Friedel-Crafts acylation of anisole with hexanoic acid catalyzed by Hβ zeolite-supported tungstophosphoric acid

Article abstract of DOI:10.1007/s11164-014-1586-9

Friedel-Crafts acylation of anisole with hexanoic acid was studied and an efficient method was established for the green synthesis of 4-methoxy phenyl hexyl ketone over an Hβ zeolite-supported tungstophosphoric acid catalyst (HPW/Hβ). The conversion of hexanoic acid was 100 % and the yield of 4-methoxy phenyl hexyl ketone reached 89.2 % under the optimized reaction conditions over HPW/Hβ, due to the synergistic effect between HPW and Hβ zeolite. Inductively coupled plasma results indicated the dealumination of the Hβ zeolite due to the treatment of tungstophosphoric acid (HPW), offering more active centers in HPW/Hβ. X-ray diffraction and Brunauer-Emmett-Teller results demonstrated that HPW was highly dispersed on the surface and in the larger pores of the Hβ zeolite. NH₃ temperature-programmed desorption results showed HPW/Hβ has larger amounts of strongly acidic sites than the Hβ zeolite, accounting for its good catalytic performance. Furthermore, this catalyst can be recycled for four times with the yield of 4-methoxy phenyl hexyl ketone maintained at roughly 70 %, demonstrating its relatively good stability.

Full text of DOI:10.1007/s11164-014-1586-9

Res Chem Intermed
DOI 10.1007/s11164-014-1586-9
Friedel–Crafts acylation of anisole with hexanoic acid
catalyzed by Hb zeolite-supported tungstophosphoric
acid
Guoyi Bai
Huixian Dong
•
Huanhuan Zhang
Jie Han
•
Tianyu Li
•
•
Received: 29 November 2013 / Accepted: 5 March 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Friedel–Crafts acylation of anisole with hexanoic acid was studied and
an efficient method was established for the green synthesis of 4-methoxy phenyl
hexyl ketone over an Hb zeolite-supported tungstophosphoric acid catalyst (HPW/
Hb). The conversion of hexanoic acid was 100 % and the yield of 4-methoxy phenyl
hexyl ketone reached 89.2 % under the optimized reaction conditions over HPW/
Hb, due to the synergistic effect between HPW and Hb zeolite. Inductively coupled
plasma results indicated the dealumination of the Hb zeolite due to the treatment of
tungstophosphoric acid (HPW), offering more active centers in HPW/Hb. X-ray
diffraction and Brunauer–Emmett–Teller results demonstrated that HPW was highly
dispersed on the surface and in the larger pores of the Hb zeolite. NH temperature-
3
programmed desorption results showed HPW/Hb has larger amounts of strongly
acidic sites than the Hb zeolite, accounting for its good catalytic performance.
Furthermore, this catalyst can be recycled for four times with the yield of 4-methoxy
phenyl hexyl ketone maintained at roughly 70 %, demonstrating its relatively good
stability.
Keywords Friedel–Crafts acylation Á Green synthesis Á Carboxylic
acid Á Tungstophosphoric acid Á Hb zeolite
Introduction
Aromatic ketones are important intermediates or products in the field of
pharmaceuticals, fragrance and flavor materials, dyes, and agrochemicals [1–3].
G. Bai (&) Á H. Zhang Á T. Li Á H. Dong Á J. Han
Key Laboratory of Chemical Biology of Hebei Province, College of Chemistry and Environmental
Science, Hebei University, No.180 Wusi East Road, Baoding City 071002, Hebei, People’s
Republic of China
e-mail: baiguoyi@hotmail.com
123

G. Bai et al.
They are usually obtained from Friedel–Crafts acylation of aromatic compounds
with various acyl chlorides or anhydrides [4] in the presence of either Lewis or
protic acids, such as H SO , ZnCl , Sc(OTf) , AlCl , and BF [5–9]. For example,
2
4
2
3
3
3
Ottoni et al. [10] have investigated the reaction of indole with different acylating
agents in the presence of different Lewis acids, and found that 3-acylindole can be
obtained in 95 % yield over SnCl at 0 °C in 2 h. Although these traditional
4
methods are usually promising, many of them still suffer from one or more
disadvantages like the use of environmentally toxic reagents, excess of catalysts,
and complicated manipulations. Therefore, in view of the increasing demand for
environmental protection, there is still a need for an ecofriendly and efficient
catalyst for the synthesis of aromatic ketones using carboxylic acids [11–13] as a
green reagent, instead of the traditional acyl chlorides and anhydrides.
Heteropoly acids (HPAs) can catalyze a wide variety of reactions due to their strong
Br o¨ nsted acidity, whereas their high solubility in polar solvents, low thermal stability,
and small surface area limit their further applications [14]. The use of a suitable
support, such as clays [15], SiO [16], ZrO , TiO [17], and carbon [18], can overcome
2
2
2
the above limitations, thus accounting for a significant increase in the surface area with
a possible increase in thermal stability. For example, Rostami et al. [19] have
developed a simple and efficient method for the synthesis of indenone derivatives via
intramolecular Friedel–Crafts reaction of azlactones catalyzed by an alumina-
supported tungstophosphoric acid (HPW). However, there are scant reports on the
Friedel–Crafts acylation with carboxylic acids catalyzed by zeolite-supported HPAs.
In a continuation of our previous work [20, 21], we here report our recent
research into the Friedel–Crafts acylation over zeolite-supported HPAs with the
acylation of anisole with hexanoic acid to 4-methoxy phenyl hexyl ketone being
chosen as a model system. An Hb zeolite-supported tungstophosphoric acid catalyst
(HPW/Hb) shows the best catalytic performance in this reaction. The activity and
stability of the catalysts were investigated and agreed well with their structures.
Experimental
Catalyst preparation
Unless otherwise stated, all reagents were purchased from Baoding Huaxin Reagent
and Apparatus Co., Ltd., and were used as received without further purification. HPW/
Hb was prepared by adding 1.4 g of Hb zeolite (Tianjin Chemist Catalyst) to 20 mL of
an aqueous solution of 0.03 g/mL HPW and then impregnated at room temperature for
4
for 4 h. Moreover, MCM-41- and TiO -supported HPW catalysts were prepared using
h. After that, the sample was filtrated, dried at 110 °C, and then calcined at 250 °C
2
the same method and are labeled as HPW/MCM-41 and HPW/TiO , respectively.
2
Catalyst characterization
Bulk compositions were identified by inductively coupled plasma analysis (ICP) on
a Thermo Vista-MPX spectrometer. X-ray diffraction (XRD) patterns were recorded
1
23

Hb zeolite-supported tungstophosphoric acid
OCH3
O
O
catalysts
OH
CH3O
Scheme 1 Friedel–Crafts acylation of anisole with hexanoic acid
on a Bruker D8 advance diffractometer with Cu Ka radiation. The Brunauer–
Emmett–Teller (BET) surface area, pore volume, and pore size were measured on a
Micromeritics Tristar II 3020 surface area and pore analyzer. NH temperature-
3
programmed desorption (NH -TPD) experiments were performed on a TP-5000
3
instrument with a thermal conductivity detector.
Catalyst activity test
Friedel–Crafts acylation of anisole with hexanoic acid, as shown in Scheme 1, was
carried out in a 25-mL round-bottom flask equipped with a condenser. In a typical
run, hexanoic acid (0.23 g, 2 mmol), anisole (2.16 g, 20 mmol), and catalyst (0.3 g)
were charged into the flask and stirred at 180 °C for 5 h. Reaction mixtures were
analyzed by gas chromatography using a 30-m SE-30 capillary column and the
1
identities of the products were confirmed by GC–MS (Agilent 5975C) and HNMR
(
Bruker Avance III 600 MHz). The used HPW/Hb was filtrated, washed with
anisole, dried at 110 °C, and further calcined at 250 °C for recycling.
Results and discussion
Catalysts selection
Considering that hexanoic acid is not active enough in the Friedel–Crafts acylation,
selection of a suitable catalyst is crucial to the efficient synthesis of 4-methoxy
phenyl hexyl ketone. Thus, some commercial HPAs, including HPW, silicotungstic
acid (HSiW), and phosphomolybdic acid (HPMo), were first examined (Table 1,
entries 1–3). It is clear that HPW was the most efficient catalyst among them;
although still lower than our expectations. Considering the benefit of support, a
series of supported HPW catalysts (30 wt% HPW) were prepared and tested
(
Table 1, entries 4–6). As envisaged, these supported HPW catalysts showed better
catalytic performance than the pure HPW, especially in the case of HPW/Hb. The
conversion of hexanoic acid and the selectivity for 4-methoxy phenyl hexyl ketone
were 82.2 and 95.4 %, respectively, over HPW/Hb, better than both the HPW
(
88.2 % conversion and 55.4 % selectivity) and the Hb zeolite (51.5 % conversion
and 90.5 % selectivity). 2-Methoxy phenyl hexyl ketone and phenyl hexanoate were
detected as the two main byproducts and in agreement with the literature [11, 22].
Furthermore, the conversion of hexanoic acid can reach 100 % while increasing the
reaction temperature from 154 to 180 °C, together with only a slight decrease on the
123

G. Bai et al.
Table 1 Catalytic results for the acylation of anisole with hexanoic acid over different catalysts
Catalyst
Catalyst
amount (g)
Conversion of
hexanoic acid (%)
Selectivity for 4-methoxy
phenyl hexyl ketone (%)
HPW
0.1
0.1
0.1
0.3
0.3
0.3
0.2
0.3
0.3
0.3
88.2
63.2
4.9
55.4
71.2
67.2
95.4
93.1
69.6
90.5
89.2
15.0
87.8
HSiW
HPMo
HPW/Hb
HPW/MCM-41
82.2
56.8
85.9
51.5
100
HPW/TiO
2
Hb zeolite
a
HPW/Hb
a
HPW
100
a
Hb zeolite
61.9
Reaction conditions: anisole-to-hexanoic acid molar ratio: 10:1, reaction temperature: 154 °C, reaction
time: 5 h
a
The reaction was performed at 180 °C
Table 2 Effect of HPW loading amount and calcination temperature of HPW/Hb on the acylation of
anisole with hexanoic acid
HPW loading
amount (wt%)
Calcination
temperature (°C)
Conversion of
hexanoic acid (%)
Selectivity for 4-methoxy
phenyl hexyl ketone (%)
1
3
5
3
3
0
0
0
0
0
250
250
250
150
350
95.2
100
100
100
85.4
84.2
89.2
75.2
76.3
85.7
Reaction conditions: anisole-to-hexanoic acid molar ratio: 10:1, reaction temperature: 180 °C, reaction
time: 5 h, catalyst amount: 0.3 g
selectivity; but its yield was still much higher than those of HPW and Hb zeolite
under the same reaction conditions. Thus, there should be some synergistic effect
between HPW and Hb zeolite [23–25], accounting for the good catalytic
performance of HPW/Hb. Finally, HPW/Hb was chosen as the appropriate catalyst
for further investigation.
As is well known, the loading amount of the active species and the calcination
temperature are two key factors on the catalytic performance of the supported
catalysts [17]. Thus, the effect of the HPW loading amount was first investigated
and the results are listed in Table 2. As can be seen, the conversion of hexanoic acid
increased from 95.2 to 100 % with the increase of the HPW loading amount from 10
to 30 %, together with the increase of the selectivity for 4-methoxy phenyl hexyl
ketone, whereas further increase of the HPW loading amount to 50 % led to the
decrease of the selectivity, implying that the excess HPW might block the pores of
1
23

Hb zeolite-supported tungstophosphoric acid
the Hb zeolite. Thus, 30 % is believed as an appropriate HPW loading amount.
Subsequently, the effect of the calcination temperature was studied. It was found
that the selectivity for 4-methoxy phenyl hexyl ketone increase from 76.3 to 89.2 %
while the calcination temperature increased from 150 to 250 °C, both together with
the complete conversion of hexanoic acid. In contrast, the conversion of hexanoic
acid markedly decreased to 85.4 % while the calcination temperature further
increased to 350 °C, ascribed to the beginning of decomposition of the Keggin
structure of HPW, as also reported by Kaur [26].
Catalysts characterization
The compositions, BET surface areas, pore volumes, and pore sizes of HPW, Hb
zeolite, and HPW/Hb with different loading amounts were measured, and selected
results are listed in Table 3. It was found that the Si/Al molar ratio increased from
1
2 to 30 while 30 wt% HPW was supported on the Hb zeolite, indicating that the
dealumination of the Hb zeolite occurred during this process, and, in turn, offered
more active centers in HPW/Hb. It is clear that the HPW possesses a very low BET
surface area (0.436 m /g), as well as very poor pore volume (only 0.0028 cm /g),
2
3
demonstrating it as a nearly non-porous solid. In contrast, Hb zeolite shows the
2
3
highest BET surface area (381 m /g) and pore volume (0.25 cm /g) in the samples
studied. The average pore size of Hb zeolite is about 2.6 nm, demonstrating that it
has some larger pores, and making HPW with an average diameter of 1.2 nm [27]
accessible into its pores. Furthermore, the BET surface area, pore volume, and pore
size of the HPW/Hb catalysts decreased with the increase of the HPW loading
amount, demonstrating the occupying of the surface and some larger pores of the
Hb zeolite by HPW, whereas their BET surface area and pore volume are still much
higher than that of HPW.
XRD patterns of HPW, Hb zeolite, and HPW/Hb with different loading amounts
are shown in Fig. 1. The characteristic diffraction peaks of HPW/Hb were at about
7
.5° and 22.5°, just like the Hb zeolite, indicating that no obvious structural changes
occurred under the treatment of HPW [21]. In contrast, no characteristic diffraction
peaks related to the typical HPW was detected in the HPW/Hb samples even while
the HPW loading amount was up to 50 %. It demonstrated that HPW was well
dispersed on the surface and some larger pores of the Hb zeolite and no crystalline
Table 3 Textural properties of the catalysts
a
Catalyst
Bulk Si/Al
BET surface
2
area (m /g)
Pore volume
3
(cm /g)
Pore size (nm)
HPW
–
0.436
339
0.0028
0.22
–
10 wt% HPW/Hb
30 wt% HPW/Hb
50 wt% HPW/Hb
–
2.6
2.5
2.4
2.6
30
–
256
0.16
176
0.10
Hb zeolite
12
381
0.25
a
Calculated from the ICP results
123

G. Bai et al.
e
d
c
b
a
10
20
30
40
50
2
Theta (degree)
Fig. 1 XRD patterns of HPW, Hb zeolite, and HPW/Hb catalysts with different loading amounts.
a HPW, b Hb zeolite, c 10 wt% HPW/Hb, d 30 wt% HPW/Hb and e 50 wt% HPW/Hb
b
a
100
200
300
400
500
600
700
Temperature of desorption (C)
3
Fig. 2 NH -TPD profiles of a Hb zeolite and b 30 wt% HPW/Hb
HPW phase occurred in the samples [14], as also proved by the ICP result with
1
4.7 wt% W being detected in the 30 wt% HPW/Hb sample.
NH -TPD analyses were then carried out to determine the number and types of
acid sites on the surfaces of the Hb zeolite and 30 wt% HPW/Hb by the
3
measurement of the amounts of NH desorbed at different temperatures, as shown in
3
Fig. 2. There is only one large peak in the range of 150–500 °C with fluctuation
between 320 and 450 °C in the NH desorption profile of the Hb zeolite, whereas
3
1
23

Hb zeolite-supported tungstophosphoric acid
Table 4 Stability of the HPW/Hb catalyst
Catalyst
Conversion of
hexanoic acid (%)
Selectivity for 4-methoxy
phenyl hexyl ketone (%)
Yield of 4-methoxy phenyl
hexyl ketone (%)
Fresh
100
89.2
86.6
87.0
88.2
87.0
89.2
73.2
69.2
73.8
67.6
The first cycle
The second cycle
The third cycle
The fourth cycle
84.5
79.5
83.7
77.7
Reaction conditions: anisole-to-hexanoic acid molar ratio: 10:1; reaction temperature: 180 °C; reaction
time: 5 h; catalyst amount: 0.3 g
two peaks in the range of 150–400 and 400–650 °C were observed in HPW/Hb,
which are assigned to the weakly acidic sites and strongly acidic sites, respectively
[
28]. Compared with the Hb zeolite, the amount of weakly acidic sites in HPW/Hb
decreases to some extent, together with a marked shift to higher temperatures, but
the total amounts of acidity of the two samples are similar due to the existence of the
strongly acidic sites in HPW/Hb. Combining with the reaction results, the strongly
acidic sites are suggested to be favor of this transformation.
Stability test
Finally, the stability of HPW/Hb was tested under the optimized reaction conditions
and the results are shown in Table 4. It was found that the recycled catalysts showed
a relatively stable catalytic performance in the four subsequent runs with a yield of
4
-methoxy phenyl hexyl ketone maintaining at roughly 70 %, although a little lower
than the fresh one. Considering 0.83 wt% W being detected by ICP in the reaction
mixture catalyzed by the fresh HPW/Hb catalyst, the leaching of the active species
into the reaction mixture should result in the decrease of the initial activity.
Conclusions
In summary, an efficient method was established for the green synthesis of
4
-methoxy phenyl hexyl ketone via the Friedel–Crafts acylation of anisole with
hexanoic acid. An HPW/Hb catalyst exhibits better catalytic performance than
HPW and the Hb zeolite in this transformation, suggesting certain synergistic effect
between them. The highly dispersed HPW on the surface and in the larger pores of
the Hb zeolite and its large amounts of strongly acidic sites were responsible for the
good catalytic performance of HPW/Hb. Moreover, HPW/Hb can be recycled for
four times with the yield of 4-methoxy phenyl hexyl ketone maintaining at roughly
7
0 %, indicating its relatively good stability.
Acknowledgments Financial support by the National Natural Science Foundation of China (20806018
and 21376060) and the Natural Science Foundation of Hebei Province (B2014201024) are gratefully
acknowledged.
123

G. Bai et al.
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