUS in UiO-66 [28], followed by oxidation and then
sulfonation [29]. Before grafting, terminal water molecules
were removed from the framework of UiO-66 by a vacuum
treatment at 423 K for 12 h. Cysteamine was used as a
thiol source. In a typical procedure, 2 g of the activated
UiO-66 was heated under reflux with 4 mmol of cyste-
amine in ethanol for 12 h. After grafting, the thiol grafted
UiO-66 (hereafter UiO-66-SH) was filtered, washed with
deionized water and ethanol, and dried in a vacuum oven at
343 K for 12 h. The immobilized thiol groups were then
oxidized with hydrogen peroxide. Typically, 1 g of UiO-
66-SH was treated with 20 g of 1 wt% H2O2 solution
dissolved in 20 ml of water/ethanol mixture at 293 K for
24 h. After oxidation, the suspension was filtered and
2.4 Catalytic Activity Test
p-Xylene acylation with benzoyl chloride was carried out
under atmospheric pressure conditions in the liquid phase.
Prior to use, UiO-66 and its functionalized catalysts, MIL-
101 and Cu-BTC were activated at 393 K for 4 h under
123

Friedel–Crafts Acylation of p-Xylene over UiO-66-SO3H
819
vacuum, and HBEA (Si/Al = 12.5) was calcined at 673 K
for 4 h. In a typical run, 100 mmol of p-xylene, 0.5 g of
tetradecane (internal standard) and 0.05 g of the catalyst
were introduced to a 50 ml three-necked round bottom
flask fitted with a condenser and thermocouple. The reac-
tion mixture was heated with continuous magnetic stirring
(800 rpm). When the temperature reached 403 K, the
reaction was started with an injection of benzoyl chloride
(acylation agent, 16.7 mmol) into the vessel using a syr-
inge and the reaction was maintained for 4 h. The reaction
product was analyzed by Agilent 6890 Gas Chromatogra-
phy equipped with a HP-5 column with a flame ionization
detector (FID).
solvothermal synthesis. Post-synthesis grafting involves the
immobilization of cysteamine on the Lewis acidic Zr nodes
in UiO-66 (Scheme 1). Oxidation of the thiol group with
H2O2 followed by sulfonation with H2SO4 produces UiO-
66-SO3H (grafting). Another type of UiO-66-SO3H was
directly synthesized using a mixture of H2BDC and its
analogue bearing a sodium sulfonate group (Scheme 2).
After completing the solvothermal reaction, the sodium ion
attached to the sulfonic group of the BDC linker was
exchanged with a proton. The different synthesis proce-
dures led to sulfonic acid groups either sitting on the Zr
sites in UiO-66-SO3H (grafting) or on the BDC linkers in
UiO-66-SO3H (solvothermal).
Table 1 lists the specific surface area and pore volume
of the UiO-66 structures. Both the surface area and pore
volume decreased significantly after incorporating bulky
SO3H moieties into UiO-66, and UiO-66-SO3H (solvo-
thermal) exhibited superior textural properties to UiO-66-
SO3H (grafting) and maintained the high porosity of UiO-
66. The sulfonation step, in particular, was found to be
critical to the porosity of UiO-66-SO3H (grafting); the
3 Results and Discussion
3.1 Synthesis and Characterization of UiO-66-SO3H
UiO-66-SO3H samples were prepared using two different
methods: post-synthesis sulfonic acid grafting and direct
HO
O
HS
S
O
NH2
4+
NH2
4+
4+
Zr
Zr
Zr
O
O
O
4+
Zr
4+
Zr
4+
Zr
O
O
O
Cysteamine
i) 1wt% H2O2, r.t. 24h
ii) 1 wt% H2SO4, r.t. 4h
EtOH reflux, 12h
UiO-66
UiO-66-SH
UiO-66-SO3H (Grafting)
Scheme 1 Preparation of sulfonated UiO-66 via post-synthesis grafting method
+Na-O3S
4+
Zr
O
O
HO
4+
Zr
CO2H
HO2C
S
0.18
0.82
O
O
DMF / CH3COOH
+
ZrCl4
+
+
Na
H
CO2H
HO2C
UiO-66-SO3H (Solvothermal)
Scheme 2 Preparation of sulfonated UiO-66 via direct solvothermal method
123

820
Y.-M. Chung et al.
Table 1 Specific surface area and pore volume of the UiO-66 and its
functionalized structures
3.0
2.0
1.0
0.0
Catalyst
Surface area (m2/g)a
Pore volume (cm3/g)b
UiO-66
1511
1432
275
0.64
0.57
0.12
UiO-66-SH
UiO-66
UiO-66-SO3H
(grafting)
UiO-66-SH
UiO-66-SO3H (Grafting)
UiO-66-SO3H (Solvothermal)
UiO-66-SO3H
(solvothermal)
975
0.44
a
Calculated by the BET (Brunauer–Emmett–Teller) equation
MP (micropore analysis) method
b
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Pore Diameter (nm)
400
Fig. 2 Pore size distribution of UiO-66 and its functionalized forms
during incorporation of sulfonic acid groups via post-synthesis
grafting and direct solvothermal methods
300
UiO-66
200
UiO-66-SH
UiO-66-SO3H (Grafting)
UiO-66-SO3H (Solvothermal)
100
0
UiO-66
0
0.2
0.4
0.6
0.8
1
UiO-66-SO3H (Solvothermal)
Relative Pressure (P/P0)
UiO-66-SH
Fig. 1 N2 adsorption–desorption isotherms of UiO-66 and its func-
tionalized forms during incorporation of sulfonic acid groups via two
different synthesis routes
UiO-66-SO3H (Grafting)
UiO-66 (Simulation)
grafting of thiol groups to UiO-66 affected the N2
adsorption–desorption isotherm only to a minor extent, as
shown in Fig. 1. The pore size distribution in UiO-66-SH
was also shifted modestly to the left after cysteamine
immobilization on the Zr CUS sites (Fig. 2). On the other
hand, UiO-66-SO3H (grafting) exhibited a significant loss
of porosity after subsequent oxidation and sulfonation of
the UiO-66-SH, as shown in Figs. 1 and 2, to such an
extent that an approximately 80 % decrease in surface area
and pore volume was observed. Although the grafting was
carried out using highly dilute H2O2 and H2SO4 solutions
compared to those used in previous studies [29, 32], it was
still difficult to maintain the textural properties of UiO-66.
The undesirable structural deformation in UiO-66-SO3H
(grafting) suggests that the MOF might become unstable in
the presence of acid or oxidant. As shown in Fig. 3,
identical characteristic XRD peaks were observed in both
the UiO-66 and UiO-66-SO3H samples, which matched
well with the simulated pattern reported [24]. The reduced
peak intensities of the functionalized UiO-66 samples
5
10
15
20
25
30
35
40
2 Theta (degree)
Fig. 3 XRD patterns of UiO-66 and its functionalized forms during
incorporation of sulfonic acid groups via two different synthesis
routes
relative to the parent UiO-66 suggest that the use of an
organic linker comprised of bulky heteroatoms or post-
synthesis grafting of such precursors might result in a
decrease in the crystallinity of UiO-66. On the other hand,
the incorporation of organic guest molecules into a porous
structure can also lead to a decrease in the XRD peak
intensities [33]. The fact that the peak intensities of UiO-
66-SH decreased even further after the oxidation and sul-
fonation treatment in UiO-66-SO3H (grafting) supports the
weak stability of UiO-66 against acid or oxidants. More
serious collapse of the UiO-66 structure was observed
when the chemical treatment time or temperature of the
UiO-66-SH was increased (not shown). UiO-66-SO3H
123

Friedel–Crafts Acylation of p-Xylene over UiO-66-SO3H
Table 2 Sulfur contents of the sulfonated UiO-66 catalysts
821
Table 3 Acid amounts of UiO-66 and its functionalized structures
based on NH3-TPD analysis
Catalyst
Sulfur content (wt%)
Catalyst
Acid amount (mmol/g)
UiO-66-SO3H (grafting)
2.33
0.81
Totala Weak-mediumb Strongc
UiO-66-SO3H (solvothermal)
UiO-66
0.115
0.094
0.092
0.051
0.089
0.023
0.043
0.077
UiO-66-SO3H (grafting)
UiO-66-SO3H (solvothermal) 0.166
a
373–673 K
b
373–593 K
c
593–673 K
(a)
(b)
(c)
In the case of UiO-66-SO3H (solvothermal), the total
number of acid sites was ca. 1.4 times higher than that of
UiO-66 and, in particular, approximately one half of the
acid sites could be classified as strong acid sites. Clearly,
the enhanced acidity of the catalyst is ascribed to the
Brønsted acid sites created by the incorporation of sulfonic
acid groups at the organic linkers. On the other hand, the
total number of acid sites including the number of strong
acid sites in UiO-66-SO3H (grafting) was less than that of
UiO-66-SO3H (solvothermal), despite the fact that sulfur
content of the former was ca. three times higher than that of
the latter. This diminished acidity in UiO-66-SO3H
(grafting) can be explained as a consequence of blocking
the Lewis acid sites in UiO-66 by grafting cysteamine on
the metal centers in one hand, and insufficient oxidation
and sulfonation of the thiol groups in the other. Incomplete
sulfonation by the use of highly dilute H2O2 and H2SO4
solutions in UiO-66-SO3H (grafting) seems inevitable to
prevent collapse of UiO-66 structure during sulfonation
step. Based on these characterization results, the direct
solvothermal approach with the optimal content of sulfonic
acid precursor was established to be superior to that based
on the post-synthesis grafting to have high crystallinity,
porosity, and acidity in UiO-66-SO3H.
400
450
500
550
600
650
Temperature (K)
Fig. 4 NH3-TPD profiles of UiO-66 and its functionalized forms.
UiO-66, UiO-66-SO3H (grafting), and UiO-66-SO3H
(solvothermal)
a
b
c
(solvothermal) exhibited clearly higher structural stability
than UiO-66-SO3H (grafting).
Elemental analysis revealed UiO-66-SO3H to have a
sulfur content of 2.33 and 0.81 wt% in the grafting and
solvothermal cases, respectively (Table 2). The grafting
method resulted in a significantly larger amount of sulfur
species incorporated onto the UiO-66 structure. The
amount of sulfonic acid incorporated into the parent UiO-
66 via direct solvothermal synthesis could be increased by
increasing the amount of bulky sodium sulfonated BDC
linker introduced but at the expense of a decrease in surface
area and pore volume. In this study, the mixed ligand ratio
of monosodium 2-sulfoterephthalate to H2BDC was opti-
mized to ca. 4.55:1 to retain the textural properties and
crystallinity of UiO-66-SO3H (solvothermal) close to those
of the parent UiO-66.
3.2 Catalytic Activity Test
MOF structures with Lewis acidity, such as Cu-BTC, MIL-
101(Cr), and MIL-101(Fe), were previously assessed as a
catalyst for the Friedel–Crafts acylation, and MIL-101(Fe)
was reported to show superior catalytic activity compared
to the other MOFs [35]. Because acylation reactions are
known to be catalyzed by both Brønsted and Lewis acids
[36], a MOF catalyst comprised of additional Brønsted acid
sites and more common Lewis acid sites is expected to be
more effective for the acylation of p-xylene with benzoyl
chloride (Scheme 3). Since UiO-66 as a Lewis acid catalyst
is established [37], the further incorporation of Brønsted
acid sites was carried out in UiO-66-SO3H. Postulated
acylation mechanisms by the Lewis acid sites on Zr or by
To measure the acid strength of the catalysts, NH3-TPD
analysis was performed. The ammonia desorption profiles
over the UiO-66 and its functionalized forms are shown in
Fig. 4 and Table 3 gives the corresponding distribution of
acid sites with different acid strength classified following
the guideline of Kim et al. [34]. While high population of
weak-medium acidity was detected in UiO-66, the number
of strong acid sites was found increased further in the
sulfonated UiO-66, regardless of the preparation methods.
123

822
Y.-M. Chung et al.
O
O
Table 4 p-Xylene acylation with benzoyl chloride using various acid
catalysts
Cl
acid cat.
403K, 4h
+
Entry Catalyst
Benzoyl chloride conv. (%)
1
2
3
4
5
6
7
8
Blank
–
a
H2SO4
68.5
46.0
36.4
21.4
17.2
40.8
p-Xylene
Benzoyl chloride
1-Benzoyl-2,5-dimethyl benzene
HBEA (Si/Al = 12.5)
Cu-BTC
Scheme 3 p-Xylene acylation with benzoyl chloride
MIL-101(Cr)
UiO-66
the Brønsted acid sites on the organic linker in UiO-66-
SO3H (solvothermal) are shown in Schemes 4 and 5,
respectively.
UiO-66-SO3H (grafting)
UiO-66-SO3H (solvothermal) 55.7
Reaction conditions catalyst (0.05 g), p-xylene (100 mmol), benzoyl
chloride (16.7 mmol), tetradecane (0.5 g), 403 K, 4 h
UiO-66 is comprised of tetrahedral and octahedral cages
in a 2:1 ratio, and their free dimensions are close to 8 and
˚
11 A, respectively [24]. Considering the triangular widow
a
0.005 mmol H2SO4 was used as a homogeneous catalyst
˚
of UiO-66 with a free diameter of *6 A, it appears that the
Table 4 lists the reaction performance of these heteroge-
neous catalysts.
cages can be accessible for a reactant, such as p-xylene
without any significant diffusion problems. To compare the
catalytic performance of UiO-66 and its sulfonated forms,
different MOF structures of Cu-BTC and MIL-101 as well
as H-BETA zeolite were also tested for the same reaction.
The selectivity to the primary acylation product of
1-benzoyl-2,5-dimethylbenzene was approximately 100 %
under the reaction condition, regardless of the catalysts
O
O
+
+
HCl
-
H
Cl
Cl
Zr 4+
Zr 4+
O
O
O
O
O
O
HO
HO
Zr4+
Zr4+
S
S
O
O
O
O
Scheme 4 Postulated Lewis acid catalyzed acylation mechanism on the metal centers of UiO-66-SO3H (solvothermal)
-
O
O
+
-
O
+
O
+
Cl
+
H
-
OH
S
O
S
-
O
S
O
O
O
O
O
O
O
- HCl
Scheme 5 Postulated Brønsted acid catalyzed acylation mechanism on the organic linker of UiO-66-SO3H (solvothermal)
123

Friedel–Crafts Acylation of p-Xylene over UiO-66-SO3H
823
60
60
50
40
30
20
10
0
UiO-66
(a)
UiO-66-SO3H (Grafting)
50
UiO-66-SO3H (Solvothermal)
40
(b)
30
20
10
0
Catalyst Filtration
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
Fig. 5 Conversion profiles of p-xylene acylation with benzoyl
chloride over UiO-66 and its functionalized forms
Fig. 6 p-Xylene acylation with benzoyl chloride over a UiO-66-
SO3H (solvothermal) and b in the filtrate solution [UiO-66-SO3H
(solvothermal) was filtered-off after 1 h reaction time]
used. No sequential reaction of the acylated product or
other side reactions, such as p-xylene isomerization, were
observed. UiO-66 shows relatively poorer catalytic activity
than those by HBEA, Cu-BTC and MIL-101(Cr), which
were close to the values reported previously [34]. The
heavy atomic weight of Zr in UiO-66 meant a relatively
small number of Lewis acid sites per gram catalyst. As
shown in Table 4 and Fig. 5, on the other hand, incorpo-
ration of sulfonic acid groups to UiO-66 improved the
catalytic performance dramatically regardless of the syn-
thetic methods used. The best catalytic performance was
obtained by UiO-66-SO3H (solvothermal) among the het-
erogeneous catalysts tested, even though homogeneous
sulfuric acid was more active (entry 2 of Table 4). The
enhanced catalytic activity of the sulfonated UiO-66
(solvothermal) must be attributed to the incorporation of
new Brønsted acid sites combined with the existing Lewis
acid sites of the Zr clusters. UiO-66-SO3H (grafting) also
exhibited higher catalytic activity than UiO-66 due pri-
marily to its relatively larger amount of strong acid sites.
However, decrease in the number of Lewis acid sites by
grafting of cysteamine on the metal centers and insufficient
increase in the Brønsted acid sites mentioned previously
seems responsible for the limited enhancement in the cat-
alytic performance. The higher surface area and pore vol-
ume in UiO-66-SO3H (solvothermal) must have also been
advantageous for the substrate diffusion compared with
UiO-66-SO3H (grafting) with poor textural properties of
after multistep post-synthesis treatments.
60
50
40
30
20
10
0
Fresh
1
2
3
4
5
Recycle Times
Fig. 7 Recycle runs of p-xylene acylation with benzoyl chloride over
UiO-66-SO3H (solvothermal)
reused five times without significant deterioration in con-
version (Fig. 7).
4 Conclusions
The acylation of p-xylene with benzoyl chloride was car-
ried out in the presence of sulfonated UiO-66 catalysts
prepared by two different routes. UiO-66-SO3H prepared
by direct solvothermal synthesis exhibited excellent cata-
lytic activity, which was 3.2 times higher in conversion
than that of UiO-66. The significant improvement in cat-
alytic activity is a consequence of the enhanced acid
character of the catalyst through the introduction of
To test the stability of the catalyst UiO-66-SO3H
(solvothermal) against leaching, a hot catalyst filtering
experiment was carried out. As shown in Fig. 6, reaction in
the filtrate without catalyst did not proceed, which clearly
indicated true heterogeneous nature of the catalysis. UiO-
66-SO3H (solvothermal) could also be recovered and
123

824
Y.-M. Chung et al.
14. Kreno LE, Leong K, Farha OK, Allendorf M, Van Duyne RP,
Hupp JT (2012) Chem Rev 112:1105
15. Lee J, Farha OK, Roberts J, Scheidt KA, Nguyen ST, Hupp JT
(2009) Chem Soc Rev 38:1450
16. Farrusseng D, Aguado S, Pinel C (2009) Angew Chem Int Ed
48:7502
17. Corma A, Garcia H, i Xamena FXL (2010) Chem Rev 110:4606
18. Dhakshinamoorthy A, Alvaro M, Garcia H (2010) Adv Synth
Catal 352:711
19. Alaerts L, Se0guin E, Poelman H, Thibault-Starzyk F, Jacobs PA,
De Vos DE (2006) Chem Eur J 12:7353
20. Schlichte K, Kratzke T, Kaskel S (2004) Microporous Meso-
porous Mater 73:81
additional Brønsted acid sites without any reduction in the
number of unsaturated Lewis acid sites at the Zr metal
centers. The catalyst could also be recycled 5 times without
significant loss of catalytic activity. UiO-66-SO3H pre-
pared by post-synthesis grafting also catalyzed acylation
more efficiently than UiO-66, but deterioration in textural
properties, reduction in the number of Lewis acid sites
caused by grafting of cysteamine on the Zr metal sites, and
smaller number of Brønsted acid sites introduced resulted
in only limited improvement in the catalytic performance.
21. Henschel A, Gedrich K, Kraehnert R, Kaskel S (2008) Chem
Commun 35:4192
22. Akiyama G, Matsuda R, Sato H, Takata M, Kitagawa S (2011)
Adv Mater 23:3294
23. Cohen SM (2012) Chem Rev 112:970
Acknowledgments This work was funded by the National Research
Foundation of Korea (NRF) supported by the Ministry of Education,
Science and Technology (2013R1A1A4A01006480; 2013005862).
24. Cavka JH, Jakobsen S, Olsbye U, Guillou N, Lamberti C, Bordiga
S, Lillerud KP (2008) J Am Chem Soc 130:13850
25. Kim M, Cohen SM (2012) CrystEngComm 14:4096
26. Chui SSY, Lo SM-F, Charmant JPH, Orpen AG, William ID
(1999) Science 283:1148
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