Tert-butylation of toluene with tert-butanol over transition metal oxide-modified HBEA zeolite

Article abstract of DOI:10.3184/174751918X15220765507016

A series of transition metal oxide-modified HBEA (M_xO_y/HBEA) zeolite catalysts were prepared by the wetness impregnation method, and investigated for the alkylation of toluene with tert-butanol to synthesise 4-tert-butyltoluene (4-TBT). Their physico-chemical properties were characterised by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller and NH₃ temperature-programmed desorption methods. M_xO_y/HBEA zeolite showed higher para-selectivity than parent HBEA due to its improved structural and acidic properties. Though the toluene conversion of M_xO_y/HBEA catalyst decreased, the 4-TBT selectivity increased significantly at 190 °C after 4 h. The narrowed pores after loading the M_xO_y prompted an increase in selectivity for 4-TBT by increasing the shape selectivity. In addition, the decrease in strong acid sites increased selectivity for 4-TBT by suppressing further isomerisation of the 4-TBT formed on acid sites. Among all the modified HBEA zeolites, Fe₂O₃/HBEA exhibited the best catalytic activity and para-selectivity. The factors affecting the reaction over Fe₂O₃/HBEA have also been investigated extensively.

Full text of DOI:10.3184/174751918X15220765507016

160 RESEARCH PAPER
VOL. 42 MARCH, 160–165
JOURNAL OF CHEMICAL RESEARCH 2018
tert-Butylation of toluene with tert-butanol over transition metal oxide-
modified HBEA zeolite
Yixian Hana, Hua Songa,b*, Yuanyuan Wanga, Xinglong Sunc and Zhigang Sund
aCollege of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing, Heilongjing 163318, P.R. China
bProvincial Key Laboratory of Oil and Gas Chemical Technology, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing,
Heilongjing 163318, P.R. China
cDaqing Petrochemical Engineering Co., Ltd., Daqing, Heilongjiang 163714, P.R. China
dStorage and Transportation Sales Branch, Daqing Oil Field Company Ltd, Daqing, Heilongjiang 163111, P.R. China
A series of transition metal oxide-modified HBEA (MxO /HBEA) zeolite catalysts were prepared by the wetness impregnation method, and
investigated for the alkylation of toluene with tert-butyanol to synthesise 4-tert-butyltoluene (4-TBT). Their physico-chemical properties
were characterised by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller
and NH temperature-programmed desorption methods. MxOyꢀ/HBEA zeolite showed higher para-selectivity than parent HBEA due to
its impr3oved structural and acidic properties. Though the toluene conversion of M O /HBEA catalyst decreased, the 4-TBT selectivity
increased significantly at 190 °C after 4 h. The narrowed pores after loading the xMyxꢀOy prompted an increase in selectivity for 4-TBT
by increasing the shape selectivity. In addition, the decrease in strong acid sites increased selectivity for 4-TBT by suppressing further
isomerisation of the 4-TBT formed on acid sites. Among all the modified HBEA zeolites, Fe2O3/HBEA exhibited the best catalytic activity
and para-selectivity. The factors affecting the reaction over Fe2O3/HBEA have also been investigated extensively.
Keywords: alkylation, transition metal oxide, HBEA, modified, toluene, tert-butanol
It is well known that butylation of toluene with tert-butanol is an
industrially important reaction as the products obtained are used
directly or as chemical intermediates in fine chemical production
as well as organic synthesis.1–3 The butylation is generally catalysed
by homogeneous acids, such as sulfuric and phosphoric acids, and a
high reaction rate can be achieved.5,6 However, the disadvantages of
these homogeneous acid catalysts, such as severe pollution, severe
corrosion and difficult separation, restrict their use in applications.
Therefore, much attention has been paid over recent years to the
development of solid acid catalysts such as zeolites, which can be
easily removed from products and reused without neutralisation.7
Zeolite HBEA is a large pore zeolite with a complex structure.
Its three-dimensional pore network has tortuous channels with 12-T
rings that intersect with linear channels to produce large internal
cavities.8 Zeolite HBEA is used in a wide variety of industrial
applications as a catalyst for organic synthesis.9–11 It has two
comparable straight channels in the [010] and [100] directions and
a third sinusoidal 12-membered-ring channel in the [001] direction.
The interconnection of these channels forms a three-dimensional
pore structure with high accessibility. Moreover, zeolite HBEA has a
high concentration of lattice defects from subtle structural disorder.
The disorder is believed to create additional acid sites located on the
internal surface, as well as additional cation exchange positions.12
It is well known that toluene alkylation with tert-butanol provides
valuable information about the acidity and channel structures
of zeolite catalysts.13 Under typical reaction conditions, toluene
can undergo not only alkylation, but also secondary reactions
(isomerisation, disproportionation, etc.). The catalytic activity
and para-selectivity is directly influenced by the acidity, while
the para-selectivity is also influenced by the channel structures of
zeolite catalysts. In our previous reports,3,4 HBEA zeolite with 12-
ring portals (0.67 nm × 0.66 nm) had less effect on the product
selectivity for 4-tert-butyltoluene (4-TBT), and therefore HBEA
showed lower para-selectivity than that found for HM, HY and
HZSM-5 in the alkylation of toluene with tert-butanol. However,
if the 12-ring portals of HBEA were narrowed slightly, the shape
selectivity could be greatly improved. The loading of metal oxide
is a highly efficient method for improving the para-selectivity by
adjusting the pore entrance and reducing the acidity. Many studies
on the metal oxide modification method for zeolites have been
carried out using rare earth metals; however, there are few studies
using transition metals.
In this study, HBEA zeolite was modified with various transition
metal oxides to increase its selectivity for tert-butylation of toluene.
These modified catalysts were characterised by X-ray diffraction
(XRD), scanning electron microscopy (SEM), Fourier transform
infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) and
NH3 temperature-programmed desorption (NH3-TPD) methods
and their catalytic activities were evaluated. Our objective was to
correlate the channel structures and acidity of the HBEA-supported
MxOy catalysts with their catalytic properties in alkylation of
toluene.
Results and discussion
Characterisation of the catalysts
The XRD patterns of the HBEA and MxOy/HBEA catalysts are
shown in Fig. 1. MxOy/HBEA maintained the main crystal structure
of the parent HBEA zeolite, and no diffraction peaks of the transition
Fig. 1 XRD patterns of MxOy/HBEA and parent HBEA catalysts. (a) MnO/
HBEA, (b) Fe2O3/HBEA, (c) Co2O3/HBEA, (d) NiO/HBEA, (e) CuO/HBEA,
(f) HBEA.
* Correspondent. E-mail: songhua2004@sina.com

JOURNAL OF CHEMICAL RESEARCH 2018 161
Fig. 2 SEM images of MxOy/HBEA and parent HBEA catalysts. (a) MnO/HBEA, (b) Fe2O3/HBEA, (c) Co2O3/HBEA, (d) NiO/HBEA, (e) CuO/HBEA,
(f) HBEA.
metal oxides were detected in the range 10–50°, which indicated
that the transition metal oxide was highly dispersed on these MxOy/
HBEA catalysts. Compared with the parent HBEA zeolite, the peak
intensity of the peak at 22.5° decreased slightly in each of the MxOy/
HBEA zeolites. This suggested that the incorporation of transition
metal oxide and collapse of the framework in the process of ion-
exchange probably changed the structural factors.
The SEM images of MxOy/HBEA and parent HBEA zeolites
in Fig. 2 show that the outer surfaces of MxOy/HBEA were
essentially unchanged compared with HBEA zeolites. The
crystal sizes of MxOy/HBEA were around 200–300 nm.
Figure 3 shows the FTIR spectra of parent HBEA and MxOy/
HBEA catalysts. The band centred at 463 cm−1 was characteristic
of the T–O (T = Si and Al) bending mode and the vibrations
centred at 525 and 573 cm−1 indicated the presence of six- and
five-membered rings. The band centred at 1089 cm−1 with a
shoulder at 1227 cm−1 was attributed to the asymmetric stretching
vibrations of Si–O–Si. It is well known that the Si–O–Si bond
shifts linearly with the aluminium content in the framework.
Compared with parent HBEA zeolite (1089 cm−1), the Si–O–Si
bond for MxOy/HBEA was shifted to the higher wavenumber of
1092 cm−1. This shift implied a change of the aluminium content
in the zeolite structure, and this phenomenon could be ascribed to
the selective extraction of aluminium atoms from the framework
during the ion-exchange process.
Fig. 3 FTIR spectra of parent HBEA and MxOy/HBEA catalysts. (a) MnO/
HBEA, (b) Fe2O3/HBEA, (c) Co2O3/HBEA, (d) NiO/HBEA, (e) CuO/HBEA,
(f) HBEA.
sites in each MxOy/HBEA decreased, while the strong acid sites
of all the MxOy/HBEA samples almost vanished. This shows
that the transition metal oxide preferentially interacted with
the strong acid sites in HBEA. In addition, the strength of the
weak acid increased slightly, such that the desorption peak of
weak acid sites in MxOy/HBEA shifted slightly towards higher
temperatures.
The NH3-TPD of parent HBEA and MxOy/HBEA zeolites
are shown in Fig. 4. Two peaks were detected at about 240 °C
and 450 °C, which were attributed to weak acid and strong acid
sites, respectively. Compared with parent HBEA, the weak acid

162 JOURNAL OF CHEMICAL RESEARCH 2018
Table 1 Physico-chemical properties of parent HBEA and MxOy/HBEA
Specific surface
Pore volume
Pore size
(nm)
Catalyst
area (m2 g−1)
(cm3 g−1)
HBEA
492.5
396.0
401.3
398.4
400.9
402.1
0.48
0.41
0.42
0.44
0.43
0.41
3.90
3.62
3.65
3.79
3.81
3.71
Fe2O3/HBEA
MnO/HBEA
Co2O3/HBEA
NiO/HBEA
CuO/HBEA
Table 2 Catalytic results for alkylation of toluene with tert-butanol over
different zeolitesa
Distribution of
products (%)
4-TBT 3-TBT Other
Toluene
Catalyst conversion
(%)
4-TBT
3-TBT
selectivity selectivity
(%)
(%)
HBEA
Fe2O3/
HBEA
MnO/
HBEA
Co2O3/
HBEA
NiO/
HBEA
CuO/
HBEA
58.4
54.7
39.3 16.7
2.4
67.3
28.6
44.6
39.3
35.4
34.3
32.6
9.8
8.8
8.7
8.7
8.3
0.3
81.5
80.5
78.5
78.5
79.1
17.9
18.0
19.3
19.9
20.1
Fig. 4 NH3-TPD of MxOy/HBEA and parent HBEA catalysts. (a) MnO/
HBEA, (b) Fe2O3/HBEA, (c) Co2O3/HBEA, (d) NiO/HBEA, (e) CuO/HBEA,
(f) HBEA.
48.9
45.1
43.7
41.2
0.8
1.0
0.7
0.3
aReaction conditions: catalyst, 1.0 g; n(tert-butanol):n(toluene), 3:1; temperature, 190 °C;
time, 4 h; cyclohexane, 60 mL.
CH
CH
3
CH
3
H C
3
CH
3
CH
3
3
H C
H C
3
3
H C
CH
3
3
+
H C
OH
H C
OH
3
3
+
CH
H C
3
3
CH
H C
3
H C
3
3
CH
H C
3
3
CH
3
CH
3
CH
3
CH
CH
CH
3
3
3
H C
3
CH
3
+
H C
3
OH
×
CH
3
H C
3
Fig. 5 Reaction scheme for the alkylation of toluene with tert-butanol.
The physico-chemical properties of parent HBEA and MxOy/
HBEA are listed in Table 1. Compared with the parent HBEA,
the specific surface area, pore volume and pore size of all
MxOy/HBEA zeolites decreased, which could be explained
by blocking of the pores due to the modification process. The
decrease of pore size after loading MxOy improves the para-
selectivity, as narrowed pores promote shape selectivity [4].
The 4-TBT can diffuse from the narrow pores of MxOy/HBEA
easily, while the diffusion of 3-TBT is restricted owing to its
steric restriction within the narrow pores of MxOy/HBEA.
Therefore, the 4-TBT selectivity over MxOy/HBEA was higher
than that found for HBEA.
The series of transition metal oxide-modified HBEA
(MxOy/HBEA) zeolites were investigated in the alkylation of

JOURNAL OF CHEMICAL RESEARCH 2018 163
toluene with tert-butanol (Fig. 5). As shown in Table 2, two
main products were produced: 4-TBT and 3-TBT. However,
2-TBT (2-tert-butyltoluene) was not detected owing to steric
hindrance of methyl and tert-butyl groups.14 In addition, a trace
amount of 3,5-di-tert-butyltoluene (3,5-DTBT) from further
alkylation of 4-TBT or 3-TBT was also detected. The toluene
conversions of all MxOy/HBEA catalysts were lower than that
of parent HBEA, and the catalytic activity decreased in the
order of parent HBEA>Fe2O3/HBEA>MnO/HBEA>Co2O3/
HBEA>NiO/HBEA>CuO/HBEA. However, these MxOy/
HBEA zeolites showed higher para-selectivity than HBEA
zeolite, especially in the case of the Fe2O3/HBEA zeolite. The
selectivity for 4-TBT was 81.5% over the Fe2O3/HBEA zeolite
at 190 °C after 4 h, which is much higher than that found
for HBEA (67.3%). Thus, Fe2O3/HHBEA was chosen as the
catalyst for further investigation.
the formation of 4-TBT; as the kinetic diameter of 4-TBT
was smaller than that of 3-TBT, it could diffuse through the
narrowed pores of MxOy/HBEA more readily. On the other
hand, the deactivation of strong acid sties caused by the loading
of MxOy increased the selectivity for 4-TBT by suppressing the
further isomerisation of 4-TBT on strong acid sites.
Optimisation of reaction conditions
Effect of amount of catalyst
Using Fe2O3/HBEA as the catalyst, the effect of catalyst loading
on toluene butylation was investigated with various loadings
of the Fe2O3/HBEA catalyst in the range from 0.3 to 1.5 g. As
shown in Fig. 6, the conversion of toluene first increased and
then decreased on increasing the amount of catalyst. This is
due to the increase in the number of acidic sites. It is generally
accepted that electrophilic aromatic substitution occurs with
the formation of a carbenium ion as the intermediate. With
an increase in the amount of catalyst, the number of acidic
sites and surface area increased, leading to the formation of
more carbenium ions per unit time, which directly increased
the reaction rate. However, the presence of more acidic sites
on the surface of zeolites accelerates secondary reactions
(isomerisation and oligomerisation), which results in the
decrease in toluene conversion and 4-TBT selectivity.
On loading of MxOy, the toluene conversion of MxOy/
HBEA catalysts decreased, which could be attributed to the
decrease of acidity of the MxOy/HBEA catalysts. However,
the 4-TBT selectivity increased significantly. On the one
hand, the narrowed pores of MxOy/HBEA catalysts promoted
70
60
50
40
30
20
90
80
70
60
50
40
Effect of tert-butanol/toluene molar ratio
The effect of the molar ratio of tert-butanol to toluene on the
toluene conversion and 4-TBT selectivity was studied by
varying the molar ratios (tert-butanol/toluene) from 1:1 to
5:1 over Fe2O3/HBEA catalysts at 190 °C, and the results are
presented in Fig. 7. When the molar ratio of tert-butanol to
toluene was increased from 1 to 3, the conversion of toluene
increased greatly from 37.2 to 54.7%. Hereafter, on increasing
the ratio to 5, the conversion decreased to 42.0%. The reason
for this was that higher tert-butanol concentrations increase
the oligomerisation of isobutylene (formed by dehydration of
tert-butanol), and excess isobutylene further reacts with each
molecule over the catalyst to form oligomers5. These oligomers
covered the acid sites, which resulted in catalyst deactivation,
and so the conversion of toluene decreased. The best 4-TBT
selectivity was obtained at a tert-butanol to toluene molar ratio
of 3:1, showing that a small excess of tert-butanol promotes
Conversion of tolunene
Selectivity of 4-TBT
0.25
0.50
0.75
1.00
1.25
1.50
Amount of Catalyst (g)
Fig. 6 Effect of amount of catalyst on alkylation of toluene with tert-
butanol. Reaction conditions: n(tert-butanol):n(toluene), 3:1; cyclohexane,
60 mL; temperature, 190 °C; time, 4 h.
70
60
50
40
30
90
80
70
60
50
60
55
50
45
40
35
85
80
75
70
65
60
55
50
Conversionof toluene
Selectivity of 4-TBT
Conversion of toluene
Selectivity of 4-TBT
1
2
3
4
5
150
160
170
180
190
200
210
tert-Butanol/toluene molar ratio
Reaction temperature (°C)
Fig. 7 Effect of molar ratio of tert-butanol to toluene on alkylation of
toluene with tert-butanol. Reaction conditions: Fe2O3/HBEA, 1.0 g;
cyclohexane, 60 mL; temperature, 190 °C; time, 4 h.
Fig. 8 Effect of reaction temperature on alkylation of toluene with tert-
butanol. Reaction conditions: Fe O /HBEA, 1.0 g; cyclohexane, 60 mL;
n(tert-butanol):n(toluene), 3:1; tim2e, 34 h.

164 JOURNAL OF CHEMICAL RESEARCH 2018
Catalyst characterisation
para-selectivity. Similar results were reported by Kostrab and
co-workers,14 who explained that a small excess of tert-butanol
is preferred to suppress the dealkylation reaction (cleavage of
tert-butyl group from the aromatic ring).
XRD patterns of catalysts were obtained on a D/max-2200PC-X-ray
diffractometer using Cu Kα radiation (λ = 0.154 nm) and operated at
40 kV, 30 mA. The scanning rate was 0.05 ° s−1 in the range from 10
to 80°.
The morphology of the HBEA as well as the particle size
distribution for the MxOy/HBEA samples were determined with a
JSM-6360LA SEM.
Infrared spectra of catalysts were recorded on a Bruker FTIR
spectrometer (Tensor 27) in a wavenumber range of 4000–400 cm−1
with 4 cm−1 resolution. Absorbance measurements were carried
out using KBr pellets containing 5% of the powder sample to be
analysed.
Nitrogen adsorption–desorption isotherms of catalysts were
measured at −196 °C using Micromeritics adsorption equipment
type NOVA2000e. The specific surface area of the catalysts was
calculated using the BET method in the partial pressure (P/P0) range
from 0.01 to 0.99. The mesopore size distribution was obtained from
the adsorption branch of the isotherm using the Barret–Joyner–
Halenda (BJH) adsorption model.
Effect of reaction temperature
The dependence of toluene conversion and 4-TBT selectivity
on the reaction temperature was examined. The experiments
using Fe2O3/HBEA catalyst were conducted at 150, 170, 180,
190 and 210 °C. As shown in Fig. 8, there is a sharp increase
in the conversion of toluene when the reaction temperature is
increased from 150 to 180 °C, and the toluene conversion then
decreases on increasing the temperature further. Based on
these results, we suggest that activated toluene molecules and
activated tert-butyl carbenium ions increase as the temperature
increases, which results in increased toluene conversion.
However, a decrease in toluene conversion was evident when
the temperature was higher than 180 °C. Sebastian et al.7 also
reported similar results. They found that butene was formed
through dehydration of tert-butanol on acidic sites, which can
oligomerise to C8 and C12 olefins and then crack to low boiling
species. As these reactions were dominant at higher reaction
temperatures, a decrease in the toluene conversion and 4-TBT
selectivity was expected.
The NH3-TPD procedure was carried out in a Quantachrome
Chembet-3000 Characterization System; 200 mg sample was used
for each measurement. The sample was pretreated in the stream at
550 °C for 60 min in dry helium and cooled to 120 °C. It was then
exposed to 10 v% NH3/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−1 from
120 to 600 °C.
Conclusions
FTIR spectra of pyridine-adsorbed (Py-IR) samples were obtained
on a Bruker FTIR spectrometer in KBr pellets. Before each
experiment, samples were pressed into thin pellets (10–30 mg) with
diameter of 16 mm and activated in situ over one night under vacuum
(10−5 Pa) at 170 °C. Pyridine was introduced in excess at 150 °C after
the activation period. The concentrations of the B and L sites were
determined from the integrated area bands of the PyH+ and PyL
species using the values of the molar extinction coefficients of both
bands.
Several transition metal oxide-modified HBEA catalysts
were prepared and investigated in the alkylation of toluene
with tert-butanol for the synthesis of 4-TBT. The introduction
of transition metal oxides into the framework structure of
the zeolite led to a loss of crystallinity, but could adjust the
pore structures and decrease the total acidity, especially the
strong acidity of HBEA. On loading with MxOy, the toluene
conversion of MxOy/HBEA catalysts decreased, while
the 4-TBT selectivity increased significantly. A possible
mechanism was proposed to explain the increased selectivity:
(1) the narrowing of the pores of MxOy/HBEA catalysts
promoted the formation of 4-TBT by increasing the shape
selectivity; and (2) the deactivation of strong acid sites caused
by the loading of MxOy increased the selectivity for 4-TBT by
suppressing the further isomerisation of 4-TBT. The effects of
various reaction parameters, such as the amount of catalyst,
the reactant molar ratio and the reaction temperature, were
studied over Fe2O3/HBEA.
Toluene alkylation with tert-butanol
The reactions were performed in a 300 mL laboratory autoclave
reactor. For a typical run, the reaction mixture consisted of tert-
butanol (283 mmol Aladdin, 98%, 28 mL), toluene (94 mmol,
Aladdin, 98%, 10 mL) and cyclohexane (Aladdin, 99%, 60 mL). The
raw material and catalyst were first added into the reactor, then, the
air having been purged with nitrogen, the pressure in the reactor was
adjusted to 1.5 MPa with nitrogen. The reaction temperature was
maintained at 180 °C and the stirring rate was controlled at 300 rpm
to overcome external diffusional limitations. After the experiment,
the pressure was reduced to atmospheric pressure, and the zeolite
catalyst was removed by filtration. Each experiment was repeated
three times, and good reproducibility of the results was found.
The samples were analysed using a GC-14C gas chromatograph
(Shimadzu, Japan), with a SE-30 capillary column (50 m × 0.25
mm × 0.25 μm) and a FID detector. Analyses were carried out with
a temperature programme from 60 to 220 °C (with a slope of 10 °C
min−1) and at 220 °C for 10 min isothermally. The reaction products
were identified by GC/MS analysis.
Experimental
Materials
Na-BEA zeolite powder (Si/Al = 25) was purchased from Zeolyst
Int., China. Ammonium nitrate, manganese(ii) nitrate, iron(iii)
nitrate, cobalt(ii) nitrate, nickel (ii) nitrate, copper(ii) nitrate, toluene,
tert-butanol and cyclohexane (all of analytical grade) were bought
from ACROS Organics.
Catalyst preparation
Received 8 March 2018; accepted 23 March 2018
Paper 1805335
https://doi.org/10.3184/174751918X15220765507016
Published online: 29 March 2018
The HBEA zeolite with a Si/Al ratio of 25 used in this study was
prepared according to the reported procedure.15 NaBEA zeolite was
used as a starting material. The HBEA zeolite was prepared by ion
exchange of NaBEA with aqueous NH4NO3. After ion exchange, the
zeolite was filtered, and dried at 110 °C for 12 h. Then the samples
were calcined at 350 °C for 4 h in an air. A series of MxOy/HBEA
catalysts was synthesised by ion exchange of HBEA zeolites with
aqueous transition metal nitrates (0.25 mol L−1) for 24 h at room
temperature.16 The resulting materials were calcined at 350 °C for 4
h in an air.
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