Conversion of α-methylstyrene over pentasil zeolites with various silica ratios

Article abstract of DOI:10.1134/S096554410805006X

It has been shown that α-methylstyrene in the presence of H-HSZ (SiO₂/Al₂O₃ = 40-120) and Fe-HSZ (SiO ₂/Al₂O₃ = 60) pentasils in a nitrogen atmosphere (80-120°C, 5-20 wt % catalyst) is converted into linear (4-methyl-2,4-diphenylpent-1-and-2-enes) and cyclic (1,1,3-trimethyl-3- phenylindan) dimers. The H-type high-silica (SiO₂/Al ₂O₃ = 120) and ferrosilicate zeolites in air facilitate the conversion of α-methylstyrene into acetophenone and α-phenylpropionaldehyde.

Full text of DOI:10.1134/S096554410805006X

ISSN 0965-5441, Petroleum Chemistry, 2008, Vol. 48, No. 5, pp. 366–370. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © N.G. Grigor’eva, R.R. Galyautdinova, A.V. Vosmerikov, L.L. Korobitsina, B.I. Kutepov, U.M. Dzhemilev, 2008, published in Neftekhimiya, 2008, Vol. 48,
No. 5, pp. 363–367.
Conversion of a-Methylstyrene over Pentasil Zeolites
with Various Silica Ratios
N. G. Grigor’evaa, R. R. Galyautdinovaa, A. V. Vosmerikova, L. L. Korobitsinab,
B. I. Kutepovb, and U. M. Dzhemileva
a Institute of Petroleum Chemistry and Catalysis, Russian Academy of Sciences, Ufa, Bashkortostan, Russia
b Institute of Petroleum Chemistry, Siberian Branch, Russian Academy of Sciences, Tomsk, Russia
e-mail: ink@anrb.ru
Received January 17, 2008
Abstract—It has been shown that α-methylstyrene in the presence of H-HSZ (SiO2/Al2O3 = 40–120) and
Fe-HSZ (SiO2/Al2O3 = 60) pentasils in a nitrogen atmosphere (80–120°C, 5–20 wt % catalyst) is converted
into linear (4-methyl-2,4-diphenylpent-1- and -2-enes) and cyclic (1,1,3-trimethyl-3-phenylindan) dimers. The
H-type high-silica (SiO2/Al2O3 = 120) and ferrosilicate zeolites in air facilitate the conversion of α-methylsty-
rene into acetophenone and α-phenylpropionaldehyde.
DOI: 10.1134/S096554410805006X
Dimers of α-methylstyrene find practical use as with a pore or cavity size of 0.6–1.2 nm generally
chain-growth regulators, rubber plasticizers, heat trans-
fer fluids, jet fuels, and lubricant and insulation bases.
exhibit the highest activity. The regioselectivity of the
formation of preferentially linear dimers (1) and (2) or
a cyclic dimer (3) depends on both the structure of a
zeolite and its acidity, the concentration and strength of
An analysis of α-methylstyrene dimerization in the
presence of zeolites of various structural types (FAU,
MOR, BEA, MTW) revealed that wide-pore catalysts acid sites [1–3].
CH3
CH
CH3
CH2
CH3
C
CH3
H3C
C
C CH3
H3C
C
C CH2
H2C
H+
+
+
2
CH3
(3)
H3C
(1)
(2)
4-Methyl-2,4-diphenylpent-1-ene 4-Methyl-2,4-diphenylpent-2-ene
1,1,3-Trimethyl-3-phenylindan
However, the dimerization of α-methylstyrene over
medium-pore MFI zeolites (channel size, 0.5–
0.56 nm), which are effective catalysts in petroleum
chemistry, has not been studied. It is known [4] that
important factors determining the acid and catalytic
properties of zeolites to a great extent are the
SiO2/Al2O3 mole ratio and modification with various
metal cations. Yoon et al. [5] found that Si- and Mn-
modified zeolites (SÄêé4-8 and ån-Älêé4-8) exhib-
ited a high activity in the dimerization of α-methylsty-
rene, being, however, less selective for linear dimers 1
and 2 than the starting catalysts. The data reported in
[6] also suggest that the introduction of iron into zeolite
Y (faujasite (FAU) structure) can accelerate the cycliza-
tion of the linear styrene dimer.
In this paper, we present the results of the study of
the conversion of α-methylstyrene over high-silica zeo-
lites (HSZ) of the MFI pentasil family with various
SiO2/Al2O3 mole ratios (40 to 120) and over the ferro-
silicate zeolite Fe-HSZ. The catalytic properties of the
test zeolites were correlated to their acid characteristics
measured by the technique of ammonia temperature-
programmed desorption (TPD).
EXPERIMENTAL
The conversion of α-methylstyrene was studied in
an isothermal batch reactor at 80–120°ë in a flow of
ambient air fed through a cooler or in nitrogen passed
through a bubbler at a rate of 3–4 l/h. The reaction mix-
ture mass was stirred at a rate of 700–800 rpm. During
the reaction, the mixture was sampled for analysis at
366

CONVERSION OF α-METHYLSTYRENE OVER PENTASIL ZEOLITES
367
Table 1. Conversion of α-methylstyrene on H-HSZ pentasils zeolites with different SiO2/Al2O3 mole ratios
Reaction conditions
gas medium [cat], wt % T, °C
80
Selectivity, %
CD LD
Catalyst
sample
AMS conver-
sion, wt %
SiO2/Al2O3
τ, h
OP
Tr
6
5
1
2
3
6
5
1
3
6
3
5
1
3
3
1
3
6
5
29.8
41.8
77.2
89.0
93.8
33.1
35.8
76.9
92.9
30.6
86.1
49.6
76.1
92.0
28.9
68.3
80.8
82.8
92.9
–
–
6.4 77.5 16.1
20.4 66.5 13.1
7.6 80.9 11.5
8.0 80.3 11.7
8.1 80.4 11.5
7.3 81.9 10.8
Air
5
120
I
40
–
Nitrogen
5
80
–
–
80
–
Air
5
5
120
–
21.9 71.8
6.3
II
60
80
–
7.1 82.0 10.9
7.6 81.6 10.8
8.8 76.1 15.1
Nitrogen
80
–
5
20
5
80
80
–
Air
–
8.0 82.7
9.3
3.8
III
120
–
22.2 74.0
–
7.3 81.6 10.8
8.2 80.5 11.3
Nitrogen
Air
5
5
5
80
80
80
–
93.8
–
1.7
2.8
1.7
9.8
9.9
8.7
3.0
5.2 85.0
5.9 84.2
5.5 86.8
8.9 88.1
IV
120
–
Nitrogen
80
–
10
120
–
Note: OP denotes oxidation products 4 and 5, CD is cyclic dimer 3; LD denotes linear dimers 1 and 2, and Tr stands for trimers.
certain intervals (1, 2, 3, and 5 h). The reaction mixture
The acid properties were studied by means of chro-
and the samples were weighed before and after the matography and ammonia temperature-programmed
experiment to determine the mass balance; the loss was desorption (TPD) [7]; the thermal desorption consists
3–4 wt %.
in the heating of a catalyst sample with preliminarily
absorbed probe molecules (gaseous ammonia) at a lin-
ear rate of 10°C/min in a helium flow. The concentra-
tion of the acid sites in the samples was determined as
the amount of the ammonia desorbed by the time of
detection of desorption peaks and was expressed in
micromoles per gram of the catalyst.
The reactant α-methylstyrene was distilled prior to
use; its purity was 99.8 wt %. The activity of the cata-
lysts was measured in terms of the α-methylstyrene
conversion (K). The selectivity for products was calcu-
lated by the formula Si = ci/ä.
The products were analyzed with a HRGS 5300
Mega Series Carlo Erba chromatograph equipped with
a flame-ionization detector, on a 25-m glass capillary
column coated with the Ultra liquid stationary phase.
To identify the products, ç- and C NMR spec-
troscopy techniques were used.
High-silica zeolites with a SiO2/Al2O3 mole ratio of
40–120 were synthesized via the hydrothermal proce-
dure from alkaline aluminosilica gels using hexameth-
ylenediamine as the organic additive. To obtain the
RESULTS AND DISCUSSION
1
13
The data presented in Table 1 show that the zeolites
with SiO2/Al2O3 mole ratios of 40, 60, and 80 possess
very close catalytic properties. In the presence of these
catalysts, α-methylstyrene reacts to give dimers (1–3)
and a small amount of trimers (4–16 wt %).
Linear isomers 1 and 2 prevail among the dimers,
active H-form (H-HSZ), the zeolites were treated with being formed with a selectivity of 76.0–84% at 80°ë.
a 25% ammonium chloride solution followed by drying As the reaction temperature is increased to 120°ë, the
at 100°ë and calcining in air at 540°ë for 6 h. The fer- rate of formation of the cyclic dimer increases; as a
rosilicate zeolite (Fe-HSZ) was prepared by adding iron result, its formation selectivity increases by a factor of
to zeolite immediately at the hydrothermal synthesis 2–3 and the selectivity for linear dimers decreases to
step (mole ratio SiO2/(Al2O3 + Fe2O3 = 60). Prior to test- 65–72%. The yield of trimers also decreases in this
ing, the zeolites samples were calcined in air at 540°ë case. The kinetic curves showing the time change in the
for 4 h.
concentration of dimers 1–3 resemble in character
PETROLEUM CHEMISTRY Vol. 48 No. 5 2008

368
GRIGOR’EVA et al.
Table 2. Transformation of α-methylstyrene in air in the ab-
In an air atmosphere, the α-methylstyrene conver-
sion decreases to ≈30% and α-methylstyrene is oxi-
dized to acetophenone (4) and α-phenylpropionalde-
hyde (5):
sence of catalysts
Amount, %
T, °C
τ, h–1
OP
AMS
CH3
O
CH3
CH3
CH2
1
5
1
5
0.1
0.5
0.3
1.2
99.9
99.5
99.7
98.8
O
80
(O2)
+
120
(4)
(5)
A study of α-methylstyrene transformations in the
absence of the catalyst showed (Table 2) that the
amount of oxidation products 4 and 5 formed under the
given conditions is much lower than that produced over
the HSZ pentasil (SiO2/Al2O3 = 120).
those obtained previously derived over zeoliteY [8]: as
the α-methylstyrene conversion increases, the concen-
tration of linear dimers passes through a maximum and
that of the cyclic dimer increases.
A comparison of the activity of H-HSZ pentasils
I-III in air shows that the value of the SiO2/Al2O3 mole
ratio barely affects the conversion of α-methylstyrene.
The main factor that determines the α-methylstyrene
conversion is the catalyst concentration. For example,
the conversion of α-methylstyrene is 30-40 wt % in the
presence of 5 wt % catalyst, reaching 94–96 wt % as the
catalyst concentration increases to 20 wt %.
The capability of pentasil in the H-form to catalyze
the oxidation of α-methylstyrene will not be surprising
if we compare the obtained results with published data
[9, 10]. Direct experimental evidence for the presence
of singlet oxygen on the surface of H-ZSM-5 zeolites
was found in [9, 10]. The authors of the cited studies
suggested that the Lewis acid sites can efficiently gen-
erate and retain singlet oxygen. According to the theory
of heterogeneous catalysis, it is singlet oxygen that
either itself is the site of the heterogeneous catalytic
oxidation or takes part in the formation of such sites. It
is known that zeolites containing transition metal cat-
ions exhibit a high catalytic activity in hydrocarbon
oxidation reactions [11]. Indeed, the examination of the
catalytic properties of the Fe-HSZ zeolite showed that
α-methylstyrene is oxidized in its presence to com-
pounds 4 and 5. However, the activity of this catalyst in
an air atmosphere is much lower than that of the HSZ
(SiO2/Al2O3 = 120), and the selectivity for the oxidation
products is below that for the dimerization product,
38.3 versus 61.7%, respectively. The selectivity for 4
and 5 decreases with an increase in the temperature and
catalyst concentration (figure).
The reaction over Fe-HSZ pentasil in a nitrogen
atmosphere yields with a high selectivity only the oli-
gomerization products of α-methylstyrene, 88%
dimers and 12% trimers.
Thus, although the activity of the Fe-HSZ zeolite in
the oxidation reaction was predetermined by the pres-
ence of the transition metal, this catalyst proved to be
less effective than the HSZ (SiO2/Al2O3 = 120) zeolite.
An ammonia TPD study of the acid properties of the
zeolites showed that all H-HSZ test samples have acid
sites of two types, the weak sites with a peak maximum
(ím‡ı) in the range 210–235°ë and the strong sites with
ím‡ı at 410–450°ë (Table 3). As the Si/Al ratio
increases from 40 to 120, the concentration of both
The catalysts were found to exhibit a higher activity
in a nitrogen atmosphere. For example, the α-methyl-
styrene conversion on catalyst I for 6 h in air did not
exceed 30 wt %, whereas it reached 94 wt % for 3 h in
nitrogen (Table 1). An increase in the reaction time to
5–6 h resulted in a 97–98 wt % conversion and a change
in the ratio of linear dimers 1 and 2 to cyclic dimer 3 in
favor of the latter. The conversion of α-methylstyrene
decreases with an increase in the silica ratio of pentasils
from 40 to 120.
Since the dimerization of α-methylstyrene is cata-
lyzed by the Brönsted acid sites (BAS), the decline in
the activity of pentasils in the reaction in air can be
associated with a decrease in the concentration of the
OH groups on the external surface of zeolite crystals
(access of α-methylstyrene molecules having a size of
0.43 × 0.72 nm to the OH groups that occur inside the
zeolite channels is restricted as the channels are
0.51−0.55 nm in diameter). In an air atmosphere, the
Brönsted acid sites on the external surface of the zeolite
crystals seem to be blocked by oxygen adsorbed on the
Lewis acid sites. Another cause for the decline in the
initial activity of zeolites can be the formation, as a
result of the oxidation processes, of high-molecular-
mass hydrocarbons tightly bound to the surface.
Interesting results were obtained in the case of
α-methylstyrene transformation over the high silica
H-zeolite with a SiO2/Al2O3 mole ratio of 120
(sample IV).
Like the previous samples I−III, this catalyst exhib- strong and weak acid sites decreases by a factor of more
its a high activity in a nitrogen atmosphere. Under these than 2. The strength of the acid sites measured as the
conditions, α-methylstyrene yields the linear dimers shift of the peak in the thermal-desorption spectrum
with a high selectivity, 87–88%.
also decreases.
PETROLEUM CHEMISTRY Vol. 48 No. 5 2008

CONVERSION OF α-METHYLSTYRENE OVER PENTASIL ZEOLITES
369
α-Methylstyrene conversion, wt %
Selectivity, %
100
(‡)
(b)
1
2
2
1
80
60
40
20
0
3
3
60
80
100
120 T, °C 0
5
10 [cat], wt %
Effect of the (a) temperature and (b) catalyst concentration on the conversion of α-methylstyrene and the product selectivity: (1) α-meth-
ylstyrene conversion, (2) total dimers, and (3) oxidation products. Reaction conditions: (a) 5 wt % Fe-HSZ, 3 h, air; (b) 80°C, 3 h, air.
Iron introduced into the zeolites has the greatest ficient for mediating the dimerization of α-methylsty-
effect on strong acid sites, decreasing their concentra- rene; therefore, all catalyst samples show nearly the
same high activity in a nitrogen atmosphere. The higher
selectivity of H-HSZ (SiO2/Al2O3 = 120) and Fe-HSZ is
due to the low yield of the cyclic dimer, a fact that is
associated with a reduction in the strength and density
of the strong acid sites responsible for the cyclization
reaction. In an air atmosphere, the Brönsted acid sites
are likely to be blocked by the surface oxygen com-
plexes with the Lewis acid sites, thus leading first to a
reduction in the catalytic activity of zeolites in an acid–
base reaction (dimerization of α-methylstyrene) and,
then, to the appearance of the activity in a redox reac-
tion (α-methylstyrene oxidation to acetophenone and
hydratropaldehyde).
tion by 143 µmol/g, whereas the amount of weak acid
sites decreases only by 41 µmol/g. Judging from the
shift in the TD peak, the strength of the ferrosilicate
acid sites is the lowest among all of the examined sam-
ples. The ammonia TPD data can be complemented by
published IR data on the acidity of the ZSM ferrosili-
cates [12]: the acidity of the OH groups located in the
channels (3630 cm–1) and on the external surface
(3730–1) is the same (their PA is 1210 kJ/mol). This
value is much less than the strength of the framework
OH groups in unmodified ZSM zeolites, which is
1180 kJ/mol.
Since an increase in the Si/Al ratio leads to a
decrease in the concentration of aluminum associated
with the Brönsted acid sites, it is clear that the
LAS/BAS ratio increases. In the case of Fe-HSZ zeo-
lite, the fall in the concentration of the Brönsted acid
sites on the external surface is due to the blocking of a
part of the OH groups by iron cations [12]. It may be
CONCLUSIONS
The influence of the Si/Al mole ratio in the range
40–120, the presence of iron cations, and the reaction
conditions on the yield and the composition of the
products of the α-methylstyrene conversion on high sil-
assumed that the amount of the Brönsted acid sites ica pentasils was investigated. It was found that the
remaining on the external surface of the zeolites is suf- activity and selectivity of the catalysts substantially
Table 3. Acid characteristics of the zeolite catalysts
Tmax, °C
Concentration, µmol/g
Catalyst
TI
TII
C1
C2
CΣ
H-HSZ (M = 40)
235
220
215
210
200
450
425
415
410
385
561
331
269
192
290
424
311
212
166
168
985
642
481
358
458
H-HSZ (M = 60)
H-HSZ (M = 80)
H-HSZ (M = 120)
2.13% Fe-HSZ (M = 60)
Note: T and T are the peak maximum temperatures for forms I and II, respectively. C , C , and C are the concentrations of the acid sites
I
II
1
2
Σ
in forms I and II and their total concentration, respectively.
PETROLEUM CHEMISTRY Vol. 48 No. 5 2008

370
GRIGOR’EVA et al.
2. N. G. Grigor’eva, E. A. Paukshtis, B. I. Kutepov, et al.,
Neftekhimiya 45, 453 (2005) [Pet. Chem. 45, 419
(2005)].
depend on the gas atmosphere (air or nitrogen) of the
reaction. In nitrogen, all test catalysts mediate the linear
dimerization of α-methylstyrene. In an air medium, the
activity decreases in the following order: H-HSZ
(SiO2/Al2O3 = 40–80) > H-HSZ (SiO2/Al2O3 = 120) >
Fe-HSZ; the oxidation products acetophenone and
α-phenylpropionaldehyde are formed with a high
selectivity over the two latter catalysts.
3. N. G. Grigor’eva, B. I. Kutepov, R. A. Makhamatkhanov,
et al., Zh. Prikl. Khim. (St. Petersburg) 35, 1574 (2002).
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ACKNOWLEDGMENTS
375 (1989).
This work was supported by the Chemistry and
Materials Science Division, Russian Academy of Sci-
ences under the program the “Development of scientific
principles of new chemical technologies with the pro-
duction of pilot batches of substances and materials.”
7. A. V. Vosmerikov and V. I. Erofeev, Zh. Fiz. Khim. 69,
787 (1995).
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Khim. Prom-st., No. 9, 31 (2004).
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