Alkylation of toluene with tert-butyl alcohol over HPW-modified HΒ zeolite

Article abstract of DOI:10.1016/S1872-2067(16)62587-1

An Hβ-supported heteropoly acid (H₃PW₁₂O₄₀(HPW)/Hβ) catalyst was successfully prepared by wetness impregnation, and investigated in the alkylation of toluene with tert-butyl alcohol for the synthesis of 4-tert-butyltoluene (PTBT). X-ray diffraction, scanning electron microscopy, transmission electron microscopy, fourier-transform infrared spectroscopy, inductively coupled plasma-optical emission spectrometry, the brunauer emmett teller (BET) method, temperature-programmed NH₃desorption, and pyridine adsorption infrared spectroscopy were used to characterize the catalyst. The results showed that loading HPW on Hβ effectively increased the B acidity and decreased the pore size of Hβ. The B acidity of HPW/Hβ was 142.97 μmol/g, which is 69.74% higher than that of Hβ (84.23 μmol/g). The catalytic activity of the HPW/Hβ catalyst was much better than that of the parent Hβ zeolite because of its high B acidity. The toluene conversion over HPW/Hβ reached 73.1%, which is much higher than that achieved with Hβ (54.0%). When HPW was loaded on Hβ, the BET surface area of Hβ decreased from 492.5 to 379.6 m²/g, accompanied by a significant decrease in the pore size from 3.90 to 3.17 nm. Shape selectivity can therefore play an important role and increase the product selectivity of the HPW/Hβ catalyst compared with that of the parent Hβ. PTBT (kinetic diameter 0.58 nm) can easily diffuse through the narrowed pores of HPW/Hβ, but 3-tert-butyltoluene (kinetic diameter 0.65 nm) diffusion is restricted because of steric hindrance in these narrow pores. This results in high PTBT selectivity over HPW/Hβ (around 81%). The HPW/Hβ catalyst gave a stable catalytic performance in reusability tests.

Full text of DOI:10.1016/S1872-2067(16)62587-1

Chinese Journal of Catalysis 37 (2016) 2134–2141
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Alkylation of toluene with tert‐butyl alcohol over HPW‐modified
Hβ zeolite
Yuanyuan Wanga, Hua Songa,*, Xinglong Sunb
a Northeastern Petroleum University, Chemistry and Chemical Engineering, Daqing 163318, Heilongjiang, China
b Daqing Petrochemical Engineering Co., Ltd., Daqing 163714, Heilongjiang, China
A R T I C L E I N F O
A B S T R A C T
Article history:
An Hβ‐supported heteropoly acid (H3PW12O40 (HPW)/Hβ) catalyst was successfully prepared by
wetness impregnation, and investigated in the alkylation of toluene with tert‐butyl alcohol for the
synthesis of 4‐tert‐butyltoluene (PTBT). X‐ray diffraction, scanning electron microscopy, transmis‐
sion electron microscopy, fourier‐transform infrared spectroscopy, inductively coupled plas‐
ma‐optical emission spectrometry, the brunauer emmett teller (BET) method, tempera‐
ture‐programmed NH3 desorption, and pyridine adsorption infrared spectroscopy were used to
characterize the catalyst. The results showed that loading HPW on Hβ effectively increased the B
acidity and decreased the pore size of Hβ. The B acidity of HPW/Hβ was 142.97 μmol/g, which is
69.74% higher than that of Hβ (84.23 μmol/g). The catalytic activity of the HPW/Hβ catalyst was
much better than that of the parent Hβ zeolite because of its high B acidity. The toluene conversion
over HPW/Hβ reached 73.1%, which is much higher than that achieved with Hβ (54.0%). When
HPW was loaded on Hβ, the BET surface area of Hβ decreased from 492.5 to 379.6 m2/g, accompa‐
nied by a significant decrease in the pore size from 3.90 to 3.17 nm. Shape selectivity can therefore
play an important role and increase the product selectivity of the HPW/Hβ catalyst compared with
that of the parent Hβ. PTBT (kinetic diameter 0.58 nm) can easily diffuse through the narrowed
pores of HPW/Hβ, but 3‐tert‐butyltoluene (kinetic diameter 0.65 nm) diffusion is restricted because
of steric hindrance in these narrow pores. This results in high PTBT selectivity over HPW/Hβ
(around 81%). The HPW/Hβ catalyst gave a stable catalytic performance in reusability tests.
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Received 29 September 2016
Accepted 27 October 2016
Published 5 December 2016
Keywords:
Alkylation
Toluene
tert‐Butyl alcohol
H3PW12O40
B acidity
Hβ zeolite
Published by Elsevier B.V. All rights reserved.
1. Introduction
devoted to development of environmentally friendly solid acid
catalysts [4,5].
4‐tert‐Butyltoluene (PTBT) is commercially important be‐
cause its derivatives, such as 4‐tert‐butylbenzoic acid and
4‐tert‐butylbenzaldehyde, are used in the fine chemical and
petrochemical industries [1–3]. Conventionally, toluene butyla‐
tion reactions are conducted over catalysts such as AlCl3, sulfu‐
ric and phosphoric acids, and boron trifluoride. However, these
acid catalysts are not environmentally benign or reusable and
corrode equipment. Considerable attention has therefore been
Zeolites are regarded as a good choice because of their
unique pore structures and acidic properties. Zeolites have
been investigated for toluene alkylation with tert‐butyl alcohol
(TBA) [4,6–10]. Zeolite β, which has a three‐dimensional inter‐
connecting pore system and high acidity, shows high catalytic
activity. Pai et al.[4] reported that the catalytic activities of Hβ
(Si/Al = 25) and HY (Si/Al = 30) were better than that of
HMCM‐22 (Si/Al = 52) in vapor‐phase butylation of toluene.
* Corresponding author. Tel: +86‐459‐6503167; Fax: +86‐459‐6506498; E‐mail: songhua2004@sina.com
DOI: 10.1016/S1872‐2067(16)62587‐1 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 12, December 2016

Yuanyuan Wang et al. / Chinese Journal of Catalysis 37 (2016) 2134–2141
2135
Mravec et al. [9] reported that the most active zeolite catalysts
2.1. Catalyst preparation
for liquid‐phase butylation of toluene were Hβ (Si/Al = 12.5)
and HM (Si/Al = 17.5). In our previous study [11], we found
that Hβ showed high catalytic activity in toluene alkylation with
TBA. However, the para selectivity, i.e., for PTBT, was 69.6%
and the meta selectivity, i.e., for 3‐tert‐butyltoluene (MTBT),
was 26.9% over Hβ. The selectivity for the desired product, i.e.,
PTBT, was therefore low. The dimensions of the 12‐ring portals
of the parent Hβ are 0.67 nm × 0.66 nm [12], and the kinetic
diameters of PTBT and MTBT are 0.58 and 0.65 nm [9], respec‐
tively. The shape selectivity has little effect on the PTBT selec‐
tivity over Hβ. However, the shape selectivity can be greatly
improved by slightly narrowing the 12‐ring portals of the par‐
ent Hβ. Modification with a metal oxide is a highly efficient
method for improving the para selectivity by reducing the
number of external acid sites and adjusting the pore entrance
size. Kostrab et al. [5] reported that cerium‐modified HM
maintained a constant high PTBT selectivity (near 90%) but
decreased the catalytic activity, i.e., improving the para selec‐
tivity by metal oxide modification usually decreases the cata‐
lytic activity. New and efficient catalysts for PTBT synthesis by
toluene alkylation with TBA that enhance the para selectivity
but also maintain or increase the catalytic activity are therefore
needed.
Zeolite‐catalyzed toluene alkylation with alcohols is gener‐
ally considered to proceed via carbonium ion mechanisms [13]
and the active sites are B acid sites [6–14]. Because of its strong
B acidity, H3PW12O40 (HPW) is a promising catalyst, with high
activity in toluene alkylation with alcohols. The specific surface
area of HPW is low (close to 5 m2/g), therefore many attempts
have been made to support HPW using various acidic or neu‐
tral solid materials [15–19]. Zeolite β is a promising support for
acid‐catalyzed alkylation reactions because of its high specific
surface area, high thermal stability, and hardness.
HPW loading is expected to improve the para selectivity
because HPW loading narrows the pores and improves the
shape selectivity. HPW loading is also expected to increase the
catalytic activity because of the increased number of B acid
sites. To the best of our knowledge, there are no reports of tol‐
uene alkylation with TBA catalyzed by Hβ‐supported HPW. It is
therefore of interest to investigate the catalytic activities of
HPW‐modified Hβ catalysts for toluene alkylation with TBA.
The aim of the present study was to evaluate the catalytic
behavior of HPW/Hβ in toluene alkylation with TBA under
liquid reaction conditions. HPW/Hβ was obtained using a wet‐
ness impregnation method and fully characterized using X‐ray
diffraction (XRD), scanning electron microscopy (SEM), trans‐
mission electron microscopy (TEM), fourier transform infrared
spectroscopy (FT‐IR), inductively coupled plasma‐optical emis‐
sion spectrometry (ICP‐OES), the brunauer emmett teller (BET)
method, temperature‐programmed NH3 desorption (NH3‐TPD),
and pyridine adsorption FT‐IR (Py‐IR) spectroscopy. Our ob‐
jective was to correlate the channel structures and acidity of
the HPW/Hβ catalyst with its catalytic properties in toluene
alkylation.
The starting material was Naβ zeolite (Zeolyst Internation‐
al). Hβ powder was prepared by impregnation of Naβ with
NH4NO3 (Sigma Aldrich, reagent grade) aqueous solution using
a reported procedure [20]. After impregnation, the zeolite was
filtered, dried at 120 °C for 12 h, and calcined at 450 °C for 4 h
to give Hβ. H3PW12O40·xH2O (HPW, Sigma‐Aldrich, reagent
grade) was dried at 200 °C in a furnace for 24 h to remove wa‐
ter. HPW‐modified Hβ was prepared by wetness impregnation
with an ethanol solution of the desired amount of HPW. The
product was aged at ambient temperature for 8 h, dried at 60
°C overnight, and calcined at 450 °C for 4 h. The prepared cata‐
lyst is denoted by HPW/Hβ.
2.2. Catalyst characterization
XRD analysis was performed using a D/max‐2200PC X‐ray
diffractometer with Cu Kα radiation at 40 kV and 30 mA, in the
scanning range 2θ = 10°–80° at a scanning rate of 4°/min.
The catalyst surface morphology was examined using SEM
(Zeiss‐ΣIGMA).
TEM examinations were performed using a JEM‐1010 in‐
strument (JEOL). Samples were dispersed in ethanol and placed
on a carbon grid before TEM examinations.
IR spectra were recorded using a Bruker FT‐IR spectrome‐
ter (Tensor 27) in the wavenumber range 4000–400 cm−1 at a 4
cm−1 resolution. KBr pellets containing 5% of powder sample
were used.
The HPW content of the catalyst was determined using
ICP‐OES (Perkin Elmer Optima 2000DV instrument).
The physicochemical properties of the supports and cata‐
lysts were investigated by the BET method using a NOVA2000e
instrument (Micromeritics). All the samples were outgassed at
200 °C until the vacuum pressure was 6 mm Hg. N2 adsorption
isotherms were recorded at −196 °C.
The catalyst acidity was measured using NH3‐TPD
(Quantachrome Chembet‐3000 characterization system). A
sample (200 mg) was pretreated at 550 °C for 1 h in dry He (50
mL/min) and cooled to 120 °C. It was then exposed to 10%
NH3/90% He for 1 h. After purging the catalyst with He for 1 h,
a TPD plot was obtained at a heating rate of 10 °C/min from
120 to 600 °C.
The Py‐IR spectra were recorded using a Bruker FT‐IR
spectrometer (Tensor 27) and a high‐temperature vacuum cell.
The sample powder was pressed into a self‐supporting wafer,
and the spectra were recorded in the wavenumber range
4000–400 cm−1 at a 4 cm−1 resolution. Before each experiment,
the sample was pressed into thin pellets (10–30 mg) of diame‐
ter 16 mm and activated in situ under vacuum (10−5 Pa) at 170
°C overnight. Excess pyridine was introduced at 150 °C after
activation. The concentrations of B sites and L sites were de‐
termined from the integrated area bands of PyH+ (around 1540
cm−1) and PyL (around 1450 cm−1) species using the molar
extinction coefficients of the bands.
2. Experimental
2.3. Catalytic tests

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Yuanyuan Wang et al. / Chinese Journal of Catalysis 37 (2016) 2134–2141
Table 1
Catalytic results for toluene alkylation with TBA over various catalysts.
Distribution of products (%)
Catalyst
Toluene conversion (%)
PTBT selectivity (%)
PTBT
37.6
26.0
28.9
3.2
MTBT
14.5
6.1
11.0
1.1
others
1.9
0.6
0.8
0.2
Hβ
HM
HY
HZSM‐5
HPW/Hβ
HPMo/Hβ
54.0
32.7
40.7
4.5
73.1
66.3
69.6
79.5
71.0
71.1
80.8
80.8
59.1
53.6
13.5
12.4
0.5
0.3
Reaction conditions: TBA/toluene molar ratio = 3/1, catalyst amount = 1.0 g, reaction temperature = 180 °C, reaction time = 4 h.
Side‐chain alkylation of toluene with TBA was performed in
a laboratory autoclave reactor (300 mL) at 180 °C. In a typical
experiment, toluene (10 mL, 94 mmol; 98%, Aladdin) and TBA
(28 mL, 283 mmol; 98%, Aladdin) were mixed with cyclohex‐
ane (60 mL, 99%, Aladdin) as a solvent. Then the zeolite cata‐
lyst (1.0 g) was added. The reaction system was sealed and
purged continuously with N2 for 30 min. The mixture was
heated to a certain temperature to start the alkylation reaction.
After the reaction, the mixture was cooled to room tempera‐
ture, and the zeolite catalyst was removed by filtration. The
liquid phase was analyzed using a GC‐14C gas chromatograph
equipped with an SE‐30 capillary column ( 0.25 mm × 50 m).
The analysis was performed at an initial temperature of 60 °C
for 2 min, a ramping rate of 10 °C/min, and a final temperature
of 220 °C, which was held for 10 min, with N2 as the carrier gas.
A number of runs were performed to check the reproducibility
of the experimental results; it was found to be excellent. Typical
errors were in the range ±2%.
54.0% and the selectivity for PTBT was just 69.6% over Hβ
zeolite, i.e., lower selectivity for the desired product, PTBT.
Based on the potential benefits of heteropoly acid modification,
heteropoly‐acid‐modified Hβ zeolites were then investigated in
this reaction (Table 1). As expected, the catalytic performances
of these modified Hβ zeolites were better than that of Hβ zeo‐
lite, especially in the case of HPW/Hβ. The toluene conversion
and selectivity for PTBT over HPW/Hβ were 73.1% and 80.8%
respectively, which are both better than the results obtained
using Hβ zeolite. HPW/Hβ therefore had the highest activity
and was chosen as the catalyst for further investigation.
The XRD patterns of the Hβ and HPW/Hβ samples are
shown in Fig. 1. The main diffraction peak from the Hβ zeolite
was observed at 2θ = 22.5°. No additional diffraction peaks
were observed in the XRD pattern of HPW/Hβ, indicating that
HPW was well dispersed on the Hβ zeolite surface and no ob‐
vious structural changes were caused by HPW loading. Howev‐
er, a slight decrease in the diffraction peak intensity was ob‐
served after HPW loading. This indicates that some loss of
crystallinity occurred after HPW loading by impregnation and
heat treatment. These processes would also cause collapse of
the Hβ framework, which is another reason for the decreased
intensity.
3. Results and discussion
3.1. Catalyst selection and characterization
Toluene is not active enough to undergo Friedel‐Crafts al‐
kylation in the absence of a catalyst. The selection of a suitable
catalyst is therefore crucial for highly efficient alkylation of
toluene with TBA. Initially, unmodified zeolites, i.e., Hβ, HM, HY,
and HZSM‐5, were used as catalysts for toluene butylation. The
results are listed in Table 1. Two monoalkylated products were
detected, namely a large amount of PTBT (the para isomer) and
MTBT (the meta isomer). However, no 2‐tert‐butyltoluene
(OTBT, the ortho isomer) was detected. PTBT is the major
product because steric hindrance of the methyl group and the
large tert‐butyl group favor formation of the para isomer. It is
also favored by shape selectivity because of the regular channel
structures of zeolite catalysts. The formation of OTBT is hin‐
dered by the methyl group ortho to the tert‐butyl group. A trace
amount of the dialkylated product, 3,5‐di‐tert‐butyltoluene,
formed by further alkylation of the monoalkylated products
PTBT and MTBT, was also detected. This can also be explained
by the steric effects of all the alkyl groups being in meta posi‐
tions.
The SEM images of the Hβ and HPW/Hβ samples in Fig. 2
show that the outer surface of HPW/Hβ was essentially un‐
changed compared with that of the Hβ zeolite. The HPW/Hβ
crystal size was around 200–300 nm.
The morphologies of the Hβ and HPW/Hβ samples were
HPW/H
H
10
20
30
40
50
2/(o)
It is clear that the Hβ zeolite was the most effective catalyst
(Table 1), this can be ascribed to the special structure of the Hβ
zeolite and its higher acidity [11]. The toluene conversion was
Fig. 1. XRD patterns of parent Hβ and HPW/Hβ catalyst.

Yuanyuan Wang et al. / Chinese Journal of Catalysis 37 (2016) 2134–2141
2137
Hβ
HPW/Hβ
500 nm
500 nm
Fig. 2. SEM images of Hβ and HPW/Hβ
HPW/Hβ
Hβ
10 nm
10 nm
Fig. 3. TEM images of Hβ and HPW/Hβ.
consistent in the TEM results (Fig. 3). TEM showed that the Hβ
zeolite had a honeycomb structure. In the HPW/Hβ catalyst,
HPW was uniformly dispersed on Hβ, and can be distinguished
as dark dots in the TEM images.
were made by Kumar et al. [25]. The dimensions of the 12‐ring
portals of the parent Hβ are 0.67 nm × 0.66 nm [12], and the
kinetic diameters of PTBT and MTBT are 0.58 and 0.65 nm,
respectively [9]. Shape selectivity therefore has little effect on
PTBT selectivity over the Hβ zeolite; the PTBT selectivity over
the parent Hβ was 69.6%, which is low. However, HPW loading
narrows the pore entrances, as confirmed by the decrease in
the average pore size from 3.90 to 3.17 nm (Table 2). Shape
selectivity could therefore play a more important role in the
product selectivity over the HPW/Hβ catalyst than in the case
Fig. 4 shows the FT‐IR spectra of HPW, HPW/Hβ, and the
parent Hβ. In the HPW spectrum, the band at 1082 cm−1 can be
assigned to PO, the broad band at 983 cm−1 can be assigned to
W =O, the band at 896 cm–1 can be assigned to OWO (cor‐
ner‐sharing oxygen), and the very broad band at 809 cm−1 can
be assigned to WOW (edge‐sharing oxygen) [21–22]. The
spectrum of HPW/Hβ showed a large band at 809 cm−1 and a
broad band at 1082 cm−1, attributed to overlapping of the
bands from HPW and the Hβ zeolite at the corresponding posi‐
tions. In addition, small bands at 983 and 896 cm–1 can be ob‐
served in the HPW/Hβ spectrum. These results confirm that
HPW was successfully supported on the Hβ zeolite and there
were strong bonding interactions between HPW and Hβ [23].
The loaded amount of HPW in HPW/Hβ, determined using
ICP‐OES, was 29.6%, which is consistent with the theoretical
amount (30.0%) (Table 2). The textural properties of the par‐
ent Hβ and HPW/Hβ are also listed in Table 2. HPW loading
decreased the BET surface area of Hβ from 492.5 to 379.6
m2/g, the pore volume decreased from 0.48 to 0.29 cm3/g, and
the pore size decreased from 3.90 to 3.17 nm. These effects are
attributed to the HPW entering the Hβ pores during the modi‐
fication process [24]. The decrease in the pore size after HPW
loading improves the para selectivity. Similar observations
HPW
1082
896
983
809
HPW/Hβ
Hβ
1400
1200
1000
800
600
400
Wavenumber (cm-1)
Fig. 4. FT‐IR spectra of HPW, HPW/Hβ, and Hβ.
Table 2
Physicochemical and acidic properties of parent Hβ and HPW/Hβ.
Content of HPW
Specific surface area Pore volume
Pore size
(nm)
3.90
B acidity
(μmol/g)
84.23
L acidity
(μmol/g)
47.17
Total acidity
(μmol/g)
131.40
B/L
ratio
1.79
4.14
Sample
(%)
—
29.6
(m2/g)
492.5
379.6
(cm3/g)
0.48
Hβ
HPW/Hβ
0.29
3.17
142.97
34.55
177.52

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Yuanyuan Wang et al. / Chinese Journal of Catalysis 37 (2016) 2134–2141
90
80
90
80
70
60
50
40
0.8
0.6
0.4
0.2
0.0
70
HPW/H
60
50
40
H
0.50
0.75
1.00
1.25
1.50
1400
1450
1500
1550
1600
Amount of catalyst (g)
Wavenumber (cm-1)
Fig. 7. Effects of catalyst amount on alkylation.
Fig. 5. Py‐IR spectra of Hβ and HPW/Hβ.
90
80
70
60
50
40
30
20
85
80
75
70
65
60
55
50
HPW/H
H
1
2
3
4
5
TBA/Toluene molar ratio
120 180 240 300 360 420 480 540 600
Temperature (C)
Fig. 8. Effects of TBA/toluene molar ratio on alkylation.
Fig. 6. NH3‐TPD curves of parent Hβ and HPW/Hβ.
of the parent Hβ. PTBT can easily diffuse from the narrow
pores of HPW/Hβ, but MTBT diffusion is restricted because of
steric hindrance in the narrow pores of HPW/Hβ. The PTBT
selectivity over HPW/Hβ (around 81.0%) was therefore higher
than that over Hβ (69.6%).
μmol/g). The larger number of B active sites on HPW/Hβ in‐
creased the catalytic activity. The toluene conversion over
HPW/Hβ reached 73.1%, which is much higher than that
achieved over Hβ (54.0%).
Fig. 6 shows that the NH3‐TPD curves of the parent Hβ and
HPW/Hβ contained two NH3 desorption peaks. The strong
peaks in the range 150400 °C are assigned to weak acid sites
and the peaks in the range 400600 °C are assigned to strong
acid sites [29–30]. Fig. 6 clearly shows that the total amount of
acid sites determined using NH3‐TPD correlates well with the
Py‐IR results; the amounts of weak and strong acid sites in
HPW/Hβ were both higher than in the parent Hβ.
The FT‐IR spectra of pyridine after desorption at 150 °C are
shown in Table 2 and Fig. 5. The strong absorption peak at
1450 cm–1 is attributed to L acid sites, and the peak at 1540
cm–1 is attributed to B acid sites [26–27]. A stronger absorption
peak, from B and L acid sites, was observed at 1490 cm–1 [28]. It
is clear that the number of L acid sites in HPW/Hβ decreased,
whereas the number of B acid sites significantly increased, after
HPW loading (see Table 2), leading to an increase in the total
number of acid sites and the B/L ratio. This is possibly because
HPW loading covers some of the acid sties on the Hβ zeolite.
However, new B acid sites are introduced by HPW loading, and
this compensates for coverage of the original B acid sites in the
parent Hβ by HPW [27]. A significant decrease in the number of
L acid sites was observed because of coverage of L acid sites by
HPW loading. Zeolite‐catalyzed alkylation of toluene with al‐
cohols is generally considered to proceed via carbonium ion
mechanisms [13] and the active sites are B acid sites [6,14]. The
data in Table 2 show that the B acidity of HPW/Hβ was 142.97
μmol/g, which is 69.74% higher than that of Hβ (84.23
3.2. Optimization of reaction conditions
3.2.1. Effects of catalyst amount
The effects of the catalyst amount on the toluene conversion
and PTBT selectivity were investigated in the range 0.51.5 g,
using HPW/Hβ as the catalyst (Fig. 7). The toluene conversion
initially increased and then decreased with increasing catalyst
amount. This is because of the increased number of acid sites.
Electrophilic aromatic substitution generally occurs via for‐
mation of a carbenium ion as the intermediate. An increase in
the catalyst amount increased the number of acid sites and the

Yuanyuan Wang et al. / Chinese Journal of Catalysis 37 (2016) 2134–2141
2139
150 to 180 °C, and the toluene conversion then decreased with
further temperature increases. Based on these results, we sug‐
gest that the numbers of activated toluene molecules and acti‐
vated tert‐butyl carbenium species increase with increasing
temperature, which results in increased toluene conversion.
However, the toluene conversion decreases above 180 °C. This
is because the decomposition rate of PTBT increases quickly as
the temperature increases. The selectivity for PTBT decreases
as the temperature increases. This is because a higher temper‐
ature favors secondary reactions, which are dominant at higher
reaction temperatures, therefore the toluene conversion and
PTBT selectivity decrease.
90
80
70
60
50
40
30
20
10
90
80
70
60
50
40
30
150
160
170
180
190
200
210
Reaction temperature (°C)
3.3. HPW/Hβ catalyst reusability tests
Fig. 9. Effects of reaction temperature on alkylation.
Reusability is an important factor for an industrial catalyst.
The reusability of HPW/Hβ was tested over four cycles; the
results are shown in Table 3. After four cycles, the selectivity
for PTBT was unchanged. However, the toluene conversion
decreased from 73.1% to 59.1%, and the PTBT yield decreased
from 59.1% to 48.0%. Drops of 19.2% for toluene conversion
and 18.8% for the PTBT yield were observed compared with
those in the first cycle. HPW/Hβ gave a relatively stable cata‐
lytic performance. HPW leaching into the reaction mixture and
deactivation of active sites during the reaction were responsi‐
ble for the decreases in the catalytic performance of HPW/Hβ.
surface area, leading to the formation of more carbenium ions
per unit of time, which directly increased the reaction rate.
However, more acid sites on the zeolite surface accelerated the
subsidiary reactions (i.e., isomerization and oligomerization),
which resulted in decreases in the toluene conversion and
PTBT selectivity.
3.2.2. Effects of TBA:toluene molar ratio
The effects of the TBA/toluene molar ratio on the toluene
conversion and PTBT selectivity were studied by varying the
molar ratios from 1/1 to 5/1 over the HPW/Hβ catalyst at 180
°C; the results are shown in Fig. 8. When the TBA/toluene mo‐
lar ratio was increased from 1 to 3, the toluene conversion
greatly increased, from 46.5% to 73.1%. When the ratio was
increased to 5, the conversion decreased to 30.0%. The reason
for this is that higher TBA concentrations increased oligomeri‐
zation of isobutylene (formed by dehydration of TBA), and the
oligomerization reaction competed with the main alkylation
reaction. The best PTBT selectivity was obtained at a
TBA/toluene molar ratio of 3/1, showing that a small excess of
TBA increased the para selectivity. Similar results were re‐
ported by Kostrab et al. [5]. They suggested that a small excess
of TBA was suppressed dealkylation (cleavage of the tert‐butyl
group from the aromatic ring).
4. Conclusions
A novel synthesis of PTBT via Friedel Crafts alkylation of
toluene with TBA using HPW/Hβ as the catalyst was developed.
The catalytic activity and product selectivity of HPW/Hβ were
better than those of the other zeolites tested in this study. The
results show that loading HPW on Hβ effectively increased the
B acidity and decreased the pore size of Hβ. The B acidity of
HPW/Hβ was 142.97 μmol/g, an increase of 69.74% compared
with that of Hβ (84.23 μmol/g). The catalytic activity of
HPW/Hβ was better than that of the parent Hβ zeolite because
its high B acidity. The toluene conversion over HPW/Hβ
reached 73.1%, which is much higher than that over Hβ
(54.0%). HPW loading on Hβ decreased the Hβ BET surface
area from 492.5 to 379.6 m2
/g, accompanied by a significant
3.2.3. Effects of reaction temperature
decrease in the pore size, from 3.90 to 3.17 nm. Shape selectiv‐
ity therefore played a more important role in the product selec‐
tivity of the HPW/Hβ catalyst than in that of the parent Hβ.
PTBT (kinetic diameter 0.58 nm) can easily diffuse through the
narrowed pores of HPW/Hβ, but the diffusion of MTBT (kinetic
diameter 0.65 nm) is restricted because of steric hindrance in
these narrowed pores. This improved the PTBT selectivity over
HPW/Hβ (around 81.0%). HPW/Hβ gave a relatively stable
catalytic performance in reusability tests.
The effects of reaction temperature on PTBT synthesis are
summarized in Fig. 9. The toluene conversion clearly increased
sharply when the reaction temperature was increased from
Table 3
Reusability testing of HPW/Hβ catalyst.
Times of
repeated use
Conversion of
toluene (%)
73.1
Selectivity to Yield of PTBT
PTBT (%)
80.8
(%)
59.1
55.9
52.2
51.6
48.0
0
1
2
3
4
69.4
64.3
64.0
59.1
80.5
81.2
80.7
81.3
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2140
Yuanyuan Wang et al. / Chinese Journal of Catalysis 37 (2016) 2134–2141
Graphical Abstract
Chin. J. Catal., 2016, 37: 2134–2141 doi: 10.1016/S1872‐2067(16)62587‐1
Alkylation of toluene with tert‐butyl alcohol over
HPW‐modified Hβ zeolite
Yuanyuan Wang, Hua Song *, Xinglong Sun
Northeastern Petroleum University; Daqing Petrochemical
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The improved catalytic activity over HPW/Hβ can be attributed to
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