12-Tungstophosphoric acid niched in Zr-based metal-organic framework: a stable and efficient catalyst for Friedel-Crafts acylation

Article abstract of DOI:10.1007/s11426-017-9182-0

Heteropolyacids (HPA) are well known for their versatile solid acid catalysis in diverse chemical reactions, however they suffer from low surface area (<10 m²/g) and leaching into the reactions media, which reduce their prospects as industrial catalyst. Herein, a novel hybrid material HPW@Zr-BTC, composed of 12-tungstophoric acid (HPW) and Zr^IV-benzene tri-carboxylate (Zr-BTC) metal-organic framework (MOF), was prepared via one-pot solvothermal method. Excellent HPW loading up to 32.3 wt% was achieved, and HPW@Zr-BTC composite proved to be highly stable, besides the crystalline morphology of Zr-BTC was intact. The catalytic activity of the hybrid composite was explored via Friedel-Crafts acylation of anisole with benzoyl chloride. The 28.2 wt% HPW@Zr-BTC showed excellent catalytic performance, with 99.4% anisole conversion and 97.6% yield (pmethoxybenzophenone) under solvent free conditions. Excellent retention of catalytic activity was achieved after at least five consecutive runs due to non-observable HPW leaching. The promising activity and stability of the catalyst forecasted its potential industrial applications.

Full text of DOI:10.1007/s11426-017-9182-0

SCIENCE CHINA
Chemistry
•ARTICLES• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . https://doi.org/10.1007/s11426-017-9182-0
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12-Tungstophosphoric acid niched in Zr-based metal-organic
framework: a stable and efficient catalyst for Friedel-Crafts
acylation
Latif Ullah1,2, Guoying Zhao1*, Zichen Xu1, Hongyan He1,
Muhammad Usman1,3 & Suojiang Zhang1*
1Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process Engineering, State Key Laboratory of Multiphase
Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China;
2University of Chinese Academy of Sciences, Beijing 100049, China;
3Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran 3126, Saudi Arabia
Received August 7, 2017; accepted November 20, 2017; published online March 2, 2018
Heteropolyacids (HPA) are well known for their versatile solid acid catalysis in diverse chemical reactions, however they suffer
from low surface area (<10 m2/g) and leaching into the reactions media, which reduce their prospects as industrial catalyst.
Herein, a novel hybrid material HPW@Zr-BTC, composed of 12-tungstophoric acid (HPW) and ZrIV-benzene tri-carboxylate
(Zr-BTC) metal-organic framework (MOF), was prepared via one-pot solvothermal method. Excellent HPW loading up to 32.3
wt% was achieved, and HPW@Zr-BTC composite proved to be highly stable, besides the crystalline morphology of Zr-BTC was
intact. The catalytic activity of the hybrid composite was explored via Friedel-Crafts acylation of anisole with benzoyl chloride.
The 28.2 wt% HPW@Zr-BTC showed excellent catalytic performance, with 99.4% anisole conversion and 97.6% yield (p-
methoxybenzophenone) under solvent free conditions. Excellent retention of catalytic activity was achieved after at least five
consecutive runs due to non-observable HPW leaching. The promising activity and stability of the catalyst forecasted its
potential industrial applications.
dodeca-tungstophosphoric acid, Zr-BTC, metal-organic frameworks, one-pot solvothermal synthesis, heterogeneous
catalysis, anisole acylation, high temperature XRD
Citation:
Ullah L, Zhao G, Xu Z, He H, Usman M, Zhang S. 12-Tungstophosphoric acid niched in Zr-based metal-organic framework: a stable and efficient
catalyst for Friedel-Crafts acylation. Sci China Chem, 2018, 61, https://doi.org/10.1007/s11426-017-9182-0
1 Introduction
stophoric acid (HPW) is the strongest and most stable solid
acid reported [1]. HPW has successfully been applied to
catalyze a number of chemical processes [6–9], like oxida-
tion [10–12], formalin dimerization [1], Friedel-Crafts (FC)
reactions [13,14], and formic acid degradation [15]. How-
ever, leaching in reaction media and low surface area
(<10 m2/g) hinder their way to industrial application. One
approach to address these shortcomings is to access the
HPW-based heterogeneous catalysis, by supporting/en-
capsulating heteropolyacids on/inside a porous matrix such
as silica [16], carbon [17], MCM-41 [18], and porous zir-
In chemical transformations, such as petroleum refining and
fine chemicals production [1,2], solid acid catalysts play a
vital role to lower the process cost and check environmental
problems of homogeneous acid catalysts. In this regard,
heteropolyacids (HPA) because of their strong and versatile
solid acid nature, been applied in multiple chemical pro-
cesses [1,3–5]. Amongst the heteropolyacids, 12-tung-
*Corresponding authors (email: gyzhao@ipe.ac.cn; sjzhang@ipe.ac.cn)
© Science China Press and Springer-Verlag GmbH Germany 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . chem.scichina.com link.springer.com

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conia [19,20]. Hitherto, because of the low surface area and
small pore volume of such conventional porous supports,
only a limited amount of HPW could be loaded, besides
leaching of the polyanions is still a problem.
chosen because of its importance in organic transformations
that include C–C bond coupling reactions [40]. Traditionally,
FC acylation has been catalyzed homogeneously by H2SO4,
HF or AlCl3, which causes severe environmental problems,
thus environmentally benign heterogeneous catalysts are
greatly desired for such reactions [41–43]. To the best of our
knowledge, this is the first attempt to encapsulate heteropoly
acids into Zr-BTC, and explore its catalytic activity in acy-
lation reactions.
Metal-organic frameworks (MOFs), are relatively new
crystalline hybrid materials, known for their large surface
area, ordered and tunable porosity, and low structure density
[21–23]. The Cu and Cr-based MOFs have recently received
much attention for heteropoly acids encapsulation [24–30].
Although fascinating results have been reported, even though
the Cu and Cr-based MOFs are sensitive to aqueous and
acidic media [31,32], CrVI is carcinogenic and is considered a
highly unwanted pollutant (LTV level of 0.05 mg/L) in wa-
ter. Therefore, it is imperative to find new hosts to support
heteropoly acids, especially MOFs having large surface area
and environmentally benign metal body that is resistant to
corrosive solvents. Previously, ZrO2 was used as support for
HPW to catalyze Friedel-Crafts acylation [19], transester-
ification for biodiesel synthesis and glycerol valorisation
[20]. Zirconium MOFs having large surface area, acid re-
sistive properties, considerable thermal stability and en-
vironmentally hazard-free zirconium metal [33–36], have
shown potential prospects in this regard. However, only a
few studies have reported heteropoly acids supported in Zr-
based MOFs [37,38]. It was perceived, that zirconium-based
MOFs would be a good choice to allow high HPW loading
and to suppress the polyanion leaching into the reaction
media.
Zr-BTC was chosen for this study due to its large surface
area (~2300 m2/g), good stability and pertinent porous
structure [33]. Zr-BTC has two types of pores, as shown in
Scheme 1. The smaller tetrahedral pores ~0.5–0.7 nm size,
and the relatively larger adamantane type pores with internal
diameter 1.8–2.0 nm, which were rationally a good choice to
host the HPW moieties that have the size of ~1.2 nm [39].
Herein, we report the encapsulation of 12-tungstophoric
acid [H3PW12O40] in the porous Zr-BTC via one-pot sol-
vothermal strategy, to prepare HPW@Zr-BTC hybrid ma-
terials, and onward explore its catalytic performance in
Friedel-Crafts acylation of anisole with benzoyl chloride as a
model acid catalyzed reaction. Friedel-Crafts acylation was
2 Experimental
2.1 Preparation of Zr-BTC
Zr-BTC was synthesized following the method adopted from
Furukawa et al. [44]. In a typical reaction, ZrOCl2·8H2O
(6.46 g, 20 mmol) and trimesic acid (1.47 g, 7 mmol) linker
were dissolved in (600 mL) pre-mixed solvent containing
50% N,N-dimethylformamide (DMF) and 50% formic acid
in a gas tight glass bottle. Modulator benzoic acid 2 eqv.
(based on Zr salt) was also added, as reported elsewhere [45].
The precursors were sonicated for 10 min followed by stir-
ring at room temperature until no suspended particles were
visible. The mixture was then placed in a pre-heated oven for
two days at 120 °C. Zr substituted POMs have previously
been reported for monolacunary POMs [46], for example in
Dawson type POMs [47] forming [PW11O39Zr(μ-OH)2]3− and
[Zr(α2-P2W17O61)2]16− monomeric complexes; neither was
observed in this case. Upon completion of the reaction, the
white crystals were thoroughly washed with fresh DMF, and
then with acetone. The crystals were evacuated at 60 °C for
8 h, followed by activation at 110 °C for 6 h.
2.2 Preparation of HPW@Zr-BTC
Synthesis of the hybrid HPW@Zr-BTC was achieved fol-
lowing the same procedure as Zr-BTC, with the addition of
12-tungstophophoric acid in the synthesis media. Different
millimoles of 12-phosphotungstic acid (HPW) was added to
the mixture of ZrOCl2·8H2O and BTC under constant stirring
and variable loadings of the HPW were achieved (18.6 wt%
to 32.3 wt%), as confirmed by inductively coupled plasma
optical emission spectrometer (ICP-OES).
3 Results and discussion
3.1 Crystalline and morphological studies
The XRD patterns for the Zr-BTC pristine and its HPW
encapsulated composites are shown in Figure 1. The XRD
peaks obtained for pure Zr-BTC are in good agreement with
the calculated MOF-808 type, corresponding to CCDC#
1002672 in the Cambridge Crystal Structural Database [44].
Scheme 1 Structure representation of Zr-BTC, dodeca-tungstophosphoric
acid and anisole acylation reaction (color online).

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The SEM results (Figure 2) revealed the pristine Zr-BTC
to possess well defined octahedral microcrystals having
average crystal diameter of 200 nm. The micrographs sug-
gested, that up to a certain limit HPW addition had little
impact upon the crystals, but as the HPW content increased
beyond 28.2 wt%, the crystal shape changed quickly towards
distortion. This might be the result of low pH of the system
which resulted in protonation of BTC, therefore inhibiting
the coordinating ability of BTC, and leading to crystals
shrinkage and ultimately crystal disappearance [49]. No
polyanions agglomeration was observed, as evident from
XRD (Figure 1) and energy dispersive spectrometer (EDS)
elemental mapping (Figure S1, Supporting Information on-
line), which showed well dispersed HPW inside the host
framework.
Figure 1 Powder XRD Bragg’s peaks of Zr-BTC simulated (CCDC#
1002672) (a), Zr-BTC pristine (b), 18.6 wt% HPW@Zr-BTC (c), 25.2 wt%
HPW@Zr-BTC (d), 28.2 wt% HPW@Zr-BTC (e), 32.3 wt% HPW@Zr-
BTC (f) and >32.3 wt% HPW@Zr-BTC (g) (color online).
3.2 FTIR spectroscopy
The two sharp peaks at the 8.36° (311) and 8.74° (222) for
the pristine Zr-BTC revealed that the material obtained had
high crystallinity. No separate ZrO2 peaks were observed,
which indicated good reproducibility of the MOF material.
From the XRD patterns indicated in Figure 1, the addition of
HPW up to (18.6 wt%) into the Zr-BTC revealed improved
crystallinity, that could be due to the anionic templating ef-
fect of oxometallates; discussed elsewhere [30,48]. Also, it
could be seen that further addition of HPW into the host
hardly affected the crystalline morphology of the pristine Zr-
BTC up to a threshold amount of HPW (25.2 wt%
HPW@Zr-BTC). With further increase in the amount of
HPW, the intensity of the representative peak at 8.36° (311)
declined drastically, due to low pH of the medium [49,50],
that protonated the linkers, inhibiting coordination with the
metal. Additional HPW loading resulted in greatly damaging
the Braggs peaks of Zr-BTC, which was also verified from
the scanning electron micrographs (SEM) shown in Figure 2
(vide infra).
To confirm whether the HPW had been successfully en-
capsulated into the Zr-BTC, Fourier transform infrared
spectroscopy (FTIR) studies were conducted. FTIR spectra
for the bulk HPW, pristine Zr-BTC, and HPW@Zr-BTC
samples are shown in Figure 3. The observed vibration bands
between 1300–1750 cm−1, corresponding to the carboxylate
linkers [37] of the pristine Zr-BTC were well maintained in
the hybrid samples. Additionally, new peaks appeared in the
region of 600–1200 cm−1 for the hybrid HPW@Zr-BTC
composite samples, which were assigned to the P–O, W–O–
W and W=O vibration bands in HPW [51]. The peaks at υ
(cm−1): 1050 (PO4−), 823, 881 and 942 (W–O–W) and 960
(W=O) cm−1 stretches were all in good agreement with the
earlier works [49,50].
3.3 BET surface and textural studies
Table 1 summarizes the textural properties, such as BET
Figure 2 SEM micrographs of Zr-BTC pristine (a), HPW@Zr-BTC composites 18.6 wt% (b), HPW@Zr-BTC composites 25.2 wt% (c), HPW@Zr-BTC
composites 28.2 wt% (d), HPW@Zr-BTC composites 32.3 wt% (e), and HPW@Zr-BTC composites >32.3 wt% (f).

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Figure 3 FTIR spectra for HPW bulk (a), Zr-BTC pristine (b), 18.6 wt%
HPW@Zr-BTC (c), 25.2 wt% HPW@Zr-BTC (d), 28.2 wt% HPW@Zr-
BTC (e), and 32.3 wt% HPW@Zr-BTC (f) (color online).
surface area and pore volume of pristine Zr-BTC and
HPW@Zr-BTC composites. Consistent with the increase in
HPW loadings, the surface area of the composite structures
decreased from 2058.6 to 1065.6 m2/g. Figure 4(a) shows
type-1 isotherms, which reveal the microporous nature of Zr-
BTC with no associated capillary condensation. The iso-
therms for the HPW@Zr-BTC composites showed no ob-
servable sign of swelling, which confirmed that the pores
were not damaged by HPW incorporation into the host as-
sembly. Also the size of the micropores were not less than
0.5 nm, that might have inhibited the tensile strength
amongst the adjacent pores to damage the pores integrity
[52]. The pore size distribution of the materials was calcu-
lated from the desorption branch of the isotherm using
Horvath-Kowazoe (HK) method, which revealed two types
of micropores; smaller pores of 0.5–0.7 nm and larger pores
of ~1.8 nm, in well agreement with the Ref. [33] (Figure 4
(b)). The larger pores of ~1.8 nm size in the Zr-BTC edifice
were available to accept HPW, and upon the introduction of
HPW their size decreased, whereas the smaller pores (~
0.5–0.7 nm) remained almost unaffected, as shown in Figure
4(b).
Figure 4 (A) BET sorption isotherms for Zr-BTC Pristine (a), 18.6 wt%
HPW@Zr-BTC (b), 25.2 wt% HPW@Zr-BTC (c), 28.2 wt% HPW@Zr-
BTC (d), and 32.3 wt% HPW@Zr-BTC (e). (B) Pore size distribution
calculated from adsorption peak using Hovarth-Kowazoe (HK) method
(color online).
was observed, before and after encapsulation (Figure 5). For
tungsten W6+ before encapsulation, two principal peaks were
obtained at 35.70 and 37.75 eV corresponding to W 4f7/2 and
W 4f5/2, respectively. The peak ratio between them was about
4:3 which showed that the HPW was in its fully oxidized
(W3O and WO2) state [53]. For the 28.2 wt% HPW@Zr-
BTC, the tungsten peaks were almost of the same height and
ratio but slightly shifted towards higher binding energies,
with W4f7/2 and W4f5/2 peaks that appeared at 36.55 and
38.75 eV, corresponding to the stronger interaction of HPW
with the host Zr-BTC [19]. In addition to these two peaks,
another distinctive W4+ (W5P3/2) peak at 42.60 eV was ob-
served, which indicated the formation of WO3 [53]. The
phosphorus P 2p peaks for bulk HPW and encapsulated
samples are also shown in Figure 5. The fundamental peaks
3.4 XPS analysis
To understand the interaction of HPW with the host struc-
ture, X-ray photoelectron spectroscopy (XPS) studies were
conducted and the behaviour of tungsten and phosphorus
Table 1 Surface studies for the Zr-BTC pristine and the HPA@Zr-BTC samples
Entry
Sample type
Surface area (BET) (m2/g)
Pore size (nm)
1.814
Pore volume (cm3/g) a)
HPW:Zr-BTC (wt%)
0:100
1
2
3
4
5
Zr-BTC (Pristine)
2058.6
1633.6
1284.5
1117.7
1065.6
1.432
1.195
1.114
0.993
0.765
18.6 wt% HPA@Zr-BTC
25.2 wt% HPA@Zr-BTC
28.2 wt% HPA@Zr-BTC
32.3 wt% HPA@Zr-BTC
1.713
18.6:81.4
1.691
25.2:74.8
1.654
28.2:71.8
1.653
32.3:67.7
a) Calculated by using Horvath-Kowazoe (HK) method.

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Figure 5 X-ray photoelectron spectra for phosphorus (P 2p) and tungsten (W 4f) of HPW, before and after encapsulation. Results for 28.2 wt% HPW@Zr-
BTC (color online).
for phosphorous P 2p3/2 and P 2p1/2 in Keggin structure were
observed at 134.75 and 135.65 eV, respectively, which were
slightly shifted to 134.85 and 135.80 eV after encapsulation.
The slight shift in binding energy for the phosphorus octa-
hedra showed that the Keggin topology of the HPW was
retained inside the MOF, yet it was interactive with the host
matrix [19]. This was also corroborated by 31P-NMR (Fig-
ure 8, vide infra).
3.5 TGA analysis
TGA studies were conducted to substantiate the thermal
stability of the Zr-BTC and its corresponding HPW@Zr-
BTC composites. From the thermographs in Figure 6, the
Figure 6 TGA thermographs for pristine Zr-BTC (a), 18.6 wt%
HPW@Zr-BTC (b), 28.2 wt% HPW@Zr-BTC (c), 32.3 wt% HPW@Zr-
BTC (d), and bulk HPW (e) (color online).
bulk HPW followed one-stepped, whereas the pristine Zr-
BTC and HPW@Zr-BTC composites followed two-stepped
thermal degradation. The two-stepped thermal degradation is
considered a typical feature for MOF materials as reported
elsewhere [31,54]. The bulk HPW revealed weight loss at
around 135 °C, mainly due to dehydration, and beyond that
point there was no evident weight loss till 700 °C. For Zr-
BTC, the observable and prolonged weight loss around
70 °C was obvious, due to the evaporation of embedded
moisture and guest molecules. The second trough for the
pristine Zr-BTC emerged at around 330 °C, which was re-
ferred to as linker sites opening and structure degradation
[34]. The thermographs for the HPW@Zr-BTC composite
(18.26, 28.2 and 32.3 wt%) displayed less dehydration and
guest removal as compared to the pristine Zr-BTC, indicat-
ing that HPW had occupied the free space previously
available for the guest. For the 18.3 and 28.2 wt% HPW@Zr-
BTC composites, the linker sites opening was delayed to 348
and 362 °C, respectively, showing enhancement in stability
of the Zr-BTC after the introduction of heteropoly acids.
TGA findings were corroborated by HT-XRD (high-tem-
perature XRD) for 28 wt% HPW@Zr-BTC (Figure 7). The
HT-XRD gave a detailed account of the phase behaviour of
the material under investigation. It was apparent that the

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4 Catalytic tests
The catalytic activity of the HPW@Zr-BTC composite was
adequately explored by Friedel-Crafts acylation of anisole
with benzoyl chloride as an acylating agent in line with
Scheme 2. All reactions were undertaken in liquid phase
batch reactor at 120 °C and at atmospheric pressure for 6 h
under constant stirring with anisole to benzoyl chloride
(BoC) molar ratio 1:2, respectively. The major product ob-
tained was p-methoxybenzophenone (yield~98%), while o-
methoxybenzophenone was produced as minor product
(Table 2). With pristine Zr-BTC, the anisole conversion was
only 37.77%, whereas the HPW@Zr-BTC composites
showed excellent activity, owing to the availability of strong
active sites. The 28.2 wt% HPW@Zr-BTC gave best results
amongst all the hybrid catalysts employed giving 99.4%
anisole conversion. Furthermore, the low HPW loaded
sample, i.e., 18.6 wt% gave merely 68% conversion which
sequentially increased with the increase in the HPW content,
due to availability of more active sites (Figure 9). In addition
to this, it was observed that the catalytic activity of 32.2 wt%
HPW@Zr-BTC was lesser than its 28.2 wt%, which might
be due to pore blockage by HPW, as revealed by BET surface
area and pore size distribution (Figure 4). Such trends have
also been reported by Lingaiah et al. [19] and by Ammar et
al. [49]. The possible structural representation and catalysis
of the hybrid material are shown in Scheme 3, also proposed
by Su et al. [20] for their HPW supported ZrO2.
Figure 7 High temperature XRD for 28.2 wt% HPW@Zr-BTC (color
online).
enhancement in the thermal stability of Zr-BTC by the in-
corporation of HPW, was due to the templating effect of
heteropoly anions [31,48,55,56]. In contrast, the onset of the
thermal degradation for 32.3 wt% HPW@Zr-BTC appeared
earlier than for the pristine Zr-BTC, i.e., at 318 °C. Thus, it
could be inferred that up to a certain limit the heteropoly
anions stabilized the Zr-BTC structure, however as the
amount of HPWexceeded this threshold value; the low pH of
the medium protonated the BTC linker which consequently
let to structure deterioration.
3.6 Solid state 31P-NMR
Solid state 31P-NMR was employed to understand the
structural integrity, retention of Keggin topology as well as
crystallinity of the composite material. Accordingly, we
conducted solid state 31P-NMR for the HPW@Zr-BTC
composites (18.6 and 28.2 wt%) to know the effect on the
Keggin structure after encapsulation (Figure 8). One can find
the literature chemical shift value for HPW i.e., −15.5 ppm,
corresponding to the tetrahedral phosphorus, surrounded by
wolfram octahedrae at the periphery [57]. The crests at
−14.41 ppm for the 18.6 wt% HPW@Zr-BTC showed dis-
tortion of the Keggin topology due to stronger interactions
with the host, and such trends were also reported by Koz-
hevnikov et al. [50] and Su et al. [20], independently. For
28.2 wt% HPW@Zr-BTC, the characteristic peak at
−16.35 ppm was noticed, which was referred to the tetra-
hedral [PO4]4− in the Keggin structure. The slight deviation
from the mean value reflected to the fact that the Keggin
structure was retained yet interactive with the host matrix
[19,20]. The shoulder peak at −14.87 ppm for 28.2 wt%
sample showed the stronger interaction of the support with
the heteropoly anions [48]. These findings were also corro-
borated by XPS results (vide supra).
Figure 8 Solid state of 31P-NMR 18.6 wt% HPW@Zr-BTC (a) and 28.2
wt% HPW@Zr-BTC (b) (color online).
Scheme 2 Anisole acylation with benzoyl chloride.

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Table 2 Reaction inferences of 28.2 wt% HPW@Zr-BTC catalyzed anisole acylationa)
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Entry
Catalyst content (wt%)
HPW content (wt%)
Zr-BTC content (wt%)
Conversion (%)
37.77
Yield (%)
34.01
67.05
78.43
87.8
1
2
3
4
5
6
1.5
3.0
4.5
6.0
7.5
8.0
0.429
0.858
1.287
1.716
2.145
2.288
1.071
2.142
3.213
4.284
5.355
5.712
72.76
83.07
91.87
96.36
94.92
97.96
99.37
a) Reaction conditions: anisole to BoC molar ratio 1:2 moles, 120 °C for 6 h.
modified with Ga and In chloride/oxide [60]. Patil et al. [61]
claimed 95% selectivity for their borate zirconia catalyst. In
connection to these results, the bulk HPW gave 100% con-
version in our study, but lower yield for the para isomer due
to over oxidation and formation of methyl/phenyl benzoates,
besides we were unable to recover HPW after the reaction.
Henceforth, the promising yield (~98%) for the FC acylation
of anisole with benzoyl chloride, show the competitiveness
of the 28.2 wt% HPW@Zr-BTC hybrid catalyst against the
reported and conventional catalytic systems.
5.2 Effect of reaction temperature
Figure 9 Catalytic activity of pristine Zr-BTC (a), 18.6 wt% HPW@Zr-
BTC (b), 25.2 wt% HPW@Zr-BTC (c), 32.2 wt% HPW@ZR-BTC (d), and
28.2 wt% HPW@Zr-BTC (e) (Reaction conditions: anisole to BoC molar
ratio 1:2 moles, temperature 120 °C, time 6 h) (color online).
Since temperature plays a vital role in any chemical reaction,
we subsequently investigated the effect of temperature in our
system from 100, 110, 120, 130 and 140 °C. It was found that
the optimum temperature for this reaction was 120 °C
(Figure 10). Further increase in reaction temperature did not
reveal any significant change on the anisole conversion due
to establishment of reaction equilibria. However, decrease in
reaction temperature greatly affected the course of reaction.
At 100 °C, only 45% conversion and 86% selectivity were
observed as compared to 72% conversion and 92.3% se-
lectivity at 110 °C and 99.4% conversion 98.5% selectivity
at 120 °C. Further increase to 130 and 140 °C resulted in
small rise in the conversion at the expense of selectivity due
to the formation of etheric side products.
5 Optimization of reaction variables
5.1 Effect of catalyst amount
It is reported that Friedel-Crafts acylation requires an extra
stoichiometric amount of acid catalyst due to complexation
with the oxygen atoms [58]. This extra catalyst consumption
caused cost ineffectiveness vis-à-vis inevitable environ-
mental risks. As shown in Table 2, the amount of catalyst
significantly affects the rate of reaction, i.e., 34% to 99.4%
conversion observed from 1.5 to 8.0 wt% for the 28.2 wt%
HPW@Zr-BTC in a relatively shorter reaction time. Further
increase in the catalyst amount had no significant effect upon
anisole conversion. Earlier studies reported different catalyst
for anisole acylation (Table 3), like AlPW12O40 could give
94% yield with benzoyl chloride [49]. Ammar et al. [49]
reported 94.5% yield, for the same reaction with dodeca-
phosphotungstic acid encapsulated in ZIF-67 metal-organic
framework. In another work, the selectivity of p-isomer was
reported to be 90% for the H-Beta type zeolites previously
5.3 Re-usability of the catalyst
The re-usability of a catalyst is of prime importance, con-
sequently HPW@Zr-BTC catalyst was investigated for its
re-usability and ease of separation from the reaction media.
After the completion of a typical reaction (Sect. 4), the cat-
alyst was removed by simple centrifugation (7000 r/min for
5 min), washed with dichloromethane to remove traces of
adsorbed reagents and dried overnight at 60 °C before acti-

8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ullah et al. Sci China Chem
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Scheme 3 Schematic representations of HPW interaction with the Zr-BTC host: catalyst for anisole acylation (color online).
Table 3 Anisole acylation with benzoyl chloride by different catalysts (comparison)
Entry
Catalyst type
28.2 wt% HPW@Zr-BTC
Zr-BTC
Catalyst content (wt%)
Anc)/BoCd) (mol/mol)
Tempture (°C)
Yield (%)
97.9b)
25.5b)
96.7b)
65.5b)
1
2
3
4
5
6
7
8
9
8
8
0.5
0.5
0.5
0.5
0.8
0.5
0.6
2.0
0.8
120
120
120
120
150
120
60-70
80
HPW
8
H-beta
8
H-Beta
20
10
4
92.8 [61]
94.5 [49]
94.0 [59]
90.0 [60]
87.8 [61]
26.5wt% PTA@ZIF-67
AlPW12O40
20% In2O3-Hβa)
20
20
a)
B2O3/ZrO2
150
a) Nitrobenzene as solvent; b) this study; c) an-anisole; d) BoC-benzoyl chloride.
runs giving 99.4%, 92.7%, 92.1%, 90.5% and 89.2% con-
version under the same reaction conditions (within experi-
mental error) (Figure 11).
5.4 Catalyst leaching tests
Leaching of heteropoly acids into the reaction media is the
bottleneck to their applicability on larger scale. To confirm
whether the active material had leached into the reaction
medium or not, controlled test reactions were carried out.
Here, the reaction assembly as previously described was split
into two parts after 4 h on the reaction. One part was run
undisturbed, while the second part was filtered to remove the
solid catalyst particles, and the reaction was carried out
keeping the other reaction conditions same. As shown in
Figure 12, the part which had been filtered has stopped
further reacting, which proved that the catalyst HPW@Zr-
BTC was robust and resistant to leaching. To further validate
these findings, inductively coupled plasma (ICP) analyses
were done to determine the tungsten traces in the reaction
Figure 10 Temperature effect upon the catalytic activity of 28.2 wt%
HPW@Zr-BTC (reaction conditions: anisole to BoC molar ratio 1:2, re-
action time 6 h) (color online).
vation at 110 °C for 6 h. It was observed that the catalyst was
easily separable from the reaction medium, showing the
heterogeneity of the catalyst. In this way the spent 28.2 wt%
HPW@Zr-BTC catalyst was applied in at least 5 consecutive

9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ullah et al. Sci China Chem
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
silient catalytic sites as revealed by anisole acylation, with
28.2 wt% HPW@Zr-BTC showed the best catalytic results
(anisole conversion 99.4%). No leaching and hence no ob-
vious reduction in catalytic activity was observed during five
consecutive runs, which indicated the permanency of HPW
guest inside the Zr-BTC framework host.
Acknowledgements This work was supported by the National Key Re-
search and Development Program of China (2016YFB0601303), the Na-
tional Natural Science Foundation of China (51374193, 21676278), Key
Program of National Natural Science Foundation of China (9143420), and
Chinese Academy of Sciences, State Administration of Foreign Experts
Affairs (CAS/SAFEA) International Partnership Program for Creative Re-
search Teams (20140491518). Latif Ullah acknowledged the Chinese
Academy of Sciences (CAS) and the World Academy of Sciences (TWAS)
for providing with the opportunity to pursue research work under the um-
brella of CAS-TWAS Presidents’ Fellowship.
Figure 11 Catalyst re-usability for 28.2 wt% HPW@Zr-BTC (Reaction
conditions: anisole to BoC molar ratio 1:2 moles, temperature 120 °C, time
6 h) (color online).
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available
online at http://chem.scichina.com and http://link.springer.com/
journal/11426. The supporting materials are published as submitted,
without typesetting or editing. The responsibility for scientific ac-
curacy and content remains entirely with the authors.
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