Friedel-Crafts alkylation of toluene with tert-butyl alcohol over Fe2O3-modified Hβ

Article abstract of DOI:10.1039/c6ra21054f

Fe₂O₃ (x)/Hβ catalysts with different Fe₂O₃ loadings (x) were successfully prepared and characterized by XRD, SEM, TEM, ICP, BET, NH₃-TPD and Py-IR. With selective alkylation of toluene with tert-butyl alcohol to produce 4-tert-butyltoluene as a probe reaction, the effects of the different Fe₂O₃ loadings on channel structures, acidity and catalytic performance over Hβ were studied. The results showed that modification of Hβ with Fe₂O₃ could adjust pore structures and decrease the total acidity, especially the strong acidity of Hβ. The parent Hβ exhibited the highest toluene conversion of 58.4% with the lowest PTBT selectivity of 67.3%. The low para-selectivity over parent Hβ could be attributed to the low shape-selective action of 12-ring portals of Hβ and the isomerization of the formed PTBT to MTBT. Upon loading of Fe₂O₃, the toluene conversion of Fe₂O₃ (x)/Hβ catalysts decreased slightly, but the PTBT selectivity increased significantly. A toluene conversion of 54.7% with PTBT selectivity of 81.5% was observed over Fe₂O₃ (20%)/Hβ at 190 °C after 4 h. The narrowed pores after loading the Fe₂O₃ were beneficial to increasing the selectivity to PTBT, since PTBT with a lower kinetic diameter (0.58 nm) can diffuse through the narrowed pores of Fe₂O₃ (x)/Hβ more easily than MTBT (0.65 nm). In addition, the decrease in number of strong acid sites and deactivation of acid sites on the surface are beneficial to increasing the selectivity to PTBT by suppressing further isomerization of the formed PTBT on acid sites.

Full text of DOI:10.1039/c6ra21054f

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ARTICLE
Friedel-Crafts alkylation of toluene with tert-butyl alcohol over
Fe O -modified Hβ
2
3
a
a,b,
c
Yuanyuan Wang , Hua Song *, Xinglong Sun
Received 00th August 2016,
Accepted 00th August 2016
Fe
BET, NH
probe reaction, the effects of the different Fe
were studied. The results showed that modification of Hβ with Fe
2
O
3
(x)/Hβ catalysts with different Fe
-TPD and Py-IR. With selective alkylation of toluene with tert-butyl alcohol to produce 4-tert-butyltoluene as
loading on channel structures, acidity and catalytic performance over Hβ
could adjust pore structures and decrease the total
2 3
O loading (x) were successfully prepared and characterized by XRD, SEM, TEM, ICP,
3
DOI: 10.1039/x0xx00000x
2
O
3
www.rsc.org/
2
O
3
acidity, especially the strong acidity of Hβ. The parent Hβ exhibited the highest toluene conversion of 58.4% with the
lowest PTBT selectivity of 67.3%. The low para-selectivity over parent Hβ could be attributed to low shape-selective action
of 12-ring portals of Hβ and isomerization of the formed PTBT to MTBT. Upon loading of Fe
Fe (x)/Hβ catalysts decreased slightly, but the PTBT selectivity increased significantly. The toluene conversion of 54.7%
with PTBT selectivity of 81.5% were observed over Fe (20%)/Hβ at 190 °C after 4 h. The narrowed pores after loading
the Fe were benefit to increase selectivity to PTBT, since PTBT with lower kinetic diameter (0.58 nm) can diffuse
through the narrowed pores of Fe (x)/Hβ more easily than MTBT (0.65 nm). In addition, the decrease of strong sites and
2 3
O , the toluene conversion of
2 3
O
2
O
3
2 3
O
2
O
3
deactivation of acid sties on surface are benefit to increase selectivity to PTBT by suppressing further isomerization of the
formed PTBT on acid sties.
.
7
activity. For example, Pai et al. found Hβ (25) and HY (30) had
Introduction
better catalytic activity than HMCM-22 (52) in vapour phase
8
butylation of toluene. Mravec et al. found the most active zeolite
Acid catalyzed reactions such as Friedel-Crafts alkylation is an
important process in organic synthesis, fine chemical production as
catalysts for the liquid-phase butylation of toluene were Hβ (12.5)
9
1
-3
and HM (17.5). In our previous report , we also found Hβ (25)
well as in petrochemical process . The butylated products of
toluene, particularly the 4-tert-butyltoluene (PTBT) has great
commercial significance as its derivatives 4-tert-butylbenzaldehyde
showed high catalytic activity in alkylation of toluene with TBA.
However, the para-selectivity (4-tert-butyltoluene, PTBT) of 69.5%
and the meta-selectivity (3-tert-butyltoluene, MTBT) of 26.9% over
Hβ were observed at 180 °C after 4h. It showed that the selectivity
to desired product PTBT over Hβ zeolite was low.
4
-6
and 4-tert-butylbenzoic acid are widely used in the industry
Conventionally, toluene butylation reactions are carried out with
alkylating agents via AlCl , sulfuric acid and phosphoric acid as
.
3
A petrochemical process is evaluated not only the basis of
conversion of a given reactant, but also in terms of selectivity to
desired product. Since the dimensions of 12-ring portals of parent
catalysts. However, these acid catalysts are not environmentally
benign, not re-usable and also lead to corrosion of equipments.
Therefore, considerable attention has been devoted to
development of solid acid catalysts which are environmentally
friendly.
1
0
Hβ are 0.67 nm × 0.66 nm , while the kinetic diameters are
8
0
.58nm for PTBT and 0.65nm for MTBT , respectively. The shape-
selective action has less effect on the product selectivity to PTBT
over Hβ zeolite. However, the shape-selective action could be
greatly improved if the 12-ring portals of parent Hβ were narrowed
slightly. Zeolite Hβ contains numerous lattice defects due to subtle
structural disorder, which is believed to create additional acid sites
It is well known that toluene is not active enough in the
Friedel-Crafts alkylation in the absence of a catalyst. So, selection of
a suitable catalyst is crucial to highly efficient alkylation of toluene
with tert-butyl alcohol (TBA). Zeolites are regarded as a good choice
due to their pore structures and unique acid properties. Among
1
1
on internal surfaces as well as extra cation exchange positions
.
different zeolites, zeolite
β with a three-dimensional (3D)
Modification by metallic oxide is a highly efficient method for
improving the para-selectivity by reducing the external acid sites
and adjusting pore entrance.
interconnecting pore system and high acidity shows a high catalytic
a.
2 3
As far as we know, the use of Fe O modification of Hβ zeolites
College of Chemistry & Chemical Engineering, Northeast Petroleum University,
Daqing 163318, Heilongjiang, China.
as catalysts in tert-butylation of toluene has not been reported. The
aim of the present study was to evaluate the catalytic behaviour of
Fe O supported on Hβ for the alkylation of toluene by TBA under
b.
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry
&
Chemical Engineering, Northeast Petroleum University, 163318 Daqing, China.
Heilongjiang
c.
Daqing Petrochemical Engineering Co.
China;
，Ltd.
，
Daqing 163714
，
，
2
3
liquid reaction conditions. For this purpose, Fe O (x)/Hβ samples
2
3
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containing 8–25% Fe
2
O
3
were prepared and were fully temperature vacuum cell. The sample powder was pressed into a
characterized by XRD, SEM, TEM, FT-IR, BET, TPD and Py-IR. Our self-supporting wafer, and the spectra were recorded in
a
−1
-1
objective was to correlate the channel structures and acidity of the wavenumber range of 4000–400 cm with a 4 cm resolution.
Hβ supported Fe
2
O
3
catalysts with their catalytic properties in Before each experiment, material samples were pressed into thin
pellets (10–30 mg) with diameter of 16 mm and activated in situ
alkylation of toluene.
−5
during one night under vacuum (10 Pa) at 170 °C. Pyridine was
introduced in excess at 150 °C after the activation period. The
concentrations of the B sites and L sites were determined from the
Experimental
+
−1
Materials
integrated area bands of the PyH (located at around 1540 cm )
−1
and PyL (around 1450 cm ) species using the values of the molar
extinction coefficients of both bands.
Naβ zeolite powder (Si/Al=25) was purchased from Zeolyst Int.,
China. Ammonium nitrate, ferric nitrate, toluene, tert-butyl alcohol
and cyclohexane (all analytical grade) were bought from ACROS
Organics and used without further purification.
Toluene alkylation with tert-butyl alcohol
The side-chain alkylation of toluene with TBA was carried out in a
laboratory autoclave reactor (300 ml) at 190 °C. At typical
experiment, 10 ml of toluene (94 mmol, Aladdin, 98%) and 28 ml of
tert-butyl alcohol (283 mmol Aladdin, 98%) were mixed with 60 ml
of cyclohexane (Aladdin, 99%) solvent. Then 1.0 g of zeolite catalyst
Catalyst preparation
The Hβ zeolite with Si/Al radio of 25 used in this work was prepared
1
2
according to the reported procedure . Naβ zeolite was used as a
starting material. The Hβ zeolite was prepared by ion exchange of
(
1.35%) was added. The reaction system was sealed and purged
Naβ with NH
was filtered, and dried at 110 °C for 12 h. Then, the samples were
calcined at 550 °C for 4 h an air atmosphere. Fe -modified Hβ
zeolites support with the mass fraction of Fe (4%, 8%, 16%, 20%
and 25%) were synthesized by ion exchange of Hβ zeolites with
4 3
NO aqueous solution. After ion exchange, the zeolite
with continous N purging for 30 min. Then the mixture was heated
2
to a certain temperature to start alkylation reaction. After reaction,
the mixture was cooled down to room temperature, and the zeolite
catalyst was removed by filtration. The liquid phase was analyzed
on GC-14C gas chromatograph equipped with SE-30 capillary
column (φ 0.25 mm × 50 m). The measurement was performed at
the initial temperature of 60 °C for 2min, the ramping rate of 10 °C
2 3
O
2
O
3
1
3
Fe(NO
resulting materials were calcined at 350 °C for 4 h in an air
atmosphere. The obtained Fe -modified Hβ zeolites were
denoted as Fe (x)/Hβ, where x standed for the mass fraction of
Fe in %.
Catalyst characterization
3 3
) aqueous solutions for 24 h at room temperature . The
2
O
3
/
min, the final temperature of 220 °C for 10 min, with nitrogen as
2
O
3
carrier gas. During the course of the investigation, a number of runs
were repeated to check for reproducibility in the experiment
results, which were found to be excellent. Typical errors were in the
2
O
3
XRD analysis of material powders were carried out on a D/max- range of ± 2%.
200PC-X-ray diffractometer using Cu Kα radiation under the
2
setting conditions of 40 kV, 30 mA, scan range from 10 to 80 at a
rate of 4 °/min.
Results and discussion
The surface morphological details of catalysts were studied by Physicochemical characterization
scanning electron microscopy (SEM, Zeiss-ΣIGMA).
The tungsten and magnesium contents of all catalysts were
determined by inductively coupled plasma–optical emission
spectrometry (ICP–OES) using
instrument.
a PerkinElmer Optima 2000DV
Nitrogen adsorption–desorption were recorded at −196 °C
using Micromeritics adsorption equipment of NOVA2000e. Prior N
2
adsorption, the powder samples were degassed under secondary
vacuum for 1 h at 90 °C and 10 h at 180 °C. The specific surface area
2
of each solid (SBET in m
/
g) was calculated from the adsorption
isotherms using the BET method. The total pore volume (Vtot) was
calculated from the adsorbed volume of nitrogen at P/P equal to
.99.
Samples acidity were measured by NH
programmed desorption (NH -TPD) using
0
0
3
temperature-
Fig 1. XRD of parent Hβ and Fe
a) parent Hβ, (b) Fe (8%)/Hβ, (c) Fe
Fe (25%)/Hβ
2 3
O (x)/Hβ
3
a
Quantachrome
(
O
2 3
2 3 2 3
O (16%)/Hβ, (d) Fe O (20%)/Hβ and (e)
Chembet-3000 Characterization System. A 200 mg sample was pre-
treated at 550°C for 1 h in dry helium (50 mL/min) and cooled to
2 3
O
1
20 °C. It was then exposed to 10 v% NH
the catalyst with He for 1 h, a TPD plot was obtained at a heating The peak positions of Hβ were not changed even at high Fe
rate of 10 °C/min from 120 to 600 °C.
content. These results showed that the structure of Hβ was
IR spectra with pyridine adsorption of samples were recorded retained after Fe modification. The peaks of Fe were not
using a Bruker FT-IR spectrometer (Tensor 27) together with a high observed at Fe
3
2 3
/He for 1 h. After purging Fig.1 showed typical XRD patterns of parent Hβ and Fe O (x)/Hβ.
2
O
3
2
O
3
2 3
O
2
O
3
contents less than 20%. This result indicated that
2
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the Fe
corresponding to Fe
the Fe contents up to 25%. In addition, the crystallite size of measurement were close to their corresponding theoretical
Fe (20%)/Hβ, which was calculated using the Debye–Sherrer amounts. However, the amount of Fe in spent Fe (20%)/Hβ
equation applied to the reflection at 2θ of 22.5°, was estimated to was 19.5%, which was slightly lower than that found in the fresh
2
O
3
was highly dispersed. Nevertheless, the peaks
As can be seen from Table 1, the actual loading amounts of
1
4-16
2
O
3
appeared at 2θ of 33.1° and 35.6°
when Fe
2
O
3
in the fresh Fe (x)/Hβ materials obtained from ICP
2 3
O
2
O
3
2
O
3
2
O
3
2 3
O
1
7
be 249 nm
Fig. 2 (a) showed the scanning electron micrograph of fresh responsible to the low amount of Fe
Fe (20%)/Hβ sample. The crystal of fresh Fe (20%)/Hβ showed Fe (20%)/Hβ.
typical agglomerates of small cubic particles with sizes around 200- Table 1 The structural parameters of parent Hβ zeolite and Fe
.
2 3
one (19.8%). The leaching of Fe O into the reaction mixture was
2
O
3
in the spent
2
O
3
2
O
3
2 3
O
2 3
O (x)/Hβ zeolites
c
3
00 nm, which was in good agreement with the calculated
crystallite size of Fe (20%)/Hβ from XRD analysis. Fig. 2 (b)
showed the scanning electron micrograph of spent Fe (20%)/Hβ
sample. There were some carbonaceous deposits on the surface of
the spent Fe (20%)/Hβ zeolite as compared with the fresh one.
Content of
BET surface
Pore volume
Pore size
(nm)
Zeolites
Hβ
a
b
2
3
2
O
3
Fe O3 (%)
2
area (m /g)
(cm /g)
2
O
3
-
492
0.48
0.46
0.43
0.41
0.31
0.20
3.90
3.81
3.75
3.62
3.36
4.58
Fe
2 3
O (8%)/Hβ
8.2
446
2
O
3
Fe
2
Fe
2
Fe
2
O
3
O
3
O
3
(16%)/Hβ
16.0
19.8
25.3
425
(20%)/Hβ
(25%)/Hβ
396
352
spent Fe
2 3
O (20%)/Hβ
19.5
177
a
b
c
Deduced from ICP analysis.
Specific surface area calculated by the BET method.
Total pore volume determined at P/P = 0.99.
0
The N
2
adsorption/desorption curves in Fig.4 obtained from parent
(x)/Hβ samples showed the typical structure of
2 3
Hβ and Fe O
mesoporosity, namely type-IV isotherms with well-defined
hysteresis loops. A typical H4 type hysteresis loop was observed in
0
the P/P range from 0.4 to 0.9, indicating the existence of large
1
8, 19
mesopores
.
Fig.2. SEM images obtained Fe
2 3
O (20%)/Hβ
(
a) fresh Fe (20%)/Hβ; (b) spent Fe O
2
O
3
2 3
(20%)/Hβ
The transmission electron microscope of Fe
was shown in Fig.3. Fe (20%)/Hβ showed a honeycomb structure.
The Fe was highly dispersed on Hβ, which could be distinguished
as dark dots in TEM images.
2 3
O (20%)/Hβ catalyst
2 3
O
2
O
3
Fig.4. N
a) parent Hβ; (b) Fe
Fe (25%)/Hβ
2
adsorption/desorption curves of Hβ and Fe
2 3
O (x)/Hβ
(
2 3
O
(8%)/Hβ; (c) Fe (16%)/Hβ; (d) Fe O (20%)/Hβ and (e)
2
O
3
2 3
2 3
O
The textural properties including BET surface area, pore volume,
and pore size of parent Hβ and Fe (x)/Hβ samples distributions
were summarized in Table 1. Parent Hβ zeolite possessed the
2
O
3
2
3
highest specific surface area (492 m /g), pore volume (0.48 cm /g)
and pore size (3.90 nm). The increase of the Fe loading from 8%
to 25% led to a decrease in the specific surface area from 446 m /g
2
O
3
2
2
3
3
to 352 m /g, the total pore volume from 0.46 cm /g to 0.31 cm /g
and the pore size from 3.81 nm to 3.36 nm. The decrease of pore
2 3
volume and pore size after loading Fe O is expected to improve the
2 3
Fig.3. TEM images obtained Fe O (20%)/Hβ
para-selectivity, since narrowed pores are benefit to shape-
selective action. The severe decrease in specific surface area and
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2 3
total porous volume observed for the Fe O (25%)/Hβ solid sample the B/L ratio first increased then dropped. It meant that L acid
could be explained by blocking of pores due to the high loading of decreased more drastically than B acid. This may be attributed the
Fe
Fe
2
O
O
3
. The pore size decreased in the order: parent of Hβ＞ electronic structure of transition metal elements. Most of the
(8%)/Hβ＞Fe (16%)/Hβ＞Fe (20%)/Hβ＞Fe (25%)/Hβ. transition metal elements has empty orbit, which can be used as
2
3
2
O
3
2
O
3
2 3
O
The specific surface area and pore volume of the spent the “transferring station” for electron. This special electronic
2
3
Fe
2
O
3
(20%)/Hβ decreased remarkably to 177 m /g and 0.20 cm /g,
structure can make transition metal cation to interact with the
framework via a polarizing or inductive effect, withdraw electrons
which could be caused by carbonaceous deposits on the surface
and inside the pores of the spent Fe (20%)/Hβ zeolite. The
increase in pore size (4.58 nm) observed for the spent
Fe (20%)/Hβ further confirmed blockage of micropores during
2
O
3
20
from O-H bonds, thereby increase the B acidity . As a result, the
increased B acidity can compensate for the original B acidity
2
O
3
covered by Fe
acid of Fe (25%)/Hβ were sharply reduced compared to those of
Fe (20%)/Hβ, which can be attributed to blocking of pores of Hβ
owing to the high Fe loading in the former. As proposed above
based on the N adsorption analysis, thus decreasing the total acid
amount. The B acid amount decreased in the order of parent Hβ＞
2 3
O , to some degree. In addition, both B acid and L
the reaction by carbonaceous deposition.
2
O
3
Acidity characterization
2
O
3
The acid properties of zeolites are important in determining the
catalytic activity and product selectivity for any catalyzed reaction.
2
O
3
2
Acidic measurement was achieved by NH -TPD and Py-IR
3
characterization. NH -TPD curves of parent Hβ and Fe O (x)/Hβ
3
2 3
Fe
2
O
3
(8%)/Hβ＞Fe
2
O
3
(16%)/Hβ＞Fe
2
O
3
(20%)/Hβ＞Fe
2
O
3
(25%)/Hβ.
were showed in Fig.5.
Two peaks could be detected at 240 °C and 460 °C in the curve Table 2 Acidic properties of catalysts determined by Py-IR at 150 °C.
of parent Hβ, which were attributed to weak acid sites and strong
acid sites, respectively. The number of weak acid sites was
obviously greater than that of strong acid sites. Compared to parent
Hβ, the desorption peak of weak acid sites in Fe O (x)/Hβ shifted
a
b
B acid
L acid
Total acid
(μmol/g)
B/L
Catalysts
(
μmol/g)
84.2
(μmol/g)
47.2
ratio
parent Hβ
131.4
125.7
120.1
112.7
1.78
1.84
1.92
2.02
1.94
2
3
Fe
2 3
O (8%)/Hβ
81.4
44.3
slightly toward higher temperatures with the increase of Fe O
2
3
Fe
Fe
Fe
2
O
2
O
2
O
3
(16%)/Hβ
78.9
41.2
loading. It indicated that the strength of weak acid increased
slightly. The acid amount changed after the addition of Fe O too,
3
(20%)/Hβ
75.4
37.3
2
3
3
(25%)/Hβ
63.2
32.5
95.7
the weak acid sites in Fe O (x)/Hβ decreased gradually with
2
3
a
b
−1
increasing of x, while strong acid sites of all the Fe O (x)/Hβ almost
Deduced from the intensity of the band located at around 1540 cm .
2
3
−1
vanish. This result showed that Fe O preferentially acted with the
Deduced from the intensity of the band located at around 1450 cm .
2
3
strong acid sites of Hβ zeolite. In addition, the loading of Fe O is
2
3
benefit to increase selectivity to PTBT by suppressing the further
isomerization of formed PTBT to MTBT and this will be discussed
later in section 3.3. The total acid amount decreased in the order of
3
.3 Catalytic activity evaluation
parent Hβ ＞ Fe O (8%)/Hβ ＞ Fe O (16%)/Hβ ＞ Fe O (20%)/Hβ ＞ Table 3 presented the results of toluene conversion and PTBT
2
3
2
3
2 3
Fe O (25%)/Hβ.
selectivity in the alkylation of toluene with TBA. Two monoalkylated
products were detected. One was PTBT (para-isomer), which is in
large amount, and the other was MTBT (meta-isomer). However,
the OTBT (ortho- isomer) was not detected. The PTBT is desirable
product since para-position is favored by the influence of the steric
hindrance of methyl group and voluminous tert-butyl group on one
hand, and by shape-selective action due to the regular channel
structure of zeolite catalysts on the other hand. The formation of
OTBT is hindered by ortho-position of methyl group and tert-butyl
group. Trace amount of di-alkylated product 3,5-di-tert-
butyltoluene (3,5-DTBT) from further alkylation of the
monoalkylated products PTBT and MTBT was also detected. This
can be explained by the same steric effects, where all alkyl groups
are in the meta-position.
2
3
Fig.5. NH3-TPD patterns of Hβ and Fe
2 3
O (x)/Hβ
Table 3 Products selectivity and catalyst activity
(
(
a) parent Hβ; (b) Fe
25%)/Hβ
2 3 2 3 2 3 2 3
O (8%)/Hβ; (c) Fe O (16%)/Hβ; (d) Fe O (20%)/Hβ and (e) Fe O
Toluene
Distribution of products
(%)
PTBT
selectivity
(%)
MTBT
selectivity
(%)
Catalyst
conversion
Table 2 showed Py-IR spectra of parent Hβ and Fe O (x)/Hβ. It
2
3
(
%)
PTBT
39.3
43.4
44.2
44.6
37.8
MTBT
16.7
10.9
10.2
9.8
other
2.4
could be clearly seen from Table 2 that the total acid amount
parent Hβ
58.4
56.0
55.5
54.7
46.3
67.3
28.6
determined by Py-IR correlated well with the NH -TPD results and
3
Fe O
2
3
(8%)/Hβ
(16%)/Hβ
(20%)/Hβ
(25%)/Hβ
1.7
77.5
19.5
the relative amounts of B acid and L acid of Hβ changed significantly
after loading Fe O . Upon increasing the Fe O content, both the B
Fe
Fe
Fe
2
O
2
O
2
O
3
1.1
79.6
18.4
2
3
2
3
3
0.3
81.5
17.9
acid and L acid decreased. This is possibly because Fe O would
2
3
3
8.3
0.2
81.6
17.9
cover some acid sites on Hβ zeolite. However, it’s worth noting that
4
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Reaction conditions: n (TBA): n(toluene)-3:1, catalyst-1.0 g, temperature-190°C, time-4 the catalyst, as sketched in Fig.7, and explains the increased
h, cyclohexane-60 ml
selectivity in three ways. i) One reason because the narrowed pore
size upon loading the Fe . The pore size of Fe (x)/Hβ catalyst
Of the catalysts investigated, parent Hβ exhibited toluene was decreased with increasing the Fe loading (Table 1). The
conversion of 58.4% at 190 °C after 4 h. The toluene conversion of narrowed pores of Fe (x)/Hβ catalysts were benefit to formation
all the Fe (x)/Hβ catalysts were slightly lower than that of parent of PTBT. Since the steric restriction with narrowed pores of
Hβ. In addition, with increasing the Fe loading the toluene Fe (x)/Hβ, the PTBT with lower kinetic diameter (0.58 nm) can
conversion of Fe (x)/Hβ catalyst was decreased. This result diffuse through the narrowed pores of Fe (x)/Hβ more easily than
agrees with the trend of B acidity of Fe (x)/Hβ catalyst (Table 2), MTBT with higher kinetic diameter (0.65 nm). ii) The decrease in
which also decreased with increasing the Fe loading. As is well strong acid sites could improve the para-selectivity. As discussed in
known, the alkylation of toluene with alcohols catalyzed by zeolites section 3.2 (Fig.5), the weak acid sites in Fe (x)/Hβ decreased
is commonly considered to proceed via carbonium ion mechanisms gradually with increasing of x, while strong acid sites of all the
2
O
3
2 3
O
2 3
O
2
O
3
2 3
O
2
O
3
2 3
O
2
O
3
2 3
O
2 3
O
2
O
3
2
O
3
2
1
22, 23
and the B acid sites are active sites
acidity leaded to a lower catalytic activity. The toluene conversion loading of Fe
of 54.7% was observed over Fe (20%)/Hβ catalyst. With further suppressing the further isomerization of PTBT on acid sites. iii) In
increases the Fe content to 25%, a sharp decrease in toluene addition, the increase in PTBT selectivity could also be attributed to
conversion was observed. This is probably due to sharp decrease in the deactivation of acid sties on surface of Fe (x)/Hβ caused by
B acid sites amount, which caused by blockage of pores by excess the loading of Fe . The isomerization of the formed PTBT to MTBT
Fe . In addition, the severely decrease in pore size would lead to runs more easily over the active sites on the external surface, and
poor adsorption and diffusion of reactants and products, and this is less acid sties on surface reduce the isomerization reaction. As a
. Therefore, Lower B Fe
2
O
3
(x)/Hβ almost vanish. Less strong acid sties caused by the
2
O
3
is benefit to increase selectivity to PTBT by
2
O
3
2 3
O
2
O
3
2
O
3
2
O
3
2 3 2 3
another reason for the poor catalytic activity for Fe O (25%)/Hβ result, loading Fe O on Hβ catalysts could enhance and retain
catalyst.
constant high para-selectivity.
The selectivity to PTBT over parent Hβ is only 67.3%, which
much lower than those found for Fe (x)/Hβ (around 80.0%). As
2
O
3
shown in Fig.6, the lower selectivity for PTBT over Hβ can be
explained in three ways. i) The low para-selectivity can be
attributed to the pore size of Hβ zeolite. The dimensions of 12-ring
portals of parent Hβ are 0.67×0.66 nm, while the kinetic sizes are
0.58nm for PTBT and 0.65nm for MTBT. Therefore, the shape-
selective action has less effect on the product selectivity to PTBT
over Hβ zeolite. ii) The high strong acid amount of the Hβ (Fig.4) is
also responsible to the low para-selectivity. Because the MTBT is
the most thermodynamically favored, the kinetically favored PTBT is
gradually transformed to MTBT by isomerization on acid sites. And
2
4,
2 3
Fig.7 The mechanism for high PTBT selectivity over Fe O (x)/Hβ zeolite
the isomerization reaction occurs easily on the strong acidic sites
2
5
.
iii) The high of acidity of external surface also promotes
isomerization of the formed PTBT to MTBT. The isomerization of
formed PTBT to MTBT runs more easily over the active sites on the
external surface, where it is not sterically hindered as in the pores
of zeolite. The above three reasons lead to the low PTBT selectivity
over parent Hβ.
Conclusions
In the present work, Fe
2
O
3
(x)/Hβ catalysts with different Fe
2 3
O
loading were successfully prepared, and the effects of Fe
2
O
3
loading
on the catalytic properties for alkylation toluene with TBA were
studied. The acidity characterization results showed that
modification of Hβ with Fe
strength. However, the weak acid sites in Fe
gradually with increasing of x, while strong acid sites of all the
Fe (x)/Hβ almost vanish. Both the B acid amount and total acid
amount decreased in the order of parent Hβ＞Fe (8%)/Hβ＞
Fe (16%)/Hβ ＞ Fe (20%)/Hβ ＞ Fe (25%)/Hβ. The textural
2
O
3
could increase the weak acid
2 3
O
(x)/Hβ decreased
2
O
3
2
O
3
2
O
3
2
O
3
2 3
O
properties characterization results showed that parent Hβ zeolite
2
possessed the highest specific surface area (492 m /g), pore volume
Fig.6 The mechanism for low PTBT selectivity over parent Hβ zeolite
3
(
0.48 cm /g) and pore size (3.90 nm). The increase of the Fe
2
O
3
loading from 8% to 25% led to a decrease in the pore size from 3.81
The PTBT selectivity over all the Fe O (x)/Hβ catalysts were
2
3
nm to 3.36 nm. The pore size decreased in the order of parent Hβ＞
around 80.0%, showing that upon loading of Fe O , the PTBT
2
3
Fe
2
O
3
(8%)/Hβ＞Fe
2 3 2 3 2 3
O (16%)/Hβ＞Fe O (20%)/Hβ＞Fe O (25%)/Hβ.
selectivity increased significantly.
A possible mechanism was
The catalytic activity results showed that the toluene conversion
proposed to explain the effect of Fe O on the PTBT selectivity of
2
3
over parent Hβ, which possessed the largest B acidity (84.2 μmol/g),
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DOI: 10.1039/C6RA21054F
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exhibited the highest activity of 58.4% at 190 °C after 4 h. However, 15.
the PTBT selectivity over parent Hβ was the lowest (67.3%). The
dimensions of 12-ring portals of parent Hβ are 0.67 nm × 0.66 nm,
M. Mauvezin, G. Delahay, B. Coq, , S. Kieger, a. J. C. Jumas
and J. OlivierFourcade§, J. Phys. Chem. B, 2001, 105, 928-
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35.
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6.
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while the kinetic sizes are 0.58 nm for PTBT and 0.65nm for MTBT.
Therefore, the shape-selective action has less effect on the product
selectivity to PTBT over Hβ zeolite. In addition, further
isomerization of formed PTBT to MTBT on acid sites also decreased
the para-selectivity. The toluene conversion of 54.7% with PTBT
1
7.
2
41-249.
1
8.
S. S. Vieira, Z. M. Magriotis, M. F. Ribeiro, I. Graça, A.
Fernandes, J. M. F. M. Lopes, S. M. Coelho, N. A. V. Santos
and A. A. Saczk, Micropor. Mesopor. Mater., 2015, 201,
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selectivity of 81.5% was observed over Fe
90 °C after 4h. The selectivity to PTBT was improved significantly
from 67.3% to 81.5%. A possible mechanism was proposed to
explain the increased selectivity in three ways. i) The narrowed 19.
pores of Fe (x)/Hβ catalysts were benefit to formation of PTBT,
since the PTBT with lower kinetic diameter (0.58 nm) can diffuse
through the narrowed pores of Fe (x)/Hβ more easily than MTBT
with higher kinetic diameter (0.65 nm) because of its steric
restriction with narrowed pores of Fe (x)/Hβ. ii) The deactivation
of acid sties caused by the loading of Fe is benefit to increase
selectivity to PTBT by suppressing the further isomerization of PTBT
on acid sites. As a result, the Fe (x)/Hβ catalysts could enhance
and retain constant high para-selectivity. However, a significant
deactivation of catalysts containing 25% of Fe
was observed and 24.
attributed to a pore blocking phenomenon by excess Fe . With
further increases the Fe content to 25%, a sharp decrease in
2 3
O (20%)/Hβ catalyst at
1
2
O
3
2
2
2
2
0.
1.
2.
3.
R. Carvajal, P. J. Chu and J. H. Lunsford, J. Catal., 1990,
2
O
3
1
25, 123-131.
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2
O
3
2
O
3
2 3
O
2
O
3
2 3
O
2
5.
2 3
O
toluene conversion was observed, which is mainly due to sharp
decrease in B acid sites amount, which caused by blockage of pores
2 3
by excess Fe O .
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