Dimerization of α-methylstyrene on high-silica zeolites

Article abstract of DOI:10.1134/S0965544106050057

α-Methylstyrene dimerization was studied on high-silica β, ZSM-12, and TsVN zeolites. The acid properties of the zeolites were studied by IR spectroscopy. It was revealed that the catalytic properties of the zeolites in α-methylstyrene dimerization depend on both the acidic and structural characteristics of the catalysts. The highest activity in the reaction was exhibited by zeolite β, as the maximum amount of acid sites were found in its structural units (inside channels, on the outer surface). Zeolite ZSM-12 had the highest selectivity for linear dimers. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S0965544106050057

ISSN 0965-5441, Petroleum Chemistry, 2006, Vol. 46, No. 5, pp. 332–337. © Pleiades Publishing, Inc., 2006.
Original Russian Text © N.G. Grigor’eva, R.R. Galyautdinova, E.A. Paukshtis, B.I. Kutepov, M.I. Tselyutina, U.M. Dzhemilev, 2006, published in Neftekhimiya, 2006, Vol. 46,
No. 5, pp. 360–365.
Dimerization of a-Methylstyrene
on High-Silica Zeolites
N. G. Grigor’evaa, R. R. Galyautdinovaa, E. A. Paukshtisb, B. I. Kutepova,
M. I. Tselyutinaa, and U. M. Dzhemileva
a Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, Russia
b Institute of Petroleum Chemistry and Catalysis, Russian Academy of Sciences, Ufa, Bashkortostan, Russia
e-mail: ink@anrb.ru
Received March 6, 2006
Abstract—α-Methylstyrene dimerization was studied on high-silica β, ZSM-12, and TsVN zeolites. The acid
properties of the zeolites were studied by IR spectroscopy. It was revealed that the catalytic properties of the
zeolites in α-methylstyrene dimerization depend on both the acidic and structural characteristics of the cata-
lysts. The highest activity in the reaction was exhibited by zeolite β, as the maximum amount of acid sites were
found in its structural units (inside channels, on the outer surface). Zeolite ZSM-12 had the highest selectivity
for linear dimers.
DOI: 10.1134/S0965544106050057
INTRODUCTION
Prior to experiments on α-methylstyrene dimeriza-
tion, all catalyst samples were calcined in air at 540°ë
for 4 h.
The linear dimers of α-methylstyrene (4-methyl-
2,4-diphenylpent-1- and 2-enes) and their hydroge-
nated derivatives hold promise as synthetic lubricating
oils and radiation-resistant and insulating materials
[1−3].
The catalytic properties of the zeolites in α-methyl-
styrene dimerization were studied in an isothermal
batch reactor at 20–120°ë and atmospheric pressure in
the presence of 0.1–50.0 wt % zeolite catalysts. The
reactant α-methylstyrene was distilled before the
experiments; its purity was 99.8 wt %. The activity of
the catalysts was measured as the conversion (K) of
α-methylstyrene, and the selectivity for products was
calculated by the formula Si = ci/ä.
α-Methylstyrene dimers can be successfully synthe-
sized in the presence of type Y zeolites, on which the
selectivity for linear dimers is 90–92% at a feedstock
conversion of 95–100% [4, 5].
It is known that high-silica zeolites differ from zeo-
lites Y in structure, as well as in the concentration and
strength of acid sites; therefore, it was of interest to
study the dimerization of α-methylstyrene in the pres-
ence of high-silica zeolites β, ZSM-12, and TsVN. The
obtained data on the catalytic properties of the zeolite
catalysts were correlated with their acid characteristics,
which were determined by IR spectroscopy.
The synthesized hydrocarbons were analyzed on a
Tsvet chromatograph equipped with a flame-ionization
detector, a 2 m × 2 mm metal column packed with 5%
SE-30-coated Chromaton HMDS, in the mode of tem-
perature programming from 50 to 280°C at a rate of
8 K/min, and detectors and evaporator temperatures of
250 and 300°C, respectively, using helium as a carrier
gas at a flow rate of 30 ml/min.
The acid properties of the zeolite specimens were
studied by IR spectroscopy of adsorbed CO [6]. The IR
spectra were recorded on a Shimadzu 8300 Fourier-
transform spectrometer with a resolution of 4 cm–1 and
a number of spectra accumulation equal to 50. The
investigation procedure was described earlier [4].
EXPERIMENTAL
The H-forms of β (SiO2/Al2O3 = 18), ZSM-12
(SiO2/Al2O3 = 34) and TsVN (SiO2/Al2O3 = 28.4) zeo-
lite specimens were investigated in this work. The zeo-
lites were synthesized at the Angarsk Catalyst and
Organic Synthesis Plant (Russia).
RESULTS AND DISCUSSION
The sample of β zeolite available in the Nç4 form
The dimerization of α-methylstyrene on zeolites
was converted into the H form by calcination at 540°ë. yields dimers I–III and trimers:
332

DIMERIZATION OF α-METHYLSTYRENE
333
H2C
CH3
C
2
+
H
Trimers
C
CH3
H3C
C
CH2
+
H3C
+
–H
+
+
–H
–H
CH3
C
CH3
CH2 C CH2
H3C
CH
C CH3
H3C
C
CH3
CH3
H3C
I
II
III
The results of α-methylstyrene dimerization in the portionally to the amount of the catalyst. An increase in
presence of the test zeolites are given in Table 1 and temperature slightly influences the conversion. These
Figs. 1–3. It is seen that the main products are dimers findings allow us to suppose that the reaction proceeds
of α-methylstyrene; the amount of trimers does not on the outer surface of zeolite crystals, not inside the
exceed 15%. The linear isomers dominate the dimer channels.
fraction under the given conditions.
The highest activity was exhibited by zeolite β; the
It was found that TsVN zeolite exhibits the lowest dimerization of α-methylstyrene on this zeolite pro-
catalytic activity in the reaction (Fig. 1). The conver- ceeds even at 20°ë at an α-methylstyrene conversion
sion of α-methylstyrene on this catalyst increases pro- higher than 95 wt %.
Table 1. α-Methylstyrene dimerization on zeolite catalysts
Reaction conditions
α-MS con-
version,
wt %
Selectivity, %
linear
dimer/cyclic
dimer
Catalyst
[cat], wt %
T, °C
τ, h
linear dimer cyclic dimer
trimers
β
1
2
60
20
20
20
80
40
80
60
80
80
120
2.5
4
98.1
94.3
95.0
95.1
91.0
24.6
98.2
94.4
5.1
89.2
86.0
82.6
81.9
94.1
94.6
92.9
93.7
88.1
78.6
87.2
2.9
3.4
3.9
4.7
3.6
2.0
4.0
3.3
7.1
6.6
7.1
7.9
10.6
13.6
13.4
2.3
30.8
25.3
21.2
17.4
26.1
47.3
23.2
28.4
12.4
11.9
12.3
5
3
10
1
0.5
4
ZSM–12
TsVN
5
3
3.4
5
3
3.1
0.5
6
3.0
5
50
10
3
4.8
3
77.8
32.5
14.8
5.7
6
PETROLEUM CHEMISTRY Vol. 46 No. 5 2006

334
GRIGOR’EVA et al.
α-methylstyrene conversion, wt %
1
100
80
60
40
20
(‡)
(b)
1
2
2
3
3
0
10
20
30
[cat], wt %
0
20
40
60
80
100 120
T, °C
Fig. 1. Dependence of α-methylstyrene conversion over zeolites (1) β, (2) ZSM-12, and (3) TsVN on (a) the catalyst concentration
and (b) temperature; reaction conditions: (a) T = 80°C; τ = 1 h; (b) the amount of catalyst is 10 wt %, τ = 1 h.
Selectivity, %
100
80
60
40
20
(‡)
(b)
2
2
3
3
1
1
1*
1*
3*
3*
2*
60
2*
20
0
20
40
80
100 120
T, °C
0
10
30
[cat], wt %
Fig. 2. Influence of the (a) catalyst concentration and (b) temperature on the selectivity for α-methylstyrene dimers: (1) β, (2)
ZSM-12, and (3) TsVN; (1–3) selectivity for linear dimers; (1*–3*) selectivity for cyclic dimer; reaction conditions: (a) T =
80°C, τ = 1 h; (b) the amount of catalyst is 10 wt %, τ = 1 h.
The conversion of α-methylstyrene on zeolite ZSM-12 common feature for all test catalysts. Zeolite ZSM-12
is determined to a great extent by the reaction tempera- exhibits the highest selectivity for linear dimers: the
ture: the conversion is negligible (20–30 wt %) at 20°ë selectivity Sld is 90–94% at an α-methylstyrene conver-
and reaches 100 wt % as the temperature increases to sion of 95–100 wt %, and the yields of the cyclic dimer
60–80°ë.
and trimers are minimal (2–6 and 2–4%, respectively).
The selectivity on TsVN zeolite is close to that on
ZSM-12.
From Fig. 2, it is seen that the product selectivity
depends on the type of zeolite and reaction conditions.
A decrease in the selectivity for linear dimers with an
It was found that the selective synthesis of linear
increase in temperature or catalyst concentration is a dimers (85–90%) on zeolite β is possible only at low
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DIMERIZATION OF α-METHYLSTYRENE
Amount of linear dimers, wt %
335
temperatures (20–40°ë) and catalyst concentrations
100
up to 5 wt %. An increase in temperature, as well as in
the catalyst concentration, results in a sharp decrease
in the selectivity for linear dimers and increases the
yield of the cyclic dimer. Figure 3, which shows a
change in the ratio of linear to cyclic dimers in the cat-
alyzates obtained at 80°C in the presence of 10 wt %
catalyst, clearly demonstrates that the rate of cyclic
dimer formation on zeolite β is much higher than that
on ZSM-12.
2
80
60
40
20
1
The IR data on the acid properties of β, ZSM-12,
and TsVN zeolite specimens are given in Table 2.
The spectra of OH groups of the test zeolites display
absorption bands at 3615 cm–1 (due to vibrations of
bridging Al–OH–Si groups in channels), 3738–
3745 cm–1 (correspond to vibrations of bridging OH
groups on the outer surface of crystals and weakly
acidic terminal SiOH groups), 3670–3700 cm–1 (due to
AlOH groups of extra-framework aluminum). In addi-
tion, the IR spectrum of zeolite β shows absorption
bands at 3780–3790 cm–1 due to the vibrations of termi-
nal AlOH groups.
0
10
20
30
Amount of cyclic dimer, wt %
Fig. 3. Change in the concentration of α-methylstyrene
dimers in the reaction mixture: (1) β, (2) ZSM-12; reaction
conditions: T = 80°C, the amount of catalyst is 10 wt %.
As follows from the intensity of the absorption band
at 3745 cm–1, the maximum amount of OH groups cor-
responding to this band is present in zeolite β and the
minimum amount is in zeolite TsVN.
From the data presented in Table 2, it follows that
the highest concentration of Bronsted acid sites (BAS)
is on β zeolite, and the strength of acid sites on all zeo-
lites is the same.
During adsorption on the surface of a zeolite sam-
ple, carbon monoxide reacts with OH groups, forming
H-bonded complexes. In this case, the OH absorption
bands are shifted to lower frequencies. For all speci-
mens, there are three bands relevant to hydrogen-
bonded OH groups: at 3278 cm–1 (vibrations of bridg-
ing OH groups in channels), 3430 cm–1 (bridging OH
At low temperatures, Lewis acid sites (LAS) also
form complexes with CO, which give rise to IR absorp-
tion bands at 2186–2230 cm–1.
The maximum amount of LASs (in terms of both
groups on the outer surface), and 3640 cm–1 (SiOH type and concentration) was found for the zeolite β
groups).
specimen. The IR spectrum of this sample exhibits a set
Table 2. Acid properties of zeolite catalysts
Site type
Specimen
LAS
BAS
ν
CO, cm–1
NCO, µmol/g
QCO,* kJ/mol
ν
CO, cm–1
NH,** µmol/g
PA, kJ/mol
β
2232
2224
12.3
43.0
22.0
49.0
244.0
4.0
55.0
51.0
2175
2166
128
51
1165
1165
2214–2206
2197
46–42.0
37.5
2193
35.5
ZSM–12
TsVN
2228
53.0
2174
2162
75
31
1165
1165
2215
0.9
46.5
2189
1.1
33.5
2186
4.2
32.0
2174
2164
104
31
1165
1165
* Q = 10.5 + 0.5 (ν 2143) [6].
CO
CO
** N is the concentration averaged over the CO and OH band (initial and after CO adsorption).
H
PETROLEUM CHEMISTRY Vol. 46 No. 5 2006

336
GRIGOR’EVA et al.
of 5 bands characteristic of zeolites: at 2193 cm–1 (CO 450°ë under high-vacuum conditions. Under low-tem-
complexes adsorbed on the alumina phase), 2200 cm–1 perature (<100°ë) α-methylstyrene dimerization con-
(nuclei of the alumina phase), 2215 cm–1 (BAS-local- ditions at atmospheric pressure, Lewis acid sites can
ized nuclei of the alumina phase), 2223 cm–1 (presum- react with water molecules to form a Bronsted site
ably, tetrameric oxide–hydroxide aluminum clusters), unless special measures are taken (e.g., sealing) to pre-
and 2228–2232 cm–1 (BAS-bonded dimeric oxide clus- vent the contact of the reaction mixture with the envi-
ters of aluminum).
ronment. Thus, it is likely that zeolite β in actuality con-
tains a much greater number of Bronsted sites AC than
was determined by IR spectroscopy. Second, according
to [6], Lewis sites in high-silica zeolites are situated in
places of connection of microcrystals, i.e., on the outer
zeolite surface, and, hence, are the most accessible to
reacting molecules. The great amount of easily accessi-
ble acid sites can ensure the effect of high activity of
this catalyst. Note that the role of these acid sites
undoubtedly increases with a rise in the reaction tem-
perature and that they make the main contribution to the
reactions of intramolecular alkylation and cyclization
leading to the formation of the cyclic dimer.
The total LAS concentration on the zeolite β speci-
men was 370.3 µmol/g. About 80% of this amount falls
to the share of CO complexes with nuclei of the alu-
mina phase. The strength of these LASs as determined
in terms of the heat of adsorption, Qëé, is low (35.5–
37.5 kJ/mol). The concentration of the strongest Lewis
acid sites (Qëé = 51.0–55.0 kJ/mol) is considerably
higher than that on other specimens.
The IR spectrum of the zeolite ZSM-12 specimen
exhibits the same absorption bands as in the case of
zeolite β, although their intensity is considerably lower
and an additional band appears at 2173 cm–1. Corre-
spondingly, the LAS concentration in this zeolite was
62 times lower.
The spectrum of zeolite TsVN sharply differs from
that of the aforementioned two specimens and exhibits
only one absorption band at 2186 cm–1. The concentra-
tion and strength of Lewis acid sites on this specimen
was minimal.
A comparison of the acidic and catalytic properties
of the test zeolites shows that the zeolites can be
arranged in the following order in terms of concentra-
tion of acid sites (both BAS concentration and the total
concentration of acid sites): β > TsVN > ZSM-12. The
activity of the catalysts in α-methylstyrene dimeriza-
tion decreases in the order β > ZSM-12 ꢀ TsVN.
Since the strength of Bronsted acid sites is the same
(~1165 kJ/mol) in all test zeolites, the obtained depen-
dence of the catalyst activity may mean that either not
all acid sites of zeolite TsVN are accessible to α-meth-
ylstyrene molecules or the formed products cannot be
desorbed onto the outer zeolite surface and, thus, block
the zeolite channels that contain the main amount of
acid OH groups inside. Zeolite TsVN has a porous
structure similar to that of zeolite ZSM-5, i.e., a system
of channels of two types: straight channels with a round
10-membered ring 0.56 nm in diameter and sinusoidal
(elliptic) channels with a pore opening diameter of 0.51
× 0.55 nm. Obviously, it is difficult for α-methylstyrene
molecules, which have dimensions of 0.43 × 0.72 nm,
to penetrate inside zeolite channels, and is particularly
impossible for α-methylstyrene dimers to diffuse inside
the channels. Owing to these constraints, the transfor-
mation of α-methylstyrene proceeds on active sites
located on the outer zeolite surface (about 30% of total
amount of OH groups).
The different selectivities for linear and cyclic
dimers on ZSM-12 and β-zeolites are associated with
the peculiarities of the acid properties of the catalysts,
as well as with their porous structure. Taking into
account the obtained data on acidity, this difference
may be explained by the fact that the amount of OH-
groups on the outer surface of β-zeolite is 1.7 times that
on ZSM-12. Therefore, conditions for the formation of
bulkier molecules of cyclic dimer and trimers are favor-
able on zeolite β.
The appearance of a molecular-sieving effect of
zeolites also cannot be excluded. It is known [7] that the
porous structure of zeolite β represents a three-dimen-
sional system of two-type straight channels connected
with each other and formed by 12-membered rings with
mouth dimensions of 0.55 × 0.55 nm and 0.64 ×
0.75 nm. In zeolite ZSM-12, straight channels are also
formed by 12-member rings with a mouth size of 0.55 ×
0.62 nm. Apparently, the cyclic dimer of
α-methylstyrene more readily forms and diffuses from
wide straight channels of zeolite β than from the nar-
rower channels of ZSM-12. However, the pore size of
zeolite ZSM-12 is optimal for the formation and diffu-
sion of linear dimers.
CONCLUSIONS
It was found that the catalytic properties of zeolites
β, ZSM-12, and TsVN in α-methylstyrene dimerization
are determined by differences in concentration and
strength of acid sites, as well as by the molecular-sieve
properties of the catalysts (channel size, location of
acid sites inside the channels or on the outer surface of
crystals).
The high activity of zeolite β seems to be deter-
Zeolite β exhibits the highest catalytic activity in the
mined by several factors. First, as has been mentioned reaction, as it possesses the maximum total concentra-
above, zeolite β has a high concentration of Lewis sites tion of Bronsted and Lewis acid sites occurring in wide
(twice that of Bronsted sites). Their presence in the zeo- channels (BAS) and on the outer surface of crystals
lite was detected after calcination of the specimen at (LAS).
PETROLEUM CHEMISTRY Vol. 46 No. 5 2006

DIMERIZATION OF α-METHYLSTYRENE
337
Zeolite ZSM-12 is the most selective for linear
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