Comparison of microporous/mesoporous and microporous HZSM-5 as catalysts for Friedel-Crafts alkylation of toluene with ethene

Article abstract of DOI:10.1039/c4ra02056a

In this work we investigated the effect of mesopores in a standard zeolite used as a catalyst for Friedel-Crafts alkylation of toluene with ethene. A cationic polymer was used for templating mesopores in a microporous ZSM-5 framework. The mesopore-containing zeolite was compared with a regular zeolite with only micropores with respect to conversion, yield and selectivity. The two NaZSM-5 materials were prepared with the same Si/Al molar ratio and diffuse reflection infrared Fourier transform spectroscopy (DRIFT-FTIR) confirmed that the acidity of the ion-exchanged forms (HZSM-5) were identical. Scanning electron microscopy (SEM) and dynamic light scattering (DLS) were used to determine the particle size of the zeolites, which was similar for the two HZSM-5 materials and nitrogen sorption was used to determine the surface area and pore size distribution. X-ray diffraction (XRD) analysis displayed typical crystalline diffraction patterns for the ZSM-5 framework for both the microporous/mesoporous and the microporous ZSM-5 materials. The results from catalytic testing show an increase in the overall conversion of toluene for the zeolite that contains mesopores. Furthermore, a higher product yield (C ₉) is obtained for this catalyst. The increase in yield and conversion is most likely due to the mesopores; however, incorporation of mesopores in the microporous ZSM-5 framework gives only minor effects on selectivity with respect to mono- vs. dialkylation, and ortho:meta:para ratio. Consequently, this work shows that the presence of mesopores in a microporous ZSM-5 framework is beneficial for the reaction in terms of conversion of starting material and reaction yield but does not markedly affect the product composition.

Full text of DOI:10.1039/c4ra02056a

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Comparison of microporous/mesoporous and
microporous HZSM-5 as catalysts for Friedel–
Crafts alkylation of toluene with ethene
a
Cite this: RSC Adv., 2014, 4, 28786
Zebastian Bohstrom, Hanna Harelind,b Borje Gevert,a Sven-Ingvar Anderssona
*
¨
¨
¨
a
*
and Krister Holmberg
In this work we investigated the effect of mesopores in a standard zeolite used as a catalyst for Friedel–
Crafts alkylation of toluene with ethene. A cationic polymer was used for templating mesopores in a
microporous ZSM-5 framework. The mesopore-containing zeolite was compared with a regular zeolite
with only micropores with respect to conversion, yield and selectivity. The two NaZSM-5 materials were
prepared with the same Si/Al molar ratio and diffuse reflection infrared Fourier transform spectroscopy
(DRIFT-FTIR) confirmed that the acidity of the ion-exchanged forms (HZSM-5) were identical. Scanning
electron microscopy (SEM) and dynamic light scattering (DLS) were used to determine the particle size
of the zeolites, which was similar for the two HZSM-5 materials and nitrogen sorption was used to
determine the surface area and pore size distribution. X-ray diffraction (XRD) analysis displayed typical
crystalline diffraction patterns for the ZSM-5 framework for both the microporous/mesoporous and the
microporous ZSM-5 materials. The results from catalytic testing show an increase in the overall
conversion of toluene for the zeolite that contains mesopores. Furthermore, a higher product yield (C9)
is obtained for this catalyst. The increase in yield and conversion is most likely due to the mesopores;
however, incorporation of mesopores in the microporous ZSM-5 framework gives only minor effects on
selectivity with respect to mono- vs. dialkylation, and ortho : meta : para ratio. Consequently, this work
shows that the presence of mesopores in a microporous ZSM-5 framework is beneficial for the reaction
in terms of conversion of starting material and reaction yield but does not markedly affect the product
composition.
Received 9th March 2014
Accepted 10th June 2014
DOI: 10.1039/c4ra02056a
www.rsc.org/advances
Thus, both microporous and mesoporous catalysts have
specic desirable properties, as well as specic drawbacks
Introduction
associated with the character of the pores. It would therefore be
advantageous to combine, in one material, the favorable prop-
erties of microporous catalysts with those of mesoporous cata-
lysts, thereby obtaining a uniquely versatile catalyst. Efforts
have recently been made to prepare mesoporous zeolites.6 It has
been shown that depending on the preparation route and the
synthesis conditions zeolites with different populations of
micro- and mesopores can be obtained. In this work we
prepared microporous zeolites with mesopores penetrating the
structure. The crystalline micropore structure is of MFI-type
(ZSM-5). The performance of this microporous/mesoporous
ZSM-5 zeolite has been studied in the Friedel–Cras alkylation
of toluene with ethene. In addition, we prepared a conventional
microporous ZSM-5 zeolite and compared its catalytic activity
with that of the microporous/mesoporous ZSM-5. The two ZSM-
5 zeolites were prepared with the same Si/Al weight ratio in
order to ensure that differences in acidity and activity of the
catalysts are due to differences in structure, not in composition.
The optimum conversion of toluene into ethyltoluene over
Microporous zeolites are extensively used as catalysts in
industry.1 However, microporous catalysts are oen associated
with efficiency problems such as short lifetime and poor mass
transport.2 The small pores are easily clogged by carbonaceous
material, in particular when the reaction temperature is high.
Mesoporous catalysts do not exhibit the same mass transport
and lifetime limitations as microporous catalysts.3 On the other
hand, the use of mesoporous catalysts in industrial processes is
oen restricted by the low hydrothermal stability of the mate-
rial.4 Furthermore, large pore catalysts sometimes lack the
isomer selectivity that can sometimes be obtained with micro-
porous catalysts.5
aDepartment of Chemical and Biological Engineering, Applied Surface Chemistry,
¨
Chalmers University of Technology, SE-412 96 Goteborg, Sweden. E-mail: kh@
chalmers.se; zebastian.bostrom@chalmers.se; Fax: +46 31 16 00 62; Tel: +46 31 772
29 69
bDepartment of Chemical and Biological Engineering and Competence Centre for
¨
Catalysis, Chalmers University of Technology, SE-412 96 Goteborg, Sweden
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zeolitic catalysts is reported to lie in the temperature range from under atmospheric pressure. Before the experiments the xed
300 C to 350 ꢀC.7
bed reactor was packed with quartz wool, followed by a layer of
glass pellets and another layer of quartz wool. Next, the catalyst
powder was added to the reactor and packed tightly. The cata-
lyst powder (4.0 g) consisted of a mixture of zeolite powder
(1.25 g) and silicon powder (2.75 g). Subsequently, quartz wool
was added on top of the catalyst powder and nally a layer of
glass pellets was applied (Table 1).
ꢀ
Experimental
Materials and reagents
Sodium aluminate (Sigma-Aldrich, technical), tetrapropy-
lammonium hydroxide solution (Sigma-Aldrich, 1 M in H2O),
tetraethyl
orthosilicate
(Sigma-Aldrich)
and
poly-
The reactor was heated to the reaction temperature and
ethene was introduced into the system over heated liquid
toluene. The ethene–toluene gas mixture (1 : 1) was fed into the
reactor and then separated in a cooling trap with cold glycerol
(for ow rates, see Table 2). The gas leaving the cooling trap was
periodically analysed and directly fed into a Perkin Elmer Clarus
500 GC equipped with a ame ionization detector (FID-GC) and
a thermal conductivity detector (TCD). The condensed products
collected in the cooling trap were analysed qualitatively with a
GC-MS setup and quantitatively with a GC-FID setup.
(diallyldimethylammonium chloride)(Aldrich, 65 wt% in H2O)
were used to prepare the NaZSM-5 zeolites. Ammonium nitrate
(A.C.S. reagent $ 98%), LiBO2$H2O (Aldrich) and nitric acid
(Sigma-Aldrich, A.C.S. 70%) were used to ion-exchange the
zeolites from Na-form to H-form. Toluene (Sigma-Aldrich,
99.9%) and ethene gas (AGA, 95%, containing 3 ꢁ 2 mol%
nitrogen) were used for the alkylation reaction. 2-Ethyltoluene
(purum, $98%, FLUKA), 3-ethyltoluene (purum, $98%, FLUKA)
and 4-ethyltoluene ($95%, FLUKA) were used for identication
of relative amounts of ethyltoluene isomers in the liquid
samples produced from the continuous reactor.
Analysis techniques
Preparation of ZSM-5 zeolites
Determination of the specic surface area was performed on an
ASAP 2010 instrument, using nitrogen adsorption and calcu-
lated with the BET (Brunauer–Emmett–Teller) method.10 The
pore size distribution was calculated from the isotherms using
the BJH (Barett–Joyner–Halenda) procedure.11 All samples were
The microporous and microporous/mesoporous ZSM-5 zeolites
were prepared by essentially following the procedure described
by Wang et al.8 Sodium aluminate (1.12 g), tetraethyl orthosili-
cate (39.76 g) and tetrapropylammonium hydroxide (56.0 g)
were added to distilled water (140.0 g) and aged at 100 ꢀC
ꢀ
dried at 225 C in a vacuum oven for approximately 3 h before
the measurement.
under stirring for
3
h. Then the polymer poly-
(diallyldimethylammonium chloride) (10 wt%, 42.0 g) was
added to the mixture. For the preparation of the microporous
ZSM-5 no polymer was added. Aer stirring at room tempera-
ture for 48 h, the reaction mixture was transferred into an
autoclave and crystallised at 180 ꢀC for 144 h. The product was
Samples for scanning electron microscopy (SEM) were run
on a LEO, Ultra, 55 FEG, SEM equipped with an Oxford Inca
EDX system operated at 1–2 kV. A secondary electron detector
was used for detection. Samples for SEM were prepared by
dispersing a small amount of zeolite in ethanol and then
grinding the mixture and placing a droplet onto the metallic
sample holder.
ꢀ
collected by centrifugation and calcined at 550 C for 5 h with
1 h ramping.
Samples for transmission electron microscopy (TEM), run on
a JEOL 1200 EX II instrument at 120 kV, were prepared by
placing a drop of an ethanol dispersion of the zeolitic material
onto a copper Holey grid.
Preparation of the zeolites in H-form
The Na-zeolites were converted into H-zeolite following the
procedure described by Punyapalakul et al.9 Ammonium nitrate
solution (1.0 M) was mixed with the zeolite at a ratio of 1 : 20,
solid : liquid. The mixture was stirred at 60 C for 2 h, ltered
and washed with Milli-Q water (18.2 MU cm). This exchange
In situ FTIR (Fourier Transform Infrared) spectroscopy
measurements were carried out using a BioRad FTS 6000
spectrometer equipped with a Harrick Praying Mantis DRIFT
(Diffuse Reection Infrared Fourier Transform) reaction cell.12
The sample was put in the DRIFT cell and the gases; Ar, NH3
and O2 were introduced via mass ow controllers (Bronkhorst
Hi-Tech) to the cell. The samples were initially pre-treated in O2
ꢀ
procꢀedure was repeated. The solid was collected and dried at
ꢀ
100 C for 12 h and then calcined at 400 C for 3 h with a 6 h
ramping. The degree of ion-exchange was determined with
atomic absorption spectroscopy. The rst ion-exchange cycle
resulted in a 42% exchange of the sodium content in the
sample. Aer two cycles 87% of the sodium had been
exchanged.
ꢀ
(8%) at 500 C for 30 minutes and then evacuated in Ar for 15
minutes (keeping the total ow rate constant at 200 ml minꢂ1).
ꢀ
Adsorption of NH3 (1000 ppm) was performed at 25 C during
30 minutes, followed by evacuation in Ar (6 scans per min,
1 cmꢂ1 resolution). Background spectra were collected in Ar
Friedel–Cras alkylation reactor setup
The Friedel–Cras alkylation reactions were performed in a (6 scans per min, 1 cmꢂ1 resolution).
xed bed down-ow reactor with an internal diameter of 10 mm
X-ray powder diffraction (XRD) was performed on a LynxEye
and a length of 175 mm. The reactor was equipped with a AXS D8 ADVANCE q/2q diffractometer, equipped with a
heating jacket and a high sensitivity temperature controller. linear detector. The runs were performed at 40 kV and 40 mA,
The setup was designed to carry out a vapour phase reaction in monochromatic mode with G(111) CuKa1 radiation
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(l ¼ 1.5406 A, step size 0.050, step time 366 s and primary slit 275 ꢀC. Toluene, para-ethyltoluene, meta-ethyltoluene and
˚
width 0.2 mm).
ortho-ethyltoluene were injected into the GC-MS setup to
Atomic absorption spectroscopy was performed on a Thermo identify the specic retention times of each of these
Scientic ICE 3000 Series AA spectrometer. The solid catalyst compounds.
(100 mg) was mixed with LiBO2$H2O (1.0 g) in a platinum
crucible and the mixture was heated at 1000 ꢀC for 13 minutes.
When the crucible had cooled to room temperature, aqueous
Results and discussion
The microporous and the microporous/mesoporous ZSM-5
zeolites
nitric acid (3 wt%) was added (1.5–3.0 ml) and the mixture in
the platinum crucible was stirred at 300 rpm for 6 h. The
content in the crucible was added to a 25 ml ask and aqueous
nitric acid (3 wt%) was added to the 25 ml mark. A ame was
used to atomize the sample and a sodium hollow cathode lamp
was used for the irradiation.
Dynamic light scattering (DLS) was performed on a BI-90
Brookhaven Instruments particle sizer. Samples were prepared
by taking a small fraction of the solid material and dispersing it
into a glass beaker containing 5.0 ml ltered (0.2 mm lter)
Milli-Q water (18.2 MU cm). The beaker was immersed in a
sonic bath for 1 minute. 3.0 ml of this solution was then
transferred into another glass beaker containing 3.0 ml ltered
(0.2 mm lter) Milli-Q water. This beaker was then immersed
into a sonic bath for 1 minute and 2.5 ml of this solution was
transferred into the quarts cuvette with a syringe tted with a
1.2 mm lter and analysed.
Two different types of zeolites were prepared, microporous
NaZSM-5 and microporous/mesoporous NaZSM-5. The zeolites
were prepared with the same Si/Al molar ratio. In Fig. 1 the
diffractograms of the two NaZSM-5 zeolites are shown. The
crystalline structure displayed is typical for the ZSM-5 zeolite
type.13 In addition, no difference in intensities was observed
between the two ZSM-5 zeolites. The microporous NaZSM-5
displayed a nitrogen sorption isotherm of Type I with a H4
hysteresis characteristic for microporous materials,14 see Fig. 2.
The microporous/mesoporous NaZSM-5 displayed a Type IV
sorption isotherm and a H3 hysteresis. The Type IV sorption
isotherm is typical for mesoporous materials.14 H3 hysteresis
has been interpreted as an intermediate between H1 and H4
hysteresis curves.14 The hysteresis ranging from 0.8 to 0.95 seen
for the microporous/mesoporous NaZSM-5 zeolite is indicative
of hierarchical mesopores.8 In situ DRIFT experiments with
ammonia adsorption followed by argon ushing were per-
formed in order to characterize the surface acidity of the
samples and the results are shown in Fig. 3. When ammonia is
adsorbed on the microporous HZSM-5 sample multiple peaks at
3395, 3275 and 1288 cmꢂ1 evolve. These peaks can, according to
Wang et al. and Tsyganenko et al., be ascribed to NH3 adsorbed
on Lewis acid sites.15 Another group of peaks at 1572 and
1510 cmꢂ1 can be attributed to NH2 surface species formed
concurrently with OH groups during ammonia dissociation.15
Furthermore, the negative peaks at 3689 and 3610 cmꢂ1 are
indicative of blocking of surface OH species by adsorbed NH3.15
Similar peaks evolve upon addition of ammonia to the micro-
porous/mesoporous HZSM-5. No signicant differences are
observed for the two samples, indicating that the acidic sites are
similar. This is not surprising since the composition of the two
Flame ionization and thermal conductivity gas chromatog-
raphy (FID-GC and TCD-GC, respectively) was performed on a
PE Clarus 500 GC and used to monitor the gaseous products
formed during the reaction. The instrument was tted with a
standard RGA Model 1115 Analyser GC column set. The
injector temperature was 50 ꢀC, helium was used as carrier gas
and nitrogen was the actuator gas. Both gases were kept at a
pressure of 6.2 bars. The gases were used at a ow rate of
0.91 ml minꢂ1 and the splitow was 50 ml minꢂ1. The TCD
detection temperature was 200 ꢀC and the current used for the
detector was 40 mA. The FID detection temperature was 250 ꢀC.
Samples from the continuous reactor (5.0 ml) were introduced
from a 0.125 and a 1.0 cm3 gas loop valve into the GC. The
analysis was performed during 15 minutes at a temperature of
60 ꢀC. The liquid product formed during the reaction was
analysed qualitatively with GC-MS and quantitatively with a
GC-FID setup. The GC-MS analysis was performed with a
HP5890 gas chromatograph coupled to a thermo trace mass
spectrometer and the GC was tted with an Agilent J&W DB-
5MS, 30 m ꢃ 0.25 mm ꢃ 1.0 mm column. The analysis was
performed with an injector temperature of 200 ꢀC, helium was
used as the carrier gas at a ow of 1.2 ml minꢂ1, the splitow
was 40 ml minꢂ1 and the scans performed at a mode of 40–
ꢀ
300 amu at an ion source temperature of 200 C. The temper-
ature program used started at 40 ꢀC and was maintained at that
temperature for 3 min, then ramped to 125 ꢀC at a rate of 4 ꢀC
minꢂ1, then increased from 125 ꢀC to 150 ꢀC at a rate of 2.5 ꢀC
minꢂ1, and nally went from 150 ꢀC to 200 ꢀC at a rate of 5.0 ꢀC
minꢂ1. For the quantitative GC-FID analysis the same GC
column and temperature program was used for the ꢀqualitative
GC-MS analysis. The injector temperature was 275 C, helium
was used as carrier gas at a ow of 0.92 ml minꢂ1 and the
Fig. 1 XRD diffractograms of the (a) microporous and (b) the micro-
splitow was 40 ml minꢂ1. The FID detection temperature was porous/mesoporous NaZSM-5.
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Fig. 2 Nitrogen sorption isotherms of microporous NaZSM-5 and microporous/mesoporous NaZSM-5.
zeolites is the same. Fig. 4 shows scanning electron microscopy presented above one can conclude that the microporous and the
(SEM) images of the microporous and the microporous/meso- microporous/mesoporous ZSM-5 zeolites are very similar in
porous NaZSM-5 zeolites, as well as transmission electron terms of particle size, specic surface area and chemical
microscopy (TEM) micrograph of the microporous/mesoporous composition, including acidic surface sites. However, as indi-
NaZSM-5. The SEM images show that the particle size is similar cated by both the nitrogen sorption analysis and the electron
for the two zeolites. The morphology of the particles is very microscopy images the ZSM-5 prepared with a polymer
different, however. Whereas the microporous zeolite particles template contains pores not only in the micro size but also in
have a smooth surface and round edges, the microporous/ the meso size range. Thus, any difference in yield, rate or
mesoporous zeolite particles are very rough. They appear to selectivity of an organic reaction where these materials are used
consist of granules glued together. The TEM image analysis as catalyst can be attributed to the presence of mesopores in one
indicates that the granules are porous or hollow. From the data of the zeolites.
Fig. 3 DRIFT spectra showing surface NH3 and NH4+ species after adsorption of ammonia during 30 minutes followed by argon flushing during
30 minutes.
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Table 1 Zeolite characteristics obtained from nitrogen sorption data, dynamic light scattering (DLS) and scanning electron microscopy (SEM)
Nitrogen sorption
Average particle size (nm)
Calculatedb
(r, nm)
Measuredc
(r, nm)
Pvol
Microvol
Vtot
a
c
d
a
as,BET
Zeolite
(m2 gꢂ1
)
(cm3 gꢂ1
)
(cm3 gꢂ1
)
(cm3 gꢂ1
)
DLSe
SEMf
Micro/mesoporous ZSM5
Microporous ZSM5
380
372
1.32
0.91
3.61
—
0.10
—
0.13
0.11
0.25
0.17
312 ꢁ 26
328 ꢁ 38
350 ꢁ 80
340 ꢁ 110
a
b
c
Calculated from nitrogen sorption using the BET method. Calculated 2Vtot/SBET
.
Calculated from the adsorption branch of the nitrogen
d
e
sorption isotherm using the BJH method. Calculated from t-plot. Particle size determined from DLS analysis presented as means of 4
independent runs. f Particle size determined with SEM. The values reported are means of 50 particles.
in the gaseous fraction from the reaction was negligible.
Whereas the conversion of ethene is quantitative already at 325
ꢀC with the microporous/mesoporous catalyst, reaction over the
conventional microporous zeolite did not reach full ethene
conversion until the temperature reached 375 ꢀC. This indicates
a slightly higher activity of the catalyst that contains mesopores.
The gures for toluene conversion give the same picture; the
catalyst with mesopores seems to be more active. The
The Friedel–Cras alkylation reaction
Alkylation of toluene over zeolitic catalysts has been the topic of
many studies. The activity and selectivity of conventional
microporous zeolites, in particular HZSM-5 zeolites, have been
investigated indepth.16 In this study we have investigated and
compared the conversion, yield and regioselectivity of the
microporous and the microporous/mesoporous HZSM-5
zeolites as catalysts for the Friedel–Cras alkylation of toluene
with ethene. However, before any catalytic testing were per-
formed the mesoporous/microporous and microporous ZSM-5
zeolites thermal and hydrothermal stability was investigated. It
was found that the mesoporous/microporous ZSM-5 was stable
under temperatures and conditions used for catalytic testing.
Alkylations at high temperatures in continuous ow-bed
reactors usually give rise to several side reactions that occur in
parallel to the target alkylation reaction. Transalkylation,
disproportionation, dealkylation and catalytic cracking are
examples of such side reactions17 and they are illustrated in
Scheme 1 for the toluene–ethene system. Furthermore, previous
studies at our laboratory have shown that ethene starts to
undergo zeolite-catalyzed oligomerization at temperatures
around 320 ꢀC. Catalytic cracking over HZSM-5 zeolites is
generally performed at temperatures ranging from 550 ꢀC to
650 ꢀC.18 However, catalytic cracking occurs also at lower
temperatures.19 Suza et al. recently reported that at 350 ꢀC there
was 10–13% cracking conversion of natural gasoline in a
continuous ow reactor.19b All these side reactions give rise to
other products than the target product of the Friedel–Cras
reaction, i.e., ethyltoluene (and possibly some diethyltoluene)
and these products will all appear in the liquid fraction from the
reaction. GC analysis showed that the yield of C5–C11 products
Scheme
1 Friedel–Crafts alkylation of toluene with ethene and
possible side reactions.
Fig. 4 (a and b) are SEM images of the microporous and the microporous/mesoporous NaZSM-5, respectively. (c) is a TEM image of the
microporous/mesoporous NaZSM-5.
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Table 2 Yield of C5–C11 hydrocarbons obtained with the microporous/mesoporous HZSM-5 and microporous HZSM-5 at various conditions
Catalyst
Microporous/mesoporous HZSM-5
Microporous HZSM-5
Temperature (ꢀC)
325
30
32
350
30
61
375
30
70
400
30
65
375
15
70
375
60
68
325
30
46
350
30
49
375
30
49
400
30
41
Flow (ml minꢂ1
)
Toluene conversiona (%)
Ethene conversionb (%)
>99
>99
>99
>99
>99
97
93
95
>99
>99
Products (wt%)
C5
C6
C7
C8
C9
C10
C11
C12
n.d.
n.d.
73
n.d.
25
1
1
n.d.
n.d.
24
5.9
0.4
38
3.5
40
5.6
5.1
0.5
n.d.
38
5.7
0.4
30
3.8
46
8.0
6.0
0.1
n.d.
45
5.2
0.1
32
12
37
5.7
5.9
2.0
0.1
34
9
4.4
0.3
28
5.7
44
9.8
6.2
1.6
n.d.
42
3.9
0.2
30
10
38
7.7
6.7
3.4
0.1
37
8
4.0
0.3
54
0.5
27
6.7
6.5
n.d.
n.d.
23
1.2
0.3
56
0.2
34
2.4
5.9
n.d.
n.d.
33
0.4
0.1
57
n.d.
34
2.0
6.5
n.d.
n.d.
34
0.2
n.d.
66
n.d.
30
0.1
3.7
n.d.
n.d.
29
C13
Ethyltoluene C9
Ortho
—
3
6
9
1
6
6
7
Meta
48
52
20.3
55
42
6.4
58
36
6.2
59
32
4.8
61
30
5.8
62
30
4.6
54
45
3.4
56
38
4.7
57
37
4.2
56
37
6.6
Para
Mono/di ratioc
a
b
c
Calculated from C7, C9 and C11 molar fraction in the liquid samples. Calculated from in situ GC with TCD detector. Mol ratio. n.d. ¼ not
detected.
conversion of toluene is clearly temperature dependent, in
No attempts have been made to derive the formation
particular for the microporous/mesoporous catalyst, but it mechanism for the C8 and C10 fractions. Several of the pathways
never reaches above 70%. The obvious interpretation of these shown in Scheme 1, as well as ethene oligomerization, may lead
results is that ethene is a very reactive reactant, which partici- to such products. The fact that the difference in yield for the two
ꢀ
ꢀ
pates in reactions other than the Friedel–Cras alkylation, seet catalysts is larger at 375 C than at 325 C indicates that pore
he discussion above. When it comes to product distribution, the clogging is a deactivation mechanism. It seems reasonable that
C7 and the C9 fractions dominate, for both catalysts and at all the catalyst that contains mesopores is more resistant to clog-
temperatures. The C7 fraction is unreacted toluene and the C9 ging by high molecular weight carbonaceous materials than the
fraction is mainly, but not exclusively, ethyltoluene. The relative catalyst with only micropores. The drop in reaction yield with
ꢀ
amount of the C9 fraction, and also of ethyltoluene specically, both the catalysts when the temperature is raised to 400 C is
seems to reach a maximum at around 375 ꢀC for both the probably due to catalytic cracking of the reaction product
catalysts. The fact that the yields of the C9 and the C11 fractions becoming important, as was discussed above. Table 2 gives the
are lower at 400 ꢀC than at 375 ꢀC for both the catalysts indicates relative amounts of the three regioisomers of ethyltoluene
that catalytic cracking of monoethyl- and diethyltoluene obtained with the two catalysts at the different temperatures. As
becomes important at the higher temperature. The drop in yield can be seen, both the microporous/mesoporous and the
when going from 375 ꢀC to 400 ꢀC is particularly pronounced for microporous HZSM-5 give very little alkylation in ortho position.
the mesopore-containing catalyst, which suggests that this This is in agreement with previous studies on ethylation
material is a more active cracking catalyst. However, the yields of toluene with ethene over HZSM-5 zeolite.16a Whereas
are generally higher for the microporous/mesoporous catalyst the thermodynamic equilibrium for ethylation of toluene is
than for the microporous catalyst, which is in line with the para : meta : ortho 30–35 : 50 : 15–20 (values are temperature
values for conversion of toluene. As can be seen from Table 2, dependent), alkylation with ethene over HZSM-5 gives a much
several other fractions, in particular C8, C10 and C11, are lower yield of the ortho isomer.16e Unmodied HZSM-5 has been
generated in non-negligible amounts.
reported to give a higher relative amount of the meta isomer and
The C11 fraction can be attributed to dialkylation, i.e., to a somewhat lower relative amount of the para isomer. The
formation of diethyltoluene and the ratio of monoalkylation to regioselectivity also depends on the reaction temperature and
dialkylation has been calculated and is also given in Table 2. on the acidity of the catalyst. A decrease in temperature results
One may have anticipated that the larger pores of the meso- in an increase in the relative amount of the para isomer.7a The
¨
pores-containing catalyst would have resulted in more dia- para selectivity also increases when the Bronsted acidity is
lkylation but that was evidently not the case. On the contrary, reduced.16c By a proper choice of catalyst and reaction condi-
reaction over microporous/mesoporous HZSM-5 at the lowest tions a para selectivity well above 90% can be achieved. These
temperature (325 ꢀC) gives a very high ratio of mono- to dia- deviations from the thermodynamical isomer ratio are caused
lkylation and reaction over microporous HZSM-5 at the same by the fact that ortho-ethyltoluene has the largest and para-
temperature gives the lowest ratio.
ethyltoluene the smallest minimum dimension. The product
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ratio will depend on the relative rates of diffusion of the
regioisomers. It has been demonstrated that the diffusion of the
para isomer of ethyltoluene in the pores of a zeolite can be
several orders of magnitude higher than that of the bulkier
ortho and meta isomers.20 The more slowly moving ortho and
meta isomers remain within the zeolite and once the para
isomer is generated as a result of the random isomerization that
occurs under the inuence of the acidic catalyst it rapidly
diffuses out. The net result is that the relative amount of para-
ethyltoluene can become very high and the relative amount of
ortho-ethyltoluene, which is the most voluminous and therefore
the most slowly diffusing isomer, virtually zero. Table 2 show
that the ortho to meta to para ratio did not differ dramatically for
the two types of catalysts. Thus, the large pores in the micro-
porous/mesoporous material did not result in a higher relative
yield of the more bulky ortho and meta isomers. For the catalyst
with mesopores there was a substantial increase in the meta to
para ratio with an increase in temperature. This is in agreement
with previously reported trends.7a Reactions over the regular
microporous catalyst gave almost the same meta to para ratio at
the different temperatures, however. The effect of the ethene
ow rate was investigated for microporous/mesoporous HZSM-
5 at 375 ꢀC. As can be seen in Table 2, neither the conversion of
toluene, nor the product composition was much inuenced by
this parameter under the conditions studied.
Notes and references
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The microporous/mesoporous HZSM-5 catalyst gave a slightly
higher conversion than the conventional microporous zeolite in
the Friedel–Cras alkylation of toluene with ethene. The yield of
ethyltoluene was also somewhat higher with the former catalyst.
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Acknowledgements
Cecilia Thorstensson at AkzoNobel Surface Chemistry is
acknowledged for help with the GC-MS analysis. The Swedish
Research Council is acknowledged for nancial support.
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